Displacement Patterns of Cemetery Monuments in Ferndale, CA, During the MW 6.5 Offshore Northern California Earthquake of January 10, 2010

Displaced and toppled monuments in a cemetery are an effective means of assessing local ground motion during an earthquake. The MW 6.5 Offshore Northern California earthquake of January 10, 2010, was felt throughout northwestern California and caused moderate damage in coastal communities between Petrolia and Eureka. The earthquake was generated by left-lateral strike slip on a NE-trending fault within the subducting Gorda plate. Peak horizontal ground accelerations of -0.440g (E) and 0.279g (N) and vertical ground acceleration of -0.122g (up) were recorded in Ferndale, CA, on the North American plate 37km east southeast of the epicenter. We measured displaced and toppled monuments in the Ferndale cemetery as a means of assessing ground motion during the January 10, 2010 Offshore Northern California earthquake. The cemetery occupies a hillside that slopes gently to the northwest, and a dormant landslide underlies the cemetery. Approximately 30% of the monuments were displaced during the earthquake. Affects included toppled columns and urns; headstones, columns and large tomb covers that slid and rotated and relative to monument bases; tilted retaining walls and headstones; and liquefaction-related settling (or, less commonly, uplift) of monuments. We measured translation and rotation of 79 monuments displaced from their bases during the earthquake. Toppled monuments do not display a preferred orientation. Seven of the 18 toppled monuments fell to the southeast, but toppling occurred in all directions. For monuments that were displaced but not toppled, 1-10 cm of northwestward translation and 3-8° of clockwise rotation were most common; however, virtually all directions of translation and both clockwise and counterclockwise rotations and were recorded. Damage was not evenly distributed geographically. In general, damage was concentrated in the northern, topographically lower, part of the cemetery. Counterclockwise rotation of monuments occurred mainly along the northeastern margin of the cemetery. Twelve of the measured monuments consist of three of more segments that rotated in different directions and/or slid different distances than adjacent segments. Our measurements therefore record the cumulative displacement of each monument segment, but many values are undoubtedly less than the maximum offset experienced during earthquake shaking. We infer that a combination of direct ground shaking from seismic waves, local liquefaction, and possibly earthquake-induced remobilization of the dormant landslide underlying the cemetery, all influenced displacement of monuments during the earthquake. One outcome of our data analysis will be insight on the patterns of movement that earthquakes produce in dormant landslide topography.