Stress Drop Values Research Articles

Estimation of source characteristics, site effects and path attenuation in the Sichuan Basin, China, using a non-parametric generalized inversion technique

SUMMARY A non-parametric generalized inversion technique for the S-wave amplitude spectrum on both the horizontal and vertical components was adopted to calculate the source parameters, site effects and path attenuations. In this scheme, 1647 three-component records corresponding to 78 earthquakes with Ms values of 2.9–6.0 recorded by 58 strong-motion stations in the Sichuan Basin were used. In the non-parametric generalized inversion technique, a reference station was adopted to remove the trade-off between the site and source terms. Then, the simple model R−0.5 was selected to describe the geometric spreading characteristics of the Sichuan Basin region. The frequency-related quality factor models for the horizontal and vertical components are estimated as Q(f) = 129.9872f 1.1119 and Q(f) = 132.54f 1.1236, respectively. In addition, the inverted source spectra are well matched with Brune's model, in which the stress drop values range between 0.3 and 3.5 MPa with a mean value of 1.36 MPa. In addition, a comparison between the site effects estimated from horizontal-to-vertical and non-parametric generalized inversion methods shows that the geological environment significantly amplifies the vertical component of ground motion. Finally, a residual related to distance and magnitude indicates that there is no obvious dependence of the scatter on distance or magnitude. The source, path and site parameters estimated in this inversion can be adopted in ground motion simulations, which could aid in the study of seismic disasters and risk assessment in the Sichuan Basin.

Rupture process of the 2019 Mw 6.8 Davao Del Sur earthquake, Philippines from geodetic and seismic data

Because of its complex tectonic setting, the Philippines experiences vigorous seismic activity. On December 15, 2019, an Mw 6.8 earthquake struck Davao del Sur, the Philippines, severely damaging infrastructure and adversely affecting the population. In this study, we used Interferometry Synthetic Aperture Radar (InSAR) and teleseismic broadband data to explore the earthquake’s source parameters, rupture process, and tectonic characteristics through a Bayesian modeling approach and Coulomb stress analysis. An analysis of InSAR suggested that the mainshock occurred on an NW–SE oriented single plane within the Cotabato fault system, with the Makilala-Malungon fault being the causative fault. The Joint inversion revealed that the rupture initiated in the middle crust (depth: approximately 16.5 km), exhibiting a blind, left-lateral strike-slip fault rupture propagation with minimal reverse slip and no surface rupture. The slip was concentrated in a distinct asperity spanning a depth of 3–24 km, with the peak slip reaching approximately 1.8 m. The source time function of this event indicated a single pulse with a total source duration of approximately 17 s. The static stress drop value was low, which may be related to the brittle condition of the crust. Coulomb stress changes calculations suggested earthquake risk in the future.

Source parameter and tectonic implications of small earthquakes originating in South India

A data collection of 80 local earthquakes (1.8 < ML < 3.5) recorded by several seismic stations beneath South India from February 2009 to October 2012 was studied to estimate the source parameter characteristics. The result shows that the seismic moments (M0) vary from 8.95×1011to 6.56×1013 Nm, while source radii (r) are between 120 and 150 m. The source radius seems to be independent of magnitude and smaller within a major part of the region. This can be due to local earthquakes that may originate in the region from either the brittle shear-failure mechanism on faults or the presence of weakened zones in this region. The estimated stress drops values range from 0.20 to 3 MPa for most of the events and shows an increasing trend with the seismic moment, indicating a wide range of strength of crustal rocks. Few lower crustal event exhibits slightly elevated stress drop (4–10 MPa) values, and these cannot be solely attributed to a single model; instead, it appears that the potential contributing factors vary area wise. The corner (fc) and high cut (fmax) frequency values are bit scattered with the seismic moment, and the possible explanation would be either a complex rupture process or the involvement of a long period spectrum in the component. Both fmax and fc show a decreasing trend against seismic moments, indicating that both are caused by a source process and independent of epicentral distances and focal depths. However, source displacement D(0.006 and 0.04 m) and radiated seismic energy (Es) increases linearly with the seismic moment and is an indication of the size dependency feature. We established various empirical relationships between source parameters, including MW - ML and log(M0) - ML and proposed the ML- MW relationship for the study region, which is MW∝ 0.62ML. Overall, the present study indicates that most source parameters tend to vary with the size of the earthquake and generally follow the global model of small magnitude earthquakes. The information, we gained through this study provides insight into earthquake size, source physics, and that can help the scientific community significantly, to better understand, mitigate, and respond to the seismic hazards posed by earthquakes in the studied area.

