Active Fault Traces Research Articles

Identification of active fault topography along the Kishtwar Fault, Jammu and Kashmir, Northwest Himalaya, India

Kishtwar region of Jammu and Kashmir, India has been investigated to identify the traces of active faults preserved in the topography of the area. The objectives were achieved by geomorphic mapping with the help of satellite imageries (Cartosat-1 with 2.5 m resolution), detailed geological field investigations, shallow sub-surface geophysical mapping and the optical luminescence dating. Active tectonic landforms mapped in this study include Kishtwar rhombohedral basin, fault scarps, drainage offset, deformed recent sediments, uplifted lacustrine sediments and faulted river terraces. The evidence of active extensional topography reported here is from the compressional tectonic regime of NW Himalaya and is represented by the ~ 10 km long and 2–3 km wide Quaternary basin, interpreted as a tectonic graben. The basin is vaulted by normal faults, where master fault and the antithetic fault show continuous and en echelon behaviour respectively. The presence of normal fault system bounding the basin/graben, lead to hypothesize that the releasing bend of Kishtwar strike-slip fault is responsible for their origin. Detailed field investigation along these faults led to highlight their active nature and future prospects for palaeoseismic investigations. The active nature of these faults is supported by dip-slip movement, fault scarp, fault rupture and displacement in the Holocene and recent sediments. The spatial distribution of these features has been confirmed with the help of Ground Penetrating Radar (GPR) survey. Optically Stimulated Luminescence (OSL) dating of the deformed units demonstrate that the displacement along the Kishtwar fault has occurred between 16.35 ± 2.13 to 13.14 ± 0.91 ka and 13.14 ± 0.91 to 7.89 ± 0.71 ka.

A systems-based approach to parameterise seismic hazard in regions with little historical or instrumental seismicity: active fault and seismogenic source databases for southern Malawi

Abstract. Seismic hazard is commonly characterised using instrumental seismic records. However, these records are short relative to earthquake repeat times, and extrapolating to estimate seismic hazard can misrepresent the probable location, magnitude, and frequency of future large earthquakes. Although paleoseismology can address this challenge, this approach requires certain geomorphic setting, is resource intensive, and can carry large inherent uncertainties. Here, we outline how fault slip rates and recurrence intervals can be estimated by combining fault geometry, earthquake-scaling relationships, geodetically derived regional strain rates, and geological constraints of regional strain distribution. We apply this approach to southern Malawi, near the southern end of the East African Rift, and where, although no on-fault slip rate measurements exist, there are constraints on strain partitioning between border and intra-basin faults. This has led to the development of the South Malawi Active Fault Database (SMAFD), a geographical database of 23 active fault traces, and the South Malawi Seismogenic Source Database (SMSSD), in which we apply our systems-based approach to estimate earthquake magnitudes and recurrence intervals for the faults compiled in the SMAFD. We estimate earthquake magnitudes of MW 5.4–7.2 for individual fault sections in the SMSSD and MW 5.6–7.8 for whole-fault ruptures. However, low fault slip rates (intermediate estimates ∼ 0.05–0.8 mm/yr) imply long recurrence intervals between events: 102–105 years for border faults and 103–106 years for intra-basin faults. Sensitivity analysis indicates that the large range of these estimates can best be reduced with improved geodetic constraints in southern Malawi. The SMAFD and SMSSD provide a framework for using geological and geodetic information to characterise seismic hazard in regions with few on-fault slip rate measurements, and they could be adapted for use elsewhere in the East African Rift and globally.

