Earthquake Stress Drop Research Articles

The Impact of Source Time Function Complexity on Stress-Drop Estimates

ABSTRACT Earthquake stress drop—a key parameter for describing the energetics of earthquake rupture—can be estimated in several different, but theoretically equivalent, ways. However, independent estimates for the same earthquakes sometimes differ significantly. We find that earthquake source complexity plays a significant role in why theoretically (for simple rupture models) equivalent methods produce different estimates. We apply time- and frequency-domain methods to estimate stress drops for real earthquakes in the SCARDEC (Seismic source ChAracteristics Retrieved from DEConvolving teleseismic body waves, Vallée and Douet, 2016) source time function (STF) database and analyze how rupture complexity drives stress-drop estimate discrepancies. Specifically, we identify two complexity metrics—Brune relative energy (BRE) and spectral decay—that parameterize an earthquake’s complexity relative to the standard Brune model and strongly correlate with the estimate discrepancies. We find that the observed systematic magnitude–stress-drop trends may reflect underlying changes in STF complexity, not necessarily trends in actual stress drop. Both the decay and BRE parameters vary systematically with magnitude, but whether this magnitude–complexity relationship is real remains unresolved.

Stress drops of intermediate-depth intraslab earthquakes beneath Tohoku, northern Japan

We calculated stress drops for 2875 small intraslab earthquakes at intermediate depths beneath Tohoku, Japan. We applied an S-coda-wave spectral ratio method to almost 900,000 event pairs. Detailed velocity values for the oceanic crust (OC) were adopted from previous observational studies. The median stress drops in the OC are about half those in the oceanic mantle (OM). The median stress drop for earthquakes in the OC decreases from depths of 70 to 120 km and increases from 120 to 170 km. Our preferred interpretation is that the rigidity in the OC decreases and then increases with depth due to combined effects of the dehydration associated with the eclogite formation and the increasing temperature with depth. These depth variations are consistent with results of a similar study beneath Hokkaido. The median stress drops in the oceanic plate beneath Tohoku are generally smaller than those beneath Hokkaido. Previous studies imaging the seismic structure at shallow depths and b-value analyses of intraslab earthquakes indicate that the near-trench region of the oceanic plate off Tohoku is more hydrated than that off Hokkaido. Taken together, these results suggest that differences in the degree of hydration of the oceanic plate in the near-trench regions could produce the different behaviors of stress drops of intermediate-depth earthquakes observed in Tohoku and Hokkaido.Graphical

Magma intrusion process during pre-magmatic period (2010−2013) of Sinabung volcano as revealed by seismicity of volcano-tectonic and hybrid earthquakes

Sinabung volcano in Indonesia reactivated, with a series of phreatic eruptions, in August and September 2010. These eruptions were followed by various types of magmatic eruption at the summit, including lava dome growth, pyroclastic density currents, lava flow, and vulcanian eruptions, starting in December 2013. Prior to the magmatic eruptions, Sinabung exhibited two sequences of phreatic eruption and an increase in the seismicity of volcanic earthquakes. This study clarifies the progress of magma intrusion during the pre-magmatic period based on the hypocenter distribution, waveform similarity, seismic moment, rupture length, and stress drop for volcano-tectonic (VT) and hybrid earthquakes. It was found that the hypocenters of VT earthquakes are distributed north and northwest of the summit at a depth range of 1–10 km below sea level. Deep seismicity (4–10 km) alternated with shallow seismicity (2–4 km). The last shallow seismicity, from July to mid-December 2013, was different from previous seismicity in terms of higher intensity, as determined from the seismic moment, a temporary increase in the rupture length, and the migration of hypocenters towards the summit. The seismicity of VT earthquakes was altered by a swarm of hybrid earthquakes in mid-December. The hybrid earthquakes were smaller, in terms of seismic moment (mostly <3 × 1010 Nm), than the VT earthquakes. Their hypocenters were concentrated in the shallowest depth range (−0.5 to 1.5 km below sea level) directly below the summit, their source process was repeatable (they could be grouped into six earthquake families), their dominant peak was in a low-frequency range (2.5–4.5 Hz), and they had a relatively low stress drop (<0.15 MPa). This suggests that the swarm of hybrid earthquakes was induced by a frequent repeated fracture of fluid-filled cracks due to the intrusion of magma up to directly below the summit. The transition of the earthquake family and the change in its source parameters, namely an increase in the stress drop prior to the appearance of a lava dome and then a slight decrease, may reflect a gradual change in internal pressure in the hypocentral zone through the magma intrusion process.

