Earthquake Rupture Propagation Research Articles

In-situ rock shattering and strain localization along a seismogenic fault in dolostones (Monte Marine fault, Italian Central Apennines)

In-situ shattered rocks are often associated with seismogenic fault zones, but their mechanism of formation is still matter of debate, partly because of the limited number of field studies. Here we describe the characteristics of in-situ shattered rocks distribution along the NW-SE-striking seismogenic Monte Marine Fault (MMF) in the Italian Central Apennines. In the studied area, the MMF cuts through Mesozoic carbonates, is exhumed from <3 km depth and consists of two >5 km-long major hard-linked segments with normal kinematics. The linkage between the two fault segments occurs along a ∼2 km-long step-over zone with E-W trending faults and oblique-slip kinematics. To the northwest, fault-related shear deformation is localized in a ∼5 m-thick cataclastic fault core and off-fault deformation is dominated by in-situ shattered rocks up to ∼40 m-thick. Instead, in the step-over zone to the southeast, the in-situ shattered rocks are up to ∼500 m thick, particularly where MMF crosscuts older low-angle thrust faults.We integrated detailed field structural surveys with microstructural and grain size distribution analyses of the fault rocks to assess the mechanism of (1) formation of in-situ shattered rocks and, (2) progressive localization of shear deformation along the MMF. The obtained results, after the viability of several formation mechanisms (mechanical models) have been reviewed, support the hypothesis that the formation of in-situ shattered rocks was associated with the propagation of (multiple) seismic ruptures (mainshocks and aftershock sequences) within a mechanically heterogeneous fault zone. Heterogeneity is due to the occurrence of preexisting damage related to previous earthquakes, but also inherited from the older low-angle thrust faults. Therefore, we suggest that the origin of these shattered rocks is more compatible with seismic related processes than only with quasi-static fault growth models. On the other hand, the cataclastic fault core derived from the progressive accommodation of shear deformation within the in-situ shattered rock volumes during several seismic cycles. We conclude that the large volumes of in-situ shattered rocks are the result of seismic-related dissipative processes in a geometrically and mechanically heterogeneous fault zone. In this scenario, large volumes of in-situ shattered rocks are compliant low velocity zones which can influence the propagation of earthquake ruptures.

Discontinuous Surface Ruptures and Slip Distributions in the Epicentral Region of the 2021 Mw7.4 Maduo Earthquake, China

Discontinuous Surface Ruptures and Slip Distributions in the Epicentral Region of the 2021 Mw7.4 Maduo Earthquake, China

A window into thousands of earthquakes: Results from the Deep Fault Drilling Project (DFDP)

In 1941, two geologists, Harold Wellman and Richard Willett, traversed the length of Westland, mapping what would become one of the most influential continental faults in the world, New Zealand’s Alpine Fault (Wellman &amp; Willet 1942). The Alpine Fault strikes down the western edge of the Southern Alps, a youthful mountain range on New Zealand’s South Island (Fig 1). Here, collision between the Australian and Pacific Plates forms peaks over 3000 m in elevation which trap rain-laden clouds, resulting in 5–15 m of precipitation a year in central and south Westland. Driven by gravity, the rain and snow migrate into fractures and voids along the Alpine Fault, becoming heated and saturated with reactive ions along the way. Within the fault zone, these fluids play a fundamental role in the processes that drive earthquake nucleation and rupture propagation. Measurements made, and rocks recovered, from boreholes drilled during phases one and two of the Deep Fault Drilling Project (DFDP) have enabled scientists to document and quantify these processes for the first time.

Depth-dependent seismic anomalies and potential asperity linked to fluid-driven crustal structure in Garhwal region, NW Himalaya