Analysis and Comparison of Fourier and Wavelet Transforms: Application for the Study of Seismic Source Parameters, Case of the 2001 Arequipa Earthquake

The purpose of this study is to apply the spectral analysis technique to the aftershocks of the 2001 Arequipa earthquake (Mw=8.4), based on the Fourier and Wavelet transforms.The data corresponds to the world network (IRIS), which has a station called NNA that is located in Peru. Together, 10 records corresponding to replicas registered during a period of 10 days after the main event have been analyzed. The methodology used in this study was based on the SAC software that stands for "Seismic Analysis Code". As a result of the Fourier transform, the fracture radius values obtained are in the range of 0.21 to 1.15km, the stress drop values for the analyzed aftershocks are between 0.60 to 38.08Mpa, the Energy Magnitude is from 3.36 to 4.42Me; for Wavelet: the frequency content which presents the values between 0.18 to 0.30 Hz, The maximum radiated energy between 25.70 to 40.80, the values obtained are in the range of 92.18 to 146.12s. The analysis and comparison of the transforms showed that with the Fourier transform different parameters can be determined by applying spectral analysis, however, with the wavelet transform the maximum radiated energy was determined This study contributes to the literature by finding these parameters of both transforms as it is of vital importance to determine the zones of energy accumulation that would cause earthquakes of great magnitude in the future.

Assessing seismic hazard and uncertainty in Büyükçekmece using ground motion simulations

This study presents a comprehensive seismic hazard assessment for Büyükçekmece, a district in Istanbul, Turkey, situated near the seismically active North Anatolian Fault (NAF). The study utilizes stochastic ground motion simulations with the validated EXSIM algorithm to understand the potential impact of medium to large-magnitude earthquakes (ranging from MW 6.3 to 7.42) on this vulnerable community. The research employs a site-specific approach, considering unique amplification factors for each location. By conducting 50 scenario-based simulations, the study assesses the seismic hazard, highlighting the importance of comprehending variations in ground motion, even when they are small, for a more precise hazard assessment. Furthermore, this study addresses the critical issue of uncertainty, particularly concerning stress parameters and hypocenter locations. The researchers demonstrate that variability in these factors can introduce substantial uncertainty in ground motion predictions. The study provides insights into the range of potential ground motion outcomes through probabilistic assessments involving multiple scenarios and stress drop values. Notably, the results indicate that ground motion levels vary with earthquake magnitudes and underscore the significance of accounting for this variability. This research emphasizes the seismic vulnerability of Büyükçekmece and the importance of accurate ground motion simulations, offering valuable insights for earthquake preparedness and mitigation efforts in the region.

Different earthquake nucleation conditions revealed by stress drop and b-value mapping in the northern Chilean subduction zone

Stress drop is an earthquake property indicative for the characteristic relation of slip to fault dimension. It is furthermore affected by fault strength, fault topography, the presence of fluids, rupture size, slip, and velocity. In this article, the stress drop image of an entire subduction zone, namely for the seismically highly active northernmost part of Chile, is combined with mapped b-values and their corresponding magnitude distribution in order to better constrain the conditions under which earthquakes of different provenances may nucleate. The underlying recent earthquake catalog contains over 180,000 events, covering 15 years of seismicity, from which more than 50,000 stress drop estimates were computed. Their spatial average segments the subduction zone into different parts, i.e., average stress drop between seismotectonic areas is different, although this difference is small compared to the natural scatter of stress drop values. By considering stress drop variations, b-value map, magnitude distribution, and thermal models, candidate earthquake nucleation mechanisms are identified which can explain the observed distributions. This is done for two exemplary regions: (1) The plate interface, where principally lower stress drop events are found, while at the same time a high spatial heterogeneity of stress drop values is observed. This indicates relatively smooth or lubricated rupture surfaces, and locally it suggests the existence of alternating regions controlled by strong asperities, weaker material, or creep. (2) The highly active intermediate depth (ID) seismicity region, where the variation of stress drop and b-value point to a gradual change of nucleation mechanism from dehydration embrittlement at the top of the ID cloud, over dehydration driven stress transfer in its central part, to thermal runaway shear mechanisms at its bottom. In both cases, the combination of stress drop and b-value distribution helps to better understand the origin and the differences of the observed seismicity.

