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Search Results: 1 - 10 of 14731 matches for " Stress inversion "
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Stress tensor inversion in Western Greece using earthquake focal mechanisms from the Kozani-Grevena 1995 seismic sequence
A. A. Kiratzi
Annals of Geophysics , 1999, DOI: 10.4401/ag-3752
Abstract: Stress tensor inversion has been applied to estimate principal stress axes orientations in Western Greece, from 178 earthquake fault plane solutions from the Kozani-Grevena May 13, 1995 sequence. All focal mechanisms were previously determined through the deployment of a dense portable array. The magnitude range is 2.7-6.5 and the depth range is 4.0-15 km. A single stress tensor with an average misfit of 6.5°, small enough to support the assumption of stress homogeneity, can describe the stress field. The maximum compressive stress, s1, has a NNE-SSW trend (N26°E) and a nearly vertical plunge (80°) while the minimum compressive stress, s3, has a NNW-SSE orientation (N159°E) and a shallow plunge (7°) southwards. The scalar quantity, R (stress ratio) was found equal to 0.4 suggesting a transtensional regime (normal faulting with strike-slip motions) in which s2 is compressional. The identification of the fault plane from the auxiliary plane was achieved for 99 fault plane solutions out of 178 in total (56%). Vertical cross sections support previous results concerning the north dipping main fault segments and the south dipping antithetic faulting. The strike-slip motion is mainly dextral, along NNE-SSW structures, which follow the direction of the main neotectonic faults while the scarce sinistral strike-slip motion is connected to NW-SE trending zones of weakness pre-existing the old phase of compression in the Aegean. The strong strike slip motion that supports the transtensional regime probably reflects the effect of the motions of the North Anatolian Fault, taken up by normal faulting in the area of Western Greece.
Spatial segmentation characteristic of focal mechanism of aftershock sequence of Wenchuan Earthquake
QinCai Wang,ZhangLi Chen,SiHua Zheng
Chinese Science Bulletin , 2009, DOI: 10.1007/s11434-009-0367-0
Abstract: Moment tensor solutions of 88 earthquakes were determined by using the broadband waveform data recorded in six stations within 450 km around the Wenchuan Earthquake sequence by means of the time domain moment tensor inversion method. It was found that the type of the focal mechanism solution is characteristic of obvious spatial segmentation. There are six segments along the main rupture zone from southwest to northeast, where initially the focal mechanism is of main thrust type, finally of main right-lateral strike-slip type and between these two areas there is a transition zone characterized in multiple types of focal mechanisms appearing in turn. Earthquakes of left-lateral strike-slip type perpendicular to the main rupture zone occurred near Xiaoyudong Town. The stress field of each segment is inversed by means of the FMSI program, and it was found that, along the main rupture zone from southwest to northeast, the direction of the maximum principal stress is gradually changing from near EW to NW-SE, and finally changing back to near EW.
Source process and stress change associated with the 11 January, 1997 (Mw=7.1) Michoacán, Mexico, inslab earthquake
Miguel A. Santoyo,Shri K. Singh,Takeshi Mikumo
Geofísica internacional , 2005,
Abstract: We study the source characteristics and the associated coseismic stress change of the January 11, 1997 Michoacán earthquake, based on teleseismic, regional and near-source recordings. This event was located just beneath the ruptured zone of the 1985 Michoacán thrust event. From the inversion of teleseismic recordings, we obtain a nearly-vertical faulting at 35 km depth, a total source duration of 15 sec, a moment magnitude of Mw=7.1, and a mean rupture velocity of 2.8 km/sec. An analysis of teleseismic and local seismograms shows that the rupture had a strong directivity to the southeast, with a rather complex source-time function with two main subevents and a total duration of 13 sec. The dislocation distribution obtained from 2D kinematic linear inversion, using both local strong motion and broad-band teleseismic data, shows a complex rupture pattern with the main seismic moment release occurring in the southwest portion of the fault. A comparison between the coseismic Coulomb failure stress change during the main shock, and the location and mechanism of the aftershocks, suggests that post-event seismicity may have been triggered by the stress changes due to the main event.
