Reverse Faulting
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2021 ◽  
Siyu Wang ◽  
Edwin Nissen ◽  
Timothy Craig ◽  
Eric Bergman ◽  
Léa Pousse-Beltran

The Kepingtag (Kalpin) fold-and-thrust belt of the southern Chinese Tian Shan is characterized by active shortening and intense seismic activity. Geological cross-sections and seismic reflection profiles suggest thin-skinned, northward-dipping thrust sheets detached in an Upper Cambrian décollement. The January 19 2020 Mw 6.0 Jiashi earthquake provides an opportunity to investigate how coseismic deformation is accommodated in this structural setting. Coseismic surface deformation resolved with Sentinel-1 Interferometric Synthetic Aperture Radar (InSAR) is centered on the back limb of the frontal Kepingtag anticline. Elastic dislocation modelling suggests that the causative fault is located at ~7 km depth and dips ~7° northward, consistent with the inferred position of the décollement. The narrow slip pattern (length ~37 km but width only ~9 km) implies that there is a strong structural or lithological control on the rupture extent, with up-dip slip propagation possibly halted by an abrupt change in dip angle where the Kepingtag thrust is inferred to branch off the décollement. A depth discrepancy between mainshock slip constrained by InSAR and teleseismic waveform modelling (~7 km) and well-relocated aftershocks (~10-20 km) may imply that sediments above the décollement are velocity strengthening. We also relocate 148 regional events from 1977 to 2020 to characterize the broader distribution of seismicity across the Kepingtag belt. The calibrated hypocenters combined with previous teleseismic waveform models show that thrust and reverse faulting earthquakes cluster at relatively shallow depths of ~7-15 km but include abundant out-of-sequence events both north and south of the frontal Kepingtag fault.

2021 ◽  
Hong Peng ◽  
James Jiro Mori

Abstract We use the Japan Meteorological Agency (JMA) earthquake catalogue from 2001 to 2021 to investigate the spatiotemporal distribution of foreshocks for shallow mainshocks (Mj3.0–7.2) that are located onshore of Japan. We find clear peaks for the earlier small earthquakes within 10 days and 3 km prior to the mainshocks, which are considered as our definition of foreshocks. After removing the aftershocks, earthquake swarms and possible earthquakes triggered by the 2011 Mw9.0 Tohoku-oki earthquake, we find that for the 2,066 independent earthquakes, 783 (37.9%) have one or more foreshocks. There is a decreasing trend of foreshock occurrence with mainshock depth. Also, normal faulting earthquakes have higher foreshock occurrence than reverse faulting earthquakes. We calculate the rates of foreshock occurrence as a function of the magnitudes of foreshocks and mainshocks, and we have found no clear trend between the magnitudes of foreshocks and mainshocks.

2021 ◽  
Ryo Okuwaki ◽  
Stephen Hicks ◽  
Timothy Craig ◽  
Wenyuan Fan ◽  
Saskia Goes ◽  

The state-of-stress within subducting oceanic plates controls rupture processes of deep intraslab earthquakes. However, little is known about how the large-scale plate geometry and the stress regime relate to the physical nature of the deep-intraslab earthquakes. Here we find, by using globally and locally observed seismic records, that the moment magnitude 7.3 2021 East Cape, New Zealand earthquake was driven by a combination of shallow trench-normal extension and unexpectedly, deep trench-parallel compression. We find multiple rupture episodes comprising a mixture of reverse, strike-slip, and normal faulting. Reverse faulting due to the trench-parallel compression is unexpected given the apparent subduction direction, so we require a differential-buoyancy driven stress rotation which contorts the slab near the edge of the Hikurangi plateau. Our finding highlights that buoyant features in subducting plates may cause diverse rupture behavior of intraslab earthquakes due to the resulting heterogeneous stress state within slabs.

2021 ◽  
Vol 9 ◽  
Giancarlo Neri ◽  
Barbara Orecchio ◽  
Debora Presti ◽  
Silvia Scolaro ◽  
Cristina Totaro

High-quality non-linear hypocenter locations and waveform inversion focal mechanisms of recent, shallow earthquakes of the Messina Straits have allowed us to obtain the following main results: 1) seismicity has occurred below the east-dipping north-striking fault proposed by most investigators as the source of the 1908, magnitude 7.1 Messina earthquake, while it has been substantially absent in correspondence of the fault and above it; 2) earthquake locations and related strain space distributions do not exhibit well defined trends reflecting specific faults but they mark the existence of seismogenic rock volumes below the 1908 fault representing primary weakness zones of a quite fractured medium; 3) focal mechanisms reveal normal and right-lateral faulting in the Straits, reverse faulting at the southern border of it (Ionian sea south of the Ionian fault), and normal faulting at the northern border (southeastern Tyrrhenian sea offshore southern Calabria); 4) these faulting regimes are compatible with the transitional character of the Messina Straits between the zone of rollback of the in-depth continuous Ionian subducting slab (southern Calabria) and the collisional zone where the subduction slab did already undergo detachment (southwest of the Ionian fault); 5) the whole seismicity of the study area, including also the less recent earthquakes analyzed by previous workers, is compared to patterns of geodetic horizontal strain and uplift rates available from the literature. We believe that the joint action of Africa-Europe plate convergence and rollback of the Ionian subducting slab plays a primary role as regard to the local dynamics and seismicity of the Messina Straits area. At the same time, low horizontal strain rates and large spatial variations of uplift rate observed in this area of strong normal-faulting earthquakes lead us to include a new preliminary hypothesis of deep-seated sources concurring to local vertical dynamics into the current debate on the geodynamics of the study region.

