Earthquake Slip Research Articles

Cascading Geological Hazards and Risks of the 2018 Sulawesi Indonesia Earthquake and Sensitivity Analysis of Tsunami Inundation Simulations

Multiple cascading hazards triggered by the 2018 Sulawesi, Indonesia, earthquake caused various compounding consequences. A major strike-slip fault movement with along-dip components resulted in intense ground shaking, liquefaction and lateral spreading, large-scale mudflows, and tsunamis. This paper presents observations of such multi-hazard effects on buildings and infrastructure in areas along Palu Bay, based on field reconnaissance work, and discusses the main causes of the disaster by focusing upon the combined effects of the cascading geological hazards. To evaluate the tsunami risk potential of the strike-slip event, tsunami simulations for the 2018 Sulawesi earthquake are performed by considering different model settings for spatial earthquake slip distribution, rake angle, astronomical tidal effect, and co-seismic ground deformation. The numerical results indicate that the co-seismic rupture of a moment magnitude 7.5 strike-slip earthquake, having notable dip components can generate damaging tsunami waves at coastal locations of Palu Bay. The conclusion has important implications for future tsunami hazard assessments in active seismic regions where major fault systems cut across a bay or into the sea.

Seismic hazard analysis for East Malaysia; based on a proposed ground motion prediction equation

The seismic hazard analysis requires an estimation of ground motion intensity where the process needs to use a compatible ground motion prediction equation or GMPE, which provides ground acceleration estimates in a function of earthquake magnitude and distance. Hence, the effect of current equation often does not accurately represent the earthquake condition in East Malaysia region. In this study, the characteristics of low-to-moderate databases were used and derived by regression analysis in terms of horizontal peak ground acceleration (PGA). The appropriate GMPE design for East Malaysia is based on the ground motion records compiled from strike slip earthquakes that occurred within 10 to 1,350 km. This earthquake data is based on actual data recorded at a broad range of magnitude levels within a wide range of distances. The new equation is used to predict the PGA value throughout East Malaysia by probabilistic method. PSHA is a method to analyse seismic hazard assessment using probability concept by considering the uncertainties of the size, location and rate of occurrence of earthquake and the variation of ground motion characteristics. The four well-known existing attenuation functions are evaluated with current equation to highlight their limitations in magnitude and distance. With a more complete collection of earthquake databases, GMPE has become more reliable. The GMPE of peak ground acceleration for low-to-moderate earthquake at long distance was found to be logarithmically distributed. The equation provides ease in both implementation and interpretation of physical parameters with a comparable standard deviation.

Paleoseismicity of Adyr Faults: The Kokonadyr–Tegerek Fault, Southwestern Issyk-Kul Region, the Tien Shan

We have studied paleoseismicity along the adyr (piedmont) Kokonadyr–Tegerek fault zone in the SW Issyk-Kul region. We have shown that although earthquake slip continues in the tectonic contact zone between the Paleozoic rocks of the Kokonadyr–Tegerek uplift and the Quaternary deposits of the Alabash–Konuroleng trough, still most earthquake-generating ruptures visible at the surface are moving for some hundreds of meters southward from the abovementioned adyr fault. This process leads to the formation of multislip earthquake scarps and compensation grabens behind them. Radiocarbon dating identified four large Holocene earthquakes along the Kokonadyr–Tegerek fault zone that occurred during the following time spans: 1771‒1785, 1440‒1515, as well as probably 2310–745 B.C. and 6400–5300 B.C. Judging from the parameters of these ruptures, or earthquake scarps, their magnitudes were greater than or equal to 7 and the intensity of shaking was Io ≥ IX. Vigorous seismic activity has gone on along the Kokonadyr–Tegerek fault zone since at least the Middle Quaternary. The relevant evidence includes uplifted and abandoned valleys of this age in the eponymous uplift and considerable accumulations of lacustrine deposits in the northern Alabash–Konuroleng depression. The thick sedimentary sequence was caused by glacier melting on the northern slope of the Terskei Ala-Too mountain range, while the abandoned valleys indicate considerable tectonic damming. The most suitable model to explain the formation of adyrs (piedmont areas or foothills) in the southern Issyk-Kul basin seems to be a model that envisages a low-angle northward propogating or southward dipping main fault that is complicated with reverse movements. All the materials acquired in this study can be used to develop a new Earthquake Hazard Map for the Republic of Kyrgyzstan.