Stress tensor computation from earthquake fault-plane solutions: an application to seismic swarms at Mt. Etna volcano (Italy)
S. Gresta,C. Musumeci
Annals of Geophysics , 1997, DOI: 10.4401/ag-3867
Abstract: Fault-plane solutions of some tens of local earthquakes which occurred at Mt. Etna volcano during 1983-1986 have been inverted for stress tensor parameters by the algorithm of Gephart and Forsyth (1984). Three seismic sequences were focused on which respectively occurred during a flank eruption (June 1983), just after the end of a subterminal eruption (October 1984) and during an inter-eruptive period (May 1986). The application to the three sets of data of both the "approximate" and the "exact" methods evidenced the stability of results, and the stress directions are well defined in spite of the small number of events used for the inversion. The s1 obtained agrees with the regional tectonic framework, nearly horizontal and oriented N-S, only in the shallow crust, and just after the 1984 eruption. This supports the hypothesis of a tectonic control on the end of the eruptive activities at Mt. Etna. Conversely, results concerning the depth range 10-30 km are in apparent disagreement with other investigations (Cocina et al., 1997), as well as with the regional tectonics. The stress was here found homogeneous, but with s1 respectively trending ENE-WSW (June 1983) and E-W (May 1986). We suggest that the stress field could be temporarily modified by a local stress regime driven by the intrusion of uprising magma.
INVERSION/EVERSION STRENGTH DYSBALANCE IN PATIENTS WITH MEDIAL TIBIAL STRESS SYNDROME
O?uz Yüksel,Cengizhan ?zgürbüz,Metin Ergün,?etin ??legen
Journal of Sports Science and Medicine , 2011,
Abstract: The main purpose of the study is to investigate the inversion/eversion muscle strength balance of the ankle in patients with medial tibial stress syndrome (MTSS). A dysbalance of these muscles may play a role in the pathophysiology of MTSS. Another aim is to measure the medial longitudinal arch and navicular drop in patients with MTSS. A total of 11 patients diagnosed with MTSS in the outpatient clinic of Ege University School of Medicine Sports Medicine Department were enrolled in this study. The control group consisted of 11 regularly exercising individuals. The mean age of the patient group and the control group was 21. 0 ± 1.9 years (18-23 years) and 23.2 ± 2.9 years (18-27 years), respectively. A detailed exercise questionnaire was administered to all subjects. Isokinetic muscle strength testing was performed at 30°/sec and 120°/sec to assess invertor and evertor muscle strength of the ankle. Photographs of the weight bearing and non-weight bearing foot were taken to measure the medial longitudinal arch deformation and the navicular drop. At 30°/sec, the average eversion concentric strength was significantly higher in the patient group, and the inversion/eversion strength ratio was significantly higher in the control group (p < 0.05). At 120°/sec velocity, average concentric eversion strength was significantly higher in the patient group (p < 0.05). The measurements of pronation indicators did not reveal any statistically significant differences between the two groups (p > 0.05). MTSS may occur without an increase of pronation indicators like medial longitudinal arch deformation or navicular drop. In such cases, one of the predisposing factors may be the strength dysbalance of the invertor/evertor muscles in favour of the evertor muscles. This observation may be of additional value in the prevention and therapy of MTSS.
The Application of Joint Inversion in Geophysical Exploration  [PDF]
ákos Gyulai, Mátyás Krisztián Baracza, éva Eszter Tolnai
International Journal of Geosciences (IJG) , 2013, DOI: 10.4236/ijg.2013.42026
Abstract:

The paper presents a short overview about the application of joint inversion in geophysics. It gives also an alternative explanation for the term of different data sets and discusses what types of inversion procedures can be considered as joint inversion. Nowadays there are no standard standpoints using the appellation joint inversion. What is joint inversion? Based on the information matrix an answer could be given for this question what could be regarded as various types of data sets that are inverted simultaneously. We would like to expand the explanationthat is professed by many researchersof the method that regards only the simultaneous inversion of data sets based on different physical parameters as joint inversion.

On the origin of low angle normal faulting in the Southern Rio Grande Rift
Carciumaru, Dana;Ortega, Roberto;
Geofísica internacional , 2011,
Abstract: we reconstruct the stress regime in the east potrillo and franklin mountains. using modern numerical techniques the stress field and the tectonic history of this region is discussed and extensional veins were compared with the general stress field. the majority of faults were reactivated during the first rio grande rift extensional event and occurrence of extensive tilting and rotations occurred. in the east potrillo mountains, back tilting by w25°se and w45°se oriented along strikes of n30°w are required in order to obtain two homogeneous stress fields. the corrected fault plane shows a significant oblique component in both stress fields. the first stress field corresponds to a fault reactivation with σl and σ3 oriented n78°e and n69°e respectively, whereas the second stress field is related to the youngest fault oriented n64°w and n61°e respectively. in the franklin mountains, tilting is less significant; however two stress fields with similar orientations but different phi values are recognized. the post -laramide stress history of the region is controlled by different extensional events that influenced the preexisting structures and generated other young fault systems.
