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The long‐term state of stress in the subduction forearc depends on the balance between margin‐normal compression due to the plate‐coupling force and the margin‐normal tension due to the gravitational force on the margin topography. In most subduction margins, the outer forearc is largely in margin‐normal compression due to the dominance of the plate‐coupling force. The inner forearc's state of stress varies within and among subduction zones, but what gives rise to this variation is unclear. We examine the state of stress in the forearc region of nine subduction zones by inverting focal mechanism solutions for shallow forearc crustal earthquakes for five zones and inferring the previous inversion results for the other four. The results indicate that the inner forearc stress state is characterized by margin‐normal horizontal deviatoric tension in parts of Nankai, Hikurangi, and southern Mexico. The vertical and margin‐normal horizontal stresses are similar in magnitudes in northern Cascadia as previously reported and are in a neutral stress state. The inner forearc stress state in the rest of the study regions is characterized by margin‐normal horizontal deviatoric compression. Tension in the inner forearc tends to occur where plate coupling is shallow. A larger width of the forearc also promotes inner‐forearc tension. However, regional tectonics may overshadow or accentuate the background stress state in the inner forearc, such as in Hikurangi. Plain Language Summary The state of stress in the overriding plate between the trench and the volcanic arc of subduction zones depends on frictional coupling between the overriding and subducting plates and gravitational force, which causes lateral compression and tension, respectively. The trench‐ward portion of this so‐called “forearc” region is generally in compression due to the dominant effect of plate coupling, but for the arc‐ward portion, the relative importance of the two forces varies spatially. We constrain the state of stress in the forearc using earthquake data for five subduction zones and inferring results from previous studies for four other subduction zones. The results indicate that the forearc stress state seems to correlate with the downdip depth of plate coupling and the width of the forearc. A relatively shallow downdip extent of coupling in a wide forearc tends to have the arc‐ward portion in tension or neutral stress state as observed in parts of Nankai, Hikurangi, Mexico, and Cascadia although this tendency is impacted by the local tectonic settings. Key Points Focal mechanism inversion results indicate the correlation of inner forearc stress state with the downdip depth of plate coupling Margin‐normal horizontal deviatoric tension in the inner forearc tends to occur where plate coupling is shallow and the forearc is wide The variation in the inner forearc stress state does not require a variation in the subduction fault strength

The seismically active Sumatra subduction zone has generated some of the largest earthquakes in the instrumental record, and both historical accounts and paleogeodetic coral studies suggest these were large enough to transfer stress to the surrounding region, including the Great Sumatran Fault (GSF). Therefore, evaluating the stress transfer from these large subduction earthquakes could delineate segments of elevated stress along the GSF where large earthquakes may potentially occur. In this study, we investigated eight megathrust earthquakes from 1797 to 2010 and resolved the accumulated Coulomb stress changes onto 18 segments along the GSF. Additionally, we also estimated the rate of tectonic stress on the GSF segments which experienced large earthquakes. We considered two cases, with: (1) no forearc sliver movement, and (2) the forearc sliver movement suggested by recent studies. Based on the historical stress changes of large earthquakes and the increase in tectonic stress rate, we analyzed the time evolution of stress changes on the GSF. The Coulomb stress changes on the GSF due to megathrust earthquakes between 1797 and 1907 increased the Coulomb stress mainly on the southern part of GSF, which was followed by four major GSF events during 1890–1943. The estimation of tectonic stress rates using case (1) produces a low rate of stress accumulation and long recurrence intervals, which would imply that megathrust earthquakes play an important role in promoting the occurrence of GSF earthquakes. When implementing the arc-parallel sliver movement of case (2), the tectonic stress rates are much higher than case (1), with an observed slip rate of 15–16 mm/yr at the GSF consistent with a recurrence interval for full-segment rupture of 100–200 years. The case (2) result suggests that the occurrence of GSF earthquakes is dominantly controlled by the rapid arc-parallel forearc sliver motion. Furthermore, the analysis of the evolution of stress changes with time shows that some segments such as Tripa (North and South), Angkola, Musi and Manna, which have experienced full-segment rupture and are therefore likely locked, appear to have returned to stress levels similar to those prior to previous historical events, suggesting elevated earthquake hazard along these GSF segments.

A 1.6 km riser borehole was drilled at site C0009 of the NanTroSEIZE, in the center of the Kumano forearc basin, as a landward extension of previous drilling in the southwest Japan Nankai subduction zone. We determined principal horizontal stress orientations from analyses of borehole breakouts and drilling-induced tensile fractures by using wireline logging formation microresistivity images and caliper data. The maximum horizontal stress orientation at C0009 is approximately parallel to the convergence vector between the Philippine Sea plate and Japan, showing a slight difference with the stress orientation which is perpendicular to the plate boundary at previous NanTroSEIZE sites C0001, C0004 and C0006 but orthogonal to the stress orientation at site C0002, which is also in the Kumano forearc basin. These data show that horizontal stress orientations are not uniform in the forearc basin within the surveyed depth range and suggest that oblique plate motion is being partitioned into strike-slip and thrusting. In addition, the stress orientations at site C0009 rotate clockwise from basin sediments into the underlying accretionary prism.

We determined focal mechanism solutions of microearthquakes and examined the stress field in the low-seismicity region from southern Hokkaido to eastern Aomori, NE Japan. The stress fields determined in this study comprise (1) a reverse faulting stress regime in southern Hokkaido with the axis of maximum compressional stress (σ1) being sub-horizontal and trending WNW–ESE, and (2) a stress regime in eastern Aomori to Tsugaru Strait that shows temporal variations and differential stress of less than tens of MPa. The spatiotemporal variation in stress from eastern Aomori to Tsugaru Strait might reflect the effects of the upper-plate bending and the 2011 Mw 9.0 Tohoku-Oki earthquake. It also indicates that the compressional stress caused by the descending Pacific plate is relatively weak, which is similar to other areas in eastern parts of the NE Japan arc.