Earth's Free Surface Complicates Inference of Absolute Stress From Earthquake‐Induced Stress Rotations

AbstractThe stress redistribution from an earthquake can produce localized measurable rotations of the principal stress axes if the absolute level of differential stress in the crust is on the order of the earthquake stress drop. Two simple analytic solutions have been developed to estimate the differential stress from an observed stress rotation. However, each has assumptions that may not be accurate near Earth's free surface. I model synthetic earthquakes in an elastic half‐space, and show that the assumptions of the methods are accurate for strike‐slip earthquakes, and for deep dip‐slip earthquakes. However, they are incorrect for shallow dip‐slip earthquakes. I introduce a free surface correction for one of the methods for dip‐slip earthquakes. I revise an analysis of stress rotations due to great subduction zone earthquakes, including this correction. The results support the original conclusion of near complete stress drop for many shallow subduction zone earthquakes.

Source Parameter Scaling Relations for Shallow Crustal Earthquakes: Exploration With the Single Asperity Model

AbstractUnderstanding the scaling laws of source parameters is a fundamental subject in seismology. This study conducts spectral ratio analysis to estimate the source parameters for 409 shallow crustal earthquakes with Mw 3.2–6.0 in Japan. Subsequently, the source parameter scaling relations are investigated for a wide magnitude range by combining the results of this and previous studies. The spectral ratio method applied in this study provides the finite source properties of the localized area with large slip. The single asperity model, a simple heterogeneous source model with a single localized area with large slip, is used to estimate the stress drop since it can be more suitable to characterize earthquake sources than the standard homogeneous circular source. This investigation calibrates the constant to link the corner frequency and the source radius by comparing the corner frequency and source area estimated by the spectral ratio analyses. This calibrated constant is used to re‐calculate the stress drop of small earthquakes from the corner frequency estimated in previous studies. The stress drop and apparent stress increase with increasing magnitude up to Mw ∼ 5 and become magnitude‐independent for larger earthquakes. The radiation efficiency ranges typically from 0.1 to 1.0 and is independent of magnitude. The slip dependences of the stress drop, apparent stress, radiation efficiency, and fracture energy observed in this study are explained by the ones predicted from the slip‐weakening model incorporating the thermal pressurization effect. Thermal pressurization is one of the possible mechanisms explaining the observed source parameter scaling relations.

The effect of shear strain and shear localization on fault healing

SUMMARY The seismic cycle of repeated earthquake failure requires that faults regain frictional strength during the interseismic phase, when the fault is locked or undergoing quasi-static creep. Fault healing plays a central role in determining earthquake stress drop, recurrence interval, elastic radiation frequency and other source parameters. In particular, the longer a fault remains quasi-stationary, the stronger it becomes and the larger the potential stress drop can be for the next event. Here, we address the role of shear strain and strain localization on fault healing and healing rate. We performed slide-hold-slide friction experiments on quartz gouge in the double-direct shear configuration for shear strain up to 25 and hold times from 10 to 1000 s. The results show that both healing and healing rate increase nonlinearly with increasing shear strain. Frictional healing scales with volumetric strain within the laboratory fault zone. Using the volumetric strain upon reshear as a proxy for strain localization, we demonstrate that the capacity of a fault to heal is directly proportional to shear bandwidth and degree of strain localization. The more the deformation is localized, the higher are the healing and healing rate, and thus, the fault strength. Our data provide a framework for understanding variations in fault strength over the seismic cycle and the role of brecciation and strain localization on spatiotemporal variations in fault strength.

