The Effects of the Tohoku Earthquake on Regional Seismicity in Japan

Understanding Earthquake Risk in Japan Following the Tohoku-Oki Earthquake of March 11, 2011

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Table of Contents

1 Executive Summary ...... 5

2 Introduction ...... 11

2.1 The AIR Earthquake Model for Japan ...... 11

3 The Effects of the Tohoku Event on Earthquake Risk in Japan ...... 15

3.1 Japan’s Tectonic Setting—An Overview ...... 16

3.2 Implications of the Tohoku Earthquake for the State of Stress along the Japan Trench ...... 17

3.3 Implications of the Tohoku Earthquake for the State of Stress in the Kanto Plain, Sagami Trough, and Regional Faults ...... 25

4 Conclusions ...... 29

4.1 Summary of Apparent Changes in Regional Risk ...... 29

5 Research Notes – The Effects of the Tohoku Earthquake on Regional Seismicity in Japan ...... 31

5.1 Changes in Coulomb Stress Failure in the Kanto Plain and for 98 Major Faults in Japan ...... 36

6 References ...... 46

7 About AIR Worldwide ...... 49

List of Figures

Figure 1. Source Zones within the Japan Trench ...... 6 Figure 2. Faults Potentially Affected by the Tohoku-oki Earthquake (faults delineated in red are explicitly referenced in this document) ...... 8 Figure 3. AIR’s Modeled Tsunami Footprint Following the Tohoku Earthquake and Close-up of the Inundated Region near Sendai ...... 13 Figure 4. Tectonic Setting of Japan and Major Historical Earthquakes ...... 16 Figure 5. M9.0 Tohoku Earthquake and M7.9 Aftershock (contours) with Footprints of Historical Earthquakes (left panel) and Subduction Zones of the Japan Trench (right panel) ...... 18 Figure 6. Tectonics of the “Boso-oki Segment” Offshore of the Kanto Plain ...... 23 Figure 7. Surface Projection of three Rupture Scenarios under the Kanto Plain: Pacific Interface Subduction (left panel), Philippine Sea Interface Subduction (middle panel) and Philippine Sea Intraplate Subduction (right panel) ...... 26 Figure 8. Faults Potentially Affected by the Tohoku-oki Earthquake...... 30

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List of Tables

Figure 9. Tectonic Forces create Secular Stress on a Fault ...... 32 Figure 10. Schematic Diagram of a Stochastic Renewal Model used to Estimate Time-Dependent Rupture Probabilities...... 33 Figure 11. Decay of Friction Coefficient with Change in Slip Velocity...... 34 Figure 12. Seismicity Ratios for Four Different Cases ...... 35 Figure 13. Aftershock Duration and Fault Loading Rates for Selected Large Earthquakes in Different Tectonic Settings, 1 kyr = 1,000 years (Stein et al. 2009) ...... 36 Figure 14. Subduction Segments and 98 Major Faults used to Determine Changes in Coulomb Stress Failure ...... 37 Figure 15. Changes in the CFS on the Pacific Plate Interface beneath the Boso Peninsula due to Coseismic Displacement (Ozawa et al., 2011), Friction Coefficient = 0.4 ...... 38 Figure 16. Comparison of CFSC values based on coseismic displacements, coseismic and postseismic displacements, (both from Ozawa et al., 2011,) and coseismic displacement (California Institute of Technology), Friction Coefficient = 0.4...... 39 Figure 17. The rate of M ≥ 4 Earthquakes Increased Significantly after Tohoku ...... 40 Figure 18. CFS Changes at the Philippine Sea Interface for Kanto and Genroku using Right-Lateral Strike-Slip in the Sagami Trough due to Coseismic Displacement (Ozawa et al., 2011), Friction Coefficient = 0.4 ...... 43 Figure 19. CFS Changes at the Philippine Sea Interface for Kanto and Genroku using Oblique Thrust and Right-Lateral Strike-Slip in the Sagami Trough due to Coseismic Displacement (Ozawa et al., 2011), Friction Coefficient = 0.4 ...... 43 Figure 20. CFS Changes for Crustal Faults with CFSC ≥ 0.1 Bars due to Coseismic Displacement (Ozawa et al., 2011) ...... 44 Figure 21. CFS Changes for Crustal Faults with CFSC ≥ 0.1 Bar due to Coseismic and Postseismic Displacement (Ozawa et al., 2011) ...... 44 Figure 22. CFS Changes for Crustal Faults with CFSC ≤ -1.0 Bar due to Coseismic Displacement, (Ozawa et al., 2011) ...... 45

List of Tables

Table 1. Summary of Risk Changes for Subduction Zones and Faults Potentially Affected by the Tohoku-oki Earthquake ...... 9 Table 2. Number of Large Earthquakes in the Japan Trench since 1600 ...... 17 Table 3. Seismic Source Zones of the Japan Trench as Identified by HERP in the 2007 National Seismic Hazard Maps ...... 18 Table 4. Summary of Risk Changes for Subduction Zones and Faults Potentially Affected by the Tohoku-oki Earthquake ...... 30

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Executive Summary

1 Executive Summary

Japan has a long and well-documented history of large earthquakes. This has fostered a deep awareness of earthquake risk throughout the nation, which continues to develop some of the highest standards in the world for building codes and risk mitigation practices. The country’s complex and active tectonics has been the subject of extensive research leading to extremely strict and heavily enforced practices aimed at minimizing the damaging effects of earthquakes.

Yet not even Japan was prepared for the Tohoku-oki earthquake and tsunami, which devastated the country on March 11, 2011. Conventional wisdom, upheld by the scientific community and Japan’s own Headquarters for Earthquake Research Promotion (HERP), maintained that it was not possible for such an event to occur in that segment of the Japan trench—an area that has produced a history of large earthquakes but, until recently, no truly devastating ones.

Seismologists had interpreted the lack of great earthquakes in this region as a sign that the subduction zone was weakly coupled and therefore not capable of accumulating enough energy to produce an M9.0 temblor. The terrible consequences of this inaccurate supposition were made apparent during the event, which killed an estimated 20,000 people.

There is now a widespread sense of urgency in reexamining the seismicity of regions neighboring the Tohoku rupture, and in assessing the impact that this great event has had on the probability of another destructive event occurring in nearby regions. The purpose of this document is to provide guidance on these questions and to users of the AIR Earthquake Model for Japan.

The findings described in this paper are the result of a re-examination (in light of what we know now) of the existing national seismic hazard maps as published by HERP in 2007, new findings released by HERP as it undertakes a comprehensive update to those maps, and an original analysis of Coulomb failure stress changes conducted by AIR seismologists. The results of this analysis represent a preliminary quantification of the impact of those changes on fault rupture probabilities.

Provided below is a high-level summary of AIR’s findings, details of which can be found in the body of this document. The most technical discussions and details about AIR’s methodology are provided in Section 5 (Research Notes).

Implications of the Tohoku Earthquake for the Japan Trench

The greatest impact from the Tohoku event in terms of stress change is in the seismic source zones of the Japan Trench that ruptured during the earthquake and, of particular interest, the nearby subduction zone segments that did not rupture. A reference map is provided in Figure 1, and is repeated in the detailed discussions in Section 3.2.

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Executive Summary

Boso

Figure 1. Source Zones within the Japan Trench

• The two seismic zones closest to the epicenter of the earthquake are the Miyagi-oki and the southern Sanriku-oki zones (Zones 7 and 6, respectively). These zones experienced more than eight meters of slip during the Tohoku earthquake, releasing an enormous amount of energy that had been accumulating for hundreds of years in these zones. Seismologists are nearly unanimous in the conclusion that the short-term seismic risk for large earthquakes in these areas has in all likelihood been substantially reduced.

• Farther from the epicenter are zones that were at least partially ruptured. These include the Central Sanriku-oki (Zone 5), the Fukushima-oki (Zone 8), and the Ibaraki-oki (Zone 9) zones, all of which experienced over four meters of slip―an amount that is typical for large-magnitude subduction earthquakes. The amount of energy released in these areas during the event indicate that the risk here had been significantly underestimated by HERP, whose existing hazard model for these regions provided a maximum earthquake magnitude of less than 7.5. Indeed, an aftershock of the Tohoku event in Zone 9 registered M7.9. There is still a high level of uncertainty concerning the amount of coseismic slip in these areas; and therefore the short-term seismic risk for large, damaging earthquakes may be either lower or higher than the risk level estimated by HERP.

• Of the unruptured zones, one of the closest to the Tohoku earthquake is the northern Sanriku-oki seismic zone (Zone 3). The overall stress on this subduction zone fault may have increased as a result of the Tohoku earthquake. However, there is evidence that the southern part of this zone had accumulated only a small amount of seismic energy before the Tohoku event, which may have mitigated stress transfer.

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Executive Summary

• The southernmost part of the Japan Trench, which lies off the Boso peninsula (the Boso-oki segment), has not experienced a significant (damaging or fatal) rupture for several hundred years and hence HERP did not assign a zone number (it is designated “Boso” in Figure 4)1. Recent studies using GPS surveys and geophysical models have been inconclusive as to whether the segment is locked. However, because the Tohoku earthquake clearly proved that the lack of large earthquakes in the past is not reliable evidence that large earthquakes will not occur in the future, scientists will no longer rule out the possibility that the Boso-oki segment has been fully or partially locked for approximately 200-400 years and may have accumulated enough energy to generate an earthquake of magnitude 8.4-8.9; an event that would devastate the Kanto plain—and Tokyo.

Implications of the Tohoku Earthquake for the Kanto Plain, the Sagami and Nankai Troughs, and Regional Crustal Faults

To evaluate the impact of the Tohoku earthquake on source zones outside of the Japan Trench, including the Kanto Plain on which Tokyo is located, AIR analyzed the changes in Coulomb failure stress2 in conjunction with a rate-state model. The technical details of the methodology, which accounts for uncertainty in various fault parameters, are provided in Section 5; here we provide a summary of findings. The results of AIR’s analysis agree very well with the recently published results by HERP, using the same set of fault parameters given by HERP.

• The results of the AIR study indicate a large increase in the state of stress underneath the Kanto Plain along the interface between the Pacific and Philippine Sea tectonic plates—and indeed, since the Tohoku earthquake, the occurrence of M ≥ 3 earthquakes in the Boso Peninsula has measurably increased. Although the increase in seismic activity should be viewed as transient, AIR’s analysis indicates that the 30-year rupture probability of M ≥ 6.7 earthquakes here may have increased from 72%, based on HERP’s former Poissonian (time independent) estimate, to between 81% and 93%. The range reflects the uncertainties in model parameters.

A study (Sakai et al., 2011) published last fall by researchers at Tokyo University’s Earthquake Research Institute (ERI)—and which has since received considerable press attention—provided an estimated occurrence probability of 70% for an M7.0 earthquake striking in the vicinity of the Boso Peninsula within the next 4 years. AIR’s analysis produces a corresponding 4-year probability of between 23% and 28%, significantly lower than the one produced by the researchers at ERI.

