Determination of crustal structure from phase velocity of Rayleigh waves. Part II: San Francisco Bay region

摘要

The phase velocity method of measuring crustal thickness has been successfullyudapplied in southern California. Phase velocity of dispersed Rayleigh waves fromuddistant earthquakes is determined locally by use of a tripartite array of seismographudstations. Local crustal thickness is obtained by comparing the observed phaseudvelocity with an experimentally determined curve representative of the averageudcontinental crust.udIn this paper we make use of the phase-velocity data of Evernden to determineudcrustal thickness in the San Francisco Bay region of California. Evernden determinedudthe phase velocity of Rayleigh waves entering the North American continentudfrom the Pacific Ocean. He was primarily interested in studying the direction ofudapproach of these waves. His tripartite array consisted of stations at Berkeley,udSan Francisco, and Palo Alto, and a number of earthquakes were studied so that audlarge variation in direction of approach could be obtained. The data for all theudearthquakes were combined to obtain an average phase velocity for each period.udThese phase velocities are plotted in figure 1, where the region to which the dataudapply is also shown. Also plotted in figure 1 are phase-velocity curves for a 25-km.,ud35-km., and 45-km. crust having the same composition as the average crust ofudAfrica. The method by which these curves have been derived is explained in Part Iud(see fn. 1, above). By interpolating between the curves one can use each phase-velocityuddetermination to obtain a value of crustal thickness. Neglecting the lowestudthree points which fall outside the range permitted by the phase-velocity curves,udone finds a mean value of 30 ± 1 km. for crustal thickness in the San Francisco Bayudregion. The three points which were excluded fall in a period range where experimentaludphase-velocity determinations are difficult to make. Moreover, for theseudshort periods the phase velocity for the oceanic segment of the path differs greatlyudfrom that for the continental segment, with the result that refraction effects areudmost pronounced.