Tenth U.S. National Conference on Earthquake Engineering Frontiers of Earthquake Engineering July 21-25, 2014 10NCEE Anchorage, Alaska

THE 2011 M 5.8 EARTHQUAKE: GROUND MOTION, DAMAGE, AND LESSONS LEARNED

Yufang Rong1

ABSTRACT

The 23 August 2011 M 5.8 Virginia earthquake, felt across much of the east coast from Toronto to North Carolina, is the largest earthquake striking Virginia in more than a century. Many buildings in Virginia, Washington D.C., , , and experienced slight to moderate damage. The ground motion was recorded by 15 stations in Virginia, Pennsylvania, Massachusetts, , Vermont, South Carolina, and New Hampshire. However, instrumental recordings were too sparse to relate the characteristics of ground motion to building damage. We, therefore, converted abundant felt reports from the USGS “Did You Feel It?” (DYFI) to PGA using an empirical relationship. A comparison between the instrumentally recorded vs. DYFI-converted PGAs shows that the DYFI-converted ground motions agree with the instrumental records for relatively strong motion (MMI ≥ V). When the ground motion is weak (MMI < V), the DYFI-converted PGAs were much higher than observed values, suggesting a need to refine the empirical relationship for this range of ground motion. Since the DYFI intensities at most of the damaged locations were ≥ MMI V, we related the DYFI-converted PGAs at the locations to the damage of reinforced concrete, precast concrete, masonry, and reinforced masonry buildings. A review of soil properties at the damaged locations re-emphasizes that soft soil, especially artificial fill, significantly amplifies ground motions and exacerbates damage. Observed damage ratios demonstrated the need to develop damage functions based on engineering data, insurance claims and detailed damage information. This event serves as a reminder that significant earthquakes do occur in the eastern U.S. and can affect populated areas. Due to the lack of seismic design in the eastern U.S., considerable seismic risk exists. Therefore, steps need to be taken to manage risks and reduce losses.

1Senior Research Scientist, Center for Property Risk Solutions, FM Global, Norwood, MA 02062, USA

Rong Y. The 2011 Virginia M 5.8 earthquake: ground motion, damage, and lessons learned. Proceedings of the 10th National Conference in Earthquake Engineering, Earthquake Engineering Research Institute, Anchorage, AK, 2014.

THE 2011 VIRGINIA M 5.8 EARTHQUAKE: GROUND MOTION, DAMAGE, AND LESSONS LEARNED

Yufang Rong1

ABSTRACT

The 23 August 2011 M 5.8 Virginia earthquake, felt across much of the east coast from Toronto to North Carolina, is the largest earthquake striking Virginia in more than a century. Many buildings in Virginia, Washington D.C., Maryland, Pennsylvania, and New Jersey experienced slight to moderate damage. The ground motion was recorded by 15 stations in Virginia, Pennsylvania, Massachusetts, New York, Vermont, South Carolina, and New Hampshire. However, instrumental recordings were too sparse to relate the characteristics of ground motion to building damage. We, therefore, converted abundant felt reports from the USGS “Did You Feel It?” (DYFI) to PGA using an empirical relationship. A comparison between the instrumentally recorded vs. DYFI- converted PGAs shows that the DYFI-converted ground motions agree with the instrumental records for relatively strong motion (MMI ≥ V). When the ground motion is weak (MMI < V), the DYFI-converted PGAs were much higher than observed values, suggesting a need to refine the empirical relationship for this range of ground motion. Since the DYFI intensities at most of the damaged locations were ≥ MMI V, we related the DYFI-converted PGAs at the locations to the damage of reinforced concrete, precast concrete, masonry, and reinforced masonry buildings. A review of soil properties at the damaged locations re-emphasizes that soft soil, especially artificial fill, significantly amplifies ground motions and exacerbates damage. Observed damage ratios demonstrated the need to develop damage functions based on engineering data, insurance claims and detailed damage information. This event serves as a reminder that significant earthquakes do occur in the eastern U.S. and can affect populated areas. Due to the lack of seismic design in the eastern U.S., considerable seismic risk exists. Therefore, steps need to be taken to manage risks and reduce losses.

