New insight into the 24 January 2020, Mw 6.8 Elazığ earthquake (Turkey): An evidence for rupture-parallel pull-apart basin activation along the East Anatolian Fault Zone constrained by Geodetic and Seismological data

In this study, we investigated the main features of the causative fault of the 24 January 2020, Mw 6.8 Elazığ earthquake (Turkey) using seismological and geodetic data sets to provide new insight into the East Anatolian Fault Zone (EAFZ). We first constrained the co-seismic surface deformation and the rupture geometry of the causative fault segment using Interferometric Synthetic Aperture Radar (InSAR) interferograms (Sentinel-1A/B satellites) and teleseismic waveform inversion, respectively. Also, we determined the centroid moment tensor (CMT) solutions of focal mechanisms of the 27 aftershocks using the regional waveform inversion method. Finally, we evaluated the co-seismic slip distribution and the CMT solutions of the causative fault as well as of adjacent segments using the 27 focal solutions of the aftershocks, superimposed on the surface deformation pattern. The CMT solution of the 24 January 2020Elazığ earthquake reveals a pure strike-slip focal mechanism, consistent with the structural pattern and left-lateral motion of the EAFZ. The rupture process of the Elazığ event indicated that the rupture is started at 12 km around the hypocenter, and then propagated bilaterally along the NE-SW but mainly toward the southwest. The rupture slip has initially propagated toward the southwest (first 10 s) and northeast (4 s), and again toward the southwest (9 s). Maximum displacement is calculated as 1.3 m about 20 km southwest of the hypocenter at 6 km depth (centroid depth). The rupture stopped to down-dip around 20 km depth toward the southwest. The distribution of the slip vectors indicates that the rupture continued mostly through a normal oblique movement. Most of the moment release was released SW of the hypocenter and the rupture reached up to around 50 km. The focal mechanisms of analyzed 27 aftershocks show strike-slip, but mostly normal and normal oblique-slip faulting with an orientation of the tensional axes (NNE-SSW), indicating a normal oblique-slip, “transtensional” stress regime, parallel-subparallel to the strike of the EAFZ, consistent with SW-rupture directivity and co- seismic deformation pattern. Finally, based on the co-seismic surface deformation compatible with the distributional pattern of normal focal solutions, normal and normal oblique-slip focals of the aftershocks evidence the rupture-parallel pull-apart basin activation as a segment boundary of the left-lateral strike-slip movement of the EAFZ.

Structure, Faulting and Gas Accumulation: Southeast Wanganui Basin, New Zealand

<p>There has been low interest in petroleum exploration in the Wanganui Basin as it lacks known hydrocarbon source rock of sufficient age or burial depth. However, the onshore Southeast Wanganui Basin has many occurrences of methane-rich biogenic gas found in shallow water wells. This project used three studies across the Horowhenua area to examine the faulting style in the Southeast Wanganui Basin where it is bounded by the Tararua range- front, and how this faulting relates to the accumulation of gas deposits in the shallow sedimentary section. South of Levin the Tararua range front steps laterally near Muhunoa East Road. A previous seismic reflection line identified a deep intra-basement arrival, which could have been either a low-angle thrust fault or side-swipe from a pull-apart basin at the step in the Tararua range front. Two seismic lines and a gravity survey found no sub-vertical drops in basement depth which would indicate the presence of a pull-apart basin or a favourable surface off which a laterally travelling seismic wave could reflect. The intra-basement arrival on the previous seismic line was therefore interpreted to be from an intra-basement low-angle thrust fault. Also two biogenic gas sites also were surveyed. A shallow gas reservoir east of Levin on Wallace Road, abutting the Tararua range front, had been discovered when a water well was drilled; and a potential reservoir southwest of Sanson was located when an aerial survey identified a domed structure with high resistivity. In both areas biogenic gas was thought to be trapped in buried sand dunes at a depth of approximately 20 m. Shallow seismic refraction and reflection methods and amplitude variation with offset analysis were used to map both reservoir bodies and confirm the presence of biogenic gas.</p>

