Earthquake Model Research Articles

A Diffuse Interface Method for Earthquake Rupture Dynamics Based on a Phase‐Field Model

AbstractIn traditional modeling approaches, earthquakes are often depicted as displacement discontinuities across zero‐thickness surfaces embedded within a linear elastodynamic continuum. This simplification, however, overlooks the intricate nature of natural fault zones and may fail to capture key physical phenomena integral to fault processes. Here, we propose a diffuse interface description for dynamic earthquake rupture modeling to address these limitations and gain deeper insight into fault zones' multifaceted volumetric failure patterns, mechanics, and seismicity. Our model leverages a steady‐state phase‐field, implying time‐independent fault zone geometry, which is defined by the contours of a signed distance function relative to a virtual fault plane. Our approach extends the classical stress glut method, adept at approximating fault‐jump conditions through inelastic alterations to stress components. We remove the sharp discontinuities typically introduced by the stress glut approach via our spatially smooth, mesh‐independent fault representation while maintaining the method's inherent logical simplicity within the well‐established spectral element method framework. We verify our approach using 2D numerical experiments in an open‐source spectral element implementation, examining both a kinematically driven Kostrov‐like crack and spontaneous dynamic rupture in diffuse fault zones. The capabilities of our methodology are showcased through mesh‐independent planar and curved fault zone geometries. Moreover, we highlight that our phase‐field‐based diffuse rupture dynamics models contain fundamental variations within the fault zone. Dynamic stresses intertwined with a volumetrically applied friction law give rise to oblique plastic shear and fault reactivation, markedly impacting rupture front dynamics and seismic wave radiation. Our results encourage future applications of phase‐field‐based earthquake modeling.

Probabilistic Tsunami Hazard Assessment Considering the Sequence of the First and Second Earthquakes Along the Nankai Trough

The Earthquake Research Committee (ERC) of the Headquarters for Earthquake Research Promotion conducted a probabilistic tsunami hazard assessment due to large earthquakes along the Nankai Trough for the next 30 years. Utilizing the basic data of earthquake source models and calculated maximum tsunami heights, the authors propose a method to evaluate the hazard curves of the first and second earthquakes separately, considering the sequence of earthquake occurrence in one cycle of large earthquake activities along the Nankai Trough. First, based on the relative weights allocated to the 176 occurrence patterns of 79 earthquake source regions, the weights for the 2,720 characterized earthquake fault models (CEFMs) are calculated. The hazard curve of the first earthquake is evaluated using the 2,720 sets of maximum tsunami heights and weights under the condition that one of the CEFMs causes an earthquake. Next, the conditional hazard curves for the possible second earthquakes conditional on each individual first earthquake are calculated. Finally, the hazard curve for the second earthquake is evaluated as a weighted average of the conditional hazard curves. Numerical examples are shown for 15 sites. The first earthquake accounts for about 60% or more of the total hazard evaluated by ERC, and its contribution increases with increasing maximum tsunami height. The first and second earthquakes account for 80%–90% of the total hazard.

Site classification scheme based on geological age and genesis for Xinjiang and the Capital Metropolitan areas of China

The site classification map is one of the vital inputs in earthquake disaster scenario prediction and risk modeling that provides a perspective of proxy-based methods. In China, a systematic empirical relationship between geological age, genesis, and site class in the practice of site classification has yet to be established. In this study, data from 1:500,000 geological maps and thousands of engineering boreholes in the Xinjiang Uygur Autonomous Region and the Capital Metropolitan area in China were collected, and different types of geological unit were classified according to 10 types of geological age and 18 types of geological genesis. A site classification scheme was proposed on the basis of geological age, geological genesis, statistical properties of boreholes, and lithological descriptions. Borehole characteristics were investigated in terms of their proportions among the different site classes, and the mean and standard deviation of V S30. Accordingly, a chart correlating four site classes (B, C, D, and E) with each type of geological age and genesis was developed that could be used as a practical tool in regional site classification. Site classification maps were delineated in both areas using the proposed scheme and compared with those derived from the topographic slope-proxy method and another geology-proxy method, and the results showed that it is effective in identifying the site class, especially for class E sites, and probably class B that inferred by the qualitative evaluation. Moreover, the applicability of the method has also been confirmed in areas outside the study area. It is anticipated that it could provide a technical template for nationwide site classification using geological maps.