• Also of interest is the Sagami Trough, which was the source of the M7.9 Taisho (Great Kanto) earthquake of 1923, and the M8.2 Genroku earthquake of 1703. The results of AIR’s analysis indicate that the Tohoku earthquake did not have a large impact in the Sagami trough; the size of stress changes on the trough is small. However, these results are sensitive to the assumed rupture

1 HERP did identify two events related to this zone (in 1909 and 1953). Neither, however, caused damage or fatalities. 2 Coulomb failure stress is the stress that is produced from opposing forces, such as tectonic plates pushing against one another. It is typically measured in “bars” where a bar is roughly equivalent to the atmospheric pressure at sea level, or more precisely, 0.987 of the atmospheric pressure at sea level.

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Executive Summary

mechanisms for the Kanto and Genroku earthquakes and to the detail of the slip distribution at the southern end of Tohoku rupture, about which there is considerable uncertainty. AIR’s analysis indicates that the 30-year time-dependent probability for earthquakes similar to the 1923 Kanto event has potentially increased from .76% to 1.1%-1.6%; thus a probability that was low before Tohoku remains low, despite a significant percentage increase. For earthquakes similar to the 1703 Genroku event, the time-dependent probability also remains very low.

• The changes in the Coulomb failure stress on the Nankai Trough (see Figure 4)—including the Tokai segment of the Nankai Trough, which is closest to Tokyo—are quite small. Any change in rupture probabilities is therefore negligible.

• Of the 98 crustal faults analyzed by AIR, the one most affected by the Tohoku earthquake is the Futaba fault, which lies in Fukushima prefecture. AIR’s analysis shows this fault underwent a mean positive Coulomb stress change of 6.5 bars—a significant amount, although there is also significant uncertainty in the result. Yet the Tohoku event does not appear to have increased the seismicity in the vicinity of this fault.3 One possible explanation could be a rather strong coupling of the fault, a proposition that is supported by the fault’s extremely long recurrence interval of approximately 10,000 years. Other faults, including the Tachikawa, Miura and Atera faults (see Figure 2), also showed stress increases, while a number of other faults in northern Honshu (not shown) show large negative stress changes.

Figure 2. Faults Potentially Affected by the Tohoku-oki Earthquake (faults delineated in red are explicitly referenced in this document)

3 It is important to keep in mind the large uncertainties inherent in establishing a predictable cause and effect relationship between stress changes on a fault and changes in resulting seismicity; uncertainties that are due mainly to the lack of detailed knowledge of the faulting parameters and the frictional states at depth.

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Executive Summary

Implications of the Tohoku Earthquake for AIR Modeled Industry Loss Estimates

AIR’s analysis of the impacts of stress transfer have shown that the two most critical regions of risk that have been impacted by the Tohoku event are the Kanto Plain and, to a much lesser extent, the section of the Sagami Trough where the 1923 Kanto event took place. Based on the revised estimates of rupture probabilities provided in this paper, AIR has quantified their marginal impact on modeled losses using the existing AIR Earthquake Model for Japan. While the impact of these shifts in probabilities is most pronounced in the Tokyo region, it is instructive to look at its impact for the entire country. Using the lower and upper bounds of AIR’s probability estimates as constraints, the insurable, or ground up, industry losses for all of Japan increased on an average of about 15% for return periods between 1 and 30 years; and about 2% for return periods between 100 to 250 years. Using the upper bound alone, the maximum change in ground up losses for return periods up to 30 years is about 25%.

These estimates should in no way be deemed an early indication of the update to the AIR Earthquake Model for Japan scheduled for release in 2013. That update will incorporate a comprehensive set of enhancements—including the explicit modeling of tsunami and liquefaction risk—as well as additional updates on rupture probabilities from HERP and the wider scientific community. However, this information does translate the scientific modeling of the effects of stress transfer and its associated uncertainties into a perspective on risk.

It is also extremely important to point out that the indications of modeled loss changes provided above pertain to industrywide exposures. Results for any particular portfolio may deviate significantly from industry results depending on the extent to which a company’s own exposure deviates from industry averages in terms of construction types, occupancies, and perhaps most importantly, geographic distribution (location).

The intent of this paper has been to guide those who use the existing AIR Earthquake Model for Japan to think about risk in Japan more holistically. While much remains to be done in order to complete the model update, we hope that the analysis results provided in this paper helps eliminate some of the uncertainties. A summary of these results is provided in Table 1 below.

Table 1. Summary of Risk Changes for Subduction Zones and Faults Potentially Affected by the Tohoku-oki Earthquake

Seismic Zone or Fault Potential Notes Risk change Name Number Northern Sanriku-oki 3 No Change or Increase Positive Impact from Tohoku HERP has only the Gutenberg-Richter Central Sanriku-oki 5 Increase Distribution for this area. Southern Sanriku-oki 6 Decrease Tohoku Main Rupture Area Miyagi-oki segment 7 Decrease Tohoku Main Rupture Area Fukushima-oki 8 Uncertain

Ibaraki-oki 9 Increase Previously Underestimated by HERP

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Executive Summary

Not modeled by HERP; Positive Impact Offshore of the Boso Peninsula Increase from Tohoku Pacific-Philippine Sea Plate Interface under the Kanto 12 Increase Positive Impact from Tohoku Plain Sagami Trough Subduction (1923 Kanto- Uncertain or small increase Type Interplate) in the western segment Nankai Trough No or Neglibable Change Futaba Fault Tachikawa Fault Increase Positive Impact from Tohoku Atera Fault – Northern Segment

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Introduction

2 Introduction

The magnitude 9.0 earthquake that struck Tohoku-oki in Japan on March 11, 2011, and the destruction wrought by the ensuing tsunami, took the scientific community by surprise. Indeed the magnitude of the event was unprecedented in Japan’s long earthquake history, and the possibility of such an occurrence—certainly in this part of the Japan Trench—was simply not contemplated by the Headquarters for Earthquake Research Promotion, the organization that develops Japan’s seismic hazard maps.

In the aftermath of the Tohoku-oki (henceforth “Tohoku”) earthquake, attention has turned to the possible impact this event may have had on earthquake risk in Japan. In particular, what does the scientific community know about whether and where the stresses relieved by the Tohoku-oki earthquake (hereafter referred to as the Tohoku earthquake) have been transferred to neighboring faults and subduction zone segments—and what effect has that transference had on rupture probabilities? Questions have also been raised about how to interpret and use results from the AIR Earthquake Model for Japan in light of the Tohoku earthquake. This paper addresses both issues.

Although damage from this event is most closely associated with the massive tsunami—which in places reached a height of more than 30 meters and demolished nearly all structures within its footprint—by AIR’s estimate, the tsunami was responsible for only about 30% of overall insured losses from this event. Shake damage was far more widespread—and it is worth remembering that shake damage would have been significant within the area subsequently impacted by the tsunami. Damage due to liquefaction was also considerable, although given such vivid images of the tsunami and its after effects, the impact of liquefaction has not yet received the attention it deserves.

While much has been written about the Tohoku earthquake—and will be for years to come—there is, as yet, very little actual quantification of the impact that the earthquake has had on the distribution of seismic risk in Japan. Nevertheless, this paper provides some context to recent scientific studies of possible stress transfer resulting from the earthquake—including studies conducted at AIR—in an attempt to reduce the considerable uncertainty that currently exists.

2.1 The AIR Earthquake Model for Japan The AIR Earthquake Model for Japan captures the effects of ground shaking and fire-following on insured properties in Japan. It supports literally dozens of construction and occupancy classes in residential, commercial, and industrial lines of business. It also supports a number of specialty lines, including marine cargo, marine hull, inland transit, aviation, railway property, thermal power plants, construction all risk, movable all risk, personal accident, and automobile. In addition, the AIR model supports a broad range of Japan-specific policy conditions, including both single-location and multi- location first loss policies, reduced indemnity policies, as well as step endowment policies that are commonly used by Japan’s mutual insurance companies.

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Introduction

Liquefaction is not explicitly modeled4, nor are the effects of tsunami. However, in response to the Tohoku earthquake, AIR is currently planning an update of the model that will explicitly include the perils of tsunami and liquefaction for all lines of business. That update is currently scheduled for release in 2013.

Performance of the AIR Model Following Tohoku Earthquake

The current AIR Earthquake Model for Japan—and the one in force at the time of the Tohoku earthquake—follows the recommendations of the 2007 National Seismic Hazard Maps of Japan’s Headquarters for Earthquake Research Promotion, or HERP.5 The HERP maps provide specific information on the magnitudes and occurrence rates of earthquakes affecting Japan. They also provide information on the segmentation of the subduction zone and scenarios for how those segments may cascade.

The March 2011 Tohoku earthquake ruptured four contiguous segments (7 of 8 source zones) of the subduction zone off northern Japan. As noted before, such a scenario was not contemplated by HERP, or by the scientific community at large.6 It was, by far, the largest event in Japan’s historical record, which includes more than 36,000 historical earthquakes that have occurred since AD 679. Significantly, because AIR bases its model on the HERP report (with uncertainty added), a scenario similar to the Tohoku event was also not captured in the AIR model.

Fortunately, AIR does not simply rely on “like events” from the model’s stochastic catalog to estimate losses in real time, which in this case would have been highly problematic. Instead, a major advantage of AIR’s approach to real-time loss estimation is that the actual parameters as reported by seismological agencies are used.

On March 12, 2011—within 24 hours after the occurrence of the M9.0 Tohoku earthquake—AIR provided insured loss estimates due to ground shaking and fire-following using reported parameters— including preliminary estimates of magnitude, epicentral location, rupture length and depth—and the AIR Earthquake Model for Japan.

However, while highly sensitive instruments inform these preliminary reports, they are still preliminary, and may change—sometimes significantly—over time. Estimates of losses can be highly sensitive to such uncertainties. In the case of the Tohoku earthquake, for example, uncertainties in the southern extent of the rupture plane—towards the high concentrations of exposure in Tokyo and Chiba prefectures—had major implications for modeled losses.

4 The exception is the marine cargo line, for which liquefaction is modeled. More generally, however, to the extent that modeled losses have been calibrated to actual reported losses that include damage from liquefaction, the impact of that peril is captured implicitly. 5 The HERP maps are the equivalent of the USGS seismic hazard maps on which AIR’s US earthquake models relies. 6 A more thorough discussion of why the magnitude of the Tohoku earthquake took the scientific community by surprise can be found in the AIR Current “Rethinking the Unthinkable: Modeling Unprecedented Ruptures Like the Great Tohoku Earthquake.”

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Introduction

Although AIR researchers can make rough estimates of losses using the “source parameters” of magnitude, depth, and rupture length and direction, more refined estimates can only be made if actual ground motion recordings are available. After the Tohoku earthquake, Japan’s national seismic network remained offline for nearly a week. Once back online, AIR used the recorded ground motion data to constrain the simulated ground motion, and integrated that with the best available estimate of slip distribution to achieve more reliable estimates of shake damage.

