Inversion Of Focal Mechanism Data Research Articles

Seismicity and active stress field derived from the inversion of focal mechanism data, South Atlas of Tunisia

Seismicity and active stress field derived from the inversion of focal mechanism data, South Atlas of Tunisia

Source characteristics and b-values of the Tuz Gölü Fault Zone in Central Anatolia, Turkey

We investigate significant earthquake sequences in the observation period between 2000 and 2018 along the Tuz Gölü Fault Zone in Central Anatolia. These sequences include a series of strong events with moment magnitudes (Mw) >5.0, e.g. 30 July 2005 Bala-Ankara Mw 5.2, 20 December 2007 Bala-Ankara Mw 5.7 and 26 December 2007 Mw 5.6 Afşar-Ankara earthquakes. The mainshocks are shallow focus strike-slip events with minor normal component at depths of 12–15 km. Focal depths of aftershocks range from 2 to 30 km. The focal mechanisms of the aftershocks are mainly normal faulting with variable strike-slip components. The geometry of focal mechanisms reveals a normal faulting regime with E-W and also NE-SW trending directions of T-axis in the entire activated region. A stress tensor inversion of focal mechanism data is performed to acquire a more accurate picture of the Tuz Gölü Fault Zone stress field. The stress tensor inversion results display a predominant normal stress regime with a NNE–SSW oriented maximum horizontal compressive stress (SH). According to high-resolution hypocenter relocations of the Tuz Gölü Fault Zone seismic sequences, eight clusters are revealed. The aftershock activities in the observation period between 5 March 2000 and 2 June 2018 extend from NW to SE direction. Seismic cross-sections show that a complex pattern of the hypocenter distribution with the activation of Tuz Gölü Fault Zone segments. The western clusters are associated with a fault plane trending mainly N-S and dipping towards almost vertical, while the eastern clusters are related to a fault plane trending NW and dipping towards SE direction. The best constrained focal depths are mainly confined in the crust (depth < 30 km) and range from 2 to 30 km. In this study, we compare the results of an analysis of b-values from seismicity and stress tensors of focal mechanism solutions to understand their coupling in terms of faulting and earthquake hazard implications. In particular, this comparison allows to investigate the spatiotemporal correlation between b-values and stress distributions and it is therefore able to locate fault segments that have a high potential of generating large earthquakes. According to recent seismicity, b-values range from 0.5 to 1.5 along the Tuz Gölü Fault Zone in Central Anatolia. The maximum horizontal compressive stress (SH) indicates N-S and NE-SW directions while the minimum horizontal compressive stress (Sh) displays E-W. These results represent that low stress variances are correlated with high b-values. Low b-values are predominant in areas with high stress rates, consistent with a high heterogeneity of focal mechanisms.

An example of triggered earthquakes in western Turkey: 2000–2015 Afyon-Akşehir Graben earthquake sequences

We investigate significant earthquake sequences in the observation period between 2000 and 2015 along the Afyon-Akşehir Graben system in western Anatolia. These sequences include a series of strong events with moment magnitudes (Mw) larger than 5.5, e.g. Eber (Mw 5.8), Sultandağı (Mw 6.5) and Çay (Mw 5.8) earthquakes. The mainshocks are shallow focus normal events at depths of 8–15km. Focal depths of aftershocks range from 2 to 30km. The focal mechanisms of the aftershocks are mainly normal faulting with variable strike-slip components. The geometry of focal mechanisms reveals a normal faulting regime with NNE-SSW trending direction of T-axis in the entire activated region. A stress tensor inversion of focal mechanism data is performed to acquire a more accurate picture of the Afyon-Akşehir Graben stress field. The stress tensor inversion results display a predominant normal stress regime with a NW–SE oriented maximum horizontal compressive stress (SH). According to high-resolution hypocenter relocations of the Afyon-Akşehir Graben seismic sequences, six clusters are revealed. The aftershock activities in the observation period between 17 January 2000 and 26 October 2015 extend from east to west direction. Seismic cross-sections show that a complex pattern of the hypocenter distribution with the activation of Sultandağı Fault segments. The western clusters are associated with a fault plane trending mainly E-W and dipping towards SE, while the eastern clusters are related to a fault plane trending NW-SE and dipping towards vertical. The best constrained focal depths are mainly confined in the crust (depth<30km) and run in the approximate depth range from 2 to 30km. Moreover, Coulomb stress analysis is also performed to estimate the stress loading for the activated region. We obtain more than 3bars for positive lobes. This result defines that the values are large enough to increase the Coulomb stress failure towards NE-SW direction.

