Subduction Megathrust Research Articles

Subducted carbon weakens the forearc mantle wedge in a warm subduction zone

Subducting oceanic plates carry large amounts of carbon into the Earth’s interior. The subducted carbon is mobilized by fluid and encounters ultramafic rocks in the mantle wedge, resulting in changes to the mineral assemblage and mechanical properties of the mantle. Here, we use thermodynamic modeling of interactions between carbon-bearing multi-component fluids and mantle rocks to investigate the down-dip variation in mineral assemblage in the forearc mantle along subduction megathrusts. We found that fluids rich in aqueous carbon are preferentially generated in a warm subduction zone (e.g., Nankai, SW Japan), causing a change in mineral assemblage from serpentine-rich at the mantle wedge corner to talc + carbonate-rich at greater depths. The transition caused by the infiltration of aqueous carbon may influence the depth of the boundary between the seismogenic and aseismic zones, and the down-dip limit of episodic tremor and slip.

Morphological differences across the Shumagin-Semidi fault segments control slip behaviors and tsunami genesis in the Aleutian-Alaska subduction zone

Rupture behaviors of a subduction megathrust define the slip type, the extent and the associated tsunami hazard. They are, however, difficult to be defined precisely due to limited fault-zone observations. Here, we integrate GNSS, tsunami-waveforms, seismic-profiles, and earthquake-cycle modeling to delineate the slip-extent of the 2020 Mw 7.8 Simeonof and the 2021 Mw 8.2 Chignik earthquakes in the Semidi segment; and to understand the possible structural and mechanical control on the distinct rupture behaviors of this segment and its neighboring Shumagin segment at the Aleutian-Alaska subduction zone. We show that both the Simeonof and Chignik earthquakes slipped a compact area at depth between ∼20 and 40 km that is well constrained by the combination of GNSS and tsunami-waveform data. We explain the distinct slip behaviors associated with the Semidi and Shumagin segments by highlighting the morphological changes in the fault along the strike direction. Beneath the Shumagin Island, we identify a structural-mechanical boundary that separates the megathrust into Semidi (east) and Shumagin (west) two segments. Semidi is gentle and curved; while Shumagin is steep and planar. The Semidi segment produces spatially-heterogenous stress field, and generates partial, full, complex ruptures as indicated in modeled cycles and in historical seismic observations. Meanwhile the Shumagin segment, coincides with the ocean-continent transition boundary – the Beringian margin, tend to generate slow-slip-events, tremors, otherwise, generates small or moderate seismicity as indicated in the modeled cycles and in seismic records since 1750. Our findings indicate that Semidi is likely to rupture in a chaotic fashion with major or large earthquakes, resulting a greater tsunami hazard like the 1938 Mw 8.2 event. The tsunami potential in the Unimak segment may also remain high.

Temperatures of Vein Formation Associated With Plate Interface Deformation Constrained by Oxygen and Clumped Isotope Thermometry

AbstractTectonic mélanges, characterized by conditions reflective of modern subduction fault zones, preserve mineral veins formed through mass transfer, a mechanism influencing the slip behavior of subduction megathrusts. In this study, we apply secondary ion mass spectrometry quartz‐calcite oxygen isotope thermometry and clumped isotope thermometry to examine the temperatures of vein formations in six mélange units in the Cretaceous Shimanto belt and one mélange in the Kodiak accretionary prism. Calcite in the veins exhibits δ13CPDB values ranging from −17.2‰ to −6.8‰, indicative of a carbon source mixing with sedimentary carbonate and organic matter. δ18OSMOW values of calcite range from +11.1‰ to +17.2‰; quartz yields δ18OSMOW values of +14.9‰ to +21.7‰. Oxygen isotopic signatures in minerals reveal that most vein‐forming fluids are significantly affected by rock buffering, while some retain isotopic compositions of seawater and meteoric water. Temperature estimates, derived from both thermometers, fall within the range of 100–250°C. Notably, vein temperatures remain constant across diverse vein types and mélange units with distinct maximum temperatures. The combined temperature records and fluid isotopic compositions imply vein formations at shallower depths linked to the incorporation of seawater, meteoric water, and fluid released from early dehydration reactions. At greater depths, vein formations are associated with fluid released from clay dehydration and long‐distance fluid flow. Reduced vein formations between 250 and 350°C may correlate with a shift to fluid‐unsaturated conditions resulting from clay hydration reactions. Our study highlights potential mechanical and hydraulic variations within the thermal conditions of 100–350°C along the plate boundary driven by fluid‐mineral interactions.

