Seismic Coupling Research Articles

Seismic input method coupling ground motion and fault dislocation for near-fault engineering sites

Seismic input method coupling ground motion and fault dislocation for near-fault engineering sites

Makran Subduction Zone: A Review and Synthesis

This review synthesizes existing research to elucidate the factors driving the distinct tectonic behaviors in the western and eastern Makran subduction zone, focusing on seismic activity, uplift rate, convergence rate, coupling, and subduction angle. The literature identifies the asymmetry in pressure and the variation in subduction angles between the western and eastern parts of the Makran as key factors in defining the region’s tectonic patterns. The western region has a steeper subduction angle, resulting in lower pressure, reduced coupling, and decreased seismic activity. This disparity arises from different interactions between the subducted and overriding plates. This article offers an overview of the Makran subduction zone, identifies some knowledge gaps, and suggests directions for future research to improve our understanding of this complex geological region. The review highlights the need for more comprehensive GPS stations and targeted studies on subduction dip angles to better understand the region’s tectonic dynamics.

FaultQuake: An open-source Python tool for estimating Seismic Activity Rates in faults

In regions experiencing ongoing aseismic deformation, fault’s Activity Rate (AR) calculations often lead to an overestimation of hazard potential. This study proposes a novel methodology that integrates the Seismic Coupling Coefficient (SCC) into the fault Seismic Activity Rate (SAR) calculation process to discriminate seismic moment rates. We introduce FaultQuake, an open-source Python tool equipped with a Graphical User Interface (GUI), designed to implement this methodology and accurately estimate SAR for faults. These activity rates can be included in Probabilistic Seismic Hazard Assessment (PSHA) frameworks and assist in differentiating the seismic and aseismic deformation. FaultQuake also presents an innovative embedded workflow, the Optimal Value Computation Workflow (OVCW), based on Conflation of Probabilities (CoP), for calculating the Maximum Magnitude (Mmax) from the empirical relationships and the observed magnitudes (Mobs) assigned to a single fault. This enhancement improves the estimation of seismic moment rates and the SAR calculation process. FaultQuake outputs are provided in the format of OpenQuake engine input files to facilitate the PSHA process. We present a sample case study focusing on the PSHA of a region in southern Iran characterized by a substantial aseismic deformation to illustrate the practical application of FaultQuake in seismic hazard analysis. Peak Ground Acceleration (PGA) maps for 10% and 2% Probabilities of Exceedance (PoE) are plotted to compare PGAs with and without applying the FaultQuake algorithm. The results provide an enhanced view of the area’s hazard with mitigation of the overestimation, resulting in more representative hazard maps. The source codes of FaultQuake are available at the FaultQuake GitHub repository, contributing to the computer and geoscience community.

Present-day quantification of seismic coupling along the décollement level beneath the Potwar Plateau region in Pakistan western Himalaya

Using GNSS horizontal surface velocities and Sentinel-1 interferometry line-of-sight velocities, we quantify the current velocity field of the Potwar Plateau Salt Range fold-and-thrust belt. From this velocity field indicating a creep of the Potwar Plateau along the Main Himalayan Thrust, we inferred a weak subhorizontal décollement level formed by a massive Precambrian salt layer. To the south of the Plateau, the Salt Range is uplifted along the Salt Range Thrust, conditioned by the presence of an inherited normal fault. The Kalabagh Fault, which forms the western boundary of the Salt Range and Potwar Plateau, exhibits a creep rate of 3.3 mm/yr. Numerical modelling enabled us to characterise the slip distribution and coupling along the faults, showing the presence of a large asperity along the décollement level beneath the Potwar Plateau and several asperities along the eastern part of this basal thrust. The model also indicated an alternation of coupled and decoupled zones along the Kalabagh Fault, suggesting that this strike-slip fault can be characterised by creep and the occurrence of earthquakes and/or slow slip events. Considering the lack of instrumental and historical large-magnitude earthquakes in this area, the Main Himalayan Thrust and Kalabagh Fault are likely to be affected by earthquakes of magnitude Mw larger than 7.5. A slip rate of 20 mm/yr is modelled along the southern and superficial parts of the Salt Range Thrust, which is larger than the 14 mm/yr slip rate along the Main Himalayan Thrust at depth. This observation suggests the existence of a southward flow of massive salt along the Salt Range Thrust.

