Research Paper THEMED ISSUE: Subduction Top to Bottom 2

GEOSPHERE Subduction zone megathrust earthquakes Susan L. Bilek1 and Thorne Lay2 1Department of Earth and Environmental Science, New Mexico Tech, Socorro, New Mexico 87801, USA GEOSPHERE; v. 14, no. 4 2Department of Earth and Planetary Sciences, University of California–Santa Cruz, Santa Cruz, California 95064, USA https://doi.org/10.1130/GES01608.1 ABSTRACT the subduction zone trench to the downdip limit of the sliding plate boundary, 19 figures; 2 tables defined by the onset of ductile flow in the mantle wedge. The megathrust itself Subduction zone megathrust faults host Earth’s largest earthquakes, along may involve a shear zone rather than a single surface, and repeated ruptures CORRESPONDENCE: Susan​.Bilek@​nmt​.edu with multitudes of smaller events that contribute to plate convergence. An of the plate-boundary contact may occur on parallel faults within the shear understanding of the faulting behavior of megathrusts is central to seismic zone. The overall plate convergence is accommodated by spatially varying CITATION: Bilek, S.L., and Lay, T., 2018, Subduction and tsunami hazard assessment around subduction zone margins. Cumula- stick-slip earthquake failure, episodic slow slip, and continuous aseismic slid- zone megathrust earthquakes: Geosphere, v. 14, no. 4, p. 1468–1500, https://​doi​.org​/10​.1130​/GES01608.1. tive sliding displacement across each megathrust, which extends from the ing. Heterogeneous frictional properties of the megathrust and dynamic inter- trench to the downdip transition to interplate ductile deformation, is accom- actions control how any given region behaves, including the possibility that Science Editor: Shanaka de Silva modated by a combination of rapid stick-slip earthquakes, episodic slow-slip some regions of stable sliding might transition to stick-slip failure during high- Guest Associate Editor: Laura M. Wallace events, and quasi-static creep. Megathrust faults have heterogeneous fric- strain-rate loading. Large-scale deformation of the underthrusting plate occurs tional properties that contribute to earthquake diversity, which is considered in subduction zones, with the plate bending and straightening as it subducts Received 26 August 2017 here in terms of regional variations in maximum recorded magnitudes, Guten- beneath either oceanic or continental lithosphere, such that the forces acting Revision received 19 April 2018 Accepted 19 June 2018 berg-Richter b values, earthquake productivity, and cumulative seismic mo- on each megathrust and the resulting seismicity are affected by the broad tec- Published online 6 July 2018 ment depth distributions for the major subduction zones. Great earthquakes tonic configuration of each plate boundary. All of these deformation processes on megathrusts occur in irregular cycles of interseismic strain accumulation, can produce hazards, from seismic shaking to submarine slumping and tsu- foreshock activity, main-shock rupture, postseismic slip, viscoelastic relax- nami generation. Here, we focus on megathrust faulting, which produces the ation, and fault healing, with all stages now being captured by geophysical greatest hazards. monitoring. Observations of depth-dependent radiation characteristics, large Based on the elastic rebound theory of earthquake occurrence and the rel- earthquake slip distributions, variations in rupture velocities, radiated energy atively steady loading provided by long-term plate convergence in subduction and stress drop, and relationships to aftershock distributions and afterslip zones, one might expect the stick-slip failure of megathrust faults to be at least are discussed. Seismic sequences for very large events have some degree of somewhat regular. This does appear to hold for many regions, where irregular regularity within subduction zone segments, but this can be complicated by “cycles” of long intervals of interseismic strain accumulation, sudden earth- supercycles of intermittent huge ruptures that traverse segment boundaries. quake sliding, and postseismic adjustment are apparent for well-documented Factors influencing variability of large megathrust ruptures, such as large-scale sequences of great earthquakes (M > 8) in regions such as the Nankai Trough plate structure and kinematics, presence of sediments and fluids, lower-plate of southwest Japan, along southern Chile, and along the Kuril Islands (e.g., bathymetric roughness, and upper-plate structure, are discussed. The diver- Ando, 1975; McCann et al., 1979; Lay et al., 1982). In many places, the historical sity of megathrust failure processes presents a suite of natural hazards, in- record of great earthquakes is limited, with too few events to establish any reg- OLD G cluding earthquake shaking, submarine slumping, and tsunami generation. Improved monitoring of the offshore environment is needed to better quantify Trench and mitigate the threats posed by megathrust earthquakes globally. upper plate crust seismogenic zone

OPEN ACCESS m) 25 crust shallow slip INTRODUCTION large/great earthquakes mantle tsunami earthquakes Depth (k 50 slow slip slow slip The contact surfaces between underthrusting and overriding plates in tremor subduction zones are called megathrust faults (Fig. 1). Over time, aside from 050 100 150 200 secondary inelastic deformation or block motions of the surrounding plates, Distance from Trench (km) This paper is published under the terms of the the total displacement across the entire megathrust surface must equal the Figure 1. Megathrust fault zone within a generalized subduction zone, highlighting the diverse CC‑BY-NC license. relative plate convergence. This slip budget extends over the depth range from slip modes observed in the shallow seismogenic, or earthquake-producing, region.

© 2018 The Authors

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ularity, or such limited information about early events that it is hard to establish that may be accompanied by seismic tremor or small repeating earthquakes, their relationship to more recent well-documented ruptures. Nonetheless, the but the full deformation occurring over days to months can only be resolved time interval since the last great earthquake in a given region and that event’s geodetically. Deformation in the upper plate of a frictionally locked megathrust, inferred size and spatial extent have quite successfully guided designation of with landward compression, can be resolved by global positioning system “seismic gaps” as regions with potential for future great earthquakes (e.g., (GPS) and Global Navigation Satellite System (GNSS) sensors operating over McCann et al., 1979; Nishenko, 1991), albeit without time predictability (e.g., years to decades, and these data sets now provide direct measurement of the Lay, 2015). Smaller events appear to be irregular in occurrence and to interact interseismic process and the ability to establish whether a seismic gap is actu- strongly with nearby ruptures, causing the seismic gap approach to be less ally accumulating strain that will result in a future large earthquake or whether useful than for great events. This perspective is not lacking in contention, as aseismic sliding is releasing the strain. Geodesy also enables characterization some researchers deny any degree of cyclicity in earthquake occurrence (e.g., of upper-plate sliver or block motions that may result from oblique conver- Kagan and Jackson, 1995; Kagan et al., 2012), but for very large earthquakes, gence. This is now recognized as quite common, and it must be accounted for these objections do not appear to be valid. Regularity in great earthquake cy- when evaluating the megathrust strain energy budget (e.g., McCaffrey et al., cles has long been recognized to be complicated by intermittent triggering of 2000; Wallace et al., 2004; La Femina et al., 2009). Precursory sliding near a fu- adjacent megathrust segments that otherwise tend to fail separately in smaller ture hypocenter, postseismic slip around main-shock rupture zones, relocking events, notably in southwest Japan (Ando, 1975) and Ecuador-Colombia of a ruptured zone, and viscoelastic deformation after an event are processes (Kanamori and McNally, 1982), with synchronization of rupture possibly oc- that have been sensed by onshore and, increasingly, offshore geodetic mea- curring in supercycles of huge earthquakes on time scales that greatly exceed surements, greatly expanding our view into the seismic strain accumulation recorded history (e.g., Sieh et al., 2008). Such unprecedented events have low, and release process and the ways in which relative plate motion is accommo- but nonzero probability for many regions, and it is difficult to reliably evaluate dated across the entire megathrust (e.g., Ito et al., 2013; Yokota et al., 2016). In the worst-case potential and its associated probability of occurrence, given the this overview, we highlight some of the basic seismological characteristics of limited temporal span of the seismological and paleoseismological records. major subduction zones and salient findings of recent investigations of large Surprises that stem from the limited historic record for megathrust events earthquakes and slow deformation on megathrusts. We conclude with consid- have included great earthquakes striking in regions lacking any known prior eration of the seismic hazards posed by megathrust earthquakes. great event and having disrupted subduction zone structure. This includes examples such as subduction of a young ridge at a triple junction where a great earthquake struck in the Solomon Islands in 2007 (e.g., Taylor et al., 2008; SEISMICITY CHARACTERISTICS OF MEGATHRUST FAULTS Furlong et al., 2009), and the great 2006 Kuril Islands rupture (e.g., Ammon et al., 2008), which struck in a region with discontinuous arc structure and an Earthquake occurrence reveals many attributes of subduction zone unusual forearc basin. Both areas had been argued to potentially be perma- megathrusts, although it is good to keep in mind that cumulative seismic slip nently aseismic, an assertion also still invoked for regions of subducting ridges appears to account for less than half of total convergence across the global such as the Tehuantepec and Carnegie Ridges along South America, where set of plate-boundary faults in the seismic record. Variations in earthquake there is no history of great earthquakes. Confidence in such assertions must size, number, and depth distribution indicate distinct properties of megathrust be assessed with caution, and the use of geodetic measurements to assess faults that guide comparisons of processes in different subduction zones. upper-plate strain is the most promising approach to evaluating actual seismic potential in such regions. Great megathrust earthquakes have also now been observed to have trig- Largest-Magnitude Ruptures gering interactions with great intraplate faulting (e.g., Ammon et al., 2008; Lay et al., 2010b, 2017), and cascading failures on the relevant megathrust (e.g., Lay Measured seismic magnitudes for shallow subduction zone earthquakes et al., 2010a), presenting further challenges to hazard assessment and rapid span from near zero at the low end, depending on availability of regional seis- warning procedures. Very large earthquakes have also been documented to mic monitoring, to the largest observed event, the 1960 Chile moment magni-

rupture the shallowest part of the megathrust (e.g., Polet and Kanamori, 2000; tude (Mw) = 9.5 earthquake. Globally, earthquake catalogs have improved with Lay and Bilek, 2007), where it had been thought that aseismic deformation of time, with fairly complete catalogs back to 1900 for events larger than M ~ 7, and wet sediments on the plate boundary would preclude stick-slip sliding. near-complete recording since the mid-1970s of events larger than M ~ 5.0. To Deployment of high-quality geodetic networks in subduction zones has provide a sampling of diverse megathrust activity, we considered the seismicity added important constraints on coseismic slip during nearby megathrust characteristics for nine major subduction zones (Fig. 2; Table 1) and identified earthquakes, along with capturing deformation prior to and after large rup- the largest observed earthquakes in each zone using two readily available earth- tures of the plate boundary. This has revealed the existence of slow-slip events quake catalogs, the U.S. Geological Survey National Earthquake Information

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90° 120° 150° 180° –150° –120° –90° –60°

1964 60° Aleutian-Alaska 60° 1938 Kuril-Kamchatka 1923 1965 1957 2006 1952 1946 1986 Figure 2. Great ( 8.0) earthquakes from 1918 2007 Mw ≥ 1958 1963 1900 to 2016 presumed to be on subduc- 1958, 2003 1994 Honshu 1960 1968 1933 tion zone megathrusts (green circles) and some important intraplate ruptures (red 1944, 1946 2011 30° 1923 30° circles) discussed in the text, from the U.S. Mexico-Middle America Geological Survey National Earthquake Information Center catalog (USGS-NEIC) 1932 Marianas 1995 catalog. Events mentioned in the text are Sumatra 1985 labeled with dates. Some events in the 2004 Southwest Pacific subduction zones have 2005 uncertain faulting parameters, but most 0° 2012 1906 0° 2007 2007 are likely thrust events in the small subduc- 2013 1940 1917 1966 Peru tion zones in the region. No great events 2009 1942 in the Marianas subduction zone have 2007 2001 1919 2014 been recorded. Note that regions such as 1995 Cascadia, New Zealand, Izu-Bonin, Luzon, Tonga-Kermadec 1922 −30° 1976 −30° Central America, and the Himalayan front 1906 2015, 1943 are not represented because great events 1917 Chile 1985 2010 have not happened in these regions in the 1960 (2) seismological time period of record. Histori­ cal great ruptures have been documented in Cascadia and the Himalayas.