Megathrust locking encoded in subduction landscapes.

Locked areas of subduction megathrusts are increasingly found to coincide with landscape features sculpted over hundreds of thousand years, yet the mechanisms that underlie such correlations remain elusive. We show that interseismic locking gradients induce increments of irreversible strain across the overriding plate manifested predominantly as distributed seismicity. Summing these increments over hundreds of earthquake cycles produces a spatially variable field of uplift representing the unbalance of co-, post-, and interseismic strain. This long-term uplift explains first-order geomorphological features of subduction zones such as the position of the continental erosive shelf break, the distribution of marine terraces and peninsulas, and the profile of forearc rivers. Inelastic yielding of the forearc thus encodes short-term locking patterns in subduction landscapes, hinting that megathrust locking is stable over multiple earthquake cycles and highlighting the role geomorphology can play in constraining Earth's greatest source of seismic hazard.

Subduction intraslab-interface fault interactions in the 2022 Mw 6.4 Ferndale, California, earthquake sequence.

The Mendocino triple junction-the intersection of the Pacific, North American, and Gorda plates-activates a collection of disparate faults that reconcile Cascadia subduction with San Andreas transform motion. The 20 December 2022 Mw 6.4 Ferndale, California, earthquake occurred within this complex zone as strike-slip faulting within the subducting Gorda slab. Here, we analyze the seismic and geodetic signatures of the mainshock and aftershock sequence to illuminate its role within complex tectonic surroundings. We find aftershocks on varied fault structures within the uppermost Gorda slab, yet seismicity on the subduction interface itself was notably absent. Nevertheless, we identify small but coherent postseismic deformation that is well modeled by aseismic slip on this interface, likely triggered by stresses generated at the updip limit of coseismic rupture. This sequence demonstrates the potential for interactions between intra-slab earthquakes and slip on the subduction megathrust, highlighting the need to consider this and other subduction zones as coupled systems of interacting faults.

Large-scale submarine landslides in the Barberton Greenstone Belt, southern Africa—Evidence for subduction and great earthquakes in the Paleoarchean

Abstract New mapping of the Barberton Greenstone Belt in South Africa shows that the central part is a pseudo-stratigraphy made of shallow-water and deep-water siliciclastic and volcanic slide blocks, with individual blocks ranging in size from tens of meters to >10 km in length. The outcrop pattern and scale are remarkably similar to those of large-scale Miocene to recent submarine landslides in New Zealand along the active Hikurangi subduction zone that are periodically triggered by earthquakes on the subduction megathrust, providing evidence for megathrust earthquakes in the Paleoarchean.

Co-seismic Ground Deformation Monitoring on Brebes-Segment of Baribis-Kendeng Fault Zone Based on Sentinel-1 Data

Co-seismic deformation is a sudden change in the earth’s surface in a fault zone. Therefore, with modern satellite imagery, it is possible to investigate. Synthetic Aperture Radar (SAR) data from Mission Sentinel-1 is one example of satellite imagery that can be used to identify deformation. We employ the interferometry method by utilizing the Interferometric synthetic aperture radar Scientific Computing Environment (ISCE) and another InSAR software to monitor the co-seismic ground deformation in the form of line-of-sight (LOS) displacement. This research focused on the Brebes Segment of the Baribis-Kendeng Fault Zone and applied the interferometry method for the March 22, 2015, Balapulang earthquake (M4.7) and December 11, 2020, Brebes earthquake (M4.2). The earthquake event choice is based on the shallow depth of the hypocenter, which represents the earthquake originating from the fault in the crustal, not the megathrust subduction. The results show that despite the earthquake occurring under the thick quaternary sediment of the north shore of Java Island, the deformation caused by the co-seismic is still emerging to the surface. The LOS displacement was detected within the range of 6 cm from the ascending and descending track of the satellite sensors. Furthermore, this result may provide new information on the Baribis-Kendeng Fault zone, specifically the Brebes Segment.