Active faults of El Salvador

In this work we present a review of the current state of knowledge of the active faults in El Salvador and its seismo-tectonic implications. An updated map of active fault traces is combined with the spatial analysis of shallow seismicity, focal mechanisms, recent geodetic GPS velocities, local strain estimations and morpho-tectonic features to provide a synoptic view of the active tectonics of the El Salvador. The major faults selected as potential seismic sources bound tectonic blocks or regions that include GPS sites with consistent relative velocity vectors. We propose several active tectonic domains along El Salvador controlled by three current deformation regimes: a crustal block in the forearc sliver dominated by rigid westward translation with the faster and more homogeneous GPS velocities; three zones dominated by E-W distributed extensional coaxial deformation; and two bands with a deformation compatible with transtensional regime. In the Western sector of the ESFZ, GPS velocities and local structure suggest that E-W extension concentrates along the NNW-SSE oriented Santa Ana volcanic axis and it could connect to the north with the extensional region of the Ipala Graben. This is consistent with to the eastward shift of the North America-Cocos-Caribbean triple junction and “the closing of the zipper” proposed in recent models that progressively slows down the strike-slip movement along the northern limit of the forearc sliver. In the central sector large GPS velocity gradient parallel to the volcanic arc may be associated to complex and discontinuous structure of the ESFZ driving slowdown of the westward movement of local tectonic blocks. The southeaster sector of the ESFZ is an incipient large pull-apart structure affecting a pre-existing extensional N-S oriented fabric that induces two tectonic subdomains, the eastern one undergo E-W coaxial extension and the western one characterized by transtensional strain regime. We propose a new structural segmentation of the ESFZ considering that this fault zone includes all those structures that accommodate the relative velocity between the forearc sliver and the Chortís block. The 38 active faults with surface traces that are mapped for more than 5 km long proposed in this work, with maximum potential magnitude Mw ranging from 5.98 to 7.94, will contribute to improve regional and local seismic sources databases and seismic hazard assessment.

Geomorphic assessment of the factors contributing to the evolution of landforms in Ukhaldhunga area, Kosi River valley, Kumaun Himalaya, Uttarakhand

This paper documents the various factors contributing to the evolution of landforms in the Kosi River valley, Ukhaldhunga area of the Kumaun Himalaya, Uttarakhand. The area falls in the major tectonic boundary between the Lesser Himalaya and the sub-Himalaya. Passing through the Parewa-Ukhaldhunga-Dabara area, this tectonic contact is known as the Main Boundary Thrust (MBT), which is characterized by presence of varied landforms. Study of the development of geomorphology, particularly those evolved in the MBT zone, suggests that the morphological features are intimately associated with the tectonic activities related to the movement of rocks along the MBT and the surface processes such as fluvial and mass movement. The various landforms observed in the MBT zone include strath terraces, fill terraces, paleochannels and V-shaped valley that pertain to fluvial genesis; gravity induced landforms such as landslide debris and colluvial fans; and tectonic landforms such as linear fault traces, swerving/swing of river course and linear depressions. Signature of recent tectonic activity along the MBT is observed between Dabara and Khakrakot. The activity is evident in the form of 8-km-long active fault trace trending NW-SE. Normal fault related landforms are observed in the hanging wall block of the MBT. Neotectonic strath terraces and fill terraces are well developed in the MBT zone. Multiple events of recurrence of landslides are evident from exposed section of landslide debris fans. The bedrocks are highly sheared, suggesting successive tectonic movements along the MBT. As a consequence, tectonic wedges have developed in the bedrocks. These wedges have facilitated for the recurrence of a number of landslides. The anthropogenic activities have also aggravated the slope stability. Truncation of older debris fan surfaces by an erosional/fault scarp is observed and the scarp separates the older fans from the youngest debris fan deposit. The scarp runs parallel to the MBT and Kosi River. Swerving of the Kosi River is as a result of the tectonic forcing along active lineaments/faults. The trend of lineaments of the maximum and high density is NE-SW, which is transverse to the trend of the NW-SE trending Himalayan fold-thrust belt.

Geological and geomorphic evidences of neotectonic activity along the Himalayan Frontal Thrust, Nahan Salient, NW Himalaya, India

