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After the occurrence of the 2011 Mw9.0 off the Pacific coast of Tohoku earthquake, an unusual shallow normal‐faulting earthquake sequence occurred near the Pacific coast at the Ibaraki‐Fukushima prefectural border. We have investigated why normal‐faulting earthquakes were activated in northeast (NE) Japan, which is otherwise characterized by E–W compression. We computed the stress changes associated with the mainshock on the basis of a finite fault slip model, which showed that the amount of additional E–W tensional stresses in the study area was up to 1 MPa, which might be too small to generate normal‐faulting earthquakes in the pre‐shock compressional stress regime. We thus determined focal mechanisms of microearthquakes that occurred in the area before the mainshock, which indicated that the pre‐shock stress field in the area showed a normal‐faulting stress regime in contrast to the overall reverse‐faulting regime in NE Japan. We concluded that the 2011 Tohoku earthquake triggered the normal‐faulting earthquake sequence in a limited area in combination with a locally formed pre‐shock normal‐faulting stress regime. We also explored possible mechanisms for localization of a normal‐faulting stress field at the Ibaraki‐Fukushima prefectural border. Key Points Finding of a locally formed extensional stress regime at northeast Japan Pre‐shock extensional stress regime is needed to trigger the normal‐faulting EQs Implication for the occurrence of past repeated megathrust EQs

After the 2011 Mw 9.1 Tohoku earthquake, numerous intraplate earthquakes occurred beneath the outer slope of the Japan Trench. Based on ocean bottom seismograph observations, these earthquakes occurred in the oceanic crust and uppermost mantle of the Pacific plate at depths shallower than about 40 km and had normal‐faulting focal mechanisms at all depths. Before the 2011 earthquake, normal‐faulting earthquakes beneath the outer trench slope occurred only at depths shallower than 20 km, whereas those at depths of around 40 km had reverse‐faulting mechanisms. These observations suggest that the stress regime in the Pacific plate was changed by the 2011 earthquake. The tensional stresses that now extend to depths of about 40 km may play an important role not only in the occurrence of large normal‐faulting earthquakes but also in hydration of the uppermost mantle of the incoming Pacific plate prior to the subduction. Key Points OBS observations for outer trench slope earthquakes after the 2011 Tohoku EQ Normal‐faulting earthquakes in oceanic crust and mantle of the incoming plate Stress regime in the Pacific plate was changed by the 2011 Tohoku earthquake

We provide field data of coseismic ground deformation related to the 6 April Mw 6.3 L'Aquila normal faulting earthquake. Three narrow fracture zones were mapped: Paganica‐Colle Enzano (P‐E), Mt. Castellano‐Mt. Stabiata (C‐S) and San Gregorio (SG). These zones define 13 km of surface ruptures that strike at 130–140°. We mapped four main types of ground deformation (free faces on bedrock fault scarps, faulting along synthetic splays and fissures with or without slip) that are probably due to the near‐surface lithology of the fault walls and the amount of slip that approached the surface coseismically. The P‐E and C‐S zones are characterized by downthrow to the SW (up to 10 cm) and opening (up to 12 cm), while the SG zone is characterized only by opening. Afterslip throw rates of 0.5–0.6 mm/day were measured along the Paganica fault, where paleoseismic evidence reveals recurring paleo‐earthquakes and post‐24.8 kyr slip‐rate ≥ 0.24 mm/yr.

We present a 1:25,000 scale map of the coseismic surface ruptures following the 30 October 2016 M w 6.5 Norcia normal-faulting earthquake, central Italy. Detailed rupture mapping is based on almost 11,000 oblique photographs taken from helicopter flights, that has been verified and integrated with field data (>7000 measurements). Thanks to the common efforts of the Open EMERGEO Working Group (130 people, 25 research institutions and universities from Europe), we were able to document a complex surface faulting pattern with a dominant strike of N135°-160° (SW-dipping) and a subordinate strike of N320°-345° (NE-dipping) along about 28 km of the active Mt. Vettore-Mt. Bove fault system. Geometric and kinematic characteristics of the rupture were observed and recorded along closely spaced, parallel or subparallel, overlapping or step-like synthetic and antithetic fault splays of the activated fault systems, comprising a total surface rupture length of approximately 46 km when all ruptures were considered.