Probabilistic Tsunami Hazard Analysis for Vancouver Island Coast Using Stochastic Rupture Models for the Cascadia Subduction Earthquakes

Tsunami hazard analysis is an essential step for designing buildings and infrastructure and for safeguarding people and assets in coastal areas. Coastal communities on Vancouver Island are under threat from the Cascadia megathrust earthquakes and tsunamis. Due to the deterministic nature of current megathrust earthquake scenarios, probabilistic tsunami hazard analysis has not been conducted for the coast of Vancouver Island. To address this research gap, this study presents a new probabilistic tsunami hazard model for Vancouver Island from the Cascadia megathrust subduction events. To account for uncertainties of the possible rupture scenarios more comprehensively, time-dependent earthquake occurrence modeling and stochastic rupture modeling are integrated. The time-dependent earthquake model can capture a multi-modal distribution of inter-arrival time data on the Cascadia megathrust events. On the other hand, the stochastic rupture model can consider variable fault geometry, position, and earthquake slip distribution within the subduction zone. The results indicate that the consideration of different inter-arrival time distributions can result in noticeable differences in terms of site-specific tsunami hazard curves and uniform tsunami hazard curves at different return period levels. At present, the use of the one-component renewal model tends to overestimate the tsunami hazard values compared to the three-component Gaussian mixture model. With the increase in the elapsed time since the last event and the duration of tsunami hazard assessment, the differences tend to be smaller. Inspecting the regional variability of the tsunami hazards, specific segments of the Vancouver Island coast are likely to experience higher tsunami hazards due to the directed tsunami waves from the main subduction zone and due to the local underwater topography.

Along-strike seismotectonic segmentation reflecting megathrust seismogenic behavior

Abstract Understanding the along-strike seismogenic behavior of megathrusts is crucial to anticipating seismic hazards in subduction zones. However, if and how spatiotemporal frictional heterogeneity (high and low kinematic coupling) at depth feeds back into the upper-plate deformation pattern and how the upper-plate elastic signals and permanent records may correlate have yet to be fully understood. Hence, we mimic subduction megathrust seismic cycles using an analog seismotectonic model of an elastoplastic wedge overlying a frictionally heterogeneous megathrust. Coseismically, the zone above the down-dip limit of the aseismic and seismogenic patches undergoes extension and contraction, respectively, while the strain state shows a switch in polarity from coseismic to interseismic. The down-dip limit of the creeping zone produces permanent along-strike extension or contraction, depending on the frictional barrier strength. Our experiments show that the frictional locking heterogeneity generates more segmented along-strike strain patterns elastically (short term) than permanently (long term). Moreover, our results suggest that along-strike upper-plate strain patterns could serve as a proxy for interpreting persistent lateral variations of seismogenic behavior in subduction megathrusts.

Long‐Term Offshore Borehole Fluid‐Pressure Monitoring at the Northern Cascadia Subduction Zone and Inferences Regarding the State of Megathrust Locking

