Earthquake Spectra

On April 24, 1984, at 21:15 UTC, a moderate earthquake occurred on the Calaveras fault in the vicinity of Halls Valley, California (epicenter 37° 19.2'N, 121° 41.9'W). The earthquake had a mean local magnitude of 6.2 ± 0.20 and a maximum Modified Mercalli Intensity of VIII. It is probably a repeat of the 1911 Coyote earthquake.

The rate of seismicity from historical catalogs, compiled at the U. C. Seismographic Stations yields an average recurrence time of 160±60 years for an M _L ≥6.2 event occurring in a 30 km segment of the Calaveras fault. Other evidence from the repeat times of earthquakes suggests a recurrence period of 75 years for a total magnitude of 6.0 or larger.. These estimates are in reasonable agreement.

A seismic moment of 1.1×10²⁵ dyne-cm for the mainshock was estimated from broadband displacement seismograms recorded at Berkeley (Δ ≈ 79 km). The moment and the rupture area inferred from the aftershocks suggest a mean fault displacement of 30 cm. The aftershock sequence through December 31, 1984, contained four earthquakes of magnitude 4.0 or greater and the sequence b-value was 0.81 ± 0.03.

Seventy-two strong-motion accelerograph stations, mostly from within the permanent networks of the U. S. Geological Survey and the California Division of Mines and Geology, were triggered during the Morgan Hill earthquake, April 24, 1984. Among the more interesting of the records are the large acceleration (1.29g) at Coyote Lake dam, the Gilroy array spanning the region between the San Andreas and Calaveras faults, the record from Hall's Valley (epicentral distance 4 km), and records from buildings, a bridge, and a dam within 20 km of the epicenter. Digitized data from 16 stations are available on tape.

The 1984 Halls Valley (Morgan Hill, California) earthquake had a complex seismic source. Velocities of the major seismic phases measured from continuous broadband seismograms at Berkeley Seismographic Station (BKS) and Richmond Field Station (RFS) show unambiguously that the earthquake is predominantly a double event with the second source hypocenter located approximately 17 km southeast of the mainshock hypocenter given by Bolt, Uhrhammer and Darragh (1985). The southeasterly fault rupture of the first source and the location of the focus of the second source have critical implications for the observed spatial variation of the recorded accelerograms. Of particular engineering interest, the high frequency 1.29g pulse of horizontal ground acceleration measured at Coyote Lake dam can be explained primarily as due to the second source and constructive interference of the principal S waves from the two sources.

The response of the equipment commonly found in power plants and industrial facilities to the Morgan Hill earthquake of April 24, 1984 is reviewed. Brief descriptions of the general effects of the earthquake are given, including strong ground motion recordings and impacts on commerical and residential structures and their contents. Four sites near San Jose and seven sites near Morgan Hill are examined in detail. These include two wineries, a community college, an industrial park, a software development laboratory, a chemical manufacturing complex, and an electrical substation. Both damaged and undamaged equipment is examined. Generally equipment performed quite well in this moderate earthquake. Seismic damage to well-engineered systems and structures was minimal to non-existent.

United Technologies Chemical Systems Division, a manufacturer of rocket engine propellants, was situated in close proximity to the Calaveras fault. Soil conditions and fault rupture mechanisms produced anomalously high ground motion at the site. The main plant complex insisted of more than 130 buildings, constructed within the past 25 years to building code standards that considered earthquakes.

Damage to buildings was not extensive and was limited to the structures founded in the valley on loose alluvial deposits. Most of the damage was associated with nonstructural building elements and equipment. Estimated repair costs exceeded $1.25 million. No lives were lost and only minor injuries were experienced. Important observations include: (1) soil amplification effects increased damage, and (2) that persons who had received blast response training responded far better than those who were not exposed to such training.

Two hospitals constructed under the provisions of the Hospital Act and six public school buildings constructed under the provisions of the Field Act were investigated following the Morgan Hill earthquake and all were observed to have performed quite well during the Morgan Hill earthquake. Maintaining function in hospitals in an important aspect of the Hospital Act; neither of the two hospitals inspected suffered any loss of function.