A MW 6.3 earthquake struck on April 6, 2009 the Abruzzi region (central Italy) producing vast damage in the L'Aquila town and surroundings. In this paper we present the location and geometry of the fault system as obtained by the analysis of main shock and aftershocks recorded by permanent and temporary networks. The distribution of aftershocks, 712 selected events with ML ≥ 2.3 and 20 with ML ≥ 4.0, defines a complex, 40 km long, NW trending extensional structure. The main shock fault segment extends for 15–18 km and dips at 45° to the SW, between 10 and 2 km depth. The extent of aftershocks coincides with the surface trace of the Paganica fault, a poorly known normal fault that, after the event, has been quoted to accommodate the extension of the area. We observe a migration of seismicity to the north on an echelon fault that can rupture in future large earthquakes.

The Mw 7.0 mainshock of the 2016 Kumamoto earthquake sequence was triggered by dextral rupture of the Futagawa fault within the Aso volcanic region, Southwestern Japan. To reproduce its faulting patterns and to reveal the geological and geophysical characteristics of the fault and surrounding lithological units, we report the results of a multiple‐borehole drilling program penetrating the Futagawa fault zone. By combining core descriptions with geophysical logs, we identified >200 m of normal faulting displacement along the currently dextral strike‐slip Futagawa fault. Considering previous kinematic and chronological studies of the fault, we interpret that the Futagawa fault dominantly slipped as a normal fault in a short period (∼300–87 ka) before switching to its current transtensional (dominant strike‐slip) regime ∼87 ka caused by a local change in the stress field associated with the termination of the Aso caldera‐forming eruptions. In the main borehole, three damage/slip zones were penetrated at depths of ∼354, 461, and 576 m. The 461 damage zone was identified as ∼45 m in vertical thickness and thicker than the other damage zones (∼3–6 m vertically) and was characterized by high fracture density and the presence of strike‐slip slickenlines. Depth profiles of physical properties revealed different patterns near the three damage zones; both the resistivity and the P‐wave velocity showed stronger deterioration at the 461 damage zone than the others. Based on these geological and geophysical observations, we suggest that the 461 damage zone is the primary candidate for seismogenic faulting during the 2016 Kumamoto earthquake mainshock. Plain Language Summary The Mw 7.0 mainshock of the 2016 Kumamoto earthquake sequence was caused by slipping along the Futagawa fault within the Aso volcanic region of Southwestern Japan. To investigate its faulting patterns, the geological structure and geophysical properties of the fault and surrounding units, we report the results of the Futagawa fault drilling project in which multiple boreholes were drilled through the fault zone. We found >200 m of normal faulting displacement along the currently strike‐slip Futagawa fault. Taking previous studies together, we interpret that the Futagawa fault dominantly slipped as a normal fault in a short period (∼300–87 kyr ago) before switching to its current strike‐slip mode ∼87 kyr ago caused by termination of the Aso caldera‐forming eruption sequence. In the ∼700 m‐deep main borehole, three damage zones were identified. The second damage zone at ∼461 m depth was more strongly damaged than the others. In addition, the physical properties revealed different change patterns near the three damage zones and showed strongest deterioration at the 461‐m damage zone. Based on geological and geophysical observations, we suggest that this damage zone is the primary candidate that caused the 2016 Kumamoto earthquake mainshock. Key Points Multiborehole drilling through the currently dextral Futagawa fault zone reveals >200 m of cumulative normal‐sense displacement The switch from normal to dextral motion was probably due to termination of the Aso caldera‐forming eruption sequence at ∼87 ka Geological and geophysical data suggest the fault zone at ∼461 m depth experienced dextral‐sense slip during the 2016 Kumamoto earthquake

We report a sequence of crustal quakes that began after the Mw = 8.8 thrust‐subduction Maule earthquake that affected the Central Chile margin on 27 February 2010. This activity lasted by several months, having the most important events on 11 March 2010 (Mw = 6.9 and Mw = 7.0) with normal focal mechanisms. Seismicity shows a rupture oriented along a NW‐striking and SW‐dipping normal fault from the surface down to the interplate contact. Seismicity can be correlated with neotectonics extensional structures similarly oriented in the region, which have coexisted with NNE‐SSW reverse faults since the late Pliocene, even though both have older periods of activity since the Paleozoic. This crustal rupture would have been triggered by the high Coulomb stress change produced by the Maule earthquake, enhanced by likely fluid presence along weakened zones of the forearc crust as evidenced by high Vp/Vs ratio. The occurrence of relevant neotectonic activity in coincidence with short‐term deformation suggests a relationship with long‐term tectonic features of this region, which would have been acting as a barrier during the interseismic period, increasing the strain accumulation and triggering contractional faulting in the crust, as well as producing high slip patches during great subduction ruptures favoring triggering of crustal extensional faulting. Crustal faulting in Pichilemu suggests that this kind of events should be considered in seismic hazard analysis despite the absence of historical crustal seismic activity before the Maule earthquake. Key Points Normal faulting induced by the big Chilean earthquake Extensional tectonics during coseismic Compressional crustal tectonics during interseismic