Real‐Time Fault Tracking and Ground Motion Prediction for Large Earthquakes With HR‐GNSS and Deep Learning

AbstractEarthquake early warning (EEW) systems aim to forecast the shaking intensity rapidly after an earthquake occurs and send warnings to affected areas before the onset of strong shaking. The system relies on rapid and accurate estimation of earthquake source parameters. However, it is known that source estimation for large ruptures in real‐time is challenging, and it often leads to magnitude underestimation. In a previous study, we showed that machine learning, HR‐GNSS, and realistic rupture synthetics can be used to reliably predict earthquake magnitude. This model, called Machine‐Learning Assessed Rapid Geodetic Earthquake model (M‐LARGE), can rapidly forecast large earthquake magnitudes with an accuracy of 99%. Here, we expand M‐LARGE to predict centroid location and fault size, enabling the construction of the fault rupture extent for forecasting shaking intensity using existing ground motion models. We test our model in the Chilean Subduction Zone with thousands of simulated and five real large earthquakes. The result achieves an average warning time of 40.5 s for shaking intensity MMI4+, surpassing the 34 s obtained by a similar GNSS EEW model. Our approach addresses a critical gap in existing EEW systems for large earthquakes by demonstrating real‐time fault tracking feasibility without saturation issues. This capability leads to timely and accurate ground motion forecasts and can support other methods, enhancing the overall effectiveness of EEW systems. Additionally, the ability to predict source parameters for real Chilean earthquakes implies that synthetic data, governed by our understanding of earthquake scaling, is consistent with the actual rupture processes.

Estimation of Uncertainty in the Average Rate of Earthquakes Exceeding a Magnitude Threshold

AbstractWe propose a method to estimate the uncertainty of the average rate of earthquakes exceeding a magnitude threshold in a future period of given length based on observed variability of the earthquake process in an existing catalog. We estimate the ratio R of the variability to that of a stationary Poisson process. R is estimated from subsets of the catalog over a wide range of timescales. The method combines the epistemic uncertainty in estimating the rate from the catalog and the aleatory variability of the rate in future time periods. If R is stable over many timescales, there is a solid basis for estimating the uncertainty of earthquake rate estimates. In the 2022 revision of the New Zealand National Seismic Hazard Model (NZ NSHM), estimation of the total shallow earthquake rate over the next 100 yr and its uncertainty is an important element. Using a 70 yr New Zealand catalog with hypocentral depths ≤40 km and standardized magnitudes M ≥ 4.95, we find stable estimates of R for timescales from 3 days to 2.4 yr. This gives a standard error of 0.95 on the estimated annual rate of M ≥ 4.95, in the next 100 yr. R becomes unstable and has poor precision for longer subperiods. We investigate potential causes using synthetic catalogs with known inhomogeneities. Analysis of International Seismological Centre-Global Earthquake Model (ISC-GEM) catalog, to investigate the effect of higher magnitude thresholds, shows that R is lower for M ≥ 6.95 than for M ≥ 5.45. The ISC-GEM catalog restricted to New Zealand gives comparable stable estimates of R to the NZ NSHM 2022 catalog for M ≥ 5.45 and lower estimates than the NZ NSHM 2022 catalog for M ≥ 4.95. We also verify that magnitude standardization of the New Zealand GeoNet catalog has reduced the uncertainty of rate estimates by decreasing R throughout the entire range of timescales.