Sensitivity of Probabilistic Tsunami Hazard Assessment to Far-Field Earthquake Slip Complexity and Rigidity Depth-Dependence: Case Study of Australia

Probabilistic Tsunami Hazard Assessment (PTHA) often proceeds by constructing a suite of hypothetical earthquake scenarios, and modelling their tsunamis and occurrence-rates. Both tsunami and occurrence-rate models are affected by the representation of earthquake slip and rigidity, but the overall importance of these factors for far-field PTHA is unclear. We study the sensitivity of an Australia-wide PTHA to six different far-field earthquake scenario representations, including two rigidity models (constant and depth-varying) combined with three slip models: fixed-area-uniform-slip (with rupture area deterministically related to magnitude); variable-area-uniform-slip; and spatially heterogeneous-slip. Earthquake-tsunami scenarios are tested by comparison with DART-buoy tsunami observations, demonstrating biases in some slip models. Scenario occurrence-rates are modelled using Bayesian techniques to account for uncertainties in seismic coupling, maximum-magnitudes and Gutenberg-Richter b-values. The approach maintains reasonable consistency with the historical earthquake record and spatially variable plate convergence rates for all slip/rigidity model combinations, and facilitates partial correction of model-specific biases (identified via DART-buoy testing). The modelled magnitude exceedance-rates are tested by comparison with rates derived from long-term historical and paleoseismic data and alternative moment-conservation techniques, demonstrating the robustness of our approach. The tsunami hazard offshore of Australia is found to be insensitive to the choice of rigidity model, but significantly affected by the choice of slip model. The fixed-area-uniform-slip model produces lower hazard than the other slip models. Bias adjustment of the variable-area-uniform-slip model produces a strong preference for ‘compact’ scenarios, which compensates for a lack of slip heterogeneity. Thus, both heterogeneous-slip and variable-area-uniform-slip models induce similar far-field tsunami hazard.

Earthquake‐Scaling Relationships from Geodetically Derived Slip Distributions

AbstractEmpirical earthquake scaling relationships describe expected relationships between moment magnitude and various spatial descriptors of the earthquake rupture (along‐strike length, down‐dip width, rupture area, and peak and mean slip). These scaling relationships play important roles in many seismological, geological, and hazards‐assessment applications. Historically, scaling relationships were defined from various seismological criteria, such as teleseismic finite‐fault models or aftershock distributions. The proliferation of earthquake slip distributions from geodetic observations presents an opportunity to reassess earthquake scaling relationships using observations that more directly sample the spatial characteristics of an earthquake than seismological observations. Here, we present a database of 111 earthquake slip distributions from 73 different earthquakes that were derived from geodetic observations. The earthquakes range in magnitude from Mw 5.3 to 9.1. We extract common spatial descriptors from these slip distributions in four different ways to account for biases introduced by inversion regularization, and we regress these spatial descriptors with moment magnitude to derive new empirical scaling relationships. We additionally assess the shape characteristics of the slip distributions and report the average earthquake shape. We find that our scaling relationships differ in important ways from previous studies, and we show that these differences originate from our use of a geodetic slip‐distribution database rather than from methods for extracting spatial descriptors. Notably, we find that geodetic slip distributions systematically predict smaller fault areas than seismically derived scaling relationships. Because geodetic source inversions are likely contaminated to some degree by aseismic afterslip, this relationship suggests that seismologically determined scaling relationships systematically overpredict earthquake dimensions. We find that fault length, fault width, peak slip, and mean slip differ from previous studies in ways that are more complex and magnitude dependent. Given the high‐model resolution afforded by geodetic observations, our earthquake scaling relationships derived from geodetic slip distributions provide improved constraints on empirical scaling relationships.