Geologic structure of the northern margin of the Chihuahua trough: Evidence for controlled deformation during Laramide Orogeny
Carciumaru, Dana;Ortega, Roberto;
Boletín de la Sociedad Geológica Mexicana , 2008,
Abstract: in this article we studied the northern part of the laramide foreland of the chihuahua trough. the purpose of this work is twofold; first we studied whether the deformation involves or not the basement along crustal faults (thin- or thick- skinned deformation), and second, we studied the nature of the principal shortening directions in the chihuahua trough. in this region, style of deformation changes from motion on moderate to low angle thrust and reverse faults within the interior of the basin to basement involved reverse faulting on the adjacent platform. shortening directions estimated from the geometry of folds and faults and inversion of fault slip data indicate that both basement involved structures and faults within the basin record a similar laramide deformation style. map scale relationships indicate that motion on high angle basement involved thrusts post dates low angle thrusting. this is consistent with the two sets of faults forming during a single progressive deformation with in - sequence - thrusting migrating out of the basin onto the platform. we found that the style of deformation in the chihuahua trough is variable. in places such as the east potrillo mountains and indio mountains is typical of the thin- skinned style, associated with the cordilleran thrust belt, while in other places, the thick - skinned deformation present is typical of the laramide orogeny in the southern rocky mountains. the franklin mountains record the transition from thick- to thin - skinned deformation. we notice that this difference in the style of deformation is related to the thickness of the cretaceous section within the chihuahua trough. on the other hand, the orientation of the shortening direction can be explained based on the geometry of the trough and especially the strike of its eastern margin. along strike variations in shortening direction and kinematics are controlled by the curved northeast margin of the trough and reflect stress reorientation along the weak
Stress tensor computations at Mount St. Helens (1995-1998)
C. Musumeci,S. D. Malone,E. Giampiccolo,S. Gresta
Annals of Geophysics , 2000, DOI: 10.4401/ag-3681
Abstract: Fault plane solutions of 459 events occurring between 1995 and 1998 at Mount St. Helens (State of Washington, Northwest U.S.A.) were considered in order to infer the state of stress beneath the volcano. These events occurred in two distinct depth zones. The shallower zone is between 2 and 5.5 km, with shocks clustering in a tight cylindrical distribution about 1 km in radius directly beneath the crater. The deeper events are spread over a larger volume from 5.5 to 10 km depth and surround an aseismic zone below and slightly west of the lava dome. Faulting is characterized by a mixture of strike-slip, reverse and normal faults with maximum compression axes which do not cluster around a single direction. In the deep zone, between 5.5 and 10 km, P axes define a wheel-spoke pattern pointing radially away from the center of the aseismic zone. The 459 fault plane solutions were inverted for stress tensor parameters using the algorithm of Gephart and Forsyth. The inversion of the whole data set revealed that faulting was not produced by a uniform stress distribution. The subdivision of the zone into smaller volumes significantly reduced misfit and confidence areas of the solutions, whereas temporal subdivision of the sample did not lead to significant improvements in terms of stress uniformity. We suggest that the inhomogeneous stress field is consistent with a varying pressure source originating from the inferred crustal magma chamber and a thin conduit extending above it.
Seismological aspects of the Cairo earthquake, 12th October 1992
K. M. Abou Elenean,H. M. Hussein,A. S. Abu El-Ata,E. M. Ibrahim
Annals of Geophysics , 2000, DOI: 10.4401/ag-3655
Abstract: On 12 October 1992 a significant earthquake (MB = 5.8) occurred southwest of Cairo in the vicinity of the Dahshour region. For the mainshock, an average moment release M 0 = 9.70E17 n-m; a fault length = 13.8 km; an average displacement = 0.22 m; a stress drop = 0.760 MPa and maximum spectral magnitude = 5.78 were obtained. Spectral magnitude calculations were used in this analysis. The distribution of the well-recorded aftershocks over 15 months using a temporary seismic network installed immediately after the mainshock shows a zone of concentrated activity. Three composite focal mechanism solutions are constructed, using P wave polarity data for 30 earthquakes. These solutions have a mechanism, involving normal and right lateral strike slip motion along E-W to ENE-WSW trending fault plane. The P-wave polarity data of the individual 30 earthquakes are inverted to determine the stress tensor. The stress field estimated is extensional with s3 in the direction of NNE. The stress pattern determined from the inversion is in good agreement with the one estimated from the three composite fault plane solutions, while the neotectonic situation in Northern Egypt supports ENE-WSW extensional movement. However, the region of study has suffered both extensional tectonic activity of the Northern Red Sea rift zone and the compressional tectonic activity along the Hellenic arc. The inconsistency of the principal tension directions may belong to the interaction between the extensional and compressioal tectonics.
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