Characterizing the stress heterogeneity and crustal complexities are essential to understand the important aspects of rupture development and its propagation in a tectonically active region. We analyze a continuous seismic data (1999–2020) to estimate stress drops, b-value, fractal dimension (Dc) and focal mechanisms using a local network of 14 broadband stations. Seismic periodicity on a Himalayan major thrust above locked zone, involves long period of stress accumulation and interseismic strain. There exist a good spatio-temporal-depth correlation between the estimated results which indicates the positive anomaly at 12–14 km (stress transition) beneath the Chamoli region. Interestingly, this region shows maximum geodetic strain rate with comparatively high-stress drop, low coefficient friction value and very low b- (0.583 ± 0.02) and Dc- (1.20 ± 0.01) values. Majority of moderate to strong earthquakes and swarms activity have occurred in this region with maximum crustal accommodation. It is also observed that the significant temporal decrease in b-value always followed by moderate earthquakes. Here shallow crustal potential seismogenic asperity controls the earthquake's stress drop and rupture propagation. The observed correlations strongly support the evidence of entrapped fluid-faults interaction which alters pore pressure and generates numerous ruptures. Also, the role of pressurized entrapped fluids and roughness of fault-plane responsible for low friction zone and microseismicity. Continuous stress accumulation along the fault-plane within the asperity could release significant amount of energy in future.

Along-Strike Variation in the Shallow Velocity Structure beneath the Chenghai Fault Zone, Yunnan, China, Constrained from Methane Sources and Dense Arrays

Abstract The fine structure of the fault zone and the surrounding area is the basis for understanding the process of earthquake nucleation and rupture propagation. To obtain the high-resolution structure of the Chenghai fault (CHF) and the nearby basins, we deployed two dense arrays and excited eight methane sources across the fault from October to November 2020. Based on the 611 P-wave travel times, we obtained the shallow velocity structure beneath the arrays using the simul2000 travel-time inversion program, and the results are as follows: (1) the shallow velocity structure beneath the CHF is very complex, with obvious velocity contrasts on both the sides of the regional fault; (2) low-velocity zones (LVZs) beneath the CHF show clear along-strike variations. The LVZs extend to ∼500 m in depth with widths of ∼2 km and ∼5 km beneath the Qina and Pianjiao arrays, respectively, which are consistent with the Quaternary sediments, and the velocity contrasts along the interface of the LVZ can reach 20%–50%; and (3) the distribution of shallow surface tectonic geomorphology is mainly controlled by regional fault activities that are formed under the combined action of regional near-east–west stretching and clockwise rotation of microblocks. Our results can help improve cognition and seismic hazard assessment for potential earthquakes on the CHF, as well as lay the foundation for understanding the seismic wave velocity variation mechanism in the fault zone.

Semi-automated template matching and machine-learning based analysis of the August 2020 Castelsaraceno microearthquake sequence (southern Italy)

The accurate characterization of microearthquake sequences allows seismologists to better understand the physical processes involved in earthquake nucleation and rupture propagation and to gain insights on fault geometry at depth. Standard procedures for seismic sequences analysis are based on manual detection and phase-picking, requiring a huge amount of work from expert seismologists, particularly in the case of microseismic events. Here we show how the investigation of a low-magnitude foreshock-mainshock-aftershock sequence, occurred in August 2020 close to Castelsaraceno village (southern Italy), greatly benefited from the application of a semi-automated template matching and machine-learning based workflow. The phase-picking was automatically performed through a deep-learning algorithm on 202 microearthquakes detected between July and October 2020, followed by an automatic multi-step absolute and relative earthquake location procedure. The 72 relocated events of the seismic sequence were clustered in time (7–12 August) and in a narrow range of depths (10–12 km). The Ml 2.1 foreshock doublet and the Ml 2.9 mainshock identified a persistent asperity. The joint analysis of aftershocks distribution, the mainshock focal mechanism, and the geology of the study area suggest the occurrence of the sequence along a NNE-SSW left-lateral, transtensional fault in the brittle portion of the crystalline basement.

Microscale Spatial Variations in Coseismic Temperature Rise on Hematite Fault Mirrors in the Wasatch Fault Damage Zone

