Earthquake Rates Research Articles

Characteristics of the foreshock occurrence for Mj3.0 to 7.2 shallow onshore earthquakes in Japan

We use the Japan Meteorological Agency (JMA) earthquake catalogue from January, 2001 to February, 2021 to investigate the spatiotemporal foreshock occurrence for shallow (within 30 km depth) onshore earthquakes (Mj3.0 to 7.2). We find clear peaks for the numbers of small earthquakes within 10 days and 3 km prior to the larger earthquakes, which are considered as our definition of foreshocks. After removing the aftershocks, earthquake swarms and possible triggered earthquakes by the 2011 Mw9.0 Tohoku-oki earthquake, we find that for the 2066 earthquakes (mainshocks), 783 (38%) have one or more foreshocks. There is a decreasing trend of foreshock occurrence rate with mainshock depth. Also, normal faulting earthquakes have higher foreshock occurrence rate than reverse faulting earthquakes. We calculate the earthquake occurrence rate as a function of the magnitudes of foreshocks and mainshocks, and we have found no clear trend between the magnitudes of foreshocks and mainshocks.Graphical

An Efficient, Analytic Solution Using Order Statistics for Probabilistic Seismic-Hazard Assessment without the Poisson Assumption

ABSTRACT Standard approaches to probabilistic seismic-hazard assessment (PSHA) assume that earthquakes are random, independent events that follow a Poisson distribution of occurrences in a given time period (Cornell, 1968). To overcome the limitations of the Poisson assumption, such as ignoring earthquake clustering, we introduce an analytic method for PSHA that uses order statistics to allow for arbitrary distributions of earthquake occurrence. Cornell (1968) used the Poisson assumption to achieve a computationally efficient method that enables users to explore the impact of parameters used in earthquake occurrence and ground-motion models. We apply our order statistics method to the highly clustered seismicity associated with caldera collapses at Kīlauea and explore the general implications of non-Poisson behavior for PSHA. We find that non-Poisson behavior has the greatest impact for high probabilities of exceedance, low-mean rates of occurrence, and multiple exceedances. Those conditions can be important for applications such as operating standards for buildings and infrastructure engineering, standards for temporary structures and during construction, the insurance industry, the design of earthquake early warning, and to assess hazards due to clustered processes such as aftershock sequences and earthquake swarms. The commonly used rate of exceedance hides the difference between the hazard due to a non-Poisson distribution and a Poisson distribution with the same mean rate of earthquakes. Thus, including non-Poisson behavior in PSHA means that we must plot and discuss PSHA results as the probability and not the rate of exceedance.

Climatic controls on travertine deposition in southern Tibet during the late Quaternary

Large volumes of travertine deposits are preserved at hydrothermal spring sites on the Tibetan Plateau (TP). Yet, most of these deposits are under-researched with respect to their diagenetic and depositional history and there is still very limited understanding of the tectonic and climatic influences on travertine precipitation in the arid high-altitude setting of Tibet. In this study, a detailed uranium-series dating campaign was carried out for the Qiusang travertine (~4270 m above sea level), southern Tibet that has been previously dated back to 486 thousand years ago (ka). Based on 42 new 230 Th/U ages, combined with geomorphological and sedimentological investigations, several travertine zones were identified and distinct travertine depositional phases constrained: 11.7–6.8 ka and ~ 13.4 ka (zone 1), 128–122 ka (zone 2), ~193 ka (zone 3), ~292 ka and 324 ka (zone 4), >317 ka (zone 5), ca. 415 to 470 ka (zone 6) and ca. 419 to 445 ka (zone 7). Comparison of these depositional phases with local and regional proxy records suggests that travertine accumulation at Qiusang occurred during main interglacials when monsoon precipitation peaked on the TP. This coincidence, together with a sensitive response of Tibetan hydrothermal spring activity to meteoric recharge, implies that climate controls the precipitation of large travertine volumes on orbital timescales on the plateau. We propose that (i) tectonic activity is of subordinate importance and influences travertine precipitation on the TP only episodically and on significantly shorter (i.e. centennial to millennial) timescales related to the recurrence rates of large earthquakes and that (ii) intensive monsoonal-driven groundwater recharge is required on top of tectonic activity for generating volumetrically significant travertine accumulations. Because of the high precipitation rates typical for hydrothermal spring carbonates, we conclude that travertine deposits on the TP could be utilized as valuable high-resolution proxy records of peak monsoon conditions in the currently arid to semi-arid landscape. Furthermore, the Qiusang travertine zone 7 is terraced and the travertine layers adjusted to a paleo-riverbed elevation ~30 m above the current river, allowing us to constrain fluvial incision to ~0.07 m/ka for the south-central sector of the TP since the Mid-Pleistocene. The abundant travertine occurrences in Tibet in combination with uranium-series dating can thus also provide detailed insights into earth surface dynamics and landscape evolution on the world highest plateau. • A detailed 230 Th/U dating campaign was conducted for the Qiusang travertine, Tibet. • Travertine deposition is restricted to interglacial conditions with peak monsoon. • Climatic controls on Tibetan travertine growth are on top of tectonic influences. • U Th ages of terraced travertines constrain a fluvial incision rate of ~0.07 m/ka.

