High Pore Fluid Pressure Research Articles

Aftershock sequence of the 2017 Mw 6.5 Jiuzhaigou, China earthquake monitored by an AsA network and its implication to fault structures and strength

SUMMARY We deployed a seismic network near the source region of the 2017 Mw 6.5 Jiuzhaigou earthquake to monitor aftershock activity and to investigate the local fault structure. An aftershock deployment of Array of small Arrays (AsA) and a Geometric Mean Envelop (GME) algorithm are adopted to enhance detection performance. We also adopt a set of association, relocation and matched-filter techniques to obtain a detailed regional catalogue. 16 742 events are detected and relocated, including 1279 aftershocks following the Mw 4.8 aftershock. We develop a joint inversion algorithm utilizing locations of event clusters and focal mechanisms to determine the geometry of planar faults. Six segments were finally determined, in which three segments are related to the Huya fault reflecting a change in fault dip direction near the main shock hypocentre, while the other segments reflect branches showing orthogonal and conjugate geometries with the Huya fault. Aftershocks were active on branching faults between the Huya and Minjiang faults indicating that the main shock may have ruptured both major faults. We also resolve a fault portion with ‘weak strength’ near the main shock hypocentre, which is characterized by limited coseismic slips, concentrated afterslip, low aftershock activities, high b-value and high sensitivity to stress changes. These phenomena can be explained by fault frictional properties at conditional stable sliding status, which may be related to the localized high pore-fluid pressure produced by the fluid intrusion.

Crust and upper-mantle seismic anisotropy variations from the coast to inland in central and Southern Mexico (2): correlations with tectonic tremor

SUMMARY Seismic anisotropy in the flat slab region of Mexico is compared with tectonic tremor (TT) activity. The anisotropy is observed in three separate horizontal layers using a novel technique with receiver functions. Those layers are identified as the continental crust and the subducted flat oceanic slab and a thin (∼10 km thick) remnant mantle wedge between those two layers. The TT is located in two zones: (1) the Sweet Spot where most of the tremor is observed (∼160–180 km from the coast) and (2) the Transient Zone (∼80–110 km from the coast). Anisotropy within each layer is observed to be different within each of the tremor zones than just outside them. The changes are explained as due to hydration within those zones. Water releasing phase changes have previously been modelled to occur within those two zones in the subducted slab (Manea & Manea). Water rising through each of the layers should generate the observed differences in anisotropy in those zones as the fast polarization direction and split times can differ between dry and hydrated material. This observation also correlates with the many observations of high pore fluid pressure associated with TT.

Cataclastic and crystal-plastic deformation in shallow mantle-wedge serpentinite controlled by cyclic changes in pore fluid pressures

Episodic tremor and slip (ETS) events in the forearc mantle wedge of a warm subduction zone may reflect mixed brittle–ductile deformation of serpentinite in association with high pore fluid pressures. To understand deformation mechanisms and processes occurring in the hydrated mantle wedge, we examined in the Sanbagawa metamorphic belt, SW Japan, an antigorite serpentinite shear zone derived from mantle wedge that was formed under pressure and temperature conditions that correspond to the ETS regions. The serpentinite underwent multiple extensional (mode I) and extensional–shear (mode I–II) failure events at supralithostatic pore fluid pressures (Pf). Such failure events led to drops in Pf (several MPa) and formation of a distributed ‘fault–fracture mesh’. Antigorite precipitation in the fracture openings contributed to an increase in Pf until the failure condition was reached again, and thereby antigorite kinetics controlled the recurrence interval of seismic events. We also suggest that under the low-Pf conditions that facilitate intracrystalline plasticity rather than cataclasis, the newly precipitated antigorite aggregates (localized along shear bands) deform by dislocation creep at a high strain rate and high shear stress, resulting in the transient, accelerated viscous creep that may characterize slow slip transients.