In this paper, we report geomorphic signatures of neotectonic activity identified from the area around Nahan Salient along Himalayan Frontal Thrust (HFT), using remote sensing and field investigations. We document two newly discovered exposures of HFT fault zone and active fault traces along the mountain front at Khetpurali, Nahan Salient, NW Himalaya, India. We demonstrate that HFT fault zone at Khetpurali is not blind, instead it is exposed at the surface along with highly deformed, folded strata in the hanging wall and its geometry is quite variable with the possibility of along strike-segmentation. The geometry of fold and various deformational features preserved in the hanging wall indicate a process of fault-propagation-fold. This is in contrast to the previous observation of fault-bent-folding within the Nahan Salient. Two-three levels of river terraces are preserved and truncated in the hanging wall, along with the active fault scarps which are linked to uplift along HFT fault zone. The Khetpurali Anticline (KA), is a fault-propagating-fold formed due to displacement and shortening along HFT which dips 30° towards north and strike WNW-ESE. The uplift (U) along the KA is calculated to be ~73 m and accommodated 7.3% shortening of the original length, before the fault tip truncated the fold. Various deformational structures have been identified within the hanging wall around-tip damage zone of ~80 m perpendicular to HFT fault zone and in the core of KA. Northwest of Khetpurali, a south-facing hogback structure ~100 m high is formed due to erosion of forelimb of the anticline. At Khetpurali, tectonic control due to folding of strata and uplift of river terraces is evident as the Dudhgarh River forms V-shaped profile near the thrust, compared to the wide floodplain before and after crossing the HFT fault zone. Along strike segmentation of HFT and associated anticlines is observed as the surface trace of HFT is offset at Khetpurali, Trilokpur, and Sabilpur due to the strike-slip faults. The study reveals that HFT has been truncated by tear faults and has different subsurface geometries in study area which may have implication on seismic hazard in the Nahan Salient where three thrusts; Himalayan Frrontal Thrust (HFT), Nahan Thrust (NT) and Main Boundary Fault (MBF) are located at relatively shorter distance of ~6 km as compared to the surrounding recesses.

CCAF-DB: the Caribbean and Central American active fault database

Abstract. A database of ∼250 active fault traces in the Caribbean and Central American regions has been assembled to characterize the seismic hazard and tectonics of the area, as part of the Global Earthquake Model (GEM) Foundation's Caribbean and Central American Risk Assessment (CCARA) project. The dataset is available in many vector GIS formats and contains fault trace locations as well as attributes describing fault geometry and kinematics, slip rates, data quality and uncertainty, and other metadata as available. The database is public and open source (available at: https://github.com/GEMScienceTools/central_am_carib_faults, last access: 23 March 2020), will be updated progressively as new data become available, and is open to community contribution. The active fault data show deformation in the region to be centered around the margins of the Caribbean plate. Northern Central America has sinistral and reverse faults north of the sinistral Motagua–Polochic fault zone, which accommodates sinistral Caribbean–North American relative motion. The Central Highlands in Central America extend east–west along a broad array of normal faults, bound by the Motagua–Polochic fault zone in the north and trench-parallel dextral faulting in the southwest between the Caribbean plate and the Central American forearc. Faulting in southern Central America is complicated, with trench-parallel reverse and sinistral faults. The northern Caribbean–North American plate boundary is sinistral off the shore of Central America, with transpressive stepovers through Jamaica, southern Cuba and Hispaniola. Farther east, deformation becomes more contractional closer to the Lesser Antilles subduction zone, with minor extension and sinistral shear throughout the upper plate, accommodating oblique convergence of the Caribbean and North American plates.

Morphotectonic Kinematic Indicators along the Vigan-Aggao Fault: The Western Deformation Front of the Philippine Fault Zone in Northern Luzon, the Philippines

The Vigan-Aggao Fault is a 140-km-long complex active fault system consisting of multiple traces in the westernmost part of the Philippine Fault Zone (PFZ) in northern Luzon, the Philippines. In this paper, its traces, segmentation, and oblique left-lateral strike-slip motion are determined from horizontal and vertical displacements measured from over a thousand piercing points pricked from displaced spurs and streams observed from Google Earth Pro satellite images. This work marks the first instance of the extensive use of Google Earth as a tool in mapping and determining the kinematics of active faults. Complete 3D image coverage of a major thoroughgoing active fault system is freely and easily accessible on the Google Earth Pro platform. It provides a great advantage to researchers collecting morphotectonic displacement data, especially where access to aerial photos covering the entire fault system is next to impossible. This tool has not been applied in the past due to apprehensions on the positional measurement accuracy (mainly of the vertical component). The new method outlined in this paper demonstrates the applicability of this tool in the detailed mapping of active fault traces through a neotectonic analysis of fault-zone features. From the sense of motion of the active faults in northern Luzon and of the major bounding faults in central Luzon, the nature of deformation in these regions can be inferred. An understanding of the kinematics is critical in appreciating the distribution and the preferred mode of accommodation of deformation by faulting in central and northern Luzon resulting from oblique convergence of the Sunda Plate and the Philippine Sea Plate. The location, extent, segmentation patterns, and sense of motion of active faults are critical in coming up with reasonable estimates of the hazards involved and identifying areas prone to these hazards. The magnitude of earthquakes is also partly dependent on the type and nature of fault movement. With a proper evaluation of these parameters, earthquake hazards and their effects in different tectonic settings worldwide can be estimated more accurately.