Time-Independent Grid-Based Forecast Model for M ≥6.0 Earthquakes in Southeastern Tibetan Plateau Using GNSS Strain Rates and Seismicity

ABSTRACT Earthquake forecasting models play a vital role in earthquake occurrence assessment. Despite improved availability of seismic and geodetic data and processing techniques to produce high-resolution catalogs and deformation history, the implementation of earthquake forecasting models with seismic and geodetic data remains a challenge. In this study, we utilize seismicity and Global Navigation Satellite Systems (GNSS) data to propose time-independent grid-based regional earthquake likelihood models for southeastern Tibetan plateau (RELM-TibetSE). First, we solve the GNSS velocity field and strain rates from 1999 to 2017, deriving geodetic moment rates and introducing empirical correction coefficients to balance them with historical seismic moment rate. Subsequently, we employ a truncated Gutenberg–Richter law and Poisson process to calculate time-independent probabilities for M ≥ 6 earthquakes in 0.2° × 0.2° cells. The grid-based forecasting models indicate that the 30-year probability for M ≥ 6 earthquakes exceeds 1% in more than one-third of the entire study area, highlighting prominently higher earthquake occurrence in these regions. Probability distribution exhibits significant spatial variations. Finally, the predictive performance of the forecasting models is validated based on historical seismicity. The validation indicates that all RELM-TibetSE exhibit good predictive capability relative to a spatially uniform model. The RELM-TibetSE incorporating principal strain rates outperforms those involving maximum shear strain rate in forecasting seismicity. And the differences in forecasting performance between the RELM-TibetSE accounting for spatially varied seismogenic thickness and rigidity and those with uniform thickness and rigidity are not significant. The forecasting models also exhibit better predictive performance for seismic source areas than for epicenters. Moreover, the optimal model highlights zones with higher earthquake occurrence, including the zones about 50 km wide across the Ninglang fault, the zones across the southwestern segment of the Lijiang–Xiaojinhe fault, the China–Myanmar borderland north of the Nantinghe fault, and so on. Therefore, it is justified to conduct multidisciplinary rigorous observations to capture the potential nucleation process of future large earthquakes in these zones.

Effects of Deck-Abutment Pounding on the Seismic Fragility Curves of Box-Girder Highway Bridges

ABSTRACT Earthquake-induced pounding in bridges is a complex contact phenomenon in which the dynamic responses of structures, including collisions between deck and abutments, are strongly related to structural properties and earthquake excitation. The goal of this study is to develop and compare the seismic fragility curves of overall system and individual components of regular and irregular box-girder highway bridges in two cases: with pounding and without pounding. For this purpose, four levels of altitudinal irregularity, ranging from regular to highly irregular, are considered. To extend the results for all bridges in the same class, different sources of uncertainties related to earthquakes, structural geometries, and material properties are taken into account. The analytical fragility curves have been developed based on nonlinear time history analyses in OpenSees finite element software for the cases with and without pounding effects. The process has been repeated for each two-, three-, and four-span classes at four irregularity levels. The fragility function parameters for the two cases with and without pounding have been compared for all classes considered in this study. Using fragility functions, this paper clarifies the interactive roles of irregularity and pounding between deck and abutments for seismic vulnerability of multi-span box-birder highway bridges. The results indicate that collisions often show an adverse effect on all structural components. It has also been observed that the detrimental effect of pounding on seismic fragility is more apparent in irregular bridges when compared with regular ones. In addition, the study introduces a conversion coefficient to clarify the effects of pounding on the fragility of bridge components and the overall system. This coefficient can be applied in both conventional analytical methods like static or simplified analysis and technical earthquake models like HAZUS, adjusting fragility values for pounding and irregularity effects.

