The 23:19 aftershock of the 15 October 1979 Imperial Valley earthquake: More evidence for an asperity
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Abstract The well-recorded strong ground motion data for the 23:19 aftershock of the 15 October 1979 Imperial Valley earthquake provide a good opportunity to study the high-frequency source characteristics and the path effects at near-source distances. The best-fitting point source model has a strike-slip mechanism, N40°W, which is nearly identical to the main event. The estimated stress drop is extremely high, roughly 500 bars, with a triangular time history (0.1, 0.1 sec) but with a moment of 1.0 × 1024 dyne-cm. A double-source model found by inversion fits the high-frequency data better but indicates complex faulting: the first source (with strike = N319°E, dip = 42°NE, and slip angle = 165°) has a moment of 0.7 × 1024 dyne-cm, the second source (with strike = N324°E, dip = 82°SW, and slip angle = 181°) lies about 0.5 km to the north and has a seismic moment twice that of the first source. Source dimensions appear very small for this amount of energy release. Many of the anomalous behaviors observed at certain stations for the main event are, also, present in the aftershock data. These features are examined in terms of path effects.Keywords:
Seismic moment
Asperity (geotechnical engineering)
Point source
Source model
Abstract The well-recorded strong ground motion data for the 23:19 aftershock of the 15 October 1979 Imperial Valley earthquake provide a good opportunity to study the high-frequency source characteristics and the path effects at near-source distances. The best-fitting point source model has a strike-slip mechanism, N40°W, which is nearly identical to the main event. The estimated stress drop is extremely high, roughly 500 bars, with a triangular time history (0.1, 0.1 sec) but with a moment of 1.0 × 1024 dyne-cm. A double-source model found by inversion fits the high-frequency data better but indicates complex faulting: the first source (with strike = N319°E, dip = 42°NE, and slip angle = 165°) has a moment of 0.7 × 1024 dyne-cm, the second source (with strike = N324°E, dip = 82°SW, and slip angle = 181°) lies about 0.5 km to the north and has a seismic moment twice that of the first source. Source dimensions appear very small for this amount of energy release. Many of the anomalous behaviors observed at certain stations for the main event are, also, present in the aftershock data. These features are examined in terms of path effects.
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In an attempt to examine the characteristic behavior of fault asperities (large slip areas), we comparatively studied two large earthquakes: the Tokachi-oki earthquake (M 7.9) of May 16, 1968 and the Sanriku-oki earthquake (M 7.5) of December 28, 1994, which have a partially common source area. Both the strong motion records at a regional network and the teleseismic body waves at global networks were analyzed to determine the detailed spatio-temporal distribution of moment release. The aftershock distribution, which may provide us with a more reliable location of asperity, was also re-examined using the same underground structure and the same algorithm for both events.The total seismic moment, Mo, and the source duration, T are obtained as: Mo=3.5×1021Nm; T=90s for the 1968 event, and Mo=4.4×1020Nm; T=60s for the 1994 event. It is also shown that the 1968 event consists of more than two asperities, one of which took a role of asperity again for the 1994 event. The distribution of relocated aftershocks, which fringe the major asperities, strongly supports this fact. A simple calculation indicates that the seismic coupling is almost perfect (100%) in this common asperity. We thus propose that there exist characteristic sites for asperities where fault slip occurs only as a seismic event, and that the individual asperities usually manifest M 7 class earthquakes but sometimes synchronize to cause M 8 class earthquakes.
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Abstract A waveform inversion study was conducted to reveal the rupture process of the 2007 Noto Hanto, Japan, earthquake using near-source strong ground motion records. To avoid uncertainty in the subsurface structure in and around the source region when calculating Green’s functions, I used the aftershock records as empirical Green’s functions. To ensure that the path and the site effects are shared between the ground motions from the mainshock and those from aftershocks, the mainshock fault plane was divided into four domains, each of which was allocated to one of the aftershocks used as the empirical Green’s function events. The result of the inversion indicated that the rupture process included the break of two asperities. The first asperity, located close to the JMA hypocenter, was broken soon after the rupture started. This was followed by an extension of the rupture and the subsequent break of the second shallow asperity. The total moment of the mainshock was estimated to be 1.96E+19 N m, which is roughly consistent with the moment estimated by the F-net (1.36E+19 N m). Agreement between the observed and synthetic ground motions is quite satisfactory.
