Assessing the Nation's Earthquakes The Health and Future of Regional Seismograph Networks (1990) / Chapter Skim
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4. The Need for Regional Seismic Networks
4. The Need for Regional Seismic Networks
Pages 25-31
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From page 25... ...
The USNSN will open important new avenues for seismological research; by itself, however, it will be insufficient to meet the nation's seismic data needs. The seismic stations of the USNSN are planned to be of very high quality (see Chapter 5~; but the number of stations deployed will be smallonly 100 to 150 USNSN stations are proposed compared to the more than 1,500 stations now deployed in existing regional networks.
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From page 26... ...
estimated that such a system could have provided the Los Angeles region more than a minute's warning before the great Fort Tejon earthquake of 1857, an event with a significant probability of recurrence in the next several decades. Upgraded dense regional networks could play an important role in postearthquake disaster response and recovery.
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From page 27... ...
The following case studies document two earthquake sequences: the Whittier Narrows earthquake, which occurred within an existing regional network, and the Painesville, Ohio, earthquake, which occurred outside network coverage. CASE STUDY: THE 1987 WHITTIER NARROWS EARTHQUAKE IN THE LOS ANGELES METROPOLITAN AREA, CALIFORNIA The moderate-sized (magnitude 5.9)
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From page 28... ...
These data have also been the basis of detailed studies that have greatly improved understanding of the tectonics of the Los Angeles basin and of the seismic hazards facing the Los Angeles metropolitan area. The Whittier Narrows earthquake occurred at 7:42 a.m.
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From page 29... ...
had shown that the anticline that was the surface expression of the main shock fault at Whittier Narrows extended westward across the full width of the Los Angeles basin and seaward into Santa Monica Bay. Moreover, analysis of the shape of the anticline at Whittier Narrows and at another site near downtown Los Angeles strongly suggested that a thrust fault was buried beneath the anticline at those locations.
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From page 30... ...
the Painesville earthquake was the largest event known to have occurred in the region; (2) in situ stress measurements in the Paleozoic sedimentary units overlying crystalline basement indicated the presence of high levels of deviatoric stress so that preexisting favorably oriented faults would be close to failure; (3)
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From page 31... ...
~ focal depth confidently constrained to the S-km centroid depth estimate, for example, would put the hypocenter about 3 km deep in crystalline basement. Without appeal to special fracture or joint pathways or pore fluid—especially if an absence of microearthquakes could be confidently established for the intervening crustal volume a causal connection between the waste disposal wells and the main shock could be ruled out.
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