Faulting Parameters of Earthquakes (4.1 <= ML <= 5.3) in the Peninsular Ranges of Baja California, Mexico

Abstract

Seismic moment ( M ) is a parameter related to the physical process that takes place at the source (Aki 1966; Aki and Richards 1980). Sometimes it is possible to calculate M from the constant spectral amplitude level at low frequencies, but in other cases this is not possible. For instance, for M > 4.0 earthquakes (recorded at short distances), it is difficult to determine the flat portion of the spectrum due to instrumental limitations. For these cases, waveform modeling is an alternative way to evaluate M . In addition to determining this source parameter, it is also possible to obtain the faulting geometry and the depth of the hypocenter through waveform modeling. In the Peninsular Ranges of Baja California (PRBC), few studies have been conducted to determine earthquake faulting parameters. Some examples are the studies of Nava and Brune (1983) (1975 ML 5.1 earthquake) and Gonzalez (1987) (1985 ML 5.0 earthquake). However, studies of the seismic source have not been attempted for earthquakes of lower magnitude ( M < 5). We computed the faulting parameters of five small-to-moderate earthquakes that occurred between 1991 and 1996 in the PRBC, via waveform modeling of broadband seismograms recorded at regional distances (180 to 245 km). Our modeling was important in constraining the fault plane orientation obtained from first-motion polarities recorded for those earthquakes. In this modeling, we considered epicenter-to-station paths of the Peninsular Ranges of southern California and Baja California tectonic regions. Historically, moderate size earthquakes (5 ≤ M ≤ 7) have occurred in the PRBC, mainly along the San Miguel fault. Examples of these earthquakes are the 1954 earthquakes ( M 6.0 and 6.4; Leeds 1979) and the 1956 earthquakes ( ML 6.8, 6.4 and 6.3; Shor and Roberts 1958). Castro (1983) used synthetic seismograms to calculate the seismic …