The 1984 Morgan Hill, California, Earthquake

Bakun, William H.; Clark, Malcolm M.; Cockerham, Robert S.; Ellsworth, William L.; Lindh, A. G.; Prescott, W. H.; Shakal, A. F.; Spudich, Paul

doi:10.1126/science.225.4659.288

Cited by 52 publications

(14 citation statements)

References 8 publications

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“…Thus it is likely that this fault segmentation acted to terminate rupture of the 1984 Morgan Hill earthquake to the southeast. This possibility had been recognized previously [ Bakun et al , 1984; Beroza and Spudich , 1988]; however, the fault trace at the surface and at depth do not correspond very closely and it was not necessarily clear from the seismicity what the depth extent of the fault offset was. From the relocated seismicity it is clear that the fault discontinuity extends over the same range as slip in the main shock.…”

Section: A Fault Discontinuitymentioning

confidence: 87%

High‐resolution image of Calaveras Fault seismicity

et al. 2002

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[1] By measuring relative earthquake arrival times using waveform cross correlation and locating earthquakes using the double difference technique, we are able to reduce hypocentral errors by 1 to 2 orders of magnitude over routine locations for nearly 8000 events along a 35-km section of the Calaveras Fault. This represents $92% of all seismicity since 1984 and includes the rupture zone of the M 6.2 1984 Morgan Hill, California, earthquake. The relocated seismicity forms highly organized structures that were previously obscured by location errors. There are abundant repeating earthquake sequences as well as linear clusters of earthquakes. Large voids in seismicity appear with dimensions of kilometers that have been aseismic over the 30-year time interval, suggesting that these portions of the fault are either locked or creeping. The area of greatest slip in the Morgan Hill main shock coincides with the most prominent of these voids, suggesting that this part of the fault may be locked between large earthquakes. We find that the Calaveras Fault at depth is extremely thin, with an average upper bound on fault zone width of 75 m. Given the location error, however, this width is not resolvably different from zero. The relocations reveal active secondary faults, which we use to solve for the stress field in the immediate vicinity of the Calaveras Fault. We find that the maximum compressive stress is at a high angle, only 13°from the fault normal, supporting previous interpretations that this fault is weak.

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Section: A Fault Discontinuitymentioning

confidence: 87%

High‐resolution image of Calaveras Fault seismicity

et al. 2002

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“…It is well known that radiated seismic energy is concentrated in the rupture direction due to the so‐called directivity effect. Because the rupture direction of the 1984 Morgan Hill earthquake is to the SE (Bakun et al 1984), we expect larger ground motions and hence larger temporal changes for station in that direction. For example, stations CAL, CSC, and CMH are all close to active FZs, but the observed temporal changes are not as large as that of CCO, which could be due to the fact that these stations were not in the rupture direction of the Morgan Hill main shock, and hence experienced relatively small strong ground motions.…”

Section: Discussionmentioning

confidence: 99%

“…It has generated at least 14 earthquakes with M > 5 since 1850 (Oppenheimer et al 1990; Manaker et al 2005), with the most recent M w 5.6 event near Alum Rock on 2007 October 31. The largest event was the M L 6.2 1984 April 24 Morgan Hill earthquake that ruptured the central portion of the Calaveras fault (Bakun et al 1984; Fig. 1).…”

Section: Background and Repeating Cluster Identificationmentioning

confidence: 99%

Depth extent of damage zones around the central Calaveras fault from waveform analysis of repeating earthquakes

Zhao¹,

Peng²

2009

Geophysical Journal International

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S U M M A R YWe systematically investigate spatial variations of temporal changes and depth extent of damage zones along the Calaveras fault that ruptured during the 1984 Morgan Hill earthquake by the waveform analysis of 333 sets of repeating earthquakes. We use a sliding window waveform cross-correlation technique to measure traveltime changes in waveforms generated by each repeating cluster. We find clear traveltime delays in the S-and early S-coda waves for events immediately after the Morgan Hill main shock. The amplitudes of the time delays decrease logarithmically with time since the main shock, indicating a time-dependent recovery (healing) process following the abrupt coseismic temporal changes. The largest temporal changes are observed at station CCO that is the closest to the rupture zone of the Morgan Hill main shock. The time delays at this station are larger for clusters in the top 6 km, and decrease systematically at larger depth. In comparison, the time delays observed at other five stations are much smaller, and do not show clear relationship with hypocentral depth. We suggest that the temporal changes at these five stations mostly occur in the top few hundred metres of the near-surface layers, while the temporal changes at station CCO are likely associated with the damage zone around the Calaveras fault that is well developed in the top few kilometres of the upper crust. Our results are consistent with the inference of a widespread damage and non-linearity in the near-surface layers associated with strong ground motions of nearby large earthquakes, and localized damages and flower-type structures around active faults based on previous studies of fault zone structures and recent 3-D numerical simulations.

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“…Whereas slip in the Coyote Lake event apparently was concentrated in a circular zone at 5-to-10 depth (Liu and Helmberger, 1983), slip in the Morgan Hill event was concentrated at the ends of the 15-to-20 km long rupture, with little slip near the center of the rupture length (Hartzell and Heaton, 1986). Nearly all of the strong ground motions were generated in the 1-meter slip at the end of the rupture (Bakun et al, 1984). Hazard models should encompass these examples and conform, in a statistical sense, to the distribution of nearly-constant 30-bar stress drops documented for crustal strike-slip earthquakes in numerous publications (e.g., Thatcher and Hanks, 1973;Hanks and Bakun, 2002).…”

Section: Working Group 2002mentioning

confidence: 99%

“…CC is characterized by abundant microseismicity (Bakun, 1980(Bakun, , 1984others, 1986: 1985;Oppenheimer et al, 1990), which may reflect the high rate of creep. The simple average creep rate from 1968 to 1999 is 16.3 mm/yr (Galehouse and Lienkaemper, 2002), although this value includes slip possibly triggered by the 1989 Loma Prieta earthquake (Galehouse, 1997).…”

Section: Working Group 2002mentioning

confidence: 99%

Earthquake probabilities in the San Francisco Bay region: 2002-2031

2003

Open-File Report

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The 1984 Morgan Hill, California, Earthquake

Cited by 52 publications

References 8 publications

High‐resolution image of Calaveras Fault seismicity

High‐resolution image of Calaveras Fault seismicity

Depth extent of damage zones around the central Calaveras fault from waveform analysis of repeating earthquakes

Earthquake probabilities in the San Francisco Bay region: 2002-2031

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