Introduction

The M 5.8 earthquake struck Virginia at 17:51 UTC (1:51 p.m. local time) on August 23, 2011. The USGS located the event at 37.936°N, 77.933°W, 60 km northwest of the state capital, Richmond, and 135 km southwest of Washington, D.C. (Fig. 1). It had a focal depth of 6 km. The earthquake is one of the largest earthquakes in Virginia in historical times [1]. It occurred in the previously recognized Central (CVSZ) (Fig. 1). Ground shaking was felt along the east coast, as far as Toronto to the north and North Carolina to the south. Small CVSZ earthquakes have been persistently felt by people since at least 1774. The previous largest historical earthquake in the zone happened in 1875 with an estimated magnitude of 4.8. The 1875 earthquake caused bricks to fall from chimneys, broke plaster and windows, and overturned furniture. The most recent felt earthquake before the 2011 M 5.8 event occurred in December 2003. Located about 20 km southwest of the M 5.8 shock, it had a magnitude of 4.5 and caused minor damage. The higher seismicity rate of CVSZ relative to other mid-Atlantic locations is

1 Senior Research Scientist, Center for Property Risk Solutions, FM Global, Norwood, MA 02062, USA reflected in the national seismic hazard maps [2].

Figure 1. Epicenters of the 8/23/2011 Virginia M 5.8 earthquake (red star) and historical earthquakes from 1534 to 2006 [2] (green circles). Central Virginia Seismic Zone is displayed in brown. Thin dark red lines represent the mapped geological faults [3].

The 2011 earthquake occurred as reverse faulting on a north- or northeast-striking plane at a shallow depth. The regional geologic structure and faults strike southwest-northeast, parallel to the [3] (Fig. 1). The faults are old and most of them are considered inactive. Because no surface rupture from the 2011 earthquake has been discovered and the area is not covered by detailed geological maps, it is difficult to designate a causative for this earthquake. Stress drop exerts a strong control on ground motions. High stress drop leads to larger intermediate-to-high frequency ground motions, which are important to mid-rise and low- rise structures. Some preliminary studies show that this event had a stress drop as high as ~300 bar [4, 5], whereas the stress drop of a typical earthquake in western North America is only ~100 bar. With such high stress drop and low attenuation through the old and stable North American craton, the earthquake was widely felt along the east coast. Moderately heavy damage (MMI VIII) occurred in a rural region of Louisa County in central Virginia. Light to moderate damage was observed from central Virginia to southern Maryland including the Washington D.C. area. Minor damage was found in Delaware, Pennsylvania, and New Jersey. Due to the sparse distribution of seismic stations in the eastern U.S., the instrumentally recorded ground motion data from this earthquake are scarce. However, the “Did You Feel It?” (DYFI) program [6] developed by the U.S. Geological Survey (USGS) provided abundant intensity data based on felt reports, which allow us to relate the building damage with ground motions to examine building performance and summarize lessons learned from this earthquake.

Ground Motions

“Did You Feel It?” The DYFI program developed by USGS collects people’s experiences and observations for earthquakes they have felt by answering a questionnaire [6]. The Modified Mercalli Intensity (MMI) at each location is determined by applying a simple algorithm to convert the responses to MMI. Atkinson and Wald [7] converted recorded peak ground velocity (PGV) data of some historical earthquakes into MMI using a linear regression relationship. After overlaying the PGV-converted MMI on the DYFI-converted MMI, they concluded that the DYFI intensity data are a good measure of earthquake ground motion. For the 2011 Virginia earthquake, DYFI MMI data have been determined for about 8600 locations (one zip code is taken as one location) [8] (Fig. 2). The intensity at the epicentral region is as high as VII-VIII. Some locations in Washington D.C., , and experienced intensity V-VI.

Instrumentally Recorded Ground Motions

The instrumentally recorded ground motions are valuable for studying earthquake source characteristics, verifying existing ground motion prediction equations (GMPEs), and developing new GMPEs. Before the Virginia earthquake, there were only a few instrumental records for moderate size earthquakes in the eastern U.S. and none for large events. The Virginia earthquake is the largest instrumentally recorded earthquake in the eastern U.S., and the ground motion data collected from the event will contribute to the ground motion database of eastern North America.

The recorded ground motions at 14 stations can be found on the website of Center for Engineering Strong Motion Data (http://www.strongmotioncenter.org/). The locations of the stations and the recorded PGA (g) and SA (g) at 0.3 s, 1.0 s are illustrated in Fig. 2. The PGA is larger than 0.1 g at three stations closest to the epicenter. The SA becomes very small at periods ≥1.0 s because the earthquake magnitude is not very large. Note that the 0.3-s SA at the VA Reston Fire Station is 0.225 g, much higher than the values of the two recordings closer to the epicenter. The VA Reston Fire Station is about 120 km northeast from the epicenter, and 20 km west of Washington D.C. Many damaged properties are near this area. The reasons for such high ground motion are not yet clear due to limited information at this time, although soil amplification is suspected. Ground motions recorded at the North Anna Nuclear Power Plant (NAPP), 18 km from the epicenter (Fig. 2), have a PGA of 0.26 g [9]. The record at this station is the closest recording of the M 5.8 main shock.

