Shallow Seismic Activity Research Articles

Finite-Source Model of the 8 July 2021 M 6.0 Antelope Valley, California, Earthquake

Abstract We present a finite-source coseismic slip model of the 2021 Mw 6.0 Antelope Valley earthquake based on the joint inversion of regional seismic waveforms, Interferometric Synthetic Aperture Radar (InSAR), and Global Navigation Satellite Systems data. The results show that the mainshock rupture was dominated by normal slip along a nearly north–south-trending fault dipping to the east. The rupture lasted for ∼10 s, with primarily unilateral propagation toward the south. Most coseismic slip is found to be within a depth range between 6 and 10 km, with apparently no slip reaching the surface. Surface projection of the modeled fault plane matches well with the southern extension of the previously mapped Slinkard Valley fault (SVF). Aftershocks one year after the mainshock are mostly distributed within a relatively narrow band of 2–3 km thickness around the up-dip portion of the inferred coseismic rupture plane. There is little aftershock activity below 10 km, suggesting a relatively shallow brittle-to-ductile transition in this area. Aftershocks are also clustered at shallow depth beneath several branches of the Antelope Valley faults to the east of the mainshock rupture, including the Mw 4.4 event on 27 August 2021, which produced clear coseismic surface deformation observed by InSAR. Most aftershocks, immediately up-dip of the coseismic rupture and to the east beneath the Antelope Valley faults, are in areas of substantial coseismic Coulomb stress increase, particularly when assuming that all faults in this area dip to the east. This suggests that like the SVF that hosted the mainshock, the Antelope Valley faults in this area also dip to the east. There is little to no postseismic deformation seen from InSAR observations ∼2 months after the mainshock. The lack of clear coseismic and postseismic slip on the shallow portion of the fault suggests the potential for future shallow seismic activity.

The damaging character of shallow 20th century earthquakes in the Hainaut coal area (Belgium)

Abstract. The present study analyses the impact and damage of shallow seismic activity that occurred from the end of the 19th century until the late 20th century in the coal area of the Hainaut province in Belgium. This seismicity is the second-largest source of seismic hazard in north-western Europe after the Lower Rhine Embayment. During this period, five earthquakes with moment magnitudes (Mw) around 4.0 locally caused moderate damage to buildings corresponding to maximum intensity VII on the European Macroseismic Scale 1998 (EMS-98). Reviewing intensity data from the official macroseismic surveys held by the Royal Observatory of Belgium (ROB), press reports and contemporary scientific studies resulted in a comprehensive macroseismic intensity dataset. Using this dataset, we created macroseismic maps for 28 earthquakes, established a new Hainaut intensity attenuation model and a relationship linking magnitude, epicentral intensity and focal depth. Using these relationships, we estimated the location and magnitude of pre-1985 earthquakes that occurred prior to deployment of the modern digital Belgian seismic network. This resulted in a new updated earthquake catalogue for the Hainaut area for the 1887–1985 period, including 124 events. A comparison with other areas worldwide where currently similar shallow earthquake activity occurs suggests that intensity attenuation is strong in Hainaut. This high attenuation and our analysis of the cumulative effect of the Hainaut seismicity indicate that current hazard maps overestimate ground motions in the Hainaut area. This reveals the need to use more appropriate ground motion models in hazard issues. Another strong implication for earthquake hazard comes from the reliability of the computed focal depths that helps clarifying the hypotheses about the origin of this seismicity. Some events were very shallow and occurred near the surface up to a depth not exceeding 1 km, suggesting a close link to mining activities. Many events, including the largest shallow events in the coal area before 1970, occurred at depths greater than 2 km, which would exclude a direct relationship with mining, but still might imply a triggering causality. A similar causality can also be questioned for other events that occurred just outside of the coal area since the end of the mining works.