Fluid‐Induced Aseismic Slip May Explain the Non‐Self‐Similar Source Scaling of the Induced Earthquake Sequence Near the Dallas‐Fort Worth Airport, Texas

AbstractNumerous studies have reported the occurrence of aseismic slip or slow slip events along faults induced by fluid injection. However, the underlying physical mechanism and its impact on induced seismicity remain unclear. In this study, we develop a numerical model that incorporates fluid injection on a fault governed by rate‐and‐state friction to simulate the coupled processes of pore‐pressure diffusion, aseismic slip, and dynamic rupture. We establish a field‐scale model to emulate the source characteristics of induced seismicity near the Dallas‐Fort Worth Airport (DFWA), Texas, where events with lower‐stress drops have been observed. Our numerical calculations reveal that the diffusion of fluid pressure modifies fault criticality and induces aseismic slip with lower stress drop values (&lt;1 MPa), which further influence the timing and source properties of subsequent seismic ruptures. We observe that the level of pore‐pressure perturbation exhibits a positive correlation with aseismic‐stress drops but a reversed trend with seismic‐stress drops. Simulations encompassing diverse injection operations and fault frictional parameters generate a wide spectrum of slip modes, with the scaling relationship of moment (M0) with ruptured radius (r0) following an unusual trend, , similar to observed in the DFWA sequence. Based on the consistent scaling, we hypothesize that the lower‐stress‐drop events in the DFWA may imply less dynamic ruptures in the transition from aseismic to seismic slip, located in the middle of the broad slip spectrum, as illustrated in our simulations.

A Comprehensive Stress Drop Map From Trench to Depth in the Northern Chilean Subduction Zone

AbstractWe compute stress drops for earthquakes in Northern Chile recorded between 2007 and 2021. By applying two analysis techniques, (a) the spectral ratio (SR) method and (b) the spectral decomposition (SDC) method, a stress drop map for the subduction zone consisting of 51,510 stress drop values is produced. We build an extended set of empirical Green’s functions (EGF) for the SR method by systematic template matching. Outputs are used to compare with results from the SDC approach, where we apply cell‐wise obtained global EGF's to compensate for the structural heterogeneity of the subduction zone. We find a good consistency of results of the two methods. The increased spatial coverage and quantity of stress drop estimates from the SDC method facilitate a consistent stress drop mapping of the different seismotectonic domains. Albeit only small differences of median stress drop, strike‐perpendicular depth sections clearly reveal systematic variations, with earthquakes at different seismotectonic locations exhibiting distinct values. In particular, interface seismicity is characterized by the lowest observed median value, whereas upper plate earthquakes show noticeably higher stress drop values. Intermediate depth earthquakes show comparatively high average stress drop and a rather strong depth‐dependent increase of median stress drop. Additionally, we observe spatio‐temporal variability of stress drops related to the occurrence of the two megathrust earthquakes in the study region. The presented study is the first coherent large scale 3D stress drop mapping of the Northern Chilean subduction zone. It provides an important component for further detailed analysis of the physics of earthquake ruptures.

