Source studies over a wide range in earthquake magnitude

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Virginia Polytechnic Institute and State University

The concept of similarity (that earthquake source parameters obey scaling relations) and the empirical linear relation between magnitude and the log₁₀ of the number of events (the Gutenberg-Richter relation) describe well the behavior and recurrence of many earthquake data sets. The universality of these relations are tested herein using a suite of earthquakes from the southeast comer of Hokkaido Island, Japan. Within this active seismic region, over 11,100 events ranging in magnitude from 0 to 7.1 were cataloged by the Hokkaido University network in the period 1976-1986 with epicentral distances of less than 50 km from the Carnegie broadband station KMU. Two subsets of the events are examined herein: crustal earthquakes, those with locations shallower than 45 km and above the top of the subducting Pacific plate, and subduction earthquakes, those with locations below 60 km within the subducting plate.

The frequency of occurrence versus magnitude relations for both the crustal and subduction events are nonlinear with a definite decrease in the number of detected events for lower magnitudes, but the subduction events have proportionally more large earthquakes and fewer small earthquakes than the crustal data suite. A completeness analysis indicates that the catalogs are complete to less than magnitude 2, which is clearly in the nonlinear region, suggesting that the observed curvature of the frequency-magnitude curves is not due to incompleteness of the catalogs. Hence, a single, linear Gutenberg-Richter relation is inadequate for describing the frequency of occurrence of these events. The ratio of the frequency-magnitude curves gives a remarkably linear relation from magnitude 1 to magnitude 5, indicating that in terms of fitting these frequency-magnitude curves to higher order polynomials, the crustal and subduction data sets have identical higher order coefficients, and their curvature difference is caused by only the constant and linear coefficients. A possible cause for the difference in the recurrence relations is the increased lithostatic load with depth.

In an attempt to gain insight into the frequency of occurrence characteristics of the data, the seismic energy release of the crustal and subduction regions was calculated as a function of time. Evaluation of the energy release versus time indicates that there was a precursory energy decrease prior to the mL = 7.1 event in 1982. Analysis of energy release appears to be a potentially useful and relatively objective technique for studying precursory quiescence.

Using data from the Carnegie broadband station KMU, seismic source scaling relations were derived for 21 crustal and 24 subduction events. Using Q-corrected SV and SH amplitude spectra and assuming an average focal mechanism, spectral parameters (zero frequency level and comer frequency) were estimated using the objective technique of Snoke (1987). Cepstral filtering was employed both to remove the effect of multiple arrivals, as well as to increase the objectivity with which parameters were determined. The resulting moment versus magnitude relations indicate a significant change in slope around magnitude 3.5 and moment 2 x 10²⁰ dyne-cm. Brune radii average 0.3 km over the range 10¹⁸ to 10²¹ dyne-cm, and increase from 0.6 to 2 km over the range 10²² to 10²⁵ dyne-cm. Log Brune stress drop was found to be linearly correlated with log moment with a slope of approximately unity below 10²¹ dyne-cm, and highly correlated but with a slightly smaller slope above that point. For lower moments, stress drops increase with moment from 0.035 bar to 10 bar, and for higher moments, stress drops range from 10-822 bar, with most values near 100 bar. These variations of radius and stress drop with moment for moments below 10²¹ dyne-cm is inconsistent with the similarity hypothesis.

Stress drop versus moment relations were compared with those from a study for the Matsushiro region, Japan, which is characterized by a shallow, localized crustal seismicity. No significant difference is found between the scaling relations for the crustal earthquakes, subduction earthquakes, and Matsushiro earthquakes taken separately, even though the tectonic stress is expected to be quite different in the three regions. We conclude that the calculated scaling relations are not directly determined by the tectonic stress.