−60° −60°

90° 120° 150° 180° –150° –120° –90° –60°

TABLE 1. MAXIMUM AND MEDIAN EARTHQUAKE MAGNITUDES FROM THE GLOBAL CENTROID MOMENT TENSOR (GCMT) CATALOG

Latitude range Longitude range Max. Mw Max. Mw Median Mw Median Mw Region (°) (°) Trench strike (all) (megathrust) (all) (megathrust) Alaska 49.17N–61.65N 165.06E–149.11W 250 ± 40° 8.08.0 5.35.4 Central America 12N–20N; 106.12W–94.44W; 290 ± 30° 8.08.0 5.45.4 7.6N–12N 83.47W–94.44W Chile 18S–48S 66W–78W 0 ± 30° 8.88.8 5.55.5 Japan 34.5N–41N 140E–145E 190 ± 30° 9.19.1 5.35.3 Kuril/Kamchatka 41N–55.67N 141.47E–163.70E 210 ± 40° 8.38.3 5.35.3 Marianas 10.45N–25.10N 138.17E–150.5E 240 ± 30° (10–13N); 7.77.1 5.35.3 160 ± 40° (13N–22N); 145 ± 30° (22N–25.1N) Peru 2.2S–18.5S 82.78W–67.25W 330 ± 30° 8.48.4 5.35.4 Sumatra 5S–15N 90E–108E 320 ± 30° 9.09.0 5.25.3 Tonga-Kermadec 14.3S–35.3S 176.6E–170.5W 200 ± 30° 8.18.0 5.35.4 Note: Depth range is 0–100 km, and time window is 1 January 1976 to 7 December 2016 for all regions. The subset designated megathrust is for thrust events with focal

mechanism consistent with local trench strike determined from bathymetric data. Mw is moment magnitude.

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Center catalog (USGS-NEIC, https://earthquake​ .usgs​ .gov​ ), from 1900 to 2016, Great Earthquake Catalog (1900–2016) and the Global Centroid Moment Tensor catalog (GCMT, http://www​ ​.globalcmt​ .org), from 1976 to 2016. The USGS-NEIC catalog includes the largest-­magnitude We first considered instrumentally recorded, great (M ≥ 8.0), shallow earthquakes over the last century, drawing upon redetermined magnitude and plate-boundary thrust-faulting events in nine major subduction zones (Fig. 2; location estimates from the International Seismological Centre–Global Earth- Table 1). For earthquake seismologists, most of these earthquakes are immedi- quake Model (ISC-GEM; http://www​ ​.isc.ac​ .uk​ /iscgem/​ ). The GCMT catalog pro- ately identifiable by their years and place names, as they represent major tec- vides relatively uniformly determined estimates of each event’s moment tensor tonic events that have sometimes produced devastating shaking and tsunami and seismic moment. We utilized the focal mechanisms to identify events that damage. Along the Alaska-Aleutian subduction zone, six great earthquakes

were most likely located on the subduction megathrust (Figs. 3–11). spanned the arc in 1964 (Alaska, Mw 9.2), 1965 (Rat Islands, Mw 8.7), 1957 (Fox

60°

55° Figure 3. Thrust faulting distribution and Mw -frequency statistics for the Alaska-­ Aleutian subduction zone from the 1976– 2016 Global Centroid Moment Tensor (GCMT) catalog. Top: Map of earthquakes that have thrust mechanisms (lower-hemi- sphere best double-couple mechanisms), most of which are consistent with the lo- cal trench orientation, along with all earth- 50° 100 km quakes in the catalog for this region (small red circles). Bottom: Earthquake cumula- tive magnitude distributions for all GCMT 165° 170° 175° 180° –175° –170° –165° –160° –155° –150° earthquakes in the region (left) and for the thrust earthquakes shown in above map 2.0 1.5 (right); the number of earthquakes in each All Thrust set is n, and b is the Gutenberg-Richter­ 1.5 n = 1425 n = 893 b value estimated by the maximum like- b = 0.82 1.0 b = 0.79 lihood method discussed in the text. The GCMT catalog is considered complete for 1.0 Mw ≥ 5.2, and we used this data set for the 0.5 b value calculation (dark-blue circles). 0.5 0.0 0.0 –0.5 –0.5

–1.0 –1.0 log(#Events/year Magnitude > Mw ) –1.5 –1.5 4.0 4.5 5.05.5 6.0 6.5 7.07.5 8.0 8.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 Mw Mw

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20°

18°

16°

14°

12°

10° Figure 4. Thrust faulting earthquake distri-

bution and Mw -frequency distributions for 100 km the Mexico–Middle America subduction 8° zone from the Global Centroid Moment Tensor (GCMT) catalog. See description of Figure 3 for details. –106° –104° –102° –100° –98° –96° –94° –92° –90° –88° –86° –84° 2.0 1.5 All Thrust n = 1242 n = 659 1.5 1.0 b = 0.76 b = 0.71 1.0 0.5 0.5 0.0 0.0 –0.5 –0.5 –1.0 –1.0 –1.5 –1.5

log(#Events/year Magnitude > Mw ) –2.0 –2.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 4.55.0 5.56.0 6.57.0 7.58.0 8.5 Mw Mw

Islands, Mw 8.6), 1946 (Mw 8.6), 1938 (Alaskan Peninsula, M 8.3), and 1986 (Fox Along Mexico and Middle America, the largest seismically recorded earth-

Islands, Mw 8.0). The 1986 event reruptured a western portion of the much quakes have been much smaller, and they occurred along the coast of Mex-

larger 1957 earthquake. The 1946 event ruptured between the 1957 and 1938 ico—an M 8.1 earthquake in 1932 (Jalisco) and two Mw 8 earthquakes in 1985 events and produced a disproportionally large tsunami, leading to it being (Michoacan) and 1995 (Colima). However, the largest documented historical identified as a “tsunami earthquake” by Kanamori (1972). These events have event occurred along the coast from Guerrero to Tehuantepec in 1787 with essentially filled in most of the Alaska-Aleutian plate boundary during this cen- M ~ 8.6 (Suárez and Albini, 2009), apparently rupturing across multiple zones tury. An historical great event in 1788 ruptured the 1938 zone and Kodiak Island of more recent smaller events, so the limited time span of the seismologi- portions of the 1964 ruptures (e.g., Briggs et al., 2014), indicating that bound­ cal catalog must be kept in mind. The largest recorded earthquake in Peru

aries between recent ruptures have not been persistent. was the 2001 (Arequipa, Mw 8.4) event, which ruptured within approximately

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–18° 2.0 All 100 km 1.5 n = 1175 –20° b = 0.77 1.0 –22° 0.5 –24° 0.0

–26° –0.5

–28° –1.0 –1.5 –30° log(#Events/year Magnitude > Mw ) –2.0 Figure 5. Thrust-faulting earthquake distri- 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0 bution and M -frequency distributions for –32° Mw w 1.5 the Chile subduction zone from the Global ) Thrust Centroid Moment Tensor (GCMT) catalog. See description of Figure 3 for details. –34° 1.0 n = 721 b = 0.73 –36° 0.5

–38° 0.0 –0.5 –40° –1.0 –42° –1.5 log(#Events/year Magnitude > Mw –44° –2.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0 –78°–76°–74°–72°–70°–68°–66° Mw

two-thirds of the zone of the 1868 (Arica, M ~8.5–9.0) earthquake. Other great began with interplate normal faulting but triggered Mw 8.0 thrusting on the

­Peruvian earthquakes occurred in 1940 (Mw 8.2), 1966 (Mw 8.1), 1942 (Mw 8.1), northern Tonga megathrust (Beavan et al., 2010; Lay et al., 2010b; Fan et al.,

and 2007 (Pisco, Mw 8.0). The Chile subduction zone has experienced 10 great 2016). Other great shallow events in the region in 1917 (Kermadec, M 8.2), 1919

events: 1960 (Valdivia, Mw 9.5), 2010 (Maule, Mw 8.8), 1922 (Atacama, M 8.5), (Tonga, M 8.1), and 1917 (Samoa, M 8.0) all appear to have been intraplate

2015 (Illapel,­ Mw 8.3), 1906 (Valparaíso, M 8.2), 2014 (Iquique, Mw 8.2), 1960 events (Meng et al., 2015b). The subduction zones along Vanuatu, Solomon Is-

(Bio-Bio, Mw 8.1), 1943 (Coquimbo, M 8.1), 1985 (Valparaíso, Mw 8.0), and 1995 lands, Sulawesi, Taiwan, and the Philippines all have maximum recorded shal-

(Antofagasta, Mw 8.0). Of these, only the 1943 and 2015 events rupture zones low thrusting earthquake magnitudes of about M ~8.1. Maximum magnitudes overlapped significantly. The region south of the 2014 Iquique event last rup- for recorded events along the Marianas subduction zone have been less than tured in 1877 (Iquique, M ~8.8), and this remains a major seismic gap. The 1922 M 8. While it is conceivable that very-low-probability events might rupture the rupture zone along Atacama likely has substantial strain accumulation relative entire megathrust contact in each of these island-arc regions, it may be that the to surrounding regions, as geodetic measurements indicate that the mega­ apparently low coupling and high fraction of aseismic convergence preclude thrust is pervasively locked in the region (Métois et al., 2014). this from happening, and seismic hazard assessments can focus on ruptures Along the Tonga-Kermadec arc, the largest known interplate thrust event of the size historically observed. In contrast, the Sumatra subduction zone has

occurred in 1976 (Kermadec, Mw 8.0). The 2009 Samoa earthquake (Mw 8.1) experienced several great thrust earthquakes since 2004, in 2004 (Sumatra-­

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2.0 All 1.5 n = 1257 b = 0.81 1.0

40° 0.5

0.0

–0.5

–1.0

–1.5 log(#Events/year Magnitude > Mw) Figure 6. Thrust faulting earthquake 38° –2.0 4.0 5.0 6.07.0 8.0 9.0 distribution and Mw -frequency distribu- Mw tions for the Honshu, Japan, subduction 1.5 zone from the Global Centroid Moment ) Thrust Tensor (GCMT) catalog. See description 1.0 n = 720 of Figure 3 for details. b = 0.75 0.5

0.0 36° –0.5

–1.0

100 km –1.5 log(#Events/year Magnitude > Mw –2.0 4.0 5.0 6.0 7.08.0 9.0 140° 142° 144° Mw

Andaman, Mw 9.15), 2005 (Nias, Mw 8.6), and 2007 (Bengkulu, Mw 8.4). Multiple subduction zone, including an Mw 9.0 event in 1952 (Kamchatka), and the 1963

historical great thrust earthquakes of up to M ~9 struck the region prior to 1900 (Kuril Islands, Mw 8.5), 1923 (Kamchatka, M 8.4), 2006 (Central Kuril Islands, Mw

as well. Great intraplate strike-slip ruptures with Mw 8.6 and 8.2 struck in 2012 8.3), 2003 (Hokkaido, Mw 8.3), 1994 (Kuril Islands Mw 8.3), 1958 (Kuril Islands,

in the Indo-Australian deformation zone in the oceanic plate south of Sumatra. Mw 8.3), 1952 (Hokkaido, Mw 8.1), and 1918 (Central Kuril Islands, M 8.1) events.