2021 ◽  
Vol 11 (1) ◽  
Brijesh K. Bansal ◽  
Kapil Mohan ◽  
Mithila Verma ◽  
Anup K. Sutar

AbstractDelhi region in northern India experiences frequent shaking due to both far-field and near-field earthquakes from the Himalayan and local sources, respectively. The recent M3.5 and M3.4 earthquakes of 12th April 2020 and 10th May 2020 respectively in northeast Delhi and M4.4 earthquake of 29th May 2020 near Rohtak (~ 50 km west of Delhi), followed by more than a dozen aftershocks, created panic in this densely populated habitat. The past seismic history and the current activity emphasize the need to revisit the subsurface structural setting and its association with the seismicity of the region. Fault plane solutions are determined using data collected from a dense network in Delhi region. The strain energy released in the last two decades is also estimated to understand the subsurface structural environment. Based on fault plane solutions, together with information obtained from strain energy estimates and the available geophysical and geological studies, it is inferred that the Delhi region is sitting on two contrasting structural environments: reverse faulting in the west and normal faulting in the east, separated by the NE-SW trending Delhi Hardwar Ridge/Mahendragarh-Dehradun Fault (DHR-MDF). The WNW-ESE trending Delhi Sargoda Ridge (DSR), which intersects DHR-MDF in the west, is inferred as a thrust fault. The transfer of stress from the interaction zone of DHR-MDF and DSR to nearby smaller faults could further contribute to the scattered shallow seismicity in Delhi region.

Solid Earth ◽  
2021 ◽  
Vol 12 (6) ◽  
pp. 1389-1409
Sarah Mader ◽  
Joachim R. R. Ritter ◽  
Klaus Reicherter ◽  

Abstract. The region around the town Albstadt, SW Germany, was struck by four damaging earthquakes with magnitudes greater than 5 during the last century. These earthquakes occurred along the Albstadt Shear Zone (ASZ), which is characterized by more or less continuous microseismicity. As there are no visible surface ruptures that may be connected to the fault zone, we study its characteristics by its seismicity distribution and faulting pattern. We use the earthquake data of the state earthquake service of Baden-Württemberg from 2011 to 2018 and complement it with additional phase picks beginning in 2016 at the AlpArray and StressTransfer seismic networks in the vicinity of the ASZ. This extended data set is used to determine new minimum 1-D seismic vp and vs velocity models and corresponding station delay times for earthquake relocation. Fault plane solutions are determined for selected events, and the principal stress directions are derived. The minimum 1-D seismic velocity models have a simple and stable layering with increasing velocity with depth in the upper crust. The corresponding station delay times can be explained well by the lateral depth variation of the crystalline basement. The relocated events align about north–south with most of the seismic activity between the towns of Tübingen and Albstadt, east of the 9∘ E meridian. The events can be separated into several subclusters that indicate a segmentation of the ASZ. The majority of the 25 determined fault plane solutions feature an NNE–SSW strike but NNW–SSE-striking fault planes are also observed. The main fault plane associated with the ASZ dips steeply, and the rake indicates mainly sinistral strike-slip, but we also find minor components of normal and reverse faulting. The determined direction of the maximum horizontal stress of 140–149∘ is in good agreement with prior studies. Down to ca. 7–8 km depth SHmax is bigger than SV; below this depth, SV is the main stress component. The direction of SHmax indicates that the stress field in the area of the ASZ is mainly generated by the regional plate driving forces and the Alpine topography.

2021 ◽  
Vol 148 ◽  
pp. 106825
Chaofan Yao ◽  
Chuan He ◽  
Jiro Takemura ◽  
Kun Feng ◽  
Deping Guo ◽  

2021 ◽  
Vol 114 (1) ◽  
Edwin Gnos ◽  
Josef Mullis ◽  
Emmanuelle Ricchi ◽  
Christian A. Bergemann ◽  
Emilie Janots ◽  