Quaternary Activity of the Beihewan Fault in the Southeastern Beishan Wrench Belt, Western China: Implications for Crustal Stability and Intraplate Earthquake Hazards North of Tibet

AbstractWe present evidence for the Late Quaternary activity of the Beihewan Fault (BHWF), along the southeastern margin of the Beishan block, western China. Field observations and analysis of UAV‐derived DEMs and Google Earth images reveal an ~10‐km‐long active fault trace in a region previously considered tectonically inactive. Offset landforms and trench exposures reveal that the fault is dominantly strike‐slip, with local thrust or normal displacements. Average rates of left‐lateral motion and thrusting since the Holocene are estimated to be ~2.69 and ~0.35 mm/a, respectively. The most recent surface‐rupturing event generated ~2.5 m of strike‐slip offset since 6.2–8.6 ka; with estimated M = 6.3–6.9. Modeling and two‐dimensional inversion of new magnetotelluric data indicate that the BHWF is a subvertical low‐resistivity zone that penetrates into the lower crust. Existing geological and geophysical evidence do not support subsurface linkage between the BHWF and Altyn Tagh–Jinta‐Nanshan Fault system further south in the Hexi Corridor. Close examination of high‐resolution satellite imagery west and east of the BHWF reveals other Quaternary surface fault scarps, pressure ridges, offset drainages, and truncated lithological strike belts. Collectively, these features constitute a previously unrecognized 150‐km‐long sinistral deforming belt along the southeastern Beishan and northern Hexi Corridor boundary that we call the Southeast Beishan Wrench Belt. Recognition of Quaternary activity along the BHWF within the larger Southeast Beishan Wrench Belt challenges long‐held assumptions that the Beishan region is a relatively stable and tectonically inactive block within the Late Cenozoic Indo‐Eurasian deformation field north of Tibet.

Active tectonics along the Cul-de-Sac – Enriquillo plain and seismic hazard for Port-au-Prince, Haiti

The active faults in Haiti were not well known and no detailed mapping of active fault traces was available before the 2010, M7.0 earthquake. The lack of detailed fault mapping hindered the interpretation of the event and the seismic hazard assessment. Here, using high-resolution LIDAR topography, aerial photographs, bathymetric charts, together with geological data complemented with field observations, we carried out a morphotectonic analysis at a variety of scales. We analyzed the drainage network at the northern front of the Massif de la Selle – Sierra de Bahoruco (MSB) by mapping the fans of the main rivers and their associated drainage basins. We found that the areas of all the fans were compatible with the sizes of their drainage basins, except the paleofan of Port-au-Prince. We interpret that this paleofan has been offset by 7.9±0.3km across the main strike-slip Enriquillo-Plantain Garden Fault (EPGF) and we estimate a minimum horizontal slip rate of ∼3mm/yr over the Pleistocene. Moreover, in the Cul-de-Sac – Enriquillo plain, within which the capital city of Port-au-Prince is located, we mapped numerous NW-SE to WNW-ESE-striking Quaternary folds and thrust faults. Some of the thrust faults are north-dipping and located to the southern front of the Matheux-Neiba Lower Miocene fold. Other thrusts, such as those near Ganthier, Jacquet, Nan Cadastre, and Port-au-Prince, are south-dipping and located along the northern front of the MSB Lower Miocene fold. Previously, Saint Fleur et al. (2015) showed that the 2010 Haiti earthquake activated both one of the young thrusts (Lamentin) and the EPGF, consistent with oblique convergence between the Caribbean and the North American plates.