Sensitivities of Geodetic Source Analyses to Elastic Crust Heterogeneity Constrained by Seismic Tomography for the 2017 Mw 6.5 Jiuzhaigou, China, Earthquake

ABSTRACTThe 2017 Mw 6.5 Jiuzhaigou earthquake (JE) struck a rugged area of the Jiuzhaigou Valley in eastern Tibet that has experienced frequent seismic activity over the last few decades. We use finite‐element models (FEMs) and Sentinel‐1 Interferometric Synthetic Aperture Radar observations to characterize the earthquake source. The FEM domain accommodates a heterogeneous (HET) distribution of realistic crustal materials inferred by regional seismic tomography data. The HET‐derived source configurations yield a significantly smaller misfit, at the 95% confidence level, than that estimated for a homogeneous (HOM) half‐space. The former generally requires a lower degree of smoothing constraint, highlighting that the HET solutions are systematically more compatible with the surface observations than the HOM solutions. The magnitudes of induced Coulomb failure stress change (ΔCFS) estimated by the HET solution drastically differ (by >0.1 MPa) from those calculated by the HOM solution. The postearthquake stability of near‐field faults is generally overestimated by the HOM estimations, whereas some localities of negative ΔCFSHOM are predicted with positive ΔCFSHET. These results highlight the sensitivities of both slip and stress estimations to the complexity of the adopted elastic modeling domain, leading to more accurate aftershock hazard assessments. The HET‐resolved seismic rupture reveals two major slip asperities of magnitude up to 0.83 m distributed along the fault strike, which is coherent with the aftershock distribution. Two aftershock clusters are consistently found near or below these two peak‐slip zones, which are imaged by the HET model but absent in the HOM solution. The JE hypocenter and aftershocks are bounded below by a negative velocity anomaly (ΔVP, ΔVS down to −4%) at ∼18 km depth. Such low‐velocity layers of reduced strength may be relevant to the vertical distribution of seismicity and earthquake slip, which provide insights into assessing the seismic hazards and aftershock‐prone areas of the eastern Tibetan margin.

Biomarker Thermal Maturity Reveals Localized Temperature Rise From Paleoseismic Slip Along the Punchbowl Fault, CA, USA

AbstractThe Punchbowl Fault, California, USA, is an example of a simple fault zone that has a relatively narrow fault core with further slip localization to principal slip zones. We sampled the principal slip zone, fault core, and wall rocks and conducted hydrous pyrolysis experiments to analyze biomarker thermal maturity within the Punchbowl Fault. Using biomarker maturity as a proxy for temperature rise, we show that the existing principal slip zone experienced greater temperature rise than the surrounding fault core and wall rock, and therefore, we infer that earthquake slip localized along this layer. Furthermore, evidence of slight thermal maturity within the ultracataclasite suggests that the fault core is made up at least in part of reworked former principal slip zones. Using a wide range of possible layer thicknesses, we find that the maximum temperature range during a single earthquake could have varied from ~460 to 1,060 °C at 1‐m/s slip velocity. However, not all samples from within the principal slip zone show a temperature rise, indicating that layer thickness, slip, or shear stress varied during slip. Our temperature estimate also allows us to constrain the frictional energy dissipated during the earthquake to 2.2–25 MJ/m2. Our study demonstrates that localized slip structures can be directly linked to seismicity and that small changes in earthquake or fault parameters can lead to changes in temperature (and likely fault strength) at small scales.

Shortening Accommodated by Thrust and Strike‐Slip Faults in the Ili Basin, Northern Tien Shan

AbstractThe Tien Shan accommodates a significant portion of the India‐Eurasia N‐S convergence. In its northern part a zigzag pattern of mountain ranges bounds the western Ili Basin. The role of this basin in the overall shortening and the regional kinematics is not well understood. Geodetic data and instrumental seismicity are not sufficient to infer the role of individual faults and fault systems. We analyze GPS data and earthquake slip vectors and present the results of fault mapping based on remote sensing and field campaigns in the western Ili Basin. These observations indicate that E‐W thrust faults are active at the basin margins, and oblique and strike‐slip faults, both in the basin and in the Paleozoic rocks within the mountain ranges, have been active in the Late Quaternary. We propose a regional tectonic model in which the left‐lateral strike‐slip faults at the NW margin of the basin accommodate ~3‐mm/year NE‐SW shear. Smaller right‐lateral oblique faults transfer the motion in between the left‐lateral faults, and also take up shortening by rotations about vertical axes. We see the onset of internal deformation within the Ili Basin, although it has a strong basement. Our kinematic model is consistent with geodetic data, earthquake seismology, historical, and prehistorical surface faulting, and describes the first‐order features of active deformation that can be observed in the northern Tien Shan. Our study illustrates the importance of combining these different data sets to understand the regional tectonics.

