High Pore Fluid Pressure Research Articles

Interseismic Locking of the Xiaojiang Fault May Be Controlled by Pore Fluid Pressure

AbstractAlthough fault locking state has been widely acquired with space geodetic observations, the mechanisms controlling fault locking are poorly known. Here, we infer the locking state of the Xiaojiang fault with a viscoelastic deformation model and integrated GNSS data, based on which we probe the mechanism that may control the locking pattern of the fault. Our results reveal four highly locked asperities along the Xiaojiang fault, which are separated by weak locking zones. Relying on the inferred locking degree and spring water temperature along the fault, we construct a model to connect the fault hydraulic conductivity with the cooling process of upwelling water. Model results suggest that high spring water temperature correlates with high hydraulic conductivity. Further, we use an experimental law to infer that the obtained high hydraulic conductivity may associate with high pore fluid pressure, which in turn weakens the fault frictional strength and leads to weak locking.

Linking the spatiotemporal distribution of static stress drops to source faults in a fluid-driven earthquake swarm, northeastern Noto Peninsula, central Japan

We investigated stress drops during an earthquake swarm in northeastern Noto Peninsula, central Japan, which is characterized by ongoing seismic activity in four clusters. We focused on the spatiotemporal distribution of the static stress drop and its relationship with the source faults of the earthquake swarm. Employing the empirical Green’s function method, we estimated static stress drops for 90 earthquakes of MJMA 3.0–5.4. We obtained logarithmic mean stress drops of 13 MPa and 19 MPa from P-wave and S-wave analyses, respectively, which were typical values for crustal earthquakes. We comprehensively analyzed the spatiotemporal distribution of static stress drops in the northern cluster due to the abundance of available data and clarity of fault structures there. We observed larger static stress drops for earthquakes along shallow portions of the source faults, as defined by the hypocentral distribution during a given period. Conversely, we observed smaller static stress drops for earthquakes at medial parts along the faults. These results suggest higher fault strength at shallower parts along the faults and reduced fault strength at medial parts. We attribute the high fault strength at shallow parts to low pore fluid pressure after only limited fluid diffusion near the fault terminus. In contrast, we attribute the reduction in fault strength at medial parts to high pore fluid pressure within the fault following penetration by migrating fluids.Graphical

Geological fingerprints of deep slow earthquakes: A review of field constraints and directions for future research

Abstract Slow earthquakes, including low-frequency earthquakes, tremor, and geodetically detected slow-slip events, have been widely detected, most commonly at depths of 40–60 km in active subduction zones around the Pacific Ocean Basin. Rocks exhumed from these depths allow us to search for structures that may initiate slow earthquakes. The evidence for high pore-fluid pressures in subduction zones suggests that they may be associated with hydraulic fractures (e.g., veins) and with metamorphic reactions that release or consume water. Loss of continuity and resulting slip at rates exceeding 10−4 m s–1 are required to produce the quasi-seismic signature of low-frequency earthquakes, but the subseismic displacement rates require that the slip rate is slowed by a viscous process, such as low permeability, limiting the rate at which fluid can access a propagating fracture. Displacements during individual low-frequency earthquakes are unlikely to exceed 1 mm, but they need to be more than 0.1 mm and act over an area of ~105 m2 to produce a detectable effective seismic moment. This limits candidate structures to those that have lateral dimensions of ~300 m and move in increments of <1 mm. Possible candidates include arrays of sheeted shear veins showing crack-seal structures; dilational arcs in microfold hinges that form crenulation cleavages; brittle-ductile shear zones in which the viscous component of deformation can limit the displacement rate during slow-slip events; slip surfaces coated with materials, such as chlorite or serpentine, that exhibit a transition from velocity-weakening to velocity-strengthening behavior with increasing slip velocity; and block-in-matrix mélanges.