Global Drivers and Transport Mechanisms of Lunar Rockfalls

AbstractThe long‐ and short‐term drivers and transport mechanisms of lunar rockfalls are currently not well understood, but could provide valuable information about the geologic processes that still shape the surface of the Moon today. Here, we compare the global distribution of rockfalls with relevant geophysical data, such as seismic, topographic, thermal, gravity anomaly, and tidal displacement data sets. Rockfalls appear to predominantly occur (a) on equator‐facing slopes and thus in regions with large thermal amplitudes, (b) on slope angles well above‐average (Δ ∼ 10°), and (c) in regions with above‐average rock abundance. We do not observe a qualitatively or statistically relevant relation between rockfall abundance, monitored Apollo‐era shallow seismic activity, and the distribution of visible tectogenetic features. Informed by our global analysis, we conduct a targeted, in‐depth study of 687 rockfall boulders and trajectories in 13 sites across the Moon, including 7 craters, 2 volcanic vents, 2 tectonic structures, and 2 unclassified geomorphic regions. We identify four different source region types, where the type appears to control the occurrence of rockfalls. The source region type in turn is controlled by surface age rather than geomorphic context. We find that rockfall trajectories are mainly controlled by the trigger energy and the geometry of the slope. Our results suggest that erratic small‐scale impacts (mainly in old, Imbrian‐Nectarian, shallow terranes), aided by solar‐induced thermal fatigue of fractured bedrock (mainly in young, Copernican‐Eratosthenian steep terranes), were the dominant, global‐scale long‐ and short‐term drivers of rockfalls in the Moon's recent geologic past.

PRIMORYE EARTHQUAKE on April 12, 2014, KP=11.9 (Far Eastern Russia)

The article presents instrumental and macroseismic data of the earthquake that occurred on April 12, 2014 in the Primorye Region of the Far Eastern Federal District of the Russian Federation. Primorye refers to areas with a weak of shallow seismic activity. This relatively small magnitude M=4.5 earthquake is a rare occurrence in this region. It caused a significant macroseismic effect over an unexpectedly large area. The highest seismic intensity as large as 5 degrees was observed in the settlements nearest to the epicenter – Mezhgorye, Krylovka and Maryanovka. 36 minutes after the main event, an aftershock was recorded with an epicenter 6.5 km southeast of the main shock location, felt by the inhabitants of the settlement of Krylovka. According to the data obtained, the focal mechanism of the earthquake might be treated as the strikeslip fault type with the nodal planes of the sublatitudinal and sublongitudinal extension. In view of tectonics, the earthquake and its aftershock epicenters might be related to a nameless NW striking fault located near Mezhgorye Settlement and linking the Krylovsky and Chernorechensky faults.

Relationship between Observed Seismicity and Water Level Fluctuations in Polyphyto Dam Area (North Greece)

The spatial-temporal seismicity in Aliakmona river area is presented for the period 1974-2010 (NW Greece). This study area, which was classified as low seismicity, presents a particular interest due to the unexpected strong earthquake of Ms=6.6 which occurred between the cities of Kozani–Grevena, at a distance of 18 km from the southern edge of Polyphyto lake in 1995. According to [1], seismic hazard changes have been identified SE of the Polyphyto dam after the impoundment of the lakes. In this study, we examined the possible correlation between the impoundment and the water level fluctuations due to the three artificial lakes which are established in the region and the observed seismicity using data from the catalogues of the National Observatory of Athens (NOA) (see details for seismological data on Mc, RMS, etc. in [2,3]). For the latter purpose, spatial and temporal distribution maps were developed as well as correlation diagrams between water level variations in respect to monthly seismicity for distances 10 Km, 20 Km, 30 Km, 40 Km and 50 Km around the artificial lakes using the ESRI ArcMap 10 software. A remarkable change is observed in seismic activity in the vicinity of reservoirs for the period commencing 10 years prior to the first filling in comparison with the period 10 years after. This increased seismicity is correlated to the daily rate of water level fluctuations (dh/dt) for the period 1984-2010 reveals the presence of mainly shallow seismic activity with focal depths of 0-5 km in a high percentage (67%). This protracted seismicity which is mainly located in the SW area of Polyphyto lake, has characteristics of the second type of induced seismicity according to Talwani [4], and seems to be controlled by the water level fluctuations of Polyphyto reservoir due to the mechanism of pore pressure diffusion.

