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Teleseismic observations of the 1976 Friuli, Italy earthquake sequence

SEISMOLOGICAL LABORATORY DIVISION OF GEOLOGICAL AND PLANETARY SCIENCES CALIFORNIA INSTITUTE OF TECHNOLOGY, PASADENA, CALIFORNIA 91125

Abstract

Teleseismic long-period body and surface waves radiated by the May 6, 1976 Friuli, Italy earthquake and its principal aftershock of September 15, 1976 (09h 21 m) are studied to determine source characteristics. Focal mechanisms along with geological evidence suggest that both events represent the underthrusting of the Friuli Plain beneath the southern Alps. The depths of both earthquakes, estimated by matching synthetic body-wave seismograms to observations, are found to lie between 6 and 10 km. It is not possible to discern any evidence of source directivity in the observed main shock body waves. Synthetic seismogram calculations which include rupture effects suggest that the fault length of the main shock could not be much larger than 16 km for a unilaterally propagating fault or about 24 km for a symmetrical rupture assuming a rupture velocity of 3.0 km/sec. Observations of 100-sec Rayleigh waves confirm the focal mechanism deduced from body waves, but suggest that the seismic moment of the main shock is 5 x 10²⁵ dyne-cm compared to 2.9 x 10²⁵ dyne-cm estimated from body waves. The P-wave moment of the aftershock is 1 x 10²⁵ dyne-cm. The aftershock studied in this paper is one of four large events which occurred in the epicentral area of the main shock more than 4 months after the main shock. These aftershocks had a combined seismic moment over twice that of the main shock. It is suggested that these earthquakes represent deformation in a different part of the seismic zone than the main shock. Large earthquakes with series of severe aftershocks are well known in the historical record of the Friuli region. The average displacement and stress drop are estimated to be 33 cm and 12 bars for the main shock and 33 cm 24 bars for the aftershock.

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