The Honshu, Japan, subduction zone has hosted three great thrust earth- The 2006 Kuril Islands event appears to have triggered the 2007 (Mw 8.1) outer-­

quakes since 1900, with the largest being the 2011 (Tohoku, Mw 9.1) event in rise normal fault rupture (Ammon et al., 2008; Lay et al., 2009), given that it im-

2011, followed by the 1968 (Tokachi-oki, Mw 8.2) and 1960 (Sanriku, Mw 8.0; mediately activated outer-rise faulting aftershocks. The 2003 Hokkaido rupture possibly an overestimated magnitude) earthquakes. The 2011 event appears to zone closely overlaps the 1952 zone. have reruptured a significant portion of the 1896 Meiji Sanriku tsunami earth- The variation in largest recorded earthquake size between regions is very quake zone on the shallow portion of the megathrust near the trench from substantial, and many efforts have been made to establish the factors influenc- 38°N to 40.3°N (e.g., Yamazaki et al., 2018; Lay, 2017). Other significant north- ing the distribution of great earthquakes, as discussed later. Within the seis- ern Japan earthquakes occurred in 1933 (Sanriku outer-rise normal faulting, mological record, only a few recent great events have reruptured a region with M 8.4) and 1923 (Kanto oblique thrust along the Sagami Trough, M 8.1). Nine seismological recordings of prior great earthquake activity (1986 Aleutians, great thrust earthquakes have occurred along the Hokkaido-Kuril-Kamchatka 2015 Illapel, 2003 Hokkaido). Because the prior events (1957, 1943, 1952, re-

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2.0 All 1.5 n = 1666 b = 0.83 1.0 54° 0.5

0.0

52° –0.5 –1.0 –1.5 Figure 7. Thrust faulting earthquake distri- 50° log(#Events/year Magnitude > Mw) bution and -frequency distributions for –2.0 Mw 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 the Hokkaido-Kuril-Kamchatka subduction Mw

) 1.5 zone from the Global Centroid Moment Thrust Tensor (GCMT) catalog. See description of 48° n = 1203 Figure 3 for details. 1.0 b = 0.80 0.5 46° 0.0

–0.5 44° –1.0

42° –1.5 100 km log(#Events/year Magnitude > Mw –2.0 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 142° 144° 146° 148° 150° 152° 154° 156° 158° 160° 162° Mw

spectively) all have limited seismological data and relatively poor constraints GCMT Catalog (1976–2016) on their rupture characteristics, it is very difficult to directly evaluate the sta- bility of repeated great ruptures of a given portion of the relevant megathrust. During the relatively short 40 yr time interval spanned by the GCMT cata- It is important to recognize that only ~118 yr of reliable seismological rec­ log, the largest-magnitude megathrust earthquakes occurred in Sumatra (2004

ords exist, and many earlier great events have been documented around the Mw 9.2), northern Japan (2011 Mw 9.1), and central Chile (2010 Mw 8.8). In Fig- Pacific subduction zones by historical accounts and paleoseismic analyses ures 3–11, we show GCMT faulting mechanisms for likely megathrust events (e.g., Beck et al., 1998; Kelleher, 1972; Lay et al., 1982; McCann et al., 1979; (those having shallow-dipping thrust fault solutions) for our subset of nine

Scholz and Campos, 1995, 2012). In some cases, such as for the 1837 and subduction zones, along with regional Mw -frequency distributions for shallow 1737 events in the 1960 Chile rupture zone, the size of the historic events is GCMT events. The maps display the variation in megathrust geometry and being reevaluated based on detailed examination of the record, modifying width among the major subduction zones. Table 1 lists details of the regions inferences of segmentation and regularity (e.g., Cisternas et al., 2017). The and magnitude distributions from this catalog. Cascadia subduction zone is known to have had many great earthquakes, but the most recent was in 1700, and so it is not represented by the seismological b Values catalogs. Similarly, the Himalayan front hosts great thrust events, but none has occurred during the seismological record. Southwest Japan did have Frequency-magnitude distributions are often characterized by the slope of well-characterized great earthquakes in 1944 and 1946, and documentation the linear fit to the logarithmic number of earthquakes equal to or greater than and field observations exist for numerous events in the same region dating a given magnitude (Gutenberg and Richter, 1944). This slope, the b value, is back over 1000 yr (e.g., Ando, 1975). typically around 1 for global and regional earthquake catalogs, although there

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1.5 All n = 732 1.0 24° b = 1.03 0.5

0.0 22° –0.5

–1.0 20° –1.5 Figure 8. Thrust faulting earthquake distri- –2.0

log(#Events/year Magnitude > Mw) bution and -frequency distributions for 18° 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 Mw Mw the Marianas subduction zone from the 1.0 Thrust Global Centroid Moment Tensor (GCMT) n = 181 catalog. See description of Figure 3 for 0.5 details. 16° b = 1.07 0.0

14° –0.5 –1.0

–1.5 12° 100 km –2.0 –2.5 log(#Events/year Magnitude > Mw) 4.5 5.0 5.5 6.0 6.5 7.0 7.5 140° 142° 144° 146° 148° 150° Mw

can be some variation. Variability in b value has been used to assess variations timates of the GCMT catalog completeness (Ekström et al., 2012; Wetzler et al., in stress conditions on the fault (e.g., Frohlich and Davis, 1993; Schorlemmer 2017). This choice appears to be adequate for all of the cases shown here, and

and Wiemer, 2005; Schorlemmer et al., 2005; Ghosh et al., 2008). We used the use of a higher Mc value did not change the relative values significantly. GCMT catalog centroid locations, magnitudes, and focal mechanisms for all In all regions except for the Marianas, we found b values < 1 for both the events with depths <100 km. We computed b values for all GCMT events in total and megathrust populations (Table 2). The numbers of events in the pop- each region, as well as for all thrust events, most of which are likely to have oc- ulations and the magnitude ranges spanned by the data sets (see Figs. 3–11) curred on the subduction megathrusts based on location and faulting mecha- are limited for the GCMT catalog, and most magnitude distributions only have

nism for each region (Figs. 3–11). The mechanism selection criteria for designa- well-sampled distributions up to Mw ~ 7.5 in the figures (lower in the Marianas). tion as “likely located on the megathrust” are a 45° to 90° plunge of the tension These data limitations give rise to significant uncertainty in theb values of axis, and a strike within 30° of the regional trench strike. Using the data sets ~0.05, so regional differences are suggested, but not well resolved. Apart from of all events or only thrust events, we computed b values using the ­Aki-Utsu the Marianas and Tonga, the b values for the megathrust events (0.7–0.8) were equation (Aki, 1965; Utsu, 1965): found to be systematically lower than for the full populations (0.76–0.88) by an average of 0.07. The Tonga subduction zone has b values of 0.92 and 0.93 for log e b = 10 , (1) the full data set and the megathrust, respectively, while the sparse Marianas  dM  MM−− data set has corresponding b values of 1.03 and 1.07, respectively. Reduction  c 2  of the b value for the quasi-two-dimensional, single-fault-plane thrust faulting where M is the mean magnitude of the regional distribution for events above data set relative to the three-dimensional total distribution is plausibly the re-

a completeness threshold Mc, and dM = 0.1 (controlled by rounding of the sult of having more faulting geometries in the greater volumes sampled by the

magnitudes; Table 2). We set Mc = 5.2 for both populations, based on prior es- full data set, although the differences are marginally resolved. The high thrust-

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1.5 All 1.0 n = 491 b = 0.87 –4° 0.5

0.0 –6° –0.5

–8° –1.0 –1.5 log(#Events/year Magnitude > Mw) Figure 9. Thrust faulting earthquake distri- –10° –2.0 bution and Mw -frequency distributions for 4.555.06.5 .0 6.577.08.5 .0 8.5 the Peru subduction zone from the Global ) Mw Centroid Moment Tensor (GCMT) catalog. Thrust See description of Figure 3 for details. n = 187 –12° 0.5 b = 0.73

0.0 –14° –0.5 –16° –1.0

–18° 100 km log(#Events/year Magnitude > Mw –1.5 5.065.56.0 .5 7.0 7.5 8.0 8.5 –82° –80° –78° –76° –74° –72° –70° –68° Mw

fault b values for the Tonga (Fig. 11) and Marianas (Fig. 8) zones, which have Regional Seismic Productivity steeply dipping and narrow megathrust interfaces, are likely due to inclusion

of many thrust faulting intraplate events by our criteria, since intraslab com- We also used the GCMT Mw -frequency relations for Mc = 5.2 to evaluate pression events are known to occur in these regions (e.g., Meng et al., 2015a). overall seismic productivity variations during the 1976–2016 time period using Previous studies of b values in subduction zones have also found b < 1 in both the full set of events, as well as only thrust faulting events. The Japan shallow megathrust zones. Along the northern Japan subduction zone, Tor- and Tonga-Kermadec subduction zones are most productive overall, with ~1.5 mann et al. (2015) estimate b values of 0.5–0.9 at depths between the surface earthquakes of any mechanism per kilometer length along strike of the subduc- and ~100 km, although they did not specifically separate out events occurring tion zone. Sumatra is next, with ~0.7 events/km, followed by Kuril-Kamchatka on the megathrust zone versus upper-plate or intraslab events. Nishikawa and (0.6 events/km), Chile (0.5 events/km), Mexico–Middle America (0.4 events/km), Ide (2014) computed b values for all of the subduction zones considered here, Marianas and Alaska-Aleutians (0.3 events/km), and Peru (0.2 events/km). although with more spatial subdivisions within each subduction zone and no Japan and Tonga-Kermadec also produce the most megathrust events, separation by mechanism. Given the data and methodological differences in at ~0.9 and ~0.7 events/km, respectively. Kuril-Kamchatka is next at 0.4 b value calculation, comparison of absolute values is difficult. However, their events/km, and several others group together at ~0.2–0.3 events/km (Chile, lowest b values (~0.8 for parts of Central America and Peru) correspond to Sumatra, Mexico–Middle­ America, and Alaska-Aleutians). Peru and Marianas relatively low b values in our analysis, and their highest b values, exceeding 1, produced the fewest megathrust events in this time period, at ~0.09 and 0.08 were also found for the Marianas and Tonga-Kermadec regions. events/km, respectively.

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2.0 All 14° 1.5 n = 1783 b = 0.88 1.0 12° 0.5

10° 0.0 –0.5

8° –1.0

–1.5 6° log(#Events/year Magnitude > Mw) Figure 10. Thrust faulting earthquake dis- –2.0 tribution and -frequency distributions Mw 4.0 5.06.0 7.08.0 9.0 for the Sumatra-Andaman subduction Mw zone from the Global Centroid Moment 4° 1.5

) Thrust Tensor (GCMT) catalog. See description of n = 691 Figure 3 for details. 1.0 b = 0.74 2° 0.5

0° 0.0

–0.5 –2° –1.0

–4° 100 km –1.5 log(#Events/year Magnitude > Mw –2.0 90° 92° 94° 96° 98° 100° 102° 104° 106°108° 4.0 5.0 6.07.0 8.0 9.0 Mw

The regional differences in seismic productivity are also manifested in period, cumulative moment is largely concentrated in the 15–25 km depth large event aftershock productivity, with lower values found in Mexico–Middle range (Fig. 12). The histograms are dominated by the largest earthquakes,

