Gerstenberger_et_al-2001-Geophysical_Research_Letters_bvalue_hypothesis

Gerstenberger_et_al-2001-Geophysical_Research_Letters_bvalue_hypothesis

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GEOPHYSICAL RESEARCH LETTERS, VOL. 28, NO. 1, PAGES 57-60, JANUARY 1,2001 A systematic test of the hypothesis that the b value varies with depth in California Matt Gerstenberger, Stefan Wiemer and Domenico Giardini Institut fiir Geophysik, Eidgen6ssische Technische Hochschule H6nggerberg, Ziirich, Switzerland. Abstract. We spatially mapthe shallow to deep b value ratio in the crust in California. Previous studies of the frequency magnitude distribution, as a function of depth, for selected crustal regions indicated that b decreases from b > 1.1 in the 0-5 km depth range to b < 0.8 in the depth range 7-15 km. Our detailed mapping confirms that this pattern can be estab- lished at the 99% significance level for about 32% of the entire seismically active crust.About 2% of the crust displays the opposite b-gradient. One such areais the San Francisco Bay Area. We speculate that differences in stress levels are the main factor controlling the depth dependency of b. These re- sults confirm that the b value should not always be considered a constant in studies such as seismic hazard estimations. Introduction The relative size distribution of earthquakes is an essential input parameter needed to perform probabilistic hazard analysis. Along with the activity rate, this size distribution determines how frequently earthquakes of a given size are to be expected in a given time interval. A simple equation, commonly known as the Gutenberg-Richter Law [Gutenberg and Richter, 1944; Ishimoto and lida, 1939], log•oN = a-bM, relates three parameters, the number of earthquakes N, their activity measured by a, and their size distribution described by b. Every seismicity map shows clearly that the activity rate varies spatially and temporally; however, there is no consensus on the questions of whether or not significant spatial or temporal variations also existin the relative sizedistribution of events. Some seismologists [Kagan, 1999; Frohlich and Davis, 1993] argue that the b valueis essentially constant, and casestudies showing variability of b suffer from errors in the data and a lack of statistical rigor. However, other authors have pointed out that if sufficiently high quality data are available, at least the spatial variability of the b values can be established to high significance level for various tectonic regimes [Mori and Abercrombie, 1997; Amelung and King, 1997; Wiemer and Katsumata, 1999; Wiemer et al., 1998; Wiemer and Wyss, 1997]. In this paper we address one aspect of this discussion by testing the following hypothesis: The b value for the shallowest part of the crust is significantly higher thanfor the bottom part of the crust. This hypothesis was originally proposed by Wyss [1973], and further studied by Wyss and Wiemer[1997], who showed a characteristic dependency of b on hypocentral depths for the Parkfield and Morgan Hill Copyright 2001 bythe American Geophysical Union.
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  • Spring '16
  • Southern California, Wiemer, California Earthquake Data

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