A significant driver of loss from the Tohoku earthquake, however, was the devastating tsunami— currently a peril that is not modeled by AIR. To estimate property losses from the tsunami, AIR researchers used high-resolution wave height and elevation data to create an inundation footprint. This was compared to known flooded regions collected from aerial photography and satellite imagery. Further validation was undertaken using the Princeton Ocean Model (POM)—a numerical grid point model used for a wide variety of oceanographic applications worldwide.

Figure 3. AIR’s Modeled Tsunami Footprint Following the Tohoku Earthquake and Close-up of the Inundated Region near Sendai

The estimate obtained from the model indicated that insured shake and fire-following damage to properties ranged between USD 15 billion and USD 35 billion. On March 24, AIR was able to revise this estimate using ground motion data from the Kyoshin network (K-NET), which was offline for a week following the earthquake. The revised estimate, which ranged between USD 20 billion and USD 30 billion and included tsunami and liquefaction losses, continues to compare well with available information.

Japan’s Financial Service Agency (FSA) issued an insured loss estimate of 2.2 trillion JPY (property lines only, net of government recoveries), which is 27.5 billion USD (using the exchange rate of 1 USD = 80 JPY). Swiss Re has estimated insured losses of 35 billion USD and Munich Re has estimated insured

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Introduction

losses of up to 40 billion, but both of these include sources of loss across all lines of business, some of which were not included in AIR’s estimate. In the months since the Tohoku earthquake, many insurance companies have reported that their final losses are in line with expectations based on AIR modeled loss estimates.

Assessing Risk Using the AIR Model Post-Tohoku

The purpose of catastrophe models is to prepare companies for the financial impacts of future catastrophes. Because no model can precisely capture the exact parameters of a future event, the AIR models provide probabilities of loss rather than event probabilities.

Despite the historic magnitude of the Tohoku event, the modeled insured loss estimate of 20-30 billion USD has an annual exceedance probability of 1.7% to 2.5%, which equates to a roughly 40 to 80-year loss—well within the range to which most companies manage their risk.

Certainly, the effect of the Tohoku event on seismic risk in Japan has added an additional level of uncertainty that is difficult to quantify. However, observations of the damage and the results from AIR’s model can provide insight. The devastation within the tsunami footprint (caused by both ground shaking and tsunami) was contained within the inundated areas while damage was fairly moderate in other locations. The actual amount of damage from the tsunami, which was entirely confined within the tsunami footprint, accounted for only about a third of the total damage from the earthquake. Most of the damage was due to ground shaking, which was much more widespread even if it was not as severe at individual locations.

It is important to note that there are other seismic sources in the Japan region that could produce far greater losses than the Tohoku earthquake. While it is not possible for a catastrophe model to perfectly capture the exact parameters of every future event, a well-constructed model will capture the full range of potential losses.

Meanwhile, AIR continues to study the causes and effects of the Tohoku earthquake and its implications for regional seismicity. Preliminary findings are discussed in the sections that follow. These address the likelihood of a repeat event and of damaging aftershock activity, not only in the area of the Tohoku earthquake, but around the Tokyo area as well, and of the uncertainty involved in assessing any change in risk.

Meanwhile, too, HERP has begun the process of updating the National Seismic Hazard Maps in light of the Tohoku earthquake. Already they have assigned a return period of 600 years to the Tohoku earthquake, which means that the time-dependent probability of a recurrence in the next 10, 30, and 50 years is near 0%. HERP has also identified five inland faults for which the probability of rupture may have increased, although they have not released any quantitative information on changes to the probabilities.

More quantitative information is expected from HERP by the end of 2012—and will be incorporated in the 2013 update to the AIR Earthquake Model for Japan.

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The Effects of the Tohoku Event on Earthquake Risk in Japan

3 The Effects of the Tohoku Event on Earthquake Risk in Japan

In any discussion of earthquake risk in Japan post-Tohoku, we need to consider two aspects: 1) risk factors that have always been there but that have heretofore gone unmodeled—namely, tsunami and liquefaction, and; 2) actual physical changes in the risk landscape that resulted from the earthquake.

While the Kobe earthquake of 1995 can be considered a classic shake event, the Tohoku earthquake can reasonably be considered a tsunami event. Although shake damage was also very substantial simply as a result of the large magnitude and the geographic extent of the earthquake’s shake footprint, the Tohoku earthquake left no doubt about the vulnerability of Japan to tsunami. Furthermore, given that the damage from tsunami is an insured peril, any comprehensive view of insured seismic risk in Japan can no longer ignore tsunami risk.

While the tsunami has received most of the attention, liquefaction damage was also substantial, particularly around the Tokyo Bay area, which is characterized by uncompacted, liquefiable soil. Indeed, the Christchurch earthquake in New Zealand has shown that this peril, which previously was considered a secondary peril, can become the primary peril under certain circumstances. Given the extensive evidence of liquefaction in Chiba prefecture, near Tokyo Bay, liquefaction must also be explicitly considered in future modeling efforts.

As AIR prepares to update the AIR Earthquake Model for Japan—an update that will incorporate not only the physical changes that Tohoku has made to the risk landscape in Japan, but also explicit capabilities for modeling tsunami and liquefaction risk—it should be understood that the impact on modeled losses of adding the tsunami and liquefaction perils is likely to dominate the impact of any changes in rupture probabilities that have resulted from the Tohoku earthquake itself.

As for the actual physical risk landscape post-Tohoku, AIR researchers are carefully examining the implications of the event on our understanding of maximum possible magnitudes and the potential stress changes across Japan’s very complex tectonic environment. It is only through a better understanding of these changes that rupture probabilities in the AIR model can be updated. Section 3.2 below will address the impact of the Tohoku earthquake on segments of the Japan Trench, several of which were the source of the Tohoku event. Section 3.3 addresses the impact on other seismic sources, including those in the Kanto Plain, the Sagami Trough and regional crustal faults. We start, however, in Section 3.1, with an overview of regional tectonics that will provide the context for the discussions that follow.

It is important to note at the outset that because the Tohoku earthquake occurred less than a year ago, the current seismicity rate as reported by Japan’s monitoring network is transient. It should be interpreted within the context of the changes in stress along the ruptures and how that affects the seismicity.

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The Effects of the Tohoku Event on Earthquake Risk in Japan

3.1 Japan’s Tectonic Setting—An Overview Japan’s extremely complex tectonic setting is dominated by subduction zone activity although shallow intraplate earthquakes also occur throughout the country. Offshore of the eastern part of the country is the Japan Trench, the site of the Tohoku earthquake (Figure 4). This site is where one of the oldest tectonic plates in the world, the Pacific plate, subducts underneath the Okhotsk and Philippine plates at a rate of about 8 cm/year; a rate that is faster than that of most of the active subduction zones in the world that cause mega-earthquakes. The northern part of the Japan Trench connects with the Kurile- Kamchatka subduction zone while farther south it reaches the Mariana Trench in central Japan.

Figure 4. Tectonic Setting of Japan and Major Historical Earthquakes

Central Japan is the site of the Kanto Plain, which is the most populated area of the country and contains Tokyo as well as several other major cities and ports. It is also a transitional area for the Japan Trench and here the Pacific plate subducts under the Philippine Sea plate at the Mariana Trench, at 6 cm/year. The Philippine Sea plate subducts under the Amurian plate in the Nankai Trough and under the Okhotsk Plat at the Sagami Trough. The Amurian plate also converges with the Okhotsk plate in central Japan and the eastern Sea of Japan. Due to its relatively fast movement, the Pacific plate has sunk over 600 km, deep enough to penetrate the upper mantle. However, the thrust earthquakes it

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The Effects of the Tohoku Event on Earthquake Risk in Japan

generates only occur at shallower depths of 10-55 km, where the rock is cool enough to withstand elastic energy and does not change shape.

Despite Japan’s tectonic setting and long history of large earthquakes, the Tohoku event is the first subduction earthquake with a magnitude exceeding 8.5 that has occurred in the Japan Trench since 1900. The largest known historical earthquake before the Tohoku event happened in 1896 and is believed to have had a magnitude of 8.5.

Table 2 lists the number of earthquakes with magnitudes of 7.0 and higher that have occurred in the Japan Trench over the past 400 years.

Table 2. Number of Large Earthquakes in the Japan Trench since 1600

Total Number of Number of Events Magnitude Events since since 1900 1600

≥ 7.0 69 41

≥ 7.5 23 11

≥ 8.0 6 3

Although the frequency of very large events at M8.0 and higher is much lower than in other trenches such as the Chilean, Aleutian, Kamchatka, and others, it is not the result of fault creep due to a weakly- coupled interface. Instead, the convergence zones between the Pacific and Okhotsk plates is very strong, which has decreased the number of extremely powerful events in the Japan Trench, but also opens the possibility of extremely large events on the order of the Tohoku earthquake.

3.2 Implications of the Tohoku Earthquake for the State of Stress along the Japan Trench The Tohoku earthquake was the largest event on record to occur in the Japan Trench; it ruptured about 500 km of the trench’s 800-km total length, extending from Iwate to Ibaraki prefecture. The rupture area is estimated at more than 500,000 square kilometers. Its footprint dwarfs those of other large historical earthquakes in Japan, as shown in the left-hand panel of Figure 5. The colored contours of the Tohoku footprint in the figure show the amounts of coseismic slip—that is, the relative movement along the fault plane at the time of the earthquake. The largest slip occurred offshore of Miyagi prefecture. By contrast, the single pale yellow area off Ibaraki prefecture is the footprint of the largest aftershock from this event, the M7.9 earthquake that occurred about 30 minutes after the main shock.

In order to assess how the seismic risk in areas along the Japan Trench has been affected by the Tohoku earthquake, we need to know whether all of the accumulated seismic energy in the ruptured area was released, or just a portion of it. If only a portion, then another event could occur relatively soon—its size depending on how much energy remains locked in the fault.

For purposes of developing the 2007 National Seismic Hazard Maps, HERP divided the Japan Trench into eight seismic source zones, as shown in the right-hand panel of Figure 5 and listed in Table 3.

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These zones were delineated based on the pattern of historical earthquake ruptures, and a characteristic earthquake model (a model that defines the magnitude and return period of the characteristic, or representative, earthquake) was developed for each source zone.

Figure 5. M9.0 Tohoku Earthquake and M7.9 Aftershock (contours) with Footprints of Historical Earthquakes (left panel) and Subduction Zones of the Japan Trench (right panel)

Table 3. Seismic Source Zones of the Japan Trench as Identified by HERP in the 2007 National Seismic Hazard Maps

Zone Name

3 Northern Sanriku-oki 5 Central Sanriku-oki 7 Miyagi-oki 6 Southern Sanriku-oki 8 Fukushima-oki 9 Ibaraki-oki Southern Japan Trench, Offshore of the Kanto Plain (this Un-numbered is the area—a “gap”—that lies roughly between Zone 12 and Zone 4 in Figure 3) 4 Eastern Japan Trench

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The Tohoku earthquake ruptured seven of the eight seismic source zones in the Japan Trench. However the amount of seismic energy released in each source zone by the Tohoku earthquake varied significantly. The total amount of energy released can be approximated by the coseismic slip. Because the amount of slip varied along the rupture plan, as seen in the left-hand panel of Figure 5, we can assume that the impact of the Tohoku earthquake on the eight seismic source zones may also be different.