Ground motion simulations of the 1887 Pinal de Amoles (MW = 5.53) and the 1989 Landa de Matamoros (MW = 4.94) earthquakes, Querétaro, Mexico

Ground motions characteristics of the 1887 Pinal de Amoles (MW = 5.53) and the 1989 Landa de Matamoros (MW = 4.94) Querétaro earthquakes were studied. For this purpose, the specific barrier model (SBM) was implemented in the context of the stochastic ground motion representation of earthquakes. The SBM was calibrated by modeling the peak ground acceleration (PGA) and the Fourier acceleration spectra (FAS) of the 11 May 2016 (MW = 4.80) Guadalajara earthquake. The calibration results showed that the SBM and selected source and path parameters were able to reproduce synthetic PGA and spectra in good agreement with observed waveforms; in particular, a stress drop of 160 bars fits the results better. Several rupture scenarios for the 1887 and 1989 events were performed by obtaining synthetic seismograms considering different values of stress drop (10, 30, 50, 100, 150, 200, 300, and 500 bars). For a typical stress-drop value of 100 bars, the PGA at distances of 10 and 200 km are in the following intervals: 2.57–295 cm/s2 and 1.72–186 cm/s2 for the Pinal de Amoles and Landa de Matamoros earthquakes, respectively. The results obtained in this study contribute toward a better understanding of strong ground motions in the region.

Dynamic Rupture Models of the 2016 ML 5.8 Gyeongju, South Korea, Earthquake, Constrained by a Kinematic Rupture Model and Seismic Waveform Data

ABSTRACT The ML 5.8 earthquake that jolted Gyeongju in southeastern Korea in 2016 was the country’s largest inland event since instrumental seismic monitoring began in 1978. We developed dynamic rupture models of the Gyeongju event constrained by near-source ground-motion data using full 3D spontaneous dynamic rupture modeling with the slip-weakening friction law. Based on our results, we propose two simple dynamic rupture models with constant strength excess (SE) and slip-weakening distance (Dc) that produce near-source ground-motion waveforms compatible with recorded ones in the low-frequency band. Both dynamic models exhibit relatively large stress-drop values, consistent with previous estimates constrained by source spectrum analyses. The fracture energy estimates were also larger than those predicted by a scaling relationship with the seismic moment. The dynamic features constrained in this study by spontaneous rupture modeling and waveform comparison may help understand the source and ground-motion characteristics of future large events in southeastern Korea and thus the seismic hazard of the region.

Electro-plastic effect in Ti-6Al-4V: An experimental and numerical study

Electric pulse aided deformation is gaining importance in plastic deformation processes because of its ability to form difficult-to-form materials like Ti-6Al-4V at much lower temperatures than hot/superplastic forming processes. Applying electric pulses with suitable parameters during plastic deformation reduces the flow stress near instantaneously (stress-drop) due to thermal (expansion and softening) and electro-plastic effects. To quantify the electro-plastic effect, one needs to predict thermal effects accurately. In the present work, electrically assisted uniaxial tensile tests on Ti-6Al-4V are carried out both in elastic and plastic regions. Flow stress reduction due to thermal effects are predicted using finite element analysis. Comparison of predicted thermal effects with that of experimentally measured in elastic region revealed that they are in excellent agreement, as it is well known that thermal expansion only plays a role in the elastic region. In the plastic region, a considerable difference between measured (thermal and athermal) and predicted (only thermal effects) stress-drop values is observed, and this difference is due to the electro-plastic effect. The effect of different process parameters on electro-plastic effect is studied, and the same is quantified.