Loss-Based Performance Assessment and Seismic Network Optimization for Earthquake Early Warning

ABSTRACT The goal of earthquake early warning (EEW) is to issue an alert before the damaging seismic waves of an earthquake hit a given exposure. We develop a framework to evaluate the EEW performance in a loss-based context. We use warning time as a key performance indicator and determine statistics of warning time by loss severity to assess the rate and consistency with which an EEW system can deliver timely alerts. In the second part of this work, we develop a Genetic Algorithm approach to optimize an existing sensor network by proposing sites for new stations to enhance the EEW performance in damaging earthquakes. We demonstrate this framework for Switzerland using 2000 realizations of a 50-yr-long stochastic earthquake catalog, which samples the earthquake rate forecast of the Swiss Hazard Model in space and time. For each of the almost 24k earthquake scenario ruptures (5.0 ≤ M ≤ 7.4), we predict shaking intensities and losses (here, fatalities and injuries) at the largest Swiss cities. We find that the current Swiss Seismic Network could provide positive warning times to the affected sites for about 80% of very damaging earthquakes (≥100 fatalities) and for around 85% of earthquakes with ≥10 fatalities. Warning times of >5 s could be achieved for about 40%–55% of very damaging earthquakes and >10 s for about 35%. For around 50% of events with ≥1 fatality (≥10 injuries), EEW could provide >15 s of warning. The greatest benefit of EEW is expected in Zürich, where the population density and, consequently, absolute long-term expected losses are highest and the warning times for damaging earthquakes are often long (>15 s). Densifying the Swiss Seismic Network with additional stations can increase warning times in selected scenarios by up to 5 s. However, because it is already very dense (7 ± 5 km interstation distance), decreasing data latencies (currently ∼2 s) may be more important.

On the Relation Between Anomalous Clouds and Earthquakes in Italian Land

In 1997 Russian scientist Morozova found some cloud anomalies possibly related to active fault systems and earthquakes. Now, 24 years later, the correlation between clouds and earthquakes is still controversial, and in this paper we checked systematically the satellite images in Italy from 2010 to 2013. The correlation between earthquakes and cloud anomalies was statistically examined by assuming different various leading times and magnitudes. The result showed that when the leading time interval was set to 23≤ΔT≤45 days and the magnitude is bigger than or equal to M4.7, 70% of earthquakes were preceded by cloud anomalies. Poisson random test showed that anomaly appearance rate (AAR) and earthquake occurrence rate (EOR) was much higher than those derived in a random situation, which means it is nearly impossible to deny the correlation between cloud anomaly and land earthquakes in Italy. Error matrix analysis showed this method provides 75% overall accuracy for the 52 total cases. Analysis also showed that cloud method provides a very high AAR value and similar EOR value compared with other earthquake prediction methods based on ionospheric or skin temperature data. The physical mechanism of cloud anomaly was likely caused by electric field, which linked active fault, atmosphere circuit conduction current, and cloud anomaly, and thus provides a reasonable hypothesis of cloud anomaly.