Weakening Mechanisms of Alpine Fault Gouge in High‐Velocity Shear Experiments

AbstractThe Alpine Fault, New Zealand, is a large plate boundary fault with history of major seismic events that frequently ruptured to the surface. To better understand earthquake slip at shallow depth, we analyzed the frictional properties of gouge samples collected at three field exposures distributed along 40 km of the fault trace. The samples are rich in phyllosilicates (30–55%) and quartz/feldspar (41–64%), with small amounts of calcite (4–12%). The gouge samples were sheared in a confined rotary cell under room conditions at a normal stress of 2–3 MPa, under alternating slip velocity steps from 0.002 to 1.47 m/s. We found initial friction coefficients of 0.6–0.8 and gentle velocity strengthening during early slip with short displacement and low velocity. A drastic weakening of more than 50% reduction of the frictional strength occurred after displacements of 2–5 m as slip velocity exceeds ~0.2 m/s, and there was no strength recovery even when slip velocity reduced back to low levels. This weakening was associated with marked temperature rise and intense CO2 and H2O emissions. Based on continuous monitoring of gas pressure, CO2, and H2O concentrations within the gouge chamber, our analysis revealed that the dynamic weakening was triggered by gouge‐zone pressurization, which was driven by thermal decomposition of calcite and dehydration of absorbed water. The pressurization process was enhanced by slip localization and establishment of a low‐permeability shear zone. We further envision that the gouge zone pressurization is likely to cause local overpressure and internal fluidization within of the fine‐grain gouge layer that leads to dynamic weakening.

The 2016 M7 Kumamoto, Japan, Earthquake Slip Field Derived From a Joint Inversion of Differential Lidar Topography, Optical Correlation, and InSAR Surface Displacements

AbstractObservations of surface deformation within 1–2 km of a surface rupture contain invaluable information about the coseismic behavior of the shallow crust. We investigate the oblique strike‐slip 2016 M7 Kumamoto, Japan, earthquake, which ruptured the Futagawa‐Hinagu Fault. We solve for variable fault slip in an inversion of differential lidar topography, satellite optical image correlation, and Interferometric Synthetic Aperture Radar (InSAR)‐derived surface displacements. The near‐fault differential lidar pose several challenges. The model fault geometry must follow the surface trace at the sub‐kilometer scale. Integration of displacement datasets with different sensitivities to the 3D deformation field and varying spatial distribution permits additional complexity in the inferred slip but introduces ambiguity that requires careful selection of the regularization. We infer a Mw earthquake. The maximum slip of 6.9 m occurred at 4.5‐km depth, suggesting an on‐fault slip deficit in the upper several kilometers of the crust that likely reflects distributed and inelastic deformation within the shallow fault zone.

A non-Gaussian random field model for earthquake slip

The present study proposes a methodology to generate stochastic slip field with desired probability structure and spectral characteristics. The characterization utilizes 100 tsunamigenic and 130 non-tsunamigenic slip models available in the SRCMOD database. First, the cumulative distribution function (CDF) of marginal slip distributions is obtained. Then, it compares the estimated CDFs with nine standard distribution functions based on model fitting criteria like Akaike Information Criterion, mean squared error, correlation coefficient, and QQ plots. The analysis showed that the majority of slip distributions follow generalized Pareto or truncated exponential distribution functions. For the parameters of CDF estimated from slip fields, an empirical relation is proposed as a function of magnitude (Mw) to predict the corresponding values. As the next step, to understand the correlation structure of the slip field, semi-variograms are estimated along strike and along downdip for all rupture models considered in the study. A total of nine theoretical variogram models is compared with the semi-variogram of each slip field. From the analysis based on model fitting criteria, most of the slip field is observed to follow a stable variogram model. An empirical equation is developed to predict the sill and range of stable variogram function for a given Mw. Then, an iterative method based on translational field theory is proposed to obtain an ensemble of non-Gaussian slip fields for a given Mw and type of source. The generated sample fields from the proposed method match with the prescribed non-Gaussian distribution and variogram structure. In the sequel, the method estimates the variogram and the corresponding spectral density of the underlying Gaussian field according to translation field theory. The scaling relations and the proposed iterative methodology can be used to obtain an ensemble of possible earthquake slip fields.