Coupling of source rock gas expulsion intensity and reservoir quality in time and space-a key to determine gas accumulation in Upper Paleozoic tight sandstones of the Mugua gas field, Ordos Basin, China

Research on gas accumulation mechanisms and variations in gas saturation is limited in the newly established Mugua gas field in the Ordos Basin. Understanding these aspects is vital for reduce drilling uncertainty and exploration risk. This study employed a variety of methods, including: (1) the use of organic geochemical examinations to determine the gas expulsion intensity of hydrocarbon source rocks; (2) fluid inclusion analyses to determine the time of gas accumulation; (3) a combination of optical microscopy, scanning electron microscopy observations, pore-simulation and permeability tests, mercury-intrusion capillary pressure experiments, and nuclear magnetic resonance studies to assess the properties of the reservoir quality and identify the tight sandstone rock type; (4) wireline logging and interpretive data analyses to research the gas saturations features; and (5) establish reservoir quality evolution model to systematically study this problem. Integrating fluid inclusion homogenization temperatures with burial and thermal histories indicates that gas accumulation in the Mugua gas fields occurred throughout the Early Jurassic to Late Cretaceous period. Five diagenetic facies have been identified: dissolution, quartz-cemented, clay mineral filling, tightly compacted, and carbonate-cemented facies. The reservoir quality evolution model reveals that the five diagenetic facies experienced densification before the period of gas accumulation. Therefore, for gas accumulation in tight sandstones, it is essential that mature and gas-generating source rocks, provide sufficient expulsion forces to drive gas into the rocks. The mechanism of gas accumulation in the Mugua gas field's tight gas sandstone reservoirs involves the process occurring in sandstones that are either near or interbedded with source rocks, under high pore-fluid pressure generated by gas production. Compared to other Upper Paleozoic gas fields in the Ordos Basin, the gas expulsion intensity in the Mugua gas field is lower, ranging from 6.12–8.39 × 108 m3/km2. Due to the low gas expulsion intensity, the Shihezi 8 Member's tight sandstone reservoirs, situated away from the source rock, typically show low gas saturation. Conversely, the dissolution and quartz-cemented facies within the Benxi, Taiyuan, and Shanxi Formations, intimately interbedded with hydrocarbon source rocks, exhibit good reservoir quality. They offer ideal conditions for gas accumulation, making them prime candidates for exploration and development.

Complex rupture evolution of the 2007 Martinique earthquake: a non-double-couple event in the Caribbean Sea

SUMMARY A large non-double-couple component of a tectonic earthquake indicates that its rupture likely was complex and likely involved multiple faults. Detailed source models of such earthquakes can add to our understanding of earthquake source complexity. The 2007 Martinique earthquake in the Caribbean Sea is one of the largest recent earthquakes with a known large non-double-couple component. It was an intermediate depth intraslab earthquake within the South American plate where it is subducting beneath the Caribbean plate. We applied potency density tensor inversion (PDTI) to teleseismic P waves generated by the 2007 Martinique earthquake to model its source processes and focal mechanism distribution. We identified two focal mechanisms: a strike-slip mechanism with a north–south tension axis (T-axis), and a downdip extension (DDE) mechanism with an east–west T-axis. Rupture by the DDE mechanism was predominant in the northern part of the source region and strike-slip rupture in the southern part. These two focal mechanisms had approximately parallel pressure axes (P-axes) and approximately orthogonal T-axes. The seismic moments released by both types of rupture were almost equal. These results indicate that the 2007 Martinique earthquake had a large non-double-couple component. We identified five subevents with two predominant directions of rupture propagation: two strike-slip subevents propagated to the southeast and three DDE subevents propagated to the east. Although the directions of propagation were consistent for each focal mechanism, each subevent appears to have occurred in isolation. For example, the rupture of one DDE subevent propagated from the edge of the source region back towards the hypocentre. Complex ruptures that include multiple subevents may be influenced by high pore fluid pressure associated with slab dehydration. Our results show that PDTI can produce stable estimates of complex seismic source processes and provide useful information about the sources of complex intermediate depth intraslab earthquakes for which fault geometry assumptions are difficult.

Simulation Study on the Mechanical Effect of CO2 Geological Storage in Ordos Demonstration Area

In order to understand the long-term process of CO2 storage and demonstrate its safety, multi-field coupled numerical simulation is considered a crucial technology in the field of geological CO2 storage. This study establishes a site-specific homogeneous thermo-hydro-mechanical coupling model based on TOUGH-FLAC3D coupling program using actual stratigraphic data from the Ordos demonstration area. The analysis investigates the transport behavior of CO2 within the formation considering pore permeability homogeneity, incorporates redistribution of effective stress and rock deformation, and provides a mechanical evaluation of the effectiveness of CO2 sequestration at this specific site. The findings indicate that: (1) the sealing effect of the cap rock depends on the difference of permeability between the reservoirs. The greater the permeability difference, the better the sealing effect. (2) High pore fluid pressure can lead to a decrease in the effective stress of rocks, causing deformation. After simulation calculations, the maximum deformation of rocks can reach 7.79 mm within a decade of CO2 injection. (3) Under the condition of continuous CO2 injection, the pore pressure will not be able to dissipate quickly and will continue to rise, and eventually shear failure will occur in the rock layer, but it is mainly concentrated in the lower part of the cap rock.