Cross-effects of loading rate and cumulative fault slip on pre-seismic rupture and unstable slip rate of laboratory earthquakes

The fact that the same fault may host different rupture or slip modes was partly attributed to the change in strain rate of the fault in previous studies. However, contradictory observations were obtained in the previous experimental studies, that is, the loading rate can promote or inhibit the rapid instability of faults. Although the two opposite effects of the loading rate were separately explained in the previous studies, the contradiction remains unresolved. Based on an experiment conducted on a granodiorite sample, we use the cumulative fault slip to quantitatively describe the slip-driven changes in fault properties and study the cross-effects of loading rate and cumulative fault slip on the pre-seismic rupture speed, unstable slip rate, and friction coefficient drop of the laboratory earthquakes (stick-slip events). The experimental results show that under the same loading rate, the exponential propagating rupture speed, the unstable slip rate, and the friction coefficient drop of the stick-slip events increase with increasing cumulative fault slip. As a result, the loading rate has opposite effects on the exponential propagating rupture speed, the unstable slip rate, and the friction coefficient drop under different amounts of the cumulative fault slip. This provides an explanation for the observation contradiction that the loading rate can promote or inhibit the rapid instability of the fault. The experimental results also reveal that the steady rupture speed is approximately proportional to the loading rate but weakly depends on the cumulative fault slip. Finally, the significance of the experimental results for understanding the earthquake and faulting mechanics is discussed.

The Analysis of the Aftershock Sequences of the Recent Mainshocks in Alaska

The forecasting of the evolution of natural hazards is an important and critical problem in natural sciences and engineering. Earthquake forecasting is one such example and is a difficult task due to the complexity of the occurrence of earthquakes. Since earthquake forecasting is typically based on the seismic history of a given region, the analysis of the past seismicity plays a critical role in modern statistical seismology. In this respect, the recent three significant mainshocks that occurred in Alaska (the 2002, Mw 7.9 Denali; the 2018, Mw 7.9 Kodiak; and the 2018, Mw 7.1 Anchorage earthquakes) presented an opportunity to analyze these sequences in detail. This included the modelling of the frequency-magnitude statistics of the corresponding aftershock sequences. In addition, the aftershock occurrence rates were modelled using the Omori–Utsu (OU) law and the Epidemic Type Aftershock Sequence (ETAS) model. For each sequence, the calculation of the probability to have the largest expected aftershock during a given forecasting time interval was performed using both the extreme value theory and the Bayesian predictive framework. For the Bayesian approach, the Markov Chain Monte Carlo (MCMC) sampling of the posterior distribution was performed to generate the chains of the model parameters. These MCMC chains were used to simulate the models forward in time to compute the predictive distributions. The calculation of the probabilities to have the largest expected aftershock to be above a certain magnitude after a mainshock using the Bayesian predictive framework fully takes into account the uncertainties of the model parameters. Moreover, in order to investigate the credibility of the obtained forecasts, several statistical tests were conducted to compare the performance of the earthquake rate models based on the OU formula and the ETAS model. The results indicate that the Bayesian approach combined with the ETAS model produced more robust results than the standard approach based on the extreme value distribution and the OU law.