Tectonic stress pattern in the Chinese Mainland from the inversion of focal mechanism data

The tectonic stress pattern in the Chinese Mainland and kinematic models have been subjected to much debate. In the past several decades, several tectonic stress maps have been figured out; however, they generally suffer a poor time control. In the present study, 421 focal mechanism data up to January 2010 were compiled from the Global/Harvard CMT catalogue, and 396 of them were grouped into 23 distinct regions in function of geographic proximity. Reduced stress tensors were obtained from formal stress inversion for each region. The results indicated that, in the Chinese Mainland, the directions of maximum principal stress were ∼NE–SW-trending in the northeastern region, ∼NEE–SWW-trending in the North China region, ∼N–S-trending in western Xinjiang, southern Tibet and the southern Yunnan region, ∼NNE–SSW-trending in the northern Tibet and Qinghai region, ∼NW–SE-trending in Gansu region, and ∼E–W-trending in the western Sichuan region. The average tectonic stress regime was strike-slip faulting (SS) in the eastern Chinese Mainland and northern Tibet region, normal faulting (NF) in the southern Tibet, western Xinjiang and Yunnan region, and thrust faulting (TF) in most regions of Xinjiang, Qinghai and Gansu. The results of the present study combined with GPS velocities in the Chinese Mainland supported and could provide new insights into previous tectonic models (e.g., the extrusion model). From the perspective of tectonics, the mutual actions among the Eurasian plate, Pacific plate and Indian plate caused the present-day tectonic stress field in the Chinese Mainland.

Source Mechanisms and Stress Fields of the 15–16 June 2013 Crete Earthquake Sequence Along Hellenic Subduction Zone

15 June 2013 M w 6.1 off-shore southern Crete earthquake and its aftershock sequence along Hellenic Subduction Zone are examined. Centroid moment tensors (CMTs) for 40 earthquakes with moment magnitudes (M w) between 3.5 and 6.1 are determined by applying a waveform inversion method. The mainshock is shallow focus thrust event with a minor strike-slip component at a depth of 20 km. The seismic moment (M o) of the mainshock is estimated as 2.07 × 1018 Nm, and rupture duration of the mainshock is 4 s. The focal mechanisms of aftershocks are mainly thrust faulting with a strike-slip component. The geometry of the moment tensors (M w ≥ 3.5) reveals a thrust faulting regime with N–S trending direction of P axis in the entire activated region. According to high-resolution CMT solutions of the off-shore southern Crete earthquake sequence, one main cluster consisting of 40 events is revealed. The aftershock activity in the observation period between 15 June and 15 July 2013 extends from N to S and NW to SE directions. Seismic cross sections indicate a complex pattern of the hypocenter distribution with the activation of two segments. The subduction interface is clearly revealed with high-resolution hypocenter source relocation and moment tensor solution. The best-constrained focal depths indicate that the aftershock sequence is mainly confined in the upper plate (depth < 30 km) and is ranging from about 5–28 km depth. A stress tensor inversion of focal mechanism data is performed to obtain a more precise picture of the off-shore southern Crete stress field. The stress tensor inversion results indicate a predominant thrust stress regime with a NE–SW-oriented maximum horizontal compressive stress (S H). According to variance of the stress tensor inversion, to first order, the southern Crete region is characterized by a homogeneous interplate stress field. We also investigate the Coulomb stress change associated with the mainshock to evaluate any significant enhancement of stresses along southern Crete and surrounding regions. Positive lobes with stress more than 0.3 bars are obtained for the mainshock, indicating that these values are large enough to increase the Coulomb stress failure toward WNW–ESE direction.