Stabilizing Effect of High Pore Fluid Pressure on Fault Growth During Drained Deformation

AbstractDilatant hardening is an accepted model for the stabilizing effect of high pore fluid pressure on fault slip and operates when deformation is undrained. To test whether high pore fluid pressure impedes fault propagation under drained conditions, we deformed highly permeable Darley Dale sandstone using strain rates of 10−4 s−1, 10−5 s−1, and 10−6 s−1, respectively. For each strain rate, we compared the inelastic behaviors and faulting styles among rocks deformed under different pore fluid pressures (Pf) (2–180 MPa). The confining pressure (Pc) was attuned to the pore fluid pressure throughout deformation to maintain a constant differential pressure (Pc − Pf) of 10 MPa. In samples deformed at 10−4 s−1 and 10−5 s−1, faulting behaviors were similar regardless of the magnitude of pore fluid pressure. However, when the strain rate was lowered to 10−6 s−1, we observed prolonged stress drops and slower slip velocities in samples deformed under high pore fluid pressures. In samples deformed at 10−6 s−1, we demonstrate that chemically assisted subcritical crack growth played an important role during faulting. A quantitative microstructural analysis revealed that slow faulting at slow strain rates was accompanied by pervasive microcracking and diffuse shear bands, which suggests pervasive subcritical cracking enabled slow faulting under drained conditions at the sample length scale. High pore fluid pressure may have facilitated slow faulting chemically by increasing the rate of subcritical cracking, mechanically via localized dilatant hardening, or both. Our results provide insight into the mechanics of faulting in natural settings where subcritical cracking is prevalent.

An alternative to the fault-valve model

This paper proposes a mechanism called the mode-switching model that is presented as an alternative to the fault-valve model. This mechanism is relevant to open-flow, low-porosity, fluid-saturated systems deforming by pressure solution creep. As opposed to most constitutive models discussed in the geological literature, the yield envelope is capped at high normal stresses, as demonstrated by experimental studies. A low-permeability rock has relatively high pore fluid pressure for a given input fluid flux. This increases the dissolution rate for quartz that in turn leads to a higher-permeability rock, low fluid pressure for the same flux and decreased quartz solubility and deposition, returning to a low permeability. This cycle continues indefinitely so long as the rock mass is stressed, a fluid flux is applied, and pressure solution operates. The high fluid pressure drives the Mohr stress circle to the tensile end of the yield envelope resulting in crack-seal and extensional veins. The low fluid pressure drives the Mohr stress circle to the cap end of the yield envelope resulting in laminated veins in rocks undergoing mineral reactions with large net volume losses coupled with solute transfer. Failure at the cap end of the yield envelope results in displacement discontinuities inclined at high angles to σ 1. Previously, these orientations have been taken to represent reactivated normal faults, an integral component of the fault-valve process. In the model presented, the yield surface prohibits the system ever reaching super-lithostatic pressures. The process of effective stress-driven switching between tensile and cap ends of the yield envelope arises from competition between dissolution and deposition, and is independent of any seismic events, fault reactivation or the episodic breaching of an impermeable seal. It provides a unifying, self-consistent concept for the interpretation of joints, faults and veins in hydrothermal systems.

Shallow Low‐Velocity Layer in the Hyuga‐Nada Accretionary Prism and Its Hydrological Implications: Insights From a Passive Seismic Array

AbstractShear wave velocity (Vs) estimations of accretionary prisms can pose unique constraints to the physical properties of rocks, which are hard to obtain from compressional velocities (Vp) alone. Thus, it would help better understand the fluid processes of the accretion system. This study investigates the Vs structure of the Hyuga‐nada accretionary prism using an array of ocean‐bottom seismometers (OBSs) with a 2 km radius. Teleseismic Green's functions and a surface wave dispersion curve are inverted to one‐dimensional Vs structures using transdimensional inversion. The results indicate the presence of a low‐velocity zone 3–4 km below the seafloor. The reduced Vs corresponds to a reduced Vp feature obtained in a previous seismic refraction survey, and the high Vp/Vs ratio suggests the presence of high pore fluid pressure. In addition, the resolved lithological boundary exhibits a sharp offset that extends laterally across the OBS array. We attribute this offset to a blind fault below while acknowledging other possibilities, such as due to mud diapirism or intense fracturing. The predicted fault is located at the Kyushu–Palau Ridge flank, oriented roughly parallel to the ridge axis, and thus likely caused by ridge subduction. This fault may act as a fluid conduit, contributing to the formation of a fluid reservoir beneath the compacted sediment layers.