Pore and Mineral Fabrics Control the Elastic Wave Velocities of Metapelite With Implications for Subduction Zone Tomography

AbstractSeismic tomography is used to infer compositional variations and environmental conditions in active subduction zones. In some regions, low P‐wave and S‐wave velocities and high are documented near and along the plate boundary, and are interpreted to reflect high pore fluid pressures in metasedimentary rocks. However, there are limited experimental data with which to constrain the in‐situ wave speeds of metasedimentary rocks, particularly under elevated pore pressure. To determine whether the velocities of metasedimentary rocks are consistent with low wave speeds and high , we measured the ultrasonic velocities of the Orocopia schist, a greenschist facies metapelite, in the laboratory. Velocities were measured under dry and saturated conditions, at effective pressures between 1 and 136 MPa, parallel and normal to foliation, and at room temperature. We find that, under fluid‐saturated conditions, the velocities of the Orocopia schist and are consistent with anomalous values in subduction zones ( km/s, km/s, and ) at effective pressures less than 10 MPa and normal to foliation. We used differential effective medium models and microscopy to quantify the contributions of mineralogy, mineral fabric, porosity, and pore shape to , , and their anisotropies. We find that, although mineralogy and mineral anisotropy impart some control on the wave velocities, aligned, small aspect ratio pores (0.005–0.03) are required to explain the measured velocities and high . The low and and high in subduction zones can be explained by phyllosilicate‐rich metapelitic rocks under near‐lithostatic pore pressure.

Heterogeneous Sediment Input at the Nankai Trough Subduction Zone: Implications for Shallow Slow Earthquake Localization

AbstractSubducted sediment plays a key role in modulating pore pressure and seismic behavior at subduction zones. We investigated the seismic character of incoming sediments to test how sediment and basement variations relate to the along‐strike changes within the accretionary prism and plate boundary conditions at the Nankai Trough. High‐resolution seismic data reveal for the first time the presence of countourite mounded drifts in the Shikoku Basin. These features have probably introduced permeability heterogeneities into an otherwise homogenous mud‐dominant unit. Additionally, we found that normal faults in this unit are more extensive than previously documented, which probably enhances along‐strike fluid transport. The wedge taper is more correlative with the thickness of the mud‐dominant facies than the turbidite thickness. This may be due to the permeability heterogeneities associated with contourite deposits and normal faults, or due to the absence of thick turbidite deposits. Turbidite deposits can either aid the drainage of the margin where they are not confined by basement topography, or contribute to high pore fluid pressures where they are confined by less permeable mudstone or basement topography. When confined turbidite deposits and contourite mounded drifts are subducted, they may contribute to localized compartments of excess pore pressure which provide the necessary conditions for slow slip behavior. We determined that along‐strike variations in seismic behavior are likely related to the subducting basement topographic and sediment characteristics, which vary on a local (<10 km) scale.

Constraints From Exhumed Rocks on the Seismic Signature of the Deep Subduction Interface.

Low Velocity Zones (LVZs) with anomalously high V p‐V s ratios occur along the downdip extents of subduction megathrusts in most modern subduction zones and are collocated with complex seismic and transient deformation patterns. LVZs are attributed to high pore fluid pressures, but the spatial correlation between the LVZ and the subduction interface, as well as the rock types that define them, remain unclear. We characterize the seismic signature of a fossil subduction interface shear zone in northern California that is sourced from the same depth range as modern LVZs. Deformation was distributed across 3 km of dominantly metasedimentary rocks, with periodic strain localization to km‐scale ultramafic lenses. We estimate seismic velocities accounting for mineral and fracture anisotropy, constrained by microstructural observations and field measurements, resulting in a V p/Vs of 2.0. Comparable thicknesses and velocities suggest that LVZs represent, at least in part, the subduction interface shear zone.