Shallow seismic activity and young thrust faults on the Moon

The discovery of young thrust faults on the Moon is evidence of recent tectonic activity, but how recent is unknown. Seismometers at four Apollo landing sites recorded 28 shallow moonquakes between 1969 and 1977. Some of these shallow quakes could be associated with activity on the young faults. However, the epicentre locations of these quakes are poorly constrained. Here we present more-accurate estimates of the epicentre locations, based on an algorithm for sparse seismic networks. We found that the epicentres of eight near-surface quakes fall within 30 km of a fault scarp, the distance of the expected strong ground shaking. From an analysis of the timing of these eight events, we found that six occurred when the Moon was less than 15,000 km from the apogee distance. Analytical modelling of tidal forces that contribute to the current lunar stress state indicates that seven near-apogee events within 60 km of a fault scarp occur at or near the time of peak compressional stresses, when fault slip events are most likely. We conclude that the proximity of moonquakes to the young thrust faults together with evidence of regolith disturbance and boulder movements on and near the fault scarps strongly suggest the Moon is tectonically active. Shallow moonquakes detected at four Apollo landing sites between 1969 and 1977 occurred during maximum stress and in close proximity to young faults, suggesting that the Moon is tectonically active, according to reanalyses of the seismic data and tidal force modelling.

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.

Analysis of La Dehesa paleo-landslide. Central Pre-Andes of Argentina

The main objective of this paper is to consider the influence of Quaternary faults as likely triggering factor for rockslides occurrence in the Central Pre-Andes, a region with intense shallow seismic activity. A rockslide deposit was selected as study case, placed in the western flank of La Dehesa and Talacasto (DT) range (31°3′37″ S and 68°46′ 8″ W). Applied methodology includes the characterization of main discontinuities, reconstruction of the topography using a high-resolution digital elevation model, safety factor calculation along the sliding surface and, Newmark displacements estimation for three different hypothetical seismic scenarios, recreated from existing neotectonic local information. Equilibrium-limit method's results confirm that study case, La Dehesa rockslide (LDR), had a stable and safe slope's configuration under static conditions. However, a seismic horizontal coefficient between 0.2 and 0.3 decreases safety factor below the safety threshold. Newmark's displacements for different seismic reconstructed scenarios varies between 4.1 and 15.9 cm, values that agreed with a coherent failure process, likely triggered by Pleistocene to Holocene seismogenic sources in Central Pre-Andes. LDR trigger could be assigned mainly to an earthquake related to La Dehesa Quaternary fault (LDF) activity; however, similar movements produced by neighboring faults should not be discarded. LDR triggering related to climatic conditions is despised. Finally, the methodology presented in this work is easy to reproduce and may be applied to other rockslides located in the mountainous areas of the Central Pre-Andes of Argentina.

Present-day crustal motion around the Pamir Plateau from GPS measurements

We have used geodetic techniques to improve constraints on the crustal motion of the Pamir Plateau. Three campaigns of Global Position System data acquisition between 2011 and 2015 demonstrate that, in association with the India–Asia collision, a complex pattern of crustal motion exists in the Pamir Plateau. In a north–south direction from the Indian Plate to the Hazak Block, the crust has absorbed ~35mm/yr of shortening, of which ~35% is distributed around the Hindu Kush region (~12mm/yr), and another ~35% is taken up around the Alai Valley (also ~12mm/yr). Global Position System measurements also show ~5mm/yr of shortening between the Pamir Plateau and the Tajik Basin, whereas between the Pamir and the Tarim Basin, an ~10mm/yr extension rate is observed. With respect to the stable Eurasian Plate, the Pamir rotates counterclockwise at a rate of ~1.822°Myr−1, with an Euler pole positioned about the west end of the Tajik Basin (37.03±0.74°N, 65.89±0.12°E). The strain rate field calculated from Global Position System velocities reveals that the crustal motion is consistent with localized deformation around the Hindu Kush and the Alai Valley, the latter representing a zone with strong shallow seismic activity.