Figure 2. Instrumentally recorded and USGS DYFI ground motions [8]. The black star represents the epicenter. Stations that recorded the ground motions of the Virginia earthquake are indicated by black triangles. The three numbers at each station are, respectively, the PGA (g) and SA (g) at 0.3 s and 1.0 s. The DYFI intensity at each zip code is shown by colors. The location of the North Anna Power Plant (NAPP) is indicated by one of the black triangles in the insert map. The hatched box represents the main damage area where most of the damaged properties in this study are located.

Convert DYFI Intensity to PGA

Unfortunately, the recorded ground motion data are not adequate to correlate with the damage at locations from where claims have been received, because only a few stations are within the main damage area (Fig. 2), and none of the damaged locations are near those stations. However, the DYFI database offers the potential to relate the ground motion and damage caused by this earthquake.

Wald et al. [10] developed a regression relationship between MMI and PGA (% g) using observations from eight significant earthquakes in California:

= 3.66 log 1.66, for . (1)

For MMI < V, there were not sufficient data to constrain the relationship. As a result, Wald et al. imposed the relationship [10]:

= 2.20 log +1.00, for MMI < V. (2)

The imposed relationship needs to be refined as more data become available [10].

We converted DYFI intensities at the stations having instrumentally recorded ground motions for the 2011 Virginia earthquake using the above relationships. Figure 3 compares the instrumentally recorded PGA vs. DYFI-converted PGA. It shows that the DYFI-converted ground motions agree with the instrumental records for the relatively strong motion. When the ground motion is weak (MMI

1

0.1

0.01 PGA (g) DYFI PGA from

0.001 0.001 0.01 0.1 1 Recorded PGA (g)

Figure 3. Comparison of DYFI-converted and instrumentally recorded PGA values at the stations shown in Fig. 2. The red symbols are the stations within the main damage area (see Fig. 2). The blue symbols represent the stations outside the main damage area.

Damage

Damage to Properties

The damaged properties in this study are distributed in Virginia, Washington D.C., Maryland, Pennsylvania, and New Jersey. Light to moderate structural damage such as separation of concrete masonry unit (CMU) walls and chimneys was observed from central Virginia to southern Maryland, including the Washington D.C. area. Minor damage, such as wall cracks and falling ceiling tiles, was found in all of the five states. About 40% of the damaged properties are office buildings. Other occupancies include hospitals, manufacturing, government and university, and apartments. Details can be further described as follows: • About 45% of the damaged properties are in central and . All of them were built after 1990. Most of the damage was cracks in walls and façades with a few cases of automatic sprinkler leakage. • About 30% of the damaged properties are in Washington D.C., and all of them were built before 1990. Damage includes separation of CMU walls, cracks in brick chimneys and façades, cracks in CMU walls, cracked ceilings and windows, and fallen bricks and plaster. • Maryland has about 20% of the damaged properties. Similar to Washington D.C., most of the damaged buildings were built before 1990. Damage includes sprinkler leakage, brick chimney separation, concrete shear wall separation, cracks in CMU walls and stairwells, and tiles falling from ceiling. • New Jersey and Pennsylvania have about 5% of the damaged properties. The damage is mainly cracks in walls.

Site Conditions

Local site conditions can increase the severity of damage from an earthquake. In general, structural damage from earthquakes is greater in areas underlain by soft soils, alluvium, or water- saturated unconsolidated soils, and less in areas underlain by bedrock.

We estimated the site conditions at the damaged locations. The site classes are estimated from two methodologies: 1) calculated from topographic slope [11, 12]; and 2) output from the AIR CLASIC/2 software [13]. Using the topographic slope method, 54% of the damaged locations sit on NEHRP C (very dense soil and soft rock) site class, and 46% of locations on NEHRP D (stiff soil) site class. Using the AIR model, 42% of the locations are on NEHRP D and 35% on NEHRP C. Neither the AIR model nor the topographic slope method included artificial fill. However, there is plenty of landfill along the banks of the Potomac River in the Washington D.C. and Virginia area, and significant fill of up to 30 feet was placed for the construction of the western half of the National Mall [14]. In Fig. 4, we have circled the locations that are known to be on artificial fill in the Washington D.C. area [14], and all of them have a relatively high damage ratio.