In order to assess how the Tohoku earthquake may impact the different source zones, we need to know not only the coseismic slip distribution of the Tohoku earthquake, but also the seismic energy that had accumulated in each zone before the earthquake. The seismic energy accumulated in a source zone depends on the energy released by previous earthquakes and the rate of seismic energy accumulation along the plate boundary, which is a function of the relative slip rate and seismic coupling between the plates7.

In the following sections, we discuss the impact of the Tohoku earthquake on the seismic risks in each of the seismic source zones in the Japan Trench based on the coseismic slip distribution of the Tohoku earthquake, historical earthquake ruptures, the earthquake risk model used by HERP, and plate coupling.

Northern Sanriku-oki Zone (Zones 3)

The northern Sanriku-oki zone (Zone 3 in Figure 5) did not rupture during the Tohoku earthquake. According to HERP, the seismic risk in this zone is relatively high. Historically, Zone 3 has experienced earthquakes of magnitudes ranging between 7.1 and 7.6 with a mean recurrence interval of about 11 years—relatively frequent. In the 2007 seismic hazard maps, this was the only source zone in the Japan Trench where a characteristic earthquake larger than magnitude 8 was assigned. It is the location of the 1968 Tokachi-oki M8.2 earthquake which, prior to the Tohoku event, was the largest historical earthquake in the Japan Trench recorded by modern instruments.

The southern part of this zone was ruptured by the 1994 M7.8 Sanriku-oki earthquake—the largest in a series of earthquakes ranging in magnitude from 6.5 to 7.8 that occurred between 1989 and 1995. Kawasaki et al. (2001) has recently shown that the energy that had been accumulating in the southern part of this zone since the 1968 Tokachi-oki earthquake may have completely been released by the 1994 earthquake and, since then, by slow slip due to silent earthquakes8 and aseismic deformation. Recent

7 Seismic coupling refers to the interaction between two plates that resist relative plate motion.Plates may be locked or they may move past each other relatively smoothly, or aseismically. The degree of coupling determins the amount of energy that accumulates before that energy is released in the form of an earthquake. The coupling coefficient is the ratio of the amount of energy released by earthquakes to the total energy accumulated along the interface due to plate motion. A coupling coefficient of 0% means all of the energy is released aseismically while one of 100% means no energy has been released.

8 Silent, or slow, earthquakes release energy over a long period that can last for months, unlike typical earthquakes, which release a large amount of energy in seconds, or minutes. Silent earthquakes do not cause seismic waves or ground-shaking.

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GPS studies also suggest that the southern part of Zone 3 is weakly coupled. Therefore very little seismic energy has accumulated in the southern part of the zone, which could have acted as a barrier to the northward propagation of the Tohoku earthquake rupture. The lack of seismic energy accumulation in the southern part of Zone 3 may also explain why aftershock activity from the Tohoku event has been low here.

Since the northern part of this zone did not rupture, its seismic risk remains at pre-Tohoku earthquake levels. While it is possible that the Tohoku earthquake could have increased the stress on the fault in this zone—as it is the closest neighboring segment to the Tohoku rupture—it is also possible that the southern part of the zone may have reduced the impact of the Tohoku event by preventing the rupture from extending further.

Central Sanriku-oki Zone (Zone 5)

The Central Sanriku-oki source zone (Zone 5) has produced no known large earthquakes. Indeed, HERP did not have a characteristic earthquake model for this zone, which points to the perceived low probability for large interplate events in Zone 5 in the 2007 seismic hazard maps.

However, the Tohoku earthquake ruptured the eastern half of Zone 5 producing 4 to 16 meters of coseismic slip. An M7.4 aftershock also occurred here. Therefore, we can conclude that this zone is at least partially coupled and is capable of generating large earthquakes—and that HERP had underestimated the seismic risk.

Since this zone was partially ruptured, the amount of coseismic slip in the western half is probably less than 4 meters. However, the actual coseismic slip cannot be accurately resolved and hence is quite uncertain. A significant amount of deformation has occurred in the western part of Zone 5 since the Tohoku earthquake—as much as 0.5 to 1 meters (Ozawa et al., 2011). This further complicates the situation. Ultimately, therefore, the current risk in this zone remains highly uncertain, but given the large aftershock and several meters of coseismic deformation, it is most likely higher than the HERP recommendation prior to the Tohoku event.

Southern Sanriku-oki and Miyagi-oki Zones (Zones 6 and 7)

The zones that experienced the largest coseismic slip, or energy release, during the Tohoku earthquake were the Southern Sanriku-oki (Zone 6) and Miyagi-oki (Zone 7) source zones. Both zones experienced at least 4 meters of slip and the largest slip may have reached 30 to 60 meters (Ozawa et al. 2011; Simons et al., 2011; Lay et al., 2011; Ammon et al., 2011). This amount of slip implies that part of the Japan Trench in this area had been strongly coupled, or locked, for 375 to 750 years. Indeed, the area that experienced the largest coseismic slip is consistent with near 100% coupling, as had been suggested by some GPS studies (e.g. Mazzotti et al. 2000; Nishimura et al., 2004; Suwa et al., 2006).

Historically, the Miyagi and southern Sanriku-oki zones have been ruptured repeatedly by earthquakes of magnitudes less than 8.0, with a mean return period for large earthquakes of 37 years in the Miyagi- oki zone and 104.5 years in the southern Sanriku-oki zone. The 30-year probability of a large

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earthquake in these two zones is 98% in the Miyagi-oki zone and 79% in the southern Sanriku-oki zone, according to HERP. Therefore earthquake risk in these two zones was deemed to be high before the Tohoku earthquake.

Yet the actual seismic energy released by the Tohoku earthquake in these two zones was much higher than expected. This indicates that historical earthquakes here had ruptured only part of the source zones, primarily in Zone 7. The main part of Zone 6 may have not been ruptured in at least 375 years. The coseismic slip distribution in Zone 7 is smaller than in Zone 6, which is consistent with the observation that Zone 7 had repeatedly been ruptured in the last 200 year, hence there was less energy accumulation before the Tohoku rupture. Given the large amount of slip that occurred in these two zones, it is very unlikely that another large event will occur here any time soon. Therefore seismic risk in these two zones should be reduced.

Fukushima-oki Zone (Zone 8)

The Tohoku earthquake also ruptured the eastern half of the Fukushima-oki source zone (Zone 8). The slip is estimated to have been over 4 meters, which represents a significant amount of energy. The coseismic slip in the western half of the Fukushima-oki zone is probably less than 4 meters; however it cannot be accurately resolved. Hence it is unclear if any significant amount of seismic energy was released here by the Tohoku earthquake.

The western half of the Fukushima-oki zone was ruptured by a series of M7.0 to 7.5 earthquakes in 1938. If the subduction zone is fully locked in this region, 5 to 6 meters of slip could have accumulated since 1938. On the other hand, if the subduction zone is less than 100% coupled, as indicated by some GPS studies (e.g., Nishimura et al., 2004; Suwa et al., 2006), then the total accumulated slip would most likely be less than 5 meters. This amount of slip would be close to the lower limit of model resolution in most recent studies of the Tohoku earthquake rupture. Therefore one cannot reliably infer how the risk has changed after the Tohoku earthquake.

In its 2007 seismic hazard maps, HERP assumed the return period of the characteristic earthquake in Zone 8 is 400 years. This implies that the 30-year probability of a large earthquake in this zone is quite low—just 7%— given that it last ruptured in 1938.

However, HERP’s assessment of earthquake risk in this zone is likely an underestimation. A recurrence interval of 400 years for a magnitude 7.4 earthquake would mean that only 1% of the total energy accumulated along the plate boundary is actually released by earthquakes, with the remaining energy being released by aseismic deformation. This is inconsistent with recent GPS studies that suggest more than 50% coupling along this segment of the Japan Trench (e.g. Nishimura et al., 2004; Suwa et al., 2006). In addition, the seismic energy released during the Tohoku earthquake in this zone alone is equivalent to a magnitude 8 earthquake, which again shows that HERP had underestimated the risk for this zone.

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Ibaraki-oki Zone (Zone 9)

The Ibaraki-oki zone (Zone 9) was partially ruptured by the Tohoku earthquake. The main rupture extended to the western part of the zone. The largest aftershock (M7.9) also occurred in the middle of the zone. Coseismic slips in the main rupture area and the large aftershock area both reached more than 4 meters, implying that seismic energy has been accumulating in the zone for at least 50 years.

Prior to the Tohoku earthquake, the Ibaraki zone had exhibited so little seismic activity that HERP had assumed that only earthquakes of magnitude less than 7 will occur here. The characteristic magnitude assumed by HERP was 6.8, with a mean return period of 15.5 years. HERP’s seismicity model for this zone would imply that less than 1% of the total plate motion in the region produces earthquakes.

Although plate coupling in Zone 9 may have been lower than in Zones 6 and 7, recent GPS studies suggested a coupling coefficient of 25% to 50% for this part of the Japan Trench (Nishimura et al., 2004; Suwa et al., 2006). This would imply that HERP’s seismicity model may have significantly underestimated the seismic risk for this zone.

The M7.9 aftershock is another piece of evidence that the seismic risk in this zone was underestimated by HERP. If the 25%-50% coupling suggested by GPS data were accurate for this zone, this would indicate that the M7.9 aftershock and other measurable coseismic slip in this zone would have released less than half of the seismic energy accumulated in the last 200 years during which no large events occurred. Therefore the existence of areas that remain unruptured in this zone is possible, and the risk of another large earthquake therefore cannot be ruled out. However, because of uncertainties in the coseismic distribution of the Tohoku earthquake obtained from various studies, and in the degree of plate coupling obtained from GPS studies, actually quantifying such a risk is extremely challenging.

The Southern Japan Trench, Offshore of the Kanto Plain (“Boso-oki Segment”)

South of Zone 9, from the southern end of the Tohoku rupture to where the Japan Trench meets the Sagami Trough and the Mariana Trench (Figure 5), and east of the Kanto Plain (Zone 12 Figure 5), is an area about 180 km long that hereafter will be referred to as the Boso-oki, or Boso, segment (offshore from the Boso Peninsula) of the Japan Trench. In Figure 5, it is the area roughly between Zone 12 and Zone 4, although it encroaches into Zone 4.

Historically, the Boso segment has been very quiet. Although HERP did identify two events related to this zone, they caused no damage, and hence HERP did not formally identify it as a seismic zone (nor assign a zone number). There have been no known damaging or fatal interplate earthquakes here since at least 1677. Indeed, the only large historical event that might have occurred in the segment is the 1677 earthquake, with an estimated magnitude close to 8, which produced tsunami damage in the Boso Peninsula. Very little is known, however, about the 1677 event.

The tectonics of this area is illustrated in Figure 6, which shows the surface projection (shaded area) of the intersection of the Japan Trench with the Mariana Trench and the Sagami Trough.