America, Peru, and Chile and higher values found in Tonga, Sumatra, Japan, namely, the 2004 Mw 9.2 Sumatra-Andaman, 2010 Mw 8.8 Maule, Chile, and

and Southwest Pacific island arcs (e.g., Singh and Suárez, 1988; Wetzler et al., 2011 Mw 9.1 Tohoku, Japan, earthquakes, and the moment for these events 2016). In general, island-arc megathrusts that host great earthquakes tend to is actually distributed over a depth range rather than at the GCMT centroid be more productive than continental arcs. depth. The other subduction zones have less cumulative seismic moment in the GCMT time period, and we found more diversity in the depth range of peak moment in each region (Fig. 13). For example, both the Alaska and Cumulative Seismic Moment versus Depth Peru subduction zones had a peak moment in the 20–30 km range, but the Mexico–Middle America zone seismic moment peaked at shallower depth, We compiled cumulative seismic moment from the GCMT database between 10 and 20 km. Both the Kuril and Tonga subduction zones have for all thrust-mechanism earthquakes and for all likely megathrust events recorded multiple peaks in moment depth distribution. Kuril moment peaks with strike aligned with the trench. Seismic moment is predominantly asso­ have been recorded at both 5–10 km and 20–30 km depths. There is another ciated with the likely megathrust events. For Chile, Japan, and Sumatra, peak in the Kuril zone that is much deeper, between 60 and 65 km, although

the three subduction zones with Mw ≥ 8.8 events during the 1976 to 2016 the corresponding thrust-mechanism events have strikes inconsistent with

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2.5 All 2.0 n = 3513 –16° b = 0.92 1.5

–18° 1.0 0.5 –20° 0.0 –0.5 –22° –1.0

–1.5 –24° log(#Events/year Magnitude > Mw) Figure 11. Thrust faulting earthquake dis- –2.0 tribution and Mw -frequency distributions 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 for the Tonga-Kermadec subduction zone Mw from the Global Centroid Moment Tensor 2.0 –26° Thrust (GCMT) catalog. See description of Fig- ) n = 1739 ure 3 for details. 1.5 b = 0.93 –28° 1.5

–30° 0.5

0.0 –32° –0.5

–34° –1.0 100 km log(#Events/year Magnitude > Mw –1.5 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 178° 180° –178°–176°–174°–172°–170° Mw

TABLE 2. REGIONAL b VALUE CALCULATIONS BASED ON GLOBAL CENTROID MOMENT TENSOR (GCMT) CATALOG DATA

Region Number of all earthquakes b value (Aki-Utsu, Mc = 5.2)Number of megathrust earthquakes b value (Aki-Utsu, Mc = 5.2) Alaska-Aleutians 1425 0.82 8930.79 Central America 1252 0.76 6590.71 Chile 1175 0.77 7210.73 Japan 1257 0.81 7200.75 Kuril/Kamchatka 1666 0.83 1203 0.80 Marianas 732 1.03 1811.07 Peru 491 0.87 1880.73 Sumatra 1783 0.88 6910.74 Tonga-Kermadec 3513 0.92 1739 0.93 Note: Depth range is 0–100 km, and time range is 1 January 1976 to 7 December 2016 for all regions. Megathrust earthquakes are thrust events with focal mechanism strike within the trench strike range listed in Table 1. The b value estimates based on maximum likelihood using Aki-Utsu equation (Aki, 1965; Utsu, 1965) are included, with

magnitude of completeness (Mc) based on formal GCMT estimate of Mc = 5.2.

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0 0

20 20 m) 40 m) 40 Depth (k Depth (k

60 All Thrust Thrust 60 All Thrust Thrust (Trench-Strike) (Trench-Strike) Chile Alaska Japan Central America 80 Sumatra 80 Kuril Marianas Tonga Peru 100 29 29 29 29 29 29 100 01 10 2 10 3 10 4 10 5 10 6 10 28 28 28 28 29 02 10 4 10 6 10 8 10 1 10 Seismic Moment (dyne-cm) Seismic Moment (dyne-cm) Figure 12. Cumulative seismic moment release with depth in the subduction zones that have Figure 13. Cumulative seismic moment release with depth in the indicated subduction zones. produced the largest events Mw ≥ 8.8 since 1976. Moment, centroid depth, and focal mechanism are from the Global Centroid Moment Tensor (GCMT) catalog. Solid lines indicate those thrust Moment, centroid depth, and focal mechanism are from the Global Centroid Moment Tensor mechanism events with focal mechanism strikes that fall within the regional trench strike listed (GCMT) catalog. Solid lines indicate those thrust mechanism events with focal mechanism in Table 1; dashed lines represent moment from all thrust mechanism earthquakes in the speci- strikes that fall within the regional trench strike listed in Table 1; dashed lines represent moment fied region (which mostly overlap indistinguishably for these regions). from all thrust mechanism earthquakes in the specified region.

the local trench orientation, so they are likely intraslab events. The Tonga tics. These rare shallow ruptures tend to have anomalous properties, with rela- subduction zone includes a shallow peak at 10–15 km, and a deeper one at tively low rupture velocity, low radiated energy, and large slip that generates a 40–45 km; again, the deeper one is likely to be from intraslab compressional strong tsunami. Such events have struck along the Aleutians, Japan, the Kuril events (e.g., Meng et al., 2015b). We believe that all of the significant inter- Islands, Peru, Nicaragua, El Salvador, New Zealand, Java, and Sumatra, so plate seismic moment release is at depths less than 50 km in our selected they are widespread, albeit infrequent events (e.g., Kanamori, 1972; Polet and regions, and less than 60 km globally. Kanamori, 2000; Bilek and Lay, 2002). In addition, the occurrence of several great (M > 8) earthquakes since 2004 has provided excellent recordings by large numbers of broadband seismo- Depth Variations in Radiation Characteristics graphs, leading to observations of large slip but very little in the way of coher- ent bursts of high-frequency radiation in the shallowest part of the relevant In recent decades, many observations have indicated depth dependence subduction zone, but many bursts of high-frequency radiation originating in along megathrusts for a variety of seismic characteristics. The recognition of the deeper regions of the seismogenic zone, where there is lower coseismic tsunami earthquakes (Kanamori, 1972) that produced much larger tsunamis slip (e.g., Lay et al., 2012). In the following sections, we outline several of the than would be expected for a typical earthquake with the same surface wave depth-dependent seismic characteristics that have been noted for megathrust

magnitude (MS) prompted focused research on depth-dependent characteris- earthquakes.

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Source Duration found positive mb-Mw perturbations for deeper megathrust events, reflecting enhanced deep short-period radiation in Japan, northern Sumatra, and central Tsunami earthquake ruptures have significantly longer rupture durations Chile. Little to no apparent depth trend was observed in the scattered data for

than is typical for similar-sized events. For example, the 1992 Mw 7.7 Nicaragua the other subduction zones in their study. tsunami earthquake had source time function duration estimates of 100–150 s

(e.g., Kanamori and Kikuchi, 1993; Ihmlé, 1996b), and the 2010 Mw 7.8 Men- tawai event had duration estimates of 80–125 s (Yue et al., 2014a; Newman Spatial Variations of Slip et al., 2011). Motivated by these long source duration observations for the tsunami Recent well-recorded large and great earthquakes have displayed exten-

earthquakes, Bilek and Lay (2002) examined over 500 earthquakes (Mw 5.0–7.5) sive diversity in rupture characteristics, with differences in radiated energy within 14 circum-Pacific subduction zones and found that some events, but depending on depth of the rupture patch, both bilateral and unilateral rup- not all, in the shallowest 15 km of the megathrust had long moment-scaled ture expansion, and highly variable slip distribution complexity. Hundreds of

source durations (to an equivalent Mw = 6.0 reference), similar to scaled dura- events in subduction zones now have finite-fault solutions, derived from a mix tions of seven tsunami earthquakes. Results for individual subduction zones of teleseismic, strong motion, geodetic, and tsunami observations. The USGS- indicate several events with long durations (10–20 s) located in the uppermost NEIC routinely computes high-quality solutions for large shallow earthquakes 15 km, with events deeper than 15 km having shorter (mean ~5–6 s) durations. (Hayes, 2017), and many contributions are collected in the SRCMOD database El Hariri et al. (2013) examined relocated subduction zone earthquakes with (http://equake​ ​-rc.info​ /SRCMOD/​ ). Some solutions are well resolved, others improved depth constraints and classified the events into shallow (<26 km) have severe trade-offs between kinematic constraints and model parameter- and deep (>26 km) groups based on overall longer moment-scaled durations ization. Point source time functions are routinely determined by the SCARDEC of the shallow group. El Hariri et al. (2013) also focused on regional compari­ method (e.g., Vallée and Douet, 2016). Efforts are under way to synthesize the sons, finding weak to no correlation between longer duration and reported accumulated data sets (e.g., Kanamori, 2014; Lay, 2015; Ye et al., 2016b, 2016c; amounts of subducted sediment, presence of bathymetric features, or regions Denolle and Shearer, 2016; Meier et al., 2017; Melgar and Hayes, 2017; Hayes, of observed afterslip, all factors that have been proposed to affect rupture 2017), and we will not attempt to summarize the multitude of studies. Large complexity and duration. shallow coseismic slip occurred in the 2015 Illapel (e.g., Li et al., 2016; Melgar More recent work suggests some depth variation in duration estimates us- et al., 2016), 2011 Tohoku (e.g., Lay et al., 2011b; Iinuma et al., 2012; Ozawa et al., ing global data sets. Denolle and Shearer (2016) examined spectral ratios for 2012; Satake et al., 2013; Romano et al., 2014; Bletery et al., 2014; Melgar and over 900 thrust earthquakes (M > 5.5) and found weak dependence of scaled Bock, 2015; Lay, 2017), 2010 Maule (e.g., Vigny et al., 2011; Yue et al., 2014b; source duration with increasing depth on the fault. Similarly, Ye et al. (2016c) Yoshimoto et al., 2016; Maksymowicz, et al., 2017), and 2004 Sumatra (e.g., found longer-duration earthquakes in the shallowest (<18 km) portion of the Ammon et al., 2005; Rhie et al., 2007; Fujii and Satake, 2007) events, accompa-

subduction megathrust using finite fault determinations for over 100M w > 7 nying slip on the downdip portions of the megathrusts. In other cases, such as earthquakes. Tsunami earthquakes, as well as a few other shallow events, the 2014 Iquique, Chile (e.g., Lay et al., 2014; Hayes et al., 2014b), 2012 Nicoya,

were found to have scaled durations (to an equivalent Mw = 6.0 reference) of Costa Rica (e.g., Yue et al., 2013; Liu et al., 2015), 2003 Tokachi-oki, Japan (e.g., between ~8 and 19 s, i.e., significantly larger than the average of the rest of the Miyazaki and Larson, 2008; Romano et al., 2010), and 2007 Pisco, Peru, rup- events in their study (~5.6 ± 1.5 s). tures (e.g., Lay et al., 2010a; Sladen et al., 2010), slip was concentrated on the central or deeper portion of the rupture zone, with no shallow coseismic slip. Several subduction zones have experienced aseismic slip transients both

mb/Mw before and after the main shocks. The 2011 Tohoku and 2014 Iquique earth- quakes had large numbers of migrating repeating earthquakes during active Recent observations of depth dependence in high-frequency radiation foreshock sequences, with the repeating earthquakes providing a proxy for (e.g., Lay et al., 2012) suggest that other seismic characteristics, such as the shallow aseismic slip that migrated toward the eventual main-shock hypo-

short-period body wave magnitude (mb), may also vary with depth relative to centers (e.g., Kato et al., 2012; Ruiz et al., 2014; Hayes et al., 2014b; Kato and

the long-period Mw. Tsunami earthquakes, given their shallow slip with limited Kakagawa, 2014; Meng et al., 2015a). Socquet et al. (2017) describes GPS data

high-frequency radiation, can be identified by their relatively lowm b relative analysis that supports the presence of a slow slip event in the 2014 Iquique re-

to Mw measurement (e.g., Kanamori, 1972; Kanamori and Kikuchi, 1993; Lay gion during the 8 mo prior to the main shock. Aseismic slip triggered by great

et al., 2011a). Rushing and Lay (2012) examined the differences between mb earthquakes is also common (e.g., Chlieh et al., 2007; Hsu et al., 2006; Ozawa

and Mw for a large catalog of likely megathrust events with Mw ≥ 5 in several et al., 2011; Hayes et al., 2014a; Uchida et al., 2015). Aseismic, or slow-slip, subduction zones (Chile, Japan, Sumatra, Kuril, Aleutians, Sumba, Peru). They transients are not limited to regions of recent great earthquakes, as these have