Moment tensor inversion of microearthquakes along the Santorini-Amorgos zone: Tensile faulting and emerging volcanism in an extensional setting

The Santorini-Amorgos zone is an area rich in microseismicity at the center of the Hellenic volcanic arc. The microseismicity of the zone is distributed along the Santorini-Amorgos ridge and Kolumbo submarine volcano. In this study, we utilized crustal events that were recorded by temporary networks during September 2002 to July 2004 and October 2005 to March 2007, and also by the permanent network from 2011 to 2020. These events were inverted for their moment tensors by using P-wave polarities as well as SV/P and SH/P amplitude ratios, yielding 74 well-constrained moment tensor solutions. Most of these moment tensors have significant CLVD and isotropic components that are positively correlated to each other (R2 = 0.68). Tensile faulting due to high pore pressure is considered as the most likely cause of the observed non-DC components. The positive and negative non-DC components observed in Kolumbo may be generated by the opening and closing of cracks beneath the shallow (6–7 km) magma chamber due to a steady migration of magmatic fluids from the deep reservoir into the chamber. In Anydros, most of the microearthquakes have positive non-DC components associated with the opening of cracks. It is possible that the extensional deformation and high pore fluid pressure in the area opens subvertical cracks that become pathways for upward migrating fluids. The upward migration of magmatic fluids in an extensional regime such as the Santorini-Amorgos zone can also be viewed as an indication of emerging volcanic activity in this area.

The stabilizing effect of high pore-fluid pressure along subduction megathrust faults: Evidence from friction experiments on accretionary sediments from the Nankai Trough

Pore-fluid pressure is an important parameter in controlling fault mechanics as it lowers the effective normal stress, allowing fault slip at lower shear stress. It is also thought to influence the nature of fault slip, particularly in subduction zones where areas of slow slip have been linked to regions of elevated pore-fluid pressure. Despite the importance of pore-fluid pressure on fault mechanics, its role on controlling fault stability, which is determined by the friction rate parameter (a−b), is poorly constrained, particularly for fault materials from subduction zones. In the winter of 2018-19 the accretionary complex overlying the Nankai Trough subduction zone (SW Japan) was drilled as part of Integrated Ocean Drilling Program (IODP) Expedition 358. Here we test the frictional stability of the accretionary sediments recovered during the expedition by performing a series of velocity-stepping experiments on powdered samples (to simulate fault gouge) while systematically varying the pore-fluid pressure and effective normal stress conditions. The Nankai gouges, despite only containing 25% phyllosilicates, are strongly rate-strengthening and exhibit negative values for the rate-and-state parameter b. We find that for experiments where the effective normal stress is held constant and the pore-fluid pressure is increased the Nankai gouges become more rate-strengthening, and thus more stable (an increase in (a−b) of ∼6 × 10−5 MPa−1 with increasing pore-pressure). In contrast, when the pore-fluid pressure is held constant and the effective normal stress is varied, there is minimal effect on the frictional stability of the gouge. The increase in frictional stability of the gouge at elevated pore-fluid pressure is caused by an evolution in the rate-and-state parameter b, which becomes more negative at high pore-fluid pressure. These results have important implications for understanding the nature of slip in subduction zones and suggest the stabilizing effect of pore-fluid pressure could promote slow slip or aseismic creep on areas of the subduction interface that might otherwise experience earthquake rupture.

The 2019 September 24, Mw = 6, Mirpur earthquake, NW Himalaya: Geodetic evidence for shallow, near-horizontal décollement rupture of the Main Himalayan Thrust

We present a source model for the 2019 Mw = 6 Mirpur earthquake, NW Himalaya using Interferometric Synthetic Aperture Radar (InSAR) measurements. Bayesian inversion of InSAR data from both ascending and descending orbits suggests that the earthquake ruptured a shallow (Depth ~ 5 km), near-horizontal (Dip ~2.5°) up-dip portion of the décollement of the Main Himalayan Thrust (MHT). The distributed slip model suggests a compact rupture terminating the up-dip end at the base of the Main Frontal Thrust (MFT) with higher slip (> 0.4 m) around the hypocentre, equivalent to a moment magnitude of Mw = 6. A shallow up-dip rupture of the MHT through a moderate magnitude earthquake is atypical as Himalayan earthquakes generally originate at the down-dip portion of the MHT and propagate towards south. We estimate a low effective coefficient of friction of 0.06 ± 0.02 from the slip model and suggest that high pore fluid pressures and/or a weak, lubricated portion of décollement could have caused a local, near-horizontal rupture at the base of the MFT. The 2019 Mirpur earthquake released a small fraction of the accumulated strain energy since the 1555 Kashmir earthquake. Coseismic Coulomb stress change analysis suggests a significant increase in stress on the locked, up-dip portion of the MHT and the frontal fold-thrust system. These findings compel a revisit of the seismic hazard assessment of the northwestern Himalaya.

