Static stress drop in the Mw 9 Tohoku‐oki earthquake: Heterogeneous distribution and low average value
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Abstract Static stress drop distribution and its average value over the rupture area contain important information on the mechanics of large earthquakes. Here we derive static stress drop distributions from 40 published rupture models for the 2011 M w 9 Tohoku‐oki earthquake that are based on various multidisciplinary observations. Average stress drop value over the fault area encompassed by the 5 m coseismic slip contour is not unusually large for each rupture model; the mean for the 40 models is 2.3 ± 1.3 MPa, assuming a uniform rigidity 40 GPa. The value for the entire rupture zone and with a more realistic rigidity structure will be even lower. In the majority of the models, local stress drop in parts of the rupture zone well exceeds 20 MPa. The heterogeneous stress change distribution, with large stress drop being accompanied by large stress increase, leads to the small average for the earthquake.Keywords:
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The complex broadband P ‐wave displacement recordings of the 570 km deep March 9, 1994, M w 7.6 Fiji Island and the 660 km deep June 9, 1994, M w 8.2 Bolivia earthquakes are analyzed to estimate the spatial and temporal characteristics of their ruptures. We model relative arrival times and amplitudes of coherent subevents in P waveforms, and perform waveform analyses. We resolve: (1) short rupture durations relative to their seismic moments, 12 and 40 s for the Fiji and Bolivia earthquakes, respectively; (2) small areas of main moment release, for both earthquakes 30 × 40 km²; (3) high stress drops compared to shallow earthquakes, with the stress drop of the Bolivia event (283 MPa) being significantly higher than the stress drop of the Fiji event (26 MPa); (4) small changes in mechanism (up to 10° changes in fault plane orientation); and (5) a steeply dipping rupture plane for the Fiji earthquake, and a shallowly dipping rupture plane for the Bolivia earthquake. Details of the ruptures are difficult to resolve using teleseismic P waveforms.
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The Lleyn Peninsular earthquake which occurred at 0656 on 1984 July 19 was recorded at three medium aperture seismological array stations located at teleseismic distances. From these recordings an estimate is made of the hypocentre, origin time, magnitude and fault-plane solution of the earthquake. The fault-plane solution was determined using the Pearce algorithm and indicates that the focal mechanism is predominantly strike-slip. The fault-plane solution was used to generate synthetic seismograms for comparison with the observed to confirm the nature of the source and in particular the depth of focus which was estimated to be 20.5 km. It is concluded that the determination of the earthquake parameters using only three teleseismic seismograms is in good agreement with the results obtained from an analysis of 45 local and regional seismological stations.
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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.
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Several earthquakes, which occurred near Kikai-jima from October 18 to November 1, 1995, are examined by means of teleseismic body waves. All the focal mechanisms determined for these earthquakes reveal normal-faults having a down-dip extension within a subducting slab. The main source parameters of the largest event are : (strike, dip, rake) = (208°, 75°, -92°); the seismic moment=5.9×1019 [Nm] (Mw=7.1); fault area =6.0×108 [m2];dislocation=1.5 [m]; rupture duration=23 [s]; stress drop =10 [MPa]. In this event the rupture propagated unilaterally to SW direction. The second largest event ruptured the NE region, where the fault plane did not smoothly connected but formed a step to the main fault plane. It may be noteworthy that these earthquakes did not trigger any significant inter-plate earthquakes, suggesting that the adjacent plate interface may be intrinsically aseismic.
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Abstract The 28 June 1992 Big Bear earthquake occurred at 15:05:21 GMT and is considered to be an aftershock of the earlier Mw = 7.3 Landers earthquake. From overall aftershock locations and long-period focal studies, rupture is generally assumed to have propagated northeast. No surface rupture was found, however, and the mainshock locations determined from both strong motion and TERRAscope data are mutually consistent and do not lie on the assumed fault plane. Further, directivity analysis of records from the TERRAscope array suggests significant short- and long-period energy propagating northwest along the presumed antithetic fault plane. This observation is supported by significant early aftershocks distributed along both the presumed rupture plane and the antithetic plane to the northwest. An empirical Green's function (eGf) approach using both the Mw = 5.2, 28 June 1992 14:43 GMT foreshock and the Mw = 5.0 17 August 1992 aftershock produces consistent results and suggests that the Big Bear event comprised at least two substantial subevents. From the eGf results, we infer that the second and possibly a third subevent occurred on the presumed (northeast striking) mainshock rupture surface, but that significant moment release occurred on the antithetic northwest striking surface. We present results from line-source fault modeling of broadband displacement recordings of the Big Bear mainshock, which indicate that a two-fault event is necessary to produce the observed waveforms. The limitations imposed by the mainshock location and directivity analysis require that the initial rupture be towards the northwest, striking 320°. This was followed approximately 4 sec later by bilateral rupture along a northeast-southwest fault that strikes 50° east of north.
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abstract The ML 6.0 Point Mugu, California earthquake of February 21, 1973 and its aftershocks occurred within the complex fault system that bounds the southern front of the Transverse Ranges province of southern California. P-wave fault plane solutions for 51 events include reverse, strike slip and normal faulting mechanisms, indicating complex deformation within the 10-km broad fault zone. Hypocenters of 141 aftershocks fail to delineate any single fault plane clearly associated with the main shock rupture. Most aftershocks cluster in a region 5 km in diameter centered 5 km from the main shock hypocenter and well beyond the extent of fault rupture estimated from analysis of body-wave radiation. Strain release within the imbricate fault zone was controlled by slip on preexisting planes of weakness under the influence of a NE-SW compressive stress.
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