The 16 April 2015 M w 6.0 offshore eastern Crete earthquake and its aftershock sequence: implications for local/regional seismotectonics

We examine the 16 April 2015 M w 6.0 offshore eastern Crete earthquake and its aftershock sequence in southern Aegean Sea. Centroid moment tensors for 45 earthquakes with moment magnitudes (M w) between 3.3 and 6.0 are determined by applying a waveform inversion method. The mainshock is shallow focus thrust event with a strike-slip component at a depth of 30 km. The seismic moment (M o) of the mainshock is estimated as 1.33 × 1018 Nm, and rupture duration of the mainshock is 3.5 s. The focal mechanisms of aftershocks are mainly thrust faulting with a strike-slip component. The geometry of the moment tensors (M w ≥ 3.3) reveals a thrust-faulting regime with NE–SW-trending direction of T axis in the entire activated region. According to high-resolution hypocenter relocation of the eastern Crete earthquake sequence, one main cluster consisting of 352 events is revealed. The aftershock activity in the observation period between 5 January 2015 and 7 July 2015 extends from N to S direction. Seismic cross sections indicate a complex pattern of the hypocenter distribution with the activation of three segments. The subduction interface is clearly revealed with high-resolution hypocenter relocation and moment tensor solution. The best constrained focal depths indicate that the aftershock sequence is mainly confined in the upper plate (depth <40 km) and are ranging from about 4.5 to 39 km depth. A stress tensor inversion of focal mechanism data is performed to obtain a more precise picture of the offshore eastern Crete stress field. The stress tensor inversion results indicate a predominant thrust stress regime with a NW–SE-oriented maximum horizontal compressive stress (S H). According to variance of the stress tensor inversion, to first order, the Crete region is characterized by a homogeneous interplate stress field. We also investigate the Coulomb stress change associated with the mainshock to evaluate any significant enhancement of stresses along Crete and surrounding regions. Positive lobes with stress more than 3 bars are obtained for the mainshock, indicating that these values are large enough to increase the Coulomb stress failure toward NE–SW and NW–SE directions, respectively.

Seismotectonics of the Antalya Basin and surrounding regions in eastern Mediterranean from 8 to 28 December 2013 Mw 5.0–5.8 earthquake sequence

The 8–28 December 2013 Mw 5.0–5.8 Antalya Basin earthquake sequence in eastern Mediterranean is examined. Centroid moment tensors for 16 earthquakes with moment magnitudes (Mw) between 3.6 and 5.8 are determined by applying a waveform inversion method. All earthquakes are shallow focus thrust events at a depth of 40–45km. The seismic moments (Mo) of the earthquakes are estimated as 4.10×1016–5.54×1017Nm and rupture durations of the mainshocks are 20–22s. The focal mechanisms of the aftershocks are mainly thrust faulting with a strike-slip component and reveal NW–SE trending direction of T-axis in the entire activated region. According to high-resolution hypocenter relocation of the Antalya earthquake sequence, seven main clusters are revealed. The aftershock activity in the observation period between 1 December 2013 and 23 January 2015 extends in an N to S direction. A seismic cross-section indicates that a complex pattern of the hypocenter distribution with the activation of seven segments. The westernmost cluster (cluster 1) is associated with a fault plane trending mainly WNW–ESE and dipping vertical, while the cluster 5 is related to a fault plane trending NNE–SSW and dipping towards SSE. The best constrained focal depths indicate that the aftershock sequence is mainly confined in the crust (depth<40km) and are operating in the approximate depth range from 3 to 110km. A stress tensor inversion of focal mechanism data is performed to obtain a more precise picture of the Antalya Basin stress field. The stress tensor inversion results indicate a predominant thrust stress regime with a NE–SW oriented maximum horizontal compressive stress (SH). According to variance of the stress tensor inversion, to first order, the Antalya Basin is characterized by a homogeneous interplate stress field. The Coulomb stress change associated with two mainshocks are also investigated to evaluate any significant enhancement of stresses along the Antalya Basin and surrounding regions. Positive lobes with stress of more than 0.4 bars are obtained for two mainshocks, indicating that these values are large enough to increase the Coulomb stress failure towards NE–SW and NW–SE directions, respectively.

Present-day tectonic regime and stress patterns from the formal inversion of focal mechanism data, in the North of Central–East Iran Blocks

The kinematic models and the associated orientation of compressional stress in northeastern Iran have long been the subject of debate. Previously proposed models have relied on the interpretation of overall fault geometry and geological fault slip data. These models generally suffer from low resolution of the temporal and spatial changes in the stress field and geodynamic regime. In the recent years there has been a significant increase in the number of focal mechanisms available for the study area, and it is now possible to estimate the present-day stress field in relative detail based on the formal stress inversion method. We compiled 59 earthquake focal mechanisms from the Global/Harvard CMT catalogue and various other sources. These we grouped into 7 separate regions (boxes). For each region reduced stress tensors have been obtained by formal stress inversion using the Win-Tensor program (Delvaux and Sperner, 2003). The present-day stress state obtained from earthquake focal mechanisms shows that the North of Central–East Iran Blocks (NCEIB) is presently subjected to a transpressional tectonic regime (R′ = 1.78 ± 0.26), with an N37 ± 7° E direction of horizontal principal compression. This stress state is consistent with the direction of convergence between the Arabian and Eurasian plates. In this part of northeastern Iran this convergence is accommodated by clockwise rotations. The Doruneh fault system (DFS) curves ESE–WSW, perpendicular to relative motion. This orientation results in oblique compression with a significant along-belt component of left-lateral shear in the western and central segments and belt-perpendicular compression in the eastern segment.

