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Thomas H Heaton - One of the best experts on this subject based on the ideXlab platform.
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reply to comment on models of stochastic spatially varying stress in the crust compatible with Focal Mechanism data and how stress inversions can be biased toward the stress rate by deborah elaine smith and thomas h heaton by jeanne l hardebeck
Bulletin of the Seismological Society of America, 2015Co-Authors: Deborah Smith, Thomas H HeatonAbstract:In her comment (Hardebeck, 2015) on our stress heterogeneity article (Smith and Heaton, 2011), Hardebeck suggests a different Focal‐Mechanism error distribution than what we used in our 2011 article and suggests that this new error distribution will reduce our estimates of stress heterogeneity. In response to this, we have rerun our calculations three ways: (1) with the original Mechanism error distribution from Smith and Heaton (2011), (2) with a Mechanism error distribution similar to the one presented by Hardebeck (2015), and (3) with a Mechanism error distribution derived from repeating earthquake statistics. We find the two new Mechanism error models, relative to the original Mechanism error distribution, reduce the heterogeneity ratio ( HR ) estimates by approximately 35%–40% (using Hardebeck’s suggested distribution) and by approximately 8%–10% (using the repeating earthquake based error distribution). Applying these two new Mechanism error distribution models helps parameterize the estimates of stress heterogeneity amplitude but does not change the main novel points of the Smith and Heaton (2011) article. Namely, we find that Focal‐Mechanism data are still compatible with a heterogeneous stress that is more dissimilar at large interevent distances and more correlated at small interevent distances and that a heterogeneous stress can bias traditional stress inversions toward the stressing rate function. Last, we demonstrate that as the size of the stress inversion region decreases and as the maximum variability of the heterogeneous stress decreases, the normalized stress inversion bias also decreases. This is consistent with taking the model of Smith and Heaton (2011) to the limit where region size decreases to a point source; however, most stress inversions may require dimensions closer to the outer scale of the stress (∼60 km for southern California) and hence experience significant stress inversion biasing toward the stressing …
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models of stochastic spatially varying stress in the crust compatible with Focal Mechanism data and how stress inversions can be biased toward the stress rate
Bulletin of the Seismological Society of America, 2011Co-Authors: Deborah Smith, Thomas H HeatonAbstract:Evidence suggests that slip in earthquakes and the resultant stress changes are spatially heterogeneous. If crustal stress from past earthquakes is spatially heterogeneous, then earthquake Focal Mechanisms should also be spatially variable. We describe the statistical attributes of simulated earthquake catalogs, including hypocenters and Focal Mechanisms, for a spatially 3D, time-varying model of the crustal stress tensor with stochastic spatial variations. It is assumed that temporal variations in stress are spatially smooth and are primarily caused by plate tectonics. Spatial variations in stress are assumed to be the result of past earthquakes and are independent of time for periods between major earthquakes. It is further assumed that heterogeneous stress can be modeled as a stochastic process that is specified by an autocorrelation function. Synthetic catalogs of earthquake hypocenters and their associated Focal Mechanisms are produced by identifying the locations and times at which the second deviatoric stress invariant exceeds a specified limit. The model produces a seismicity catalog that is spatially biased. The only points in the grid that exceed the failure stress are those where the heterogeneous stress is approximately aligned with the stress rate. This bias results in a Focal-Mechanism catalog that appears less heterogeneous than the underlying stress orientations. Comparison of synthetic Focal-Mechanism catalogs with catalogs of real earthquakes suggests that stress in the crust is heterogeneous. Stochastic parameters are estimated which generate distance dependent spatial variations in Focal Mechanisms similar to those reported by Hardebeck (2006) for southern California.
Deborah Smith - One of the best experts on this subject based on the ideXlab platform.
