Active Fault Plane Research Articles

Characterization of the active fault deformation zone of the Chegualin Fault in the alluvial plain of southwestern Taiwan

The activity of a creeping active fault poses significant challenges to engineering structures due to surface deformation. Therefore, quantifying the strain concentration caused by an active fault, delineating the extent and location of the Active Fault Deformation Zone (AFDZ), estimating long-term deformation trends, and predicting future deformations are crucial in the field of engineering geology.This study comprehensively integrates multi-timescale analytical methods by incorporating detailed geodetic data, rupture surveys, morphotectonic analysis, geological borehole data, biochronological data, radiocarbon dating, and Holocene uplift rate analysis to identify or confirm the locations of active faults and the long-term evolution trends of active deformation zones. Based on our findings, three active fault planes are identified, with one possibly being the main fault with the highest activity. Furthermore, by comparing long-term deformation rates derived from isochrone lines with short-term deformation rates obtained from leveling, we observe a risk of slip rate deficit. These findings have significant implications for engineering geology. As a general contribution, our study can serve as a site screening strategy for similar locations. Regarding the infrastructure we targeted, we provide critical input parameters for further numerical models (such as the trishear model) to simulate surface deformation, and offer essential design parameters for structures. Considering potential coseismic deformation on the investigated fault, these results are fundamental for future investigations or mitigation plans.

Source rupture process of the March 18th, 2021, Mw6.0 Béjaia (Algeria) earthquake associated with the Western segment – A link with the August 1856 Djidjelli earthquakes (Io = VIII-IX, M ≥ 6)

A strong offshore earthquake (Mw6.0) struck Béjaia city (eastern Algeria) on March 18th, 2021. This earthquake was followed by several aftershocks among the Mw5.2 that occurred 13 min after the main shock. Moreover, another earthquake (Mw5.0) occurred in the same zone one year later on March 19th, 2022. Near-field digital accelerograph records were used to study the earthquake and its related aftershocks. First, the March 2021 (Mw6.0) main shock, six of its main aftershocks, and the March 19th, 2022 (Mw5.0) earthquake were located. These epicentres are distributed in a 10 km-long and 2 to 3 km-wide NE–SW-trending area, with depths ranging between 8 km and 14 km. Second, using waveform inversion, the seismic moment and the focal mechanism of the three events (the March 18th, 2021, main shock and its strongest aftershock (Mw5.2) that occurred 13 min after the main shock and the March 19th, 2022 (Mw5.0) earthquake) were determined. These focal mechanisms exhibit reverse faulting with a short lateral component. Third, the source rupture process of the March 18th, 2021 (Mw6.0), earthquake was calculated from waveform inversion to obtain the moment–release distribution on a finite fault. The nodal plane oriented N74E seems to be associated with the activated fault plane. Considering the seismotectonic framework of the region, the fault that activated during the 2021 earthquake sequence is offshore. This fault, called the Western Segment, which is situated in the western part of the reverse fault system, is also at the origin of the Djidjelli historical earthquakes of August 21st, and 22nd, 1856 (Io = VIII-IX, M ≥ 6.6).

Structure and kinematics of active faulting in the northern domain of Western and Central Alborz, Iran and interpretation in terms of tectonic evolution of the region

Field evidence, seismicity, and geodetic data are combined to define the tectonic evolution along the northern domain of the Western and Central Alborz ranges, which have experienced significant earthquakes. One of our main focuses is to present the geometric-kinematic characteristics of active fault planes in the North Alborz fault zone and its subsidiary faults as a basis for seismic hazard assessment. To evaluate the seismotectonics in the region, three active segments of the seismic zone are identified. These segments primarily exhibit reverse mechanisms and left-lateral strike-slip components and act as the boundaries of neotectonic stress domains around the transition zone between Alborz and the South Caspian Basin. The spatial distribution of the strike-slip and dip-slip faulting over the Western and Central Alborz range supports our understanding of the geodynamic processes of the region. Our map of local active faults reveals five main systems with N–S, NE–SW, ENE–WSW, E–W, and NW–SE directions. Field studies indicate that strike-slip movements predominate and are consistent with a ∼WNW-ESE-oriented left-lateral wrench system. Pure left-lateral strike-slip faults trending E-W, extensional faults trending NE–SW, and compressional faults trending NW-SE are identified. Regional field investigations reveal spatial variations in the fault zone width of the northern domain of the Western and Central Alborz. The findings demonstrate that fault zone structures and their spatial distribution along the width of the area are strongly affected by the strain partitioning process in the transpressional zone. A spectacular well-exposed transpressional zone from the Western and Central Alborz region is described in detail, where outer domains are faults with a dominant reverse mechanism, and the inner domain is a wrench zone that includes faults with a dominant strike-slip mechanism. The seismic activity in the Northern Alborz region is mainly concentrated along the North Alborz fault zone and its subsidiary fault planes, of which some are identified for the first time in this research. Significant seismic events have occurred at locations where these newly identified fault planes interact. The maximum values of the geodetic strain axes in Western and Central Alborz are consistent with the area of high seismicity.

