Seismicity of California's North Coast

Aftershocks and Triggered Events of the Great 1906 California Earthquake

“…Determining accurate epicenters for earthquakes external to any local network is difficult (Lee and Stewart, 1981); error ellipses for such epicenters typically are elongated with major axes perpendicular to the near edge of the network. Similarly, the confidence contours for location from intensity data for offshore earthquakes generally are elongated perpendicular to the coast, the edge of the network of MMI observation sites (Bakun, 2000). The inability of the method to usefully con -126˚W 125˚W 124˚W 123˚W 122˚W 121˚W 120˚W 38˚N 39˚N 40˚N 41˚N 42˚N 43˚N 6 126˚W 125˚W 124˚W 123˚W 122˚W 121˚W 120˚W 38˚N 39˚N 40˚N 41˚N 42˚N 43˚N 0 strain the source locations notwithstanding, Bakun (2000) demonstrated that M I 's at the (instrumentally determined) epicenters of offshore earthquakes generally agree with the instrumental magnitudes.…”

“…While the method is useful for most onshore and near-coast offshore events, Bakun (2000) established that the confidence contours for location generally fail to usefully constrain the source regions for earthquakes located more than a few tens of kilometers offshore. Bakun substantiated this problem while analyzing earthquakes off California's north coast, and it seems logical that this problem would exist any time an epicenter is far offshore Note that the 95% confidence-level contour is off the map; the entire area shown is within the 95% confidence contour .…”

“…The method can be summarized in the following three steps: Meltzner and Wald (1999). The trial source location for which rms[M I ] is minimum is the point source of seismic energy that best satisfies the available intensity data (Bakun, 2000). This location, called the "intensity center," corresponds more to the moment centroid than to the epicenter (Bakun, 1999a).…”

The Tejon Pass Earthquake of 22 October 1916: An M 5.6 Event on the Lockwood Valley and San Andreas Faults, Southern California

“…The level of confidence can be assigned to each contour based on the number of MMI observations (e.g., table 1 in Meltzner and Wald [1999]). The trial source location for which rms[M I ] is minimum is the point source of seismic energy that best satisfies the available intensity data (Bakun, 2000). This location, called the intensity center, corresponds more to the moment centroid than to the epicenter (Bakun, 1999).…”

“…The magnitude associated with a particular trial source location can be read from the magnitude contours for the grid, which appear as dotted black lines in Figure 5. In cases where the epicenter is known, M I at the epicenter is the best estimate of M W for the earthquake (e.g., Bakun, 2000). The statistical uncertainty in M W appropriate for the desired level of confidence is taken from Bakun and Wentworth (1999) and is listed in Table 2.…”