Seismicity of the 24 May 2014 Mw 7.0 Aegean Sea earthquake sequence along the North Aegean Trough

The northern Aegean Sea was hit by a large size (Mw=7.0) earthquake on 2014 May 24. Centroid moment tensor solutions for 40 events with moment magnitudes (Mw) between 3.3 and 7.0 are computed by applying a waveform inversion method on data from the Turkish and Greek broadband seismic networks. The time span of data covers the period between 2014 May 24 and 2014 June 26. The mainshock is a shallow focus strike–slip event at a depth of 15km. Focal depths of aftershocks range from 6 to 30km. The seismic moment (Mo) of the mainshock is estimated as 4.60×1019Nm. The calculated rupture duration of the North Aegean Sea mainshock is 40s. The focal mechanisms of the aftershocks are mainly strike–slip faulting with a minor normal component. The geometry of focal mechanisms reveals a strike–slip faulting regime with NE–SW trending direction of T-axis in the entire activated region. A stress tensor inversion of focal mechanism data is performed to acquire a more accurate picture of the northern Aegean Sea stress field along the North Aegean Trough. The stress tensor inversion results indicate a predominant strike–slip stress regime with a NW–SE oriented maximum principal compressive stress (σ1). In the development of the North Aegean Trough in Aegean Sea is in good agreement with the resolved stress tensors. With respect the newly determined focal mechanisms, the effect of the propagating of the North Anatolian Fault into Aegean Sea is very clearly pronounced. According to high-resolution hypocenter relocation of the North Aegean Sea seismic sequence, three main clusters are revealed. The aftershock activity in the observation period between 2014 May 24 and 2014 July 31 extends from the mainshock cluster from NE to the SW direction. Seismic cross-sections indicate that a complex pattern of the hypocenter distribution with the activation of seventeen segments. The eastern cluster is associated with a fault plane trending mainly ENE–WSW and dipping vertical, while the western is related to a fault plane trending NE–SW and dipping towards NNW. The best constrained focal depths indicate that the aftershock sequence is mainly confined in the crust (depth⩽30km) and is operating in the approximate depth range from 1.5 to 30km. Consequently, Coulomb stress analysis is performed to calculate the stress transfer and correlate it with the activated region. Positive lobes with stress more than 3bars are obtained, indicating that these values are large enough to increase the Coulomb stress failure towards ENE–WSW direction.

Stress Inversion of Focal Mechanism Data Using a Bayesian Approach: A Novel Formulation of the Right Trihedra Method

Research Article| March 18, 2015 Stress Inversion of Focal Mechanism Data Using a Bayesian Approach: A Novel Formulation of the Right Trihedra Method Luca D’Auria; Luca D’Auria aIstituto Nazionale di Geofisica e Vulcanologia, Sezione di Napoli Osservatorio Vesuviano, via Diocleziano, 328, 80124 Napoli, Italyluca.dauria@ov.ingv.it Search for other works by this author on: GSW Google Scholar Bruno Massa Bruno Massa bUniversità degli Studi del Sannio, Dipartimento di Scienze e Tecnologie, via dei Mulini, 59a, 82100 Benevento, Italy Search for other works by this author on: GSW Google Scholar Author and Article Information Luca D’Auria aIstituto Nazionale di Geofisica e Vulcanologia, Sezione di Napoli Osservatorio Vesuviano, via Diocleziano, 328, 80124 Napoli, Italyluca.dauria@ov.ingv.it Bruno Massa bUniversità degli Studi del Sannio, Dipartimento di Scienze e Tecnologie, via dei Mulini, 59a, 82100 Benevento, Italy Publisher: Seismological Society of America First Online: 14 Jul 2017 Online ISSN: 1938-2057 Print ISSN: 0895-0695 © 2015 by the Seismological Society of America Seismological Research Letters (2015) 86 (3): 968–977. https://doi.org/10.1785/0220140153 Article history First Online: 14 Jul 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Luca D’Auria, Bruno Massa; Stress Inversion of Focal Mechanism Data Using a Bayesian Approach: A Novel Formulation of the Right Trihedra Method. Seismological Research Letters 2015;; 86 (3): 968–977. doi: https://doi.org/10.1785/0220140153 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySeismological Research Letters Search Advanced Search We propose a Bayesian approach for the determination of the stress field from focal mechanism datasets. This method is a revision of the right trihedra method (RTM), used for both fault striation and focal mechanism data. The new probabilistic formulation of the RTM method (BRTM) allows a quantitative estimation of the confidence regions for the principal stress axes. Using an appropriate graphical representation, the method is able to provide simultaneous information about the stress field and its reliability. Fractures developed in rocks can preserve information about the paleostress field responsible for their own genesis. This information consists of superficial structures... You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The 10 June 2012 Fethiye Mw 6.0 aftershock sequence and its relation to the 24–25 April 1957 Ms 6.9–7.1 earthquakes in SW Anatolia, Turkey

