Earthquake Source Properties Research Articles

Rupture complexity of a moderate-sized (mb6.0) earthquake: Broadband body-wave analysis of the North Yemen earthquake of 13 December 1982

AbstractThe occurrence of the North Yemen earthquake of 13 December 1982 provides an important opportunity to study source properties of earthquakes in the southern Arabian Peninsula. Although moderate in size (mb 6.0), this earthquake is the largest to have occurred in the region since the deployment of the Global Digital Seismograph Network. P waves recorded at teleseismic distances can be obtained from stations of this network and of the Graefenberg array that are flat to displacement and velocity in the frequency range from 0.01 to 5.0 Hz. The broad bandwidth of the data is necessary for deriving a dynamic description of the earthquake as well as the static properties of the source. In contrast, inversions using only long-period data provide much less insight into the complexity of rupture. By fitting the displacement pulse shapes with synthetics, we find that the Yemen earthquake was a complex rupture consisting of two events that occurred about 3.0 sec apart. The depth of both hypocenters is estimated to be 7 km. The focal mechanisms of the events each have a nodal plane that strikes 340° and dips 60° to the east. However, the slip directions of the focal mechanisms are in slightly different directions, 230° and 300°, respectively. The total moment of the earthquake is 3.0 ×1025 dyne-cm, with the second event having three times the moment of the initial event. By inverting P-wave arrival time differences at each station, we find the second hypocenter is located 7.5 km from the initial hypocenter in a direction that is nearly colinear with the NNW-striking nodal plane of the focal mechanisms of each event. The trend of extensional ground cracks in the epicentral region and the predominant strike of composite focal mechanisms of locally recorded aftershocks are also in a NNW direction. From the consistency of these observations, we infer that the east-dipping NNW-striking nodal planes are the fault planes. The static stress drop of the first and second events are 44 and 31 bars, respectively. Inverting the energy flux in the velocity waveforms, we find the energy radiated by the earthquake is 4.8 ×1020 dyne-cm. This implies an average apparent stress of 4 bars.At different times during the aftershock sequence, seismicity tended to cluster about one or another of two different planes which, although having roughly the same strike, had conjugate dips. One of these planes, delineated late in the aftershock sequence, coincides with the inferred fault plane for the main shock. The complexity of the main shock and of the aftershock sequence suggests that strain accumulated on a system of strongly interdependent faults. As the region became critically loaded, the effect of a major rupture was to critically stress faults adjacent to the initial nucleation. The second event of the main shock represents the rapid release of stress from the fault system, while the ensuing aftershock sequence represents the gradual release of stress.

Moment tensor inversion of complex earthquakes

Summary. Moment tensor inversion methods can be applied with success in the determination of source properties of simple earthquakes. However, these methods utilize the assumption of a point source, which is inadequate for modelling many complicated, shallow earthquakes. For complex earthquakes, an inversion using finite faulting models is desirable but the number of parameters involved requires that a good starting model be found or that independent constraints be placed on some of the parameters. A method is presented for low-pass filtering both the data and Green's functions, passing only signals with wavelengths greater than the dimension of the entire fault. The filter tends to smooth complications in the waveforms and allows application of the point source moment tensor inversion. This method is applied to body waves from the 1978 Thessaloniki, Greece, earthquake, the 1971 San Fernando earthquake and to a multiple-point source synthetic model of the San Fernando event. For the Thessaloniki event, although a multiple-source mechanism has been suggested, inversion results before and after filtering were essentially identical, indicating that a point source mechanism is sufficient in modelling the long-period, teleseismic body waves. In the case of the San Fernando earthquake, the point source Green's functions were incapable of simultaneously modelling the P- and SH-waves. Inversion of P-waves alone resulted in extreme parameter resolution problems, but allowed constraint in one axis of the moment tensor and suggested an overall source time function. Inversion of a synthetic San Fernando data set yielded similar results, but allowed an investigation of the shortcomings of the method under controlled circumstances. Although the results may require substantial interpretation, the method presented represents a simple first step in the analysis of complex earthquakes.

Source Properties of Shallow Earthquakes in New Zealand and their Tectonic Associations

Summary Seismic moments estimated from the surface-wave parameter AR, together with local magnitudes ML have been used to determine the source parameters of 270 shallow New Zealand earthquakes between 1965 and 1973 which were well recorded on long-period seismographs at Wellington. Average displacements across the fault plane, referred to a common seismic moment, show differences several times larger than errors likely in their estimation. The displacements display a regular regional pattern. In the northern seismic zone displacements are small between the Hikurangi Trench and the east coast of the North Island and also on the continental side of the volcanic front. Between these two regions, and thus over most of the North Island, displacements are large. The whole central region of the South Island has small displacements. The Fiordland region in the south-west, which has island-are affinities but no volcanism or active faulting, is one of predominantly large displacements.