Frequency of occurrence of moderate to great earthquakes in intracontinental regions: Implications for changes in stress, earthquake prediction, and hazards assessments

We investigate the departure of shallow intracontinental earthquakes from the frequency-magnitude relationship log ((n) over dot(M)) = a - b M. We consider seven large continental regions but exclude subduction zones. In each, ''active'' and ''stable'' areas are differentiated. Within active areas we separately categorize earthquakes on faults with high slip rates. We use data sets from 1978-1994 and 1900-1994, which are complete for moment magnitudes M-w greater than or equal to 5.3 and greater than or equal to 7.0, to calculate changes in b value with M-w. For all active intracontinental regions combined except Asia, b changes at 98% confidence from 0.90 to 2.1 at a corner magnitude M-c of 6.9 to 7.0. The distribution for stable regions indicates a similar change. Values of M-c are associated with downdip widths of rupture of about 23 to 27 km, which are smaller than those for subduction zones. In each case, M-c marks a transition from unbounded to bounded earthquakes. In Asia, events from 1958 to 1994 also delineate a similar change near M-c 7.0. Earthquakes in Asia of 7.0 less than or equal to M-w less than or equal to 8.1 from 1900 to 1957, however, define a b value of 1.0 and a moment release rate 5.6 times higher than that from 1958 to 1994. Those anomalous values indicate that a large region of Asia was close to failure, i.e., near a self-organized critical state, during the earlier period. We associate those changes with the giant Himalayan earthquake in 1950 of M-w 8.6. Regional variations in seismic strain release rates correlate with the types of processes at adjacent plate boundaries, Extrapolating rates of activity of the past 95 years to M-w greater than or equal to 8.0 indicates that such events should be very rare in stable areas.