Comparison of Ground-Shaking Hazard for Segmented versus Multifault Earthquake-Rupture Models in Aotearoa New Zealand

Abstract Multifault ruptures are common for historical earthquakes, and here we consider their impact on seismic hazard. We compare ground-shaking hazard forecasts from the 2022 Aotearoa New Zealand National Seismic Hazard Model (NZ NSHM 2022), which incorporates many multifault ruptures (referred to as the multifault model) with modeled hazard from a simpler model of characteristic earthquakes on individual faults or fault segments (referred to as the segmented model). The multifault model includes very-low-probability rupture lengths of up to ∼1100 km and a mean of 221–234 km, whereas the segmented model primarily comprises rupture lengths of <200 km (mean, 43–51 km) and the maximum of 414 km. The annual rates of Mw 6.9–7.5 earthquakes are more than an order of magnitude higher for the segmented model (0.132–0.24/yr; recurrence times ∼4–7 yr) than the multifault model (0.027/yr; recurrence times 37 yr). Conversely, the rates of earthquakes are similar for segmented and multifault models at Mw>7.5 (0.018–0.031/yr; recurrence times 32–56 yr). Despite differences in rupture lengths and annual rates of earthquakes, the calculated ground-shaking hazard at 10% probability of exceedance (PoE) in 50 yr for the segmented model differs by <55% compared with the multifault model for 95% of sites across Aotearoa New Zealand. For 50% of sites, the modeled hazard differs by <20% between the two models. If a distributed seismicity model (DSM) is included in the hazard calculations, 95% of sites differ in modeled hazard by <18%, and 50% of sites differ by <2.2%. In most areas, seismic hazard at 10% PoE in 50 yr is greater for the segmented model than the multifault model, with notable exceptions along the central Alpine fault in the western South Island and the Taupō volcanic zone in the central North Island.

Evidence of Seattle Fault Earthquakes from Patterns in Deep-Seated Landslides

ABSTRACT Earthquake-induced landslides can record information about the seismic shaking that generated them. In this study, we present new mapping, Light Detection and Ranging-derived roughness dating, and analysis of over 1000 deep-seated landslides from the Puget Lowlands of Washington, U.S.A., to probe the landscape for past Seattle fault earthquake information. With this new landslide inventory, we observe spatial and temporal evidence of landsliding related to the last major earthquake on the Seattle fault ∼1100 yr before present. We find spatial clusters of landslides that correlate with ground motions from recent 3D kinematic models of Seattle fault earthquakes. We also find temporal patterns in the landslide inventory that suggest earthquake-driven increases in landsliding. We compare the spatial and temporal landslide data with scenario-based ground motion models and find stronger evidence of the last major Seattle fault earthquake from this combined analysis than from spatial or temporal patterns alone. We also compare the landslide inventory with ground motions from different Seattle fault earthquake scenarios to determine the ground motion distributions that are most consistent with the landslide record. We find that earthquake scenarios that best match the clustering of ∼1100-year-old landslides produce the strongest shaking within a band that stretches from west to east across central Seattle as well as along the bluffs bordering the broader Puget Sound. Finally, we identify other landslide clusters (at 4.6–4.2 ka, 4.0–3.8 ka, 2.8–2.6 ka, and 2.2–2.0 ka) in the inventory which let us infer potential ground motions that may correspond to older Seattle fault earthquakes. Our method, which combines hindcasting of the surface response to the last major Seattle fault earthquake, using a roughness-aged landslide inventory with forecasts of modeled ground shaking from 3D seismic scenarios, showcases a powerful new approach to gleaning paleoseismic information from landscapes.

Pemodelan Percepatan Tanah sebagai Parameter Hazard Gempabumi menggunakan Metode Deterministic Seismic Hazard Analysis di Kota Manado

Research has been carried out on modeling ground acceleration as an earthquake hazard parameter in Manado. This study aims to create an earthquake modeling scenario using Deterministic Seismic Hazard Analysis to determine the estimated PGA value and the intensity value of a significant earthquake. This study utilizes the BMKG SHAKEMAP software to determine the PGA value which is then converted into a modified Mercalli intensity value. A total of 13 modeling scenarios were built in this study. The results show that the scenario of significant earthquake modeling in Manado City and its surroundings using the 2018 BMKG catalog input shows varying results. This depends on the earthquake source parameters as well as the distance from the source to the site. Scenario model 10 is the most representative because it has the same intensity value as the June 20, 1991 earthquake. The resulting PGA value shows the area that has the highest PGA in Manado comes from the 3rd model (8.1% g) and the lowest from the model to 12 (1.4 % g). Due to the limited historical intensity data from the catalog, only 8 models out of 13 models were tested for root mean square error verification ranging values from 0 to 0.25.