Detection and interpretation of local surface deformation from the 2018 Hokkaido Eastern Iburi Earthquake using ALOS-2 SAR data

We identified and analyzed surface displacements associated with the 2018 Hokkaido Eastern Iburi Earthquake in northern Japan using satellite radar interferograms from the Advanced Land Observing Satellite 2. The data generally show elastic deformation caused by the main earthquake as well as numerous complex surface displacements that cannot be explained by the motion of the seismic source fault. We identified three distinct phenomena: linear surface displacements representing secondary earthquake faults west of the epicenter, surface deformation caused by liquefaction in urban and coastal areas, and coherence changes in the interferograms due to landslides in mountainous areas and liquefaction in urban areas. The linear surface displacements show reverse fault motion with low dip angles and appear to be a geographic extension of known active faults; however, it is unlikely that these displacements were directly connected to the source fault of the main earthquake. Although there is no evidence that they generated strong seismic waves at the time of the main earthquake, there is a possibility that they represent active fault traces and could be the sources of large earthquakes in the future. Therefore, such linear surface displacements can be used to identify potentially dangerous hidden active faults. The interferograms reveal that liquefaction in urban areas occurred in low areas artificially filled during past residential development. Coherence-change maps drawn from the interferograms were useful for detecting liquefaction, but their high sensitivity limited their application for landslide detection in mountainous areas; the phase noise deviation method was more practical for purposes such as rapid response or mitigation. Our methods have the potential to allow improved mapping of local hazards in other areas and can be applied to urban planning and/or safety assessments.

Late Pleistocene left-lateral slip rates of the Gulang Fault and its tectonic implications in eastern Qilian Shan (NE Tibetan Plateau), China

The late Pleistocene deformation pattern in the eastern Qilian Shan remains controversial. Study of the geometry and slip rate of the fault holds crucial value for better understanding the regional deformation pattern. The active E-W trending Gulang and Tianjing Shan Fault Zone is located in the southern boundary of the Tengger Desert which plays a significant role in tectonic deformation and slip transfer in the NE Tibetan Plateau. Geomorphic mapping of the active fault traces reveal that the Gulang fault is composed of two segments, separated by the Guanjia Tai pull-apart basin. Offset geomorphic features indicate that the Gulang fault appears to be primarily sinistral strike-slip with no reverse component. Based on offset gullies/terraces and optically stimulated luminescence dating of offset-related sediments, we obtain a consistent late Pleistocene slip rate of ~2.5 mm/a along the western and eastern segments of the Gulang Fault. Meanwhile, the slip vectors indicate the Gulang fault redistributes strain and transforms displacement along the Haiyuan fault, accommodating the westward motion of the Gobi-Alashan block. Moreover, a devastating earthquake of Ms 8.0 struck Gulang on 23rd May 1927. Geomorphically fresh fault scarps and surface rupture zone are observed at Gulang Fault, we modify the Modified Mercalli Intensities (MMI) and revise the isoseismal map for the Gulang earthquake. Combining with the high-resolution deep seismic-reflection data, the Gulang fault may be the source of the earthquake. Regionally, GPS velocities indicate that faulting, crustal shortening, and basin/block rotation accommodate deformation of the plateau's northern margin. In conclusion, the fault geometry, kinematics, and regional GPS velocities reflect that this region experiences eastward movement of the northeastern Tibetan Plateau relative to the Gobi-Alashan.

Distribution of landslides along the Andean active orogenic front (Argentinean Precordillera 31–33° S)

As one of the main environmental effects of earthquakes, the distribution of landslides could reflect regional neotectonic activity. To that end, we carried out an inventory of landslides along the Precordillera range (31°-33°S) comprising the most seismically active region of Argentina. Main Quaternary deformation is concentrated in this Western central part of the country associated with active faults linked to intense shallow seismic activity (<35 km depth). During the last 150 years, the region has suffered at least six major earthquakes with a magnitude greater than Ms ≥ 7.0. The main focus of this research is to analyse the distribution of landslides along this Andean active orogenic front and characterize the response of collapses. We analysed morphological parameters of these landslides. According to our results, collapses match with traces of active Quaternary faults in this region. Landslides are clustered however, being more densely splayed in the centre of the study area and decreasing gradually towards the North and the South. We propose that the Juan Fernandez Ridge trace promotes a higher rate of deformation in this area, matching with higher historical seismicity and distribution of landslides.