Non-planarity, scale-dependent roughness and kinematic properties of the Pidima active normal fault scarp (Messinia, Greece) using high-resolution terrestrial LiDAR data

Terrestrial LiDAR (TLS), photogrammetric and field data were collected during the years 2014, 2015 and 2017, at a 20-m long limestone scarp locality along the N–S striking, 70° west-dipping, active Pidima fault (Messinia, SW Peloponnese, Greece). This locality presents an artificially exhumed portion of an active fault plane, thus providing a unique opportunity to study kinematics and limestone scarp morphology (including its curvature and roughness). The survey of 2015 offered a high-density point cloud of the fault scarp with 6-mm working resolution, creating a very close to real life representation 3D model. We found that scarp geometry is non-planar with increasing convexity up-dip and increasing northwesterly dip-directions from north towards south, along strike. Non-planarity is also recorded from the morphological data along strike with the appearance of several, slip-parallel troughs and ridges with a mean distance (half-wavelength) of 0.75 m. The fault-plane roughness (absolute values) is scale dependent. The roughness configuration attains a slip-parallel pattern for observation scales above 1-cm. At observations scales less than 1-cm a slip-normal pattern is weakly visible. The obtained TLS results agree with field data (manual compass measurements) and macroscopic observations. We infer an earthquake magnitude of 6.2 ± 0.2 for the last event along the Pidima fault based on the measured thickness of the smooth stripe that was imaged by t-LiDAR along the non-exhumed surface of the scarp.

Reply to the “Comment on “The May 1 20 (MW 6.1) and 29 (MW 6.0), 2012, Emilia (Po Plain, northern Italy) earthquakes: New seismotectonic implications from subsurface geology and high-quality hypocenter location” by Carannante et al., 2015” by Bonini L., et al.

In their comments Bonini et al. argue that our seismotectonic interpretation of the Emilia 2012 seismic sequence does not agree with observations, and follow three lines of arguments to support their statement. These concern the structural interpretation of seismic reflection profiles, the relationship between seismogenic sources and seismicity patterns, and the fit of inferred fault geometry to InSAR observations. These lines of arguments are mostly repeating what has been previously presented by the same authors, and none of them, as discussed in detail in our reply, presents a strong case against our structural interpretation, that, we are convinced, does not conflict with the available data. The two adjacent rupture surfaces outlined by accurately relocated aftershocks are an indication of the presence of two different active fault planes. Interpretation of seismic profiles supports seismological observation and indicates the occurrence of relevant along-strike changes in structural style. These pieces of information have been integrated to build a new seismotectonic interpretation for the area of the Emilia 2012 seismic sequence. Analysis of geodetic data from the area of the Emilia earthquakes has produced very different models of the fault planes; unlike what has been stated by Bonini et al., who see a difficult fit to InSAR data for the fault planes we have identified, the most recent results are consistent with our interpretation that see a steep fault in the upper 8–10km under the Mirandola anticline. We point out that the geological structures in the subsurface of the Ferrara Arc do change along strike, and the attempt of Bonini et al. to explain both the May 20 and May 29 sequences using a single cross section is not appropriate.