One tune, many tempos: Faults trade off slip in time and space to accommodate relative plate motions

Analysis of incremental slip rates from the four major strike-slip faults of the Marlborough fault system (MFS) of northern South Island, New Zealand, provides a first-ever record at the scale of an entire plate-boundary fault system of how relative plate motions are accommodated in time and space. This record, which spans the past 350–450 m of relative plate motion and ca. 12–14 ky, demonstrates that the fault system as a whole accommodates a steady plate-boundary slip rate, with the MFS faults “keeping up” with the overall rate of relative Pacific-Australia plate motion at relatively short displacement (10 s of meters) and time (102–103 yr) scales. These results affirm the often-assumed but until now unproven assumption that the relative plate-motion rate provides a robust basic constraint on both geodynamical models and analyses of system-level seismic hazard at these scales. In marked contrast, the incremental slip rates of each of the four main Marlborough faults are highly variable through time, marked by coordinated accelerations and decelerations spanning 4–6 earthquakes and several millennia as the faults trade off slip to accommodate a steady relative plate motion rate. These results suggest that (a) the weakest fault in the system will slip faster than average while adjacent mechanically complementary faults slip more slowly, and (b) that these patterns switch back and forth through time, likely reflecting reversible changes in the strength (i.e., resistance to shear) of the individual faults as they collectively accommodate relative plate motion. Interestingly, the periods of fast slip on the MFS faults exhibit ∼20–25 m of displacement, suggesting that these may record periods of fast slip on a weakened fault/ductile shear zone that continues until it uses up all locally stored elastic strain energy, thus potentially approaching local complete stress drop, albeit during a few tens of meters of rapid fault slip during multiple earthquakes, rather than during a single event. This hypothesis is consistent with typical earthquake stress drops of ∼1–10 MPa and estimates of depth-averaged crustal shear stress of a few 10 s of MPa, such as might be released in clusters of 4–6 earthquakes. These results emphasize the need to analyze the collective behavior of the entire fault systems, rather than just individual faults, to understand the mechanics of the system. Moreover, these patterns suggest a potential path forward for more accurate estimation of time-dependent seismic hazard, with the possible incorporation of current position of a fault within a fast- or slow/no-slip period into the probability analysis, as well as a means of potentially estimating crustal shear stress.

Quantifying Site Effects and Their Influence on Earthquake Source Parameter Estimations Using a Dense Array in Oklahoma

AbstractWe investigate the effects of site response on source parameter estimates using earthquakes recorded by the LArge‐n Seismic Survey in Oklahoma (LASSO). While it is well known that near‐surface unconsolidated sediments can cause an apparent breakdown of earthquake self‐similarity, the influence of laterally varying site conditions remains unclear. We analyze site conditions across the 1825‐station array on a river plain within an area of 40 km by 23 km using vertical ground motions from 14 regional earthquakes. While the source radiation pattern controls P‐wave ground motions below 8 Hz, the surface geology correlates with P‐wave ground motions above 8 Hz and S‐wave ground motions at 2–21 Hz. Stations installed in alluvial sediments have vertical ground motions that can exceed three times the array median. We use the variation of ground motion of regional earthquakes across the array as a proxy for site effects. The corner frequencies and stress drops of local earthquakes (ML = 0.01–3) estimated using a standard single‐spectra approach show negative correlations with the site‐effect proxy, while the seismic moments show positive correlations. In contrast, the spectral‐ratio approach effectively shows no correlation. The overall bias is small as expected for this relatively homogeneous structure; accurate estimation of site‐related biases requires at least 30 stations. Correcting for site‐related biases reduces the standard deviations of the source parameters by less than 13% of the total variations. Remaining variations are partially associated with source directivity and model misfits— as small earthquakes can have complex ruptures.

