The intensity of local ground-shaking depends on several factors besides the magnitude of the earthquake , one of the most important being soil conditions . For instance , thick layers of soft soil ( such as fill ) can amplify seismic waves , often at a considerable distance from the source , while sedimentary basins will often resonate , increasing the duration of shaking . This is why , in the 1989 Loma Prieta earthquake , the Marina district of San Francisco was one of the most damaged areas , though it was nearly 100 km from the epicenter . Geological structures were also significant , such as where seismic waves passing under the south end of San Francisco Bay reflected off the base of the Earth 's crust towards San Francisco and Oakland . A similar effect channeled seismic waves between the other major faults in the area
1989 Loma Prieta earthquake
1989 Loma Prieta earthquake
Seismic magnitude scales