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been observed in both shallow and deep portions of the seismogenic zone ary slip budget, the diversity of recent large earthquakes on megathrusts has in many subduction zones around the globe (e.g., Dragert et al., 2001; Obara presented surprises in terms of event location, size, and hazard for multiple et al., 2004; Brudzinski et al., 2007; Schwartz and Rokosky, 2007; Correa-Mora plate boundaries. The 2004 Sumatra earthquake ruptured over a much greater et al., 2009; Jiang et al., 2012; Saffer and Wallace, 2015; Wallace et al., 2016). length (>1300 km) of the plate boundary than had been considered likely (e.g., Many of the great earthquakes in the last 39 yr have occurred in previ- Shearer and Bürgmann, 2010), and the 2011 Tohoku, Japan, earthquake rup- ously recognized seismic gaps (McCann et al., 1979; Nishenko, 1991), although tured with unexpectedly huge slip, extending to the trench, along with rerup- some only partially filled the gap, leaving residual strain accumulation to be turing regions of recent, smaller earthquakes (e.g., Lay and Kanamori, 2011; released in future events (Fig. 14). Targeted instrumentation of specific iden- Fujiwara et al., 2011; Iinuma et al., 2012; Lay, 2017). Both events generated tified seismic gaps prior to great earthquakes in 2010, 2014, and 2015 along devastating tsunamis and reaffirmed the potential for events larger than have Chile, in 2005 and 2007 along Sumatra, in 2016 along Ecuador, and in other been documented in recorded history to strike in various regions. locations, has resulted in unprecedented seismic and geodetic data sets for The 2015 Illapel earthquake ruptured the Chilean megathrust close to previ- recent events. However, even given the general constraint of the plate-bound- ous earthquakes in 1880 and 1943, and so the 2015 event has been suggested

Figure 14. Seismic gaps, as defined by McCann et al. (1979), and recent great earthquakes (1979–2016). Colored lines indicate timing of great earthquake occur- rence prior to gap definitions in 1979, with red shading indicating longest time since great earthquake. Focal mechanisms of great earthquakes between 1979 and 2016 are from the Global Centroid Moment Ten- sor (GCMT) catalog.

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to be a “characteristic earthquake” for this segment of the margin (e.g., Tilman 2007a). LFEs are deficient in high-frequency energy, with dominant signals et al., 2016; Klein et al., 2017). In contrast, the 2014 Iquique earthquake spanned around 1–8 Hz, durations of ~0.3 s, and magnitudes of ~1011 Nm, with locations only about 20% of the northern Chile seismic gap, with most of the 1877 M and mechanisms reflecting slip on the megathrust (e.g., Shelly et al., 2007; ~ 8.8 earthquake rupture zone remaining unbroken (e.g., Hayes et al., 2014b; Ide et al., 2007b). In many cases, these LFEs occur as swarms that comprise Meng et al., 2015a; Cesca et al., 2016). Earthquakes along northern Japan were much of the seismic nonvolcanic tremor observed in subduction zone settings common in the last century, mainly in the M 7–8 magnitude range, but the 2011 (e.g., Ide et al., 2007b). LFEs were initially detected and located on the deeper

Mw 9 Tohoku earthquake ruptured a region where the last giant event occurred portion of seismogenic zones (e.g., Katsumata and Kamaya, 2003; Shelly et al., in A.D. 869, along with multiple rupture zones of previous individual ruptures 2006; Brown et al., 2009; Bostock et al., 2012), but they have now also been

(e.g., Koper et al., 2011; Lay, 2015). The 2010 Mw 8.8 Maule earthquake ruptured observed in shallow portions of accretionary prisms (e.g., Obana and Kodaira, a seismic gap in a region that last ruptured in 1835 in an M ~8.5 earthquake 2009; Yama­shita et al., 2015; Nakamura, 2017). (e.g., Moreno et al., 2010), but the main slip extended beyond that rupture, VLFEs represent even slower slip, with durations of ~20 s, magnitudes overlapping the rupture zone of a large event in 1928, but not the adjacent 1985 up to ~1014 Nm, and frequencies of ~0.02–0.05 Hz, i.e., even more deficient in rupture zone. Along the Sumatra megathrust, the great earthquakes in 2005 high-frequency energy than LFEs (e.g., Ide et al., 2007a). Similar to LFEs, these and 2007 reruptured regions that had previously broken in 1797, 1833, 1861, events appear to be located throughout the seismogenic zone (Fig. 1), from the and 1907 (e.g., Lay et al., 2005; Konca et al., 2008; Chlieh et al., 2008). The 2007 deep transition between the seismogenic and aseismic zones (e.g., Ito et al., Peru earthquake occurred in a region last possibly ruptured in 1687 or 1746 and 2007, 2009) to the updip end of the seismogenic zone, in some cases, very only partially filled a seismic gap between the 1996 and 1974 events (Pritchard close to the trench (e.g., Ito and Obara, 2006; Asano et al., 2008; Matsuzawa and Fielding, 2008). The 2007 Solomon Islands earthquake ruptured in a very et al., 2015; Araki et al., 2017), and within the accretionary prism (e.g., Obara complex triple-junction plate-boundary zone, with no prior record of large and Ito, 2005; Ito and Obara, 2006). earthquakes (Taylor et al., 2008; Furlong et al., 2009; Chen et al., 2017). Along At the slowest end of the slip spectrum, there are slow slip events (SSEs),

the central Kuril Islands, the 2006 Mw 8.3 earthquake ruptured a portion of the which are observed geodetically along many subduction zones. These slip 18 20 megathrust between the 1963 Mw 8.5 and 1952 Mw 9 earthquakes identified as a events can be quite large, reaching an equivalent seismic moment of 10 –10 seismic gap that may have partially failed in 1918 (e.g., Lay et al., 2009). Nm, with durations over 105–107 s (days to months; e.g., Ide et al., 2007a). SSEs are often, but not always, accompanied by nonvolcanic tremors and are Spectrum of Slip Velocities grouped into the episodic tremor and slip (ETS) category (e.g., Dragert et al., 2001; Kostoglodov et al., 2003; Obara et al., 2004; Hirose and Obara, 2006; Observations of slip processes for subduction zone megathrusts have ex- Schwartz and Rokosky, 2007; Beroza and Ide, 2011). These were also initially panded greatly beyond early measurements of earthquake rupture front ve- observed in the vicinity of the deepest portions of the seismogenic zone (Fig. locities, which were on the order of 75%–95% of the shear wave velocity in the 15), but more recent observations indicate that shallower megathrust SSEs slip zone expansion of the earthquake (e.g., Kanamori and Brodsky, 2004). Tsu- occur as well (Saffer and Wallace, 2015, and references therein), including very nami earthquakes appear to have very low rupture velocities, such as the 1–1.5 close to the trench (Araki et al., 2017). km/s rupture velocity of the 1992 Nicaragua tsunami earthquake (e.g., Kikuchi and Kanamori, 1995; Ihmlé, 1996a, 1996b) and the 1.25–1.5 km/s rupture veloc- Radiated Energy, Apparent Stress, and Stress Drop ity for the 2010 Mentawai event (e.g., Lay et al., 2011a; Newman et al., 2011). Kanamori et al. (2010) and Bilek et al. (2011, 2016) suggested that other events Choy and Boatwright (1995) determined radiated energy from global shal- in the shallow region of tsunami earthquakes may also exhibit low rupture low earthquakes (M > 5.8) and used it to compute apparent stress (given by the

velocities. Variable rupture speeds may exist within an individual earthquake, ratio of radiated energy to seismic moment, ER /M0, multiplied by the average such as the 2010 Maule (Kiser and Ishii, 2011) event, and rupture velocities may rigidity). They found that subduction zone thrust events have the lowest val- lie along a wide continuum between “typical” and “slow” speeds, such as the ues (~0.29 MPa) of apparent stress for any tectonic category, although with re- ~1.5–2 km/s rupture velocity estimated for the 2013 Santa Cruz Islands earth- gional variations between 0.15 MPa (Kermadec) and 0.42 MPa (Peru-Ecuador). quake (Lay et al., 2013; Hayes et al., 2014a). Supershear earthquakes have not Newman and Okal (1998) determined radiated energy and energy-to-­ been observed in the shallow portion of subduction zones, possibly because moment ratios for over 50 teleseismic earthquakes, highlighting the 1992 the events tend to have mixed mode ruptures. Nicaragua,­ 1994 Java, and 1996 Peru tsunami earthquakes as having distinctly At rupture velocities much slower than for tsunami earthquakes, a fam- low values for moment-scaled radiated energy relative to other earthquakes ily of slip processes, including low-frequency earthquakes (LFEs) and very in their data set. Okal and Newman (2001) examined other earthquakes low-frequency earthquakes (VLFEs), involves slip events with velocities or- within these three tsunami earthquake-producing regions, finding additional ders of magnitude lower than typical earthquake rupture velocities (Ide et al., “slow” earthquakes present only near the 1960 Peru tsunami earthquake zone.

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10 TremorSSE 20 Contours 30 of plate Megathrust –1 40 earthquake 5 cm year 50 interfaces 60 70 Alaska km km 0 200 400600 Mexico km Costa Rica km 0200 400 600 0200 400600 −156° 400 400 400 20°

−154°58° km ° km 12 km 200 −88° 200 Guerrero Oaxaca 200 −152° 1964 18° Figure 15. Examples of tremor and slow- slip events (SSE) located at the downdip portion of the seismogenic zone, modified 0 0 0 −102 −100 −98 −88 − −84° from Beroza and Ide (2011). –150° 58° ° 60° 10° 8° 86° ° ° ° −148° −146° −144° −142 °

Nankai km Cascadia km 34° 0 200 400 600 800 1,000 0200 400600 8001,000 1,200

400 –124° 400

130° Tokai km –126 km 32 ° ° Shikoku Kii 200 200 1946 Nankai 1944 Tonankai 1923 –128 Kanto Boso °

32° 34° 0 0 ° °

50° 48° 46 44° 42° 40° 132° 134° 136 138° 140°

­Convers and Newman (2011) expanded the catalog of Newman and Okal (1998) 2011), and the 2015 Mw 8.3 Illapel, Chile, event (e.g., Melgar et al., 2016; Yin by determining estimates of radiated seismic energy for 342 earthquakes from et al., 2016). Ye et al. (2016c) documented modest depth dependence in high-­ 1997 to 2010, finding shallow subduction zone thrust earthquakes generally frequency radiation for the 100+ large events, with lower spectral decay rates deficient in radiated energy. They found additional energy-deficient events in for deeper events on subduction zone megathrusts. the rupture zones of the 1992 Nicaragua and 2006 Java tsunami earthquakes. Allmann and Shearer (2009) determined stress drop for over 1700 global Venkataraman and Kanamori (2004) and Ye et al. (2016c) also demonstrated earthquakes, finding little depth variation in the full global catalog, but region- that tsunami earthquakes have low moment-scaled radiated energy compared ally significant depth-dependent stress drops. They also found along-strike to other subduction zone earthquakes. variations, in particular, very low values along portions of the Central America For events within a given region, recent great earthquakes that have subduction zone and near the hypocenter of the 2004 Sumatra earthquake. been well recorded by large seismic networks have a depth dependence in Similar to the depth dependence of source duration discussed in the “Source the frequency of radiated seismic energy (e.g., Lay et al., 2012). The deeper Duration” subsection herein, Denolle and Shearer (2016) found very weak parts of the megathrust produce significantly more high-frequency energy depth dependence for stress drop and scaled energy using their global cata­ than the shallower megathrust for most of the largest earthquakes in recent log. They observed regional variations, with lower stress drop in general in

