Seismogenic Sources Research Articles

Recent and active faulting along the exposed front of the Northern Apennines (Italy): New insights from field and geochronological constraints

Establishing genetic links between active shallow faulting and deep seismogenic sources is challenging, especially in areas where seismogenic faults lack clear and readily interpretable geological evidence at the surface. The architecture of the Pedeapenninic margin of the Northern Apennines (Italy) reflects a regional-scale and complex NE-verging blind thrust system, which is dissected by transpressive/transtensive faults resulting from active NE-SW orogenic compression. The local geological framework is defined by allochthonous ocean-derived units resting atop Pliocene-to-present successions exposed along the Northern Apennines margin and to the NE of it, while the innermost chain sector mostly contains Adria-related units. We present results from field structural-geological investigations to identify and characterize potential active faults along the margin. There, the Pliocene-to-present units are faulted and folded, indicating that tectonic activity is still on-going, thus contributing to the local seismic hazard. Top-to-the NE and SW normal faults are common in the area and deform the Pliocene-to-present succession together with NE-SW strike-slip and transpressional/transtensional faults. Based on field evidence, we define four potentially active thrust segments affecting Middle Pleistocene to Holocene deposits exposed along the margin. Calcite U-Th dating on samples from faults extend the most recent datable tectonic activity back to the Middle Pleistocene. Paleostress analysis from inversion of fault-slip data from the most recent identified striated fault planes constrains a NE-SW shortening direction parallel to the Apennines regional migration direction. A distinct but coaxial extensional stress regime, recorded by structures measured within Plio-Pleistocene formations, was also identified. Our results offer a sound starting point for future investigations aimed at improving our understanding of active and seismogenic faulting in the area, so as to create robust Probabilistic Seismic and Fault Displacement Hazard Assessment (PSHA and PFDHA) models that can implement refined seismic hazard maps benefitting from structural-geological deterministic inputs in addition to the classic seismological constraints.

Assessment of earthquake location uncertainties for the design of local seismic networks

The ability to estimate earthquake source locations, along with the appraisal of relevant uncertainties, is paramount in monitoring both natural and human-induced micro-seismicity. For this purpose, a monitoring network must be designed to minimize the location errors introduced by geometrically unbalanced networks. In this study, we first review different sources of errors relevant to the localization of seismic events, how they propagate through localization algorithms, and their impact on outcomes. We then propose a quantitative method, based on a Monte Carlo approach, to estimate the uncertainty in earthquake locations that is suited to the design, optimization, and assessment of the performance of a local seismic monitoring network. To illustrate the performance of the proposed approach, we analyzed the distribution of the localization uncertainties and their related dispersion for a highly dense grid of theoretical hypocenters in both the horizontal and vertical directions using an actual monitoring network layout. The results expand, quantitatively, the qualitative indications derived from purely geometrical parameters (azimuthal gap (AG)) and classical detectability maps. The proposed method enables the systematic design, optimization, and evaluation of local seismic monitoring networks, enhancing monitoring accuracy in areas proximal to hydrocarbon production, geothermal fields, underground natural gas storage, and other subsurface activities. This approach aids in the accurate estimation of earthquake source locations and their associated uncertainties, which are crucial for assessing and mitigating seismic risks, thereby enabling the implementation of proactive measures to minimize potential hazards. From an operational perspective, reliably estimating location accuracy is crucial for evaluating the position of seismogenic sources and assessing possible links between well activities and the onset of seismicity.

Seismogenic Quasi-Stationary Electric Fields and Currents from Large-Scale Sources on the Earth’s Surface: Comparison of Model Representations

A comparative analysis of various model representations of seismogenic quasi-stationary electric fields/currents from large-scale sources on the Earth’s surface was carried out. It has been established that previously proposed analytical models of seismogenic quasi-stationary sources of electric field/current with field/current amplitudes continuously decreasing to zero at infinity are consistent with extreme values of electric field/current experimentally observed in epicentral zones before earthquakes. It is also shown that sharply spatially limited models of seismogenic sources of quasi-stationary electric fields/currents in the epicentral zones of future earthquakes on the Earth’s surface lead to their values being an order of magnitude or more greater than those actually observed.