Citizen Earthquake Science in Taiwan: From Science to Hazard Mitigation

Taiwan is located at the convergent plate boundary between the Eurasian and the Philippine Sea plates. As a result, intense earthquake activity and associated surface deformation are manifesting in this region. To implement and promote citizen earthquake science in Taiwan, we have developed several web-based platforms with multi-purpose themes, such as earthquake science information, popular science education, and crowdsourcing systems. First, with the rapid earthquake report issued from the Taiwan Central Weather Bureau (CWB), the available near real-time scientific results obtained from the Taiwanese seismology community are collected and published to a platform, the Taiwan Earthquake Science Information System (TESIS). The scientific information archived at TESIS includes CWB earthquake reports, focal mechanisms, shake maps, and finite source models (for strong earthquakes). All real-time results are integrated into a GIS system with background geospatial information, such as geological maps, traces of active faults, background seismicity, and inter-seismic GPS velocity fields. Second, by collaborating with Stanford University to maintain a regional Quake-Catcher Network (QCN) server in Taiwan, we have promoted citizen seismology in Asia by bringing earthquake information and scientific knowledge to the public. More than 200 school teachers have already installed the QCN sensors in Internet-enabled computers. Through two web-based educational platforms, users are able to access the guidelines and further interact with the recorded waveforms. Third, we also developed an earthquake damage reporting system – the Taiwan Scientific Earthquake Reporting (TSER) system – to encourage the citizen to collect field observation for significant earthquake-induced ground damages such as surface fault rupture, landslide, rock fall, liquefaction, and landslide-triggered dam or lake. The TSER system is constructed under the Ushahidi mapping platform, which has been widely used in crowdsourcing for the geospatial archiving of events. Trained high school teachers and public volunteers can send their ground damage observations, including photographs, through the TSER system. Most of these products and online systems are now being operated by the Taiwan Earthquake Research Center (TEC). With these newly developed platforms and materials, we aim to not only raise earthquake awareness and preparedness, but also encourage public participation in earthquake science in Taiwan.

Seismic faults and sacred sanctuaries in Aegean antiquity

The ancient destructive capability of earthquake faults is well chronicled by historians and their cultural impact widely uncovered by archaeologists. Archaeological and geological investigations at some of the most renowned sites in the ancient Greece world, however, suggest a more nuanced and intimate relationship between seismic faults and past human settlements. In the Aegean’s karstic landscape earthquake fault scarps act as limestone ramparts on which fortifications, citadels and acropoli were constructed, and underlying fault lines were preferred pathways for groundwater movement and egress. The vital purifactory or therapeutic role of natural springs in the ritual practices of early settlements implies that the fault lines from which they leaked may have helped position the nascent hubs of Greek cities. Equally, the tendency for earthquakes to disrupt groundwater patterns and occasionally shut down persistent springs provides a hitherto unrecognized mechanism for the abrupt demise of those same settlements. Votive niches, carvings, reliefs and inscriptions on fault surfaces suggest important sacred sanctuaries, particularly those with oracular functions, may have been deliberately built astride active fault traces and venerated as direct connections to the chthonic realm (‘the underworld’). Regionally, the Aegean’s distributed network of tensional faulting, circulating geothermal waters and deep-seated de-gassing sets the tectonic framework for the springs and gases that infuse the ancient Greek netherworld of caves, chasms, chambers, and sacred grottos. The possibility that seismic faults may have constituted the fulcrum of prominent sacred places means that, for all their obvious destructiveness, earthquakes may have had an unacknowledged cultural significance in Greek antiquity.

High‐Precision Analysis of an Aftershock Sequence Using Matched‐Filter Detection: The 4 May 2015ML 6 Wanaka Earthquake, Southern Alps, New Zealand

ABSTRACT The 4 May 2015 M L 6.0 Wanaka earthquake was the largest earthquake in the Central Otago region of New Zealand’s Southern Alps for 70 years and the largest shallow event in the region since national records began. The earthquake occurred within 50 km of the alpine resort towns of Wanaka and Queenstown (combined population 35,000), at 9.1 km depth, and was felt widely across South Island, although no significant damage was reported. We utilize waveforms recorded on a temporary broadband network and the national network to conduct matched‐filter analysis of the aftershock sequence. We use the mainshock and 99 aftershocks distributed through the sequence as template events for the detection of further aftershocks. We detect 2544 aftershocks over 26 days, 27 times more than reported by the national network alone. Magnitudes of detected aftershocks range from M L 5.8 (34 s after the mainshock) to 0.3 and follow a Gutenberg–Richter exponential relationship with a b ‐value of 0.807±0.02 and a magnitude of completeness of M L 1.4. We compute high‐precision relocations of detected events using cross‐correlation‐derived pick corrections. Our relocated aftershocks highlight a steeply northwest dipping (∼70°) fault striking at 250°, which allows us to identify the structure responsible for the earthquake in a region where no active fault traces are mapped. This orientation is consistent with the mainshock focal mechanism for a dextral strike‐slip earthquake on a subvertical plane subparallel to the plate convergence direction (249°). Analysis of focal mechanism slip vectors shows that aftershocks principally occurred on secondary synthetic Riedel shears oriented 25° clockwise from the principal slip zone, possibly controlled by local structures. Aftershock locations migrate in the down‐dip direction away from the mainshock, slowing with a 1/log(time) relationship, consistent with previous numerical models of afterslip within the fault plane.