­decades, such as the 2010 Maule Chile Mw 8.8 (e.g., Kiser and Ishii, 2011; Koper western Pacific subduction zones as compared to eastern Pacific zones, and

et al., 2012), 2011 Tohoku Mw 9.1 (e.g., Koper et al., 2011; Ide et al., 2011; Ishii, very low stress drops for earthquakes along the Alaska-Aleutian subduction

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zone. Ye et al. (2016c) found little depth dependence of stress drop for major variations that appeared to link areas of high-stress-drop events with zones and great megathrust events deeper than 15 km, but there is an order of mag- of little coseismic slip in 2011, south of the region of maximum main-shock nitude scatter in stress drop values with little regional coherence. Our compi- slip. Bilek et al. (2016) used spectral ratio techniques to compute corner fre- lations of these various global studies also suggest weak depth dependence in quencies, seismic moment, and stress drop for a series of earthquakes that

both ER /M0 and stress drop, with large scatter (Figs. 16 and 17). occurred within and adjacent to the 1992 Nicaragua tsunami earthquake rup- On regional scales, spatial variations in stress drop for small earthquakes ture zone. Earthquakes from 2005 to 2006 that occurred within the 1992 ­rupture are observed and may be useful for probing the stress state of a megathrust. zone exhibited significantly lower corner frequencies and stress drop than Uchide et al. (2014) described stress drops computed for over 1500 small earth- those events that occurred adjacent to the 1992 rupture zone, suggesting that

quakes within the Tohoku region of Japan prior to the 2011 Mw 9.1 earthquake. regional variations in the megathrust zone that affect rupture processes may They found increasing stress drop at 30–60 km depths, and strong along-strike persist for many years afterward, if not permanently.

10–2 104 Denolle and Shearer (2016) Ye et al. (2016, group 2 events) Ye et al. (2016, group 1 events)

10–3 1000

100 10–4

10 10–5 o Stress Drop (MPa) /M r

E 1

10–6

0.1

10–7

0.01 Ye et al. (2016, group 1 events) Ye et al. (2016, group 2 events, Vr 3.0 km/s) 10–8 Denolle and Shearer (2016) El Hariri et al. (2013) 01020304050607080 0.001 Depth (km) 01020304050607080

Figure 16. Radiated energy (Er) over seismic moment (MO) estimates for global subduction zone Depth (km) earthquake populations from Denolle and Shearer (2016) and Ye et al. (2016b). The Ye et al. (2016b) study included only megathrust ruptures; group 1 events are those with independent Figure 17. Stress drop estimates for global compilations of shallow subduction zone earth- rupture model constraints; rupture characteristics for group 2 events were determined using a quakes from Ye et al. (2016b), Denolle and Shearer (2016), and El Hariri et al. (2013). Stress drop

range of rupture velocities in kinematic inversions of teleseismic body waves. estimates from Ye et al. (2016b) include the group 2 event rupture models with a Vr of 3 km/s.

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Aftershock Distribution tion zones in order to compare it with other parameters, such as slab age and dip, to understand the large-scale controls on coupling. With the availability of Compilations of aftershock locations relative to areas of high slip during longer earthquake catalogs, more detailed seismic observations of individual large earthquakes of all types suggest that aftershocks tend to locate at bound- earthquakes, and increasingly widespread geodetic observations, other stud- aries between areas of high slip and low slip and/or at edges of large-slip re- ies have focused on smaller spatial scales to examine coupling and the rela- gions (e.g., Das and Henry, 2003; Wetzler et al., 2018). This observation has tionship to seismic activity. We summarize both scales of observations here. been supported by aftershock catalogs for many of the M > 8 earthquakes, such as the recent Sumatra (e.g., Hsu et al., 2006), Chile (Hayes et al., 2014b; Yue et al., 2014b; Li et al., 2016), and Tohoku (e.g., Asano et al., 2011) earth- Large-Scale Subduction Zone Parameters quakes. The 2013 Santa Cruz Islands earthquake appears to have had few after- shocks on the megathrust fault, and this event may have triggered an adjacent Uyeda and Kanamori (1979) described end-member subduction models aseismic slip episode following the main shock (Hayes et al., 2014a). The 2014 with distinct back-arc processes. They noted that zones with active back-arc Iquique earthquake produced many aftershocks, but the aftershocks had low b spreading, such as the Marianas, have fewer large and great earthquakes than values, despite the occurrence of a large aftershock (Hayes et al., 2014b). There zones without back-arc spreading, such as the great earthquake-producing is a weak tendency for high-stress-drop megathrust ruptures with a given mo- Chilean margin. As an explanation for these end-member behaviors, they sug- ment to have lower aftershock production, which is attributed to the smaller gested each zone has a different level of plate coupling caused by either time main-shock rupture dimensions (Wetzler et al., 2016). The source dimensions evolution of the subduction geometry and progressive decoupling or differ- of the main slip zone for the 2014 Iquique event were indeed unusually small ences in the motion of the overriding plate. for a great earthquake. In their broader review of plate coupling in many subduction zones around Aftershocks occur off of the main megathrust rupture plane as well. Nor- the globe, Scholz and Campos (1995) suggested that the strongest coupling mal faulting aftershocks have been observed after many large megathrust rup- occurs where large megathrust normal forces exist, controlled by plate age, tures (Fig. 18), particularly those that rupture very shallowly near a trench (e.g., slab length and dip angle, and velocity of plate motion. Subduction zones Kanamori, 1971; Christensen and Ruff, 1988; Lin and Stein, 2004; Ammon et al., with active back-arcs, and old, steeply dipping subducting lithosphere such 2008; Lay et al., 2009; Asano et al., 2011; El Hariri and Bilek, 2011; Bilek et al., as the Marianas, southern Tonga, and Izu-Bonin have the lowest normal force 2011; Ide et al., 2011; Kato et al., 2011; Lange et al., 2012; Rietbrock et al., 2012; acting on the interface in this model, and so they have the weakest coupling Yue et al., 2014b; Wetzler et al., 2017; Sladen and Trevisan, 2018). Some of these and smaller maximum-magnitude earthquakes. In contrast, the Chile, Peru, intraplate extensional events are within the outer rise, seaward of the trench, ­Sumatra, Alaska, and Cascadia subduction zones have higher normal forces or on plate-bending related faults below the megathrusts, while others are and plate coupling. Scholz and Campos (2012) revisited this model with up- found within the upper plate adjacent to the zone of high slip (e.g., Farías et al., dated seismic catalogs and coupling estimates finding a similar correlation, 2011; Hicks and Rietbrock, 2015). Events with rupture deeper on the mega­thrust with the larger earthquakes occurring in regions of higher normal forces and activate relatively few intraplate aftershocks (Wetzler et al., 2017; Sladen and higher coupling. They also took advantage of the large number of geodetically Trevisan, 2018), but in some cases, they have large afterslip on the updip re- based coupling estimates to resolve significant along-strike coupling varia- gion of the megathrust (Fig. 18). tions within individual subduction zones. Because of the importance of slab dip angle on normal forces and coupling, various studies have examined dip angles in the context of other subduction FACTORS CONTRIBUTING TO MEGATHRUST SPATIAL zone parameters. For example, a comparison of slab dip in over 150 geomet- HETEROGENEITY AND SLIP VARIABILITY rically simple subduction zone transects suggests that the absolute motion of the overriding plate has the strongest correlation with slab dip, much larger As demonstrated in the previous section, earthquake observations now than factors such as the age and thermal structure of the slab, the slab pull span a wide range of slip speed and complexity. Here, we outline a range of force, or convergence rate (Lallemand et al., 2005). Back-arc spreading is only possible factors leading to these diverse slip observations. found in areas where the deeper portion of the slab dips at greater than 50°, Key questions about where slip can occur in subduction zones often start and back-arc shortening is prevalent in areas with slab dips of less than 31° with an examination of seismic coupling. This can be defined and estimated in (Lallemand et al., 2005). Curvature of the megathrust is strongly correlated a variety of ways (e.g., Wang and Dixon, 2004), but it fundamentally involves a with average dip angle of the megathrust, but it is also influenced by dip of the short time window estimate of how much of the plate motion is released in fast deeper slab, which may cause shear strength to vary across the plate bound- processes that generate seismic energy (e.g., Scholz and Campos, 2012). Var- ary. More shallowly dipping, flatter boundaries are able to support larger ious studies have estimated seismic coupling for large segments of subduc- earthquakes due to more homogeneous strength (e.g., Bletery et al., 2016).

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A

Figure 18. Examples of range of outer-­ rise seismicity characteristics follow- ing large and great megathrust earth- quake ruptures, reprinted from Sladen and Trevisan (2018), with permission from Elsevier. In several large to great earthquakes with slip distributions (green-blue shading) indicating slip to the trench, significant numbers of normal faulting aftershocks occurred (red circles). In areas with little to no B rupture to the trench, normal-faulting aftershocks were less prevalent. Global CMT—Global Centroid Moment Tensor catalog. ISC, International Seismologi- cal Centre; WR, Wharton Ridge. Events such as HaidaGwaii and Tocopilla are discussed in Sladen and Trevisan (2018).

C

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The nature of the upper plate, being either continental or oceanic, also appears in upper-plate Vp/Vs and inferred fluid pressure variations along a small seg- to be important, as are factors such as the presence of back-arc spreading and ment of Costa Rica, with lower Vp/Vs in the northwestern portion of the Nicoya trench roll-back (Scholz and Campos, 2012). Peninsula relative to the higher Vp/Vs and higher fluid pressures along the southeastern segment of the peninsula. They suggested that these upper-plate variations influence the seismogenic zone behavior. Role of Megathrust Fluids These variations in fluid content and pressure are also linked to variations in slip processes. Along the Nankai subduction zone, the high fluid pressures Fluids play a critical role in many subduction zone processes, from fault in the shallow zone may inhibit near-trench slip (Tobin and Saffer, 2009), and slip to volcanic activity (e.g., Peacock, 1990). We focus here on the aspects of the along-strike variability at depth may allow for nucleation of tsunami earth- fluids that influence slip processes on the megathrust, from great earthquakes quakes in fluid-poor regions, with rupture into fluid-rich zones later in the to the slow-slip and tremor episodes observed in many regions. Fluids enter event (Park et al., 2014). Along central Costa Rica, the seismicity appears to the shallow seismogenic system either within the pore space of the subducting preferentially occur in the fluid-poor zones (Bangs et al., 2015), and along the igneous crust and sediment, or bound in hydrous minerals. Normal faulting Nicoya Peninsula, Costa Rica, the areas of low fluid pressures host small-mag- associated with bending of the subducting plate is likely to produce enhanced nitude earthquakes with higher apparent stresses than are found in the high- hydration of the crust and upper mantle seaward of the subduction zone (e.g., fluid-pressure­ areas (Audet and Schwartz, 2013; Stankova-Pursley et al., 2011). Ranero et al., 2003; Naliboff et al., 2013; Korenaga, 2017; Grevemeyer et al., Other studies have compared seismic observations of Vp/Vs and fluid pres- 2018). These fluids are released either through compaction in the shallowest sure estimates to geodetic coupling and rupture details of specific earthquakes. 5–7 km or in dehydration reactions at higher pressures and temperatures (e.g., For example, Moreno et al. (2014) examined a portion of the 2010 Maule Chile Saffer and Tobin, 2011). The fluids move through the system by a variety of rupture zone and found a positive correlation between areas of high Vp/Vs and pathways, including faults, fractures, and permeable strata within the mega­ low geodetically determined coupling. Huang and Zhao (2013) and Yamamoto thrust and overriding plate (e.g., Carson and Screaton, 1998), or they remain et al. (2014) found significant Vp/Vs heterogeneity in the shallow megathrust trapped by low-permeability sediments, affecting the seismogenic zone by zone of the 2011 Tohoku rupture, suggesting a high Vp/Vs ratio in the very near- increasing fluid pressures (e.g., Saffer and Bekins, 2002). Excess fluid pres- trench area of high coseismic slip, as well as downdip of the hypocenter of the sures within the megathrust zone modify the effective normal stress, which is event, with low Vp/Vs just updip of the hypocenter. linked to overall fault strength and generation of both typical earthquakes and slower-slip­ processes (e.g., Saffer and Tobin, 2011). Slow-Slip and Tremor Processes