An offshore non-ergodic ground motion model for subduction earthquakes in Japan Trench area

With the improvement of the world’s largest seafloor observation network (S-net) and the increase in the quantity and quality of records, the ergodic assumptions can be further relaxed in the modeling of offshore ground motion models (GMMs). This allows accounting for systematic and repeatable source, site, and path effects to further understand the characteristics of offshore ground motion in the Japan Trench region. We developed an offshore ergodic backbone GMM for subduction earthquakes and classified the sites into four categories using horizontal–vertical response spectral ratio to investigate site amplification. The offshore ergodic GMM is applicable for subduction earthquake scenarios with moment magnitudes ranging from 4.0 to 7.4 and rupture distances ranging from 10 to 300 km. Comparing offshore ergodic GMMs with onshore GMMs for subduction earthquakes, we found that offshore GMMs were significantly different from onshore GMMs, especially in the long-period and unburied states. Then a new offshore non-ergodic GMM was developed based on the offshore ergodic GMM. The systematic and repeatable source and site effects were captured by the spatially varying coefficients represented by Gaussian processes, while the systematic and repeatable path effects were captured by cell-specific anelastic attenuation proposed by Dawood and Rodriguez-Marek (2013), calculated with the Cohen-Sutherland computer graphics algorithm. The non-ergodic GMM revealed systematic and repeatable source, site, and path effects that were not captured by the ergodic GMM. Moreover, the non-ergodic GMM showed reduced aleatory variability and epistemic uncertainty on ground motion estimation compared to ergodic GMM. The reduction of aleatory variability and epistemic uncertainty had a significant impact on probabilistic seismic hazard analysis. Quantifications of these results are contributed to conduct reasonable seismic design and seismic risk assessment for marine engineering in offshore regions vulnerable to strong subduction earthquakes.

Long-term patterns of earthquakes influenced by climate change: Insights from earthquake recurrence and stress field changes across the Korean Peninsula during interglacial periods

This study examined the temporal patterns of paleo-earthquake recurrences on the Korean Peninsula, situated near the Eurasian Plate boundary, by using electron spin resonance (ESR) dating to ascertain the ages at which fault gouge patches were reactivated. The findings reveal that the paleo-earthquakes in each of the fault zones (Keumwang, southern part of Wangsukcheon, Yangsan, and Ulsan) display quasiperiodic patterns. The observed patterns align with a time-dependent earthquake recurrence model. A periodicity of approximately 100 kyr could represent a regional baseline for fault activity. Over the past 650 kyr, the periodicity of paleo-earthquakes has coincided with the interglacial periods of marine isotope stages (MIS) 15, 13, 11, 9, and 7. On the Korean Peninsula, it is hypothesized that long-term differential stress changes, resulting from sea-level rises in the Pacific Ocean, have played a significant role in driving Quaternary fault activities during these interglacial stages. Consequently, this study suggests that the ongoing global warming trend could potentially increase the earthquake frequency on the Korean Peninsula.