反射法地震探査および重力探査から明らかになった仙台平野南部の伏在活断層

The southern part of the Sendai Plain is located between the Futaba active fault zone and the Nagamachi-Rifu active fault zone. In the existing research, only 6.5km long of the active fault trace was estimated along the eastern foot of Medeshima Hills because the geomorphic surface of the Medeshima Hills is warping toward east. Besides the above, no other active fault trace had been clearly identified in this area. To reveal the subsurface structure, we conducted 5.3-km-long of seismic reflection survey across the southern part of the Sendai Plain, near the mouth of the Abukuma river, which is the northern extension of Futaba active fault zone. The result of seismic profile analyzed with a standard CMP method shows the existence of concealed active fault beneath the Sendai Plain and 30-40m of dislocations in seismic reflectors from Miocene to Quaternary layer. The activity of the concealed fault is estimated as a C-class (Long-term rate of faulting, 0.01-0.1mm/yr). The survey of relative gravity measurement at intervals of 200 m was also conducted, including the seismic reflection survey line. Steep gradient of the Bouguer gravity anomaly were observed in the vicinity of the concealed active fault that is revealed from the seismic reflection survey. Analysis with the 2-D density structure analysis demonstrates that steep gradient of Bouguer gravity anomaly was responsible for large deformation of pre-Miocene basement rocks by repeated faulting of the concealed active fault. In addition, to discuss a tectonic evolution of the Sendai Plain, we made topographic profiles, using 1m-DEM and 2m-DEM data across the plain. Topographies of these profiles show that discontinuity of 1.5-2.2m on both sides of the westernmost beach ridge in the southern part of the Sendai Plain. Active faulting of the concealed fault has possibly contributed to these topographic discontinuities.

Characterization of slow slip rate faults in humid areas: Cimandiri fault zone, Indonesia

AbstractIn areas where regional tectonic strain is accommodated by broad zones of short and low slip rate faults, geomorphic and paleoseismic characterization of faults is difficult because of poor surface expression and long earthquake recurrence intervals. In humid areas, faults can be buried by thick sediments or soils; their geomorphic expression subdued and sometimes undetectable until the next earthquake. In Java, active faults are diffused, and their characterization is challenging. Among them is the ENE striking Cimandiri fault zone. Cumulative displacement produces prominent ENE oriented ranges with the southeast side moving relatively upward and to the northeast. The fault zone is expressed in the bedrock by numerous NE, west, and NW trending thrust‐ and strike‐slip faults and folds. However, it is unclear which of these structures are active. We performed a morphometric analysis of the fault zone using 30 m resolution Shuttle Radar Topography Mission digital elevation model. We constructed longitudinal profiles of 601 bedrock rivers along the upthrown ranges along the fault zone, calculated the normalized channel steepness index, identified knickpoints and use their distribution to infer relative magnitudes of rock uplift and locate boundaries that may indicate active fault traces. We compare the rock uplift distribution to surface displacement predicted by elastic dislocation model to determine the plausible fault kinematics. The active Cimandiri fault zone consists of six segments with predominant sense of reverse motion. Our analysis reveals considerable geometric complexity, strongly suggesting segmentation of the fault, and thus smaller maximum earthquakes, consistent with the limited historical record of upper plate earthquakes in Java.

