 As the largest earthquake in California since the October 1989 event in the Santa Cruz Mountains , the mainshock near Petrolia produced some of the highest ground motions ever recorded ( at that time ) by the California Strong Motion Instrumentation Program ( CSMIP ) . Fourteen existing CSMIP stations comprising 84 strong motion sensors recorded the event , ten of which were ground response stations . The remaining four were located on structures , including a Highway 101 overpass in Rio Dell , a dam , a one @-@ story supermarket in Fortuna , and a 5 @-@ story residential building in Eureka . The supermarket , residential building , and dam were 28 , 50 , and 75 kilometers distant from the epicenter respectively , and recorded peak accelerations of .46g , .34g , and .15g. An accelerograph at the Painter Street overpass ( a concrete bridge , 24 km ( 15 mi ) from the epicenter ) recorded a free field acceleration of .55g and an instrument on the structure saw an amplified peak of 1.23g during the mainshock . 
