Surface Waves Several types, travel along the earth's surface or on layer boundaries in the earth. The slowest waves but the ones that do the damage in large earthquakes. 1 to 12 We can plot earthquake intensity by gathering reports from observers. Although the reports will be subjective, and vary somewhat, most observers will agree on the intensity criteria, for example, feeling the quake while driving. For very strong quakes, damage provides fairly objective measures of intensity.
Soils here are very shallow - usually less than a meter to bedrock.
Observers here felt mostly a sharp jolt. In contrast, the high intensity in central California coincides with the Central Valley, where young and unconsolidated sediments are kilometers deep. Unconsolidated material shakes like jelly in an earthquake. Note how intensity VI follows the shoreline of San Francisco Bay, where there are also thick unconsolidated sediments. At left is an isoseismal map for San Francisco itself .