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Tectonically, the Boso segment of the Japan Trench is quite complex. A series of sea mounds intersects the Japan Trench near the northern edge of the segment. The northern edge also represents the northernmost reach of the Philippine Sea plate. Thus there is a physical transition in the plate boundary; here the Pacific plate comes in contact with the Philippine Sea plate instead of the Okhotsk plate (Figure 4). The slip rate is reduced from the Pacific-Okhotsk plate motion rate of about 8 cm/year to about 6 cm/year at the boundary between the Pacific and Philippine Sea plates, indicating a slower rate of energy accumulation in this segment.

Figure 6. Tectonics of the “Boso-oki Segment” Offshore of the Kanto Plain

Because of the absence of large historical ruptures in this area, HERP only considered the Pacific plate subduction zone interface in the depth range of 40 – 80 km, which lies in the Kanto Plain region (Zone 12 in Figure 5). The maximum characteristic earthquake magnitude for Zone 12 is assumed to be 7.2.

The seismic source zone near the trench axis (Zone 4) also extends into the Boso segment, and accounts for certain tsunamigenic interplate earthquakes far offshore in the trench. HERP’s seismic model did not consider any scenarios that involve the rupture of the entire Boso segment. In the Japan Trench generally, typical large interface earthquakes occur in the depth range of 20-40 km, but such an earthquake in this part (Boso segment) of the Japan Trench was not contemplated by HERP. This means that HERP considered that the part of the subduction zone in the Boso segment to be unlocked (0% coupling).

Whether HERP is right in its assumption is of critical importance. At present the only evidence that the Boso segment is unlocked is the lack of earthquake activity in the area. Other evidence of the degree of plate coupling for this segment is either missing or inconclusive. Unlike the central and northern segments where dense GPS data collected onshore can determine the degree of coupling along the plate

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interface, the GPS data in the Kanto Plain cannot help determine the degree of plate coupling along the Pacific-Philippine Sea plate boundary. In the Kanto area, the Philippine Sea plate subducts under Honshu Island and the Pacific plate subducts under the Philippine Sea plate at a greater depth. Therefore, crustal motion in the Kanto region detected by GPS and other geodetic techniques are dominated by the subduction of the Philippine Sea plate under Honshu.

Nishimura et al. (2006) recently developed a physical model (an elastic dislocation model) to interpret the GPS and leveling survey data in the Kanto Plain, east Tokai, and the Izu islands. They were able to estimate the interseismic slip rate along the Philippine Sea plate subduction zone fault well, but they were unable to resolve the slip rate along the Pacific-Philippine Sea plate interface. Their result indicates that the complexity of the tectonics, the limitations of the currently available model and the limitations of the data all make it difficult to determine the degree of coupling along the Pacific- Philippine Sea plate interface.

Therefore, one cannot rule out that this segment of the plate boundary may be partially or even fully locked. Indeed, the Tohoku earthquake itself suggests that the lack of large earthquakes over the course of 200 to 400 years cannot act as proof that a segment is unlocked.

What is the potential risk if the Pacific–Philippine Sea plate is, in fact, fully or partially coupled? If we assume, for example, that this segment has in fact been 50% to 100% coupled for 200-400 years, the total slip accumulated along the plate boundary would range from 6 to 24 meters, even at this slow rate of plate motion. By comparison, the devastating 1923 M8 Kanto earthquake, the 1944 M7.9 Tonankai earthquake, and the 1946 M8.0 Nankai earthquake all had maximum coseismic slips of 6-8 meters (Tanioka, 2004; Nyst et al., 2005). The possible total accumulated slip in the Boso segment is similar to, or larger than, the maximum slips released by these events.

Indeed, the potential seismic moment accumulated is equivalent to a moment magnitude ranging from 8.4 to 8.9. A magnitude 8.4 to 8.9 earthquake would be the largest ever to be experienced in the Kanto Plain, and would cause significant shake damage in the greater Tokyo area. Moreover, it could also generate large tsunami waves around the Boso and Izu Peninsulas, resulting in further damage.

Great earthquakes like Tohoku can be followed, for long periods, by large aftershocks. But they can also transfer large amounts of stress to neighboring segments, triggering their rupture. The M9.3 2004 great Sumatra earthquake led to the rupture of the southern neighboring segment with an M8.5 earthquake in 2005. The 1944 Tonankai earthquake was followed two years later by the M8.0 Nankai earthquake along the segment to its south. And again in 1854, the M8.4 Ansei Tokai earthquake was followed by another M8.4 earthquake one day later in the neighboring segment.

The Boso segment of the Japan Trench is the immediate southern neighbor of the Tohoku rupture. A simple stress transfer calculation indicates that the stress on the plate interface in the depth range of about 30-40 km has increased by 2 to 4 bars (see Section 3.3), which is significant. Needless to say, the Tohoku earthquake will greatly promote the release of any accumulated seismic moment, and hence will significantly increase the risk of failure in the form of a great earthquake in the Boso segment—if that segment is indeed locked.

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Moreover, there would be few physical barriers to stop such a rupture from propagating southward into the Mariana Trench. If the Boso segment holds a very large slip accumulation, the potential earthquake could propagate well beyond the segment’s 180 km length and into the Mariana Trench for hundreds of kilometers, resulting in a much larger event. The operative word here, however, is “if.”

3.3 Implications of the Tohoku Earthquake for the State of Stress in the Kanto Plain, Sagami Trough, and Regional Faults Section 3.2 discussed stress changes along the length of the Japan Trench—the source of the March 2011 Tohoku earthquake. In this section we turn our attention to other nearby earthquake sources, including the Kanto Plain on which Tokyo sits, the Sagami Trough, which lies south of the Japan Trench, and (shallow) crustal faults throughout Japan.

Kanto Plain

Because of the major concentration of exposure in the Kanto Plain region (as noted above, this is where Tokyo lies), this is the region of particular interest in any discussion of stress transfer resulting from the Tohoku earthquake.

The Kanto Plain region lies above the convergence zone between the Pacific and Philippine Sea plates, a complex tectonic setting with both shallow and deep plate interfaces. These interfaces have been the source of many of the large destructive earthquakes in Japan’s history, including the M7.9 earthquake at Taisho, Kanto in 1923, and the M8.2 earthquake at Genroku, Kanto in 1703, and several intermediate- depth earthquakes under the Boso Peninsula.

HERP has identified a very elaborate series of seismic sources for different types of earthquakes that occur in this region. Considering its relative proximity to the Tohoku rupture, researchers are greatly interested in evaluating the potential impact of the Tohoku earthquake on the rupture probabilities of the different types of earthquakes in and under the Kanto Plain.

AIR conducted a detailed study of the changes in Coulomb failure stresses9on regional faults, the technical details of which are provided in Section 5. To apply this analysis to the Kanto Plain region, AIR researchers developed a set of rupture scenarios to study the stress impact of the Tohoku event on the Pacific-Philippine Sea plate interface, and on intraslab earthquakes that occur within the Pacific plate under the Boso peninsula. Figure 7 illustrates surface projections (orange-shaded areas) for three of these rupture scenarios, at three different plate interfaces beneath the Kanto Plain.

The results of the analysis indicate that the Tohoku earthquake has increased the state of Coulomb failure stress on both the Pacific-Philippine plate interface and Pacific intraslab faults in this region.

9 Coulomb failure stress is the stress on a fault surface produced from opposing forces, such as tectonic forces that work to rupture the fault and frictional stresses that resist rupture. It is typically measured in “bars” where a bar is roughly equivalent to the atmospheric pressure at sea level, or more precisely, 0.987 of the atmospheric pressure at sea level.

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Indeed, the 30-year probability of an M≥6.7 earthquake has potentially increased from 72% to approximately 81-93%, depending on the assumptions (see Section 5 for details).

Since the Tohoku earthquake, observations of seismicity within the Boso Peninsula clearly indicate a four- to five-fold increase in the rate of M ≥ 3 earthquakes, which correlates well with the high positive Coulomb failure stress changes produced from the AIR analysis. When this information is directly applied to a rate-state model10 (see Section 5 for technical details) the resulting estimates of rupture probabilities agree well with the results based on the Coulomb failure stress changes.

Kanto Plain

Figure 7. Surface Projection of three Rupture Scenarios under the Kanto Plain: Pacific Interface Subduction (left panel), Philippine Sea Interface Subduction (middle panel) and Philippine Sea Intraplate Subduction (right panel)

A Note on a Recent Study from Japan on the Probability of a Major Earthquake near the Kanto Plain

A study (Sakai et al., 2011) that was published last fall by researchers at Tokyo University’s Earthquake Research Institute (ERI)—and which received considerable press attention just last month11—provided an estimated occurrence probability of 70% for an M7.0 earthquake striking in the vicinity of the Boso Peninsula within the next 4 years. The research pointed to the above-mentioned increase in the frequency of earthquakes with magnitudes of 3.0 and higher in this area, and used the Gutenberg- Richter distribution12 to extrapolate an estimated increase in the post-Tohoku seismicity rate of larger magnitude earthquakes in the area.

The difficulty with this approach is that the rate of seismicity after Tohoku, over a short time window of less than a year, is transient, which makes the Gutenberg-Richter distribution an inappropriate tool for predicting the rates of large-magnitude earthquakes. Instead, the transient rate information needs to be interpreted within the context of Coulomb stress changes using the rate-state friction model, as

10 The rate-state (RS) model, proposed by Dieterich (1986, 1992, and 1994), is a physical model that describes the frictional behavior of cut surfaces, cracks, or fractures in response to changes in the shear and normal stresses. Unlike the CFS model that initiates rupture when the shear stress on the fault surface reaches a critical level, the RS model initiates rupture when the slip velocity on the fault surface reaches a high level that causes the fault’s surface motion to accelerate until it eventually ruptures. The formulation of the RS model is based on the results of many years of laboratory work on rocks and other types of materials. 11 BBC, Big Tokyo earthquake likely "withing the next few years," 23 January 2012 12 The Gutenberg-Richter (GR) relationship provides a correlation between the number of earthquakes in an area over a fixed time period to the magnitude of the earthquakes.

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discussed in this section. Using a statistical model such as Gutenberg-Richter to extrapolate the rate of large M7.0 earthquakes from the transient rate of small magnitude earthquakes could lead to results that are strongly biased.

As noted above, the dominant sources of seismicity for large-magnitude earthquakes under the Boso Peninsula are the interface between the Pacific and Philippine Sea tectonic plates and intraslab activity. The majority of the earthquakes in this area occur at depths greater than 30 km. Using the Hi-Net13 catalog for earthquake activity between March 11, 2010 (one year prior to Tohoku) and January 5, 2012, AIR estimated both the pre- and post-Tohoku seismicity rates at depths between 30 and 100 km and between 0 and 100 km. The results show an increase in the rate of M≥3.0 earthquakes on the order of between 430% and 550%, averaged over one year.