Repeating Large Earthquakes along the Mexican Subduction Zone

Abstract Repeating large earthquakes (M ≥ 7), waveforms for which are nearly identical, have been identified only on the Mexican subduction thrust near Acapulco. These earthquakes occurred on 1962 (Ms 7.0) and 2021 (Ms 7.0, Mw 7.0). Here, we report on two more sequences of three repeating large earthquakes each in eastern and western Oaxaca, Mexico. The repeating earthquakes in eastern Oaxaca occurred on 23 March 1928 (Ms 7.5), 1965 (Ms 7.6, Mw 7.5), and 2020 (Ms 7.4, Mw 7.4), and in western Oaxaca on 4 August 1928 (Ms 7.4), 1968 (Ms 7.2, Mw 7.3), and 2018 (Ms 7.2, Mw 7.2). Galitzin seismograms of the earthquakes in each sequence at DeBilt, The Netherlands or at Strasbourg, France are strikingly similar for at least 2600 s after the P-wave arrival. Similarity of waveforms of earthquakes in each sequence and tests with seismograms of events locations for which are accurately known suggest that their source areas were less than 10–20 km of each other. Moment-rate functions of these events are remarkably simple. We also document quasi-repeating earthquakes in central Oaxaca on 17 June 1928 (Ms 7.6) and 29 November 1978 (Ms 7.6, Mw 7.6). Such events have similar locations with large overlap in primary slip but are not identical. Recently, Michoacán–Colima earthquakes of 1973 (Ms 7.5, Mw 7.6) and 2022 (Ms 7.6, Mw 7.6) were reported as quasi-repeaters. Repeating or quasi-repeating large earthquakes imply that they are known for all the other events in the sequence if we know the location and gross source parameters of one of them. This permits the estimation of recurrence periods and the delineation of seismic gaps with greater confidence. Repeating and quasi-repeating large earthquakes in Oaxaca, an unique observation, shed new light on seismic hazard of the region, provide further support for the characteristic earthquake model, and reveal remarkably persistent behavior of ruptures through multiple earthquake cycles.

Directionality and polarization of response spectral ordinates in the 2023 Kahramanmaras, Türkiye earthquake doublet

Until recently, the orientation of maximum horizontal spectral response was generally believed to not have a predominant orientation at rupture distances greater than 5 km. However, a recent study found that the orientation of maximum spectral response for strike-slip earthquakes in the NGA-West2 database tends to occur close to the epicentral transverse orientation, that is, an orientation perpendicular to a line connecting the epicenter to the station. This article investigates directionality in the 6 February 2023 Türkiye doublet earthquakes ( Mw 7.8 and 7.5) with strike-slip faulting. The orientation of the maximum response of 5%-damped linear elastic oscillators was studied. The spatial distribution of the level of polarization, which in this article refers to the amount of directionality, and intensities at specific orientations were also studied. The maximum spectral response was found to occur systematically close to the epicentral transverse orientation, consistent with previous observations for other strike-slip earthquakes. For the Mw 7.8 event where the location of maximum slip was relatively far from the epicenter, it was found that the orientation of maximum spectral response is, on average, closer to the maximum slip transverse orientation (i.e. perpendicular to a line connecting the station to the surface projection of the point of maximum slip) when compared to the epicentral transverse orientation over most period ranges. This suggests that the maximum slip transverse orientation may be a better estimator for determining the orientation of maximum spectral response in large-magnitude strike-slip earthquakes, although further study using more events is warranted. Polarized motions were observed over large geographical areas, and the orientation of maximum spectral response was found to be close to the epicentral or maximum slip transverse for Joyner–Boore distances up to the farthest studied (400 km). These findings further support the case for the development of orientation-dependent ground motion models for strike-slip earthquakes.

Ground-Motion Model for Significant Duration Constrained by Seismological Simulations