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On August 11, 2012,within several minutes, two shallow destructive earthquakes with moment magnitudes of 6.5 and 6.4 occurred in Varzagan, Azerbaijan-e-Sharghi Province, in the northwest of Iran In this study, the Empirical Green Function (EGF) method was used for strong ground motion simulationto estimate the source parameters and rupture characteristics of the earthquakes. To simulate the first earthquake, two aftershocks with magnitudes of 5.6 and 5.2 were used as the EGFs. In the second event, an aftershock with a magnitude of 5 was used as the small event. The size of the main fault caused by the first event was about 18 km in length and 10 km in width. Also, the size of the asperity in the second earthquake was about 16 km in the strike direction and 11 km in the dip direction. The durations of the ruptures in the first and second events were more than 9 and 10s, respectively. The estimated fault plane solution showed strike-slip faulting for the first earthquake and a reverse mechanism with a strike-slip component for the second one. Strike, dip and rake of a causative fault of the first and second earthquakes were determined as 270, 81 and 175 degrees and 230, 57 and 134 degrees, respectively. In addition, the stress drop in the first and second events was calculated to be about 22 and 34 bar, respectively.
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We explore a recently developed procedure for kinematic inversion based on an elliptical subfault approximation. In this method, the slip is modelled by a small set of elliptical patches, each ellipse having a Gaussian distribution of slip. We invert near-field strong ground motion for the 2004 September 28 Mw 6.0 Parkfield, California, earthquake. The data set consists of 10 digital three-component 18-s long displacement seismograms. The best model gives a moment of 1.21 × 1018 N m, with slip on two distinct ellipses, one with a high-slip amplitude of 0.91 m located 20 km northwest of the hypocentre. The average rupture speed of the rupture process is ∼2.7 km s−1. We find no slip in the top 5 km. At this depth, a lineation of small aftershocks marks the transition from creeping above to locked below, in the interseismic period. The high-slip patch coincides spatially with the hypocentre of the 1966 Mw6.0 Parkfield, California, earthquake. The larger earthquakes prior to the 2004 Parkfield earthquake and the aftershocks of the 2004 earthquake (Mw > 3) also lie around this high-slip patch, where our model images a sharp slip gradient. This observation suggests the presence of a permanent asperity that breaks during large earthquakes, and has important implications for the slip deficit observed on the Parkfield segment, which is necessary for reliable seismic hazard assessment.
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A rupture model with varying rupture front expansion velocity for the March 11, 2011, Tohoku-Oki earthquake was obtained by the joint inversion of high-rate Global Positioning System (GPS) data and ocean bottom GPS/acoustic (OB-GPS) data. The inverted rupture velocity with a complex distribution gradually increases near the hypocenter and shows rapid rupture expansion at the shallowest part of the fault. The entire rupture process, which lasted 160 s, can be divided into three energy release stages, based on the moment rate function. The preferred slip model, showing a compatible relationship with aftershocks, has a primary asperity concentrated from the hypocenter to the trench and a small asperity located on the southern fault. Source time functions for subfaults and temporal rupture images suggest that repeated slips occurred in the primary rupture, which is consistent with that from seismic waveforms. Our estimated maximum slip and total seismic moment are ~65 m and 4.2 × 1022 Nm (Mw 9.0), respectively. The large slip, stress drop, and rupture velocity are all concentrated at shallow depths, which indicates that the shallow part of the fault radiated high-frequency as well as low-frequency seismic waves.