The local topographic slope of the accretionary prism is often used together with the critical taper theory to determine the effective friction on subduction megathrust. In this context, extremely small topographic slopes associated with extremely low effective basal friction (μ≤0.05) can be interpreted either as seismically locked portions of megathrust, which deforms episodically at dynamic slip rates or as a viscously creeping décollement. Existing mechanical models of the long-term evolution of accretionary prism, sandbox models, and numerical simulations alike, generally do not account for heat conservation nor for temperature-dependent rheological transitions. Here, we solve for advection–diffusion of heat with imposed constant heat flow at the base of the model domain. This allows the temperature to increase with burial and therefore to capture how the brittle–ductile transition and dehydration reactions within the décollement affect the dynamic of the accretionary prism and its topography. We investigate the effect of basal heat flow, shear heating, thermal blanketing by sediments, and the thickness of the incoming sediments. We find that while reduction of the friction during dewatering reactions results as expected in a flat segment often in the forearc, the brittle–ductile transition results unexpectedly in a local increase of topographic slope by decreasing internal friction. We show that this counterintuitive backproduct of the numerical simulation can be explained by the onset of internal ductile deformation in between the active thrusts. Our models, therefore, imply significant viscous deformation of sediments above a brittle décollement, at geological rates, and we discuss its consequences in terms of interpretation of coupling ratios at subduction megathrust. We also find that, with increasing burial and ductile deformation, the internal brittle deformation tends to be accommodated by backthrusts until the basal temperature becomes sufficient to form a viscous channel, parallel to the décollement, which serves as the root to a major splay fault and its backthrust and delimits a region with a small topographic slope. Morphologic resemblances of the brittle–ductile and ductile segments with forearc high and forearc basins of accretionary active margins, respectively, allow us to propose an alternative metamorphic origin of the forearc crust in this context.

This work explores the characteristics and the seismogenic potential of crustal faults on the overriding plate in an area of high seismic hazard associated with the occurrence of subduction earthquakes and shallow earthquakes of the overriding plate. We present the results of geomorphic, structural, and fault kinematic analyses conducted on the convergent margin between the Cocos plate and the forearc region of the overriding North American plate, within the Guerrero sector of the Mexican subduction zone. We aim to determine the active tectonic processes in the forearc region of the subduction zone, using the river network pattern, topography, and structural data. We suggest that in the studied forearc region, both strike-slip and normal crustal faults sub-parallel to the subduction zone show evidence of activity. The left-lateral offsets of the main stream courses of the largest river basins, GPS measurements, and obliquity of plate convergence along the Cocos subduction zone in the Guerrero sector suggest the activity of sub-latitudinal left-lateral strike-slip faults. Notably, the regional left-lateral strike-slip fault that offsets the Papagayo River near the town of La Venta named “La Venta Fault” shows evidence of recent activity, corroborated also by GPS measurements (4–5 mm/year of sinistral motion). Assuming that during a probable earthquake the whole mapped length of this fault would rupture, it would produce an event of maximum moment magnitude Mw = 7.7. Even though only a few focal mechanism solutions indicate a stress regime relevant for reactivation of these strike-slip structures, we hypothesize that these faults are active and suggest two probable explanations: (1) these faults are characterized by long recurrence period, i.e., beyond the instrumental record, or (2) they experience slow slip events and/or associated fault creep. The analysis of focal mechanism solutions of small magnitude earthquakes in the upper plate, for the period between 1995 and 2008, revealed that frequent normal faults, sub-parallel to the trench, could be reactivated in the current stress field related to the Cocos subduction. Moreover, these features could also be reactivated by subduction megathrust earthquakes.

We investigate the spatial variation in the state of stress in the overriding plate in the forearc region of northern Cascadia. The key forces that act on the forearc region are the plate coupling force, the northward push of the Oregon block, the gravitational collapse force on elevated topography, and buoyancy of the serpentinized mantle wedge. The latter two forces are expected to cause margin-normal tension at shallow depths in the inner forearc. In this study, we compile available earthquake focal mechanism solutions and determine spatial variations of stress orientation through focal mechanism inversion. The results indicate the maximum compressive stress axis is margin-normal in the outer forearc due to shear stress at the subduction interface and margin-parallel throughout the inner forearc with varying orientations of the intermediate and minimum compressive stress axes throughout the inner forearc, consistent with previous studies. Using a 3-D finite-element code for lithospheric deformation, we explore the effects of gravitational collapse force, the buoyancy of the serpentinized mantle wedge corner, the plate coupling force, the shape of the slab, and the Oregon push on the stress field. The results indicate the buoyancy of the serpentinized mantle wedge corner and the shape of the slab may be as important of contributors to the the state of stress in the inner forearc as the plate coupling force whereas the gravitational collapse force and the Oregon push have a minimal effect on the state of stress in the inner forearc.

Tectonic stress fields in the overriding plate at convergent plate margins are complex and vary on local to regional scales. Volcanic arcs are a common element of overriding plates. Stress fields in the volcanic arc region are related to deformation generated by subduction and to magma generation and ascent processes. Analysis of moment tensors of shallow and intermediate depth earthquakes in volcanic arcs indicates that the seismic strain field in the arc region of many convergent margins is subhorizontal extension oriented nearly perpendicular to the arc. A process capable of generating such a globally consistent strain field is induced asthenospheric corner flow below the arc region.