Along‐Strike Variations of Alaska Subduction Zone Structure and Hydration Determined From Amphibious Seismic Data

AbstractWe develop a 3‐D isotropic shear velocity model for the Alaska subduction zone using data from seafloor and land‐based seismographs to investigate along‐strike variations in structure. By applying ambient noise and teleseismic Helmholtz tomography, we derive Rayleigh wave group and phase velocity dispersion maps, then invert them for shear velocity structure using a Bayesian Monte Carlo algorithm. For land‐based stations, we perform a joint inversion of receiver functions and dispersion curves. The forearc crust is relatively thick (35–42 km) and has reduced lower crustal velocities beneath the Kodiak and Semidi segments, which may promote higher seismic coupling. Bristol Bay Basin crust is relatively thin and has a high‐velocity lower layer, suggesting a dense mafic lower crust emplaced by the rifting processes. The incoming plate shows low uppermost mantle velocities, indicating serpentinization. This hydration is more pronounced in the Shumagin segment, with greater velocity reduction extending to 18 ± 3 km depth, compared to the Semidi segment, showing smaller reductions extending to 14 ± 3 km depth. Our estimates of percent serpentinization from VS reduction and VP/VS are larger than those determined using VP reduction in prior studies, likely due to water in cracks affecting VS more than VP. Revised estimates of serpentinization show that more water subducts than previous studies, and that twice as much mantle water is subducted in the Shumagin segment compared to the Semidi segment. Together with estimates from other subduction zones, the results indicate a wide variation in subducted mantle water between different subduction segments.

Seismogenic depth and seismic coupling estimation in the transition zone between Alps, Dinarides and Pannonian Basin for the new Slovenian seismic hazard model

Abstract. Seismogenic depth and seismic coupling are important inputs into seismic hazard estimates. Although the importance of seismic coupling is often overlooked, it significantly impacts seismic hazard results. We present an estimation of upper and lower seismogenic depth and expected hypocentral depth and seismic coupling in the transition zone between the Alps, Dinarides and Pannonian Basin, characterized by a complex deformation pattern, highly variable crustal thickness, and moderate seismic hazard, supporting the development of the 2021 seismic hazard model of Slovenia. The hazard model was based on three seismic source models: area source model, fault source model and smoothed seismicity (point) source model. We estimated the lower seismogenic depth using seismological and geological data and compared them. The seismological estimate was based on two regional earthquake catalogues prepared for this study. In the area source model, estimates of lower seismogenic depth from seismological data are deeper or equal to the ones derived from geological data, except in one case. In the fault source model, we analysed each fault individually and chose seismological lower depth estimates in 12 among 89 faults as more representative. The seismogenic thickness for each individual fault source was determined for seismic coupling determination. The seismic coupling was assessed by two approaches, i.e. we chose the most trusted value from the literature, and the value determined for each fault individually by using the approach based on the updated regional fault and earthquake data sets. The final estimate of seismic coupling ranges from 0.77 to 0.38. We compared the tectonic moment rate based on long-term slip rate using different values of seismic coupling with the seismic moment rate obtained from the earthquake catalogue. The analysis is done for the whole area, as well as for the individual area zones. The analysis of N–S components of estimated slip for the largest faults in the area of west Slovenia shows that the regional geological and geodetic shortening rates are comparable. The total activity rate of three global seismic source models is compared, which gives up to a 10 % difference. Our results contribute to a better understanding of the seismic activity in the region. The presented approach for seismic coupling estimation can be applied in cases where the total slip rate is given instead of its seismic part and can be used at regional or national level. The approach is also suitable for the cross-border harmonization of the European seismic hazard modelling data.