An Adaptable Random Forest Model for the Declustering of Earthquake Catalogs

AbstractEarthquake catalogs are essential to analyze the evolution of active fault systems. The background seismicity rate, or rate of earthquakes that are not directly triggered by other earthquakes, directly relates to the stressing rate, a crucial quantity for understanding the seismic hazards. Determining the background seismicity rate is challenging because aftershock sequences may dominate the seismicity rate. Classifying these events in earthquake catalogs—known as catalog declustering—is a common practice and most declustering solutions rely on spatiotemporal distances between events, such as the nearest‐neighbor‐distance algorithm, widely used in various contexts. This algorithm assumes that the nearest‐neighbor distance (NND) follows a bimodal distribution related to the background seismicity and to the aftershocks. Constraining these two distributions is crucial to distinguish the aftershocks from the background events accurately. Recent work often uses linear splitting based on the NND, ignoring the potential overlap between the two populations and resulting in a biased identification of background earthquakes and aftershock sequences. We revisit this problem with machine‐learning algorithms. After testing several popular algorithms, we show that a random forest trained with various synthetic catalogs generated by an Epidemic‐Type Aftershock Sequence model outperforms approaches such as k‐means, Gaussian‐mixture models, and support vector machine classification. We apply our model to two different earthquake catalogs: the relocated Southern California earthquake center catalog and the GeoNet catalog of New Zealand. Our model capably adapts to these two different tectonic contexts, highlighting the differences in aftershock productivity between crustal and intermediate‐depth seismicity.

Geodetic Monitoring at Axial Seamount Since Its 2015 Eruption Reveals a Waning Magma Supply and Tightly Linked Rates of Deformation and Seismicity

AbstractAxial Seamount is a basaltic hot spot volcano with a summit caldera at a depth of ∼1,500 m below sea level, superimposed on the Juan de Fuca spreading ridge, giving it a robust and continuous magma supply. Axial erupted in 1998, 2011, and 2015, and is monitored by a cabled network of instruments including bottom pressure recorders and seismometers. Since its last eruption, Axial has re‐inflated to 85%–90% of its pre‐eruption level. During that time, we have identified eight discrete, short‐term deflation events of 1–4 cm over 1–3 weeks that occurred quasi‐periodically, about every 4–6 months between August 2016 and May 2019. During each short‐term deflation event, the rate of earthquakes dropped abruptly to low levels, and then did not return to higher levels until reinflation had resumed and returned near its previous high. The long‐term geodetic monitoring record suggests that the rate of magma supply has varied by an order of magnitude over decadal time scales. There was a surge in magma supply between 2011 and 2015, causing those two eruptions to be closely spaced in time and the supply rate has been waning since then. This waning supply has implications for eruption forecasting and the next eruption at Axial still appears to be 4–9 years away. We also show that the number of earthquakes per unit of uplift has increased exponentially with total uplift since the 2015 eruption, a pattern consistent with a mechanical model of cumulative rock damage leading to bulk failure during magma accumulation between eruptions.

2021 US National Seismic Hazard Model for the State of Hawaii

The 2021 US National Seismic Hazard Model (NSHM) for the State of Hawaii updates the previous two-decade-old assessment by incorporating new data and modeling techniques to improve the underlying ground shaking forecasts of tectonic-fault, tectonic-flexure, volcanic, and caldera collapse earthquakes. Two earthquake ground shaking hazard forecasts (public policy and research) are produced that differ in how they account for declustered catalogs. The earthquake source model is based on (1) declustered earthquake catalogs smoothed with adaptive methods, (2) earthquake rate forecasts based on three temporally varying 60-year time periods, (3) maximum magnitude criteria that extend to larger earthquakes than previously considered, (4) a separate Kīlauea-specific seismogenic caldera collapse model that accounts for clustered event behavior observed during the 2018 eruption, and (5) fault ruptures that consider historical seismicity, GPS-based strain rates, and a new Quaternary fault database. Two new Hawaii-specific ground motion models (GMMs) and five additional global models consistent with Hawaii shaking data are used to forecast ground shaking at 23 spectral periods and peak parameters. Site effects are calculated using western US and Hawaii specific empirical equations and provide shaking forecasts for 8 site classes. For most sites the new analysis results in similar spectral accelerations as those in the 2001 NSHM, with a few exceptions caused mostly by GMM changes. Ground motions are the highest in the southern portion of the Island of Hawai’i due to high rates of forecasted earthquakes on décollement faults. Shaking decays to the northwest where lower earthquake rates result from flexure of the tectonic plate. Large epistemic uncertainties in source characterizations and GMMs lead to an overall high uncertainty (more than a factor of 3) in ground shaking at Honolulu and Hilo. The new shaking model indicates significant chances of slight or greater damaging ground motions across most of the island chain.