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We investigated the seismic activity around the northern neighbor of the 2011 off the Pacific coast of Tohoku Earthquake (MW 9.0) with special attention to a potential large aftershock in the area. We obtained a combined data set by adding our manually-picked locations to the catalog locations by the Japan Meteorological Agency. The hypocenter distribution delineates active and inactive bands of seismicity. The band of low seismicity corresponds to a zone of a large seismic slip, indicating that aftershocks occurred in peripheral neighbors of the mainshock asperity. The broad band of active seismicity along the coast corresponds to the zone of a large postseismic slip, suggesting the enhancement of the aftershock activity by the slip. Although the northern neighbor of the mainshock fault is a favored region of increased seismicity, as shown from a Coulomb stress calculation, no significant seismic activity is observed within the potential source area except along the Japan Trench and the SW corner. This implies that the zone of interplate moment release by previous large earthquakes and the subsequent slow slip acted as a barrier to the migration of both the mainshock rupture and aftershock activity. However, an aftershock area in the zone may reflect inhomogeneous moment release by past seismic and aseismic sequences.
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The great Kurile Islands underthrusting earthquake ( M w = 8.5) of October 13, 1963, was accompanied by a large foreshock and an aftershock. This sequence allows us to investigate the rupture process and fault heterogeneities along a subduction zone. We have characterized the rupture process of the main shock event by deconvolving long‐period P wave seismograms from azimuthally well‐distributed stations to obtain source time functions. Directivity associated with the three main pulses of moment release in the source functions indicates a total source duration of 93 s, a fault length of 245 km, and a rupture direction of N40°–60° E. Three asperities along the fault are identified by the variation in moment release and located as follows: The first asperity is from 0 to 60 km NE of the epicenter and has an average rupture velocity of 2.7 km/s. The second asperity is centered at 130 km NE of the epicenter and has a length scale of 40–50 km. The final asperity is from 185 to 245 km NE of the epicenter and has a rupture velocity of 3.8 km/s. The seismic moment release of the main shock determined from the P waves is less than one‐half the surface wave moment of 70 × 10 27 dyn cm. Body wave modeling of the largest foreshock (October 12, 1963; M s = 6.7) indicates a source duration of 12 s. The foreshock rupture area is adjacent to but does not significantly overlap the main shock epicentral asperity and may have been important in loading the main shock epicenter. The largest aftershock (October 20, 1963: M s = 7.2) occurred trenchward of the main shock epicentral asperity and produced an unusually large tsunami. Body wave modeling of the aftershock suggests that the anomalous ringing of the P waves is a result of reverberations in the oceanic layer and not the result of an unusually long source duration. Our preferred model includes a source duration of 24–28 s and at least part of the source located at shallow depths in the accretionary prism. The previous earthquake sequence along this portion of the plate boundary occurred in 1918 with a main shock on September 7 ( M = 8.4) and a large aftershock on November 8 ( M = 7.7) trenchward of the main shock. The 1918 event initiated approximately 200 km NE of the 1963 epicenter. Tsunami data suggest that the 1918 event ruptured the second and third asperities but not the 1963 epicentral asperity. This indicates the potential for variations in the rupture mode between successive earthquake cycles along this segment of the Kurile Islands subduction zone. The 1963 event is the only multiple asperity earthquake to have occurred along the southern Kurile arc during the most recent cycle of underthrusting earthquakes. A comparison of the asperity distribution along the Kurile arc indicates that the asperity separation along the 1963 earthquake segment is less than the separation between the other single asperity earthquakes. This suggests that the asperity distribution may be important in determining the mode of rupture. We have compared the 1963 earthquake to the great 1906 Colombia‐Ecuador earthquake ( M w = 8.8). The 1906 earthquake ruptured three asperities in a single great event similar to the 1963 earthquake. For both these multiple asperity earthquakes, the seismic moment release is larger than the sum of each individual asperity. This indicates that multiple asperity ruptures trigger larger amounts of moment release in adjacent “weaker” regions than do single asperity earthquakes.
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