Intensity Prediction Equations Based on the Environmental Seismic Intensity (ESI-07) Scale: Application to Normal Fault Earthquakes

Earthquake environmental effects may significantly contribute to the damage caused by seismic events; similar to ground motion, the environmental effects are globally stronger in the vicinity and decrease moving away from the epicenter or seismogenic source. To date, a single intensity prediction equation (IPE) has been proposed in the Italian Apennines for intensity scale dealings with environmental effects: the Environmental Seismic Intensity (ESI-07). Here, we evaluate the sensitivity of the IPE with respect to input data and methodological choices and we propose IPEs with global validity for crustal normal faults. We show the strong influence of input data on the obtained attenuation investigating the 1980 Irpinia–Basilicata (Southern Italy) earthquake. We exploit a dataset of 26 earthquakes to build an IPE considering the epicentral distance. We also propose an IPE considering the distance from the fault rupture, which is derived from a dataset of 10 earthquakes. The proposed equations are valid for normal faults up to 40 km from the epicenter/fault and may flank other models predicting ground motion or damage to the built environment. Our work thus contributes to the use of the ESI-07 scale for hazard purposes.

Seismotectonic study in the Sierras de Velasco, Ambato, and Ancasti, Eastern Sierras Pampeanas of Argentina

In this work, we study the seismic activity and tectonic regime in the region of the Sierras de Velasco, Ambato and Ancasti, belonging to the Sierras Pampeanas. These areas exhibit notable seismic activity, posing a considerable risk for populations in the region. We relocated the most important seismic events of the last twenty years and obtained their focal mechanisms. We reviewed the field evidence of neotectonic deformation, and measured kinematic indicators on Quaternary faults interpreted as seismogenic. Based on this information, we discuss the link between seismogenic sources and the most relevant morphostructural features. Our findings reveal that the seismicity is dominated by reverse focal mechanisms, exhibiting nodal planes with strikes that align with the main structures. This indicates that the main range-bounding faults are the most important seismogenic structures in the area. Some events have strike-slip movements, likely associated with the reactivation of basement fabrics. Additionally, in the Pirquitas area, we observed a normal focal mechanism, oriented perpendicular to the main structures. Our results contribute to a better understanding of current deformation dynamics and of seismic hazard in the region, and underscore the need for more detailed studies.

Probability and recurrence interval of large earthquakes (Mw ≥ 6) in the Himalayan seismicity zone

The Himalayan Mountains are one of the world’s most seismically active regions. Due to the active seismic status of the Himalayan belt and ongoing development activities in the region, it is important to keep the seismic hazard estimates updated by incorporating historical and new data. This study uses the Gutenberg-Richter relations for four seismogenic source zones (SSZ) to estimate the probability of earthquakes of a specified magnitude in a given time interval. We used exponential, Weibull, Rayleigh, and log-normal statistical models to assess the probability of earthquakes (Mw ≥ 6). The probable recurrence time of the large earthquake (Mw ≥ 6) can be estimated by maximizing the conditional probability estimates. This recurrence interval is calculated after the elapsed time since the most recent large earthquake in the region. We also apply an autoregressive (AR) forecasting technique to predict earthquake recurrence intervals. The results indicate that the approximate recurrence interval of the large earthquakes (Mw ≥ 6) may be 22.052 ± 5.65 years within the seismicity zones, as per our statistical estimates based on limited and available datasets. The results of this study may be useful for the agencies involved in assessing seismic hazards and developing mitigation and preparedness strategies in the region.