Present-day stress field pattern in the Vrancea seismic zone (Romania) deduced from earthquake focal mechanism inversion

Vrancea seismogenic zone in the South-Eastern Carpathians is characterized by localized intermediate-depth seismicity. Due to its complex geodynamics and large strain release, Vrancea represents a key element in the Carpatho-Pannonian system. Data from a recently compiled catalogue of fault plane solutions (REFMC) are inverted to evaluate stress regime in Vrancea on depth. A single predominant downdip extensive regime is obtained in all considered clusters, including the crustal layers located above the Vrancea slab. The prevalent stress regime confirms previous investigations and requires some mantle-crust coupling. The S3 principal stress is close to vertical, while S1 and S2 are horizontal, oriented perpendicularly and respectively tangentially to the Carpathians Arc bend. This configuration is present at any depth level. According to seismicity patterns, there are two main active segments in the Vrancea intermediate-depth domain, at 55 – 105 km and 105 – 180 km, both able to generate major events. The configuration of the tectonic stresses as resulted from inversion is similar in both segments. Also, high fault instability (I > 0.95) is characterizing the segments. The only notable difference is given by the friction and stress ratio parameters which drop down in the bottom segment from μ = 0.95 to μ = 0.55 and from R = 0.51 to R = 0.29. This variation is attributed to possible weakening processes activated below 100 km depth and can explain the intensification of seismicity production as earthquake rate and average energy release in the lower segment versus the upper segment.

Modulation of Ground Deformation and Earthquakes by Rainfall at Vesuvius and Campi Flegrei (Italy)

Volcanoes are complex systems whose dynamics is the result of the interplay between endogenous and exogenous processes. External forcing on volcanic activity by seasonal hydrological variations can influence the evolution of a volcanic system; yet the underlying mechanisms remain poorly understood. In the present study, we analyse ground tilt, seismicity rates and rainfall amount recorded over 6 years (2015–2021) at Vesuvius and Campi Flegrei, two volcanic areas located in the south of Italy. The results indicate that at both volcanoes the ground deformation reflects the seasonality of the hydrological cycles, whereas seismicity shows a seasonal pattern only at Campi Flegrei. A correlation analysis on shorter time scales (days) indicates that at Vesuvius rain and ground tilt are poorly correlated, whereas rain and earthquakes are almost uncorrelated. Instead, at Campi Flegrei precipitations can affect not only ground deformation but also earthquake rate, through the combined action of water loading and diffusion processes in a fractured medium, likely fostered by the interaction with the shallow hydrothermal fluids. Our observations indicate a different behavior between the two volcanic systems: at Vesuvius, rain-induced hydrological variations poorly affect the normal background activity. On the contrary, such variations play a role in modulating the dynamics of those metastable volcanoes with significant hydrothermal system experiencing unrest, like Campi Flegrei.

The Seismic Hazard Implications of Declustering and Poisson Assumptions Inferred from a Fully Time-Dependent Model

ABSTRACT We use the Third Uniform California Earthquake Rupture Forecast (UCERF3) epidemic-type aftershock sequence (ETAS) model (UCERF3-ETAS) to evaluate the effects of declustering and Poisson assumptions on seismic hazard estimates. Although declustering is necessary to infer the long-term spatial distribution of earthquake rates, the question is whether it is also necessary to honor the Poisson assumption in classic probabilistic seismic hazard assessment. We use 500,000 yr, M ≥ 2.5 synthetic catalogs to address this question, for which UCERF3-ETAS exhibits realistic spatiotemporal clustering effects (e.g., aftershocks). We find that Gardner and Knopoff (1974) declustering, used in the U.S. Geological Survey seismic hazard models, lowers 2% in 50 yr and risk-targeted ground-motion hazard metrics by about 4% on average (compared with the full time-dependent [TD] model), with the reduction being 5% at 40% in 50 yr ground motions. Keeping all earthquakes and treating them as a Poisson process increases these same hazard metrics by about 3%–12%, on average, due to the removal of relatively quiet time periods in the full TD model. In the interest of model simplification, bias minimization, and consideration of the probabilities of multiple exceedances, we agree with others (Marzocchi and Taroni, 2014) that we are better off keeping aftershocks and treating them as a Poisson process rather than removing them from hazard consideration via declustering. Honoring the true time dependence, however, will likely be important for other hazard and risk metrics, and this study further exemplifies how this can now be evaluated more extensively.