AbstractThe Cascadia subduction megathrust off the Pacific Northwest follows an “end member” seismogenic behavior, producing large (up to moment magnitude 9) but infrequent (every several hundred years) earthquakes and tsunamis. Crustal deformation associated with the ongoing plate convergence has been characterized by land‐based geodetic observations, but the state of locking across the full breadth of the seismogenic fault is poorly constrained. We report results of offshore monitoring of borehole fluid pressure, as a proxy for formation volumetric strain, at a site ∼20 km landward of the Cascadia subduction deformation front since 2010. The multi‐depth pressure records were plagued by hydrologic noise, but noise at the deepest monitoring level (303 m sub‐seafloor) abated in 2015. Subsequently, including at the times of regional large earthquakes that caused significant dynamic stressing, no persistent pressure transients are present above a threshold of 0.08 kPa imposed by unremovable oceanographic signals, corresponding to a strain detection limit of ∼16 nanostrain. Simple dislocation models using local megathrust geometry suggest a resolvable slip of <1 cm along a trench‐normal corridor beneath the borehole for a range of slip‐patch dimensions. A large slip patch can be well resolved even at considerable along‐strike distances from the borehole; for instance, ∼10 cm slip is detectable over a 200‐km strike range for a slip‐patch radius of ∼50 km. This high sensitivity for detecting slip, along with the lack of observed events, stands in stark contrast to observations at other subduction zones, and suggests that the Northern Cascadia megathrust is most likely fully locked.

Substantial Upper Plate Faulting Above a Shallow Subduction Megathrust Earthquake: Mechanics and Implications of the Surface Faulting During the 2016 Kaikoura, New Zealand, Earthquake

AbstractThe 2016 moment magnitude 7.8 Kaikoura, New Zealand, earthquake occurred at the southern end of the Hikurangi subduction zone where the upper plate above the shallow megathrust is exposed sub‐aerially. As a result, the substantial co‐seismic deformation in the upper plate above the megathrust rupture was observed geologically and geodetically. We explore the relationship between this surface faulting and the subduction megathrust rupture and find that the greatest upper plate fault slip occurred coincident (in time and location) with the megathrust rupture. Models of Coulomb stress change demonstrate that these surface faults become positively loaded as the upper plate rebounds during the megathrust event, favoring fault slip. In addition, during the megathrust rupture these faults terminate against an uncoupled subduction plate interface. We simulate the effects of decoupling at the base of these faults and find that very large fault slip is an expected consequence of this decoupling, allowing near‐complete strain release. In contrast, typical strike‐slip faults, pinned at their base, would have lower amounts of fault slip. These two conditions—increased Coulomb stress and basal decoupling—combine to produce the extreme co‐seismic upper plate faulting observed above the shallow Kaikoura megathrust earthquake. Similar conditions occur in other global subduction zones, but in most subduction zones the region above the coupled megathrust is underwater and poorly observed. Our analysis of the Kaikoura earthquake indicates a need to reevaluate patterns of strain accumulation and release in these regions, rather than assuming simple models of elastic rebound.

Effects of phyllosilicate content on the slip behavior of fault gouge: Insights from room-temperature friction experiments on quartz–talc mixtures

Shallow slow slip events (SSEs) occur on the shallowest reaches of subduction megathrust faults. To investigate how phyllosilicate fraction in quartzo-feldspathic sediments influences the style of fault slip behavior, we conducted frictional sliding experiments on various binary mixtures of quartz and talc using a biaxial testing apparatus at room temperature, room humidity, sliding velocities of 2 or 0.66 μm/s, and a normal stress of 10 MPa. With increasing talc content, the steady state coefficient of friction decreases from 0.7 to 0.2 in association with a transition from velocity-weakening (VW) to velocity-strengthening (VS) behavior. For the gouges with 0–10 wt% talc, fault slip behavior, following velocity steps, transitions from regular stick slips via sustained oscillations to attenuated oscillations with talc content. The oscillations emerge only in a narrow range of talc contents (i.e., 4–10 wt%) at near velocity-neutral conditions. The observed transition is attributed to an increase in the stiffness ratio K = k/kc, where k is the elastic stiffness of the system and kc is the critical stiffness of the fault, although K is larger than 1 for both stick-slip and sustained-oscillation behaviors, and thereby the critical stiffness criterion is not satisfied. We also find that for unstable slip events, the smaller the stress drop magnitude, the slower the peak slip velocity. Our results suggest that mixed gouges containing VW and VS materials show velocity-neutral behavior and facilitate the nucleation of shallow SSEs, although finely tuned rate-and-state friction parameters are required to allow spontaneous slow slip.