Fracturation des carbonates dans la zone de faille normale active d’Aigion (Grèce) à partir des carottes du puits: conséquences sur les propriétés de transfert de fluides

Abstract La faille d’Aigion appartient à un système de failles normales à pendage nord affectant la bordure sud du golfe de Corinthe, témoin d’une extension active d’échelle régionale caractérisant la région égéenne. Les carottes du forage AIG-10, qui a traversé cette faille, montrent la présence d’une zone endommagée et d’une gouge. Ce travail présente une analyse des lames minces faites à partir de ces carottes. Il confirme la zonation en termes de fracturation à l’approche de la faille. Loin de cette dernière, la fracturation héritée de la phase compressive de l’orogenèse hellénique est dominante, bien qu’il existe aussi une famille E-W liée à l’extension actuelle. Toutes ces fractures sont scellées et le remplissage est similaire d’un point de vue luminescence au calcaire hôte. A l’approche du coeur, la quantité de fractures liées à la faille augmente; le remplissage de ces fractures indique, en cathodoluminescence, le passage de plusieurs fluides qui seraient d’origine externe, et la dernière génération de fractures est encore ouverte. Au mur de la faille, sous 5 m d’épaisseur verticale de cataclasite et d’ultracataclasite dans les calcaires et radiolarites (coeur de faille) et 13 m d’épaisseur verticale de gouge dans les radiolarites, les observations sont plus limitées, car la présence d’un karst a restreint les possibilités de carottage. Néanmoins, on peut observer que le remplissage des fractures liées à l’extension est différent de ce qui nous a été dévoilé au toit. Ceci suggère que la faille, transversalement imperméable aujourd’hui, l’a toujours été. L’analyse de la succession des ciments met en évidence une perméabilité parallèle à la faille à la faveur d’un système de fractures et fentes jeunes et ouvertes, pendant qu’est discuté le transfert de fluides par rapport aux stades de propagation de la faille.

High-resolution imagery of active faulting offshore Al Hoceima, Northern Morocco

Two recent destructive earthquakes in 1994 and 2004 near Al Hoceima highlight that the northern Moroccan margin is one of the most seismically active regions of the Western Mediterranean area. Despite onshore geodetic, seismological and tectonic field studies, the onshore–offshore location and extent of the main active faults remain poorly constrained. Offshore Al Hoceima, high-resolution seismic reflection and swath-bathymetry have been recently acquired during the Marlboro-2 cruise. These data at shallow water depth, close to the coast, allow us to describe the location, continuity and geometry of three active faults bounding the offshore Nekor basin. The well-expressed normal-left-lateral onshore Trougout fault can be followed offshore during several kilometers with a N171°E±3° trend. Westward, the Bousekkour–Aghbal normal-left-lateral onshore fault is expressed offshore with a N020°E±4° trending fault. The N030°E±2° Bokkoya fault corresponds to the western boundary of the Plio-Quaternary offshore Nekor basin in the Al Hoceima bay and seems to define an en échelon tectonic pattern with the Bousekkour–Aghbal fault. We propose that these three faults are part of the complex transtensional system between the Nekor fault and the Al-Idrissi fault zone. Our characterization of the offshore expression of active faulting in the Al Hoceima region is consistent with the geometry and nature of the active fault planes deduced from onshore geomorphological and morphotectonic analyses, as well as seismological, geodetic and geodynamic data.

Rapid rotation of normal faults due to flexural stresses: An explanation for the global distribution of normal fault dips

AbstractWe present a mechanical model to explain why most seismically active normal faults have dips much lower (30–60°) than expected from Anderson‐Byerlee theory (60–65°). Our model builds on classic finite extension theory but incorporates rotation of the active fault plane as a response to the buildup of bending stresses with increasing extension. We postulate that fault plane rotation acts to minimize the amount of extensional work required to sustain slip on the fault. In an elastic layer, this assumption results in rapid rotation of the active fault plane from ~60° down to 30–45° before fault heave has reached ~50% of the faulted layer thickness. Commensurate but overall slower rotation occurs in faulted layers of finite strength. Fault rotation rates scale as the inverse of the faulted layer thickness, which is in quantitative agreement with 2‐D geodynamic simulations that include an elastoplastic description of the lithosphere. We show that fault rotation promotes longer‐lived fault extension compared to continued slip on a high‐angle normal fault and discuss the implications of such a mechanism for fault evolution in continental rift systems and oceanic spreading centers.