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reply to comment on models of stochastic spatially varying stress in the crust compatible with Focal Mechanism data and how stress inversions can be biased toward the stress rate by deborah elaine smith and thomas h heaton by jeanne l hardebeck
Bulletin of the Seismological Society of America, 2015Co-Authors: Deborah Smith, Thomas H HeatonAbstract:In her comment (Hardebeck, 2015) on our stress heterogeneity article (Smith and Heaton, 2011), Hardebeck suggests a different Focal‐Mechanism error distribution than what we used in our 2011 article and suggests that this new error distribution will reduce our estimates of stress heterogeneity. In response to this, we have rerun our calculations three ways: (1) with the original Mechanism error distribution from Smith and Heaton (2011), (2) with a Mechanism error distribution similar to the one presented by Hardebeck (2015), and (3) with a Mechanism error distribution derived from repeating earthquake statistics. We find the two new Mechanism error models, relative to the original Mechanism error distribution, reduce the heterogeneity ratio ( HR ) estimates by approximately 35%–40% (using Hardebeck’s suggested distribution) and by approximately 8%–10% (using the repeating earthquake based error distribution). Applying these two new Mechanism error distribution models helps parameterize the estimates of stress heterogeneity amplitude but does not change the main novel points of the Smith and Heaton (2011) article. Namely, we find that Focal‐Mechanism data are still compatible with a heterogeneous stress that is more dissimilar at large interevent distances and more correlated at small interevent distances and that a heterogeneous stress can bias traditional stress inversions toward the stressing rate function. Last, we demonstrate that as the size of the stress inversion region decreases and as the maximum variability of the heterogeneous stress decreases, the normalized stress inversion bias also decreases. This is consistent with taking the model of Smith and Heaton (2011) to the limit where region size decreases to a point source; however, most stress inversions may require dimensions closer to the outer scale of the stress (∼60 km for southern California) and hence experience significant stress inversion biasing toward the stressing …
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models of stochastic spatially varying stress in the crust compatible with Focal Mechanism data and how stress inversions can be biased toward the stress rate
Bulletin of the Seismological Society of America, 2011Co-Authors: Deborah Smith, Thomas H HeatonAbstract:Evidence suggests that slip in earthquakes and the resultant stress changes are spatially heterogeneous. If crustal stress from past earthquakes is spatially heterogeneous, then earthquake Focal Mechanisms should also be spatially variable. We describe the statistical attributes of simulated earthquake catalogs, including hypocenters and Focal Mechanisms, for a spatially 3D, time-varying model of the crustal stress tensor with stochastic spatial variations. It is assumed that temporal variations in stress are spatially smooth and are primarily caused by plate tectonics. Spatial variations in stress are assumed to be the result of past earthquakes and are independent of time for periods between major earthquakes. It is further assumed that heterogeneous stress can be modeled as a stochastic process that is specified by an autocorrelation function. Synthetic catalogs of earthquake hypocenters and their associated Focal Mechanisms are produced by identifying the locations and times at which the second deviatoric stress invariant exceeds a specified limit. The model produces a seismicity catalog that is spatially biased. The only points in the grid that exceed the failure stress are those where the heterogeneous stress is approximately aligned with the stress rate. This bias results in a Focal-Mechanism catalog that appears less heterogeneous than the underlying stress orientations. Comparison of synthetic Focal-Mechanism catalogs with catalogs of real earthquakes suggests that stress in the crust is heterogeneous. Stochastic parameters are estimated which generate distance dependent spatial variations in Focal Mechanisms similar to those reported by Hardebeck (2006) for southern California.
Taeseob Kang - One of the best experts on this subject based on the ideXlab platform.
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interaction between regional stress state and faults complementary analysis of borehole in situ stress and earthquake Focal Mechanism in southeastern korea
Tectonophysics, 2010Co-Authors: Chandong Chang, Taeseob KangAbstract:Abstract We characterize the present-day stress tensor in southeastern Korean Peninsula using two different sets of data (geotechnical in situ stress data and earthquake Focal Mechanism solutions), to understand the regional contemporary stress state and its relationship to the population of faults. Both sets of data show a comparable result of ENE–WSW maximum compression direction, which is in accord with the first order pattern of tectonic stress direction in the eastern Eurasian plate. More rigorous analyses of in situ stress as well as the inversion of Focal Mechanism show that the current stress field exhibits a systematic heterogeneity in its orientations and magnitudes, possibly caused by the influence of faults. The minimum and maximum horizontal principal stresses normalized by vertical stress at the shallow depths where stress measurements were conducted vary spatially. It turns out that the magnitude of stress field appears to be inversely correlated with the density of regional scale faults. This suggests that a stress release due to faulting may be one of the major factors that contribute to the low stress regime in the region. As a way to confirm the inference, we examine the attitudes of recently activated Quaternary faults with respect to the current stress field. A majority of the faults are oriented in the optimal directions for slip, as indicated by the overall high ratios of shear to normal stress acting on fault planes for the given stress condition, which implies that they might sustain the current stress field. The contemporary earthquake distribution indicates that the lower stressed region has a denser population of seismic activities, suggesting that fault strength in the corresponding region may be at frictional limit with the contemporary stress state. This may imply that the heterogeneity of the regional stress state is a result of the heterogeneity of the strength of faults.
Bertrand Delouis - One of the best experts on this subject based on the ideXlab platform.
