Evidence for precursory changes in the frequency-magnitude b-value

Abstract

Summary. A clear case has been established for a significant increase in the slope of the frequency-magnitude relation for periods of up to six years before a number of earthquakes in New Zealand and California. In a region of about 200 km diameter in central New Zealand the b-value increased from near unity to more than 2.0 at the end of 1968, and remained abnormally high until the beginning of 1975. An earthquake of magnitude 6.0 followed in 1977 January, within 50 km of the location which had registered the highest b-value. The anomaly was detected in contour maps of b-values windowed in space and time, then resolved more precisely in time using the CUSUM technique. It was statistically significant at a level higher than 99.99 per cent. Compared with the average rate of moment release in this region, there was a deficit of 1.3 x 10'*Nm during the anomalous period, which is equivalent to an earthquake of magnitude 6.08. This phenomenon of a high b-value has been found before all shallow New Zealand earthquakes of magnitude 6.0 or more since 1965, wherever the seismicity rate and detection threshold were adequate to resolve it. Further, the only statistically significant occurrences of high b-value were those which were followed by large earthquakes, two of magnitude 6.8 and 6.0 and a swarm of equivalent magnitude 5.3. The phenomenon has also been found before the San Fernando and Coalinga earthquakes in California, both of which were thrust events, but it is not apparent before two large Californian earthquakes with strike-slip mechanisms. Observations of high b-value are consistent with reported instances of quiescence in background seismicity. In fact, what has been interpreted as quiescence may probably be more accurately described as a localized high b-value. The duration of the anomaly is not a simple function of magnitude. Different strain rates in the regions examined may account for this. The available data suggest that careful monitoring of the b-value may provide a tool for medium-term earthquake prediction, although perhaps not in all tectonic environments.