The performance of three engineered buildings in San Jose during the 1984 Morgan Hill earthquake is reviewed. The lateral-load-resisting systems for these buildings are: (1) concrete shear walls, (2) concrete shear walls and moment frames in two orthogonal directions, and (3) perimeter steel moment frames. The concrete buildings performed satisfactorily with no damage. The steel building oscillated for a long period of time with low damping. There was some nonstructural and content damage and very limited structural damage to this building. The recorded responses of these buildings also showed excitations from two events with different characteristics.

The Morgan Hill earthquake caused limited structural damage. Most of the damage was concentrated in the town of Morgan Hill, which has a population of approximately 19,000. According to the latest available data, there are 6,024 residences; of these, 834 are mobile homes. Less than one percent of the residential structures suffered major damage. Less than eight percent suffered minor damage. Damage to residential structures has been estimated by state officials to be $3,000,000.

The greater Morgan Hill area has seven mobile home parks. Of the seven, there are three large parks and four small ones. For the most part, damage occurred at three parks: Windmill Mobile Estates, Woodland Mobile Home Park, and Madrone Manor Mobile Estates.

The overall response of all lifelines was good although there was some significant damage. In the power system there was significant damage to 500 kV circuit breakers at two substations, some other minor substation damage, and some damage to feeders in the distribution system. The substation damage did put a segment of one of the AC Pacific Interties out of service for a couple of days but the remaining intertie was able to carry the load. The loss of feeder lines caused local disruption of power for up to 5 hours. In water and sewage systems two 8 inch transit water lines broke in Morgan Hill and a break in an 8 inch pipe to a 350,000 gallon tank caused a large water loss. Five 6 inch water line couplings disengaged. There were numerous water service connection failures in the high damage area. There was no significant damage to communication systems but there was a significant downgrade in the quality of service due to system overload. Calls from outside the area were limited by AT&T. Damage to highway systems was limited to that caused by a rock slide which blocked E. Dunne Avenue near the Anderson Reservoir and damaged a nearby bridge over the reservoir. One bridge span moved one foot near an abutment and caused the adjacent pier to move out of plumb by almost one foot. The road surface on the crest of the Anderson Dam was cracked but there was no damage to the dam. In the natural gas system there was no damage to the utilities part of the system. A large fire in San Jose was caused by a gas leak in piping to a gas fired industrial heater. There was an explosion and fire in a mobile home and numerous leaks associated with toppled water heaters. There were numerous leaks beyond service connections to homes in the high damage area.

The Santa Clara Valley Water District (the District) is a public agency with flood control and water supply management authority in Santa Clara County, California. The District operates ten dams and reservoirs, two water treatment plants which supply potable water, 74 miles of large-diameter water distribution pipelines and 34 miles of small canals. Flood control facilities include 45 miles of streams channeled by levees and 17 miles of bayfront levees. Immediately after the earthquake, the District's Emergency Operations Center was activated. Major dams were inspected immediately by operations staff in accordance with standing instructions. The fundamental operations of the District were not interrupted by the earthquake in any significant way. Both water treatment plants operated continuously during and after the earthquake. Damage was limited to minor (functionally insignificant) surficial cracking at two embankment dams. Electronic distance measuring surveys indicated that a large existing landslide, on which one of the water treatment plants is located, moved as much as 13 mm as a result of the earthquake.

Only one bridge of 39 investigated was observed to sustain major structural damage. The county bridge carrying East Dunne Avenue over Anderson Reservoir, Bridge No. 37C-166, was closed to traffic by a rock slide and suffered major damage.

The Morgan Hill earthquake of April 24, 1984 tested the response structure of Santa Clara County and several cities within the County. While there was no damage to the communications and command infrastructure, the response was seriously disrupted by overloading of the telephone system, and the lack of accurate damage intelligence. Analysis of the response to the earthquake has served to emphasize the need for improved preparedness activities at the local and regional level, the involvement of individuals, businesses, and schools in preparedness planning, and for improvements in communication systems and procedures.

Fire following the April 24, 1984, Morgan Hill earthquake (M 6.2) was a primary cause of damage in the cities of Morgan Hill and San Jose, California. Delays in telephone dial tones, perceived as telephone outages, resulted in delayed reports of structural fires to fire departments. A fire at a shopping center in San Jose was the largest single loss in the earthquake, totaling approximately $1 million. Multiple non-fire specific incidents which fire departments were called upon to respond to, such as gas investigations, structural damage checks, downed power wires and medical aid, placed additional heavy demand on limited resources.