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U.S. GEOLOGICAL SURVEY, 345 MIDDLEFIELD ROAD, M.S. 977, MENLO PARK, CALIFORNIA 94025
EARTH PHYSICS BRANCH DEPARTMENT OF ENERGY, MINES AND RESOURCES, 1 OBSERVATORY CRESCENT, OTTAWA, ONTARIO, Canada
Abstract
We have estimated the source parameters of 53 microearthquakes recorded in July 1983 which were aftershocks of the Miramichi, New Brunswick, earthquake that occurred on 9 January 1982. These events were recorded by local three-component digital seismographs at 400 sps/component from 2-Hz velocity transducers sited directly on glacially scoured crystalline basement outcrop. Hypocentral distances are typically less than 5 km, and the hypocenters and the seven digital seismograph stations established all lie essentially within the boundaries of a granitic pluton that encompasses the faults that ruptured during the main shock and major aftershocks. The P-wave velocity is typically 5 km/sec at the surface and at least 6 km/sec at depths greater than about 1 km.
The events have S-wave corner frequencies in the band 10 to 40 Hz, and the calculated Brune model seismic moments range from 1015 to 1018 dyne-cm. The corresponding stress drops are generally less than 1.0 bars, but there is considerable evidence that the seismic-source signals have been modified by propagation and/or site-effects. The data indicate: (a) there is a velocity discontinuity at 0.5 km depth; (b) the top layer has strong scattering/attenuating properties; (c) some source-receiver paths differentiate the propagated signal; (d) there is a hard-rock-site P-wave "fmax" between 50 and 100 Hz; and (e) some hard-rock sites are characterized by P-wave resonance frequencies in the range 50 to 100 Hz. Comparison of this dataset with the January 1982 New Brunswick digital seismograms which were recorded at sites underlain by several meters of low-velocity surface sediments suggests that some of the hard-rock-site phenomena listed above can be explained in terms of a layer-over-a-half-space model. For microearthquakes, this result implies that spectrally determined source dimension scales with site dimension (thickness of the layer). More generally, it emphasizes that it is very difficult to accurately observe the details of seismogenic processes from the earth's surface.
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