Ultralow frictional healing explains recurring slow slip events.

Plate motion on shallow subduction megathrusts is accommodated by a spectrum of tectonic slip modes. However, the frictional properties and conditions that sustain these diverse slip behaviors remain enigmatic. Frictional healing is one such property, which describes the degree of fault restrengthening between earthquakes. We show that the frictional healing rate of materials entrained along the megathrust at the northern Hikurangi margin, which hosts well-characterized recurring shallow slow slip events (SSEs), is nearly zero (<0.0001 per decade). These low healing rates provide a mechanism for the low stress drops (<50 kilopascals) and short recurrence times (1 to 2 years) characteristic of shallow SSEs at Hikurangi and other subduction margins. We suggest that near-zero frictional healing rates, associated with weak phyllosilicates that are common in subduction zones, may promote frequent, small-stress-drop, slow ruptures near the trench.

A novel design criterion for I-shape beams of steel MRF buildings in subduction-zone earthquake-prone areas

This paper highlights the difference in the seismic response of 5-storey, 10-storey, and 15-storey steel moment resisting frame (MRF) buildings of various span lengths (6 m, 7.5 m, 9 m), located on Site class C in Vancouver, B.C., Canada, resulted under the mega-thrust subduction zone ground motions against crustal ground motions (GMs). To emphasize on the impact of GM duration on the seismic response of buildings, seven spectrally equivalent short and long duration record pair sets were selected from ground motion databases and employed in analyses. Numerical models that account for P-Δ effects and strength and stiffness degradation of steel MRF beams were used. Detailed nonlinear dynamic analyses were conducted to assess the collapse safety according to FEMA P695 (2009) methodology. The results indicated high probability of buildings’ collapse under the long duration subduction GM suite, with a 38% average reduction in the median collapse capacity. Moreover, tall MRF buildings designed according to the current building code and steel design standard did not satisfy the collapse safety criteria when subjected to subduction GMs characterized by significant duration t5-95 > 60 s, while the middle-rise buildings showed borderline pass. To overcome this drawback, a novel design criterion for I-shape MRF beams is proposed; hence, selecting MRF beam sections of Class 1 that are able to achieve aggregated fatigue coefficient greater than the proposed limit value, the occurrence of low cycle fatigue of plastic hinge region of MRF beams is delayed and the collapse safety criterium is achieved.

Frictional Behavior Downdip Along the Subduction Megathrust: Insights From Laboratory Experiments on Exhumed Samples at In Situ Conditions

AbstractSubduction megathrusts exhibit a spectrum of slip modes, including catastrophic earthquakes. Although the mechanical and frictional properties of materials sampled from subduction zones have been studied extensively, few datasets have been collected for compositions and at pressure and temperature conditions representative of those in situ. The Nankai subduction zone in southwest Japan is a well‐studied margin, and abundant data provide an opportunity to advance our understanding of fault and earthquake physics. Here, we use samples exhumed in the Shimanto and Sanbagawa Belts on Shikoku Island of southwest Japan that represent analogs for materials along the present‐day megathrust at depths of ∼5–&gt;25 km, and we shear these at their peak in situ pressure‐temperature (P‐T) conditions. Effective normal stresses range from 28 to 192 MPa, and temperatures from 105°C to 470°C. We used pore fluid pressures of 45–240 MPa, corresponding to fluid overpressure ratios λ of 0.65 and 0.90. Slip velocities of 0.1–100 μm/s were used, in order to focus on the nucleation of instability and earthquakes. We found predominantly velocity‐strengthening (inherently stable) behavior under all conditions for λ = 0.65. For λ = 0.90, velocity‐weakening behavior was observed at 350°C, with velocity‐strengthening behavior at lower and higher temperatures. The rate/state frictional stability parameter (a‐b) increases with slip velocity at temperatures up to ∼200°C and remains constant or decreases with slip velocity at higher temperatures. Overall, our results demonstrate the potentially important roles of both temperature and slip velocity in controlling the distribution of stress and frictional rheology along subduction thrusts.