AbstractCoseismic temperature rise activates fault dynamic weakening that promotes earthquake rupture propagation. The spatial scales over which peak temperatures vary on slip surfaces are challenging to identify in the rock record. We present microstructural observations and electron backscatter diffraction data from three small‐displacement hematite‐coated fault mirrors (FMs) in the Wasatch fault damage zone, Utah, to evaluate relations between fault properties, strain localization, temperature rise, and weakening mechanisms during FM development. Millimeter‐ to cm‐thick, matrix‐supported, hematite‐cemented breccia is cut by ∼25–200 μm‐thick, texturally heterogeneous veins that form the hematite FM volume (FMV). Grain morphologies and textures vary with FMV thickness over μm to mm lengthscales. Cataclasite grades to ultracataclasite where FMV thickness is greatest. Thinner FMVs and geometric asperities are characterized by particles with subgrains, serrated grain boundaries, and(or) low‐strain polygonal grains that increase in size with proximity to the FM surface. Comparison to prior hematite deformation experiments suggests FM temperatures broadly range from ≥400°C to ≥800–1100°C, compatible with observed coeval brittle and plastic deformation mechanisms, over sub‐mm scales on individual slip surfaces during seismic slip. We present a model of FM development by episodic hematite precipitation, fault reactivation, and strain localization, where the thickness of hematite veins controls the width of the deforming zones during subsequent fault slip, facilitating temperature rise and thermally activated weakening. Our data document intrasample coseismic temperatures, resultant deformation and dynamic weakening mechanisms, and the length scales over which these vary on slip surfaces.

Fracture Energy and Breakdown Work During Earthquakes

Large seismogenic faults consist of approximately meter-thick fault cores surrounded by hundreds-of-meters-thick damage zones. Earthquakes are generated by rupture propagation and slip within fault cores and dissipate the stored elastic strain energy in fracture and frictional processes in the fault zone and in radiated seismic waves. Understanding this energy partitioning is fundamental in earthquake mechanics to explain fault dynamic weakening and causative rupture processes operating over different spatial and temporal scales. The energy dissipated in the earthquake rupture propagation along a fault is called fracture energy or breakdown work. Here we review fracture energy estimates from seismological, modeling, geological, and experimental studies and show that fracture energy scales with fault slip. We conclude that although material-dependent constant fracture energies are important at the microscale for fracturing grains of the fault zone, they are negligible with respect to the macroscale processes governing rupture propagation on natural faults. ▪ Earthquake ruptures propagate on geological faults and dissipate energy in fracture and frictional processes from micro- (less than a millimeter) to macroscale (centimeters to kilometers). ▪ The energy dissipated in earthquake rupture propagation is called fracture energy ( G) or breakdown work ( Wb) and scales with coseismic slip. ▪ For earthquake ruptures in natural faults, the estimates of G and Wbare consistent with a macroscale description of causative processes. ▪ The energy budget of an earthquake remains controversial, and contributions from different disciplines are required to unravel this issue.

Fault Roughness at Seismogenic Depths and Links to Earthquake Behavior

AbstractFault geometry affects the initiation, propagation, and cessation of earthquake rupture, as well as, potentially, the statistical behavior of earthquake sequences. We analyze 18,250 (−0.27 &amp;lt; M &amp;lt; 4.4) earthquakes of the 2016–2019 Cahuilla, California, swarm and, for the first time, use these high-resolution earthquake locations to map, in detail, the roughness across an active fault surface at depth. We find that the strike-slip fault is 50% rougher in the slip-perpendicular direction than parallel to slip. 3D mapping of fault roughness at seismogenic depths suggests that roughness varies by a factor of 8 for length scales of 1 km. We observe that the largest earthquake (M 4.4) occurred where there is significant fault complexity and the highest measured roughness. We also find that b-values are weakly positively correlated with fault roughness. Following the largest earthquake, we observe a distinct population of earthquakes with comparatively low b-values occurring in an area of high roughness within the rupture area of the M 4.4 earthquake. Finally, we measure roughness at multiple scales and find that the fault is self-affine with a Hurst exponent of 0.52, consistent with a Brownian surface.

Back-projection results for the M w 7.5, 28 September 2018 Palu earthquake-tsunami

A back-projection technique allows seismologists to analyse rupture properties once seismic signals from a dense array of seismic networks are available. The observed waveforms are then traced back in space and time to the source region of an earthquake under investigation. In this study, the method utilised the back-projection image of the recorded high-frequency P-seismic waveforms filtered at 0.25-1.0 Hz by Multiple Signal Classification (MUSIC) processing to estimate the extent and the spread of earthquake rupture propagation of the M w 7.5 Palu event on 28 September 2018, generating a severe tsunami. This study aims to estimate rupture duration, its extent (the distance over which rupture propagates) and the corresponding speed, and rupture directivity (where most of the seismic energy propagates). The results revealed that the front of rupture propagates slightly offset southward at about 2.9 km/s over a distance of ~140 km away from the epicentre for about 49 s before slowing down at much smaller amplitudes after reaching the south end of Palu Bay. This finding is consistent with an NNW-SSE orientation of the active Palu-Koro Fault lying along the bay, suggesting that the strong ground motion is associated with the fault activities. This study has therefore substantial implications for enhanced earthquake and tsunami early warnings, helping the government and local authority build community resilience by warning people at risk from future possible earthquake and tsunami hazards.

