Focusing Of Seismic Waves Research Articles

Bedrock structure in Adapazari, Turkey—a possible cause of severe damage by the 1999 Kociaeli earthquake

The 1999 Kocaeli earthquake brought serious damage to downtown of Adapazari. To study why strong motions were generated at the town, a bedrock structure was investigated on the basis of Bouguer gravity anomaly, and SPAC and H/V analyses of microseisms. It was revealed that, the basin consists of three narrow depressions of bedrock with very steep edges, extending in E–W or NE–SW directions along the North Anatolia faults, and the depth to bedrock reaches 1000 m or more. Downtown of Adapazari is located 1–2 km apart from the basin-edge. It is considered that, the specific configuration of bedrock amplifies ground motions at the downtown area by focusing of seismic waves and/or interference between incident S-waves and surface-waves secondarily generated at the basin-edge. Studying 3D bedrock structure is an urgent issue for microzoning an urban area in a sedimentary basin.

Shaking Southern California

GEOPHYSICS The Southern California Earthquake Center (SCEC) was initiated in 1991 to try to understand the physical processes of earthquakes in southern California. In its third phase, the SCEC was to study the effects of different rock sites on the amplitude of ground shaking to help improve probabilistic seismic hazard analysis. Field et al. summarize these results, which are described in fourteen papers in a special issue. Site effects may be influenced primarily by nonlinear responses of the sediments to seismic waves, or by the scattering or focusing of seismic waves by basins and other structures, or both. Refined models of the subsurface geology, the velocity structure of the crust, and the paths of seismic waves through these structures form the basis for a detailed analysis of the amplitude of ground shaking at different sites. The depth of the Los Angeles basin has the largest effect on the amplitude of the ground shaking; sites located beneath the deepest part of the basin show the strongest ground motions (highest amplitude shaking) in simulations of medium- to large-magnitude earthquakes from a range of possible sources. These data and models provide a useful synthesis of the complex site effects in southern California and what is known of the physics behind earthquakes. — LR Bull. Seism. Soc. Am. 90 , S1-S244 (2000).