Earthquake Predominant Ground Motion in the San Francisco Metropolitan Area Using Data from Microtremors and the Loma Prieta Earthquake

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This project is part of the NSF/USGS initiative within the National Earthquake Hazard Reduction Program (NEHRP) to investigate the effects of the Loma Prieta earthquake of 17 October 1989. This project involves extensive measurements of long-period microtremors in the San Francisco basin. Microtremors, or microseisms, are small tremors or vibrations of the earth that are generated by natural occurrences such as the pounding of surf on the shore, or road traffic. They can be detected only by sensitive instruments and are not associated with earthquakes, but may be used to understand better the process of ground shaking due to earthquakes. The aim is to investigate the use of long-period microtremors in the San Francisco basin to identify "critical" structures having a natural frequency of vibration close to that of the predominant motion of the basin. The program consists of three phases: (1) The measurement of long-period microtremors at approximately 30 locations within the basin, with some of the sites coinciding with the locations were strong motion records were obtained during the Loma Prieta earthquake, (2) processing these data, and (3) analyzing these data and comparing them with the strong ground motion records obtained during the Loma Prieta earthquake. Special emphasis is being given to the Marina district which consists of very soft sediments and in which considerable damage occurred. All recorded data are being stored on floppy disks, which will be available upon request. In addition, the records will be accessible through electronic mail.