Maximum Magnitudes of Earthquakes in Geothermal Fields?
Abstract:We examine California geothermal fields to determine if historical earthquakes may yield evidence of a maximum magnitude limit for future events. An earthquake catalog presents a limited past sample of a population of earthquake magnitudes, whose properties can be extrapolated. For a catalog of events at each geothermal field, we postulate a limiting maximum magnitude, and ask if it is consistent with the data. If enough earthquakes near the limit should have occurred but did not, we reject the suggested magnitude limit with confidence. For a Poisson process, we are unable to reject a limiting magnitude unless the expected number of events is >3 and the number observed is zero.
Our search has begun for an earthquake catalog with a finite magnitude limit, based on real earthquake data. McGarr (2014) suggests that maximum magnitudes for injection-induced earthquakes are constrained by volume of fluid injected, and that future induced earthquake magnitudes may be limited by this relationship. For appropriate catalogs, we evaluate whether McGarr’s hypothesis may be explained by an insufficient number of events. When catalogs do not provide for a maximum magnitude limit, we will extrapolate current magnitude-frequency distributions to obtain future probabilities of occurrence for specific magnitudes over a given time period.