Hydrothermal alteration can result in pore pressurization and volcano instability

AbstractThe collapse of a volcanic flank can be destructive and deadly. Hydrothermal alteration is common to volcanoes worldwide and is thought to promote volcano instability by decreasing rock strength. However, some laboratory studies have shown that not all alteration reduces rock strength. Our new laboratory data for altered rhyodacites from Chaos Crags (Lassen volcanic center, California, USA) show that pore- and crack-filling mineral precipitation can reduce porosity and permeability and increase strength, Young's modulus, and cohesion. A significant reduction in permeability, by as much as four orders of magnitude, will inhibit fluid circulation and create zones of high pore fluid pressure. We explored the consequences of pore fluid pressurization on volcano stability using large-scale numerical modeling. Upscaled physical and mechanical properties for hydrothermally altered rocks were used as input parameters in our modeling. Results show that a high-pore-pressure zone within a volcano increases volcano deformation and that increasing the size of this zone increases the observed deformation. Hydrothermal alteration associated with mineral precipitation, and increases to rock strength, can therefore promote pore pressurization and volcano deformation, increasing the likelihood of volcano spreading, flank collapses, and phreatic/phreatomagmatic explosions. We conclude that porosity-decreasing alteration, explored here, and porosity-increasing alteration can both promote volcano instability and collapse, but by different mechanisms. Hydrothermal alteration should therefore be monitored at volcanoes worldwide and incorporated into hazard assessments.

Tomography of the 2018 Mw6.6 Hokkaido eastern Iburi earthquake area and surrounding regions, Japan

To better understand the structural heterogeneities under the Hokkaido subduction zone and its implications in the generation mechanism of the 2018 Mw 6.6 Hokkaido eastern Iburi earthquake, three-dimensional (3-D) high-resolution seismic velocities (Vp, Vs) and Poisson's ratio (σ) images were determined using 273,078 high-quality travel-times of P- and S-wave source-receiver pairs from both onshore and offshore earthquakes. Our results reveal that the 2018 eastern Iburi earthquake likely occurred at a presumed blind thrust fault with sharply varying seismic parameters. A strong low-V and high-σ body extending to 120-km depth was clearly observed northwest of the 2018 hypocenter, indicating the presence of fluids in the mantle wedge associated with the Pacific slab dehydration. Sharp contrasted structures of Vp, Vs, and σ were clearly imaged across the hypocenter of the 2018 Iburi earthquake. The sandwiched rigid zone with high-V and low-σ anomalies in the hypocenter was considered to be the subsidence of brittle crust, which is thickened and deepened due to the colliding system of the northeastern Japan and the Kuril arcs. The mechanical strength in the source rocks with high-V and low-σ was stronger than in surrounding regions where high pore-fluid pressure accumulated. Such a fluid-pressurized regime beside the hypocenter reduced the rock effective stress on the source rocks, lowered the threshold for fault failure, and consequently led to brittle failure. Fluid intrusion in the back-arc seisomogenic layer played a key role in intraplate earthquake generation in the Hokkaido subduction zone and elsewhere.

Stability Assessment of Landslide Occurred in India and Its Remedies: A Case Study

Landslide is a geological process on the surface of the earth, the downward movement of soil mass or rock on the slope region is called a landslide. Natural disaster has always been an important topic of discussion, currently India and many other countries are facing this sort of problem. Through the past study it’s seen that there were thousands of landslides occurred in India due to typhoon and heavy rainfall; Kethikal landslide near to Mangalore in India is one of the hazard occurred in 1998 due to heavy rainfall. The total height of the slope is 67 meter and its length is 140.5 meter. Due to this disaster, traffic has been diverted for few days and some houses in the top of the mountain were damaged. The analysis of this slope failure has been done in Plaxis 2D program and found that the cause of failure of the slope is due to the rise of high porefluid pressure and by the field observation found the same cause. In Plaxis 2D, a number of numerical analyses has been done with soil nailing solution and found that the factor of safety of this solution is satisfactory. Soil nailing has been found to be one of the solutions for the treatment of the unstable slope.

