3D geologický a seismický model oblasti zálivu San Francisco

The USGS is releasing a new tool to help mitigate loss of life and property -- a 3D computer model of the upper 20 miles of the Earth's crust in the greater San Francisco Bay Area that will enable researchers to accurately predict the shaking levels of past and future earthquakes. Construction of this 3D Bay Area model has been a joint effort of the USGS Earthquake Hazards Program and the USGS National Cooperative Geologic Mapping Program.

Traditional two-dimensional geologic maps, like the one seen below, show the distribution of rock units at the Earth's surface.

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.

Cutaway of a portion of the 3D model in the Santa Clara Valley area showing faults and earthquakes. Most of the earthquakes in the cluster on the left are Loma Prieta aftershocks.

A part of the 3D model showing rock types adjacent to the Hayward Fault may help scientists understand why parts of this fault are creeping while other parts seem to be locked, rupturing periodically in magnitude ~7 earthquakes.

Slicing open the Hayward Fault. By building a 3D model of the subsurface, we find that different rock bodies are grinding past one another on the Hayward Fault. Small earthquakes tend to occur in certain rock types and avoid other rock types.

Seismic velocities of rocks and sediments at shallow depths determine how strongly an area will shake. By assigning velocities to rock types in the 3D model, geologists can gain an impression of the extent of areas of low shear velocity that are most likely to experience localized strong shaking and future earthquake damage.

Seismic wave velocities at shallow depths (12.5 meters (40 feet) in this map extracted from the 3D velocity model) give an indication of the susceptibility of the shallow rock units to enhanced shaking and ground failure. Low velocity areas (warm colors) are most prone to enhanced shaking. Many of these areas are along the Bay and Delta margins and the coast - areas likely to have a shallow water table and, hence, are also subject to liquefaction.

The new 3D model will be calibrated to match the ground shaking observed during the 1989 Loma Prieta earthquake, and then will be used to simulate the ground shaking produced by the 1906 San Francisco earthquake and other potential Bay Area earthquakes. The amount of ground shaking at a site depends on the local rock formations, soil conditions, and the subsurface structure along the path of the seismic waves.

The Cypress Structure, the freeway approach to the Bay Bridge from Oakland, collapsed during the Loma Prieta earthquake, killing 42 people. At least two properties of the Earth's crust conspired to cause this collapse: it was built on loose soils that shook much more strongly than surrounding regions on stronger ground, and variations in the thickness of the Earth's crust between the epicenter of the Loma Prieta earthquake in the Santa Cruz Mountains and Oakland actually focused energy toward Oakland and downtown San Francisco.

Some important applications of the new model include:

• forecasting strong ground motions that may damage buildings and essential infrastructure or destroy levees in the Sacramento Delta

• predicting where destructive liquefaction of the ground may occur

• locating earthquakes more accurately

• defining the extent of groundwater aquifers and modeling of subsurface transport of contaminants.