Shattered Veins Elucidate Brittle Creep Processes in the Deep Slow Slip and Tremor Region

AbstractDeep Slow Slip and Tremors (SSTs) are a combination of transient clusters of tectonic tremors and slow slip associated with extremely elevated fluid pressures. SSTs are thought to reflect a transition from viscous to brittle plate interface rheology and likely exert a first‐order control on megathrust seismicity. Nevertheless, the deformation mechanisms governing the source of SSTs remain elusive. We herein document the occurrence of vein networks precipitated and brecciated within the deep SST region under blueschist‐facies conditions. These lawsonite‐rich vein sets exhibit extensive evidence of brittle deformation and are spatially related to localized, finely milled (cataclastic) shear bands. Petro‐geochemical data reveal that brittle deformation was accompanied by the injection of several ultramafic‐, mafic‐ and metasedimentary‐derived fluid pulses, imprinting characteristic Cr, high field strength elements, and light over heavy rare earth elements positive anomalies in the vein breccias while leaching light rare earth elements from the cataclastic blueschist host. Our results suggest that metamorphic veins represent zones of mechanical anisotropy within the rock volume prone to localized shearing, brittle deformation and episodic injection of externally derived fluids. These networks demonstrate the importance of former vein sets as structural heterogeneities in triggering fluid‐controlled brittle creep events. The combined effects of high pore fluid pressures and rheological heterogeneities in the form of metamorphic veins could trigger the nucleation and propagation of SSTs at the margins of this mechanically anisotropic environment, and thus determine where slip will take place along deep subduction interfaces.

Spatio‐Temporal Evolution of Aftershock and Repeater Source Properties After the 2016 Pedernales Earthquake (Ecuador)

AbstractSubduction zones are highly heterogeneous regions capable of hosting large earthquakes. To better constrain the processes at depth, we analyze the source properties of 1514 aftershocks of the 16th April 2016 Mw 7.8 Pedernales earthquake (Ecuador) using spectral ratios. We are able to retrieve accurate seismic moments, stress drops, and P and S corner frequencies for 341 aftershocks, including 136 events belonging to families of repeating earthquakes. We find that, for the studied magnitude range (Mw 2–4), stress drops appear to increase as a function of seismic moment. They are also found to depend on their distance to the trench. This is in part explained by the increase in depth, and therefore normal stress, away from the trench. However, even accounting for the shallow depths of earthquakes, stress drops appear to be anomalously low near the trench, which can be explained by a high pore fluid pressure or by inherent properties of the medium (low coefficient of friction/low rigidity of the medium) in that region. We are also able to examine the temporal evolution of source properties thanks to the presence of repeating earthquakes. We find that the variations of source properties within repeating earthquake families are not uniform, and are highly spatially variable over most of the study area. This is not the case near the trench, however, where stress drops systematically decrease over time. We suggest that this reflects an increase in pore fluid pressure near the trench over the postseismic period.

Free water input into the New Britain subduction zone in Solomon Sea estimated from reflected seismic data

AbstractIt is of great significance to examine the distribution of the fluid within the subduction zone to estimate the water flux into the subduction zone and to study the motion state of the subducting plate. The interaction of the Western Pacific Plate and the Indo‐Australian Plate controls the subduction and expansion of the Solomon Basin where the mechanisms governing plate activity remain unclear but are typically associated with high pore fluid pressures and stabilizing frictional conditions. Combining the long streamer seismic data (200) and nine well logs inside the work area, we used constrained sparse spike inversion to obtain the P‐wave impedance profile. Then we constructed reservoir rock physics models to predict the fluids distribution in New Britain Trench subduction system in Solomon Sea. The genetic algorithm was used to guide the inversion of the pore fluid distribution. The water content profile revealed the shallow structure of the New Britain Trench subduction system in Solomon Sea, including the water content within the basalt layers. We determined that in certain cases, faults identified from the reflection data were associated with low‐impedance anomalies, and this may indicate that they are zones of high porosity that act as conduits for fluid flow. The continuity of the distribution of water content under the New Britain Trench suggests that there is a large amount of free water that subducts deeply and may be important in controlling the motion state of the subducting plate. We assume that the water saturation is 1. Therefore, the free water flux entering the deep circle with the subducting plate is estimated to be 158 Tg/Ma/m totally, including 53.724 Tg/Ma/m in the sediment and 105.06 Tg/Ma/m in the basalt. This study provides insight into the structure of free water in the New Britain subduction zone and reveals the potential for the direct estimation of the 2D structural distribution of water content using geophysical survey data.