Deterministic and probabilistic Seismic Hazard Assessments for Merangin Kerinci Sumatera

This paper presents calculated ground motions for Merangin Kerinci in the Provinsi of Jambi Sumatera. The ground motions were calculated using a probabilistic seismic hazard analysis. The earthquake catalogue used in the analysis included all recorded earthquakes up to the end of July 2019, trimmed for magnitude completeness. Uncertainty in input parameters was considered using a logic tree, with branches including shallow crustal faults of the Great Sumatran Faults and Sumatera Megathrust Subduction. Ground motions are presented for the local site conditions, measured at those sites which can be classified as Site Class D/C (stiff soil to Soft Rock/Very Dense Soil). Ground motions, including Peak Ground Acceleration (PGA), and spectral accelerations at periods ranging from 0.1 s to 10.0s were calculated the site. Data are presented for return periods of 175 years, 475 years, 2,500 years, 5,000 years, and 10,000 years for PSHA, and at mean, 50th, and 84th percentile confidence levels for DSHA. Compared to the Indonesian Seismic Hazard Map (ISHM) 2017 (at DSHA at 5% damping and 84th Percentile), the deterministic PGA values calculated in this study are relatively closed, which is accounted for 0.58g found in this study compared to the range of 0.6g – 0.7 in the ISHM 2017. On the other hand, the probabilistic PGA values for a particular of 5,000 years return period (SEE), the PGA values found in this assessment is 0.73g which is lower than the range of 0.8g – 0.9g in the ISHM 2017.

Linking Upper‐Plate Fault Reactivation With the Megathrust Earthquake Cycle: The Case of the Northern Chile Outer Forearc (19°S–23°S)

AbstractCrustal faults above subduction megathrusts have been shown to have the capacity to produce moderate‐to‐large earthquakes (fault reactivation), causing severe damage for inhabited areas in the vicinity. Fault reactivation has been dynamically related to the interseismic stage of the subduction cycle and to megathrust earthquakes. In the northern Chile outer forearc (19°S–23°S), published works have demonstrated that six faults of different orientation and kinematics produced M ∼ 7 paleoearthquakes during the Late Quaternary; these lie above the interplate rupture of the 1877 M8.7 Iquique Earthquake. Here, we assess the potential of these and similar faults to experience reactivation due to stress perturbations induced by the interseismic and coseismic phases of the megathrust earthquake cycle. To do so, we compute Coulomb Failure Stress changes (ΔCFS) produced by different megathrust locking/earthquakes (real and synthetic) scenarios. Our results show that fault reactivation is enhanced by both interseismic and coseismic stages. Furthermore, they suggest that reverse slip on EW and NS faults is favored during the interseismic stage; in turn, faults of different orientations and kinematics are favored following megathrust earthquakes. The easiest way to promote the reactivation of most of the considered faults is with interplate ruptures slipping within (but not necessarily restricted to) the seismogenic zone between 20°S and 23°S. These ruptures would augment the hazard from crustal faulting in the area. These results highlight the need to continue studying the paleoseismology of these faults and their dynamic link with the megathrust seismic cycle, in this and other subduction margins.

