Seismic Source Processes and Tectonics: Observations of Four Intracontinental Earthquakes

Citation

Cipar, John Joseph (1981) Seismic Source Processes and Tectonics: Observations of Four Intracontinental Earthquakes. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/bwpb-4c04. https://resolver.caltech.edu/CaltechETD:etd-09152006-102853

Abstract

This thesis presents studies of the source processes of four shallow earthquakes and their relation to regional tectonics. In the first chapter, long-period teleseismic P and S waves from the Haicheng, China earthquake of February 4, 1975 are compared directly to time domain synthetic seismograms to infer source parameters. The P-wave focal mechanism indicates that faulting was dominantly left-lateral strike-slip along a northwest striking nodal plane (strike = 288°, dip = 78°N, rake = 342°). The strike of this nodal plane agrees with the trend of the aftershock distribution. Azimuthal variation of P-wave duration is attributed to fault rupture 22 km in a northwesterly direction, along strike of the aftershock zone. There is considerable discrepancy between the observed SH waves and synthetics computed using this model. These discrepancies are due to either structural complexities in the source region or change in fault mechanism as the rupture propagated along strike. Seismic moment, average dislocation and stress drop are computed to be 2.7 x 10²⁶ dyne-cm, 2.5 meters and 48 bars, respectively.

The remaining three chapters present a detailed examination of seismograms recorded by the 1976 Friuli, Italy earthquake (May 6, 1976, M_S = 6.5) and two major aftershocks (both on September 15, 1976 at 03h 15m, M_S = 6.0 and 09h 21m, M_S = 5.9). Teleseismic long-period body waves and surface waves radiated by the mainshock and 09h 21m aftershock are studied in Chapter Two to determine source characteristics. Focal mechanisms along with geological evidence suggest that both events represent 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. Synthetic seismogram calculations which include source directivity effects suggest that the fault length of the mainshock is approximately 16 to 24 km assuming a rupture velocity of 3.0 km/sec. Observations of 100 sec Rayleigh waves confirm the body wave focal mechanism, but suggest that the seismic moment of the mainshock 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.

In Chapter Three, short-period records are modeled to obtain additional details of the source time history. Two point sources of radiation are required to adequately model the aftershock short-period records. For the 09h 21m aftershock, the model derived from short-period records also produces good fits to the long-period data. The SP model for the 03h 15m aftershock, on the other hand, predicts long-period synthetics which do not agree with the observations. In particular, the SP moment (0.37 x 10²⁵ dyne-cm) is about 2-1/2 times smaller than the LP moment (1 x 10²⁵ dyne-cm). Adding a long-period component to the SP model considerably improves LP waveform and moment agreement. In the case of the mainshock, a reasonable fit to the observed SP data is obtained using three point sources of radiation. However, LP synthetics computed using this model do not agree with the observations, and the SP moment (0.65 x 10²⁵ dyne-cm) is a small fraction of the LP moment (3-5 x 10²⁵ dyne-cm). Time function durations indicate that the individual events inferred from the SP records are radiated from patches of the fault having radii of 2 to 4 km and stress drops in the range 35 to 276 bars. In comparison, overall stress drops estimated from LP data are found to be 12 bars (mainshock) and 24 bars (09h 21m aftershock). Strong-motion accelerograms are used to put additional constraint on the source geometry of the 09h 21m aftershock.

The 03h 15m and 09h 21m aftershocks are the culminating events of a series of large aftershocks which began on September 11, 1976. Cumulative seismic moment of the Friuli aftershock sequence was as large as the moment released by the mainshock. By comparison, aftershock moments of California earthquakes are typically 1 to 10 percent of the mainshock moment. The large size, location and focal mechanism of the aftershocks suggest that they represent failure of major stress concentrations remaining after the mainshock.

Thesis Files