Structure, Faulting and Gas Accumulation: Southeast Wanganui Basin, New Zealand

<p>There has been low interest in petroleum exploration in the Wanganui Basin as it lacks known hydrocarbon source rock of sufficient age or burial depth. However, the onshore Southeast Wanganui Basin has many occurrences of methane-rich biogenic gas found in shallow water wells. This project used three studies across the Horowhenua area to examine the faulting style in the Southeast Wanganui Basin where it is bounded by the Tararua range- front, and how this faulting relates to the accumulation of gas deposits in the shallow sedimentary section. South of Levin the Tararua range front steps laterally near Muhunoa East Road. A previous seismic reflection line identified a deep intra-basement arrival, which could have been either a low-angle thrust fault or side-swipe from a pull-apart basin at the step in the Tararua range front. Two seismic lines and a gravity survey found no sub-vertical drops in basement depth which would indicate the presence of a pull-apart basin or a favourable surface off which a laterally travelling seismic wave could reflect. The intra-basement arrival on the previous seismic line was therefore interpreted to be from an intra-basement low-angle thrust fault. Also two biogenic gas sites also were surveyed. A shallow gas reservoir east of Levin on Wallace Road, abutting the Tararua range front, had been discovered when a water well was drilled; and a potential reservoir southwest of Sanson was located when an aerial survey identified a domed structure with high resistivity. In both areas biogenic gas was thought to be trapped in buried sand dunes at a depth of approximately 20 m. Shallow seismic refraction and reflection methods and amplitude variation with offset analysis were used to map both reservoir bodies and confirm the presence of biogenic gas.</p>

Shortcut Faults and Lateral Spreading Activated in a Pull-Apart Basin by the 2018 Palu Earthquake, Central Sulawesi, Indonesia

Our understanding of pull-apart basins and their fault systems has been enhanced by analog experiments and simulations. However, there has been scarce interest to compare the faults that bound pull-apart basins with surface ruptures during earthquakes. In this study, we investigated the effects of a 2018 earthquake (Mw 7.5) on a pull-apart basin in the Palu–Koro fault system, Sulawesi Island, Indonesia, using geomorphic observations on digital elevation models and optical correlation with pre- and post-earthquake satellite images. A comparison of active fault traces determined by geomorphology with the locations of surface ruptures from the 2018 earthquake shows that some of the boundary faults of the basin are inactive and that active faulting has shifted to basin-shortcut faults and relay ramps. We also report evidence of lateral spreading, in which alluvial fan materials moved around the end of the alluvial fan. These phenomena may provide insights for anticipating the location of future surface ruptures in pull-apart basins.

Shortcut Faults and Lateral Spreading Activated in a Pull-Apart Basin by the 2018 Palu Earthquake, Central Sulawesi, Indonesia

Our understanding of pull-apart basins and their fault systems has been enhanced by analog experiments and simulations. However, there has been no opportunity to compare the faults that constitute pull-apart basins with surface ruptures during earthquakes. In this study, we investigated the effects of a 2018 earthquake (Mw 7.5) on a pull-apart basin in the Palu-Koro fault system, Sulawesi Island, Indonesia, using geomorphic observations in digital elevation models, optical correlation with pre- and post-earthquake satellite images. A comparison of active fault traces determined by geomorphology with the locations of surface ruptures from the 2018 earthquake shows that some of the boundary faults of the basin are inactive and that active faulting has shifted to basin-shortcut faults and relay ramps. We also report evidence of lateral spreading, in which alluvial fan materials moved around the end of the alluvial fan. These phenomena may provide insights for anticipating the location of future surface ruptures in pull-apart basins.