A preliminary one-dimensional crustal velocity model for Himachal Pradesh, India

A preliminary one-dimensional (1D) velocity model for Himachal Pradesh, India has been developed by utilising the P and S wave travel time data. A very steady and narrow velocity model was obtained with travel time inversion, and a range of velocity models were tested with earthquake locations to derive the best-fit velocity model. The 1D velocity model proposed for the study region has seven uniform layers with interfaces at depths of 0, 5, 10, 15, 20, 25 and 30 km with P wave velocity of 5.219, 5.314, 5.391, 5.392, 5.964, 6.071 and 6.073 km/s and S wave velocity of 2.998, 3.015, 3.134, 3.135, 3.441, 3.482 and 3.647 km/s, respectively. According to the proposed model, the Moho in this part of the Himalaya lies at 60 km depth on an average. For P and S waves, the station correction ranges from −0.88 to 1.50 and −0.58 to 3.59 s, respectively. This low variation in station residuals indicates small lateral velocity changes that confirm the accuracy and stability of the proposed 1D velocity model. Using the new derived 1D velocity model, the earthquake epicentres were relocated and we observe a shallow seismic activity in the region at <30 km depth that clearly describes the ongoing convergence of the India-Eurasia plates in the study region. This study also infers a new, highly active seismic window in the latitude range of 31.8 °N to 32.8 °N and longitude range of 76.8 °E to 78.8 °E in the study region across the Kaurik-Chango fault, a causative fault for the 1975 Kinnaur earthquake.

Seismogenic Tectonics and Dynamics of the 2011 Ms5.9 Yingjiang Earthquake in Yunnan, China

Abstract:In the southern South–North Seismic Zone, China, seismic activity in the Yingjiang area of western Yunnan increased from December 2010, and eventually a destructive earthquake of Ms5.9 occurred near Yingjiang town on 10 March 2011. The focal mechanism and hypocenter location of the mainshock suggest that the Dayingjiang Fault was the site of the mainshock rupture. However, most of foreshocks and all aftershocks recorded by a portable seismic array located close to the mainshock occurred along the N–S‐striking Sudian Fault, indicating that this fault had an important influence on these shocks. Coulomb stress calculations show that three strong (magnitude ≥5.0) earthquakes that occurred in the study region in 2008 increased the coulomb stress along the plane parallel to the Dayingjiang Fault. This supports the Dayingjiang Fault, and not the Sudian Fault, as the seismogenic fault of the 2011 Ms5.9 Yingjiang earthquake. The strong earthquakes in 2008 also increased the Coulomb stress at depths of ≤5 km along the entire Sudian Fault, and by doing so increased the shallow seismic activity along the fault. This explains why the foreshocks and aftershocks of the 2011 Yingjiang earthquake were located mostly on the Sudian Fault where it cuts the shallow crust. The earthquakes at the intersection of the Sudian and Dayingjiang faults are distributed mainly along a belt that dips to the southeast at ∼40°, suggesting that the Dayingjiang Fault in the mainshock area also dips to the southeast at ∼40°.

Long-time variation of soil CO2 fluxes at the summit crater of Vulcano (Italy)

Here, we report the first continuous data of geochemical parameters acquired directly from the active summit crater of Vulcano. This approach provides a means to better investigate deep geochemical processes associated with the degassing system of Vulcano Island. In particular, we report on soil CO2 fluxes from the upper part of Vulcano, a closed-conduit volcano, from September 2007 to October 2010. Large variations in the soil CO2 and plume SO2 fluxes (order of magnitude), coinciding with other discontinuous geochemical parameters (CO2 concentrations in fumarole gas) and physical parameters (increase of shallow seismic activity and fumarole temperatures) have been recorded. The results from this work suggest new prospects for strengthening geochemical monitoring of volcanic activity and for improving the constraints in the construction of a “geochemical model”, this being a necessary condition to better understand the functioning of volcanic systems.