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.

Re‐Examining Temporal Variations in Intermediate‐Depth Seismicity

AbstractChanges in the frequency of intermediate‐depth (60–300 km) earthquakes in response to static stress transfer can provide insights into the mechanisms of earthquake generation within subducting slabs. In this study, we use the most up‐to‐date global and regional earthquake catalogs to show that both the aftershock productivity of large earthquakes, and the changes in the frequency of intermediate‐depth earthquakes around the timing of major megathrust slip, support the view that faults within the slab are relatively insensitive to static stress transfer on the order of earthquake stress drops. We interpret these results to suggest the population of faults within the slab are much further from their failure stress than is typical for shallow fault systems. We also find that aftershock productivity varies within slabs over small spatial scales, indicating that the mechanism that enables faults to rupture at intermediate depths is likely to be spatially heterogeneous over length‐scales of a few tens of kilometres. We suggest dehydration‐related weakening mechanisms can best account for this heterogeneity.

Seismological Stress Drops for Confined Ruptures Are Invariant to Normal Stress

AbstractSeismic moment and rupture length can be combined to infer stress drop, a key parameter for assessing earthquakes. In natural earthquakes, stress drops are largely depth‐independent, which is surprising given the expected dependence of frictional stress on normal stresses and hence overburden. We have developed a transparent experimental fault that allows direct observation of thousands of slip events, with ruptures that are fully contained within the fault. Surprisingly, the observed stress drops are largely independent of both the magnitude of normal stress and its heterogeneity, capturing the independence seen in nature. However, we observe larger, normal stress‐dependent stress drops when the fault area is reduced, which allows slip events to frequently reach the edge of the interface. We conclude that confined ruptures have normal stress independent stress drops, and thus the depth‐independent stress drops of tectonic earthquakes may be a consequence of their confined nature.

Strength Recovery in Quartzite Is Controlled by Changes in Friction in Experiments at Hydrothermal Conditions up to 200°C

AbstractThe rate of fault zone restrengthening between earthquakes can be influenced by both frictional and cohesive healing processes. Friction is dependent on effective normal stress while cohesion is independent of normal stress, potentially explaining—in part—the lack of depth dependence of earthquake stress drops. Although amenable to laboratory testing, few studies have systematically addressed the normal stress dependence of restrengthening rate. This is partially due to difficulty in separating relative contributions of friction and cohesion in recovery of fault strength. We present results from a series of slide‐hold‐slide tests on thin layers (≤10 𝜇m) of ultrafine quartz gouge that develop during shearing of initially bare‐surface quartzite. Tests were conducted at 10 MPa constant pore pressure, 20–200 MPa constant effective normal stress, and temperatures of 22°–200°C. Restrengthening, defined as the difference between peak shear stress measured after resumption of sliding and steady‐state sliding shear stress, increases with the log of hold duration. The 200°C healing rate, 0.014 per e‐fold increase in time, is comparable to that determined from seismological observations along the Calaveras Fault, California. Construction of Mohr‐Coulomb failure envelopes shows that changes in cohesion are small (&lt;1 MPa) and independent of hold durations to 105 s, indicating that the increased strength is due to changes in the friction coefficient. These experimental results are inconsistent with the hypothesis that cohesive healing explains the depth independence of earthquake stress drop, but higher temperatures, longer time‐scales, and more complex mineralogy could facilitate cohesive healing in natural fault systems.