The 10 June 2012 Mw 6.0 aftershock sequence in southwestern Anatolia is examined. Centroid moment tensors for 23 earthquakes with moment magnitudes (Mw) between 3.7 and 6.0 are determined by applying a waveform inversion method. The mainshock is a shallow focus strike-slip with reverse component event at a depth of 30km. The seismic moment (Mo) of the mainshock is estimated as 1.28×1018Nm and rupture duration of the Fethiye mainshock is 38s. The focal mechanisms of the aftershocks are mainly strike-slip faulting with a reverse component. The geometry of the focal mechanisms reveals a strike-slip faulting regime with NE–SW trending direction of T-axis in the entire activated region. A stress tensor inversion of focal mechanism data is performed to obtain a more accurate picture of the Fethiye earthquake stress field. The stress tensor inversion results indicate a predominant strike-slip stress regime with a NW–SE oriented maximum horizontal compressive stress (SH). According to variance of the stress tensor inversion, to first order, the Fethiye earthquake area is characterized by a homogeneous interplate stress field. The Coulomb stress change associated with the mainshock and the largest aftershock are also investigated to evaluate any significant enhancement of stresses along the Gulf of Fethiye and surrounding region. Positive lobes with stress more than 0.4 bars are obtained, indicating that these values are large enough to increase the Coulomb stress failure towards NNW–SSE and E–W directions.

Source characteristics and Coulomb stress change of the 19 May 2011 Mw 6.0 Simav–Kütahya earthquake, Turkey

On 2011 May 19, Simav district of Kütahya province in northwest Anatolia was hit by a moderate size (Mw=6.0) earthquake. Centroid moment tensors for 41 events with moment magnitudes (Mw) between 3.5 and 6.0 are computed by applying a waveform inversion method on data from the Kandilli Observatory and Earthquake Research Institute broadband seismic network. The time span of data covers the period between 2011 May 19 and 2011 August 22. The mainshock is a shallow focus normal event at a depth of 10km. Focal depths of aftershocks range from 5 to 20km. The seismic moment (Mo) of the mainshock is calculated 1.15×1018Nm. The estimated rupture duration of the Simav mainshock is 30s. The focal mechanisms of the aftershocks are mainly normal faulting with a variable strike-slip component. The geometry of focal mechanisms reveals a normal faulting regime with NE–SW trending direction of T-axis in the entire activated region. A stress tensor inversion of focal mechanism data is performed to acquire a more accurate picture of the Simav earthquake stress field. The stress tensor inversion results indicate a predominant normal stress regime with a NW–SE oriented maximum principal compressive stress (σ1). According to high-resolution hypocenter relocation of the Simav seismic sequence, two main clusters are revealed. The aftershock activity in the observation period between 2011 May 19 and 2011 August 22 extends from the mainshock cluster to the NE direction. Seismic cross-sections indicate that a complex pattern of the hypocenter distribution with the activation of two nearly parallel faults. The northern cluster is associated with a fault plane trending mainly E–W and dipping towards SE, while the southern is related to a fault plane trending NW–SE and dipping towards SE. The best constrained focal depths indicate that the aftershock sequence is mainly confined in the crust (depth<25km) and is operating in the approximate depth range from 2 to 24km. Consequently, Coulomb stress analysis is performed to calculate the stress transfer and correlate it with the activated region. Positive lobes with stress more than 3 bars are obtained, indicating that these values are large enough to increase the Coulomb stress failure towards NW–SE direction.

