Basin depths contribute greatly to damage

Future seismic engineering design in southern California and elsewhere may have to consider the depth of the sedimentary basin below a structure.

New ground motion simulations for earthquakes on some of southern California's major faults show significant effects of the basin on both the amplitude and duration of long-period ground shaking (the "rolling motion" from earthquakes).

These new results made use of high-performance computers to calculate the effects due to a more realistic 3-D, rather than the 1-D, sub-surface earth structure. The computational requirements increase dramatically for 3-D calculations and as the desired ground motion frequencies are increased. Currently, only frequencies less than 1 cycle per second are being considered for design purposes, but as computational power increases and, in particular, our knowledge of the subsurface geologic structure improves, simulations with frequencies greater than 1 cycle/second will become possible. The critical ground shaking for most homes, small buildings, and freeway bridges occurs for higher frequencies.

The simulations include nine 'scenario' earthquakes on faults that are capable of generating moderate to large earthquakes in the near future. The scenarios include historical earthquakes as well, such as the 1993 Long Beach, 1987 Whittier Narrows, and 1994 Northridge events. The Northridge earthquake is used as validation of the simulations.

Results indicate that the 3-D simulations show amplification factors several times greater than those based on earlier 1-D models, as well as significantly longer durations. The degree of ground motion amplification generally increases with depth of the basin below a specific site. Furthermore, seismic waves that propagate into the basin from earthquakes occurring outside of the basin tend to be amplified more than waves from earthquakes on faults inside the basin.

Contacts:

Kim Olsen, UCSB (805) 893-7394 (email kbolsen@crustal.ucsb.edu); Ralph Archuleta, UCSB (805) 893-8441; Geoff Martin, USC (213) 740-9124; Steven Day, San Diego State University (619) 594-2663.

References:

Kim B. Olsen, Ralph J. Archuleta, Joseph R. Matarese. "Three-Dimensional Simulation of a Magnitude 7.75 Earthquake on the San Andreas Fault," Science, Vol. 270, p. 1628, 8 December 1995.

Olsen, K.B., and R.J. Archuleta (1996). "Three-dimensional simulation of earthquakes on the Los Angeles fault system," Bull. Seism. Soc. Am., 86:575-596.

Olsen, K.B. (1997). Site Classification and Site-Specific Amplification for Basin Effects, in: Probabilistic Seismic Hazard in Southern California: Uncertainties due to Assumptions and Models, in review.

Kim Olsen's Web Site: http://www.crustal.ucsb.edu/~kbolsen

Interview with Ralph Archuleta, SCEC Quarterly Newsletter, Vol. 3, No. 2, pp. 10-14.