Frictional Properties of the Longmenshan Fault Belt Gouges From WFSD‐3 and Implications for Earthquake Rupture Propagation

AbstractThe 2008 Mw 7.9 Wenchuan earthquake generated ∼270 and ∼80 km long surface ruptures along the Longmenshan fault belt, namely the Yingxiu‐Beichuan fault (YBF) and the Guanxian‐Anxian faults (GAF), respectively. So far, most of the frictional investigations were performed on the YBF gouge materials. Here, we present the results of rotary shear friction experiments performed on the GAF gouges recovered from the depth of ∼1.25 km of the Wenchuan Earthquake Fault Scientific Drilling project‐3 along the GAF. The fault gouges, mainly composed of quartz, illite, chlorite, and kaolinite, were sheared at slip velocities V ranging from 10−5 to 2 m/s and normal stresses from 8.5 to 10 MPa under both room humidity and wet conditions. At any imposed slip velocity, the wet gouges have an apparent friction coefficient lower than the room humidity one. In addition, enhanced velocity‐strengthening behavior at intermediate velocities (10−2 m/s &lt; V ≤ 10−1 m/s) was recognized. We characterized the products using field‐emission scanning electron microscopy combined with synchrotron X‐ray diffraction analysis. These microanalytical investigations evidence the formation of size‐reduced particles (without mineral phase changes) and R‐ and Y‐shears in the principal slip zone (PSZ). Regardless of the ambient conditions, the width of PSZ was proportional to the input frictional work density (the product of shear stress times displacement). Our results support the hypothesis that the GAF preferentially ruptures through wet fault gouges; however, the enhanced velocity‐strengthening regime at intermediate velocities may act as a barrier to slip acceleration during fault rupture propagation.

Working with Dynamic Earthquake Rupture Models: A Practical Guide

Abstract Dynamic rupture models are physics-based simulations that couple fracture mechanics to wave propagation and are used to explain specific earthquake observations or to generate a suite of predictions to understand the influence of frictional, geometrical, stress, and material parameters. These simulations can model single earthquakes or multiple earthquake cycles. The objective of this article is to provide a self-contained and practical guide for students starting in the field of earthquake dynamics. Senior researchers who are interested in learning the first-order constraints and general approaches to dynamic rupture problems will also benefit. We believe this guide is timely given the recent growth of computational resources and the range of sophisticated modeling software that are now available. We start with a succinct discussion of the essential physics of earthquake rupture propagation and walk the reader through the main concepts in dynamic rupture model design. We briefly touch on fully dynamic earthquake cycle models but leave the details of this topic for other publications. We also highlight examples throughout that demonstrate the use of dynamic rupture models to investigate various aspects of the faulting process.

Rupture propagation on heterogeneous fault: Challenges for predicting earthquake magnitude

