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Near‐field high resolution strain measurements prior to the October 18, 1989, Loma Prieta M s 7.1 Earthquake
Author(s) -
Johnston M. J. S.,
Linde A. T.,
Gladwin M. T.
Publication year - 1990
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/gl017i010p01777
Subject(s) - seismology , geology , epicenter , slip (aerodynamics) , borehole , foreshock , geodesy , aftershock , physics , geotechnical engineering , thermodynamics
High resolution strain recordings were made in deep boreholes throughout California prior to, during, and following, the October 18, 1989, M s 7.1 Loma Prieta earthquake. The nearest dilational strainmeters (sensitivity 10 10 ) and 3‐component tensor strainmeters (sensitivity 10 −9 ) were 37 km to 42 km, respectively, from the main shock. High quality data, including details of strain offsets, were recorded on both instruments through the earthquake. These data have been searched for indications of short‐, intermediate‐, and long‐term strain redistribution and/or fault slip that might have indicated imminent rupture. Short‐ and intermediate‐term changes in both tensor strain and dilational strain (≤, several nanostrain, if any) during the minutes to months before the earthquake are at least 1000 times smaller than that generated by the earthquake itself. If short‐term preseismic slip did occur at the nucleation point of the earthquake during the previous week, and if the type of slip is similar to that during the earthquake, its moment could not be more than 10 24 dyne‐cm. Stated another way, slip equivalent to that expected for a M 5.3 earthquake could have occurred in the hypocentral region without the strainmeters detecting it at these distances and azimuthal positions. Long‐term changes appear to have occurred in mid 1988 and mid 1989. These changes were both followed by M L 5 earthquakes in the hypocentral region on June 27, 1988, and August 8, 1989, respectively, and, since they correspond approximately to changes in geodetic strain rate over the epicenter, may indicate precursory strain redistribution in the epicentral region. Minor postseismic strain recovery (≈ 14%) occurred in the month following the main shock.