Segmented seismicity of the M w 6.2 Baladeh earthquake sequence (Alborz Mountains, Iran) revealed from regional moment tensors

The Mw 6.2 Baladeh earthquake occurred on 28 May 2004 in the Alborz Mountains, northern Iran. This earthquake was the first strong shock in this intracontinental orogen for which digital regional broadband data are available. The Baladeh event provides a rare opportunity to study fault geometry and ongoing deformation processes using modern seismological methods. A joint inversion for hypocentres and a velocity model plus a surface-wave group dispersion curve analysis were used to obtain an adapted velocity model, customised for mid- and long-period waveform modelling. Based on the new velocity model, regional waveform data of the mainshock and larger aftershocks (Mw ≥3.3) were inverted for moment tensors. For the Baladeh mainshock, this included inversion for kinematic parameters. All analysed earthquakes show dominant thrust mechanisms at depths between 14 and 26 km, with NW–SE striking fault planes. The mainshock ruptured a 28° south-dipping area of 24 × 21 km along a north-easterly direction. The rupture plane of the mainshock does not coincide with the aftershock distribution, neither in map view nor with respect to depth. The considered aftershocks form two main clusters. The eastern cluster is associated with the mainshock. The western cluster does not appear to be connected with the rupture plane of the mainshock but, instead, indicates a second activated fault plane dipping at 85° towards the north.

Earthquake focal mechanisms and stress orientations in the eastern Swiss Alps

This study presents an updated set of earthquake focal mechanisms in the Helvetic and Penninic/Austroalpine domains of the eastern Swiss Alps. In eight cases, based on high-precision relative hypocentre locations of events within individual earthquake sequences, it was possible to identify the active fault plane. Whereas the focal mechanisms in the Helvetic domain are mostly strike-slip, the Penninic/Austroalpine domain is dominated by normal-faulting mechanisms. Given this systematic difference in faulting style, an inversion for the stress field was performed separately for the two regions. The stress field in the Penninic/Austroalpine domain is characterized by extension oriented obliquely to the E–W strike of the orogen. Hence, the Penninic nappes, which were emplaced as large-scale compressional structures during the Alpine orogenesis, are now deforming in an extensional mode. This contrasts with the more compressional strike-slip regime in the Helvetic domain towards the northern Alpine front. Relative to the regional stress field seen in the northern Alpine foreland with a NNW–SSE compression and an ENE–WSW extension, the orientation of the least compressive stress in the Penninic/Austroalpine domain is rotated counter-clockwise by about 40°. Following earlier studies, the observed rotation of the orientation of the least compressive stress in the Penninic/Austroalpine region can be explained as the superposition of the regional stress field of the northern foreland and a uniaxial extensional stress perpendicular to the local trend of the Alpine mountain belt.

36Cl exposure dating of paleoearthquakes in the Eastern Mediterranean: First results from the western Anatolian Extensional Province, Manisa fault zone, Turkey