Tectonic stress of northeastern Indian region derived from seismic focal mechanisms and the effect of focal mechanism on stress drop: a comparative analysis with Kachchh intraplate region of India

SUMMARY In this study, the focal mechanism solutions and source parameters of recent earthquakes that occurred in the northeastern region of India have been determined. The region has very complex tectonics as it is subjected to the compressional forces from all sides, due to the collision of the Indian Plate with the Eurasian, Burma and Tibetan plates. Waveform data from deployed broad-band seismographs (BBS) and strong motion accelerographs (SMA) in the northeastern India are used to determine the focal mechanism solutions and source parameters of moderate earthquakes, respectively. The estimated focal mechanisms are used to understand the existing stress field in the region. It is found that the Shillong-Plateau as well as the Indo-Burma subduction zone is dominated by the compressional tectonic regime, Mikir Hills and Bengal basin are dominated by the trans-tension tectonic regime, and the easternmost Himalayan region is dominated by the strike-slip tectonic regime. The maximum horizontal stress direction Shmax is also determined for above subregions. The direction of Shmax is southeast in the Bengal basin, northeast in Mikir Hills and Indo-Burma subduction zone whereas it is NNE in Shillong Plateau and SSW in the eastern Himalayas. The estimated stress drop value of the earthquakes in the region ranges from 2.11 to 23.89 MPa. The relationship between the source parameters and focal mechanisms is also explored. It is found that the earthquakes with a strike-slip mechanism have the highest average stress drop (7.05 MPa) followed by reverse (6.82 MPa) and normal (5.12 MPa) in the northeastern region of India. According to the examined data set, the stress drop is found to be dependent on the type of focal mechanism, seismic moment and hypocentral depths. The comparison of the results with the Kachchh intraplate region in western India shows earthquakes in Kachchh have larger mean stress drop for all types of mechanisms. In both intraplate and interplate regions of India, the stress drop of earthquakes depends on the type of focal mechanism solution.

Variation of Proportionality Between Stress Drop and Slip, With Implications for Megathrust Earthquakes

AbstractEarthquake stress drop Δσ is related to fault slip via , where μ, D, and Lc denote shear modulus, average slip, and fault dimension. C is controlled by the system geometry, characterizes the effective stiffness of the system, and is commonly assumed to be a constant near 1. We use 3D elastostatic models to systematically investigate how C is controlled by fault burial depth, dip angle, and slip direction. We find that C decreases with smaller burial depth and dip angle, with a value for a shallow‐dipping surface‐rupturing fault roughly one‐fifth that of the deeply buried case. Our results help explain the apparent magnitude‐dependent stress drops of megathrust earthquakes in Thingbaijam et al. (2017), https://doi.org/10.1785/0120150291. There may also be implications for the apparent depth‐ and magnitude‐dependence in other source parameters, and for reducing uncertainties in the seismic and tsunami hazard assessments of megathrust earthquakes.

Stress Features Inferred From Induced Earthquakes in the Weiyuan Shale Gas Block in Southwestern China

AbstractStress features, particularly local stress field and earthquake stress drops, are important to understand mechanism of induced earthquakes. Since shale gas exploitations in 2015, the Weiyuan shale gas block has experienced frequent earthquakes. In this paper, we determine focal mechanisms of 257 events with ML &gt; 1.5 by fitting three‐component waveforms, invert for direction of maximum horizontal stress for two dense earthquake clusters, and then calculate stress drops for 17 earthquakes with moment magnitudes between 2.2 and 2.75 through the spectral ratio method. The focal mechanisms of all earthquakes are reverse faulting. The orientation of local maximum horizontal compressive stress is in the ESE direction, consistent with crustal movement indicated by GPS measurements. Both the focal mechanisms and microseismicity locations suggest the existence of steeply dipping faults with dip angles of ∼70°, which are relatively rare and difficult to slip under usual conditions, unlike major induced earthquakes with strike‐slip or low‐angle thrusting faults in the midwestern United States and western Canada. However, the steeply dipping reverse events can be induced by large pore pressure from hydraulic fracturing. The stress drops range from 2.5 to 54.7 MPa, comparable to those of potentially induced earthquakes in the midwestern United States. Our results imply earthquakes in the Weiyuan area are controlled by local tectonic stress and induced by large pore pressure from hydraulic fracturing, which advances our knowledge of reactivation of steeply dipping reverse faults.