Spatially variable model for extracting TIR anomalies before earthquakes: Application to Chinese Mainland

There are several long-term statistical researches using the Molchan diagram (MD) to prove the relation between thermal infrared (TIR) anomalies and earthquakes in different regions, however, these studies are flawed: 1) the original MD is based on the spatially uniform Poisson model, but it will offer wrong evaluations for inhomogenous systems with uneven spatial-temporal distribution of earthquakes; 2) some of these studies have de-clustered earthquake catalogs before applying MD, however, de-clustering will introduce ambiguities to final scores and it may also conceal the potential relation between precursors and the ‘removed’ events. Until now, we have to admit that the long-term statistical evidence for proving the correspondence between earthquake and TIR anomalies is still absent and the power of TIR anomalies for predicting earthquakes is limited. In this study, we use daily nighttime Outgoing Longwave Radiation (OLR) data provided by the National Oceanic and Atmospheric Administration (NOAA) to extract the TIR anomalies of Chinese Mainland (20°-54°N, 73°-135°E). The data from Jan. 2007 to Dec. 2010 is the training dataset to obtain the best parameters for extracting TIR anomalies and the best time-distance-magnitude (TDM) windows for determining the correspondence between earthquakes and TIR anomalies, and the data from Jan. 2011 to Dec. 2017 is the testing dataset. The new 3D Molchan diagram offers a score for each model with different parameters. Unlike the original MD that only deals with the rate of missed events and the size of warning space, the 3D Molchan diagram quantifies the errors including false alarms and missed predictions. We assume that the best parameters and TDM windows are spatially variable for different sub-regions, because the Signal/Noise ratio is spatially variable due to the different geological and meteorological backgrounds. Moreover, we construct a new probability prediction model based on non-seismic binary alarms. Results show that the TIR anomalies is strongly related to normal or reverse earthquakes with magnitude≥ 4.0, and the TIR anomalies caused by earthquakes should be persistent in space and time. Moreover, the spatially variable model is superior to the global invariant one. We succeed in transforming the non-seismic binary alarms into probabilistic predictions based on the TIR anomalies and Relative Intensity index, which is defined as the rate of past earthquakes occurring in each spatial cell. Moreover, our new probabilistic model is superior to the spatially inhomogeneous Poisson model. However, this new probability model is still naïve and weak, and needs to be improved in the future.

GPS Constraints on Drought‐Induced Groundwater Loss Around Great Salt Lake, Utah, With Implications for Seismicity Modulation

AbstractGreat Salt Lake (GSL), Utah, lost 1.89 ± 0.04 m of water during the 2012–2016 drought. During this timeframe, data from the Gravity Recovery and Climate Experiment mission underestimate this mass loss, while nearby Global Positioning System (GPS) stations exhibit significant shifts in position. We find that crustal deformation, from unloading the Earth's crust consistent with the observed GSL water loss alone, does not explain the GPS displacements, suggesting contributions from additional water storage loss surrounding GSL. This study applies a damped least squares inversion to the three‐dimensional GPS displacements to test a range of distributions of groundwater loads to fit the observations. When considering the horizontal and vertical displacements simultaneously, we find a realistic distribution of water loss while also resolving the observed water loss of the lake. Our preferred model identifies mass loss up to 64 km from the lake via two radial rings. The contribution of exterior groundwater loss is substantial (10.9 ± 2.8 km3 vs. 5.5 ± 1.0 km3 on the lake), and greatly improves the fit to the observations. Nearby groundwater wells exhibit significant water loss during the drought, which substantiates the presence of significant water loss outside of the lake, but also highlights greater spatial variation than our model can resolve. We observe seismicity modulation within the inferred load region, while the region outside the (un)loading reveals no significant modulation. Drier periods exhibit higher quantities of events than wetter periods and changes in trend of the earthquake rate are correlated with regional mass trends.