Seismic Velocity Heterogeneity of the Hikurangi Subduction Margin, New Zealand: Elevated Pore Pressures in a Region With Repeating Slow Slip Events

AbstractWe investigated the seismic velocity structure of the Hikurangi margin in New Zealand to uncover the physical features of the subduction zone and explore the relationships between microearthquake seismicity, seismic velocity structure, and slow slip events. Using local earthquake tomography with data collected from both temporary ocean bottom seismometers and on‐land permanent seismic stations, we used the tomography code TomoFD to iteratively perform a damped least squared inversion of absolute P and S arrival times to obtain relocated hypocenters and generate 3D velocity models for Vp and Vp/Vs. The seismic tomography images show two high Vp/Vs anomalies, one offshore and adjacent to a subducted seamount and the other beneath the North Island of New Zealand. The ∼50‐km wide offshore anomaly extends ∼10 km beneath the plate interface and lies directly beneath the area that slipped at least 50 mm during the 2 week‐long 2014 slow slip event. High Vp/Vs values may be related to high pore fluid pressures from subducted sediments, and such increases in pore fluid pressures have been suggested to trigger the occurrence of slow slip events in active subduction zones. The second onshore high Vp/Vs anomaly is located in the overlying plate and subducting slab and correlates with areas suggested by other geophysical techniques to be rich in fluids. Our seismic imaging supports interpretations that subduction processes in the Hikurangi margin are highly dependent on physical features such as subducted seamounts and fluid‐rich sediments.

Rock characteristics, internal structure and physical-chemical properties of Qianning segment in Xianshuihe fault zone

断裂带的变形行为和断层滑移机制是目前地震研究关注的热点，断裂带岩石特征、内部结构与物理化学性质是确定断层蠕滑或粘滑行为以及断层滑移机制的基础和关键。本文以鲜水河断裂带乾宁段地表出露的断裂岩为研究对象，通过野外地质调查、室内光学显微镜、扫描电镜、粒度统计、粉末X射线衍射分析（XRD）和薄片X射线荧光光谱分析（XRF）等多种研究方法，对鲜水河断裂带岩石特征、结构构造、物性、矿物成分及化学元素分布开展了详细的分析，并探讨了相关变形行为和滑移机制。分析表明：（1）断裂带核部主要由黑色断层泥、浅黄色及黄色断层角砾岩和灰色碎裂岩、灰色断层角砾岩组成，呈单核对称结构；（2）黑色断层泥厚3~5cm，具有快速滑动结构特征，表现为断层粘滑行为。断层泥可划分出13个滑移带，最窄滑移带厚约40μm，至少代表13期古地震事件；（3）断层泥主要由伊利石、高岭石和石英等矿物组成，其中边部伊蒙混层含量异常高，为最新一次古地震的主滑移带。由于伊利石和伊蒙混层（或蒙脱石）为主要黏土矿物的断层泥渗透率低、孔隙流体压力大，以及发现断层泥楔入脉，表明地震过程中断层滑动存在热增压弱化机制；（4）从断层泥不同滑动带中碎块蚀变程度和矿物分布特征来看，地震主滑动带有向碎裂岩方向迁移的趋势。推测断层在滑动过程中，更趋向于向弱矿物含量高（如伊利石、伊蒙混层）、强矿物含量低（如方解石、高岭石）的围岩一侧迁移。;The deformation behavior and slipping mechanism of fault zone are the focus of current seismic research. Rock characteristics, internal structure and physical-chemical properties of fault zone are the basis and key to determine creep- or stick-slip behavior and fault slipping mechanism. This study focuses on fault rocks from Qianning segment in Xianshuihe fault zone. Rock characteristics, structure, physical properties, mineral composition and chemical element distribution of the Xianshuihe fault zone have been analyzed in detail through field geological survey, observation of optical microscope and scanning electron microscope, particle-size statistics, powder X-ray diffraction analysis (XRD) and thin section X-ray fluorescence spectrum analysis (XRF). Related deformation behaviors and slipping mechanisms have been discussed. The results show that: (1) the core of fault zone is mainly composed of black fault gouge, light yellow and yellow fault breccia, gray cataclasite, and gray fault breccia, presenting a single-core-symmetrical structure; (2) the black fault gouge is 3~5cm thick and shows rapid-slip structure, indicating stick-slip behavior of the fault zone. The fault gouge can be divided into 13 slip zones, and the narrowest slip zone is about 40μm thick, representing at least 13 paleoearthquake events; (3) the fault gouge is mainly composed of illite, kaolinite, and quartz. The content of illite-smectite (I/S) mixed layer at the edge of fault gouge is abnormally high, which presents the primary slip zone of the latest paleoearthquake. The low permeability and high pore fluid pressure caused by main clay minerals of illite and I/S mixed layer (or smectite) in fault gouge and the discovery of fault gouge injection veins indicate that there is a thermal pressurization weakening mechanism in the fault zone sliding during the earthquake process; (4) according to the degree of fragment alteration and mineral distribution characteristics in different slip zones of fault gouge, the main seismic slip zone has a tendency to migrate towards cataclasite. It is speculated that during the sliding process, the fault tends to migrate to the side of the surrounding rock with high content of weak minerals (such as illite and I/S mixed layer) and low content of strong minerals (such as calcite and kaolinite).

