Digital recordings of aftershocks of the April 25 and 26, 1992, Cape Mendocino, California, earthquakes

Abstract

On April 25 and 26, 1992, three large earthquakes occurred near Cape Mendocino, Calif. The mainshock occurred on April 25 with M s = 7.1, the next day followed by two powerful aftershocks of M s = 6.6 and M s = 6.7. The permanent seismographic network operated by the U.S. Geological Survey located 197 aftershocks, greater than magnitude 3 during the following 3 weeks (Andrew Michael, personal commun., 1992). Most of them were located offshore (Michael, 1992, fig. 1). On April 27,1992, the author began operation of five portable digital seismographs located in a rough semi-circle approximately 50-90 km from the aftershock (fig. 1). The seismographs were operated for 12 days. The primary research goal was to contribute to aftershock locations with P- and S-arrival times and to record data to improve focal mechanism studies of the aftershocks. These goals were particularly important because the earthquakes originated in an especially complex, poorly understood tectonic regime. This report is intended to facilitate the use of the digital seismograms by other researchers. Tables 1A and B give the location and sensor specifications for each recording station. Appendix A is a narrative of road directions to the recording stations should they need to be reoccupied. Appendix B lists all of the trigger times of seismograms recorded by the network. Appendix C contains plots of the seismograms of earthquakes recorded by three or more seismographs. Appendix D is an example data file showing the format (DR-100). The data set is available directly from the author on 9-track tape.