Virginia's M5.8 Wake-Up Call: Where Elase Could It Strike?

07-Sep-11

The magnitude 5.8 earthquake in Virginia of August 23 is a present reminder of the seismic potential in the eastern US, and it offers an opportunity to consider the likelihood and possible consequences of an analogous earthquake in a more populous eastern US location. Although such an event is rare, its risks are quantifiable in the EQECAT model.

To recap, the earthquake occurred in a sparsely-populated area of Virginia but was felt from Atlanta to Toronto (Figure 1), since seismic waves are slow to attenuate in the geology of the eastern US. EQECAT's estimate of insured losses less than $100 million USD remains unchanged.

Even locations far from tectonic plate boundaries are not exempt from potentially-damaging tremblors. The rupture mechanism of the Virginia earthquake was thrust faulting along a north-northeast oriented fault plane. This mechanism is consistent with many other plate interior earthquakes, since the interiors of tectonic plates are typically characterized by compressive tectonic stress. The plates themselves act as stress guides, propagating stress from the boundaries long distances into the interior. A variety of causes have been hypothesized to explain how these stresses concentrate locally and cause earthquakes, including: density contrasts with the continental crust; local mass imbalances due to long-term erosion and deposition; and locked-in residual stresses from prior tectonic regimes, among others. No single one of these possible mechanisms applies to all plate-interior earthquakes, since they are more likely operate on a local scale.

Central Virginia is one of several seismic zones mapped by the USGS for the central and eastern US (Figure 2). For example, in the New York City metro area, where a M5.5 quake occurred in 1884, the USGS estimates a higher seismic hazard than in the Central Virginia Seismic Zone. Furthermore, a greater number of historic earthquakes have occurred in New York than in Virginia (Figure 3). A selection of these historic earthquakes follows in Table 1.

Table 1: Selection of Historical Earthquakes Affecting the Northeast US

Year	Location	Intensity/Magnitude	Comments
2011	Central Virginia	M5.8	perceived widely throughout the East
2010	Near Ottawa, Canada	M5.0	perceived widely throughout the Northeast
2002	upstate New York	M5.1
1988	Quebec, near Montreal	M5.9
1984	eastern Pennsylvania	M4.4
1983	upstate New York	M5.3
1962	Vermont	M4.2
1944	upstate New York	M5.1
1940	New Hampshire	M5.5	two earthquakes
1925	near Montreal	M6.2	Intensity VII at epicenter, perceived as far away as New York City
1884	New York City	M5.5
1871	New Jersey / Delaware border	VI and VII	Two earthquakes, approximately M5.5 to 6
1783	New Jersey	M5.3
1727, 1744, and 1755	Cape Ann, Massachusetts	VI to VII	approximately M5 to M6.5
1568	Connectivut	VI	approximately M5 to M5.5

For the large area over which the August 23 tremblor was felt, such an earthquake has a return period on the order of 50 years. However, when calculated only over the area where damage occurred, its return period is closer to 500 years. For a M5.5 in New York City, which is similarly a 500-year event for the local metro area, EQECAT's USQuake model estimates insured losses in the range of $10 to $20 billion, which exceed those from the 1994 Northridge earthquake.

Observed damage in a hypothetical 500-year New York City scenario might include damage to masonry buildings, particularly older, lower-rise buildings (Figure 4), and damage in areas of soft or liquefiable soil. Although major damage would most likely be localized to the most vulnerable building and soil types, effects of damage may be widespread. Significant business interruption may arise from temporary closures of bridges and other public infrastructure or significant damage to building contents.

Damage to an older, low-rise unreinforced masonry building in the 1989 Loma Prieta earthquake might be characteristic of damage in a hypothetical M5.5 in New York City - September 7, 2011

Uncertainty in earthquake risk for the eastern US is particularly high, due firstly to the scarcity of historic events. For example, before the M5.8 earthquake, the Central Virginia Seismic Zone had an historical maximum earthquake of only 4.8. To be comprehensive, then, loss calculations must account for unprecedented possibilities. EQECAT's USQuake Model does just that: among the full suite of hypothetical earthquakes used to estimate possible losses, the upper-bound event exceeds magnitude 7 in both New York City and central Virginia, with appropriately low frequencies for each area.

A second source of significant uncertainty arises from the widespread shaking footprint caused by an earthquake in the eastern US. In comparison to earthquakes in California, an earthquake in the East may affect an area 10 times as large for a given magnitude. Given the density of exposure in the eastern US, and the nonlinearity of loss calculations, small variations in the estimation of shaking intensity can produce large variability in loss estimations.

Returning to the example of a hypothetical M5.5 earthquake in NYC metro, an expected maximum value of ground shaking (acceleration) might be just under the threshold of significant damage to most buildings. But the EQECAT sampling methodology also accounts for the outlier possibility that the same earthquake may cause accelerations only nominally higher, but exceeding the damage threshold, which would result in losses many multiples of the original estimate. By incorporating such disproportionate consequences of slight variations in model parameters, EQECAT models are especially relevant for the rare and uncertain eastern US earthquakes.

Although a hypothetical loss exceeding $10 billion USD would be unprecedented for seismic activity in the eastern US, EQECAT's unique treatment of uncertainty makes our model particularly suited to quantifying risk from very rare events.

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