Pore pressure evolution in bed sediment overridden by debris flow: A general formulation

AbstractDebris flows can grow dramatically in volume and mobility as they override bed sediment due to the reduction in bed friction resistance caused by high pore fluid pressure (PP). However, the mechanisms that control PP evolution in overridden bed sediment are still unclear. Here, a new mathematical model clarifies how diverse styles and magnitudes of PP evolution can result from regulation of the flow diffusion and shear contraction of bed sediment. Normalization of the model equations shows that the propensity for PP generation depends on timescales of PP diffusion and deformation of the bed grains. The PP of saturated bed sediment under immobile conditions is equal to the fluid pressure at the bottom of the flow due to a non‐flux basal boundary. However, the PP of unsaturated bed sediment is lower than that of the overlying flow. PP diffusion from a debris flow into an unsaturated bed increases with the bed’s permeability and water content. The shear deformation behavior changes from undrained to drained with increasing permeability or decreasing shear velocity of the saturated bed sediment, leading to a reduced magnitude of pore pressure. By contrast, the shear deformation transitions from drained to undrained behavior with increasing permeability and water content of unsaturated bed sediment. The entrainment rate and erosion pattern of bed sediment are closely related to the PP evolution and liquefaction ratio of the bed sediment due to Coulomb‐friction shear tractions. Our models can be used to interpret the feedback of PP on the flow momentum during debris flow entrainment.

Seismic geomorphology of the Chandeleur submarine landslide in the northern Gulf of Mexico

Abstract The Chandeleur Submarine Landslide Complex occurs on the upper Mississippi Fan of the Gulf of Mexico in approximately 1100 m of water, 200 km SE of New Orleans, Louisiana. This part of the Mississippi Fan received high sedimentation throughout the Pleistocene, causing high pore fluid pressure and abundant slope failures, though few as large as the Chandeleur. Given its large size, proximity to major coastal cities and seafloor infrastructures, we examine the Chandeleur Slide to understand what led to the initial slope failure and decipher its post-failure transport behaviour using 2D and 3D multichannel seismic surveys, high-resolution bathymetric data, and well logs. We find a large sediment mass with a translational-rotational behaviour that was displaced to the south/SE up to 40 km from the source area. The Chandeleur Slide includes extensional faulting in the headscarp area and compressional structures in the toe where confined by a natural ramp-like structure. Beneath the Chandeleur Slide, we identify a regional sand-rich unit (called the Blue Unit) that is known to be overpressured. Beneath the Blue Unit we observe an upward-migrating salt diapir. We suggest one possible scenario for the origin of the Chandeleur Slide is the combined effects of an upward-migrating salt diapir impinging on an already overpressured Blue Unit, leading to the initial failure. The initial failure was followed by retrogressive headwall retreat northward, which created the prominent scarp on the seafloor. In total, the Chandeleur Slide complex covers an area of about 1000 km 2 and contains about 300 km 3 of sediment.

Connection between high pore-fluid pressure and frictional instability at tsunamigenic plate boundary fault of 2011 Tohoku-Oki earthquake

The 2011 Tohoku-Oki earthquake (M 9.0) rupture propagated along a shallow plate boundary thrust fault (i.e. decollement) to the trench, displaced the seafloor, and triggered a devastating tsunami. Physical properties of the underthrust sediments which control the rupture propagation are yet poorly known. We use a 2D seismic dataset to build velocity model for imaging and apply reverse time migration. We then calculate pore-fluid pressure along the decollement as the top boundary of underthrust sediments, and along the backstop interface as the boundary between undeformed structures in the continental plate and the severely deformed sediments in the accretionary prism. The results show that within horizontal distance of 40–22 km toward the trench, pore-fluid pressure is 82–60% higher than the hydrostatic pressure for both decollement and backstop interface. It then reduces to hydrostatic level for the backstop interface but remains 60–40% higher than hydrostatic level for the decollement, causing frictional instability in favor of fault rupture along the decollement. We report for the first time, by our knowledge, detailed seismic images of fluid-rich trapped bucket sediments, quantitative stress states, and fluid drainage conditions at shallow tsunamigenic portion of the Japan Trench, which are consistent with the seafloor and borehole observations.