Brittle Deformation During Eclogitization of Early Paleozoic Blueschist

The Tsäkkok Lens of the Scandinavian Caledonides represents the outermost Baltican margin that was subducted in late Cambrian/Early Ordovician time during closure of the Iapetus Ocean. The lens predominantly consists of metasedimentary rocks hosting eclogite bodies that preserve brittle deformation on the μm-to-m scale. Here, we present a multidisciplinary approach that reveals fracturing related to dehydration and eclogitization of blueschists. Evidence for dehydration is provided by relic glaucophane and polyphase inclusions in garnet consisting of clinozoisite + quartz ± kyanite ± paragonite that are interpreted as lawsonite pseudomorphs. X-Ray chemical mapping of garnet shows a network of microchannels that propagate outward from polyphase inclusions. These microchannels are healed by garnet with elevated Mg relative to the surrounding garnet. Electron backscatter diffraction mapping revealed that Mg-rich microchannels are also delimited by low angle (<3°) boundaries. X-ray computed microtomography demonstrates that some garnet is transected by up to 300 μm wide microfractures that are sealed by omphacite ± quartz ± phengite. Locally, mesofractures sealed either by garnet- or omphacite-dominated veins transect through the eclogites. The interstices within the garnet veins are filled with omphacite + quartz + rutile + glaucophane ± phengite. In contrast, omphacite veins are predominantly composed of omphacite with minor apatite + quartz. Omphacite grains are elongated along [001] crystal axis and are preferably oriented orthogonal to the vein walls, indicating crystallization during fracture dilation. Conventional geothermobarometry using omphacite, phengite and garnet adjacent to fractures, provides pressure-temperature conditions of 2.47 ± 0.32 GPa and 620 ± 60°C for eclogites. The same method applied to a mesoscale garnet vein yields 2.42 ± 0.32 GPa at 635 ± 60°C. Zirconium-in-rutile thermometry applied to the same garnet vein provides a temperature of ∼620°C. Altogether, the microchannels, microfractures and mesofractures represent migration pathways for fluids that were produced during glaucophane and lawsonite breakdown. The microfractures are likely precursors of the mesoscale fractures. These dehydration reactions indicate that high pore-fluid pressure was a crucial factor for fracturing. Brittle failure of the eclogites thus represents a mechanism for fluid-escape in high-pressure conditions. These features may be directly associated with seismic events in a cold subduction regime.