ABSTRACT A duration ground-motion model for crustal earthquakes based on the normalized Arias intensity (IA) is developed. Two sets of seismological simulations are used to constrain the form and scaling of the duration model. Simulations using a 3D crustal model show that an additive model for the source, path, and site terms captures the physical behavior of duration better than a multiplicative model for the site term. Stochastic finite-fault simulations are used to constrain the saturation of the large-magnitude scaling at short distances. The duration model is developed in two parts: a duration model for the time interval between 5% and 75% of the normalized Arias intensity (D5−75) and a duration model for the ratio of the D5−X/D5−75 duration for X values from 10 to 95. Together, these two models provide a more complete description of the evolution of the seismic energy with time than a single duration metric. A new aspect of the statistical model for duration is the inclusion of a random effect for the path term in addition to random effects for the source and site terms. The source and site random effects are modeled as scale factors on the duration, whereas the path-term random effect is a scale factor on the distance slope. The distribution of the duration residuals has a skewness that is between the skewness of a lognormal distribution and the symmetry of a normal distribution. The final duration aleatory variability is modeled by a power-normal distribution with an exponent of 0.3, which accounts for the amplitude dependence of the aleatory variability of the duration with smaller aleatory variability for large-magnitude events and larger aleatory variability for small-magnitude events as compared to the variability from a lognormal distribution.

A Damping Modification Factor Prediction Model for Horizontal Displacement Spectrum from Shallow Crustal and Upper-Mantle Earthquakes in Japan Accounting for Site Conditions

ABSTRACT This article presents damping modification factors (DMFs) for the horizontal component of the strong-motion records generated by the shallow crustal and upper-mantle earthquakes in Japan. This model can be used to scale a 5% damped design spectrum that does not associate with a known magnitude and rupture distance to obtain a design spectrum with the desirable damping ratios. Our previous study suggested that the site effect on DMF was significant, and we used site class as the site-effect parameter. We used a quadratic function of damping ratio in a natural logarithm scale to model the effect of damping ratios and we used fourth-order polynomials of the natural logarithm spectral period to present the effect of the spectral period when the spectral period is over 0.06 s. The between-event, between-site, and within-site standard deviations can be described by the linear function of the damping ratio in a natural logarithm scale. The between-event standard deviations are smaller than the within-event standard deviations and the between-site standard deviations are less than the within-site ones at many spectral periods. Reasonable displacement spectra can be obtained by using the DMF model from this study to scale the 5% damped displacement spectra. The differences in the DMF values from the three types of earthquakes are moderate at many spectral periods and the predicted DMF values from this study are similar to those from other studies at some spectral periods, but the differences are considerable at the other spectral periods. The variation trend of the DMF values suggests that the predicted DMF values may reach the theoretic value of 1.0 at some spectral periods over 5.0 s. Residual distribution analysis suggested that the bilinear function of magnitude and fault-top depth can be used in a model for scenario earthquakes.

Evaluating the Aftershock Duration of Induced Earthquakes

ABSTRACT As the carbon sequestration community prepares to scale up the number and size of commercial operations, the need for tools and methods to assess and mitigate risks associated with these operations becomes increasingly important. One outstanding question is whether aftershocks of induced events decay quickly after injection operations cease or if aftershock activity persists for hundreds of years before returning to background levels more akin to tectonic events (Stein and Liu, 2009). Appropriate estimates of the aftershock duration impact several operational management decisions including mitigation strategies and post-injection monitoring for seismic activity. It is hypothesized that induced earthquake rates may diminish more quickly after injection is stopped, owing to higher stressing rates from injected fluids. Alternatively, it is plausible that only the first event in the sequence is induced by increased fluid overpressures, whereas subsequent events (e.g., aftershocks) respond to the stored tectonic stresses and static and dynamic stress changes due to the mainshock (Keranen et al., 2013). If the aftershock duration can be linked to stressing rates due to injection, then it follows that operational strategies to reduce seismic hazard by reducing injection rates or volumes may be successful. However, if aftershocks of induced events are relieving stored tectonic stresses, then altering injection volumes may not alleviate ongoing seismic activity. Furthermore, knowledge of an aftershock duration could aid in the determination of an appropriate post-injection monitoring period for ongoing seismicity, which is a factor in overall operational costs. In this study, we model induced seismicity sequences in Oklahoma with a coupled Coulomb rate–state earthquake rate model (Dieterich, 1994; Kroll et al., 2017) to estimate aftershocks durations. Results for the current study indicate that elevated rates of aftershock activity following induced mainshocks return to background seismicity rates in less than five years, contrary to the tens to hundreds of years observed for tectonic aftershocks.