Research Article| November 01, 2012 36Cl exposure dating of paleoearthquakes in the Eastern Mediterranean: First results from the western Anatolian Extensional Province, Manisa fault zone, Turkey Naki Akçar; Naki Akçar † 1Institute of Geological Sciences, University of Bern, Baltzerstrasse 1-3, 3012 Bern, Switzerland †E-mail: akcar@geo.unibe.ch Search for other works by this author on: GSW Google Scholar Dmitry Tikhomirov; Dmitry Tikhomirov 1Institute of Geological Sciences, University of Bern, Baltzerstrasse 1-3, 3012 Bern, Switzerland Search for other works by this author on: GSW Google Scholar Çağlar Özkaymak; Çağlar Özkaymak 1Institute of Geological Sciences, University of Bern, Baltzerstrasse 1-3, 3012 Bern, Switzerland2Engineering Geological Department, Dokuz Eylül University, 35160 İzmir, Turkey Search for other works by this author on: GSW Google Scholar Susan Ivy-Ochs; Susan Ivy-Ochs 3Institute of Particle Physics, ETH Hönggerberg, 8093 Zürich, Switzerland Search for other works by this author on: GSW Google Scholar Vasily Alfimov; Vasily Alfimov 3Institute of Particle Physics, ETH Hönggerberg, 8093 Zürich, Switzerland Search for other works by this author on: GSW Google Scholar Hasan Sözbilir; Hasan Sözbilir 2Engineering Geological Department, Dokuz Eylül University, 35160 İzmir, Turkey Search for other works by this author on: GSW Google Scholar Bora Uzel; Bora Uzel 2Engineering Geological Department, Dokuz Eylül University, 35160 İzmir, Turkey Search for other works by this author on: GSW Google Scholar Christian Schlüchter Christian Schlüchter 1Institute of Geological Sciences, University of Bern, Baltzerstrasse 1-3, 3012 Bern, Switzerland Search for other works by this author on: GSW Google Scholar GSA Bulletin (2012) 124 (11-12): 1724–1735. https://doi.org/10.1130/B30614.1 Article history received: 03 Oct 2011 rev-recd: 14 Apr 2012 accepted: 19 Apr 2012 first online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Naki Akçar, Dmitry Tikhomirov, Çağlar Özkaymak, Susan Ivy-Ochs, Vasily Alfimov, Hasan Sözbilir, Bora Uzel, Christian Schlüchter; 36Cl exposure dating of paleoearthquakes in the Eastern Mediterranean: First results from the western Anatolian Extensional Province, Manisa fault zone, Turkey. GSA Bulletin 2012;; 124 (11-12): 1724–1735. doi: https://doi.org/10.1130/B30614.1 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 SocietyGSA Bulletin Search Advanced Search Abstract Based on historical and instrumental data, societies in the Eastern Mediterranean and Middle East have survived at least 150 large earthquakes (generally M > 6) during the past 2500 yr. Beyond this time span, an earthquake chronology is mostly unknown, which hampers the production of reliable long-term earthquake models. Since the only remaining evidence of seismic activity is a bedrock scarp, cosmogenic 36Cl is the only suitable nuclide to be applied in the determination of the seismic history and slip rate of an active limestone fault plane. In this study, we focus on the 4-m-high Mugırtepe fault scarp within the active Manisa fault zone in western Anatolia, one of the most seismically active and rapidly extending regions in the world. We analyzed 44 samples in two slightly overlapping strips, which in total covered 2.65 m of the fault scarp. In order to determine the timing and the amount of slip of the paleoseismic events, we analyzed the measured 36Cl concentrations using the Schlagenhauf Matlab® code. We used two different scenarios based on two different inherited 36Cl concentrations as constrained by our data and modeling. The best fit for the first scenario yields two seismic events, one at 13.7 ± 0.8 ka with a displacement of 0.5 ± 0.2 m and one at 7.8 ± 0.5 ka with 2.15 ± 0.35 m offset. For the second scenario, we obtained a single seismic event at 8.6 ± 0.6 ka with 2.65 ± 0.35 m of slip. These results indicate that the visible part of the Mugırtepe exposed fault scarp had achieved most of its displacement already by 8 ka. Initial surface faulting at Mugırtepe occurred not later than around 14 ka, and marked seismic activity continued until around 8 ka. Our first results from the western Anatolian Extensional Province show the ability to reveal periods of enhanced seismic activity beyond historical data using cosmogenic 36Cl. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Application of fuzzy clustering method to determining sub-fault planes of earthquake from aftershocks sequence

Earthquake rupture process generally involves several faults activities instead of a single fault. A new method using both fuzzy clustering and principal component analysis makes it possible to reconstruct three dimensional structure of involved faults in earthquake if the aftershocks around the active fault planes distribute uniformly. When seismic events are given, the optimal faults structures can be determined by our new method. Each of sub-fault planes is fully characterized by its central location, length, width, strike and dip. The resolution determines the number of fault segments needed to describe the earthquake catalog. The higher the resolution, the finer the structure of the reconstructed fault segments. The new method successfully reconstructs the fault segments using synthetic earthquake catalogs. By taking the 28 June 1992 Landers earthquake occured in southern California as an example, the reconstructed fault segments are consistent with the faults already known on geological maps or blind faults that appeared quite frequently in longer-term catalogs.