Based on the rate-state model, and using the 430% increase in activity (the “amplification factor”), the 30-year and 4-year occurrence probabilities for M ≥ 6.7 earthquakes are estimated to be 92% and 46%, respectively. For the amplification factor of 550%, the corresponding probabilities are 94% and 52%. Finally, for the amplification factor of 550% but disregarding any effects of time-decay, or transience, the 4- year occurrence probabilities for M ≥ 6.7 and M ≥ 7.0 earthquakes are roughly 62% and 25%, respectively. This result is significantly lower than the one produced by the researchers at ERI.

Sagami and Nankai Troughs

The Sagami Trough has produced at least two large destructive earthquakes: the M8.2 Genroku event in 1703 and the M7.9 Kanto event in 1923. AIR researchers investigated the Coulomb failure stress changes on rupture planes of these two historical events. The results indicate that the western part of the Sagami Trough, which is primarily associated with the Kanto-type rupture, experienced larger positive changes in Coulomb failure stress as a result of the Tohoku event than did the eastern segment. The eastern segment showed low levels of Coulomb failure stress change, which were either positive or negative depending on the assumped faulting mechanism, especially for the shallow depths.

In their 2007 seismic hazard maps, HERP’s long term rupture probabilities for another Kanto-like or Genroku-like earthquake were low in light of the fact that their historical ruptures were relatively recent when compared with their recurrence intervals. These probabilities were not changed significantly by the Tohoku earthquake.

Specifically, the results of AIR’s study indicate that the 30-year probability for another Kanto-type event has increased from 0.76% to 1.1–1.6%, depending on the assumptions. For faults like the one that produced the M8.2 Genroku earthquake, the time-dependent probability remains very low.

The changes in the Coulomb failure stress on the Nankai Trough—including the Tokai segment of the Nankai Trough, which is closest to Tokyo—are quite small, on the order of a maximum of 0.045 bars. Any change in rupture probabilities is therefore negligible.

13 Hi-Net is Japan’s high-sensitivity seismograph network, which is part of the National Research Institute for Earth Science and Disaster Prevention (NIED) in Japan.

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Regional Crustal Faults

AIR researchers also conducted the Coulomb failure stress analysis for 98 major Japan faults in Japan identified by HERP. The fault with the highest stress change is the Futaba fault in Fukushima prefecture (see left-hand panel of Figure 5 for location); here the Coulomb failure stress change as measured using AIR’s approach was 6.5 bars.

According to HERP, the most recent event on this fault occurred in the second century and it has a recurrence interval of approximately 10,000 years. With such a large recurrence interval, any time- dependent analysis has a very high level of uncertainly; and therefore AIR researchers used a Poissonian (time independent) model to calculate the rupture probability for this fault. Using the range of the estimated Coulomb failure stress change for this fault (see Section 4), AIR’s analysis indicates an increase in the 30-year Poissonian probability from .4% to 1.2%. However, it should be noted that there has been no apparent increase in seismicity on or near this fault, post-Tohoku.

A number of faults in northern Honshu show large negative stress changes. It is however difficult to examine and substantiate the impact of these changes on this area’s seismicity since this would require establishing a reduction in the rates of seismicity for these faults.

Indeed, it is important to keep in mind the large uncertainties inherent in establishing a predictable cause and effect relationship between stress changes on a fault and changes in resulting seismicity. These uncertainties are due mainly to the lack of detailed knowledge of the faulting parameters and the frictional states at depth. In the aftermath of the Tohoku event, AIR found a correlation between an increase in seismicity in certain areas post-Tohoku and positive Coulomb failure stress changes in those same areas. Findings from other areas however, defied this correlation.

For example, a M6.3 earthquake occurred on March 12, 2011, near the Tokamachi and Shinanogawa faults (Figure 8) in western Honshu, followed by a cluster of aftershocks. Using a single set of fault parameters suggested by HERP, AIR’s analysis initially indicated a reduction in stress for this fault, which was corroborated by HERP results. However, when the uncertainties were taken into consideration, the results indicated the possibility of a stress increase of close to one bar at depth. Similarly, the AIR study indicated a positive change in Coulomb failure stress of approximately one bar in the Miura fault group (which lies southwest of the Boso Peninsula), with similar values reported by HERP. However, the post-Tohoku seismicity for this fault group has not shown any significant increase.

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Conclusions

4 Conclusions

Due to Japan’s highly comprehensive seismic monitoring network, the Tohoku event is the most thoroughly recorded mega-earthquake in history and will be of primary interest at many research institutions for years to come. However, while quantifying the stress changes that resulted from the Tohoku earthquake is critically important, any overall reassessment of seismic risk in Japan must be performed in the larger context—a context that includes explicit consideration of previously unmodeled risk factors, namely, tsunami and liquefaction.

Thus, rather than focus on one event, it is important to identify all of the factors that contribute to Japan’s seismic risk. For example, the most significant factor may not be the consequences of the Tohoku earthquake, but rather the choice of modeled maximum magnitudes for the Nankai and Sagami Troughs. Any large event in those regions would have devastating consequences for Tokyo.

AIR is investing considerable resources into developing an updated view of the earthquake risk in Japan that is comprehensive in its scope and that will incorporate the latest scientific thinking in some of these very challenging areas of seismic risk assessment. In doing so, AIR is following developments in the broader scientific community, including HERP, to ensure that its own treatment of uncertainties is guided by the best scientific opinion.

AIR’s updated Japan earthquake model is slated for release in the summer of 2013. In the meantime, we recommend using the current model as a benchmark and the preliminary findings provided in this paper as guidance for companies who wish to make adjustments to the current model.

4.1 Summary of Apparent Changes in Regional Risk Table 4 summarizes, in qualitative terms, the changes in risk for different seismic sources in Japan. Quantifying the changes is extremely challenging due to the large number of parameters and uncertainties not only with respect to the Tohoku event, but also with respect to the historical record. Current increases in observed seismicity are largely of a transient nature and will change over time; however, it is useful to have a sense of the risk during this transient period as well as the long-term implications of the Tohoku earthquake.

The preliminary assessments shown in Table 4 are based on studies of historical events and how they have affected the seismicity of different regions, estimates of the degree of coupling taking place along plate interfaces, and estimates of the stress changes on crustal faults, as discussed in previous sections. The table includes some of the major faults potentially affected by the Tohoku earthquake, which are included in the faults illustrated in Figure 8.

The indications shown in Table 4 address only the stress changes that have resulted from the Tohoku earthquake. The update to the AIR Earthquake Model for Japan, scheduled for release in 2013, will encompass a broader view of risk—one that includes explicit capabilities for modeling tsunami and liquefaction risk. In that context, it should be understood that the impact on modeled losses of adding

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Conclusions

the tsunami and liquefaction perils may well dominate the impact of any changes in rupture probabilities that have resulted from the Tohoku earthquake itself.

Figure 8. Faults Potentially Affected by the Tohoku-oki Earthquake

Table 4. Summary of Risk Changes for Subduction Zones and Faults Potentially Affected by the Tohoku-oki Earthquake

Ibaraki-oki 9 Increase Previously Underestimated by HERP Not modeled by HERP; Positive Impact Offshore of the Boso Peninsula Increase from Tohoku Pacific-Philippine Sea Plate Interface under the Kanto 12 Increase Positive Impact from Tohoku Plain Sagami Trough Subduction (1923 Kanto- Uncertain or small increase Type Interplate) in the western segment Nankai Trough No or Neglibable Change Futaba Fault Tachikawa Fault Increase Positive Impact from Tohoku Atera Fault – Northern Segment

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Research Notes – The Effects of the Tohoku Earthquake on Regional Seismicity in Japan

5 Research Notes – The Effects of the Tohoku Earthquake on Regional Seismicity in Japan

Dr. Mehrdad Mahdyiar Director of Earthquake Hazard Research, AIR Worldwide

The M9.0 earthquake in Tohoku, Japan on March 11, 2011, has spawned widespread interest in understanding and evaluating the potential effects of this earthquake on the regional seismicity.

This difficult task requires an understanding of the mechanisms of stress transfer from one fault to another. To quantify this process and estimate the changes in occurrence probabilities of regional earthquakes on affected faults, a broad range of related information needs to be incorporated into the process. In order to accurately determine if the stress changes increase or decrease the likelihood of future earthquakes, reliable information is needed on the expected faulting mechanisms of all affected faults.

The general concept of stress transfer between faults can be best understood within the context of Coulomb Failure Stress (CFS)14, which is a formulation of how a fault may rupture, defined as:

CFS σµτ )(* −−−= Sp n (1)

where τ is the shear stress, σ n is the normal stress (with a positive compression), p is the fluid pore pressure, and µ is the coefficient of friction, and S represents cohesion effects..

A fault will rupture when CFS becomes positive, because at that point the active shear stress exceeds the resisting frictional stress.

A quantification of the CFS for a fault under a steady-state condition would require estimates of in situ values for all of the parameters that define equation (1), which cannot be done due to various limitations and the fact that we do not have physical access to faults at depth. However, when a large- magnitude earthquake occurs in a region, it is possible to estimate the changes in τ and σ n on nearby faults. This information can be used to evaluate the potential impact of the earthquake on the rupture probabilities of the affected faults. Figure 9 shows a conceptual model for the tectonic loading and unloading of a typical fault in time in the context of CFS. In the figure, a fault is loaded via secular stresses created by regional tectonic forces. According to the CFS model, the fault ruptures if the active shear stress on its rupture plane exceeds the resisting frictional stress.

14 Coulomb stress is the stress that is produced from opposing forces, such as tectonic plates pushing against one another. It is typically measured in “bars” where a bar is roughly equivalent to the atmospheric pressure at sea level, or more precisely, 0.987 of the atmospheric pressure at sea level.

Ernest Masson Anderson (1877-1960) developed a successful theory for faulting in 1905, on which Coulomb stress failure is based. His findings are based on the criteria developed by Charles Augustin de Coulomb (1736-1806), which states that slippage along fractures occurs when the shear stress on a fracture place reashes a critical state.

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Figure 9. Tectonic Forces create Secular Stress on a Fault

Each rupture episode reduces the state of the stress on the fault by some amount, often reported in terms of stress drop, typically in the range of 1–1,000 bars. A sudden change in the shear or normal stress on the fault, due to regional deformation caused by large nearby earthquakes, will impact CFS and thus the fault’s rupture clock. The scale of the impact is related to the size of the stress change and the rate of the secular shear stress, expressed as

∆τ c Tshift = (2) τ where τ is the rate of secular steady-state Coulomb stress on the fault and ∆τ c is the change in CFS.

For the purpose of hazard and risk analysis, Tshift needs to be translated into the change in rupture probability.

Most recent earthquake hazard analyses use a stochastic renewal model to estimate time-dependent rupture probabilities for faults. In a typical renewal model, a fault recurrence interval is treated as a random process defined by a density function such as a lognormal relationship or Brownian Passage Time. Given the elapsed time since the date of the last characteristic earthquake on a fault and using the recurrence density function the conditional rupture probability over a defined time window is calculated. The impact of Tshift on the conditional rupture probability is often formulated by either shifting the elapsed time, without changing the recurrence density function, or shifting the mean recurrence, without changing the elapsed time. The first approach assumes that the stress changes on the fault do not alter the overall statistics of the recurrence intervals, whereas the second approach assumes that the stress changes impact all recurrence intervals (i.e., increasing or decreasing the time until the next earthquake) thereby altering the mean recurrence interval. Figure 2 shows a schematic diagram of a typical renewal model.