The April–June 2007 Trichonis Lake earthquake swarm (W. Greece): New implications toward the causative fault zone

On 10 April 2007, three moderate earthquakes with Mw=4.9–5.1 occurred in the vicinity of Trichonis Lake (W. Greece). A local network composed of 12 three-component digital seismographs was installed in the epicentral area and recorded more than 1600 events. The double-difference algorithm HYPODD, incorporating both catalog and waveform cross-correlation differential travel-time data, was applied for the successful relocation of 1490 earthquakes. The latter led to the distinction of a main NW-SE trending and NE-dipping zone, as well as of three neighboring faults; a conjugate NW-SE striking and SW-dipping marginal fault mapped along the northeastern flanks of the lake; a E-W trending and south-dipping low-angle normal fault, possibly related to the major Agrinio Fault Zone (AFZ), parallel to the northern bank of the lake; a NE-SW striking and NW-dipping normal fault, likely related to a segment of the active Evinos fault, located south of the lake. Calculation of the Coulomb stress induced by the combination of the 1975 Mw=6.0 event and the three largest events of 10 April 2007 on the inferred structures, reveals that most of the seismicity lies within the “stress-loaded” region, except for the westernmost activity, which probably belongs to the deep part of the AFZ. A total of 178 reliable focal mechanisms were determined by regional and local body-wave modeling (5 largest events) and P-wave first motion polarity data. The types of the obtained focal mechanisms are predominantly normal and strike-slip, however, numerous earthquakes were found to exhibit reverse faulting. Inversion of focal mechanism data showed that the prevailing principal horizontal component σ3 is quite homogeneous throughout the activated area with a roughly NW-SE trend, parallel to the strike of the Hellenides. On the contrary, the compressional field σ1 appears in two patterns: NE-SW trending onshore and NW-SE trending beneath the lake. This apparent rotation of σ1 by 90° reveals a complex system enclosed by the suggested NW-SE trending antithetic faults in depths between 7 and 9km. The calculated stress ratios beneath the lake imply that vertical forces are close to the overburden pressure. The overall inferred stress pattern is rather linked to topographic variations, locally imposing increase or decrease of the vertical forces. The presence of the water in the lake possibly plays an additional important role, penetrating through the bedrock, reducing the friction coefficient, while the pore pressure and, consequently, the effective stress increase. Thus, shearing along mature fractures is enhanced, likely yielding the observed diversity.

Seismicity and earthquake hazard analysis of the Teton–Yellowstone region, Wyoming

Earthquakes of the Teton–Yellowstone region represent a high level of seismicity in the Intermountain west (U.S.A.) that is associated with intraplate extension associated with the Yellowstone hotspot including the nearby Teton and Hebgen Lake faults. The seismicity and the occurrence of high slip-rate late Quaternary faults in this region leads to a high level of seismic hazard that was evaluated using new earthquake catalogues determined from three-dimensional (3-D) seismic velocity models, followed by the estimation of the probabilistic seismic hazard incorporating fault slip and background earthquake occurrence rates. The 3-D P-wave velocity structure of the Teton region was determined using local earthquake data from the Jackson Lake seismic network that operated from 1986–2002. An earthquake catalog was then developed for 1986–2002 for the Teton region using relocated hypocenters. The resulting data revealed a seismically quiescent Teton fault, at M L, local magnitude > 3, with diffuse seismicity in the southern Jackson Hole Valley area but notable seismicity eastward into the Gros Ventre Range. Relocated Yellowstone earthquakes determined by the same methods highlight a dominant E–W zone of seismicity that extends from the aftershock area of the 1959 (M S surface wave magnitude) 7.5 Hebgen Lake, Montana, earthquake along the north side of the 0.64 Ma Yellowstone caldera. Earthquakes are less frequent and shallow beneath the Yellowstone caldera and notably occur along northward trending zones of activity sub-parallel to the post-caldera volcanic vents. Stress-field orientations derived from inversion of focal mechanism data reveal dominant E–W extension across the Teton fault with a NE–SW extension along the northern Teton fault area and southern Yellowstone. The minimum stress axes directions then rotate to E–W extension across the Yellowstone caldera to N–S extension northwest of the caldera and along the Hebgen Lake fault zone. The combination of accurate hypocenters, unified magnitudes, and seismotectonic analysis helped refine the characterization of the background seismicity that was used as input into a probabilistic seismic hazards analysis. Our results reveals the highest seismic hazard is associated with the Teton fault because of its high slip-rate of approximately 1.3 mm/yr compared to the highest rate of 1.4 mm/yr in southern Yellowstone on the Mt. Sheridan fault. This study demonstrates that the Teton–Yellowstone area is among the regions highest seismic hazard in the western U.S.

