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doi: 10.1007/BF00876824pmid: N/A
Body waveform modeling is used to determine the source processes of three large earthquakes (magnitude 6.8, 6.4, 6.3) occurring between February 9 and 15, 1956 along the San Miguel fault in northern Baja California, Mexico. Results of the modeling suggest that the mainshock on February 9 was responsible for the 20 km of surface faulting observed during the sequence. Although previous researchers have suggested a complex rupture history for the mainshock, uncertainty estimates of source-time function shape indicate single or double source models fit the observed waveforms equally well. The February 15 aftershock, however, appears to have consisted of two events. Locations and focal mechanisms obtained for the three events suggest that the rupture process may have been controlled by cross faults to the main trace of the San Miguel fault. The good correlation between source parameter information and the surficial geology/geometry of the San Miguel fault zone demonstrates the usefulness of waveform modeling studies in unravelling the complexities of historic multi-event earthquake sequences.
doi: 10.1007/BF00876825pmid: N/A
The mantle magnitudeM m is used on a dataset of more than 180 wavetrains from 44 large shallow historical earthquakes to reassess their moments, which in many cases had been previously estimated only on the basis of the earthquake's rupture area. We provide 27 new or revised values ofM o, based on the spectral amplitudes of surface waves recorded at a number of stations, principally Uppsala and Pasadena. Among them, and most significantly, we document a large low-frequency component to the source of the 1923 Kanto earthquake: the low-frequency seismic moment is 2.9×1028 dyn-cm, in accord with geodetic observations. On the other hand, we revise downwards the seismic moment of the 1906 Ecuador event, which did not exceed 6×1028 dyn-cm.
doi: 10.1007/BF00876826pmid: N/A
We extend to the case of intermediate and deep earthquakes our application of the mantle magnitudeM m to historical events. Because of the general lack of quantitative studies of deep earthquakes before the initiation of the Centroid Moment Tensor databank in 1977, we regard as historical all non-shallow earthquakes up to and including 1976. An analysis of 57 records from 41 events, using the Uppsala Wiechert seismometer and various long-period instruments at Pasadena, yields new moment estimates for 28 events whose moments had not previously been published. Our results correlate poorly with available traditional magnitudes, as would be expected from early saturation effects for magnitude scales measured at relatively high frequencies. They also suggest that large events (1028 dyn-cm and greater) take place in the 100–200 km depth range, but that the depth interval 350–520 km features few if any large earthquakes.
Russo, Raymond; Okal, Emile; Rowley, Keith
doi: 10.1007/BF00876827pmid: N/A
We have relocated the twenty-eight largest magnitude (4.3≤M s ≤7.3) historical (1922–1963) earthquakes of the southeastern Caribbean. We also present new focal mechanisms for seven of these events. The relocations are based on reported ISSP andS arrival times that we analyzed using generalized linear inversion techniques. The new focal mechanisms were constrained by first motionP polarities as reported by the ISS and as picked by us where records were available, and by the polarities and ratios ofSH andsSH, andSV andsSV arrivals that we determined from seismograms. The results of the relocations are commensurate with the distribution of seismicity observed in the recent era: hypocenters are shallow and intermediate in depth (0–200 km), and the events occur almost exclusively in areas known to be currently seismic. The frequent seismic activity in the vicinity of the Paria Peninsula, Venezuela, is clearly a persistent feature of the regional earthquake pattern; intermediate depth earthquakes indicative of subduction beneath the Caribbean plate occur here and along the Lesser Antilles arc. The Grenadines seismic gap is confirmed as an area of low seismic moment release throughout the historical era. Trinidad and the eastern Gulf of Paria were also largely quiescent.
Kawahara, Jun; Yamashita, Teruo
doi: 10.1007/BF00876828pmid: N/A
We theoretically study the scattering ofP, SV andSH waves by a zonal distribution of cracks, which simulates a fault fracture zone. An investigation is conducted how the geometrical properties of the crack distribution and the frictional characteristics of the crack surface are reflected in the attenuation and dispersion of incident waves, as well as in the amplitudes of the transmitted and reflected waves from the zone. If the crack distribution within the fault zone changes temporally during the preparation process of the expected earthquake, it will be important for earthquake prediction to monitor it, utilizing the scattering-induced wave phenomena.
doi: 10.1007/BF00876829pmid: N/A
A Maximum Entropy Spectral Analysis of the geological time series of the oxygen isotope 18 for the last 782,000 years showed 98,000 years as the most prominent periodicity, followed by 40,000 years of less than half the amplitude and still smaller peaks at 24,000, 67,000, 84,000, 107,000 and 786,000 years. Among the astronomical parameters, only eccentricity showed a similar periodicity viz. 96,000 years. Obliquity had a prominent periodicity at 41,000 years which was reflected in the oxygen isotope series also. Corresponding to the bimodal precession peaks (23,000 and 19,000 years), the oxygen isotope series showed a peak at 24,000 years and smaller peaks at 22,000 and 19,000 years. Our results compare well with those obtained byThomson (1990) with a more sophisticated method.
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