Coseismic ultramylonites: An investigation of nanoscale viscous flow and fault weakening during seismic slip

Faults weaken during the propagation of earthquakes due to the onset of thermally-activated mechanisms, which vary depending on the rock type. Recent experimental work suggests that carbonate-hosted faults are lubricated by viscous flow in nano-granular aggregates having ultramylonitic textures. However, their frail nature has often hindered unbiased characterisation of the textures and deformation mechanisms operating at such extreme conditions (strain rates as high as 104), which remain so far poorly investigated and understood.We explore the formation, evolution and deformation mechanisms of coseismic ultramylonites in carbonate-hosted faults generated during high velocity (1.4 ms−1), displacement-controlled shear experiments in a rotary apparatus. Microstructures were analysed using integrated SEM and TEM imaging while detailed crystallographic fabrics were investigated using the electron back-scattered diffraction (EBSD) technique.Mechanical data show that the strength of the experimental fault decays dynamically with slip, according to a characteristic four stage evolution; each stage is associated with characteristic textures. Microstructural observations show that brittle processes dominate when the fault is strong (friction coefficients >0.6). Cataclasis, aided by twinning and crystal plasticity, operates forming an extremely comminuted shear band (mean grain size ∼200 nm). As the fault starts weakening, shear localises within a well-defined principal slip zone. Here, thermally-activated grain size sensitive (GSS) and insensitive (GSI) creep mechanisms compete with brittle processes in controlling fault strength. GSI mechanisms produce strong monoclinic crystallographic preferred orientations in the slip zone, while textures and crystallographic orientations in adjacent locations do not evolve from the previous deformation stage. By the end of the transient weakening stage, the slip zone has reached a steady state thickness (30 μm) and shows a nanogranular ultramylonitic texture. The intensity of the crystallographic preferred orientation in the coseismic ultramylonite is reduced compared to the previous stage, due to grainsize sensitive creep mechanisms becoming gradually more dominant. As the experimental fault re-strengthens, upon deceleration to arrest, the ultramylonite may be partially reworked by brittle deformation.Our findings show that the crystallographic orientations of transient microstructures are preserved in the slip zone of coseismic ultramylonites and in narrow, adjacent deactivated layers, where mirror-like surfaces are located. This shows that EBSD techniques can usefully be employed to determine the deformation mechanisms of coseismic ultramylonites and their evolution during earthquake slip in both experimental and, potentially, natural faults.

An experimentally-based friction law for high-velocity, long-displacement slip-pulse events during earthquakes

Large earthquakes initiate at small nucleation sites and propagate as rupture fronts along the host fault. It is inherently challenging to resolve the complexity of fault rupture at depth, and therefore, the evolution of frictional strength during earthquakes is commonly analyzed in laboratory experiments. We experimentally demonstrate here that this evolution depends simultaneously on all slip kinematic components: displacement, velocity and acceleration. We incorporate these components in shear experiments with slip-histories that resemble the theoretical expectations for earthquake slip. These experiments led to a new friction law that fits fault behavior during high-velocity/long-displacement slip. Our numerical simulations of dynamic rupture along a planar fault that obeys this friction law reproduced a range of earthquake source features including slip-pulse, Yoffe function, Gaussian velocity, and spontaneous slip arrest. Finally, we demonstrate that this experimentally-based friction law can realistically simulate the propagation and arrest of natural earthquakes.

Characterization of Strong Motion Generation Regions of Earthquake Slip Using Extreme Value Theory

This article characterizes the regions of earthquake slip that are responsible for strong motion generation (SMG), using extreme value theory (EVT). A three-step iterative procedure involving Zipf plot, mean excess function plot and moment ratio plot is used to define the threshold value for a slip model. This threshold slip demarcates the region of SMG in the slip model. The iterative procedure ensures that slip values above the threshold slip follow generalized Pareto distribution (GPD). The regions where slip is greater than the corresponding threshold slip are defined as regions of SMG. A total of 159 slip samples filtered from the SRCMOD catalogue are analyzed in this paper. In order to identify the region of SMG, the first step is to obtain effective slip dimensions. A novel method based on strong motion duration (SMD) is proposed to obtain the effective dimensions of slip model. In the next step, the iterative procedure is employed to obtain threshold slip and the regions of SMG for all the slip samples. The corresponding area of SMG is estimated from the number of sub-faults in the region of SMG and the sub-fault dimensions. Regression analysis is carried out to establish source scaling relationships for the rupture parameters with respect to seismic moment. The considered rupture parameters are effective length, effective width, effective area, effective mean slip, threshold slip and area of SMG of the slip model. On a logarithmic scale, these rupture parameters are observed to follow a linearly increasing trend with the seismic moment. The proposed equations can be used to estimate the rupture parameters for future earthquake events.