Recurrence Times of Large Earthquakes: Assimilating the Effect of Seismic Coupling into a Renewal Model

ABSTRACT A renewal model for estimating recurrence times of large earthquakes is presented, which allows one to assimilate data from paleo, historic, and instrumental earthquake data as well as geodetic information in a simple and efficient way using three parameters: the Richter b-value, the mean recurrence time μT of paleo and historic earthquakes, and the seismic coupling coefficient χ. The core of the model is a state variable with linear drift in time reflecting Reid’s elastic rebound theory, decorated with random fluctuations modeling small and intermediate earthquakes. Furthermore, it is assumed that a fraction between 0% and 100% of the average seismic moment is released seismically, and the remaining part corresponds to aseismic processes such as creep. The model extends former renewal models of Matthews et al. (2002) and Zöller et al. (2008). Results for different regions indicate overall good performance and thus suggest that the model provides physically motivated constraints for recurrence time estimations in contrast to the standard fitting of a few data points to an assumed distribution.

Seismic coupling of structures on liquefiable and mitigated sites with drainage and ground densification methods

Seismic coupling of structures on liquefiable and mitigated sites with drainage and ground densification methods

Why does western Makran have a low seismicity rate?

The Makran Subduction Zone is segmented into western and eastern Makran where all instrumentally recorded megathrust earthquakes happened in eastern Makran. To find out how the lack of megathrust earthquakes and the observed low seismicity rate in western Makran is related to a combination of long interseismic quiescence, aseismic creep, or low rate of interseismic strain accumulation within the megathrust zone, we have complemented the five existing GPS vectors along the coasts of Makran by nine new GPS vectors and an extra eight new GPS vectors across the width of the Makran megathrust zone. Our block modeling shows that the coupling of the subducting Arabian oceanic plate with the overriding plate in western Makran is more than four times smaller than that of eastern Makran. Additionally, the maximum interseismic strain rate accumulation within the onshore part of the megathrust zone of western Makran is >7 times smaller than that in eastern Makran. We found a right-lateral motion of about ∼16 mm across the transfer zone between Zagros and Makran. We consider a much lower earthquake hazard for western Makran relative to that of eastern Makran because the overriding Lut block is moving northward causing much less strain accumulation within the megathrust zone, and because of the lower seismic coupling between the subducting and overriding plates. Our findings show a large strain rate across the transfer zone between Zagros and Makran and thus imply a much higher earthquake hazard for the transfer zone.

Interseismic creep of carbonate-hosted seismogenic normal faults: Insights from central Italy

Understanding fault behavior in carbonates is critical because they represent loci of earthquake nucleation. Models of fault-slip mode generally assume: (1) seismic sliding and aseismic sliding occur in different fault patches, (2) creep is restricted to lithology-controlled weak domains, and (3) rate-weakening patches are interseismically locked. We studied three carbonate-hosted seismogenic normal faults in central Italy by combining (micro)structural and geochemical analyses of fault rocks integrated with new seismic coupling estimates. The (upper bound) seismic coupling was estimated to be ∼0.75, which indicates that at least 25% of the long-term deformation in the study area is released aseismically in the upper crust. Microscopy and electron-backscatter diffraction analyses revealed that whereas the localized principal slip zone records seismic slip (as ultracataclastic material, calcite crystallographic preferred orientation (CPO), and truncated clasts), the bulk fault rock below behaves differently. Cataclasites in massive limestones deform by cataclastic flow, pressure solution, and crystal plasticity, along with CPO development. Foliated tectonites in micritic limestones deform by pressure solution and frictional sliding, with CPO development. We suggest these mechanisms accommodate on-fault interseismic creep. This is consistent with experimental results reporting velocity-strengthening behavior at low slip rates. We present multiscale evidence of coexisting seismic and aseismic slip along the same fault in limestones during the seismic cycle. Our results imply that on-fault aseismic motion must be added to seismic slip to reconcile the long-term deformation rates and that creep is not exclusive to phyllosilicate-bearing units. Our work constitutes a step forward in understanding fault behavior and the seismic cycle in carbonates, and it may profoundly impact future studies on seismogenic potential and earthquake hazard assessment.