Integrated geophysics and soil gas profiles as a tool to characterize active faults: the Amer fault example (Pyrenees, NE Spain)

The combined use of geophysical and soil gas composition exploration methods allows to rapidly obtain at relative low cost information that might be related to seismic activity conditions. In this study, we carried out geochemical soil gas sampling (222Rn, 220Rn and CO2), electrical resistivity tomography and seismic refraction profiles in two selected zones near the town of Amer in the Spanish Pyrenees, where the presence of recent fractures is evident in the field. Data analysis clearly reveals anomalous values for each gas at specific positions along the electrical imaging transects. Geomorphologic and hydrogeologic data and the integration of geophysical data and soil gas measurements indicate that: (1) endogene gases radon (222Rn) and carbon dioxide (CO2) are released from the meta-sedimentary basement rocks across the main fractured zones with higher permeability values, while lower Cenozoic detrital sedimentary formations act as an impervious boundary; (2) sites with highest radon concentrations (52 kBq m−3) coincide with the zones in the Amer fault showing more recent geomorphic evidence of activity, and more specifically with those areas covered by thinner surficial formations; (3) the lowest 222Rn values (0.2–0.4 kBq m−3) were recorded just on the master active fault plane. This pattern could be explained by a dilution effect resulting from high rates of soil CO2 efflux (267 g m−2 day−1); (4) soil thoron (220Rn) activity is maximum (143 kBq m−3) in areas with high surficial fracturing; (5) groundwater pumping may cause important distortions in the natural flow dynamics and in the measured concentrations of gases. The agreement between the different data (geochemical, geophysical, and hydrogeological) and field observations (geology and geomorphology) leads us to propose a preliminary tectonic-gravitational model for the study area.

Slip distribution model of two small-sized inland earthquakes and its tectonic implication in north-eastern desert of Egypt

Two slip distribution models of the 1999 and 2006 Beni-Suef earthquakes ( M w 4.5 and M w 4.2, respectively) were revealed using waveform inversion with an empirical Green’s function method. Waveform data used were recorded by the Egyptian Seismic Network’s short-period stations. To identify the active fault plane associated with each event and to estimate the best fitting rupture velocity, a set of slip distribution models was recovered on both nodal planes of the focal mechanism with a range of fixed rupture velocities. The result for the 1999 event shows that the northwest trending plane consistently provided better fitting solutions than the southwest trending plane. This implies that the slip was left-lateral on a northwest trending plane dipping toward the southwest. The 2006 event caused a slip movement on a dip-slip fault dipping towards the north. The rupture velocities were founded to be 3.5 km/s and 3.0 km/s which gave a comparatively small residual for the 1999 and 2006 earthquakes, respectively. The corresponding slip distribution models for the 1999 and 2006 events provide their seismic moments of 7.6E + 15 N m and 2.5E + 15 N m and moment magnitudes of 4.5 and 4.2, respectively. From the viewpoint of tectonics in the region, the present results imply an extensional tectonic process along the pre-existing WNW–ESE/NW–SE faults that may be transferred from the Gulf of Suez–Red Sea rift towards the northeastern desert in Egypt. This finding implies the diving of the northeastern of African plate beneath the Eurasian plate.

Application of GPS in identifying active fault plane in Western Maharashtra Peninsular shield of India

A broad overview of how GPS can be used to identify the active fault plane along which earthquakes occur in the seismic prone Western Maharashtra Peninsular shield of India is given. Indian Institute of Technology, Bombay in collaboration with Indian Institute of Geomagnetism, Mumbai was actively working in the field of crustal deformation monitoring along Western Maharashtra from year 2004–2006. A regional GPS network had been established for this purpose, periodically observed between May 2004 to May 2006, and analysed. To understand the pattern of seismicity along Western Maharashtra region, the GPS-estimated coordinates and displacements were used as an indicator to estimate and identify regions of high strain rates, the reliability of which was confirmed with real seismic data collected. These strain rates were used as a means to identify the fault plane along which the earthquakes occurred during the period of observation.