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Figure 10. Schematic Diagram of a Stochastic Renewal Model used to Estimate Time-Dependent Rupture Probabilities

Although integrating the CFS changes into a renewal model via Tshift provides a practical way of estimating rupture probability changes due to fault interaction, it does not capture all aspects of seismological observations on how faults respond to stress changes, which is best manifested by aftershock spatial and temporal distributions.

When a fault ruptures with a large-magnitude earthquake, the deformation it creates causes severe CFS changes within the region, which subsequently experiences aftershocks. Observations show that the rates of aftershocks is strongly time-dependent—decreasing over time—clearly indicating that the impact of the CFS change on seismicity is time-dependent as well. However, formulating the impact of the CFS changes on the rupture probabilities of faults by constant time shifts in their rupture clocks does not account for the observations.

A more realistic model for formulating CFS changes on faults that is compatible with the aftershock time-dependent behavior is the rate-state (RS) model, proposed by Dieterich (1986, 1992, and 1994). This is a physical model based on the concept that faulting is as a frictional instability. It describes the frictional behavior of cut surfaces, cracks, or fractures in response to changes in the shear and normal stresses. Unlike the CFS model that initiates rupture when the shear stress on the fault surface reaches a critical level, the RS model initiates rupture when the slip velocity on the fault surface reaches a high level that causes the fault’s surface motion to accelerate until it eventually ruptures. The formulation of the RS model is based on the results of many years of laboratory work on rocks and other types of materials.

During the 1970s, thanks to improvements in servo-controlled testing machines, which allowed stress levels and strain rates to be accurately controlled, great progress was made in understanding the nature of the frictional behavior of rocks and other materials on cut surfaces. It was observed that under steady-state low slip velocity, V0 , rocks and other materials have a steady friction coefficient, µ0 .

Additional observations indicated that if the slip velocity increased from V0 to V the friction coefficient first jumps to a new value defined as:

A * ln()V V0

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and then decays exponentially over a distance, Dc , by an amount defined as:

B * ln()V V0

The difference between the parameters A and B plays a key role in fault surface stability, and determines whether a fault surface after a slip velocity increase experiences a lower or higher friction coefficient than its steady-state value and whether it will experience friction instability. If (A-B) is positive, this implies velocity strengthening and frictional stability while a negative outcome implies velocity weakening and frictional instability. In other words, it was observed that after the velocity increases, the coefficient of friction does not drop immediately but for velocity-weakening conditions, it drops to a lower value than its steady state value after a critical displacement Dc has occurred.

Figure 11. Decay of Friction Coefficient with Change in Slip Velocity

The RS model indicates that the shear strength of a fracture depends on its surface slip velocity as well as several additional parameters, known as state variables, some of while describe the contact time between the surfaces. According to the RS model, the friction coefficient on a fracture surface is affected by the changes in the slip velocity along with the normal and shear stress on the surface. Sudden changes in the slip velocity or stress will impact the friction coefficients in a manner that is time- dependent.

The impact is at its highest immediately after the stress change and decreases over the time that has elapsed since the stress change. This application of the RS friction model to a time-dependent rate analysis of fault interaction was first formulated by Dieterich (1994). According to the Dieterich formulation, the seismicity rate on a fault R(t) after a stress perturbation can be formulated in terms of: the rate of seismicity before the stress change, a set of parameters that characterize the immediate jump in the seismicity rate due to the CFS change, and a relaxation parameter, ta , that controls the variation in the rate of seismicity after the stress changes with time.

The RS model predicts the highest increase in seismicity immediately after the changes in the CFS with decreasing effects over time. This is consistent with observations of aftershock behavior, and it has been shown that the RS model can provide a rate of aftershock occurrence that is consistent with Omori’s

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empirical aftershock formula15. The scale of the immediate change in the seismicity rate is controlled by: the size of CFSC, ∆τ , the normal stress on the fault, σ , and a rate-state friction parameter A.

Published research (Dieterich, 1994; Toda et al., 2003) reported an Aσ value of 0.5 bars. If τ =∆ 1bar, this translates to an increase in seismicity with a factor of 7.4 immediately after the stress change.

Figure 12 shows examples of changes in the seismicity ratio, R(t)/r, for different sets of values for and

σ σ 푎 the secular stress rate . The initial change in R(t)/r is controlled by A . A lower A results in a larger푡 initial seismicity rate change.휏̇ R(t)/r decreases with time and the rate of change is determined by . The four cases shown in the figure are as follows: 푡푎 Case 1: = 10 ; = 0.5 ; = 1

Case 2: 푡푎 = 50 푦푒푎푟푠; 퐴휎 = 0.5 푏푎푟푠; Δ휏 = 1 푏푎푟

Case 3: 푡푎 = 10 푦푒푎푟푠; 퐴휎 = 0.7 푏푎푟푠; Δ휏 = 1 푏푎푟

Case 4: 푡푎 = 100푦푒푎푟푠 퐴휎; = 0.5푏푎푟푠 Δ휏; = 1푏푎푟

푡푎 푦푒푎푟푠 퐴휎 푏푎푟푠 Δ휏 푏푎푟

Figure 12. Seismicity Ratios for Four Different Cases

 To quantify R(t) we need to obtain values for τ and either ta or Aσ , which can be calculated from one another as they are related. In many publications, the aftershock duration is taken as the best approximation for the relaxation time ta . Studies of the aftershock activity of the Kobe earthquake of 1995 (Toda et al., 1998) and the Izmit, Turkey earthquake of 1999 (Parsons et al., 2000) suggest that

ta = 23 years and ta = 35 years for these earthquakes, respectively.

Figure 13 shows a compilation of the aftershock duration in different tectonic regions. The solid line in the figure is the rate/state model prediction (Stein et al., 2009). Aftershock durations are represented by bars that indicate the span of published estimates. One-sided constraints have bars at the known value and arrows indicating the open value. For example, the New Madrid aftershocks span at least 200

15 In 1894, Fusakichi Omori (1868-1923) developed an empirical formula describing the rate of decrease in aftershock frequency over time. The relation states that the rate at which the frequency of earthquake aftershocks decreases is approximately the reciprocal of the time that has elapsed after the main shock.

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years, but the upper bound in unconstrained. Similarly, the Wasatch aftershocks span less than 600 years, but the lower bound is unconstrained

The data in Figure 13 indicates a shorter aftershock for active tectonic regions that have a higher secular stress rate, and longer for stable tectonic regions that have a slower secular stress rate. This implies that a higher secular stress load after an earthquake produces faster healing of the fault surface and slower healing when the secular stress load is low. An aftershock duration map (Toda et al., 2011) that takes into account the aftershock sequences of some past large earthquakes in Japan, and the distance from the subduction plate interface, shows ta ≅ 20 years for subduction zones and ta ≅100 years for interior crustal regions.

Figure 13. Aftershock Duration and Fault Loading Rates for Selected Large Earthquakes in Different Tectonic Settings, 1 kyr = 1,000 years (Stein et al. 2009)

5.1 Changes in Coulomb Stress Failure in the Kanto Plain and for 98 Major Faults in Japan Figure 14 shows the distribution of the subduction zones in and around Japan including the Sagami Trough and the contact planes of the Pacific and Okhotsk tectonic plates. The figure also shows the distribution of 98 major faults in Japan.

The tectonics of the Kanto region is complex. It is the region where the Pacific and Philippine plates converge forming a series of interfaces, both shallow and deep, that have been the source of many of the large destructive historic earthquakes such as the 1703 M8.2 Genroku event, the 1923 M7.9 Kanto event, and a series of intermediate-depth earthquakes under the Boso peninsula. HERP has identified a very elaborate series of seismic sources for different types of earthquakes in this region. Considering the proximity of this region to the Tohoku rupture area, scientists are greatly interested in evaluating the potential impact of Tohoku on the rupture probabilities of these different types of earthquakes.

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Research Notes – The Effects of the Tohoku Earthquake on Regional Seismicity in Japan

AIR researchers conducted a CFSC analysis for the major seismic sources identified by HERP within the Kanto region. This analysis includes the seismic source areas responsible for the Kanto and Genroku earthquakes and the interface and intraslab rupture areas at depth. The CFSC analysis is based on the slip distribution model from Ozawa, et al., (2011) and was performed for several fault rupture scenarios to account for the uncertainty in the strike, dip, and rake angles for each fault. Using the geometry of the Pacific plate interface and the coseismic and postseismic displacements from Ozawa et al. (2011), the CFSC are calculated at different locations along the fault planes. To account for uncertainties in the slip, dip, and rake angles, 1,000 CFSC scenario analyses were conducted for each fault.

Figure 14. Subduction Segments and 98 Major Faults used to Determine Changes in Coulomb Stress Failure

For each scenario, the rake angles within ± 15 degrees of the rake defined by HERP were searched for the maximum and minimum CFSC values. The study used a single friction coefficient of 0.4. While this coefficient was used in a number of other studies in Japan, the values of the friction coefficient for faults contain a certain degree of uncertainty, which could have large impact on the results of the analysis. This is especially the case for faults that experience large normal stress changes.

To evaluate the impact of the CFSC on the rupture probability of a fault, information is needed on the fault’s secular stress rateτ . This is a challening task. Different studies have implicit estimates of τ using the equations provided in the RS model formulation for the secular stress in terms of a state parameter, normal stress, and the relaxation time (Toda et al. 2011).

To obtain better estimates of τ that are more reflective of the faulting details and the tectonic setting in Japan, AIR researchers constructed a physical model for the subduction zones using an elastic dislocation model for stress analysis. The estimates of τ correlated well with the recurrence intervals for a number of faults reported in the HERP model. The estimates did not correlate well, however, for

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some faults, especially those with long recurrence intervals. The reason could be due to the fact that fault recurrence intervals in the HERP model reflect many complexities that cannot be simulated by a simple elastic model. Also, the estimates ofτ requires the researchers to assume values of friction coefficients for faults. In general, the effective friction coefficient can have different values for different faults, reflecting not only the rheology of the faults but also their geometrical complexities. Nevertheless, the results do provide direct estimates of τ on faults although the range is very similar to that reported in Toda et al. (2011).

Figure 15 shows the minimum, mean, and maximum CFSC for the Pacific interface seismic zone in the Kanto region, at depths between 40 and 80 km due to coseismic displacement (Ozawa et al, 2011). The figure corresponds to the left-hand panel in Figure 7, with the shaded area representing the surface projection of the rupture scenario of the Pacific interface subduction. The minimum and maximum values reflect the minimum and maximum CFSC values for the 1,000 slip scenarios that capture the epistemic uncertainty in the geometry and mechanisms of the interface. The results show positive CFSC over the interface with the highest values in the northeastern and the lowest in the southwestern parts of the interface.

