GREAT-BASIN AND SIERRA NEVADA 1911 figures for the Nevada and eastern California region based on the UNRSL earthquake catalog

We quantify foreshock occurrence probabilities by applying the empirical technique of Jones (1985) to the western Nevada and eastern California earthquake catalog compiled by the University of Nevada, Reno, from 1934 through 1991. The foreshock occurrence rates depend heavily on the parameters used to remove aftershocks from the catalog. It is necessary to separate the Mammoth/Mono region from the rest of the catalog to determine the parameters that most effectively remove the affershocks from the catalog. The probability that an M >__ 3.0 earthquake will be followed by an earthquake of larger magnitude within 5 days and 10 km is 10% in the Mammoth/Mono region and 6% in the Nevada region, and seems to be independent of the magnitude of the proposed foreshock. The probability that an earthquake will be followed by another one at least one magnitude unit larger is 1 to 2% in each region. These probabilities imply that the occurrence of an earthquake M > 4.0 increases the possibility of a damaging earthquake of M > 5.0 by several orders of magnitude above the low background probability. Most mainshocks occur within a few hours after a possible foreshock, and the probability that a mainshock will still occur decreases logarithmically with time after the proposed foreshock. These foreshock properties are similar to those in southern California and in other parts of the world, with the exception that the Mammoth / Mono region, a volcanic area, exhibits more swarm-like behavior than does the southern California or Nevada region. INTRODUCTION When a moderate ear thquake is felt, one of the first things most people want to know is whether a larger one will soon follow. The U.S. Geological Survey and the California Ear thquake Prediction Evaluation Council have recently employed t ime-dependent ear thquake hazard evaluation (e.g., Reasenberg and Jones, 1989; Jones, 1990; Agnew and Jones, 1991) in order to inform emergency management personnel and the public of the possibility that a moderate ear thquake could be a foreshock to a much more damaging earthquake. The Universi ty of Nevada, Reno, Seismological Laboratory (UNRSL) and the Nevada Division of Emergency management would like to be able to perform a similar service for the people in Nevada. However, the tectonic setting of the northern Basin and Range, an extensional region of diffuse seismicity that encompasses parts of eastern California as well as Nevada and much of Utah, is quite different from the strike-slip, plate-boundary setting in western California, in which most of the ear thquakes occur along a single fault or a set of faults. Therefore the probability that a given ear thquake in the Basin and Range will be a foreshock to a larger ear thquake could be significantly different from the values determined using the California catalog (Jones, 1985). Indeed, it has been suggested that the Basin and Range has a higher incidence of foreshocks than does southern California (Doser, 1990). We present herein appropriate 1910 F O R E S H O C K IN GREAT-BASIN AND SIERRA NEVADA 1911 figures for the Nevada and eastern California region based on the UNRSL earthquake catalog. DATA The data set for our study consists of the UNRSL earthquake catalog from 1934 to 1991. Some of this catalog has been summarized by Jones (1975) and by Corbett (written communication, 1985). The years before 1961 were compiled by Slemmons et al. (1965) who augmented the Townley-Allen (1939) "Descriptive Catalog of Earthquakes of the Pacific Coast of the United States, 1769-1928" with several other sources including the United States Coast and Geodetic Survey Bulletins, the Seismological Society of America Bulletins, and earthquake catalogs compiled by the California State Division of Water Resources, the University of California Seismographic Station at Berkeley, and the California Institute of Technology Seismological Laboratory. Other sources for early data included a Wiechert smoked-paper recorder operated semicontinuously from 1916 to 1959 by UNRSL, which gave reliable S P times and Richter magnitudes, and early newspapers archived in the University of Nevada, Reno, library and the University of California, Berkeley, Bancroft Library. Although the early catalog can never be considered complete, Slemmons et al. (1965) believed that the early catalog contains all known events with felt reports and those instrumentally determined events that have magnitudes of 4.0 or greater. Data from 1961 through 1963 were compiled by personnel at UNR, using locations from the University of California at Berkeley, supplemented by readings from a station operating in Reno, and two installed in Golconda and Tonopah in 1962. Data from 1964 through 1969 was provided by the U.S. Geological Survey, Denver in 1988, from the database used for a seismicity map of North America (Engdahl and Rinehart, 1988; 1991). Since 1970, locations have been determined by UNRSL personnel from stations operated by the UNRSL, and have been published in periodic bulletins (e.g., dePolo et al., 1992). Two other catalogs that include Great Basin earthquakes are available: the Utah region (e.g., Smith and Arabasz, 1991; Arabasz et al., 1992) and the southern Great Basin in the region surrounding the Nevada Test Site (Gomberg, 1991; Rogers et al., 1987; Gawthrop and Carr, 1988). The Gawthrop and Carr (1988) catalog overlaps the UNRSL catalog for the time period 1931 to 1974 and uses some of the same sources (in particular, the Slemmons et al. 1965 catalog is used as a base). A full analysis of Basin and Range hazard would include these catalogs; we have not included them in our analysis because the catalogs would need to be treated separately, as we have already had to do for the Mammoth/Mono region within the UNRSL catalog. Arabasz (personal comm., 1993) is performing such an analysis for the Utah catalog. Figure 1 shows sample station configurations at the beginning of the network (1970 to 1974) and at present. Although the number of stations in the network has increased, the locations have moved further west so that coverage is now concentrated more in the California/Nevada border region than it was 20 years ago. Since the Mammoth Lakes earthquakes in 1980 and the subsequent earthquake sequences, the UNRSL network has been concentrated in the Mammoth Lakes and Mono Lake region; consequently, the seismicity in the UNRSL catalog has been similarly concentrated. With the variable spatial coverage and completeness levels for this region and for regions outside it, it is necessary to separate the catalog to achieve stable results in the analysis. 1912 M . K . S A V A G E A N D D. M, D E P O L O