Paleoseismic evidence of the CE 1505 (?) and CE 1803 earthquakes from the foothill zone of the Kumaon Himalaya along the Himalayan Frontal Thrust (HFT), India

The importance of understanding earthquake sources in India and Nepal was underscored by the disastrous 2015 earthquakes of 25 April Gorkha (Mw 7.8) and 12 May Kodari (Mw 7.3, aftershock) in Nepal. The Kumaon-Garhwal segment experienced strong earthquakes in CE 1505 and CE 1803, probably along the Himalayan Frontal Thrust (HFT). Of these, the CE 1505 was the greatest earthquake reported from the region in historical chronicles. However, no surface ruptures related to either of 1505 or 1803 have been identified from the Kumaon-Garhwal segment, and an ambiguity remained about their ruptures dispite recent reports of CE 1505 surface rupture in Western Nepal.We used high-resolution satellite (CARTOSAT-1) data for mapping active fault traces and carried out paleoseismic studies to identify paleo-earthquakes along the HFT. A trench excavated across the Kaladungi Fault (KF), a branching fault of HFT, revealed evidence of at least three earthquakes. Event I (the oldest) occurred between BCE 467 and CE 570; Event II occurred between CE 1294–1587. We infer that the Event II was the most likely historically-reported, great Himalayan earthquake of CE 1505. Event III occurred between CE 1750–1932, and may represent the large magnitude CE 1803 (7.5>Mw<8.0) earthquake. Our findings not only help in understanding the frontal fault dynamics, but also may aid seismic hazard evaluation in India and Nepal.

Active interseismic shallow deformation of the Pingting terraces (Longitudinal Valley – Eastern Taiwan) from UAV high-resolution topographic data combined with InSAR time series

ABSTRACTWe focus herein on the location, characterization and the quantification of the most active structural feature of Taiwan: the Longitudinal Valley Fault that corresponds to the suture in between the Philippine and Eurasian Plates. In order to determine and monitor its present inter-seismic deformation, we focus on the Pingting Terraces area, situated in the South Longitudinal Valley (Eastern Taiwan). We first determine the structural geometry issued from both photo-interpretation deduced from new unmanned aerial vehicle (UAV) high-resolution Digital Terrain Model data that we acquired (34.78 km2 with 7.73 cm ground sampling distance), combined with geological field work. In order to characterize and quantify the present deformational patterns over the Pingting terraces, we used an InSAR time series Interferometry algorithm (MT-InSAR) applied to nine L-band SAR images from ALOS satellite acquired over the period 2007–2010. The unprecedented density of measurements (about 120 points per km2 for a total of 6,400 points) gives a continuous overview of the inter-seismic shallow deformation. The structural geometry combined with the mean velocity map (MT-InSAR) reveals two clear active faults situated above the scarps of the Pingting terraces and responsible for up to 7 and 20 mm/yr velocity offset along the radar line of sight. A temporal analysis of the deformation is performed with one measurement at each SAR acquisition date, giving major improvements in the characterization and quantification of the Longitudinal Valley active Fault trace.

Transition from collision to subduction in Western Greece: the Katouna–Stamna active fault system and regional kinematics

Transition from subduction to collision occurs in Western Greece and is accommodated along the downgoing plate by the Kefalonia right-lateral fault that transfers the Hellenic subduction front to the Apulian collision front. Here we present an active tectonic study of Aitolo-Akarnania (Western Greece) that highlights how such a transition is accommodated in the overriding plate. Based on new multi-scale geomorphic and tectonic observations, we performed an accurate active fault trace mapping in the region, and provide evidence for active normal and left-lateral faulting along the Katouna–Stamna Fault (KSF), a 65-km-long NNW-striking fault system connecting the Amvrakikos Gulf to the Patras Gulf. We further show that the Cenozoic Hellenide thrusts located west of the KSF are no longer active, either in field observation or in GPS data, leading us to propose that the KSF forms the northeastern boundary of a rigid Ionian Islands-Akarnania Block (IAB). Cosmic ray exposure measurements of 10Be and 36Cl were performed on a Quaternary alluvial fan offset along the KSF (~50 m left-lateral offset). A maximum abandonment age of ~12–14 ka for the alluvial fan surface can be determined, giving an estimated KSF minimum geological left-lateral slip rate of ~4 mm year−1, in agreement with high GPS slip rates (~10 mm year−1). Despite this high slip rate, the KSF is characterized by subdued morphological evidence of tectonic activity, a gypsum-breccia bedrock and a low level of seismicity, suggesting a dominantly creeping behavior for this fault. Finally, we discuss how the IAB appears to have been progressively individualized during the Pleistocene (younger than ~1.5 Ma).