Multiparametric stations for real-time monitoring and long-term assessment of natural hazards

The present work would like to illustrate a new concept of multiparametric stations to characterize the crustal fluids-tectonic interaction in specific geological contexts. The dynamics of crustal fluids in relation to tectonics is a complex and sometimes intricate issue. Several factors act and mutually influence themselves, so that in each tectonic and geological context they follow a specific behavior, and a comprehensive cause-effect rule is hard to find. Changes in water chemistry and levels and in soil flux regimes (e.g., CO2, CH4, radon) are just a few examples well documented in the literature as being pre-, co- and post-seismic modifications as well as being markers of the local tectonic stress acting in the crust. A regional study combined with a long-lasting multiparametric monitoring is needed to prepare to a seismic sequence in a given place. The field infrastructure was set up starting from the end of 2021, and multiparametric stations have been installed in correspondence of active seismogenic sources initially located in Northern Italy. Data are transmitted in real-time and archived in an ad hoc developed relational database. Monitoring is mainly focused on groundwater parameters (water level, temperature, and electrical conductivity) of aquifers showing distinct degrees of confinement and lithologies. Sites are also equipped of meteorological sensors (pressure, temperature, rain, humidity, wind speed and direction), radon sensors and surface and borehole seismic stations providing accelerometric and velocimetric data. A mud volcano field is also monitored and holds the installation of a permanent CO2 soil flux station. A statistical analysis working flow is also proposed for a preliminary evaluation of the acquired time-series. In particular, a couple of tools to detect, and thus filter, anthropogenic and meteorological effects on a groundwater level series is described. We wish to provide a model of approach to analogous study cases in other potentially seismic areas.

Seismotectonics-Driven Estimation of b-Value: Implications for Seismic Hazard Assessment

Abstract Taking full advantage of good-quality historical earthquake and seismogenic source data available for Italy (Catalogo Parametrico dei Terremoti Italiani [CPTI15] and Database of Individual Seismogenic Sources [DISS] databases), we tested the regional variability of the b-value of the Gutenberg–Richter law, focusing on the dominantly extensional, nearly 1200-km-long seismogenic corridor straddling the Apennines chain; an 18,200 km2 area generating about 45% of the seismic moment released countrywide. We carefully chose and tested the most appropriate completeness interval and completeness magnitude (Mc), and used the Lilliefors method, the Utsu test, and a bootstrap technique to verify the quality of our results and inferences. The 0.65 b-value we obtained is substantially lower than b-values used in Italian seismic hazard models, often close to 1.0; yet it predicts more accurately the observed Apennines earthquake record, suggesting that most currently adopted magnitude-frequency distributions underpredict events in the Mw range 6.0–7.1 and overpredict those in the range 5.2–5.5. We also highlight that the time, space, and magnitude distribution of the largest Italian earthquakes hardly follows the Gutenberg–Richter law: it rather favors a characteristic behavior. We contend that the b-value is too critical of a parameter for being calculated using statistically weak data sets, such as those resulting from overly detailed area-source models: one may end up characterizing low-hazard areas more accurately than high-hazard areas. Conversely, we advocate the use of fewer, larger area sources that fully exploit the current understanding of regional seismotectonics, as a way of obtaining more realistic regional hazard models.

Simulation of large plausible tsunami scenarios associated with the 2019 Durres (Albania) earthquake source and adjacent seismogenic zones