The seismic hazard from the Lembang Fault, Indonesia, derived from InSAR and GNSS data

Abstract. A growing number of large cities are located near poorly understood faults that have not generated a significant earthquake in recent history. The Lembang Fault is one such fault located near the city of Bandung in West Java, Indonesia. The slip rate on this fault is debated, with estimates ranging from 6 to 1.95–3.45 mm yr−1, derived from a GNSS (global navigation satellite system) campaign and geological measurements respectively. In this paper we measure the surface deformation across the Bandung region and resolve the slip rate across the Lembang Fault using radar interferometry (InSAR – interferometric synthetic aperture radar) analysis of 6 years of Sentinel-1 satellite data and continuous GNSS measurements across the fault. Our slip rate estimate for the fault is 4.7 mm yr−1, with the shallow portions of the fault creeping at 2.2 mm yr−1. Previous studies estimated the return period of large earthquakes on the fault to be between 170–670 years. Assuming simplified fault geometries and a reasonable estimate of the seismogenic depth we derive an estimated moment deficit equivalent to magnitude 6.6–7.0 earthquakes, indicating that the fault poses a very real hazard to the local population. Using the Global Earthquake Model OpenQuake engine we calculate ground motions for these two earthquake scenarios and estimate that 1.9–2.7 million people within the Bandung metropolitan region would be exposed to ground shaking greater than 0.3 g. This study highlights the importance of identifying active faults, understanding their past activity, and measuring the current strain rates of smaller crustal active faults located near large cities in seismic zones.

Long-Term Recurrence Pattern and Stress Transfer along the Kefalonia Transform Fault Zone (KTFZ), Greece: Implications in Seismic Hazard Evaluation

An effort is exerted to investigate the recurrence pattern of large earthquakes (Mw ≥ 6.0) in the Kefalonia Transform Fault Zone (KTFZ), Greece, by considering the incorporation of the 74-year (1948–2022) evolving stress field. Four earthquake occurrence models—the Poisson, Poisson with the incorporation of the static stress changes (Poisson + ΔCFF), Brownian passage time (BPT) and Brownian passage time with the incorporation of the static stress changes (BPT + ΔCFF)—have been applied to estimate the occurrence probabilities of nearly characteristic earthquakes for the seven fault segments of the study area. The mean recurrence time, Tr, is estimated using the physics-based seismic moment rate conservation method. The results show large variability depending upon fault parameters. Incorporating the state of stress into Tr results in both advanced and delayed recurrence patterns. The occurrence probability estimates for the next 10, 20 and 30 years indicate that the fault segment most likely to be ruptured is the Paliki North fault segment in all models. Overall, the occurrence probabilities, combined with the state of stress along the fault segments, emphasize the high seismic moment rate of the study area. The application of time-dependent models (BPT, BPT + ΔCFF) resulted in significant increases or decreases in the associated seismic hazard.

PDEM-based seismic performance evaluation of circular tunnels under stochastic earthquake excitation

ABSTRACT Tunnels are a vital component of urban infrastructure that must be robust against seismic hazards. Given the randomness of earthquake occurrence, the seismic response of tunnel structures mut be studied by stochastic analysis methods. To this end, this study proposes a probability density evolution method (PDEM)-based framework to investigate the seismic performance of a circular tunnel under stochastic earthquake excitation. First, a suite of nonstationary earthquake motions compatible with the seismic design code was derived using a stochastic earthquake model. Then, a series of nonlinear dynamic numerical simulations were conducted for a typical circular tunnel that considers the soil-structure interaction. Finally, using the tunnel inclination angle as the performance index, the probability density function of the structural response of the tunnel was solved using the PDEM to obtain the corresponding exceedance probabilities of the tunnel under various damage states. The results show that the PDEM-based framework can be applied to evaluate the seismic performance of circular tunnels and could serve as a reference on the seismic fragility of tunnels and underground structures.