STRESS ANALYSIS AND CHARACTERISTICS DUE TO THE SOUTH JAVA EARTHQUAKE, APRIL 10, 2021

The April 10, 2021, earthquake in the south of East Java was classified as destructive. The secondary impact of this earthquake was quite significant. Many houses collapsed, and not a few casualties. This earthquake is unique because usually, destructive earthquakes occur at shallow depths, but earthquakes with a magnitude of 6.1 are classified as medium-depth earthquakes at sea. The earthquake in the south of East Java is classified as an intraplate earthquake because it is located on the continental plate, not in the plate contact area. The question is whether the damage that occurred to the building was purely due to the magnitude of the stress released by the earthquake or whether there were other factors. This study uses seismogram data for the earthquake south of East Java on April 10, 2021, with a radius (∆) of 300-1000 recorded at MEEK, MORW, and ARMA stations in Australia. It calculates the amount of stress based on the stress drop, while the stress column determines the stress mechanism. Calculation of stress drop from the source spectrum is obtained by the deconvolution method, namely the seismogram component separation technique in the form of Source (f), Path (f), Site (f), and Instrument (f). The analysis of the observed displacement spectrum used the Nelder Mead Simplex nonlinear inversion method. Meanwhile, the Stress Columb calculation was obtained using the Columb 3.3 program from the United States Geological Survey (USGS). The result of this research is that the stress drop value is 1.69 MPa, with the type of focus mechanism being a thrust fault in the sea. The earthquake in the south of East Java was caused by rock activity in the intraplate. The value of the stress drop is more significant when compared to the subduction contact area. This area is of intraplate rock with various variations, and earthquakes are rare. This study aims to analyze the stress, both the magnitude of the stress drop and the mechanism of the column stress results, so that the stress caused by the earthquake can be known and why the earthquake in the south of East Java is destructive. The quake in Southeast Java is classified as dangerous, not because of the magnitude of the stress generated or its mechanism. The damage was due to the amplification of earthquake waves in the building. The injury occurred because most of the buildings were built on soft soil, especially in several areas in East Java, such as Lumajang, Pasuruan, Trenggalek, Probolinggo, Ponorogo, Jember, Tulunggagung, Nganjuk, Pacitan, and several urban areas, namely Blitar, Kediri, Malang, and Stone. So, there is a need for earthquake disaster mitigation, especially in densely populated areas that live on soft soil. This mitigation effort is to minimize the occurrence of casualties by building buildings according to earthquake-resistant standards and avoiding development in the regions that have the potential for amplification of earthquake waves.

SLIP SOURCE MODEL OF THE 1995 NEFTEGORSK EARTHQUAKE (NORTH SAKHALIN) FROM GEODETIC DATA

The May 27, 1995 Mw=7.0 Neftegorsk earthquake occurred in the north of Sakhalin Island, rupturing the Upper Piltun fault, a secondary feature of the main Hokkaido-Sakhalin regional fault zone. The fault geometry, coseismic slip model, and Coulomb stress changes in the earthquake focal area were calculated based on a finite fault modeling. We used near-field coseismic offsets at 24 points obtained by comparison between predating triangulation and GPS observations, which were collected before and after the earthquake. Our slip distribution model shows two major slip patches. Larger slip asperity (amplitude up to 6.36 m) was characterized by right-lateral strike-slip movements, which correspond to focal mechanism of the earthquake, whereas the northern segment has reverse fault mechanism with maximum slip of 2.64 m. The fault length and width, average slip and stress drop values are estimated at 78 km, 28 km, 1.91 m and 11.3 MPa, respectively. The estimated release moment is approximately 7.49×1019 N∙m equal to Mw=7.2, which is larger than that reported by the USGS and GCMT but consistent with the values reported by other researchers. The coseismic Coulomb stress changes enhanced the stress by more than 10 MPa on the southern segment of the Gyrgylaninsky fault and middle section of the Hokkaido–Sakhalin fault. Seismic risks on the nearest faults cannot be ignored in the future despite the fact that the earthquake with a magnitude of 5.8 occurred in 2010 near the Gyrgylaninsky fault. The recent GPS rates in the surroundings of the Neftegorsk surface rupture mean that the recurrence interval for similar earthquakes may be more than a thousand years.

Spectral Characteristics of Hydraulic Fracturing-Induced Seismicity Can Distinguish between Activation of Faults and Fractures

Abstract Analysis of earthquake spectra can aid in understanding source characteristics like stress drop and rupture complexity. There is growing interest in probing the similarities and differences of fault rupture for natural and human-induced seismic events. Here, we analyze waveform data from a shallow, buried geophone array that recorded seismicity during a hydraulic fracturing operation near Fox Creek, Alberta. Starting from a quality-controlled catalog of 4000 events between magnitude 0 and 3.2, we estimate source-spectral corner frequencies using methods that account for the band-limited nature of the sensor response. The stress-drop values are found to be approximately self-similar, but with a slight magnitude dependence in which larger events have higher stress drop (∼10 MPa). Careful analysis of the relative corner frequencies shows that individual fault and fracture segments experienced systematic variations in relative corner frequency over time, indicating a possible change in the stress state. Clustering analysis of source spectra based on the relative proportion of high- and low-frequency content relative to the Brune model further shows that event complexity evolves over time. In addition, the faults produce earthquakes with systematically larger stress-drop values than the fractures. Combined, these results indicate that the features activated by hydraulic fracturing experience observable changes in source behavior over time and exhibit different properties depending on the orientation, scale, and fabric of the structural feature on which they occur.