Perspectives on Clustering and Declustering of Earthquakes

Abstract Clustering is a fundamental feature of earthquakes that impacts basic and applied analyses of seismicity. Events included in the existing short-duration instrumental catalogs are concentrated strongly within a very small fraction of the space–time volume, which is highly amplified by activity associated with the largest recorded events. The earthquakes that are included in instrumental catalogs are unlikely to be fully representative of the long-term behavior of regional seismicity. We illustrate this and other aspects of space–time earthquake clustering, and propose a quantitative clustering measure based on the receiver operating characteristic diagram. The proposed approach allows eliminating effects of marginal space and time inhomogeneities related to the geometry of the fault network and regionwide changes in earthquake rates, and quantifying coupled space–time variations that include aftershocks, swarms, and other forms of clusters. The proposed measure is used to quantify and compare earthquake clustering in southern California, western United States, central and eastern United States, Alaska, Japan, and epidemic-type aftershock sequence model results. All examined cases show a high degree of coupled space–time clustering, with the marginal space clustering dominating the marginal time clustering. Declustering earthquake catalogs can help clarify long-term aspects of regional seismicity and increase the signal-to-noise ratio of effects that are subtler than the strong clustering signatures. We illustrate how the high coupled space–time clustering can be decreased or removed using a data-adaptive parsimonious nearest-neighbor declustering approach, and emphasize basic unresolved issues on the proper outcome and quality metrics of declustering. At present, declustering remains an exploratory tool, rather than a rigorous optimization problem, and selecting an appropriate declustering method should depend on the data and problem at hand.

Modelling earthquake rates and associated uncertainties in the Marmara Region, Turkey

Abstract. Modelling the seismic potential of active faults and the associated epistemic uncertainty is a fundamental step of probabilistic seismic hazard assessment (PSHA). We use SHERIFS (Seismic Hazard and Earthquake Rate In Fault Systems), an open-source code allowing us to build hazard models including earthquake ruptures involving several faults, to model the seismicity rates on the North Anatolian Fault (NAF) system in the Marmara Region. Through an iterative approach, SHERIFS converts the slip rate on the faults into earthquake rates that follow a magnitude frequency distribution (MFD) defined at the fault system level, allowing us to model complex multi-fault ruptures and off-fault seismicity while exploring the underlying epistemic uncertainties. In a logic tree, we explore uncertainties concerning the locking state of the NAF in the Sea of Marmara, the maximum possible rupture in the system, the shape of the MFD and the ratio of off-fault seismicity. The branches of the logic tree are weighted according to the match between the modelled earthquake rate and the earthquake rates calculated from the local data, earthquake catalogue and palaeoseismicity. In addition, we use the result of the physics-based earthquake simulator RSQSim to inform the logic tree and increase the weight on the hypotheses that are compatible with the result of the simulator. Using both the local data and the simulator to weight the logic tree branches, we are able to reduce the uncertainties affecting the earthquake rates in the Marmara Region. The weighted logic tree of models built in this study will be used in a following article to calculate the probability of collapse of a building in Istanbul.

Stability of the Fault Systems That Host-Induced Earthquakes in the Delaware Basin of West Texas and Southeast New Mexico

Abstract The Delaware basin of west Texas and southeast New Mexico has experienced elevated earthquake rates linked spatiotemporally to unconventional petroleum operations. Limited knowledge of subsurface faults, the in situ geomechanical state, and the exact way in which petroleum operations have affected pore pressure (Pp) and stress state at depth makes causative assessment difficult, and the actions required for mitigation uncertain. To advance both goals, we integrate comprehensive regional fault interpretations, deterministic fault-slip potential (DFSP), and multiple earthquake catalogs to assess specifically how faults of two systems—deeper basement-rooted (BR) and shallow normal (SN)—can be made to slip as Pp is elevated. In their natural state, the overall population faults in both the systems have relatively stable DFSP, which explains the low earthquake rate prior to human inducement. BR faults with naturally unstable DFSP and associated earthquake sequences are few but include the Culberson–Mentone earthquake zone, which is near areas of wastewater injection into strata above basement. As a system, the SN faults in the southcentral Delaware basin are uniformly susceptible to slip with small increases in Pp. Many earthquakes sequences have occurred along these shallow faults in association with elevated Pp from shallow wastewater injection and hydraulic fracturing. Our new maps and methods can be used to better plan and regulate petroleum operations to avoid fault rupture.