TDEM survey in an area of seismicity induced by water wells in Paraná sedimentary basin, Northern São Paulo State, Brazil

This article presents TDEM results from an area with recent induced shallow seismicity. The purpose was to do a geoelectrical mapping of sedimentary and fractured basaltic aquifers for better understanding of the hydrogeologic setting. The study area is in the Paraná basin where flood basalts are overlain by sedimentary units near the city of Bebedouro, northern São Paulo State, Brazil. 86 TDEM soundings were acquired in an area of 90 km2 in the Andes and Botafogo study areas. The soundings were chosen next to wells for calibration, and also along profiles crossing the seismically active areas. 1-D interpretation results showed the general geoelectrical stratigraphy of this part of the Paraná basin. The upper geoelectrical layer is the shallow sedimentary aquifer (Adamantina formation) with less than 80 m thickness. The second geoelectrical layer contains the upper basalts of the Serra Geral formation at about 60–80 m depths. A saturated fractured basalt zone between 100 and 300 m depths was identifiable on various TDEM soundings. This depth range corresponds to the range of hypocentral depths for more than 3000 micro-earthquakes in this area. The lower basalt layer was estimated to lie between 400 and 650 m depth. The deepest geoelectrical layer detected by various TDEM soundings corresponds to the Botucatu sandstone (Guarani aquifer). Results suggest that the high-discharge wells are located in the fractured zone in the middle basalt of the Serra Geral formation. There is a good correlation between seismically active areas, high discharge wells (> 190 m3/h), and fracture zones in the middle basalt. The results reinforce the hypothesis that the shallow seismic activity in the Bebedouro region is being triggered by high rates of groundwater withdrawal.

Shallow inland earthquakes in NE Japan possibly triggered by the 2011 off the Pacific coast of Tohoku Earthquake

Shallow seismic activity in the crust of the overriding plate west of the source area increased significantly after the 2011 Mw 9.0 Tohoku earthquake which ruptured the plate boundary to the east off northern Japan beneath the Pacific Ocean. In order to understand the cause of this distinctive change in seismicity, we have precisely relocated earthquake hypocenters for several earthquake sequences that occurred during the period March 11–April 6, 2011, following the Tohoku earthquake by the double-difference method. Hypocenter distributions were used to discriminate the fault plane from the auxiliary plane of the focal mechanisms for those earthquake sequences. We then calculated the Coulomb stress change on those fault planes caused by the 2011 Mw 9 earthquake. In all cases, the estimated Coulomb stress changes at the plausible fault planes for those post-mainshock sequences are positive. The positive Coulomb stress change is mainly due to the reduction of normal stress on the fault plane of the earthquake sequences by the large, low-angle thrust fault responsible for the 2011 Mw 9 earthquake. The present observations suggest that static stress transfer possibly triggered those post-mainshock earthquake sequences.

Long-term Seafloor Experiment with the CUMAS Module: Performance, Noise Analysis of Geophysical Signals, and Suggestions about the Design of a Permanent Network