Rapid Geodetic Analysis of Subduction Zone Earthquakes Leveraging a 3‐D Elastic Green's Function Library

AbstractThe 2018 M7.2 Pinotepa earthquake ruptured a shallow slab section along the Middle America subduction zone. We demonstrate how a geodetic Green's function (GF) library and efficient modeling algorithm can rapidly resolve earthquake slip and contribute to early warning systems. The source is characterized with InSAR data and a finite‐element model mimicking realistic slab geometry (Slab1.0) and velocity structures (CRUST2.0) that are not considered in the conventional homogeneous (HOM)‐ or layered(1‐D)‐crust solutions. The rupture is imaged 18 min after geodetic data are downloaded to a 16‐CPU‐thread workstation. Nearby Mw8.6‐megathrust scenarios are also studied with synthetic GPS data in the Guerrero/Oaxaca area. Our results show that earthquake slip solutions are sensitive to the materials of elastic‐modeling domains. Attaining smaller misfits or sums of squared errors, models by 3‐D GFs significantly better recover coseismic displacements than those estimated with 1‐D GFs or HOM GFs at 95% confidence.

Spatial and temporal distribution of earthquake ruptures in the eastern segment of the Altyn Tagh fault, China

We performed field investigations to reconstruct the paleoearthquake history of eastern segment of the Altyn Tagh fault, NW China. Investigation at the Saohu spring trench and Shibaocheng west trench sites result in the identification three well-resolved earthquakes, designated as events E1, E2 and E3, within the last ∼7000 years. Radiocarbon dates of detrital charcoal provide constraints on the timing of faulting events. The most recent surface-rupturing earthquake, event E1 dated between 2720 and 2800 years (a) before present (BP), had a rupture length of ∼ 83 km, extending from Hongliuxia to Bagexia, with a coseismic displacement of about 4.5 m. The penultimate event, E2, occurred between 4180 and 4970 a BP and may have ruptured from Subei to Bagexia with a coseismic displacement of > 7 m. The third youngest event, E3, occurred approximately 6440–7180 a BP and may have ruptured the whole segment from Subei to Kuantan mountain, with a coseismic displacement of >10 m. Holocene earthquake recurrence behavior on the eastern segment of the Altyn Tagh fault conforms to a variable slip earthquake model. Together with previous studies of slip rate, the huge displacement deficit along the eastern segment indicates a high earthquake risk for the surrounding region; the fault has the potential to trigger earthquakes of greater than M7.8, or a period of clustered earthquake activity. The displacement deficit could also indicate that sub-faults in the western Qilian Mountains have absorbed more lateral extrusion of the plateau during the Holocene, which would result in a decreasing fault slip rate and weakening of the eastern extension or outward expansion of the Altyn Tagh fault.

Amphibious surface-wave phase-velocity measurements of the Cascadia subduction zone

SUMMARY A new amphibious seismic data set from the Cascadia subduction zone is used to characterize the lithosphere structure from the Juan de Fuca ridge to the Cascades backarc. These seismic data are allowing the imaging of an entire tectonic plate from its creation at the ridge through the onset of the subduction to beyond the volcanic arc, along the entire strike of the Cascadia subduction zone. We develop a tilt and compliance correction procedure for ocean-bottom seismometers that employs automated quality control to calculate robust station noise properties. To elucidate crust and upper-mantle structure, we present shoreline-crossing Rayleigh-wave phase-velocity maps for the Cascadia subduction zone, calculated from earthquake data from 20 to 160 s period and from ambient-noise correlations from 9 to 20 s period. We interpret the phase-velocity maps in terms of the tectonics associated with the Juan de Fuca plate history and the Cascadia subduction system. We find that thermal oceanic plate cooling models cannot explain velocity anomalies observed beneath the Juan de Fuca plate. Instead, they may be explained by a ≤1 per cent partial melt region beneath the ridge and are spatially collocated with patches of hydration and increased faulting in the crust and upper mantle near the deformation front. In the forearc, slow velocities appear to be more prevalent in areas that experienced high slip in past Cascadia megathrust earthquakes and generally occur updip of the highest-density tremor regions and locations of intraplate earthquakes. Beneath the volcanic arc, the slowest phase velocities correlate with regions of highest magma production volume.