Role of Fluids in Seismic Slip Many observations of slow slip and tremors in subduction zones corre- spond to areas that appear to have high fluid pressures. Early efforts to iden- Because high fluid pressure acts to reduce effective normal stress, fluids tify and locate nonvolcanic tremors in the Nankai subduction zone resulted in play an important role in the earthquake process. Various studies have pro- locations at depths where pressures and temperatures are such that fluids can vided evidence for significant fluid content and high fluid pressures in the be generated through dehydration (Obara, 2002). Rogers and Dragert (2003) megathrust zone, often based on bright spots in seismic reflection profiles and emphasized the potential importance of fluids in their initial reporting of Cas- high Vp/Vs seismic velocity ratios, which serve as a proxy for high fluid pres- cadia episodic tremor and slip. Tremor signals along the Nicoya Peninsula, sures. We note only a few of these studies here, along with the seismic implica- Costa Rica margin, correlate with observations of seafloor fluid-flow measure- tions of these observations. Tobin and Saffer (2009) estimated high pore-fluid ments at the surface of the forearc, which is explained by slow-slip motions pressures and low effective stress along the shallow Nankai subduction zone in the shallow portion of the megathrust causing the observed transient flow in using detailed seismic reflection data. Similarly, Park et al. (2014) attributed the upper-plate sediments (Brown et al., 2005). along-strike variability in seismic reflection characteristics along the Nankai LFEs have also been linked to high fluid pressures. Shelly et al. (2006) lo- subduction zone to differences in fluid amounts along the megathrust. Bangs cated LFEs within tremors along the Nankai subduction zone in an area of high et al. (2015) used three-dimensional (3-D) seismic imaging to identify fluid-rich Vp/Vs, indicative of high pore-fluid pressures. Recent locations of LFEs in Nan- segments along the plate interface offshore central Costa Rica. Audet­ et al. kai correlate variations in LFE location with Vp and Vs anomalies. Nakajima (2009) used seismic receiver functions and Vp/Vs to suggest that the mega­ and Hasegawa (2016) found gaps in LFE occurrence in areas where they ob- thrust zone along the northern Cascadia margin has low permeability, acting served small Vp and Vs anomalies, indicative of a well-drained and low-pore- to seal high fluid pressures of the oceanic crust, and leading to significant over- pressure megathrust; the LFEs are instead concentrated in areas of higher Vp pressures in this zone. Audet and Schwartz (2013) found along-strike variations and Vs anomalies, serving as proxies for higher-pore-fluid pressure.

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Similar correlations exist for slow-slip events and high fluid pressures. Seis- Sumatra earthquake rupture zone may have allowed very shallow rupture for mic images suggestive of a high-fluid-pressure zone in the area of slow earth- that event, in contrast to the deeper slip of the 2005 Nias earthquake (Geersen quakes along the Nankai subduction zone prompted Kodaira et al. (2004) to et al., 2013). Large variations of subducted sediment along-strike in northern link the slow-slip event generation with fluids. Song et al. (2009) documented Hikurangi may affect the locations of shallow slow-slip events there (Bell et al., a very low-seismic-velocity layer in the same region as several slow-slip events 2010; Eberhart-Phillips and Bannister, 2015). Based on determination of seismic along the Mexico subduction zone; they postulated that the low velocities are velocities in the location of 2011 Tohoku rupture, Zhao et al. (2011) found that the result of a region with high pore-fluid pressure. Seismic reflection data high coseismic slip and foreshock locations are concentrated in the high-seis- suggest high fluid pressures in slow-slip zones of the Hikurangi margin (Bell mic-velocity areas; the nearby low-seismic-velocity patches, inferred to be sed- et al., 2010). Audet and Schwartz (2013) identified similar correlations among iments and high fluid content, host aseismic creep and slow thrust events. Be- high Vp/Vs, higher fluid pressures, and slow slip in the southeastern Nicoya yond the sediment thickness and composition, fluids expelled from subducted segment of the Costa Rica subduction zone. The high fluid pressures, and the sediments also appear to be an important component in the slow-slip process related decrease in the effective normal stress are commonly invoked as nec- (e.g., Peacock, 2009; Peng and Gomberg, 2010; Audet and Kim, 2016). essary for these slow-slip processes in the both the shallow (e.g., Saffer and Sediment type is also important for controlling slip behavior. Many stud- Wallace, 2015) and deep portions of the seismogenic zone (e.g., Audet and ies have focused on the frictional stability of various clay-rich and carbonate Kim, 2016). sediments­ observed entering subduction zones to identify the conditions Observations of finer-scale spatial and temporal distribution of fluid pres- under which velocity weakening behavior is possible (e.g., den Hartog et al., sure are now being made and related to slow slip and tremor processes. Kato 2012; den Hartog and Spiers, 2013; Ikari et al., 2013; Kurzawski et al., 2016). et al. (2010) found an asymmetric pattern of highest Vp/Vs in the area of slow Geersen et al. (2013) inferred that the clay-rich sediment composition in the slip along the Tokai segment of Japan, with lower Vp/Vs and fluid pressures 2004 Sumatra­ earthquake rupture zone was as important as the thickness in in the deeper segment of the fault where the LFEs and tremor occur. Frank controlling the shallow rupture. Ikari et al. (2015a, 2015b) demonstrated that et al. (2015) suggested variable fluid pressures over fairly short time and spa- samples of the smectite-rich fault material obtained from the shallow drilling tial scales based on acceleration and deceleration of LFE activity in Mexico of the 2011 Tohoku earthquake zone can deform in the laboratory at a variety of adjacent to motion during the 2006 slow-slip event. rates, from fast seismic slip to slow SSE speeds, to produce the range of slip behaviors observed in that zone.

Role of Sediments on Megathrusts Role of Rough Downgoing Plate Topography Another important input into the subduction zone is sediment, as the quan- tity and mineralogical content can affect shallow slip processes. Comparisons Yet another important input into the subduction zone is the overall rough- between locations of great earthquakes and sediment thickness highlight ness of the subducting plate. This can be smoothed with a thick sediment blan- the first-order observation that great earthquakes occur in regions with thick ket, or it can be rough, marked with seamounts, ridges, and other topographic sediment inputs (Ruff, 1989). This observation led to the idea that thick sub- features. As mentioned in the previous section, thick sediment subduction and ducted sediment packages act to smooth the plate-boundary zone, resulting smooth plates have been linked with the occurrence of great earthquakes. in uniform seismic coupling that allows earthquakes to rupture unimpeded The way in which the rough plate affects earthquake rupture appears to be for large distances on the megathrust (e.g., Ruff, 1989). This model has per- more variable. From large seamounts entering the Japan Trench (e.g., Nishi- sisted throughout the last three decades of seismicity and improved sediment zawa et al., 2009), to smaller seamounts leaving scars in the forearc in Costa thickness estimates (e.g., Heuret et al., 2012), with Scholl et al. (2015) finding Rica (e.g., Ranero and von Huene, 2000), and large seamounts and ridges the majority of great earthquakes occur in subduction zone segments with entering the subduction zone along Peru, Chile, and elsewhere (e.g., Spence sediment thickness greater than 1 km (Fig. 19). Seno (2017) observed that a et al., 1999; Robinson et al., 2006; Sparkes et al., 2010; Marcaillou et al., 2016), thickness of subducted sediment greater than 1.3 km is found for regions with these features are significant enough to likely affect megathrust earthquakes,

Mw ≥ 9.0 earthquakes, and such events do not occur if the thickness is less although exactly how is an active debate. Kelleher and McCann (1976) cor- than 1.2 km. related locations of great earthquakes with the relatively smooth bathymetry As with all of these factors, spatial variations in sediment thickness may portions of subduction zones, finding only moderate-magnitude earthquakes be relevant for understanding the details of slip patterns. Various diagenetic occurring in areas with significant subducting bathymetry such as aseismic and metamorphic processes that affect the friction and effective stress in the ridges. They suggested that subduction of these large topographic features shallow megathrust materials play important roles in defining the updip edge leads to increased buoyancy at those locations, which modifies coupling in of seismicity (Moore and Saffer, 2001). Thick and strong sediments in the 2004 those zones, reducing the ability to produce great earthquakes. On the other

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N

Thick (> 1.0 km) Trench Sediment Thin (< 1.0 km) Trench Sediment 5000 km

Figure 19. Locations of magnitude ≥ 7.5 earthquakes (stars) between 1899 and 2013 relative to sediment thickness at the trench, modified from Scholl et al. (2015).

end of the spectrum, Cloos (1992) suggested that many subduction zone thrust tion zone itself. One possibility is that both normal stress and seismic coupling earthquakes are the result of subducted seamounts acting to strongly couple increase because of the extra mass and buoyancy associated with a subducted with the overriding plate, producing asperities and earthquake slip, although seamount (Scholz and Small, 1997). Using analogue sandbox models of sea- the ability to produce large earthquakes would reflect the amount of sediment mount subduction, Dominguez et al. (2000) demonstrated significant defor- acting as seamount cover to seismogenic depths (Cloos and Shreve, 1996). mation in the upper layers of sand as the topographic feature passed at depth. In order to understand how subducted topography might affect earth- Following in the spirit of the sandbox models, Wang and Bilek (2011) presented quakes, we need to understand the fate of the topography within the subduc- a conceptual model of strong geometric features such as seamounts signifi-

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cantly deforming the surrounding upper plate with a complex network of zone correlate with the path of previously subducted seamounts (Ide, 2010). Bell fractures, supported by geologic evidence of highly fractured exhumed upper-­ et al. (2010) used seismic reflection images of the Hikurangi margin to suggest plate zones, seismic imaging, and recent numerical models (Ding and Lin, that subducted seamounts allow for lower effective stress and promote the oc- 2016). These models suggest different seismic behavior in areas of subducting currence of slow slip, while Kodaira et al. (2004) suggested a similar connection topography, where these features could act as either regions of high slip, or between subducted ridges and slow slip in the Nankai subduction zone. regions that arrest slip during a megathrust earthquake. The models are variously supported by a range of documented cases of these features acting either as barriers to rupture or as regions of high slip. Role of Upper-Plate Structure The ideas developed by Kelleher and McCann (1976) of great earthquakes avoiding areas of seamount subduction persist with consideration of up- The upper plate may also impact megathrust slip behavior. Correlations be- dated earthquake catalogs (Wang and Bilek, 2014). Instead of producing large tween forearc basins and the high-slip zones of many megathrust earthquakes earthquakes, several of these regions exhibit high levels of creep (low seismic led Wells et al. (2003) to suggest that subsidence associated with these basins ­coupling), which are a proposed consequence of seamount deformation of might be linked with areas of subduction erosion (requiring high coupling), the upper-plate model (Wang and Bilek, 2014). Yang et al. (2013) used numeri- affecting the megathrust stress state. Similarly, correlations between large cal simulations to show that subducted seamounts can act as rupture barriers negative trench-parallel gravity anomalies and the locations of great earth- over a wide range of seamount sizes and normal stress conditions. In regions quakes may also connect forearc structure with megathrust processes (Song adjacent to subducted topography, several great earthquakes have been ob- and Simons, 2003). Gravity anomalies are also associated with displacement served, leading to many studies suggesting that subducted topography acts in great events like the 2011 Tohoku earthquake, where positive gravity anoma- as a barrier to further rupture. For example, Kodaira et al. (2000) imaged a lies, representing differences in upper-plate geology, may have influenced the subducted seamount at depth in the Nankai subduction zone that they suggest frictional conditions on the plate interface (Bassett et al., 2016). Geology and

acted as a barrier in the 1946 Mw 8.3 Nankai earthquake rupture. Other ex- strength of the margin prism may also impact the amount of slip and seafloor amples of significant subducted topography acting as rupture barriers include deformation, based on numerical models showing high slip and increased along the South American margin (e.g., Perfettini et al., 2010; Sparkes et al., tsunami generation beneath compliant prisms (Lotto et al., 2017). Upper-plate 2010; Geersen et al., 2015), Sumatra margin (e.g., Chlieh et al., 2008; Henstock faulting may also provide limits on megathrust earthquake ruptures, as sug- et al., 2016), and Japan Trench (e.g., Simons et al., 2011; Duan, 2012). In some gested for Ecuador (Collot et al., 2004) and Chile (Moreno et al., 2012). cases, the subducting topography may initially act as a barrier and then fail Upper-plate variations may also affect nonvolcanic tremor occurrence.