<p indent="0mm">Predicting magnitudes and ground motion of future strong earthquakes on seismogenic faults holds critical implications on seismic hazard zonation and mitigation. Although present approaches to seismic hazard assessment can effectively identify potential regions to host strong earthquakes, it remains extremely challenging to accurately predict future earthquake magnitudes because of various heterogeneities of fault zones, such as fault geometry, medium structure, and frictional properties. In this work, we review a few aspects that play important roles in earthquake rupture propagation and extent, so as to determine the final magnitude of earthquakes. Under the condition of heterogeneous stress distribution on faults, the final magnitude of earthquakes as well as shallow slip distribution depends on where the rupture initiates. As shown in recent numerical models of dynamic rupture simulations, different hypocenters with similar stress level can lead to completely different rupture scenarios, some of which may significantly break the ground whereas others may be buried underground. Such findings shed critical light on seismic hazard preparation, because surface-breaching ruptures can lead to severe damage as vividly evidenced by the 2022 Menyuan <italic>M</italic>_w 6.7 earthquake. To date, we cannot predict future hypocenter locations according to our knowledge of earthquake nucleation and therefore future relevant investigations are highly demanded. Furthermore, the size of seismogenic zone on strike-slip faults controls whether ruptures may become “break-away” to form large earthquakes or stop spontaneously as “self-arresting” of moderate magnitude events. Observations show that aspect ratios between rupture length and down-dip width of dip-slip earthquakes are usually no more than 8. In contrast, the length/width ratios of strike-slip earthquakes may rise to 40 and have a drastic change around the rupture width of <sc>~10 km.</sc> Intrinsic mechanism of such variation in strike-slip earthquakes is attributed to the energy release rate of rupture fronts, which is controlled by the down-dip width. As such, high-resolution constraints on down-dip seismogenic width can be used to estimate the magnitude of future earthquakes on strike-slip faults. Moreover, fault zone structure significantly affects rupture directivity and slip extents. For instance, low-velocity fault zones may promote rupture propagation and thus enlarge the earthquake magnitude. Given the rapid development of seismic observations and seismic imaging techniques, high-resolution fault zone structure can be obtained from data recorded by dense arrays, including nodal network and the newly developed distributed acoustic sensing (DAS) arrays. Integrating with laboratory results of rheology from samples representing crustal rocks, the near-fault high-resolution seismic structure can be converted into rheological properties on faults, which can then be used to outline asperities for future earthquakes. This may provide a critical step to link seismic structure to earthquake potential, as the outlined asperities can be further used to derive rupture scenarios and earthquake magnitudes. To advance our understanding on this front, future work may integrate multiple sources of observations and modeling. Conducting rupture simulations with constraints from geodetic and seismic measurements, as well as laboratory frictional experiments, can be helpful in exploring potential earthquake magnitudes. By considering heterogeneities on faults with reasonable constraints, the numerical models can be useful for investigating the probable hypocenter locations of large earthquakes. Thus, the numerical results may support future near-field observations to improve monitoring of nucleation and development of future large earthquakes. Furthermore, the ground motions generated from the numerical models can also fill the data gap of near-field observations of large earthquakes and provide critical support for physics-based seismic hazard assessment.

The July 2020 Mw 6.3 Nima Earthquake, Central Tibet: A Shallow Normal-Faulting Event Rupturing in a Stepover Zone

Abstract On 22 July 2020, an Mw 6.3 earthquake with a predominantly normal-faulting mechanism struck the Yibug Caka fault zone, central Tibet, where the overall tectonic environment is characterized by left-lateral strike-slip motion. This event offers a chance to gain insight into the tectonic deformation and the cause of shallow normal-faulting earthquakes in this little studied region. Here, we use Sentinel-1A/B Interferometric Synthetic Aperture Radar data to investigate the coseismic and postseismic deformation related to this earthquake. The earthquake ruptured a previously mapped West Yibug Caka fault and is dominated by normal slip with a peak value of 1.9 m at depth of 6.9 km. Postseismic deformation analysis indicates that the observed subsidence signals of up to ∼4.7 cm are a consequence of afterslip. Most of the afterslip is confined at depths between 0.8 and 8.4 km, peaking at 0.27 m at depth of 6.1 km. The significant coseismic slip and afterslip involved in the earthquake highlights a complex interaction between the major normal fault and the secondary synthetic fault. By an integrated analysis of the inversions, regional geology geomorphology, fault kinematics, and seismicity background, we propose a tectonic model that attributes the occurrence of this normal-faulting event to the release of extensional stress in a stepover zone controlled by the northeast-striking sinistral strike-slip Riganpei Co fault and Bu Zang Ai fault. Compared with that the structural stepover often acts as a barrier to affect the propagation of earthquake rupture, our study demonstrates that the failure of a stepover may potentially induce the occurrence of earthquake along the bounding strike-slip faults.

