Frequency-magnitude relationships are determined for a source region defined by a circular epicentral area of 50 km radius centered on the Carnegie broadband seismograph at KMU in the southeast corner of Hokkaido Island, Japan. Within this region, bounded by a maximum depth of 125 km, more than 11,100 earthquakes were detected and cataloged by the Hokkaido University network during the period July 1976 through January 1987. The seismicity is divided into two effectively decoupled suites: crustal (Eurasian plate) and subduction (Pacific plate). The frequency-magnitude recurrence distributions exhibit departures from linearity which are statistically significant below md = 2.5 for the crustal data suite and md = 3.5 for the subduction data suite. Three independent tests show the nonlinearity is not caused by incompleteness; the catalog is complete down to magnitude 2.0 or less for both data suites. These observations are therefore inconsistent with the linear Gutenberg-Richter relationship, which is assumed to be valid over any magnitude range for which the data are complete. There are relatively more large events and fewer small events in the subduction suite. The logarithm of the ratio of the number of crustal events to the number of subduction events versus magnitude is monotonically decreasing and remarkably linear over the magnitude range 1.0 to 5.0. This suggests that the observed nonlinearity of these two frequency-magnitude relationships has a common origin.

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