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.

Megathrust Stress Drop as Trigger of Aftershock Seismicity: Insights From the 2011 Tohoku Earthquake, Japan

AbstractNumerous normal‐faulting aftershocks in subduction forearcs commonly follow large megathrust earthquakes. Postseismic normal faulting has been explained by stress changes induced by the stress drop along the megathrust. However, details of forearc stress changes and aftershock triggering mechanisms remain poorly understood. Here, we use numerical force‐balance models combined with Coulomb failure analysis to show that the megathrust stress drop supports normal faulting, but that forearc‐wide aftershock triggering is feasible within a narrow range of megathrust stress drop values and preseismic stress states only. We determine this range for the 2011 Tohoku earthquake (Japan) and show that the associated stress changes explain the aftershock seismicity in unprecedented detail and are consistent with the stress released by forearc seismicity before and after the earthquake.

Eastward expansion of the Tibetan plateau: Insights from stress drops of the 2021 Ms 6.4 Yangbi, Yunnan and Ms 7.4 Maduo, Qinghai earthquake sequences in China

The eastward expansion of the Tibetan Plateau has resulted in extensive seismic activities in the eastern Tibetan Plateau, along with various types of faulting processes across this region. However, the crustal stress status related to these phenomena remains unclear. On 21 May 2021, the Ms 6.4 Yangbi and Ms 7.4 Maduo earthquakes with diverse focal mechanisms occurred consecutively in the Chuandian and Bayan Har blocks in the southern and northern parts of the eastern Tibetan Plateau, providing a distinct opportunity to directly compare the stresses in both flanks of the laterally expanding plateau. In this study, we estimate the stress drops of the Yangbi and Maduo earthquake sequences for all M≥3.0 events from the Lg-wave spectra. Both the path attenuation and site effect are properly removed based on an established broadband Lg-wave attenuation model. Then, the distribution of the stress drops is refined through high-precision earthquake relocation. Quick decays in stress drops were observed after both mainshocks. The Yangbi sequence has a relatively high median stress drop value, with significantly high stress drops for some foreshocks that possibly indicate a cascade triggering mechanism for the nucleation of the mainshock. In comparison, the Maduo sequence is characterized by a higher stress drop for the mainshock and sustained large fluctuation in stress drops for aftershocks. The Maduo aftershocks extend both the eastern and western directions from the mainshock, with relatively low stress drops in the east, suggesting a large amount of energy has been spent to drive the supershear rupture during the mainshock. We also find that the stress drops are associated with fault junctions for the Yangbi sequence, and are associated with variations in strength along the fault for the Maduo sequence, which are very likely linked with patterns of crustal motion and deformation in the eastern Tibetan Plateau.

О сброшенных напряжениях в очагах умеренных и слабых землетрясений: особенности распределения во времени

An analysis has been performed of the change in stress drops over time during the period of foreshock activity of strong earthquakes for two seismically active regions with different geodynamic settings: the Northern Tien Shan and the Southern Kuril Islands. The catalogs of earthquake dynamic parameters, DP (source ones in English publications), in these regions, with a number of events, were used as initial data. The DP catalog for the Northern Tien Shan includes 183 records of source parameters of earthquakes with magnitudes of 2.6–6.0, and the catalog for the Southern Kurils – 264 records. The stress drop values throughout a general sampling were analyzed as well as that in foreshock periods of 500 days length before the strongest earthquakes. For each region 12 such meaningful events have been specified, the magnitudes were М &gt; 5 for the Northern Tien Shan, and М ≥ 6.5 for the Southern Kurils. The median average values of stress drops during 500-day period have been determined. The temporal variations of stress drops have been compared with changes in the b-value parameter (angular coefficient of earthquake recurrence plot) in the same observation periods. The computation of b-value for the case of the Northern Tien Shan involved the catalog data of KNET seismological network (1994–2021, more than 10 000 events), and the catalog of International Seismological Center (ISC, 1964–2000) for the Southern Kurils. In both cases, b-values were determined in 500-day moving interval with one day step. The computation gave the result that the well-known effect of b-value growth before strong earthquakes manifested itself explicitly in the considered regions. It has been established that such increase in b-value is accompanied by a decrease in the averaged stress drop values. The obtained results showed that the monitoring of the stress drop values can be used to identify the non stationary stage of the seismic regime.