The Campi Flegrei caldera (southern Italy) is one of the most hazardous areas in the world because it is home to several hundred thousand people live there and an important center of socio-economic activities. The caldera includes the western-most part of the city of Naples and extends into the Gulf of Pozzuoli (eastern Tyrrhenian basin; Figure 1). The main feature of the present volcanic activity of the caldera is the episodic slow and high-amplitude soil movement (bradyseism) accompanied by intense and shallow seismic activity that only occurs during the uplift phase. The most recent strong bradyseismic episode occurred between 1982 and 1984, when there was a maximum ground uplift of more than 170 cm, which was followed by slow and continuous subsidence, accompanied by rare low-amplitude uplifts (of a few centimeters). The maximum uplift was observed in the center of the caldera, in the town of Pozzuoli, while toward the caldera margins the uplift decreased gradually. The amplitude and the areal distribution of the vertical soil displacement that occurred over time in the marine sector, which covers more than one third of the caldera area, is completely unknown. During this 1982–84 bradyseismic episode, there were more than 10,000 shallow earthquakes (<4 km) with a maximum recorded magnitude of 4.2 (Aster et al. 1992). The space distribution of the earthquake hypocenters was denser on land, where the monitoring network had been deployed (Figure 1). In contrast, only a small number of the earthquakes were located in the marine sector. This can be explained primarily by a probable lesser level of seismic activity in the marine sector than on land, and secondly by the inadequate coverage of the sea area by the monitoring network configuration in place at the time. The present geophysical surveillance system that operates in the Campi Flegrei volcanic …

Seismic activity triggered by water wells in the Paraná Basin, Brazil

Triggered seismicity is commonly associated with deep water reservoirs or injection wells where water is injected at high pressure into the reservoir rock. However, earth tremors related solely to the opening of groundwater wells are extremely rare. Here we present a clear case of seismicity induced by pore‐pressure changes following the drilling of water wells that exploit a confined aquifer in the intracratonic Paraná Basin of southeastern Brazil. Since 2004, shallow seismic activity, with magnitudes up to 2.9 and intensities V MM, has been observed near deep wells (120–200 m) that were drilled in early 2003 near the town of Bebedouro. The wells were drilled for irrigation purposes, cross a sandstone layer about 60–80 m thick and extract water from a confined aquifer in fractured zones between basalt flow layers. Seismic activity, mainly event swarms, has occurred yearly since 2004, mostly during the rainy season when the wells are not pumped. During the dry season when the wells are pumped almost continuously, the activity is very low. A seismographic network, installed in March 2005, has located more than 2000 microearthquakes. The events are less than 1 km deep (mostly within the 0.5 km thick basalt layer) and cover an area roughly 1.5 km × 5 km across. The seismicity generally starts in a small area and expands to larger distances with an equivalent hydraulic diffusivity ranging from 0.06 to 0.6 m2/s. Geophysical and geothermal logging of several wells in the area showed that water from the shallow sandstone aquifer enters the well at the top and usually forms waterfalls. The waterfalls flow down the sides of the wells and feed the confined, fractured aquifer in the basalt layer at the bottom. Two seismic areas are observed: the main area surrounds several wells that are pumped continuously during the dry season, and a second area near another well (about 10 km from the first area) that is not used for irrigation and not pumped regularly. The main area displays cyclic annual activity, but the second area does not. We explain the earthquake swarms as being triggered by pore pressure diffusion in the fractured basalt layer due to additional pressure from the newly connected surface aquifer. This reaches critically prestressed areas up to a few kilometers away from the wells. During periods of continuous pumping, the reduction of pore pressure in the confined aquifer stops the seismic activity. Our study suggests that this kind of activity may be more common than previously thought and implies that many other cases of small tremors associated with the drilling of water wells may have gone unnoticed.

Water chemistry and trophic evaluation of Lake Albano (Central Italy): a four year water monitoring study

The crater lake Lake Albano is an increasingly diminishing water resource in terms of volume, the lake level has dropped more than four meters since the 1960s, and water quality resulting from elevated levels of nitrogen and phosphorus. The area of the lake, and the volcano as a whole, is also considered to be geologically hazardous due to continual shallow seismic activity, gaseous emissions and hydrothermal activity. Therefore, most research has been focussed on the geological aspects of the Albano lake system, whilst long-term limnological studies have been lacking. A meromictic classification was given to the lake, but this was based on one year studies of the surface water only. Presented and discussed are the results of a water chemistry and biological study of the full depth profile of Lake Albano from 2004 to 2008. During winter 2005-2006 the lake underwent a complete overturn, resulting in an influx of nutrient rich hypolimnetic water into the upper productive layers and oxygenated epilimnetic water into the deepest water layers. The effect of full overturn on the phytoplankton community is described and compared with those of meromictic years. The interplay between natural and anthropological processes on water quality and water usages is also discussed.