Shallow Rupture Propagation of Pleistocene Earthquakes Along the Hurricane Fault, UT, Revealed by Hematite (U‐Th)/He Thermochronometry and Textures

AbstractThe material properties and distribution of faults above the seismogenic zone promote or inhibit earthquake rupture propagation. We document the depths and mechanics of fault slip along the seismically active Hurricane fault, UT, with scanning and transmission electron microscopy and hematite (U‐Th)/He thermochronometry. Hematite occurs as mm‐scale, striated patches on a &gt;10 m2 thin, mirror‐like silica fault surface. Hematite textures include bulbous aggregates and cataclasite, overlain by crystalline Fe‐oxide nanorods and an amorphous silica layer at the slip interface. Textures reflect mechanical, fluid, and heat‐assisted amorphization of hematite and silica‐rich host rock that weaken the fault and promote rupture propagation. Hematite (U‐Th)/He dates document episodes of mineralization and fault slip between 0.65 and 0.36 Ma at ∼300 m depth. Data illustrate that some earthquake ruptures repeatedly propagate along localized slip surfaces in the shallow crust and provide structural and material property constraints for in models of fault slip.

Deterministic 3D Ground-Motion Simulations (0–5 Hz) and Surface Topography Effects of the 30 October 2016 Mw 6.5 Norcia, Italy, Earthquake

ABSTRACT The Mw 6.5 Norcia, Italy, earthquake occurred on 30 October 2016 and caused extensive damage to buildings in the epicentral area. The earthquake was recorded by a network of strong-motion stations, including 14 stations located within a 5 km distance from the two causative faults. We used a numerical approach for generating seismic waves from two hybrid deterministic and stochastic kinematic fault rupture models propagating through a 3D Earth model derived from seismic tomography and local geology. The broadband simulations were performed in the 0–5 Hz frequency range using a physics-based deterministic approach modeling the earthquake rupture and elastic wave propagation. We used SW4, a finite-difference code that uses a conforming curvilinear mesh, designed to model surface topography with high numerical accuracy. The simulations reproduce the amplitude and duration of observed near-fault ground motions. Our results also suggest that due to the local fault-slip pattern and upward rupture directivity, the spatial pattern of the horizontal near-fault ground motion generated during the earthquake was complex and characterized by several local minima and maxima. Some of these local ground-motion maxima in the near-fault region were not observed because of the sparse station coverage. The simulated peak ground velocity (PGV) is higher than both the recorded PGV and predicted PGV based on empirical models for several areas located above the fault planes. Ground motions calculated with and without surface topography indicate that, on average, the local topography amplifies the ground-motion velocity by 30%. There is correlation between the PGV and local topography, with the PGV being higher at hilltops. In contrast, spatial variations of simulated PGA do not correlate with the surface topography. Simulated ground motions are important for seismic hazard and engineering assessments for areas that lack seismic station coverage and historical recordings from large damaging earthquakes.

Coseismic slip gradient at the western terminus of the 1920 Haiyuan Mw 7.9 earthquake

Stepovers in strike-slip fault systems play important roles in controlling the propagation of earthquake ruptures, depending partially on the competition between the stepover width and the coseismic slip gradient approaching the step. The 1920 Mw 7.9 Haiyuan earthquake is the most recent major earthquake that has occurred along the Haiyuan fault. The earthquake rupture broke through multiple stepovers along the fault and finally ended at the 4 km-wide Jingtai pull-apart basin (releasing stepover) at the western end. To understand the process of this termination of the Haiyuan earthquake rupture, we conducted detailed mapping of the surface rupture geometry, coseismic slip measurement and slip gradient calculation in the vicinity of the endpoint based on the 0.2 m-resolution SfM-derived DEM along the 20 km section east of the Jingtai pull-apart basin. Combining coseismic slip measurements from this study and published slip data from fieldwork in the 1980s, we calculated slip gradients of 84–160 cm/km as the rupture approached the Jingtai releasing stepover. These values are high for the 4 km-wide Jingtai releasing stepover compared to those from a dataset of worldwide historical earthquakes compiled by Elliott et al. (2009). The high values imply that the rupture could have a relatively high likelihood of breaching through the Jingtai releasing stepover. Alternatively, the slip gradient may be overestimated. Detailed mapping and field investigation, however, show that the rupture may extend ~1.86 km farther west of the end location indicated in previous studies. We acquired the new slip gradient with lower value of 50–82 cm/km, which is still considerably high compared to dataset of Elliott et al. (2009). We speculate that the slip gradient could be larger variation along the stepover boundary to stop fault slip in different geologic setting. Another factor control slip termination is related to fault properties. Creep along the southern boundary fault of the Jingtai stepover, a velocity-strengthening region, may also have played a role in stopping the rupture at this location. Our observations further indicate that the increasing LOS velocity of InSAR data within the Jingtai stepover is probably related to fault behavior rather than a nontectonic signal of subsidence.