The MS 6.9, 1980 Irpinia Earthquake from the Basement to the Surface: A Review of Tectonic Geomorphology and Geophysical Constraints, and New Data on Postseismic Deformation

The MS 6.9, 1980 Irpinia earthquake occurred in the southern Apennines, a fold and thrust belt that has been undergoing post-orogenic extension since ca. 400 kyr. The strongly anisotropic structure of fold and thrust belts like the Apennines, including late-orogenic low-angle normal faults and inherited Mesozoic extensional features besides gently dipping thrusts, result in a complex, overall layered architecture of the orogenic edifice. Effective decoupling between deep and shallow structural levels of this mountain belt is related to the strong rheological contrast produced by a fluid-saturated, shale-dominated mélange zone interposed between buried autochthonous carbonates—continuous with those exposed in the foreland to the east—and the allochthonous units. The presence of fluid reservoirs below the mélange zone is shown by a high VP/VS ratio—which is a proxy for densely fractured fluid-saturated crustal volumes—recorded by seismic tomography within the buried autochthonous carbonates and the top part of the underlying basement. These crustal volumes, in which background seismicity is remarkably concentrated, are fed by fluids migrating along the major active faults. High pore fluid pressures, decreasing the yield stress, are recorded by low stress-drop values associated with the earthquakes. On the other hand, the mountain belt is characterized by substantial gas flow to the surface, recorded as both distributed soil gas emissions and vigorous gas vents. The accumulation of CO2-brine within a reservoir located at hypocentral depths beneath the Irpinia region is not only interpreted to control a multiyear cyclic behavior of microseismicity, but could also play a role in ground motions detected by space-based geodetic measurements in the postseismic period. The analysis carried out in this study of persistent scatterer interferometry synthetic aperture radar (PS-InSAR) data, covering a timespan ranging from 12 to 30 years after the 1980 mainshock, points out that ground deformation has affected the Irpinia earthquake epicentral area in the last decades. These ground motions could be a result of postseismic afterslip, which is well known to occur over years or even decades after a large mainshock such as the 23 November 1980, MS 6.9 earthquake due to cycles of CO2-brine accumulation at depth and its subsequent release by Mw ≥ 3.5 earthquakes, or most likely by a combination of both. Postseismic afterslip controls geomorphology, topography, and surface deformation in seismically active areas such as that of the present study, characterized by ~M 7 earthquakes. Yet, this process has been largely overlooked in the case of the 1980 Irpinia earthquake, and one of the main aims of this study is to fill such the substantial gap of knowledge for the epicentral area of some of the most destructive earthquakes that have ever occurred in Italy.

Source and strong-motion characteristics of two M\u2009>\u20096 buried earthquakes in southwest Taiwan

We used near-field strong-motion data to investigate the complex combination of source effect and site response for two recent disastrous earthquakes in southwest Taiwan. We estimated strong-motion generation areas (SMGAs) of 2.8 km × 2.8 km and 6.0 km × 4.2 km in a frequency band of 0.4–10 Hz for the 2010 Jiashian and 2016 Meinong earthquakes, respectively. The high-stress drops of 26.2 and 17.0 MPa for these two buried events were potentially related to the small dimension and deep rupture. Our results revealed that both earthquakes exhibited westward rupture directivity, whereas the 2016 Meinong event exhibited a stronger directivity effect because of the consistency between the propagation and slip directions. The localized high peak ground velocity (PGV) patch and the nonlinear site response could be attributed to the soft sediment with high pore fluid pressure and low-velocity structure beneath this region. However, the greater seismic moment and closer faulting location to the thick-mudstone-layer region for the 2016 Meinong event reinforced the strong ground shaking and serious damage over the broad area. This implies that this thick-mudstone-layer region in southern Taiwan plays a crucial role in earthquake response, and an investigation of characteristic site effects should be conducted for seismic hazard mitigation.Graphic abstract

Formation of Mg-carbonates and Mg-hydroxides via calcite replacement controlled by fluid pressure

