Showing papers on "Hypocenter published in 1978"

The foreshock activity of the 1971 San Fernando earthquake, California

Abstract: All of the earthquakes which occurred in the epicentral area of the 1971 San Fernando earthquake during the period from 1960 to 1970 were relocated by using the master-event method. Five events from 1969 to 1970 are located within a small area around the main shock epicenter. This cluster of activity is clearly separated spatially from the activity in the surrounding area, so these five events are considered foreshocks. The wave forms of these foreshocks recorded at Pasadena are, without exception, very complex, yet they are remarkably similar from event to event. The events which occurred in the same area prior to 1969 have less complex wave forms with a greater variation among them. The complexity is most likely the effect of the propagation path. A well located aftershock which occurred in the immediate vicinity of the main shock of the San Fernando earthquake has a wave form similar to that of the foreshocks, which suggests that the foreshocks are also located very close to the main shock. This complexity is probably caused by a structural heterogeneity in the fault zone near the hypocenter. The seismic rays from the foreshocks in the inferred heterogeneous zone are interpreted as multiple-reflected near the source region which yielded the complex wave form. The mechanisms of the five foreshocks are similar to each other but different from either the main shock or the aftershocks, suggesting that the foreshocks originated from a small area of stress concentration where the stress field is locally distorted from the regional field. The number of small events with S-P times between 3.8 to 6 sec recorded at Mt. Wilson each month suggests only a slight increase in activity of small earthquakes near the epicentral area during the 2-month period immediately before the main shock. However, because of our inability to locate these events, the evidence is not definitive. Since the change in the wave forms is definite the present result suggests that detailed analyses of wave forms, spectra, and mechanism can provide a powerful diagnostic method for identifying a foreshock sequence.

Directivity in the high-frequency radiation of small earthquakes

Abstract: On December 12, 1972 at 0351 and 0355 GMT, two earthquakes with magnitudes equal to 3.0 and 2.8, respectively, occurred on the Cienega Road section of the San Andreas fault in central California. The two events have the same hypocenter location and fault-plane soultion. Observed seismograms for these two events at 28 stations within about 65 km of and surrounding the epicenters are systematically different in a pattern that is consistent with different directions of rupture expansion for the two events. The 0351 GMT event preferentially radiated high-frequency (f 10 Hz) body waves to the southeast consistent with unilateral rupture propagation toward the southeast while the 0355 GMT event rupture expanded more toward the northwest.

Source characteristics of shallow earthquakes in the northern part of sanriku-oki region, japan

Abstract: Six shallow earthquakes, which occurred in the Sanriku-Oki region during the period from 1965 to 1970, are studied by using long-period P-and S-waves. Focal mechanism solutions are determined from the S-wave polarization angles. Fault planes, which are distinguished from the auxiliary planes by means of the hypocenter distribution of aftershocks, are slightly dipping toward the island arc. A linear correlation is found between the dip angle of fault plane and the distance of epicenter from the trench axis; the dip angle increases with increasing distance. The motion directions of the foot wall blocks relative to the hanging wall sides are in the range from N58°W to N72°W and are consistent with the expectation from the plate tectonics theory, the motion direction of the Pacific plate relative to the Eurasia plate. The focal depths re-determined by the use of depth phases such as pP and sP reveal that the hypocenters of the six earthquakes lie almost on a plane slightly dipping toward the island arc. The spectral analysis of P-waves is made to elucidate the dynamic source process. To do this, the effects of multiple reflection in the crust near the source region as well as beneath the station are taken into account. A unilateral bidirectional model of faulting appropriate to each event is determined by comparing the trough frequencies in the observed P-wave spectra with those expected theoretically. The fracture is found to have propagated in the motion direction of the foot wall block with a velocity between 2.0 and 3.1km/sec.

Seismic activity in sagami bay and off the boso peninsula observed by ocean bottom seismographs in 1971

Abstract: In 1971, for about a period of ten days, OBS observations were carried out in Sagami Bay, south of Tokyo, where the 1923 Kanto Earthquake (M=7.9) occurred, and where seismic activity has been very low since 1926. Although the OBS on the Sagami Trough detected one near earthquake of magnitude 0.1, it is concluded that the shallow seismic activity in Sagami Bay around the terminus of the Sagami Trough was very weak in late 1971. It is concluded that the 1923 earthquake fault posulated in Sagami Bay is still quiet at present in spite of the recent unusual crustal deformation in the adjacent region, the Boso Peninsula. Use of an OBS along with several land-based stations expanded the detection capability to the area south of the Boso Peninsula. The hypocenter distribution revealed a linear alignment extending to the southeast from the tip of the Boso Peninsula at a mean depth of about 60km. The vertical distribution of hypocenters along the linear alignment resemble the bathymetric feature of the sea floor. There was a depth discontinuity of hypocenters at the foot of the inner wall of the Sagami Trough at 34.8°N.

The Focal Mechanisms of a Small Intra Caribbean Plate Earthquake as Determined From Rayleigh Waves

Abstract: The focal mechanism and depth of a small yet tectonically important, intra-Caribbean plate earthquake have been determined using observed Rayleigh wave signals. This earthquake occurred on August 14, 1972 and is believed to be the first known hypocenter in this area of the Caribbean Sea; about 330 kilometers from any previous event. Thus, the source mechanism of this earthquake should yield important information about the stresses in the interior of the Caribbean plate. Only about 5 reliable P-wave first motions were recorded at WWSSN stations, due to the low magnitude (M b = 4.7) of this earthquake. Therefore, the focal mechanism of this event can not be determined from the body wave data alone. Nonetheless, coherent surface wave trains were recorded as far away as 7,000 kilometers. The deduced mechanism is obtained from a comparison of the observed Rayleigh wave phase from this earthquake with that of a master event (September 13, 1971: M b = 5.7) located along the northern boundary of the Caribbean plate. The mechanism which yields the best fit to both the surface wave data and the body wave data is of the strike–slip type, with the compression axis trending approximately NW–SE.

On the Accuracy of Hypocenter Determination of the Hokuriku Microearthquake Observatory

Abstract: The accuracy of a method of hypocenter determination which was carried out as a routine work at the Hokuriku Microearthquake Observatory was investigated. A telemetering observation system with seven satelite stations was completed in 1976 at the Observatory, with a time accuracy of 20 milliseconds. In this article, the accuracy of hypocenter determination is discussed principally from the viewpoint of errors of reading seismograms and the propriety of analysis methods. The accuracy of reading seismograms was examined by comparing the results read independently by two persons on a pen-recording seismogram with a paper-speed of 8 mm/sec. The results show that the reading errors caused by the reader's personality are estimated as, at most, 0.1 s for P-wave and 0.5 s for origin time converted from S-P time. Variations of hypocenters were calculated by giving observational errors At in various ways, in which At is 0A s for P arrival times or 0.5 s for the origin times. The results are as follows: Variations of epicenters are less than 2 km in the neighbourhood of the network, and are about 5 km apart from it. In the middle distant area, the operation of adding ,t1t mainly affects the depths of hypocenters. The variation of hypocenters caused by the differences of combinations of stations and of the sampling of the assumed P-wave velocity data were also examined. These results show that the variations are small enough to discuss seismic activity. From these results, it seems important for the accurate determination of hypocenters to know crustal structure as correctly as possible.