Subduction earthquake cycles controlled by episodic fluid pressure cycling

In subduction zones, fluids are often invoked to explain slip processes on the megathrust, from great earthquakes to slow-slip events and tectonic tremors. However, it is unclear how the transient evolution of pore-fluid is controlled by depth-dependent variations in hydraulic properties over a broad range of timescales concomitant with the full spectrum of seismic and aseismic slip. In this study, we leverage a newly-developed fully dynamic hydro-mechanical earthquake cycle modeling framework to simulate fluid-driven seismic and aseismic fault slip. By assimilating geological, geophysical, and laboratory data in a physics-based model of fault dynamics, we investigate the role of hydraulic properties on-fault in controlling the predominant slip mode along the subduction megathrust. Results indicate that fluid-driven shear cracks nucleate due to a competing mechanism between the compaction of pores and the dynamic self-pressurization of fluids inside the megathrust, whereas the subsequent propagation of dynamic ruptures is self-sustained by solitary pore-pressure waves. While models with uniform hydraulic properties yield to regular seismic cycles of complete megathrust ruptures, a depth-varying fault permeability leads to the emergence of complex aperiodic sequences characterized by partial and complete ruptures, aftershocks, and transient aseismic slip. Further parameter analysis shows that the slip response on-fault primarily depends on fault permeability and porosity, which in turn control the poroelastic compaction, the storage capacity, and the hydraulic diffusion length. Four slip response patterns are revealed by the parameter space, including seismic events, slow-slip events, oscillatory decay with time, and stable aseismic creep. Our findings provide new insights into the interplay between pore-fluid, mechanical, and fault slip processes, and suggest that solid-fluid interactions and the permeability architecture play a key role in controlling the predominant slip mode on subduction megathrusts.

Probabilistic tsunami hazard analysis for western Makran coasts, south-east Iran

Makran subduction zone, along the southern coasts of Iran and Pakistan, has a wide potential seismogenic zone and may be capable of generating large magnitude (M ~ 9) tsunamigenic earthquakes. Considering ambiguities exist in tsunamigenic source characterization for subduction megathrusts like Makran, where detailed geologic, seismic, and geodetic data are insufficient, the probabilistic tsunami hazard analysis (PTHA) is the most prevalent approach to handle uncertainties and estimate more reliable tsunami hazard. Here, PTHA is performed for the coastal region of the western Makran, south-eastern Iran. Using the logic tree approach, we have considered the uncertainty of maximum seismic magnitude, earthquake occurrence model, continuity of seismic zone, seismic coupling coefficient, rupture depth, presence or absence of splay faults, fault locations, and fault slip distribution in PTHA calculation for the western Makran region. We have derived uniform tsunami hazard maps for two return periods of 475 and 2475 years for two confidence levels, the mean and the 84th percentile. Ground subsidence effects are also evaluated in a probabilistic manner. According to the PTHA results, Chabahar and Sirik towns are at the highest and lowest tsunami risk, respectively.

Direct constraints on in situ stress state from deep drilling into the Nankai subduction zone, Japan

Abstract Stress state is a long-sought but poorly known parameter on subduction megathrusts and in overlying accretionary wedges in general. We used direct observations made during drilling of Integrated Ocean Drilling Program (IODP) borehole C0002 to a depth of 3058 m below the seafloor (mbsf) in the Nankai subduction zone of southwestern Japan to constrain in situ pore pressure and stress state in the deep interior of an accretionary wedge for the first time. These data included downhole pressure, active pumping tests, and logging and sample measurements. We found a nearly linear gradient in minimum horizontal principal stress (Shmin) and show that it remained consistently smaller than the vertical stress (Sv), definitively ruling out a thrust-faulting stress regime to at least 3 km depth, and to within ~2 km above the subduction megathrust. At 3000 mbsf, the estimated effective stresses were: Sv = 33 MPa, SHmax = 25–36 MPa, and Shmin = 18.5–21 MPa. We therefore interpret that the stress state throughout the drilled interval, which lies entirely in the hanging wall of the active megathrust, lies in a normal or strike-slip faulting regime (Sv ≥ SHmax &amp;gt; Shmin). Total differential stresses are below ~18 MPa. We conclude that (1) basal traction along the megathrust must be small in order to permit both locking (and frictional sliding at failure) of the décollement and such low differential stresses deep within the upper plate; and (2) although differential stresses may remain low all the way to the plate boundary at ~5000 mbsf, SHmax must transition to become greater than the vertical stress—either spatially below the base of the borehole or temporally leading up to megathrust fault rupture—in order to drive thrust motion along the plate boundary as observed in great earthquakes and in recurring very low-frequency earthquakes and slow-slip events.