EASOt-AP: An open-source MATLAB package to estimate the seismic moment, rupture radius, and stress-drop of earthquakes from time-dependent P-wave displacements

We developed a MATLAB package, to rapidly obtain the apparent EArthquake SOurce time function using the Average of near-source P-wave displacement records (EASOt-AP) and to estimate the source parameters of the seismic moment, the rupture radius, and the average static stress drop. The algorithm implemented in this package is based on a rapid and straightforward methodology that is recently developed by Zollo et al. (2021). To this purpose, EASOt-AP models a LPDT curve, which is the average P-wave displacement versus time in the logarithm scale. In this paper, the performance of the EASOt-AP has been shown by evaluating the strong motion data recorded in the near-source range of small magnitude events (2 = Mw ≤ 3) that occurred in the Irpinia region, southern Italy. The source parameters follow a self-similar, constant-stress-drop scaling with a relatively low average stress drop of about 01-0.2 MPa. This Tool is designed for an easy use, and all steps of data processing are performed completely within the numerical environment of MATLAB.

Anelastic deformation in the lower crust and its implications for tectonic dynamics in Kyushu, Southwest Japan

Most of the seismicity in inland Kyushu, southwest Japan occur in the localized intraplate shear zones which are Beppu-Shimabara Graben (BSG) and left-lateral shear zone (LSZ). One of the largest fault systems in Japan (Median Tectonic Line—MTL), and subduction zone of the Philippine Sea Plate also exist near Kyushu. These suggest that the island arc, such as Kyushu has a complex deformation process. This study estimated the anelastic strain rate in the lower crust and the interplate coupling rate via inversion analysis using Global Navigation Satellite System data to elucidate the seismotectonics. Deformation in the lower crust and interplate coupling were modeled by cuboid volumetric sources and a back slip model, respectively. To stabilize the solution, a L2 regularization constraint was applied to the anelastic strain rate. Further, constraints on direction and smoothing to the slip deficit rate were applied. These strength of the constraints were determined via the minimum value of the Akaike Bayesian information criterion. The analyses showed that the anelastic strain rate of the BSG and LSZ was ∼0.6 ×10−6 yr−1, and BSG was related to MTL activity. The findings also suggest that this anelastic deformation in the lower crust created a stress concentration zone in the upper crust of BSG and LSZ, with a stress change rate of ∼6–10 kPa·yr−1. The value is consistent with the stress drop and recurrence interval of regional earthquakes, suggesting that the seismicity around BSG and LSZ is strongly influenced by anelastic deformation in the lower crust. It is also suggested that BSG and LSZ development is influenced by slab rollback, and a subducting ridge. Lastly, a comparison between the deviatoric stress field estimated using focal mechanisms and the anelastic strain rate suggests the existence of a shear zone consisting of weak faults south of BSG.

Earthquake Stress Drop for a Circular Crack in an Anisotropic Medium

ABSTRACT The circular-crack model has been widely used in seismology to infer earthquake stress drop. A common assumption is that the background medium is isotropic, although many earthquakes occur in geologically anisotropic settings. In this article, we study the effect of anisotropy on stress drop for a circular crack model and present explicit formalism in both static and kinematic cases. In the static case, we obtain the relationship between stress drop and slip for a circular crack model in an arbitrarily anisotropic medium. Special attention is given to the transversely isotropic (TI) medium. The static formalism is useful in understanding stress drop, but not all quantities are observables. Therefore, we resort to the kinematic case, from which we can infer stress drop using recorded far-field body waves. In the kinematic case, we assume that the crack ruptures circularly and reaches the final displacement determined by the static solutions. The far-field waveforms show that the corner frequency will change with different anisotropic parameters. Finally, we calculate the stress drops for cracks in isotropic and anisotropic media using the far-field waveforms. We find that in an isotropic medium, only shear stress acting on the crack surface contributes to shear slip. However, in a TI medium, if the anisotropy symmetry axis is not perpendicular or parallel to the crack surface, a normal stress (normal to the crack surface) can produce a shear slip. In calculating stress drop for an earthquake in an anisotropic medium using far-field body waves, a large error may be introduced if we ignore the possible anisotropy in the inversion. For a TI medium with about 18% anisotropy, the misfit of inferred stress drop could be up to 41%. Considering the anisotropic information, we can further improve the accuracy of stress-drop inversion.

