USGS Introduces New 3D Seismic Velocity Model For The San Francisco Bay Area

The U.S. Geological Survey (USGS) and its partners have madesignificant advances since 1989 in their efforts to mitigate loss oflife and property. These include real-time monitoring networks,scientific research, and state-of-the-art mapping products to betterunderstand and characterize the many complex interactions thatcontribute to damaging earthquakes in the San Francisco Bay Region.

The USGS is releasing a new tool in these efforts, a 3D computermodel of the upper 20 miles of the earth’s crust in the greater SanFrancisco Bay Area that will enable researchers to recreate the shakinglevels of past and future earthquakes.

The 3D computer model represents an important scientific advance. Itcombines 100 years of surface geologic mapping with decades of researchinto the seismic properties of the rocks in the Bay Area. It is alsobased on information from boreholes and variations in the earth’sgravity and magnetic fields. It is a "fault and block" model—that is,the upper 15-20 miles of the earth’s crust has been broken up intoirregular shaped blocks, bounded by faults. Including the faults in thesubsurface provides key information, since seismic waves can reflect(bounce) off faults or can bend and be focused in certain directions asthey cross faults.

"We expect this new 3D model to revolutionize our ability toforecast the location of ‘hotspots’ – where shaking occurs mostintensely – throughout the Bay Area," said Tom Brocher, USGSseismologist and co-developer of the model.

Previous studies by the USGS and others have shown that thesubsurface structure of the earth significantly influences how stronglyan earthquake is felt locally, as well as the duration of the shaking.For example: the Cypress Structure, the freeway approach to the BayBridge from Oakland, collapsed during the Loma Prieta earthquake,killing 42 people. At least two properties of the earth’s crustconspired to cause this collapse—it was built on loose soils that shookmuch more strongly than surrounding regions on stronger ground, andvariations in the thickness of the earth’s crust between the epicenterof the Loma Prieta earthquake in the Santa Cruz Mountains and Oaklandactually focused energy toward Oakland and downtown San Francisco.

"The new 3D model is a result of the long and productivecollaboration between the California Geological Survey and USGS," saidstate Geologist John Parrish. "Its usefulness will be to test andpredict the intensity and effects of shaking in future earthquakes andto build safer structures. This will be cost saving and life-saving forresidents of the Bay Area, now and in the future."

The model also includes the subsurface shape, depth, and propertiesof basins that underlie the Santa Clara, Livermore, and Santa RosaValleys. The soft sediments in the basins trap seismic energy andgreatly enhance shaking levels relative to surrounding regions. The 3Dmodel incorporates geological knowledge in fine subsurface detail.

"The 3D velocity model will provide a much more detailed definitionof the intensity of shaking, site by site," said Chris Poland,President of Degenkolb Engineers. "There are hundreds of billions ofdollars of new construction each year in high seismic regions. The morewe can design for the proper amount of strength and durability, themore we can achieve cost efficiencies, perhaps in the billions, whilegiving people greater safety during a large, damaging earthquake."

Efforts are underway to calibrate the model to reproduce groundshaking measured by seismographs in the 1989 Loma Prieta earthquake.For the centennial commemoration in 2006, year, the model will be usedto simulate the ground shaking produced by the 1906 San Franciscoearthquake as well as other potential damaging Bay Area earthquakescenarios.

Important applications of the new model include:

* forecasting strong ground motions that may damage buildings andessential infrastructure or destroy levees in the Sacramento Delta andpredicting where destructive liquefaction of the ground may occur

* modeling permanent ground deformation produced by earthquakes (including ground subsidence that could cause flooding)

* locating earthquakes more accurately

USGS developers of the new model are Thomas Brocher, Robert Jachens,Russell Graymer, Carl Wentworth, Bradley Aagaard, and Robert Simpson.For more information about the 3D Bay Area model and some exampleimages derived from it, visit our web page at http://quake.wr.usgs.gov/research/3Dgeologic/