CORRELATION OF SHAKING INTENSITY AND DAMAGE PATTERNS WITH SITE CONDITIONS IN THE WASHINGTON, DC REGION DURING THE 2011 MINERAL, VIRGINIA EARTHQUAKE

The M5.8 Mineral Earthquake of 2011 was located about 50 km east of Charlottesville, 60 km northwest of Richmond, and 130 km southwest of Washington, DC. Minor damage was widespread, especially in unreinforced masonry (URM) structures. In the epicentral region, tens of URMs were severely damaged and several were partially collapsed. Maximum intensities VII to VIII were reported. To the east in Richmond, and to the north in Charlottesville, only minor damages occurred, with maximum intensities of V to VI. An uptick in intensity and damage was observed to the northeast in the Washington Metropolitan Region (WMR), where intensities of VII to VIII were reported despite the source distance of more than 130 km. On average, WMR damage to vulnerable structures, such as URMs, was as severe as that near the epicenter.

Reasons for the WMR amplification are not known, but we suspect a combination of seismological effects, regional geological and local site conditions, and structural deficiencies. There was a clear correlation between damage intensity and regional and localized site conditions. Outside the epicentral region, the vast majority of damage occurred along a narrow northeast trending zone that mimicked the Fall Line. Notable damage sites were also located east of the Fall Line within the deep Coastal Plain. Significant damage in the Piedmont was rare, and mainly confined to sites underlain by isolated pockets of Coastal Plain sediments.

The damage pattern was corroborated by site response analyses of typical WMR Fall Line and Coastal Plain sites. The analyses predict strong amplifications due to abrupt velocity contrasts between hard near-surface rock and overlying sediments along the Fall Line and within the Coastal Plain. This helps to explain damages to stiff URMs and similar structures prevalent in the WMR, especially along the National Mall.

This study presents a summary of damage observations and trends related to geologic and site conditions. We present the results of preliminary site response analyses that help explain our field findings, especially those related to prominent WMR sites. Implications for seismic hazard assessment and building code provisions are discussed, especially the need for region-specific modifications that better capture the effects of unique site conditions across the region.