Local Seismic Data Research Articles

San Andreas Fault Zone Head Waves Near Parkfield, California

Microearthquake seismograms from the borehole seismic network on the San Andreas fault near Parkfield, California, provide three lines of evidence that first P arrivals are "head" waves refracted along the cross-fault material contrast. First, the travel time difference between these arrivals and secondary phases identified as direct P waves scales linearly with the source-receiver distance. Second, these arrivals have the emergent wave character associated in theory and practice with refracted head waves instead of the sharp first breaks associated with direct P arrivals. Third, the first motion polarities of the emergent arrivals are reversed from those of the direct P waves as predicted by the theory of fault zone head waves for slip on the San Andreas fault. The presence of fault zone head waves in local seismic network data may help account for scatter in earthquake locations and source mechanisms. The fault zone head waves indicate that the velocity contrast across the San Andreas fault near Parkfield is approximately 4 percent. Further studies of these waves may provide a way of assessing changes in the physical state of the fault system.

The 1984 southern Yellow Sea earthquake of Eastern China: Source properties and seismotectonic implications for a stable continental area

On 21 May 1984, a strong shock ( MS = 6.3, mb = 5.7) occurred in the southern Yellow Sea which shook the densely populated coastal area of Eastern China. About 1 min before the main shock, a foreshock of mb 5.4 occurred. Because of interference from the foreshock, no reliable P -wave first motions or focal mechanisms have been obtained for the main shock from local seismic data. In this paper, long-period teleseismic data have been used to investigate the source mechanism. Fault plane solutions determined from first motions as well as P and SH waveforms show a predominant strike-slip motion with a small dip-slip component on a steeply dipping plane. The tectonic environment of this event supports the observation that margins of former rifts tend to be sites of midplate seismic source regions. The Yellow Sea developed in a cratonic environment due to early Cenozoic rifting. The major graben faults strike NNE-SSW to NE-SW and are intersected by second-order faults trending WNW-ESE. Aftershock distribution suggests that the fault which generated the main shock probably strikes WNW-ESE. The mechanism determined in this paper strikes 120°, dips 88°, and slips 28°. This solution suggests that the earthquake ruptured along a secondary fault near the intersection of a more major NNE-SSW trending fault. The epicentral area is subject to to an ENE-WSW compression and a NNW-SSE extension. The seismic moment and focal depth are determined to be 1.09 × 1025 dyne-cm and 12 ± 4 km, respectively. Relative hypocentral locations indicate that the focal depth of the foreshock is similar to that of the main shock, and the hypocenter of the foreshock is located very close to the WNW-ESE striking nodal plane of the main shock mechanism and is about 20 km away from the main shock hypocenter. Source properties of the southern Yellow Sea earthquake have been compared with those of three major earthquakes of North China. The stress drop of the Yellow Sea earthquake is determined to be about 42 bars. This value is much lower than the stress drops computed for the 1966 Hsingtai and the 1975 Haicheng earthquakes of North China. The southern Yellow Sea is characterized by short recurrence intervals, while the Hsingtai and Haicheng areas have very long recurrence intervals. The short recurrence intervals and low stress drop may reflect a lower material strength at the source region in the Yellow Sea.

Seismicity, normal faulting, and the geomorphological development of the Gulf of Corinth (Greece): the Corinth earthquakes of February and March 1981

Three major destructive earthquakes of M s 6.7, 6.4, 6.4 occurred in the eastern part of the Gulf of Corinth in February' and March 1981. Associated normal faulting was observed on both the north and the south sides of the Gulf. Examination of teleseismic, local seismic, surface faulting and geomorpbological data suggests that the first and second of these shocks activated major north-dipping normal faults. These faults control the topography and bathymetry and are related to the recent uplift and subsidence of the coastline. It is probable that the first earthquake occurred on a faultwhich outcrops underwater and the second on a fault which is visible on land. It appears that the third shock activated an antithetic normal fault dipping southwards on the north side of the Gulf and that the overall structure of the Gulf of Corinth graben is asymmetric. The northern antithetic fault clearly demonstrates the geomorphological changes associated with developing young normal faulting. In addition, we suggest that observations of recent coastline movement provide a powerful tool for identifying major active fault systems and predicting their historical role in the paleogeography of an area.

The Hibernia Structure: ABSTRACT

The Chevron et al Hibernia P-15 well discovered hydrocarbons on the Hibernia structure in late 1979. Since that time, the operator Mobil Oil Canada, Ltd., and partners (Gulf Canada Resources, Inc., Petro-Canada Exploration Inc., Chevron Standard Limited, and Columbia Gas Development of Canada, Ltd.) have drilled four appraisal wells on the feature. Results from these wells indicate the presence of a major oil accumulation. Hibernia is located on the Newfoundland Grand Banks 315 km east-southeast of St. John's. Local stratigraphy and seismic structural data indicate potentially productive hydrocarbon zones. End_of_Article - Last_Page 1659------------

Teleseismic evidence for a low‐velocity body under the Coso Geothermal Area

Teleseismic P wave arrivals were recorded by a dense array of seismograph stations located in the Coso geothermal area, California. The resulting pattern of relative residuals reveals an area showing approximately 0.2‐s excess travel time that migrates with changing source azimuth, suggesting that the area is the ‘delay shadow’ produced by a deep, low‐velocity body. Inversion of the relative residual data for three‐dimensional velocity structure determines the lateral variations in velocity to a depth of 22.5 km beneath the array. An intense low‐velocity body, which coincides with the surface expressions of late Pleistocene rhyolitic volcanism, high heat flow, and hydrothermal activity, is resolved between 5‐ and 20‐ km depth. It has maximum velocity contrast of over 8% between 10 and 17.5 km. The shallowest part of this body is centered below the region of highest heat flow; at depth it is elongate in approximately the N‐S direction. The hypothesis that this low‐velocity body is caused by the presence of partial melt in the middle crust is consistent with the local seismic, geologic, and thermal data.