We tested the hypotheses (1) that the fractal dimension, D, of hypocenters are different in a locked and a creeping segment of the San Andreas fault and (2) that the relationship D ≈ 2b holds approximately, where b is the slope of the frequency-magnitude relationship. The test area was the 30- to 50-km fault segment north of Parkfield for which two earthquake catalogs exist: the borehole High Resolution Seismic Network data, and the U.S. Geological Survey data, which have a minimum magnitude of completeness of MC 0.4 and MC 1.0-1.2, respectively. The relative location errors in the two catalogs are estimated as 0.25 km and less than 1 km, respectively. The periods of high-quality data available extend from 1987 to 1998.5 and 1981 to 2000.2, respectively, furnishing 2609 and 3775 events for analysis, in the two catalogs. In the locked part, 0.5 < b < 0.7 and 0.96 < D < 1.14, whereas in the creeping segment, 1.1 < b < 1.6 and 1.45 < D < 1.72. However, the spatial distribution of the hypocenters in the creeping segment is not well approximated by a fractal distribution. We conclude (1) that the frequency-magnitude distribution as described by b, as well as the fractal dimension (D), are different in the locked and creeping segments near Parkfield; (2) that the spatial distribution in the creeping segment is not well approximated by a fractal distribution; and (3) that the relationship D ≈ 2b holds in the locked segment, where both parameters can be measured accurately. Thus, we propose that the heterogeneity of seismogenic volumes lead to differences in D and b and that these differences, where established by high-quality data, may furnish clues concerning properties of fault zones.

You do not have access to this content, please speak to your institutional administrator if you feel you should have access.