African stress pattern from formal inversion of focal mechanism data

The kinematic models and the associated orientation of extensional stress of the East African Rift System have been subjected to much debate since a long time. In the past decades, the proposed models relied on the interpretation of the overall rift geometry, geological fault-slip data and the few focal mechanisms available. These models generally suffer of a poor time control and an underestimation of the possible changes in the stress field and geodynamic regime with time and space. In the recent years, there has been a significant increase in the number of focal mechanisms available for the entire rift system, and it is now possible to estimate the present-day stress field in relative detail based on seismotectonic data alone. We compile 347 focal mechanism data from the Global/Harvard CMT catalogue and various other sources and grouped 332 of them in 24 distinct regions (boxes) on the basis of their geographical proximity, kinematic homogeneity and tectonic setting. For each box and for the same data set, reduced stress tensors have been obtained by formal stress inversion using both the TENSOR program (Delvaux & Sperner, 2003) and the SLICK method (Michael, 1984/1987). Both inversion methods show in comparable results in terms of horizontal stress axes orientations and tectonic stress regimes, which stem for the robustness of the approach. The obtained stress pattern reflects a complex interaction between 1st order effects as different driving forces, including plate boundary forces, and 2nd and 3rd order effects as gravitational potential of topography, intra-lithospheric processes, and the influence of structural heterogeneities of the rift structures. The evidence present in the stress orientations of the 2nd and 3rd order stress pattern as the variations in the horizontal stress axes along the axis of the rift are of particular interest as they were not yet captured or shown in earlier numerical models of the stress field. Additional sources of tectonic forces in supplement to the gravitational potential energy forces as considered in the earlier models are necessary to explain the observed patterns.

Deformation of the subducted Indian lithospheric slab in the Burmese arc

Stress inversion of focal mechanism data in the Burmese arc region indicates distinct stress fields above and below 90 km along the subducted Indian lithospheric slab. In the upper part, the σ1 and σ3 axes trend NNE and ESE respectively, in conjunction with the ambient stress field of the Indian plate. However, in the lower part of the slab there is no preferred orientation of the σ1 or σ2 axes, but a very well defined σ3 axis is observed, that trends steeply in the down‐dip direction. It is inferred that while the upper part is governed by the NNE oriented horizontal plate tectonic forces, the lower part is governed entirely by tensile forces due to gravitational loading on the subducted slab. A model of attempted slab detachment at the base of the lithospheric contact zone is suggested, which is supported by results of high‐resolution seismic tomographic studies in this region.

Stress directions along the Alaska Wadati-Benioff zone from inversion of focal mechanism data

The focal mechanisms of 137 intermediate depth (60–180 km) earthquakes in the Alaska Wadati-Benioff zone (WBZ) were derived from P-wave first-motions. The study focused on three earthquake clusters referred to as Iliamna, Talkeetna, and Denali. In all three clusters, the orientations of the principal stresses (pressure ( P), null ( B), and tension ( T) axes) are similar. The P axes for most of the events show NNE-SSW trends with shallow plunge conforming to the geometry of the WBZ in the study area. The rest of the events yielded E-W oriented P axes. The T axis for most of the events plunge at a steeper angle than P axes, but parallel the dip of the WBZ. We interpret this feature as indicting the predominance of gravitational sinking of the associated underthrust lithospheric plate. The regional stress direction was estimated from the focal mechanisms using the inversion method of Gephart and Forsyth (1984). The results show that the maximum regional stress in the 60–90 km depth range plunges shallowly in the Iliamna (14°) and Talkeetna (5°) areas, and approximately in the direction of convergence (N163°E) of the Pacific and North American plates. In this depth range, similar results for the Denali area could not be obtained due to a lack of data. However, the inversion of the focal mechanism data for all three clusters for depths greater than 100 km yielded large values for both misfit and 95 percent confidence area compared with those in the depth range of 60–90 km. This may indicate the stress directions for depths greater than 100 km along the Alaska WBZ are more heterogeneous than in the depth range of 60–90 km.

