COASTAL UPLIFT ASSOCIATED WITH THE 1992 CAPE MENDOCINO EARTHQUAKE, NORTHERN CALIFORNIA

The April 25, 1992, Cape Mendocino earthquake (Ms 7.1) uplifted approx imately 24 km of the northern California coast at the southern end of the Cascadia subduction zone, uplift which resulted in coastal emergen ce that caused extensive mortality of intertidal organisms between Cap e Mendocino and Punta Gorda. We estimated the amount of uplift by meas uring the vertical extent of mortality of 14 sessile intertidal specie s on rocky sections of shore. The uplift profile along the coast is ge nerally parallel to the strike of the earthquake focal mechanism and f orms a broad, flat-topped arch approximately 24 km long with a gentle south limb and a steeper north limb. The maximum uplift of 1.4 +/- 0.2 m is near the center of the profile. The profile is a manifestation o f the more widespread domal upwarp produced by slip on an east-dipping buried thrust fault along or near the Cascadia megathrust. Small, new emergent terraces have formed where wave-cut intertidal platforms hav e been elevated. The new terraces resemble raised late Holocene benche s that probably record paleoearthquakes similar to the 1992 event. How ever, several of the late Holocene terraces are broader and more conti nuous. These terraces, which extend several tens of kilometres north a nd south of the 1002 uplift, suggest that some paleoearthquakes were m uch larger than magnitude 7.