CDMG Strong-Motion Records From the Morgan Hill, California Earthquake of 24, April 1984

OSMS 84-7

by A.F. Shakal, R.W. Sherburne, and D.L. Parke

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The Morgan Hill earthquake generated an important set of strong-motion records. Nearly fifty records were recovered by CDMG, almost half of which were obtained in extensively instrumented structures. The station at Coyote Lake Dam recorded an unusually high acceleration of approximately 1.3 g. Another noteworthy aspect of the Morgan Hill strong-motion data set is the striking difference between acceleration amplitudes at stations southeast of the earthquake epicenter relative to stations to the northwest. Analysis of the strong-motion data and the earthquake source mechanism may establish that directivity focusing contributed to the highly directional nature of the acceleration 1eve1s.

The response of structures ranging from a single-story warehouse to a twelve-story office building was recorded. Roof accelerations in excess of 15% g were recorded in seven structures; four of these had roof accelerations in excess of 25% g. In addition, some structures exhibited significantly different shaking durations compared to nearby structures of similar height. These records, from structures in San Jose, provide rich data for the analysis of structural response and soil- structure interaction during moderate shaking.

Current estimates of the earthquake location and magnitude are:
Hypocenter: 37.317N, 121.680W, 9 km depth. Magnitude: 6.2 ML (BRK)
Origin Time: 21:15:19 GMT (13:15:19 PST), 24 April 1984 (USGS)

Ground Motion Data Strong-motion records were recovered from twenty-two ground motion stations maintained by CDMG during the Morgan Hill earthquake. These stations are indicated on the location map of Figure 1. The closest station, Halls Valley, was approximately 4 km from the epicenter. The most distant station was Mendota, 130 km southeast of the epicenter. Many stations to the northwest which were closer than Mendota were not triggered, though fully operational. (Nominal trigger threshold level is between 0.5 and 1.0% g, on the vertical.)

The stations are identified in Figure 1 by a three-digit code. Table 1 cross-references this code, the station number, and the station name. Table 2, ordered by station name, provides information on the site conditions and/or structure. For convenience, the station Location map in Figure 1 has been partitioned into three areas: 1) San Jose and regions to the south, 2) the San Francisco Peninsula, and 3 ) east of the San Francisco Bay. In Tables 1 and 2 the area number of each station is indicated, as well as the page number on which the recorded accelerogram appears. Detailed information on each record is given in Table 3, again arranged by area.

The record from the Coyote Lake Dam station (which has also been called San Martin- Coyote Creek) recorded an unusually high horizontal peak acceleration of approximately 1.3 g. Because this record has large-amplitude excursions and trace crossings, a photographic enlargement of the high acceleration section record is included with the record (p. 26).

The Coyote Lake station is a few kilometers directly down the fault from the end of the aftershock distribution which extends from Halls Valley to near Anderson Lake. It should be noted that the Coyote Lake Dam station is not a good quality free-field or ground-response installation by current standards. Although housed in an especially constructed instrument shelter (Armco) , the shelter is located near the southwest abutment of the ( earth-filled ) dam and near a ridge-1ike formation extending from the valley wall. The import of these aspects may be expected to be the subject of future investigations. The location of the instrument in relation to the dam structure is shown in Figure 2, which also shows the local topography before and after the dam was constructed (from Buangan and Wahler, 1980).

To verify satisfactory operation of the Coyote Lake accelerograph, the unit was subjected to calibration and instrument-comparison tests after the earthquake. No evidence of improper operating characteristics was found in either static or dynamic tests. To learn the effects of the surroundings of the station on the record, an additional instrument is being installed in a small structure approximately 1500 f t south of the dam so comparative records may be obtained during aftershocks of the Morgan Hill event and other future earthquakes.

It should be noted that a strong motion record was obtained at the Coyote Dam station during the 1979 Coyote Lake earthquake. That record, with peak accelerations near 25% g (Porcella et al., 1979), appears to exhibit no evidence for anomalous site characteristics.

Gilroy Array
The Morgan Hill earthquake generated the second suite of strong motion records from the Gilroy Array, an east-west alignment of stations extending across the Calaveras fault near Gilroy (Figure 1). This array is a cooperative effort of the CDMG Strong Motion Instrumentation Program (SMIP) and the USGS Seismic Engineering Branch, and is currently instrumented and maintained by SMIP. In addition to the Gilroy array stations #1 through #6, CDMG has added station #7 (sited by R. McJunkin following the 1979 event) and the Gavilan College station. This array, extending from rock on the east across the alluvial Santa Clara Valley to rock on the west, provided significant records in both the 1979 event and this event. In this event, most of the array records have a high acceleration signal on the vertical component early in the record.

Another cooperative array, the APEEL strong-motion array, only recorded low amplitude records and was not triggered in its entirety.