One of the most important instabilities that may occur in a borehole is shear instability caused by high compressive stress in the borehole wall. The initial estimation of the width and depth of the failure zone around the borehole is very important in the field. In inclined boreholes, the shear instability or borehole breakout is affected by the in situ stress regime, the deviation angle of the borehole, the mechanical properties of the rock and the effect of the intermediate principal stress. In this article, an analytical model based on theory of elasticity is presented to find the breakout failure area around the inclined boreholes. Mogi-Coulomb shear failure criterion is used, in which there is also the effect of the intermediate principal stress. This model examines the failure in three-dimensional elements around the borehole for different in situ stress regime. The main finding of the analysis done in this article is that not only the deviation angle of the borehole but also the in situ stress regime has a great effect on the dimensions of the breakout. Also, the plane where the deviation angle of the borehole changes, affects the dimensions of the breakout.

Wedge-top basins represent useful tectonic elements for the characterisation of the evolution of their underlying accretionary wedge in space and time, as their final state of deformation sums up the bulk shortening and structural instability conditions of the wedge. Here, we present the geometric and kinematic patterns of deformation structures deforming the wedge-top Epiligurian basins of the Northern Apennines (Italy). Our main goals are to generate an evolutionary model to account for the syn- to post-orogenic evolution of the Epiligurian basins and to infer the building style of the Northern Apennines wedge during continental collision. Mesoscale structural analysis shows that common and widely distributed thrust and normal fault arrays deform the entire Epiligurian stratigraphic succession infilling the broadly E-vergent wedge-top basins. Thrusts are invariably cut by later NW–SE and NE-SW-striking normal and oblique fault systems characterised by fault planes that mutually intersect at all scales to form polygonal patterns. Remote sensing analysis of the tectonic structures affecting the Epiligurian formations confirms the variable orientation of both thrusts and normal faults within the different studied stratigraphic successions. As a whole, results suggest a polyphase tectonic evolution of the Epiligurian wedge-top basins during the widening of the Northern Apennines accretionary wedge towards the foreland by frontal accretion. The recognised main phases are: (i) syn-orogenic compression accommodating overall tectonic transport towards the eastern quadrants; (ii) post-orogenic extension genetically related to the extension of the inner zone of the Northern Apennines; (iii) more recent extension forming collapse-induced normal faults spatially arranged in polygonal patterns.

Neotectonic activity in the western margin of the Sierra Madre Oriental in northeastern Mexico was evaluated using geomorphological, geological, structural, and seismological analyses. We evaluated recent tectonic evidence in the basin of the Potosí Valley related to the Santo Domingo Fault System (SDFS), which is one of the largest active extensional fault zones documented so far in the transition zone between the Basin and Range and the Sierra Madre Oriental provinces. Two indices were used in the geomorphological analysis: transverse topographic basin asymmetry ( T ) and mountain front sinuosity ( Smf ). The results show that the studied basin is slightly tilted to the WSW, consistent with the typical basin deformation pattern in the Basin and Range province. Structural data collected in the field unveil a NW–SE striking fault system, geometrically and kinematically similar to the Cenozoic lineaments of the Basin and Range province. The local stress tensor calculated in this study indicates an extensional tectonic regime, suggesting S V > S NW > S NE . Analysis defines an azimuth trend of 298° and 036° for σ 2 ( S Hmax ) and σ 3 S hmin , respectively. According to the results, the SDFS is well orientated for possible future reactivation as a normal fault system. We estimate that a segmented rupture of the SDFS could produce seismic events on the order of magnitude 5  <  M w  <  6 . Considering a continuous rupture (~ 42 km) of the SDFS in a single event, an earthquake of M w 7.0 could be expected. The results are essential to estimate seismic hazards in northeastern Mexico, including important urban areas, such as Monterrey, Nuevo León, and Saltillo, Coahuila.