Influence of shear zone thickness and strike-slip faulting on tectonic tremor in the Nankai Trough, southwest Japan

To investigate the mechanism of tectonic tremor and the factors that influence its occurrence and fluctuations, we use seismic reflection data to map in detail a shear zone associated with the plate boundary fault in the Nankai accretionary prism, southwest Japan. The shear zone can be identified from its low seismic velocity and low reflection amplitude, which may signify the presence of high pore pressure, around the transition boundary from frontal outer wedge to inner wedge. A detailed comparison of the mapped shear zone and the distribution of tectonic tremor demonstrate that tremor tended to occur in the thick part of the shear zone, where high pore fluid pressure may weaken faults and thereby facilitate slow earthquakes. Furthermore tremors align on the oblique strike-slip faults that caused variation of the shear zone thickness as well as fluctuation of stress and pore pressure. The oblique strike-slip faults that control the thickness of the shear zone and tremor distributions are interpreted as features formed by the collision of an accretionary prism against a landward tectonic backstop.

High Stress Deformation and Short‐Term Thermal Pulse Preserved in Pyroxene Microstructures From Exhumed Lower Crustal Seismogenic Faults (Lofoten, Norway)

AbstractEarthquake rupture in strong, anhydrous lower continental crust requires high brittle failure stresses unless high pore fluid pressures are present. Several mechanisms proposed to generate high stresses at depth imply transient loading driven by a spectrum of stress changes, ranging from highly localized stress amplifications to crustal‐scale stress transfers. High transient stresses up to GPa magnitude are proposed by field and modeling studies, but the evidence for transient prerupture loading is often difficult to extract from the geological record due to overprinting by coseismic damage and slip. However, the local preservation of deformation microstructures indicative of crystal‐plastic and brittle deformation associated with the seismic cycle in the lower crust offers the opportunity to constrain the progression of deformation before, during and after rupture, including stress and temperature evolution. Here, detailed study of pyroxene microstructures characterizes the short‐term evolution of high‐stress deformation and temperature changes experienced before and during lower crustal earthquake rupture. Pyroxenes are sampled from pseudotachylyte‐bearing faults and damage zones of lower crustal earthquakes recorded in the exhumed granulite facies terrane of Lofoten, northern Norway. The progressive sequence of microstructures indicates localized high‐stress (at the GPa level) prerupture loading accommodated by low‐temperature plasticity, followed by coseismic pulverization‐style fragmentation and subsequent grain growth triggered by the short‐term heat pulse associated with frictional sliding. Thus, up to GPa‐level transient high stress (both differential and shear) leading to earthquake nucleation in the dry lower crust can occur in nature, and be preserved in the fault rock microstructure.

Outer trench slope extension to frontal wedge compression in a subducting plate

The occurrence of faulting in subducting plates is a major process that changes the mechanical properties of the subducting lithosphere and carries surface materials into mantle wedges. Two ocean-bottom seismometer networks deployed on the frontal accretionary wedge of the northern Manila trench in 2005 and on the outer slope of the trench in 2006 were used to detect earthquakes in the subducting plate. All available P and S manually picked phases and the waveforms of 16 short-period, three-component stations were used. Relocation was performed using the double-difference method with differential times derived from the phase-picked data. Two intraplate earthquake sequences of small-to-moderate magnitudes in the northern Manila subduction system were investigated in this study. The results revealed distinct fault planes, but a contrasting seismogeny over the northern Manila Trench. The seismicity in the frontal wedge (as measured in 2005) was mainly contributed by a fluid overpressure sequence, whereas that in the incoming plate (as measured in 2006) was contributed by the aftershocks of an extensional faulting sequence. The obtained seismic velocity models and Vp/Vs ratios revealed that the overpressure was likely caused by high pore-fluid pressure within the shallow subduction zone. By using the near-field waveform inversion algorithm, we determined focal mechanism solutions for a few relatively large earthquakes. Through the use of data obtained from global seismic observations, we determined that stress transfer may be responsible for the seismic activity in the study area during the period of 2005–2006. In late 2005, the plate interface in the frontal wedge area was unlocked by the overpressure effect due to a thrusting-dominant sequence. This event changed the stress regime across the Manila Trench and triggered a normal fault extension at the outer trench slope in mid-2006. However, in the present study, a hybrid focal mechanism solution indicating reverse and strike–slip mechanisms was implemented, and it revealed that the plate interface locked again in late 2006.Graphical