Spatio-temporal variations of seismic coupling along a transform fault: the western North Anatolian Fault Zone

SUMMARY The Main Marmara Fault (MMF) forms a major segment of the North Anatolian Fault Zone (NAFZ) in northwestern Türkiye. The MMF represents a seismic gap with currently high seismic hazard and associated risk for the Istanbul metropolitan area. Here we estimate the seismic coupling defined as the ratio of the seismic strain rate to the tectonic strain rate, for the MMF and adjacent NAFZ segments. This ratio indicates the fraction of total strain accumulated with time that is released seismically. We compare the results of seismic strain rates and coupling estimated from earthquakes included in historical and instrumental catalogues, which allows us to identify fault segments that represent a considerable seismic threat during the current seismic cycle. We find that along the main fault traces hosting the large events, seismic strain rates from the historical catalogue are of the same order as the tectonic strain rates. In contrast, coupling estimates based on seismic data from the instrumental catalogue covering also off-fault areas, are up to 100 times smaller, highlighting that most of the seismic energy is released in large earthquakes with recurrence times longer than the time covered by the instrumental catalogue. Within the Sea of Marmara, a significant portion (48%) of shear strain from the instrumental catalogue is currently being accommodated by seismic deformation. Significant variations of the seismic coupling are observed before and after the 1999 M > 7 Izmit earthquake, highlighting the different contribution of aseismic slip over different portions of the seismic cycle. A comparison of the temporal evolution of the 1999 Izmit and Düzce post-seismic deformation with seismic strain rates shows that the largest seismic strain rates coincide with the largest post-seismic deformation.

Numerical Analysis and Experimental Study on Seismic Coupling Response of Pipe-Soil System Under Bidirectional Seismic Excitation

The seismic response of buried pipeline is significantly affected by the soil around the pipeline. In this study, a numerical analysis model of the pipeline and the soil around the pipeline is built, and the finite element numerical analysis of its seismic response law is conducted to address the problem of pipeline soil seismic coupling response. A shaking table test of the seismic response of buried pipeline under two-way seismic excitation is performed based on the developed two-way layered shear continuum model soil box. The peak strain response at the middle section of the pipeline grows fastest, the peak strain of the pipeline under non-uniform seismic excitation constantly exceeds that under uniform excitation, and the axial strain of the pipeline exhibits more sensitivity to earthquake than the bending strain, as indicated by the results of numerical analysis and shaking table test. The peak value of soil acceleration response under non-uniform excitation exceeds under uniform excitation. The displacement of soil increases with the height. The pipeline soil interaction becomes more intense under non-uniform excitation, and the relative movement between pipeline and soil becomes more significant. The seismic excitation more significantly affects the axial displacement of soil. The peak value of soil acceleration response varies from larger along the pipeline to larger along the axis with the increase of the loading grade. A slight difference exists between the peak value of axial and transverse acceleration response of the pipeline. The peak value of pipeline acceleration response declines in the axial direction than the peak value of soil acceleration response, whereas it rises in the transverse direction than the peak value of soil acceleration response, especially under non-uniform excitation. As revealed by the above results, the effect of bidirectional non-uniform seismic excitation on the lateral pipeline soil interaction is enhanced, and soil is more seriously damaged.