Depth Extent and Vp/Vs Ratio of the Chenghai Fault Zone, Yunnan, China Constrained From Dense‐Array‐Based Teleseismic Receiver Functions

AbstractFault zone (FZ) structure is critical for understanding earthquake rupture propagation and assessment of seismic hazard. In this study, we develop a new 2‐D dense‐array‐based inversion method to obtain high‐resolution FZ structure model using teleseismic receiver function. In our method, coherent travel times of converted waves and their multiples are first identified from teleseismic receiver functions; then the travel times are inverted for both depth extent and ratio beneath the seismic dense array. Synthetic tests with various shallow low‐velocity‐zone (LVZ) models demonstrate the effectiveness and robustness of our new method. We apply our method to the data recorded by a linear dense array across the southern segment of the Chenghai fault (CHF) in Yunnan, southwestern China. The inversion results suggest that the depth of the LVZ beneath the CHF extends to ∼1.1 km. Furthermore, we derive the ratio simultaneously and obtain the P‐wave velocity model from a S‐wave velocity model of the LVZ that were derived from ambient noise tomography. Travel times predicted by the newly obtained model are consistent with teleseismic P‐ and S‐wave arrivals. In addition, our method shows clear improvement in stability compared with the single‐station H‐ method. With dense seismic arrays deployed globally, our method would be a useful tool in shallow low‐velocity structure imaging studies, including fault zones and sedimentary basins.

Rupture Directivity in 3D Inferred From Acoustic Emissions Events in a Mine-Scale Hydraulic Fracturing Experiment

Rupture directivity, implying a predominant earthquake rupture propagation direction, is typically inferred upon the identification of 2D azimuthal patterns of seismic observations for weak to large earthquakes using surface-monitoring networks. However, the recent increase of 3D monitoring networks deployed in the shallow subsurface and underground laboratories toward the monitoring of microseismicity allows to extend the directivity analysis to 3D modeling, beyond the usual range of magnitudes. The high-quality full waveforms recorded for the largest, decimeter-scale acoustic emission (AE) events during a meter-scale hydraulic fracturing experiment in granites at ∼410 m depth allow us to resolve the apparent durations observed at each AE sensor to analyze 3D-directivity effects. Unilateral and (asymmetric) bilateral ruptures are then characterized by the introduction of a parameter κ, representing the angle between the directivity vector and the station vector. While the cloud of AE activity indicates the planes of the hydrofractures, the resolved directivity vectors show off-plane orientations, indicating that rupture planes of microfractures on a scale of centimeters have different geometries. Our results reveal a general alignment of the rupture directivity with the orientation of the minimum horizontal stress, implying that not only the slip direction but also the fracture growth produced by the fluid injections is controlled by the local stress conditions.

Correlation of porosity variations and rheological transitions on the southern Cascadia megathrust

The unknown onshore extent of megathrust earthquake rupture in the Cascadia subduction zone represents a key uncertainty in earthquake hazard for the Pacific Northwest that is governed by the physical state and mechanical properties of the plate interface. The Cascadia plate interface is segmented into an interseismically locked zone located primarily offshore that is expected to rupture in large earthquakes, a region of aseismic slow slip at greater depth, and an intervening transition zone of uncertain rupture potential. Here we image the evolution of the ratio of seismic compressional to shear wave velocities from the locked zone to the transition zone, which is related to changes in fluid content of the plate boundary zone, using a dense onshore–offshore seismic dataset from southernmost Cascadia. The locked zone shows evidence of high fluid content implying a high porosity, yet the downdip transition zone shows an order of magnitude lower porosity. This strong variation is consistent with models that contain a ductile region between the earthquake rupture and slow slip zones that would inhibit onshore propagation of future large earthquake ruptures and hence reduce seismic hazard. The transition between the locked and slowly slipping regions of the southern Cascadia megathrust has a lower porosity than these regions, according to seismic imaging. This suggests that the transition area is ductile, which may limit rupture propogation.