Minimum Mean Maximum

Figure 15. Changes in the CFS on the Pacific Plate Interface beneath the Boso Peninsula due to Coseismic Displacement (Ozawa et al., 2011), Friction Coefficient = 0.4

Figure 16 compares the CFSC values in the same interface area for the coseismic, coseismic plus postseismic, and for slip distributions from the seismological model developed at the California Institute of Technology’s Tectonics Observatory (Wei et al., 2011), which shows overall lower results. The middle panel of Figure 16 indicates that the postseismic deformation increased the CFSC. The analysis shows that the positive CFSC values are mostly due to increases in the shear stresses rather than decreases in the normal stresses. This implies that the uncertainty in friction coefficient for the interface would not have a strong impact on the CFSC values. The estimated CFSC values range between 0.1 – 5.0 bars, which is a level that could potentially impact the seismicity.

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Coseismic Coseismic and Postseismic Wei et al., 2011

Figure 16. Comparison of CFSC values based on coseismic displacements, coseismic and postseismic displacements, (both from Ozawa et al., 2011,) and coseismic displacement (California Institute of Technology), Friction Coefficient = 0.4.

We also calculated the CFSC on the northern interface zone (middle panel in Figure 7) and on a few intraslab fault plane scenarios (right-hand panel in Figure 7) within the larger contact area. The results for the northern interface area indicate positive CFSC with a similar but slightly lower range than the eastern interface zone. The results for the intraslab fault scenarios show both positive and negative CFSC depending on the strike, dip, and rake used. We do not have the details of the strike, dip, and rake of all possible intra-slab events in this region; however, the CFSC for a few likely scenarios are positive, but with smaller values than those reported here for the interface zones.

Figure 15 shows the spatial distribution of M ≥ 4 earthquakes in northern Japan for the time period between March 2010 (about one year prior to Tohoku earthquake) and the present, with a detail of the Kanto Plain region. The figure clearly indicates an increase in the seismicity within the Kanto region, which correlates well with the positive CFSC estimates. The depth distribution of earthquakes agrees with the depth of the interface region used in our model. Using data from historical earthquake, HERP estimates the return period for M6.7 to M7.2 for the interface and intraslab earthquakes in this region to be about 23.8 years. This translates to a 5-year and 30-year Poissonian occurrence probability (OP) of 0.19 and 0.72, respectively. Using the rate-state model the 5-year and 30-year OP are estimated as 0.3 and 0.87, respectively, assuming small positive CFSC for the intra-slab events and 0.36 and 0.93, respectively (assuming similar CFSC for the both the interface and intraslab zones). The analysis takes into consideration the uncertainties in the rate-state friction, the normal stress, and the characteristic relaxation time (often the observed duration of the aftershock, after which the seismicity returns to its steady-state rate).

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Research Notes – The Effects of the Tohoku Earthquake on Regional Seismicity in Japan

M ≥ 4.0 Pre-Tohoku M ≥ 4.0 Post-Tohoku M ≥ 6.0 Post-Tohoku

Figure 17. The rate of M ≥ 4 Earthquakes Increased Significantly after Tohoku

A recent study (Sakai et al., 2011) published by researchers at Tokyo University’s Earthquake Research Institute inferred a 70% occurrence probability for M7.0 earthquakes within next four years in this area. The study was prompted by a significant increase in the observed frequency of M ≥ 3 earthquakes under the Bosto peninsula. The researchers used the Gutenberg-Richter distribution to infer the rate of seismicity for large-magnitude earthquakes from the rate of small earthquakes. The problem with this approach is that the rate of seismicity after Tohoku, over a short time window of less than a year, is transient and cannot be extrapolated through the Gutenberg-Richter distribution to estimate rates of large-magnitude earthquakes. The transient rate information needs to be interpreted within the context of Coulomb stress changes and their impact on seismicity, as discussed in this section, using the rate- state friction model. Using a statistical model such as Gutenberg-Richter to extrapolate the transient rate of small magnitudes to predict the rate for large M7.0 earthquakes provides results that could be strongly biased.

The ratio of the post-Tohoku seismicity rate to the pre-Tohoku rate is a direct measure of the rate increase that can be used in the rate-state model for rupture probability analysis. The dominant sources of seismicity for large-magnitude earthquakes under the Boso peninsula are the Pacific and Philippine interface earthquakes, most of which occur at a depth over 30 km. The seismicity ratios for depth

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distributions of 30–100 km and 0–100 km from the Hi-Net16 earthquake catalog, for events that occurred from 3/11/2010 to 1/5/2012, are estimated as 4.3 and 5.5 for M ≥ 3 earthquakes, respectively, averaged over one year. Using this information, the 30-year and 4-year occurrence probability for M ≥ 6.7 earthquakes using the rate-state model, are estimated as 92% and 46% (using 30–100 km data), respectively, and 94% and 53% (using 0–100 km data). The 30-year and 4-year occurrence probability for M ≥ 7.0 earthquakes are estimated to be between 56% and 17% (using 30–100 km data) and between 59% and 20% (using 0–100 km data).

Rupture Scenarios for the Sagami Trough

The Sagami Trough is the location of the destructive M8.2 Genroku earthquake in 1703 and the M7.9 Kanto earthquake in 1923. The mechanisms for these earthquakes are reported as strike-slip with an oblique thrust component. Based on this information, two rupture scenarios were considered for the Sagami Trough: a right-lateral strike slip, and an oblique thrust with right-lateral strike-slip and a rake of 135°. Figure 18 and Figure 19 show the CFSC distribution for the strike-slip and oblique/strike-slip mechanism using the Ozawa et al. (2011) coseismic slip distributions, respectively. The results suggest a positive CFSC at depth over the entire Sagami segment with lower CFSC values based on the slip distributions from the California Institute of Technology (results for this latter case is not shown here).

It is important to note that the slip distribution data near the southern boundary of the Tohoku rupture area is not well constrained, which implies a larger uncertainty in the CFSC estimates for the eastern end of the Sagami Trough. The results indicate that the post-seismic deformation increased the overall state of the stress on the Sagami Trough. The CFSC values within the eastern end are generally low and have a negative inclination for a number of locations, for different scenarios. In general, the right-lateral strike-slip shows higher CFSC than those from the oblique mechanisms.

The HERP report considers both the Kanto-type and Genroku-type faulting with Poissonian recurrence intervals of 400 and 2,300 years, respectively. Both scenarios are modeled as time-dependent, with equivalent Poissonian recurrence intervals of 3,900 and 10,000 years, respectively. The reduced rates reflect the fact that these earthquakes occurred recently when compared to the mean recurrence intervals. Based on the CFSC values, AIR researchers estimate that the 30-year time-dependent probability for Kanto-type faults remains low, having increased from 0. 76% to 1.1% to 1.6%, depending on the assumptions. For Genroku-type faults, the time-dependent probability remains very low.

Faults with Coulomb Stress Failure Changes ≥ 0.1 Bar

Crustal faults with CFSC values greater than 0.1 bar are illustrated in Figure 20 and Figure 21. As was discussed earlier, uncertainty in the strike, dip, and raking angles for these faults is taken into consideration. When the HERP fault parameters are used without any uncertainty, the CFSC estimates are similar to those recently released by HERP for the 98 major faults used in the study.

16 Hi-Net is Japan’s high-sensitivity seismograph network, which is part of the National Research Institute for Earth Science and Disaster Prevention (NIED) in Japan.

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The most positively affected fault is the Futaba fault with a CFSC of 6.5 bars in the Fukushima prefecture. The results of our analysis, taking into account the uncertainties, indicate a rather large variation for the CFSC on this fault, between 1.5 to 12 bars from the coseismic slip. This wide range is due to the changes in the shear stress on the fault from negative values for some strike, dip, and rake angle combinations to positive values for other combinations.

According to HERP, the most recent event on this fault occurred in the second century and it has a recurrence interval of approximately 10,000 years. With such a large recurrence interval, a time- dependent analysis would have a very high level of uncertainly so instead, a Poissonian model was used to estimate occurrence probabilities. Using the CFSC range for this fault, AIR’s analysis indicated an increase in the 30-year Poissonian occurrence probability from 0.3% to 1.2%.

The spatial distribution of M ≥ 2.0 earthquakes before and after Tohoku does not show any increase in seismicity on or near the Futaba fault. One possible explanation might be that the fault has a rather strong coupling, supported by its long recurrence interval. This would imply a higher effective friction coefficient than what is used here. Our CFSC analysis indicates a large reduction in the normal stress on the fault, which becomes even larger if the friction coefficient is higher than what is used here.

Faults with Coulomb Stress Failure Changes ≤ -1.0 Bar

Figure 22 shows the distribution of the major faults with large negative CFSC in northern Honshu. In general, it is rather difficult to examine and substantiate the impact of the negative CFS changes on a fault’s rupture probabilities as it requires one to establish a reduction in seismicity in regions with negative CFSC.

A close evaluation of the crustal seismicity after Tohoku indicates earthquake clustering in certain areas near faults for which the recent HERP analysis reports a negative CFSC. For example, an M6.3 earthquake was triggered on March 12, 2012 near the Tokamachi and Shinanogawa faults (Figure 8), along with a cluster of aftershocks. The strike of the earthquake is very similar to the strike of Tokamachi fault, which suggests the event occured on that fault. HERP, using a single set of parameters for this fault, reports a negative CFSC for both the Tokamachi and Shinanogawa faults in response to Tohoku. However, when uncertainties of ±15 degrees in the source parameters of the faults are considered, calculations indicate the possibility of close to one bar stress increase at depth, which corroborates with the triggering of the M6.3 earthquake. This is a clear example of the importance of considering uncertainties in fault parameters in analysis of this type. Obviously, it is possible to search all of Japan for faults that have the optimal combination of strike, dip, and rake angles that show a positive CFSC; however, it would be impossible to verify the existence of these faults. For CFSC analysis, the best we can do is account for the uncertainty in the source parameters of the known faults, which is what we have done in our study.

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Lowest Mean Highest

Figure 18. CFS Changes at the Philippine Sea Interface for Kanto and Genroku using Right-Lateral Strike-Slip in the Sagami Trough due to Coseismic Displacement (Ozawa et al., 2011), Friction Coefficient = 0.4

Lowest Mean Highest

Figure 19. CFS Changes at the Philippine Sea Interface for Kanto and Genroku using Oblique Thrust and Right-Lateral Strike-Slip in the Sagami Trough due to Coseismic Displacement (Ozawa et al., 2011), Friction Coefficient = 0.4

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Lowest Mean Highest

Figure 20. CFS Changes for Crustal Faults with CFSC ≥ 0.1 Bars due to Coseismic Displacement (Ozawa et al., 2011)

Lowest Mean Highest

Figure 21. CFS Changes for Crustal Faults with CFSC ≥ 0.1 Bar due to Coseismic and Postseismic Displacement (Ozawa et al., 2011)

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Figure 22. CFS Changes for Crustal Faults with CFSC ≤ -1.0 Bar due to Coseismic Displacement, (Ozawa et al., 2011)

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References

6 References

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