Depth‐Dependent Aftershock Trigger Potential Revealed by 3D‐ETAS Modeling

AbstractCurrent earthquake catalogs provide high‐precision depth values that contain valuable information. Thus, we extend the spatiotemporal Epidemic Type Aftershock Sequence model to 3D by considering hypocentral instead of epicentral distances. To explore the most appropriate parametric form of the spatial kernel, we first examine different triggering functions for the depth difference of the aftershock‐mainshock pairs identified by the Nearest‐Neighbor method, showing that a magnitude‐dependent power‐law kernel fits best the earthquakes data in Southern California. Therefore, we incorporate the corresponding kernel into the 3D‐ETAS model with space‐dependent background activity, where we additionally allow for depth‐dependent aftershock productivity. The application to Southern California shows that the model fits the data well. We also find that the aftershock productivity strongly depends on depth, similar to the seismic moment released by the mainshocks, which may be related to depth‐dependent seismic coupling.

Can the Nucleation Phase be Generated on a Sub-fault Linked to the Main Fault of an Earthquake?

We study the effects of seismic coupling, friction, viscous, and inertia on earthquake nucleation based on a two-body spring-slider model in the presence of thermal- pressurized slip-dependent friction and viscosity. The stiffness ratio of the system to represent seismic coupling is the ratio of coil spring K between two sliders and the leaf spring L between a slider and the background plate and denoted by s=K/L. The s is not a significant factor in generating the nucleation phase. The masses of the two sliders are m1 and m2, respectively. The frictional and viscous effects are specified by the static friction force, fo, the characteristic displacement, Uc, and viscosity coefficient, h, respectively. Numerical simulations show that friction and viscosity can both lengthen the natural period of the system and viscosity increases the duration time of motion of the slider. Higher viscosity causes lower particle velocities than lower viscosity. The ratios g=h2/h1, f=fo2/fo1, y=Uc2/Ucl, and m=m2/m1 are four important factors in influencing the generation of a nucleation phase. When s>0.17, g>1, 1.15>f>1, y<1, and m<30, simulation results exhibit the generation of nucleation phase on slider 1 and the formation of P wave on slider 2. The results are consistent with the observations and suggest the possibility of generation of nucleation phase on a sub-fault. Results exhibit independence of P wave at slider 2 on the shape and duration time of nucleation phase at slider 1.

Modeling Seismic Recordings of High‐Frequency Guided Infrasound on Mars

AbstractNASA's Interior Exploration using Seismic Investigations, Geodesy and Heat Transport (InSight) mission records several high‐frequency (>0.5 Hz) dispersive seismic signals on Mars. These signals are due to the acoustic‐to‐seismic coupling of infrasound generated by the entry and impact of meteorites. This dispersion property is due to infrasound propagating in a structured atmosphere, and we refer to this dispersive infrasound as guided infrasound. We propose to model the propagation of guided infrasound and the seismic coupling to the ground analytically; we use a 1D layered atmosphere on a three‐layer solid subsurface medium. The synthetic ground movements fit the observed dispersive seismic signals well and the fitting indicates that the regolith beneath InSight is about 40‐m in thickness. We also examine and validate the previously‐published subsurface models derived from InSight ambient seismic vibration data.

Effects of biaxial loading path on seismic behavior of T-shaped RC walls

T-shaped RC walls are commonly used to resist horizontal seismic excitations in multiple directions. The differences in the intensity and spectrum characteristics of two translational components of ground motion cause the wall to experience complex biaxial displacement trajectories, resulting in a different seismic response compared to the behavior derived from conventional uniaxial cyclic tests. Thus, to investigate the seismic behavior and coupling mechanism of T-shaped walls subjected to different magnitudes of loading in two orthogonal directions, this study examined five identical T-shaped walls under uniaxial loading along two principal axes and biaxial loading with three represented loading paths. The test results showed that the mechanical behavior in one direction was significantly affected by the loading in the orthogonal direction owing to internal force redistribution and additional stress caused by the biaxial coupling bending, resulting in vertical variation and intersection of their hysteretic curves. Further, the biaxial loading primarily aggravated the cracking and damage of the flange, which accelerated the performance degradation in the flange direction and reduced the effective flange width involved in resisting web direction load. In addition, the biaxial coupling effect was found to be proportional to the area enclosed by the loading path. Thus, the most severe damage and performance degradation were observed under a square path, followed by an eight-shaped path, with the cruciform path exhibiting the least effect. Consequently, based on the results, recommendations for the design of T-shaped walls subjected to biaxial earthquake actions were proposed.