Seismicity and tectonics in Jujuy Province, northwestern Argentina

The Portable Array for Numerical Digital Analysis (PANDA) network, a digitally recorded seismic array, operated for nine months in Jujuy province of northwestern Argentina. The network was deployed along the eastern edge of the Altiplano‐Puna plateau in a major N‐S thrust belt that is transitional in style between the thin‐skinned deformation of the Bolivian foreland to the north and basement‐involved deformation of the Pampean region to the south. Teleseismic locations of crustal earthquakes in the region indicate that seismicity is associated with compressional structures found near the eastern deformation front. No crustal seismicity was detected beneath the Puna plateau to the west. Peak seismicity levels beneath the foreland occurred between 20 and 25 km depth; a sharp decrease in seismicity was observed below 25 km. An estimate of 42 km for the thickness of the Jujuy foreland crust was inferred from wide‐angle Moho reflections observed on the digital seismograms. The highest concentration of crustal seismicity was located beneath Sierra de Zapla, a broad anticline immediately east of San Salvador de Jujuy. Many of the earthquakes in the 20–25 km depth range have a shallow, west dipping nodal plane as does the focal mechanism solution for a moderately large 1973 earthquake. Inversion of focal mechanism data for the orientation of principal stresses shows that maximum compression is oriented at azimuth 74°, closely paralleling both the current Nazca‐South America convergence direction and the shortening direction derived from regional Quaternary fault slip data. We interpret the earthquakes as occurring on planes of weakness first produced during Cretaceous rifting and later reactivated by Neogene compressive stresses. Crustal seismicity patterns and fault plane solutions suggest the presence of a midcrustal detachment, along which significant late Cenozoic E‐W shortening has occurred.

Change from Late Tertiary compression to Quaternary extension in southern Tibet during the India‐Asia Collision

Field studies in Southern Tibet show that subsequent to folding resulting from the India‐Asia collision (50–40 Ma), reverse faults and backthrusts formed as a result of NNE‐SSW and NNW‐SSE trending compressions. They affect conglomerates of Oligocene‐Miocene age and are probably contemporaneous with shearing on the Himalayan Main Central Thrust and folding of Paleogene basins along the northern border of Southern Tibet (25–16 Ma). Rift systems postdate compressional structures. They were active during the Pleistocene and probably during the Pliocene (5–2 Ma). Change from a compressional to an extensional regime in Tibet probably occurred during the Late Miocene (10–5 Ma). This change is tentatively correlated with uplift inferred by Chinese workers from Paleogene to present‐day variations of the Tibetan vegetation. Recent and active faulting in the High Plateau results from a WNW‐ESE striking extension; this is in agreement with Tibetan focal mechanisms. The Himalayan Lowlands are subjected to a NNE‐SSW trending compression as demonstrated by superficial deformations and seismicity. Therefore the horizontal maximum compressive stress trajectory is roughly parallel to India‐Asia convergence, σ Hmax being σ 1 in the Lowlands and σ 2 in the High Plateau. This change results from the increase of the vertical principal stress value due to the weight of the elevated mass, σ zz is σ 3 in the Lowlands and σ 1 in the High Plateau. Thus the third principal stress axis is σ 2 in the Lowlands and σ 3 in the High Plateau, allowing extension to occur in the WNW‐ESE direction. Extensional tectonics are significantly different in Southern and Northern Tibet. Southern Tibet being constrained between the Himalayan syntaxes, the E‐W extensional principal deviatoric stress value σ 3 is of rather small magnitude, and the N‐S trending σ 2 value is extensional, as demonstrated by analysis of Recent faults. Normal components prevail along all the azimuths; in the field this favors formation of rift systems of large axial extension. On the contrary, north of the Himalayan syntaxes, Northern Tibet “flows” easily eastward. The E–W extensional principal deviatoric stress value σ 3 is of higher magnitude, and the N‐S stress σ 2, is compressional, as shown by inversion of focal mechanism data. This favors strike‐slip components on faults which deviate from the N‐S direction, as expressed in the field by shorter N‐S trending graben linked with normal strike‐slip faults. These faults permit some N‐S shortening in Northern Tibet, without thickening, in response to India‐Asia collision. In high plateaus which are subjected to lateral compression such as the Andean Altiplano and Tibet, the extensional direction is basically controlled by the convergence direction, being roughly orthogonal to the latter.