Earthquake source parameters at the Sumatran Fault Zone: Identification of the activated fault plane

Earthquake source parameters at the Sumatran Fault Zone: Identification of the activated fault plane

Earthquake source parameters at the sumatran fault zone: Identification of the activated fault plane

AbstractFifteen earthquakes (Mw 4.1–6.4) occurring at ten major segments of the Sumatran Fault Zone (SFZ) were analyzed to identify their respective fault planes. The events were relocated in order to assess hypocenter uncertainty. Earthquake source parameters were determined from three-component local waveforms recorded by IRIS-DMC and GEOFON broadband lA networks. Epicentral distances of all stations were less than 10°. Moment tensor solutions of the events were calculated, along with simultaneous determination of centroid position. Joint analysis of hypocenter position, centroid position, and nodal planes produced clear outlines of the Sumatran fault planes. The preferable seismotectonic interpretation is that the events activated the SFZ at a depth of approximately 14–210 km, corresponding to the interplate Sumatran fault boundary. The identification of this seismic fault zone is significant to the investigation of seismic hazards in the region.

Post 2000-swarm microearthquake activity in the principal focal zone of West Bohemia/Vogtland: Space-time distribution and waveform similarity analysis

We present the pattern of seismic activity in the period between 2001 and 2007 for the Nový Kostel focal zone, which is recently the most active zone of the West-Bohemia/Vogtland earthquake swarm region. While the year 2001 was characterized by dying out of the 2000-swarm activity in the form of a few microswarms, almost no seismicity occurred in the period between 2002 and 2003. Since 2004 an elevated seismic activity occurs in the form of repeating microearthquake swarms. We used a relative location method to relate the hypocenter positions of the post-swarm activity to the geometry of the 2000-swarm cluster. We found that the activity has concentrated in several clusters, which have been repeatedly activated. Some clusters coincide with the position of the previous activity; the others have activated so far inactive deep segments at the southern edge of the Nový Kostel fault. Besides the shift of the hypocenters to the edges of the previously active area we observe a southward migration of the activity and an increase of maximum depths of earthquakes from 10 to 13 km. The waveform similarity analysis disclosed that some fault patches consist of only a single, repeatedly activated fault plane, while the others consist of multiple, differently oriented fault planes activated almost simultaneously. Most of the focal mechanisms are consistent with the geometry of hypocenters showing NNW-SSE trending steep fault planes with left-lateral strike-slip mechanisms and varying dip-slip component.

Source parameters of the Mw = 6.3 Aroma crustal earthquake of July 24, 2001 (northern Chile), and its aftershock sequence

The July 24, 2001, M w = 6.3 earthquake in Aroma, Chile, is one of the few moderately shallow earthquakes to occur recently in northern Chile. This study uses different seismological data (short-period, broadband, strong-motion) to locate the event and its corresponding aftershocks. In addition, it carefully constrains the focal depth using SP phase and the focal mechanism of the main-shock. Finally, a model of the strong-motion waveforms discriminates the activated fault plane among the two nodal planes. The main-shock fault plane solution obtained from the strong-motion analysis is (strike, dip, rake) = (14° ± 10°, 53° ± 15°, −163° ± 15°), which indicates a right-lateral motion on an inclined fault, in agreement with the aftershock distribution, which also indicates a fault striking N14°E and dipping about 50°E.

Faulting style and stress field investigations for swarm earthquakes in NE Baveria/Germany – the transition between Vogtland/NW-Bohemia and the KTB-site

A seismicity and stress field analysis of a region in NE Bavaria reveals a complex picture of seismic dislocation. The magnitudes are generally low, the strongest event recorded had a magnitude of 2.3. In the southern part of the area investigated, earthquakes occur very rarely. During the observation period of approximately four years, only four events, two of them forming a doublet, were recorded. Hypocentral depths in the southern part are considerably great (15 to 17 km) and indicate a mafic lower crust. The seismicity of the Marktredwitz area, located in the western extension of the Eger rift, is dominated by earthquake swarms that are strongly clustered in space and time. The swarms occurred at depths between 10 and 14 km. Precise relative relocations show clear planar arrangements of the hypocentres and enable to identify the orientation of active fault planes. A comparison of the structural and geomorphological settings reveals major similarities in the occurrence of earthquake swarms compared to the situation in the bordering Vogtland/NW-Bohemia swarm area.