Regional variations of seismic source parameters in the northern Baja California area

The basic observation of this study is that seismograms written in southern California from earthquakes in the northern Gulf of California are clearly different from the seismograms of most northern Baja California events. The principal visual difference is the large high-frequency amplitudes of the Baja sources as compared with the great long-period excitation seen on gulf seismograms. This difference is interpreted as a source effect and reflects the relatively small source dimensions and low seismic moments of the Baja earthquakes. The differences in the seismograms could result from a near-source or propagation-path effect, but the great similarity between the paths traversed and the large magnitude of the observed difference argue against this explanation. Some exceptional north Baja sources with large moments and source dimensions demonstrate that very different seismograms are produced by sources with almost identical propagation paths. Preliminary examination of seismograms from events within southern California suggests the same types of regional variations there. The northern Baja sources characteristically have dimensions that are at least a factor of 4 smaller and seismic moments that are an order of magnitude less than the same parameters for events of similar local magnitude within the gulf. Baja events have stress drops ranging up to 100 bars, whereas the average stress drop for the gulf data is near 2 bars. For ML < 5, northern Baja California earthquakes have source dimensions and surface-wave amplitudes that in some cases are only about a factor of 2 greater than source dimensions and surface-wave amplitudes for underground nuclear explosions of comparable magnitude.