Acoustic-seismic coupling model for the Short-Distance detection of Ultra-Low-Altitude aircraft.

When using seismic signals to detect ultra-low-altitude aircraft at short distances, inappropriately designed filters reduce the signal quality and detection accuracy. To solve this problem, continuous acoustic signals generated by standard sound sources were used to induce vibrations in soil. Based on the assumed incidence of spherical wavefronts, we found that frequency bands of strengthened and weakened coupling are caused by interference between acoustic coupled waves and precursory seismic waves. We estimated the seismic properties at the experimental site by scanning various parameters to optimize the similarity coefficient. Based on the model, we discussed the relationship among soil velocities, layer thicknesses, and the amplitude of the coupled signal with frequency and provided a basis for the design of filters for detecting ultra-low-altitude aircraft.

Evidence of Strong Upper Oceanic Crustal Hydration Outboard the Alaskan and Sumatran Subduction Zones

AbstractThe hydration state of subducting oceanic crust has been proposed to influence subduction zone processes like seismic coupling at the megathrust interface and arc magmatism downdip. Plate bending in the outer rise region is thought to help rehydrate the incoming oceanic lithosphere before subduction. Although numerous seismic refraction studies provide constraints on the amount of water stored in the lower crust and uppermost mantle, little information exists about how much free water is present in the upper 1–2 km of oceanic crust. Here, we present results from the application of advanced techniques to long‐offset multi‐channel seismic data acquired outboard the Alaskan and South Sumatran subduction zones. Our results show that the incoming upper crustal seismic layer, layer 2A, is significantly hydrated in both areas. Favorable conditions offshore the Alaska Peninsula promote the creation of a dense system of bending‐related faults, facilitating the infiltration of fluids that increases average water estimates to ∼3.9 wt.% H2O in the outer rise. As the crust subducts and temperature increases, some free water may react with the host rocks to form mineral‐bound water that is carried to greater depths, in agreement with elevated water contents found in arc lavas of Shumagin Gap volcanoes. Offshore Sumatra, we propose that similar layer 2A water estimates (∼3.2 wt.% H2O average) and heterogeneous hydration within 2B are associated with the ongoing, slow, complex deformation occurring in the Wharton Basin and thus potentially contribute to the presence of a long‐lived slow slip event recently inferred there at seismogenic depths.

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

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

Investigation of the Factors Controlling the Duration and Productivity of Aftershocks Following Strong Earthquakes in Greece

Strong crustal earthquakes in Greece are typically followed by aftershocks, the properties of which are important factors in seismic hazard assessment. In order to examine the properties of earthquake sequences, we prepared an earthquake catalog comprising aftershock sequences with mainshocks of Mw ≥ 5.5 from 1995 to 2021. Regional aftershock parameters were estimated to highlight variations in aftershock decay and productivity among regions with similar seismotectonic characteristics. A statistically based method of estimating aftershock duration and a metric of relative aftershock productivity to examine the variations among the different cases were employed. From the detailed analysis of the selected seismic sequences, we attempt to unravel the physical mechanisms behind deviations in aftershock duration and productivity and resolve the relative contribution of background seismicity, the Omori–Utsu law parameters and the mainshock faulting properties. From our analysis, the duration of aftershock sequences depends upon the rupture process of the mainshock, independently of its magnitude. The same applies to aftershock productivity, however, other tectonic setting (e.g., seismic coupling) or source-related (e.g., focal depth, stress drop) parameters also contribute. The estimated regional parameters of the aftershock rate models could be utilized as initial ones to forecast the aftershock occurrence rates at the early stage following a mainshock.