Reactive fluid flow can control the mineralogical, mechanical and chemical evolution of the Earth’s crust. When rocks are exposed to differential stresses (i.e., vertical stress ≠ horizontal stress ≠ pore-fluid pressure (Pf)) during reactive fluid flow, effective pressure is usually assumed to control the overall reaction process. Here, we show that fluid pressure can play an important role in mineral replacement reactions. We conducted experiments in which calcite (CaCO3) grains (fraction size 53–150 µm) reacted with a Mg-rich solution at ~ 200 °C both in a closed system and under reactive fluid flow conditions with different fluid flow rates and fluid pore pressures, but with similar confining pressure (σn = 10 or 20 MPa) and effective pressure (Pe). Under closed system, vapor-saturated pressures, the magnesite formed with large pores between the magnesite and the calcite. In the open system flow-through experiments, however, brucite (Mg(OH)2) or magnesite (MgCO3) formed, depending on pore-fluid pressure. The main reaction product was brucite at low pore-fluid pressure (0.2 MPa), but magnesite at higher pore-fluid pressures (≥ 1 MPa). Calcite dissolution and precipitation of the product mineral increased concomitantly with flow rate, but the flow rate did not affect the nature of the products. The permeability of the reacting rock was related to the reaction pathway, i.e. the nature of the products. Magnesite replaced the pristine calcite in a pseudomorphic manner, and mantled the pristine calcite with 10–100 µm wide pores. In contrast, tabular and/or platy brucite blocked the porosity and resulted in a decrease in permeability. Our results show that the pore-fluid pressure can be a significant parameter controlling the reaction products and reaction processes in volatile-rich (e.g., CO2, HCl, H2S and SO2) systems at conditions close to phase separation; these conditions occur for example in epithermal and porphyry hydrothermal systems, and in carbonate-replacement and some metamorphic environments.

Fracture, fluid and saturation effects on the seismic attributes of rocks from the Southern Negros geothermal field, Philippines

Seismic based geophysical methods are seeing increased usage in evaluating geothermal resources in order to maximize resource potential. However, interpreting geophysical data (such as velocities and dynamic modulus and fracture density/alignment) generated from geothermal reservoirs remains difficult. Here we present the results of a new laboratory study measuring seismic attributes of fresh and hydrothermally altered rocks from a Philippine geothermal field (Southern Negros Geothermal Project - SNGP) during triaxial deformation. Two types of rocks were obtained by sub-coring samples of low porosity (~1%) andesite and higher porosity (~10%) volcaniclastic samples from the SNGP. Samples were prepared with two offset drill holes to allow a natural fracture to permit fluid flow along the fracture. An embedded array of Acoustic Emission (AE) sensors allows elastic wave and induced microseismic data to be collected. We measure a significant reduction in elastic wave velocities and moduli, with the exception of Poisson's ratio, after shear fracture development. An initially pre-fractured permeability of approximately 10−17 m2 is measured. We find that the permeability decreases from 2.0 × 10−14 m2 to lower than 7.4 × 10−15 m2 as the confining pressure is increased from 5 MPa to 30 MPa. A concomitant increase in P and S-wave velocities, dynamic bulk and Young's moduli are also measured. Finally, we simulate a geothermal ‘venting’ situation by intentionally releasing the high pore fluid (water) pressure from 10 to 50 MPa to ambient pressure, generating a swarm of AE that increases in duration with higher pore pressure. We postulate that this is due to fluid phase change (liquid to gas) and movement along the natural fracture plane and damage zone.

Highly Heterogeneous Pore Fluid Pressure Enabled Rupture of Orthogonal Faults During the 2019 Ridgecrest Mw7.0 Earthquake

AbstractHere, we show that the 2019 Mw7.0 Ridgecrest mainshock as well as its Mw6.5 foreshock ruptured orthogonal conjugate faults. We invert the waveforms recorded by the dense strong motion network at relatively high frequencies (up to 1 Hz for P; 0.25 Hz for S) to derive multiple‐point source models for both events, aided by path calibrations from a Mw5.4 and a Mw5.5 earthquake. We demonstrate that the mainshock started from a shallow (3 km) depth with a Mw5.2 event and ruptured the main fault branches oriented in the NW‐SE direction. At ~11 s, two Mw6.2 subevents took place on the SW‐NE oriented fault branches that conjugate to the main fault to the NE and SW. The SW branch rupture partially overlapped with the foreshock rupture. We suggest the coseismic rupture on nearly orthogonal faults was enabled by high pore fluid pressure, which greatly weakened the immature fault system in a heterogeneous way.