later in the rupture, producing significant slip, as proposed for the 2001M w 8.4 Along Cascadia, along-strike variations in tremor location and recurrence Peru earthquake (Robinson et al., 2006). Given the position of the subducted interval are spatially linked with variations in upper-plate geologic terranes topography with depth, these features might also affect the downdip earth- (Brudzinski and Allen, 2007). Wells et al. (2017) showed a correlation between quake rupture behavior as well, as suggested by Marcaillou et al. (2016) as a upper-plate faults and reduced levels of tremor activity along Cascadia, sug- means for impeding updip rupture of the 1942 Ecuador M 7.8 earthquake. gesting that the upper-plate faults allow fluid pressures to drain from the fault Other small- to moderate-magnitude earthquakes have produced slip in at depth, limiting tremor occurrence. subducting areas with rough topography. Using high-quality seismic surveys and monitoring efforts, Mochizuki et al. (2008) located several repeating M ~ 7 earthquakes along a portion of the Japan Trench megathrust landward of a MEGATHRUST EARTHQUAKE HAZARDS subducted seamount, not at the seamount position at depth. Several studies have suggested that subducted seamounts acted as asperities in past Costa Due to their size and frequent occurrence, earthquakes on megathrust Rica M ~ 7 earthquake ruptures (e.g., Protti et al., 1995; Husen et al., 2002; Bilek faults constitute significant seismic and tsunami hazards in subduction zones et al., 2003), M ~ 7 earthquakes along the Hikurangi margin (e.g., Bell et al., around the world. Slab deformation and partitioned block faulting of the upper 2014), as well as the 2010 Maule, Chile, earthquake (Hicks et al., 2012). plate also contribute to hazards in many regions. Subducting topography has also been linked with other seismic processes, such as small-magnitude earthquake swarms, nonvolcanic tremor, and slow- slip events. Earthquake swarms along South American and Japanese mega­ Coseismic Slip and Geodetic Locking thrusts appear to correlate with locations of subducted topography (Holtkamp and Brudzinski, 2011). Tréhu et al. (2012) found evidence for small-magnitude With recent improvements in geodetic coverage by GPS and GNSS sta- earthquake clusters in areas of subducted seamounts along the Cascadia sub- tions on the upper plate of subduction zones, detailed images of hetero­ duction zone. Linear streaks of nonvolcanic tremors in the Nankai subduction geneous patterns of interseismic slip deficit (or “locking”) are now available.

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These illustrate the complexity of fault locking and creep in many subduction Solomon Islands–Woodlark Basin plates beneath the Pacific plate. This region

zones, and they provide a basis for refining the regional hazard. Comparisons was largely aseismic prior to the 2007 Mw 8.1 earthquake, which produced between prior fault locking maps and recent coseismic slip in large events slip along the plate-boundary interface for both the Australian and Solomon have led to a major advance, as relatively good correlations are observed Islands–Woodlark Basin plates, crossing the fracture zone boundary between between regions of high coseismic slip and strong plate locking when both these two plates and marking the first observed rupture across a triple junction are well resolved. For example, geodetic and paleogeodetic observations dat- (Taylor et al., 2008; Furlong et al., 2009). ing back several ­decades suggest heterogeneous patterns of strong coupling The near-trench region of the megathrust was once considered to be an along the Sumatra­ margin, and the strongly coupled zones served as the high- unlikely area to host large coseismic slip because of unfavorable frictional con-

slip regions in the 2005 Mw 8.7 Nias, and 2007 Mw 8.4 and Mw 7.9 Sumatra ditions, yet examples such as the massive coseismic slip in the near-trench earthquakes (Chlieh et al., 2008). Similar observations of strong locking found region of the 2011 Tohoku earthquake (e.g., Lay et al., 2011b; Ide et al., 2011;

in regions of high coseismic slip exist for the 2007 Mw 8.0 Pisco, Peru (Perfettini Ozawa et al., 2012; Iinuma et al., 2012) provide dramatic evidence to the con-

et al., 2010), 2010 Mw 8.8 Maule, Chile (Moreno et al., 2010), 2011 Mw 9 Tohoku trary. Very shallow megathrust slip also occurred in the 2006 Mw 8.3 Kuril earth-

(e.g., Ozawa et al., 2012), 2012 Mw 7.8 Nicoya, Costa Rica (e.g., Protti et al., 2014), quake, an area also lacking in previous interplate seismicity (Ammon et al.,

and the 2016 Mw 7.8 Ecuador (Ye et al., 2016a) earthquakes. 2008; Lay et al., 2009). These correlations between the geodetically determined locked patches and areas of high slip during great earthquakes have implications for seis- mic and tsunami hazard assessments along subduction zones. However, land- Earthquake Triggering and Stress Transfer based geodetic measurements have very poor resolution of near-trench slip deficit, as was found for the 2011 Tohoku earthquake (e.g., Lay, 2017). A major Quantification of the stress transfer and earthquake triggering potential is limitation is the paucity of offshore geodetic observations in most subduction another important consideration for assessing seismic hazard in subduction zones; without offshore observations, resolution of the spatial extent of locking zones. Earthquake triggering by transfer of stress, either dynamic or static, is and strain accumulation is very limited. An example where offshore obser- recognized in fault zones around the world (e.g., Hill et al., 1993; King et al., vations now exist in reasonable density for this to be overcome is along the 1994; Lin and Stein, 2004; Freed, 2005). Many subduction zone examples exist, Nankai region of southwest Japan. Yokota et al. (2016) presented coupling esti- such as along the Sumatra margin, with its history of great earthquakes in mates based on offshore measurements, suggesting wide areas of the shallow the eighteenth and nineteenth centuries. Modern great earthquake activity in

Nankai subduction zone have high slip deficit and are strongly locked, which this region began with the 2004 Mw 9.2 Sumatra underthrusting event. Follow- expands the regions where shallow, tsunamigenic slip is possible. Seafloor ing the 2004 event, other great megathrust earthquake ruptures progressed

measurements by GPS and acoustic methods (Sato et al., 2011; Kido et al., southward along the margin in the 2005 Mw 8.6 Nias (e.g., Konca et al., 2007)

2011), along with ocean bottom pressure sensors, provided invaluable data and the 2007 Mw 8.5 and Mw 7.9 Sumatra earthquakes (e.g., Konca et al., 2008). for resolving the shallow slip in the 2011 Tohoku earthquake. The greatly ex- This sequence brackets a locked region along the Mentawai Islands offshore panded cabled S-net offshore Honshu now provides continuous monitoring of of Padang, which last ruptured in 1797, and where a future great megathrust moderate-slip earthquakes, and data assimilation methods will enable early earthquake could occur. tsunami warning of unprecedented quality. The 2006–2007 Kuril Islands sequence is another example of coupled great earthquakes, although with diverse focal mechanisms. The first event

in the sequence, the 2006 Mw 8.4 event, occurred on the megathrust, whereas

Coseismic Slip in Aseismic Regions the second Mw 8.1 event in 2007 was a normal-faulting, outer-rise event that produced stronger shaking in Japan due to higher moment-scaled radiated The previous section outlined advances made in correlating areas of strong energy (e.g., Ammon et al., 2008). Slip from the 2006 earthquake, as well as geodetic locking with the areas of high coseismic slip in great earthquakes; from a large 1963 compressional earthquake in the trench slope region, led to however, it is also valuable to evaluate low coupling zones. Areas thought to increased static stress on optimally oriented normal faults in the region of the involve largely aseismic creep, in some cases linked to areas of rough sub- 2007 event (Raeesi and Atakan, 2009). ducting topography (e.g., Wang and Bilek, 2014), can only be confirmed as The 2009 Samoa-Tonga earthquake doublet also involved interaction deforming this way by geodetic measurements. We cannot ignore the pos- between two separate faults, but with a pattern reversed in time relative to

sibility than any subduction megathrust, including those with highly variable the 2006–2007 Kuril Islands sequence. In the Samoa-Tonga case, a Mw 8.1 structure, can host large earthquakes, even if historical seismicity does not normal-faulting,­ outer-rise event triggered rupture of the Tonga megathrust

reveal the potential. One such example is along the Solomon Islands subduc- with cumulative­ Mw of 8.0 within minutes, compounding tsunami runup and tion zone, where a triple junction marks the subduction of the Australia and fatalities (Lay et al., 2010b). This style of triggering is particularly troublesome,

GEOSPHERE | Volume 14 | Number 4 Bilek and Lay | Subduction zone megathrust earthquakes 1492 Research Paper

because the secondary underthrusting events in the sequence occurred within even behavior of some regions with long repose between events. Advances tens of seconds, hampering the ability to quickly assess the tsunami potential. in seismology and geodesy now provide information about the seismic cycle The hazard posed by tsunami earthquakes is now well recognized, but it for some regions, and over time improved assessments of seismic potential remains difficult to characterize in most regions. As mentioned herein, it is appear to be viable. Offshore geodetic monitoring will provide critical infor- very challenging to establish whether the shallow portion of a megathrust is mation about the along-dip and near-trench strain accumulation process and frictionally locked and accumulating strain using on-land geodetic data. This is the occurrence of shallow slow-slip events and stable sliding that may prevent particularly true for wide megathrusts. It is also difficult to establish the updip strain buildup. Coupled offshore and onshore seismic and geodetic monitor- limit of rupture in large megathrust ruptures to judge whether there is potential ing combined with real-time processing have the potential to establish early for tsunami earthquakes to occur subsequently, as occurred in the 2010 Men- warning systems for megathrust tsunami and strong shaking, but this will re- tawai earthquake updip from the 2007 Sumatra earthquakes. The tendency for quire extensive global investment in instrumentation and operations facilities very shallow ruptures to be depleted in short-period seismic wave radiation around the world. can cause delayed tsunami hazard reaction, even when shaking alerts coastal communities to an earthquake occurrence. The best strategy for mitigating ACKNOWLEDGMENTS this hazard appears to be offshore continuous seafloor deformation moni- T. Lay’s research on earthquakes is supported by National Science Foundation (NSF) grant EAR- toring (particularly cabled seafloor pressure sensors) and regional real-time 1245717. S. Bilek’s research on earthquakes is supported by NSF grant OCE-1434550. Reviews by source inversion procedures connected to an effective tsunami warning and Guest Editor L. Wallace and two anonymous reviewers helped us to improve the manuscript. evacuation system. Strong shaking from offshore events constitutes another major hazard, and REFERENCES CITED even less response time is available than for tsunami response. 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