Simple traditional two-dimensional geologic map of the San Francisco Bay region. The blue rectangle shows the horizontal outline of the 3D geologic map. Black dots show the epicenters of the 1906 San Francisco earthquake and the 1989 Loma Prieta event. Different colors correspond to different rock types found in the region.
The 3D computer model represents an important scientific advance. It combines 100 years of surface geologic mapping by the USGS, the California Geological Survey, and many other institutions together with decades of research into the seismic properties of the rocks in the Bay Area. It is also based on information from boreholes and variations in the Earth's gravity and magnetic fields.
It is a fault block model - that is, the upper 15-20 miles of the Earth's crust has been broken up into irregular shaped blocks, bounded by faults. Including the faults in the subsurface provides key information, since seismic waves can reflect (bounce) off faults or can bend and be focused as they cross faults.
The model also includes the subsurface shape, depth, and properties of basins that underlie the Santa Clara Valley, Livermore Valley, and Santa Rosa Plain. The soft sediments in these basins trap seismic energy and greatly enhance shaking levels relative to surrounding regions. The 3D model incorporates geological knowledge in fine subsurface detail.
Oblique view, looking from the southwest toward San Francisco Bay. The corner of the 3D Geologic Model has been cut away to show faults (red lines), basins (yellow), and other geologic rock units (various colors). (Other 3D views.)
The locations of earthquakes relative to faults and rock bodies give clues as to the locations of locked patches on the major faults. These locked patches are the portions of faults most likely to break in future large quakes.
