SCEC INSTANeT News
	Northridge Earthquake Damage Caused by Geologic Focusing of Seismic Waves

Results of research by SCEC Scientists Paul M. Davis (UCLA), Justin L. Rubinstein (UCLA), Kelly H. Liu (Kansas St. Univ.), Stephen S. Gao (Kansas St. Univ.), and Leon Knopoff (UCLA) on why Santa Monica experienced anomalously concentrated damage were published in the September 8, 2000 issue of Science. Abstract of the Science article: Despite being located 21 kilometers from the epicenter of the 1994 Northridge earthquake (magnitude 6.7), the city of Santa Monica experienced anomalously concentrated damage with Mercalli intensity IX, an intensity as large as that experienced in the vicinity of the epicenter. Seismic records from aftershocks suggest that the damage resulted from the focusing of seismic waves by several underground acoustic lenses at depths of about 3 kilometers, formed by the faults that bound the northwestern edge of the Los Angeles basin. The amplification was greatest for high-frequency waves and was less powerful at lower frequencies, which is consistent with focusing theory and finite-difference simulations.   Stephen Gao was interviewed about this study on the KPFK Evening News of Nov. 9, 2000 . To listen his explanation of the study, click here (Real Player required). The December, 2000, issue of Discover Magazine featured an article about this research. To read the article online, click here. To read the complete Science article (with figures) online, click here. You may need to pay a $5 fee for access. An earlier paper on this topic was published in the Northridge special issue of BSSA in 1996 (BSSA Vol. 86, No. 1B, pp. S209-230). The text and all the figures can be found here.   Selected images from the Science article (To read the complete Science article (with figures) online, click here. You may need to pay a $5 fee for access.)   Red-tagged (condemned) buildings (open squares) were damaged during the 1994 Northridge earthquake (star). The localized concentrations of high amplitudes of ground motion from the aftershocks of the Northridge earthquake suggested that focusing by deep geologic structures, which act like acoustic lenses, was likely to have caused the concentrated damage in Santa Monica during the main event. (Click on the image for a larger version). Finite difference simulations of focusing from a 2D curved interface (top left). Snapshots are shown of a sinusoidal pulse of unit initial amplitude. The energy converges to generate a focused pulse of amplitude 6.0 relative to an amplitude of 1.8 immediately after passing through the interface (causing an amplification of 3.3). The region is 6 km long and 3 km wide.     (A) Axial rays from 22 Northridge earthquake aftershocks through lens 1 to Santa Monica, projected onto a vertical plane running from north to south. The locations of the Potrero Canyon and Santa Monica faults are taken from Wright. It has been assumed that the focusing part of the basement-to-sediment interface is a spherical cap.       (B) Expanded version of the model. The location of the cap is determined from the inversion of the amplitude data and lies at the sediment-to-basement interface. The focusing structure corresponds to a region where the Occidental Marquez oil well taps a reservoir estimated to contain 60 million barrels of oil. SMoF, Santa Monica fault.   [Go to SCEC INSTANeT News Main Page]

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