Late Cenozoic deformation and uplift of the western flank of the Altiplano: Evidence from the depositional, tectonic, and geomorphologic evolution and shallow seismic activity (northern Chile at 19°30′S)

We analyze the west vergent thrust system (WTS) along the western flank of the Altiplano in northern Chile (18°S–21°S). In our study area (19°20′S–19°50′S), the WTS consists of three thrust propagation monocline folds (flexures) developing growth strata. The relative uplift accommodated by the flexures is rapid between 26 and 8 Ma (0.1 mm/yr), diminishing to 0.02 mm/yr after 8 Ma. Approximately 2000 m of relative surface uplift was accommodated by the flexures since the late Oligocene. Sedimentological and geomorphological analysis shows that westward tilting of the forearc occurred after 10 Ma, coeval with the shifting of deformation from the Altiplano to the sub‐Andean zone, where the underthrusting of the Brazilian Craton would have resulted in crustal thickening, surface uplift in the orogen, and westward ductile subcrustal flow. Forearc tilting is accommodated by east vergent thrusts (ETS) issued from the Benioff zone beneath the Central Depression emerging into the Western Cordillera, contributing 500–1400 m of surface uplift. The WTS connects the ETS in the brittle‐ductile crustal transition (∼25 km depth), continuing farther east as the Altiplano low‐velocity zone, configuring the western Altiplano as a crustal‐scale fault bend fold. Forearc tilting would be caused by westward ductile flow in the lower crust pushing the rigid forearc in the ETS. Meanwhile, between 19°S and 21°S, the WTS accommodates dextral strike slip, and ∼3 km of N‐S shortening occurred in the Coastal Cordillera. Transcurrence and strain partitioning are probably the result of slight plate convergence obliquity, strong coupling within the interplate zone, westward continental concavity, and high elevation opposing horizontal contraction.

The Abanico extensional basin: Regional extension, chronology of tectonic inversion and relation to shallow seismic activity and Andean uplift

The Abanico extensional basin: Regional extension, chronology of tectonic inversion and relation to shallow seismic activity and Andean uplift

Deep structure of the northeastern Japan arc and its implications for crustal deformation and shallow seismic activity

Seismic tomography studies in the northeastern Japan arc have revealed the existence of an inclined sheet-like seismic low-velocity and high-attenuation zone in the mantle wedge at depths shallower than about 150 km. This sheet-like low-velocity, high-attenuation zone is oriented sub-parallel to the subducted slab, and is considered to correspond to the upwelling flow portion of the subduction-induced convection. The low-velocity, high-attenuation zone reaches the Moho immediately beneath the volcanic front (or the Ou Backbone Range) running through the middle of the arc nearly parallel to the trench axis, which suggests that the volcanic front is formed by this hot upwelling flow. Aqueous fluids supplied by the subducted slab are probably transported upward through this upwelling flow to reach shallow levels beneath the Backbone Range where they are expelled from solidified magma and migrate further upward. The existence of aqueous fluids may weaken the surrounding crustal rocks, resulting in local contractive deformation and uplift along the Backbone Range under the compressional stress field of the volcanic arc. A strain-rate distribution map generated from GPS data reveals a notable concentration of east–west contraction along the Backbone Range, consistent with this interpretation. Shallow inland earthquakes are also concentrated in the upper crust of this locally large contraction deformation zone. Based on these observations, a simple model is proposed to explain the deformation pattern of the crust and the characteristic shallow seismic activity beneath the northeastern Japan arc.