Source parameters and scaling relations for small earthquakes in mainland Gujarat region of Western Deccan Volcanic Province, India

The return period of large earthquakes in intraplate regions is more in comparison to the interplate regions. Also, the intraplate region is less instrumented and hence, these regions are devoid of acceleration time histories of large and great earthquakes. The mainland Gujarat region is one of such intraplate regions for which acceleration time histories are not available, however, is vulnerable for earthquakes of M ∼ 6.0. Simulation is performed in these scenarios and the prerequisite is the knowledge of source parameters and scaling laws. We analyzed the spectra of the P- and S-waves recorded by the stations of Gujarat Seismic Network for the mainland Gujarat region of western Deccan Volcanic Province, India. Additionally, we developed scaling laws based on the obtained source parameters. We analyze 177 digital records from 43 local earthquakes during the period 2007–2016 with a magnitude of Ml = 2.5–4.3, which are located in a distance range of 10–250 km, using Brune’s seismic source model (Brune 1970, 1971). According to our study, the calculated stress drop values do not vary considerably with seismic moment values and are generally between 1 and 100 bar for most of the events. Therefore, the earthquakes in this region adhere to the law of self-similarity. As widely observed for conventional earthquakes, we find an inverse relationship between seismic moment and corner frequency for these events. Based on a linear regression analysis between the assessed seismic moment (M 0) and corner frequency (fc), we developed the scaling relation M 0 f c 3 = 2.29 × 1015. Scaling relations developed here are in agreement with those developed for the nearby Kachchh region and various other seismically active areas around the world. Use of regional scaling is recommended instead of global scaling laws for better estimation of seismic hazard. The source parameters and scaling laws presented here can be used for a variety of applications in mainland Gujarat, including the assessment of seismic hazards and the development of earthquake scenarios.

Parametric spectral inversion of seismic source, path and site parameters: application to northeast Italy

SUMMARY Strong ground motion prediction is a fundamental topic in the field of engineering seismology, as it provides the input for seismic hazard studies as well as for vulnerability and risk assessment. The spectral modelling approach can provide a realistic representation of ground motion behaviour, possibly including its frequency variability, as the full ground motion spectrum is modelled analytically. In its parametric form, this approach requires a careful calibration of the model, starting from empirical observations and fitting the source, path and the site-specific response assuming a predefined physically constrained functional form. This study explores the use of spectral modelling for a study area in northeast Italy, at the border with Slovenia and Austria. It is based on the parametrization of seismic source and attenuation effects, and it also allows to estimate site effects, as a by-product. The main innovation with respect to standard spectral modelling is the inclusion of dedicated uncertainty estimators in the functional form. Parametric inversion of source and path attenuation is performed on a data set corresponding to 23 events recorded by 24 stations located within the target area. The modular inversion setup allows to properly include a priori constraints in the mathematical solution to reduce trade-off between variables. Spectral amplification at each site is defined with respect to the network average rock condition, and its frequency-dependent component is estimated from residual analysis after the inversion. Inverted source parameters are comparable with reference values for the region available from literature (with seismic moments between ${10}^{13}$ and ${10}^{15}$ N·m, and related stress drop values in the range $1.5 - 15.5\ {\rm{MPa}}$); the same is also true for average attenuation properties (e.g. apparent frequency-independent attenuation quality factor ${Q}_0$ of $1145$). For a selection of stations with available characterization based on different methods, a preliminary comparison of site-specific response functions shows that both the frequency value and amplitude of the main amplification peaks are well recovered. These encouraging results open to the possibility of subsequently using the calibrated model for forward modelling purposes.