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Apparent stresses, stress drops, and amplitude ratios of earthquakes preceding and following the 1975 Hawaii M_S = 7.2 main shock

COOPERATIVE INSTITUTE FOR RESEARCH IN ENVIRONMENTAL SCIENCES AND DEPARTMENT OF GEOLOGICAL SCIENCES UNIVERSITY OF COLORADO/NOAA, BOULDER, COLORADO 80309

Abstract

S-wave spectra for approximately 300 earthquake signals (2.8 M_L 3.9) were obtained from hand-digitized Wood-Anderson records from Hilo, Hawaii. All sources were located within part of the rupture volume of the 1975 Hawaii M_S = 7.2 earthquake. The epicentral distance to Hilo was about 45 ± 3 km, the backazimuth was 180° ± 10° (south), and the sources were located at depths of 9 ± 1.4 km. The apparent stress and Brune stress drop, based on 3-sec windows of the S waves, were examined for possible temporal and spatial dependence. Most of the data showed a remarkable constancy of the average apparent stress and stress drop per event as a function of time. No measurable change in average signal content took place at the time of the main shock. A statistically significant difference in the average stress parameters per event was found between neighboring subvolumes (radius 5 km) of the 1975 aftershock zone: a volume previously proposed as asperity based on seismicity pattern interpretation contained earthquakes with higher apparent stresses of =9.0±8.8 bars compared to the neighboring volume with = 4.2 ± 6.1 bars. Seventy-eight spectra obtained from a second station showed the same difference between the two volumes as that found at station Hilo, suggesting that the signal differences should be interpreted in terms of source differences. In addition, amplitude ratios of SV/P waves measured at Hilo showed that a systematic difference of 10° in fault plane orientation existed during years before the 1975 main shock. This agrees with the rotation of the fault plane during the main rupture reported elsewhere. We hypothesize that heterogeneities along the fault plane with dimensions of about 5 km cause changes in fault plane dip, differences in average stress release per earthquake, strong contrasts in seismicity patterns, and differences in aseismic strain release by fault creep. These same heterogeneities in the fault plane apparently also control the location of major earthquakes and the complexity of their ruptures.

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