Tenth U.S. National Conference on Earthquake Engineering Frontiers of Earthquake Engineering July 21-25, 2014 10NCEE Anchorage, Alaska
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.
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 Virginia Seismic Zone (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 Appalachian Mountains [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 fault 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., Baltimore, and Philadelphia 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: