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 f rom 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. W ithin 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 a nd 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 c hanges 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 c hange. 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 earthqu akes. In Asia, events from 1958 to 1994 also delineate a similar chang e near M-c 7.0. Earthquakes in Asia of 7.0 less than or equal to M-w l ess 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 w as close to failure, i.e., near a self-organized critical state, durin g the earlier period. We associate those changes with the giant Himala yan earthquake in 1950 of M-w 8.6. Regional variations in seismic stra in release rates correlate with the types of processes at adjacent pla te 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.