Damage Ratio

Damage ratio is defined as estimated repair cost normalized by the replacement value of the building. Figure 4 summarizes observed building damage ratios vs. observed ground motions (converted from DYFI intensity data) for different construction types. Although the data are scattered, a consistent trend can be identified for reinforced masonry (RM) structures, and trends can also be identified for URM and precast concrete types. The scatter of the data demonstrates that buildings perform significantly differently even if their construction types are the same. Other factors, such as age of structure, height of building (natural frequencies of structures), and other details and quality of construction contribute to the scatter, and need to be taken into account when evaluating damageability of structures.

10.%

1.%

0.1% Unknown URM Damage Ratio Damage RM 0.01% RC Precast Concrete On Artifical Fill 0.001% 0 0.05 0.1 0.15 0.2 0.25 0.3 PGA (g)

Figure 4. Observed building damage ratio vs. observed PGA (DYFI-converted) for different construction types. The sites known sitting on artificial fill in the Washington D.C. area are circled in red.

HAZUS provides a set of PGA-based structural fragility curves for a reference spectrum [15]. The reference spectrum represents ground shaking of a M 7.0 western U.S. earthquake for site class D at site-to-source distances of 15 km or greater. We adjusted the median PGA values in the fragility curves to approximate the spectrum shape of the 2011 Virginia earthquake using a formula provided by HAZUS. We compared the observed damage with the PGA-based fragility curves for URM, RM, RC and precast concrete structures (Fig. 5). The plots demonstrate that URM, RC, and precast concrete buildings are more damageable than HAZUS suggests, and the damageability of RM buildings is fairly consistent with HAZUS.

HAZUS needs more data to calibrate damage curves, but here the uncertainty in the observed data is large because most of the damage is slight and the estimated repair costs are highly approximate. In addition, three factors are not considered in the plots: 1) many buildings were not damaged during the earthquake although they experienced the same level of ground motion as the buildings in the plots; 2) the observed damage includes both structural and non- structural portions, whereas HAZUS curves are for structural damage only; and 3) there may be issues with the equation used to adjust median PGA values in the fragility curves to approximate the spectrum shape of the 2011 Virginia earthquake. If these factors are considered, the observations and the HAZUS curves may become more consistent. However, at this time we do not have the data of those non-damaged buildings, nor can we separate structural and non- structural losses for the properties based on the available data. URM Structures RM Structures 10.% 10.%

1.% 1.%

0.1% 0.1%

0.01% 0.01% Observed Damage Ratio Damage Observed 0.001% 0.001% Observed Damage Ratio Damage Observed 0 0.1 0.2 0.3 0.4 0.5 0 0.1 0.2 0.3 0.4 0.5 Observed PGA (g) Observed PGA (g) (a) (b)

RC Structures Precast Concrete Structures 10.% 10.%

1.% 1.%

0.1% 0.1%

0.01% 0.01% Observed Damage Ratio Damage Observed Observed Damage Ratio Damage Observed 0.001% 0.001% 0 0.1 0.2 0.3 0.4 0.5 0 0.1 0.2 0.3 0.4 0.5 Observed PGA (g) Observed PGA (g) (c) (d)

Figure 5. Comparisons of observed damage ratios with HAZUS structural fragility curves for (a) URM, (b) RM, (c) RC, and (d) precast concrete structures. In each plot, the blue and red boxes represent HAZUS slight (assumed 0-2% loss) and moderate (assumed 2-10% loss) damages, respectively. The dashed blue and red lines represent the median PGAs of HAZUS slight and moderate damages. The HAZUS plot is based on low-code seismic design level. The width of the boxes represents median PGA±1. Discussion and Conclusions

The North Anna Nuclear Power Plant (NAPP) is located about 18 km away from the epicenter (Fig. 2). The recorded PGA at the plant was 0.26 g, which exceeded the Safe Shutdown Earthquake (SSE) ground motions of 0.12 g and 0.18 g for rock and soil sites [9]. The plant went on automatic shutdown immediately after the quake. Because the recorded PGA was about twice the SSE ground motion, a seismic safety issue was raised for the plant. Prior to the earthquake, the U.S. Nuclear Regulatory Commission had suggested that the probability of earthquake ground shaking exceeding the seismic design basis for some nuclear power plants in the eastern U.S. is greater than previous estimates [9]. This may apply to other structures in the eastern U.S.

Damage was observed for many reinforced concrete buildings as far as Washington D.C. and Maryland. Figure 2 shows a high MMI patch (red-orange-yellow colored area) from New Jersey to Virginia elongated along a NE-SW direction, which is consistent with the structural grain in the region (Fig. 1). Hough demonstrated that the intensities were amplified by one to two MMI units for some areas in Washington D.C. [1], which proves the importance of accounting for site amplifications. This earthquake underscores the fact that soft soil, especially artificial fill, does amplify ground motions and exacerbates damage.