Regional Spectral Characteristics Derived Using the Generalized Inversion Technique and Applications to Stochastic Simulation of the 2021 Mw 6.1 Yangbi Earthquake

ABSTRACTIn this study, 334 accelerograms of 42 small-to-moderate earthquakes recorded at 36 strong-motion stations were used to investigate the ground-motion characteristics of the southwestern margin of the Sichuan–Yunnan rhombic block (SWM-SYRB), one of the most active seismic regions in China. The high-frequency attenuation parameter was estimated using the spectral decay method, and the site component (κ0) was fitted. The κ0 estimates decrease with the increasing time-averaged S-wave velocity of the uppermost 30 m (VS30). Using the generalized inversion technique, the source spectra, quality factor (Q), and site amplification were derived from the Fourier amplitude spectra (FAS). The obtained average stress drop for earthquakes occurred in SWM-SYRB was the second largest among various boundary areas of SYRB. The inverted Q model was Q(f)=115.1f0.616. The low Q structure that extends southwestward from the Songpan–Garze block to SWM-SYRB could be responsible for the strong regional attenuation of ground motion with distance. At frequencies above 10 Hz, the average site amplifications were influenced by the high-frequency attenuation effect. The site amplification of site class D reached a factor of 6 at 0.7 Hz. Moreover, it was observed that site amplification factors can be even higher when peak ground acceleration is larger than 0.8 m/s2. Finally, the obtained parameters were used in the stochastic finite-fault simulation method to reproduce the FAS, 5%-damped pseudospectral acceleration, and time series of the 2021 Mw 6.1 Yangbi earthquake. The simulated spectra properly matched the observations in a broad frequency band of 0.1–25 Hz. Furthermore, the simulated time series could generally represent the amplitude of the S-wave portion of observed ones.

Stress Drops of Intermediate‐Depth and Deep Earthquakes in the Tonga Slab

AbstractMultiple physical mechanisms have been proposed to explain the cause of intermediate‐depth and deep earthquakes, but they are still under debate. Source parameters such as stress drop, have the potential to provide insight into their physical mechanisms. We develop a modified spectral decomposition method to analyze 1‐year seismic data from temporary land‐based and ocean bottom seismographs in a complex subduction zone. By applying this method to investigate 1,083 intermediate‐depth and deep earthquakes in the Tonga slab, we successfully resolve the source spectra and stress drops of 743 MW 2.6–6.0 earthquakes. Although the absolute stress drops are subject to the choices of source model parameters, the relative stress drops are more reliably resolved. The median stress drop of Tonga earthquakes does not change with respect to magnitude but decreases with depth by 2–3 times in two separate depth ranges of 70–250 and 400–600 km, corresponding to intermediate‐depth and deep earthquakes, respectively. The median stress drops show spatial variations, with two high‐stress‐drop (five times higher than the ambient value) regions, coinciding with strong local deformation where the Tonga slab bends or tears. In the Tonga double seismic zone at 120–300 km depths, the median stress drop appears smaller in the lower plane than in the upper plane, suggesting a slower rupture velocity or a higher fluid content in the lower‐plane region. Our results suggest that intermediate‐depth and deep earthquakes in the Tonga slab generally follow the earthquake self‐similar model and favor the fluid‐related embrittlement hypothesis for both groups of earthquakes.