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fmnear determination of Focal Mechanism and first estimate of rupture directivity using near source records and a linear distribution of point sources
Bulletin of the Seismological Society of America, 2014Co-Authors: Bertrand DelouisAbstract:Abstract The FMNEAR method is based on both the waveform inversion of near‐source seismic records and on a linear finite‐source model. The primary source parameters that are determined are the moment magnitude ( M w ), the double‐couple Focal Mechanism (strike, dip, and rake), and the distribution of seismic moment along strike, which provides a first‐order estimate of rupture length and directivity. Source depth is also explored. An advantage of the approach is the use of the finite‐source model directly within the search of the Focal Mechanism. This strategy allows the inclusion of strong‐motion records at short distances for moderate to very large earthquakes ( M w >5.5). However, for moderate to small earthquakes ( M w M w 4.5 to 9.0. Test cases incorporate continental events recorded by a large number of well‐distributed stations and less favorable cases. In its present form, the FMNEAR approach is very well adapted to continental earthquakes surrounded by seismic stations, but it can also give informative results in more difficult configurations. The FMNEAR method is now ready for fully automated determinations and is already implemented in near real time. Online Material: Details on inversion parameters, flow charts of the FMNEAR method, figures of waveform modeling and sensitivity for selected events, and table of velocity models.
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Focal Mechanism determination and identification of the fault plane of earthquakes using only one or two near source seismic recordings
Bulletin of the Seismological Society of America, 1999Co-Authors: Bertrand Delouis, Denis LegrandAbstract:A waveform inversion scheme was developed in order to explore the resolving power of one or two seismic recordings at short epicentral distance for the determination of Focal Mechanisms and the identification of the fault plane of earthquakes. Two key features are used to constrain the fault parameters with a reduced number of stations: (1) a simple finite-dimension source model and (2) the modeling of the complete displacement field, including the near-field waves. The identification of the fault plane should be possible, even with a single station, as soon as the seismograms produced by the two nodal planes of a same Focal Mechanism are significantly different, which is the general case when waveforms are controlled by source finiteness. Seven parameters, including the strike, dip, rake, and dislocation, are explored with a grid search, and the minima of the misfit error between the observed and calculated seismograms are mapped. With such an approach, it is possible to conclude about the uniqueness or nonuniqueness of the solutions. The method is tested with three earthquakes of moderate to large size for which the fault plane is well established and for which strong-motion records are available at maximum distances of a few tens of kilometers. Test events are the 1994 Northridge ( M w = 6.7, California), the 1996 Copala ( M w = 7.3, Mexico), and the 1996 Pinotepa Nacional ( M w = 5.4, Mexico) earthquakes. In the case of inversions with two stations, we find a unique solution, or a group of similar solutions, with a good estimation of the Focal Mechanism and the proper selection of the fault plane. Our results also show that in some cases a single station may be enough to recover the fault parameters. The inversion scheme presented here may be systematically applied to future earthquakes, especially to those recorded by few stations. It should be particularly useful in the case of blind faults for which the fault plane may not be identified with the help of other data.
Xianrui Li - One of the best experts on this subject based on the ideXlab platform.
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subsurface structure and spatial segmentation of the longmen shan fault zone at the eastern margin of tibetan plateau evidence from Focal Mechanism solutions and stress field inversion
Tectonophysics, 2019Co-Authors: Xianrui Li, Tobias Hergert, A Henk, Dun Wang, Zuoxun ZengAbstract:Abstract As the eastern margin of the Tibetan Plateau, the Longmen Shan fault zone (LMSFZ) is characterized by a complex structural style and strong seismicity, such as the 2008 Wenchuan Ms8.0 and 2013 Lushan Ms7.0 earthquakes. In order to better understand the subsurface geometry of the LMSFZ we inverted 391 Focal Mechanism solutions (FMS) of earthquakes (M ≥ 3.5) that occurred between 2009 and 2016 in the Longmen Shan region using seismic waveforms. Subsequently, we calculated the stress field based on the FMSs. The results show that the LMSFZ is dominated by a thrust faulting stress regime with subhorizontal NW-SE-trending maximum principal stress σ1 and subvertical minimum principal stress σ3, indicating a lateral compression due to the southeastward movement of the Bayan Har block. Changes in fault dips at ~ 10 km depth are inferred as the result of a shallow level detachment layer, which may also promote shortening of the upper crust and uplift of the Longmen Shan. Based on differences in subsurface fault geometry, stress field and geomorphological evidence, we divide the LMSFZ into two segments by Beichuan County: the southwestern segment has a listric fault geometry, while the northeastern segment is dominated by subvertical transpressional strike-slip faults. According to the FMSs results, we suggest that the Lixian fault is a tear fault accommodating different thrusting rates in the eastern margin of the Bayan Har block. The normal faulting earthquakes in the Lixian area are caused by local extension resulting from the sinistral shear movement of Lixian fault in a strike-slip faulting stress regime. It is suggested that under the continuous southeastward movement of the Bayan Har block and the obstruction of the rigid Sichuan basin, stress is accumulating along the southwestern segment of the LMSFZ, to which accordingly a high seismic hazard is ascribed and which should be carefully monitored.