A lesson learned from the event is that significant earthquakes do occur in the eastern U.S. and can affect populated areas. Earthquakes in the eastern U.S. are less frequent than in the western U.S., however, they are typically felt over an area as much as ten times larger than similar magnitude earthquakes in the west because the colder and more rigid crust in the eastern U.S. allows seismic waves to travel with less attenuation. Moreover, seismic building codes did not come into wide use in the eastern U.S. until the mid 1990s, whereas in the western U.S., seismic codes substantially improved construction as early as the 1930s. The densely populated east coast has weak structures of high monetary value, especially in the metropolitan areas of New York, Washington D.C., , Philadelphia, and Baltimore. Due to the lack of seismic design, considerable seismic risk exists.

The observed damage is inconsistent with the fragility curves provided by HAZUS. This result demonstrates the need to develop damage functions taking advantage of engineering knowledge and detailed loss data.

Acknowledgments

The author is grateful for the constructive comments and help from FM Global colleagues: Hosam Ali, Chris Deneff, Harold Magistrale, Sachin Mujawar, Jon Ragland, and William Vandall.

References

1. Hough SE. Initial assessment of the intensity distribution of the 2011 Mw 5.8 Mineral, Virginia earthquake. Seismological Research Letters 2012; 83(4): 649-657, doi:10.1785/ 0220110140. 2. Petersen MD, Frankel AD, Harmsen SC, Mueller CS, Haller KM, Wheeler RL, Wesson RL, Zeng Y, Boyd OS, Perkins DM, Luco N, Field EH, Wills CJ, Rukstales KS. Documentation for the 2008 Update of the United States National Seismic Hazard Maps. U.S. Geological Survey Open-File Report 2008–1128, 61 pp, 2008. 3. Reed JC Jr, Bush CA. About the geologic map in the National Atlas of the United States of America. U.S. Geological Survey Circular 1300, 52 pp, 2007. 4. Cramer CH, Kutliroff JR, Dangkua DT. The 2011 Mineral, VA M 5.8 Earthquake ground motions and stress drop: An important contribution to the NGA East ground motion database. Seismological Research Letters 2012; 83: 212 (Abstract). 5. Ellsworth WL, Imanishi K, Luetgert JH, Pratt TL. The Mw 5.8 Virginia Earthquake of August 23, 2011: A high stress drop event in a critically stressed crust. Seismological Research Letters 2012; 83: 212 (Abstract). 6. Wald DJ, Quitoriano V, Dengler L, Dewey J. Utilization of the Internet for rapid community intensity maps. Seismological Research Letters 1999; 70: 680-697. 7. Atkinson GM, Wald DJ. "Did You Feel It?" intensity data: A surprisingly good measure of earthquake ground motion. Seismological Research Letters 2007; 78: 362-368. 8. USGS Did You Feel It ? (DYFI), http://earthquake.usgs.gov/earthquakes/dyfi/events/se/082311a/us/index.html. 9. United States Nuclear Regulatory Commission (NRC). Letter from Chairman Gregory B. Jaczko to Congressman Edward J. Markey regarding seismic safety at the North Anna Power Station, http://www.nrc.gov/reading-rm/doc-collections/congress-docs/correspondence/2011/markey-11-08-2011.pdf. 10. Wald DJ, Quitoriano V, Heaton T, Kanamori H. Relationships between , peak ground velocity, and Modified Mercalli Intensity in California. Earthquake Spectra 1999; 15: 557-564. 11. Wald DJ, Allen TI. Topographic slope as a proxy for seismic site conditions and amplification. Bulletin of the Seismological Society of America 2007; 97: 1379-1395. 12. Magistrale H, Rong Y, Silva W, Thompson EM, A Site Response Map of the Continental U.S. In the Proceedings of the 15th World Conference on Earthquake Engineering (15WCEE), Lisbon, Portugal, September 24-28, 2012. 13. AIR Worldwide. AIR Earthquake Model for the United States, CLASIC/2 Version 13.0. AIR Worldwide Corporation, 2010. 14. Davis AM, Southworth CS, Reddy JE, Schindler JS. Geologic map database of the Washington DC area featuring data from three 30 x 60 minute quadrangles: Frederick, Washington West, and Fredericksburg. U.S. Geological Survey Open-File Report 01-227, 2001. 15. FEMA. HAZUS®MH MR4 Earthquake Model Technical Manual, http://www.fema.gov/plan/prevent/hazus, 2003.