AbstractFluid assisted Alpine fissure-vein and cleft formation starts at prograde, peak or retrograde metamorphic conditions of 450–550 °C and 0.3–0.6 GPa and below, commonly at conditions of ductile to brittle rock deformation. Early-formed fissures become overprinted by subsequent deformation, locally leading to a reorientation. Deformation that follows fissure formation initiates a cycle of dissolution, dissolution/reprecipitation or new growth of fissure minerals enclosing fluid inclusions. Although fissures in upper greenschist and amphibolite facies rocks predominantly form under retrograde metamorphic conditions, this work confirms that the carbon dioxide fluid zone correlates with regions of highest grade Alpine metamorphism, suggesting carbon dioxide production by prograde devolatilization reactions and rock-buffering of the fissure-filling fluid. For this reason, fluid composition zones systematically change in metamorphosed and exhumed nappe stacks from diagenetic to amphibolite facies metamorphic rocks from saline fluids dominated by higher hydrocarbons, methane, water and carbon dioxide. Open fissures are in most cases oriented roughly perpendicular to the foliation and lineation of the host rock. The type of fluid constrains the habit of the very frequently crystallizing quartz crystals. Open fissures also form in association with more localized strike-slip faults and are oriented perpendicular to the faults. The combination of fissure orientation, fissure quartz fluid inclusion and fissure monazite-(Ce) (hereafter monazite) Th–Pb ages shows that fissure formation occurred episodically (1) during the Cretaceous (eo-Alpine) deformation cycle in association with exhumation of the Austroalpine Koralpe-Saualpe region (~ 90 Ma) and subsequent extensional movements in association with the formation of the Gosau basins (~ 90–70 Ma), (2) during rapid exhumation of high-pressure overprinted Briançonnais and Piemontais units (36–30 Ma), (3) during unroofing of the Tauern and Lepontine metamorphic domes, during emplacement and reverse faulting of the external Massifs (25–12 Ma; except Argentera) and due to local dextral strike-slip faulting in association with the opening of the Ligurian sea, and (4) during the development of a young, widespread network of ductile to brittle strike-slip faults (12–5 Ma).

2021 ◽  
Vol 13 (9) ◽  
pp. 1752
Nikos Svigkas ◽  
Anastasia Kiratzi ◽  
Andrea Antonioli ◽  
Simone Atzori ◽  
Cristiano Tolomei ◽  

The active collision of the Apulian continental lithosphere with the Eurasian plate characterizes the tectonics of the Epirus region in northwestern Greece, invoking crustal shortening. Epirus has not experienced any strong earthquakes during the instrumental era and thus there is no detailed knowledge of the way the active deformation is being expressed. In March 2020, a moderate size (Mw 5.8) earthquake sequence occurred close to the Kanallaki village in Epirus. The mainshock and major aftershock focal mechanisms are compatible with reverse faulting, on NNW-ESE trending nodal planes. We measure the coseismic surface deformation using radar interferometry and investigate the possible fault geometries based on seismic waveforms and InSAR data. Slip distribution models provide good fits to both nodal planes and cannot resolve the fault plane ambiguity. The results indicate two slip episodes for a N337° plane dipping 37° to the east and a single slip patch for a N137° plane dipping 43° to 55° to the west. Even though the area of the sequence is very close to the triple junction of western Greece, the Kanallaki 2020 activity itself seems to be distinct from it, in terms of the acting stresses.

2021 ◽  
Jean-François Ritz ◽  
Stéphane Baize ◽  
Matthieu Ferry ◽  
Estelle Hannouz ◽  
Magali Riesner ◽  

<p>The 11-11-2019 Le Teil earthquake (Mw4.9), located in the Rhône river valley occurred along the La Rouvière fault (LRF) within the NE termination of the Cévennes faults system (CFS). This very shallow moderate magnitude and reverse-faulting event inverted an Oligocene normal fault which was not assessed to be potentially active, causing surface rupture and strong ground shaking. Its morphology shows no evidence of cumulative reverse faulting during the Quaternary. <span><span data-language-to-translate-into="fr" data-phrase-index="0">All of this information raises the question of whether the fault was reactivated for the first time since the Oligocene during the Teil earthquake, </span></span>or if it had broken the surface before, during the Quaternary period, but could not be detected. In addition, it poses the question of the potential reactivation of other faults of the CFS and other faults in metropolitan France as well.</p><p>To tackle those issues, we launched paleoseismic investigations along the LRF to analyze and characterize evidences of paleo-ruptures in Quaternary deposits. Twelve trenches were dug along the section that broke in 2019. The trenches were dug in aeolian deposits and slope colluvium lying against the ancient LRF normal fault mirror carved in the Barremian limestones. Five trenches yielded favorable Quaternary deposits to document deformation suggesting that one paleo-event, maybe more, occurred with kinematic characteristics (sense of movement, amount of displacement) similar to the 2019 event. The radiocarbon dating of the deformed units (“bulks” collected from the colluvium clayey-silty matrix) suggests, in particular, that at least one event occurred in the past 13 Ka (i.e. penultimate event prior to the Teil earthquake) . The fact that these events are not preserved in the morphology is explained by the small amount of displacement and a long return period, consistent with the low strain rate measured by GPS in this region (~10<sup>-9</sup> yrs<sup>-1</sup>). Our study shows that it is therefore fundamental to carry out more detailed paleoseismological investigations in metropolitan France, especially along ancient faults favorably oriented with respect to the present stress field. Those are already planned in the next coming months along other segments of the CFS.</p>

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