Recency of Faulting and Subsurface Architecture of the San Diego Bay Pull-Apart Basin, California, USA

In Southern California, plate boundary motion between the North American and Pacific plates is distributed across several sub-parallel fault systems. The offshore faults of the California Continental Borderland (CCB) are thought to accommodate ∼10–15% of the total plate boundary motion, but the exact distribution of slip and the mechanics of slip partitioning remain uncertain. The Newport-Inglewood-Rose Canyon fault is the easternmost fault within the CCB whose southern segment splays out into a complex network of faults beneath San Diego Bay. A pull-apart basin model between the Rose Canyon and the offshore Descanso fault has been used to explain prominent fault orientations and subsidence beneath San Diego Bay; however, this model does not account for faults in the southern portion of the bay or faulting east of the bay. To investigate the characteristics of faulting and stratigraphic architecture beneath San Diego Bay, we combined a suite of reprocessed legacy airgun multi-channel seismic profiles and high-resolution Chirp data, with age and lithology controls from geotechnical boreholes and shallow sub-surface vibracores. This combined dataset is used to create gridded horizon surfaces, fault maps, and perform a kinematic fault analysis. The structure beneath San Diego Bay is dominated by down-to-the-east motion on normal faults that can be separated into two distinct groups. The strikes of these two fault groups can be explained with a double pull-apart basin model for San Diego Bay. In our conceptual model, the western portion of San Diego Bay is controlled by a right-step between the Rose Canyon and Descanso faults, which matches both observations and predictions from laboratory models. The eastern portion of San Diego Bay appears to be controlled by an inferred step-over between the Rose Canyon and San Miguel-Vallecitos faults and displays distinct fault strike orientations, which kinematic analysis indicates should have a significant component of strike-slip partitioning that is not detectable in the seismic data. The potential of a Rose Canyon-San Miguel-Vallecitos fault connection would effectively cut the stepover distance in half and have important implications for the seismic hazard of the San Diego-Tijuana metropolitan area (population ∼3 million people).

Active faulting offshore the Maltese Islands revealed by geophysical and geochemical observations

&lt;p&gt;The Maltese Islands (central Mediterranean Sea) are intersected by two normal fault systems associated with continental rifting to the south. Because of a lack of evidence for offshore displacement and insignificant historical seismicity, the systems have been considered to be inactive. Here we integrate aerial and marine geological, geophysical and geochemical data to demonstrate that: (i) the majority of faults offshore the Maltese Islands underwent extensional to transtensional deformation during the last 20 ka, (ii) active degassing of CH&lt;sub&gt;4&lt;/sub&gt; and CO&lt;sub&gt;2 &lt;/sub&gt;occurs via these faults. The gases migrate through Miocene carbonate bedrock and the overlying Plio-Pleistocene sedimentary layers to generate pockmarks at the muddy seafloor and rise through the water column into the atmosphere. We infer that the offshore faults systems are permeable and that they were active recently and simultaneously. The latter can be explained by a transtensional system involving two right-stepping, right-lateral NW-SE trending faults, either binding a pull-apart basin between the islands of Malta and Gozo or associated with minor connecting antitethic structures. Such a configuration may be responsible for the generation or reactivation of faults onshore and offshore the Maltese Islands, and fits into the modern divergent strain-stress regime inferred from geodetic data.&lt;/p&gt;

Termination of a Trench-Linked Strike-Slip Fault Zone in the Sumatra–Java Forearc Basin and Accretionary Wedge Complex

This paper presents a review of several published seismic reflection and seismicity data and analyzes of high-resolution bathymetry data to revise the exact location and reveal detail characteristics of a strike-slip fault zone that formed the southernmost segment of the Sumatran Fault (SF). Previous works interpreted this fault segment as a horst structure to the south of a pull-apart basin. We observe a clear linear trace of dissected seafloor parallels to SF in the high-resolution bathymetric map. This structure extends from the south of a pull-apart basin in the northwest to the Sunda accretionary wedge farther southeast. This lineament exhibits a narrow valley and a linear ridge that in the subsurface are interpreted as negative and positive flower structures, respectively. The structure exhibits a vertical fault plane and appears to have deformed the accretionary wedge sediments and basement at depth. A cluster of shallow seismicity is observed along this NW-trending fault zone, indicating the activity of this zone. Here, we proposed this strike-slip fault as the Ujung Kulon Fault that marks the southeasternmost segment of the SF zone. This segment deformed the area of the Sumatra-Java forearc basin and terminated in accretionary wedge near the trench. The accumulated strain within UKF may trigger large earthquake in the future, close to the highly populated areas in the coast of Sumatra and Java.Keywords: Strike-slip fault, Sumatra Fault, Ujung Kulon Fault, segmentation, earthquake.