Campi Flegrei, Vesuvius and Ischia Seismicity in the Context of the Neapolitan Volcanic Area

Studying seismicity in a volcanic environment provides important information on the state of activity of volcanoes. The seismicity of the Neapolitan volcanoes, Campi Flegrei, Vesuvius, and Ischia, shows distinctive characteristics for each volcano, covering a wide range of patterns and types. In this study we relocated some significant volcano-tectonic earthquake swarms that occurred in Campi Flegrei and Vesuvius. Moreover, we compared the earthquake occurrence evolution, the magnitude and the seismic energy release of the three volcanoes. Also, we considered the results of seismic analysis in the light of geochemical and ground deformation data that contribute to defining the state of activity of volcanoes. In Campi Flegrei, which is experiencing a long term unrest, we identified a seismogenic structure at shallow depth in Pisciarelli zone that has been activated repeatedly. The increasing seismicity accompanies an escalation of the hydrothermal activity and a ground uplift phase. At Vesuvius a very shallow seismicity is recorded, which in recent years has shown an increase in terms of the number of events per year. Earthquakes are usually located right beneath the crater axis. They are concentrated in a volume affected by the hydrothermal system. Finally, Ischia generally shows a low level of seismicity, however, in Casamicciola area events with a moderate magnitude can occur and these are potentially capable of causing severe damage to the town and population, due to their small hypocentral depth (typically < 2.5 km). After the seismic crisis of August 21, 2017 (mainshock magnitude M = 4), the seismicity returned to a low level in terms of occurrence rate and magnitude of earthquakes. The seismicity of these three different volcanic areas shows some common aspects that highlight a relevant role of hydrothermal processes in the seismogenesis of volcanic areas. However, while the main swarms in Campi Flegrei and most of the Vesuvian earthquakes are distributed along conduit-like structures, the seismicity of Ischia is mainly located along faults. Furthermore, the temporal evolution of seismicity in Neapolitan volcanic area suggests a concomitant increase in the occurrence of earthquakes both in Campi Flegrei and Vesuvius in recent years.

Impact of economic development levels on the mortality rates of Asian earthquakes

Buildings in areas with high economic development levels have strong earthquake resistance, and the mortality rate caused by earthquakes is low. From 1990 to 2015, a total of 387 earthquakes (excluding the earthquakes that caused the tsunami) occurred in Asia. The paper took Asian countries as an example to quantitatively estimate the impact of economic development on the mortality of earthquakes. Based on spatial statistics and regression analysis, the paper obtained the mortality rate of every historical earthquake and drew the magnitude-mortality rate curves for three groups at different economic levels, namely, Group I (per capita GDP range of $12,500-$45,000), Group II (per capita GDP range of $1,700-$12,500), and Group III (per capita GDP range of $550-$1,700). With a M 6 earthquake, the mortality rate of Group I, II and III was 0.0002‰, 0.0071‰ and 0.0871‰ respectively. However, due to the powerful destructiveness of giant earthquakes, as the magnitude of the earthquakes increases, the differences among the three groups decrease. Furthermore, if the economic development of Asian countries maintains its pace of the past 20 years, the vulnerability of Groups II and III will reach the current level of Group I in the year of 2046. Then if earthquakes that have occurred in the past occur again, the death toll will be reduced from 399,509 to 107,112. In general, as the GDP per capita doubles, the earthquake mortality rate will drop by 19%–25%, and the disaster-reduction goals proposed by the Sendai Framework can be achieved.