Hot on the trail: Coseismic heating on a localized structure along the Muddy Mountain fault, Nevada

Recent advances in the use of thermal proxies to identify past earthquake slip in exhumed faults have created an extraordinary opportunity to map the geometry of past ruptures in unprecedented detail. This approach can reveal along-strike differences in the structure and speed of earthquake slip. Here, we present organic thermal maturity data collected along the Muddy Mountain thrust in Nevada to investigate where it was within the fault that earthquake slip occurred and whether this is consistent along strike. We observe large changes in thermal maturity, which represent temperature-rise variations along the fault. Modeling of thermal maturity measurements yield peak temperatures of 760–1090 °C where the principal slip zone (PSZ) is narrowest (0.8 mm). Nearby where the PSZ is thicker (1.5 mm) these temperatures did not exceed 500 °C. Based upon estimates of temperature rise within a submillimeter PSZ, mean frictional work during earthquake slip was 6.3–7.1 MJm−2. These results show that thickness of the active slipping zone can vary at small spatial scales and development of a continuously narrow (<1 mm) slip layer is not required for earthquake propagation. Therefore, even for an earthquake with spatially uniform slip, we may observe large variations in heating and the effectiveness of thermally-activated dynamic-weakening mechanisms.

High‐Velocity Friction Experiments Indicate Magnetic Enhancement and Softening of Fault Gouges During Seismic Slip

AbstractHigh‐velocity friction experiments were conducted on two natural fault gouges retrieved from the Yingxiu‐Beichuan fault zone (Sichuan, China), which accommodated the 2008 Wenchuan Mw 7.9 earthquake. The experiments simulate large earthquake slip; the rotary shear apparatus enables to gather information as a function of shear displacement (and slip velocity) in a single experiment. The two starting fault gouges are essentially paramagnetic with one containing goethite. The experimentally sheared gouges both show a significant magnetic enhancement and softening with increasing slip distance. Rock magnetic measurements reveal that magnetite was formed during the experiment due to thermochemical reactions of iron adsorbed on clay minerals (smectite, chlorite). Scanning electron microscope observations showed that the new magnetite occurs as spherical and sintered irregular aggregates. This implies a high‐temperature origin. The peak temperatures are estimated numerically to range from ~260 to ~440 °C. Also, during the experiment on the goethite‐bearing fault gouge, goethite was altered, reduced to magnetite due to the decomposition of trace organic matter present in the starting material. Magnetic susceptibility and magnetization of sheared samples are linearly increasing with the temperature rise induced by frictional heating; coercivities are decreasing. The grain size of the newly formed magnetite increases with heating temperature. This demonstrates that short‐duration frictional heating generated by fast seismic slip induces thermal alteration: neoformation of ferrimagnetic minerals leads to magnetic enhancement and softening of slip zones. Thus, rock magnetic methods are a useful tool for diagnosing the earthquake slip and estimating the temperature rise of coseismic frictional heating.

PROBABILISTIC TSUNAMI HAZARD ASSESSMENTS WITH CONSIDERATION OF UNCERTAIN EARTHQUAKE CHARACTERISTICS

This study proposes a methodology to conduct probabilistic tsunami hazard assessments (PTHA) with consideration of uncertain earthquake characteristics. The methodology adopts a stochastic framework to model the earthquake slip and location as a random field and a random vector, respectively. The stochastic framework consists of three stages: (1) the definition of consistent probability properties for uncertain earthquake characteristics, (2) the generation of samples and (3) the uncertainty propagation. To complete each stage we follow the method recently proposed in Sepúlveda et al. (2017). We illustrate our PTHA methodology by assessing the tsunami hazard in Kao Hsiung and Hong Kong, with consideration of earthquakes in the Manila Trench.