Seafloor overthrusting causes ductile fault deformation and fault sealing along the Northern Hikurangi Margin

IODP Site U1518, drilled during IODP Expeditions 372 and 375, penetrated a large-offset (∼6 km) thrust, the Pāpaku fault, rising from a megathrust that hosts recurring slow slip events along the Hikurangi margin. Although drilling intersected the fault zone at only ∼300 m below the seafloor within porous silty mudstone, it exhibits intense tectonic ductile deformation, including finely banded mudstones contorted into decimeter-scale folds; elongate mudstone clasts with grain tail complexes; stacked and truncated silt beds in distorted mudstones; and soft sediment injections. Locally, these ductile features are overprinted by brittle deformation, including normal faults, fracture arrays, and breccias. The more consolidated hanging wall is dominated by brittle structures, whereas the footwall exhibits ductile and brittle deformation that decreases in intensity with depth. The intense tectonic ductile deformation and asymmetric distribution of structures across the fault zone at Site U1518 can be explained by seafloor overthrusting. The emplacement of the hanging wall upon the footwall flat overrode high-porosity, undeformed, and previously unburied sediments, localizing shear deformation within these weak sediments. In contrast, the overconsolidated hanging wall preferentially experienced brittle deformation during folding and displacement. Interstitial pore water geochemical profiles at Site U1518 show a repetition of near-seafloor diagenetic sequences below the fault, consistent with overthrusting of previously unburied strata. The preserved diagenetic profiles in the footwall suggest that overthrusting occurred within the last 50-100 kyr, and indicate little along- or across-fault fluid flow at the location of Site U1518. Thus the Pāpaku fault appears to define a low-permeability seal that restricts footwall consolidation, maintaining locally high pore fluid pressures and low fault strength. If similar low permeability structures occur elsewhere along the margin, they could support regionally high pore pressure conditions favorable to the occurrence of SSEs on the Hikurangi megathrust fault.

Emergence of Low-Frequency Aftershocks of the 2019 Ridgecrest Earthquake Sequence

ABSTRACTLow-frequency earthquakes (LFEs) generally have relatively stronger spectral components in the lower frequency range compared with what is expected for regular earthquakes based on their magnitude. LFEs generally occur in volcanic systems or deep (>∼15 km) in plate boundary fault zones; however, LFEs have also been observed in nonvolcanic, upper crustal settings. Because there are few studies that explore the spatiotemporal behaviors of LFEs in the shallow crust, it remains unclear whether the shallow-crustal LFEs reflect local attenuation in their immediate vicinity or differences in their source mechanism. Therefore, it is important to identify shallow-crustal LFEs and to characterize their spatiotemporal activity, which may also improve our understanding of LFEs. In this study, we focus on detecting shallow-crustal LFEs and explore the possible generation mechanisms. We analyze 29,646 aftershocks in the 2019 Ridgecrest, California, earthquake sequence, by measuring the frequency index (FI) to identify candidate low-frequency aftershocks (LFAs), while accounting for the magnitude dependency of the FI. Using small earthquakes (ML 1–3) recorded in the borehole stations to minimize the attenuation effects in near-surface layers, we identify 68 clear LFAs in total. Based on their distribution and comparisons with other seismic parameters measured by Trugman (2020), the LFAs possess distinct features from regular events in the same depths range, including low corner frequencies and low stress drops. Events in the close vicinity of LFAs exhibit lower average FI values than regular aftershocks, particularly if the hypocentral distance between an LFA and its neighbors is less than 1 km. Our results suggest that LFAs are related to local heterogeneity or a highly fractured fault zone correlated with an abundance of cross faults induced by the aftershock sequence at shallow depths. Zones of high pore-fluid pressure in intensely fractured fault zones could cause the bandlimited nature of LFAs and LFEs in general.