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Abstract

Northern California (39°N to 42°N, as shown in Figure 1) is a geologically complex region that exhibits a wide variation in its characteristic seismicity. The rate of seismicity varies in zones that are approximately delineated by the major structural provinces shown in Figure 2. The seismicity rate varies by a factor of approximately 20, from very low in the northeastern corner of California, to high in the vicinity of the Gorda Escarpment and Gorda Basin just off the coast.

This paper discusses the general characteristics of the seismicity in the region recorded by the University of California Seismographic Stations (1949–1985), the U.S. Geological Survey Calnet (1980–1986), and numerous special studies.

The seismicity list is from two primary sources: Bolt and Miller (1975) prior to 1973, and the Bulletin of the Seismographic Stations (U.C. Berkeley) for 1973 through 1985. The 1949 starting date was chosen for two reasons. First, the installation of a high-sensitivity Benioff seismograph at Mineral (MIN in Fig. 2) in 1949 allowed routine detection and location down to ML 3.0 in the region. Second, the Wood-Anderson torsion seismographs at Mineral and Arcata (ARC) provided a uniform method for determining the local magnitude. Since 1949 the number and quality of seismographic stations has increased in northern California, due primarily to the interest of several groups in either regional monitoring or special studies. Most notably, the USGS has expanded the central California network northward during the past 20 years. This data is used as a supplement to the primary source.

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