We present an analysis of the hazards of potential earthquake-generated tsunamis along the Albanian–Adriatic coast. The study adopts a case study approach to model plausible tsunamigenic events associated with the 2019 Mw 6.4 Durres (Albania) earthquake source zone. The approach combines current findings on regional tectonics and scenario-based calculations of potential tsunami impacts. The study’s goal is to analyse the propagation of tsunami waves generated by identified seismogenic sources (namely ALCS002 [Lushnje] and ALCS018 [Shijak]) and determine the tsunami risk assessment for Durres City on the Albanian–Adriatic coast. The sources can generate earthquakes with maximum moment magnitudes of Mw 7.5 and Mw 6.8, which are likely to trigger tsunamis that could cause significant impacts in the region. The modelling is performed deterministically with the NAMI DANCE numerical code, including scenarios associated with the largest plausible earthquake. The model integrates bathymetry and topography datasets of large and medium resolutions. Each tsunami scenario simulation is based on the solution of the non-linear shallow water equations used to generate maximum positive wave amplitudes (water elevation), travel time, and tsunami inundation maps. In Durres City, modelling indicates that medium-sized waves could reach up to 2.5 m inland, posing a significant danger to the city’s low-lying areas. The most substantial tsunami waves are expected to impact the area within the first 10 to 20 min. Combining inundation maps and information on exposed assets allows for identifying areas where damages can be expected. In terms of human impact, a preliminary analysis shows that the study area is prone to tsunami threat, with more than 138,000 inhabitants living in vulnerable urban areas of Durres City by 2036. The model’s capacity to capture details related to the presence of buildings is limited due to constraints posed by the resolution of bathymetry and topography datasets available during this study. If refined with high-resolution bathymetry and topography datasets, our results can be considered a backbone for exposure and resilience assessment features to be integrated into preparedness or new urban development plans.

Cascading hazards of a major Bengal basin earthquake and abrupt avulsion of the Ganges River

Earthquakes present severe hazards for people and economies and can be primary drivers of landscape change yet their impact to river-channel networks remains poorly known. Here we show evidence for an abrupt earthquake-triggered avulsion of the Ganges River at ~2.5 ka leading to relocation of the mainstem channel belt in the Bengal delta. This is recorded in freshly discovered sedimentary archives of an immense relict channel and a paleo-earthquake of sufficient magnitude to cause major liquefaction and generate large, decimeter-scale sand dikes >180 km from the nearest seismogenic source region. Precise luminescence ages of channel sand, channel fill, and breached and partially liquefied floodplain deposits support coeval timing of the avulsion and earthquake. Evidence for reorganization of the river-channel network in the world’s largest delta broadens the risk posed by seismic events in the region and their recognition as geomorphic agents in this and other tectonically active lowlands. The recurrence of comparable earthquake-triggered ground liquefaction and a channel avulsion would be catastrophic for any of the heavily populated, large river basins and deltas along the Himalayan arc (e.g., Indus, Ganges, Brahmaputra, Ayeyarwady). The compounding effects of climate change and human impacts heighten and extend the vulnerability of many lowlands worldwide to such cascading hazards.

Shaken and Stirred: A Comparative Study of Earthquake‐Triggered Soft‐Sediment Deformation Structures in Lake Sediments

AbstractSubaqueous paleoseismic studies used soft sediment deformation structures (SSDS) to discern the shaking strength of past earthquakes, as the deformation degree of SSDS related to Kelvin Helmholtz Instability evolves from disturbed lamination and folds to intraclast breccia with higher peak ground accelerations (PGA). We lack comparative studies of different sediment types with SSDS related to earthquakes from different seismogenic sources to comprehend how these factors modulate earthquake‐induced deformation. Here, we compile sediment records with seven earthquake‐triggered SSDS from 10 lakes with organic‐, carbonate‐, siliciclastic‐, and diatom‐rich sediment from three subduction zones and one collisional setting. We target basin sequences with slope angles <0.65° to reduce the influence of gravitational downslope stress. We find that even minimal increases in slope angle, maximal 1°, lead to higher deformation degrees and, for some earthquakes, SSDS are only present at >0.65°. Fine‐grained clastics enhance sediment susceptibility to deformation, whereas abundant diatoms reduce it, demonstrating the influence of composition. Deformation correlates best with PGA and the vicinity of the earthquakes, suggesting that high frequency shaking promotes deformation. In addition, deformation only occurs above a minimum magnitude dependent on sediment composition, and higher deformation degrees in our studied basin sedimentary sequences only above Mw 4.9 for all sediment types, suggesting that sufficient duration of shaking—magnitude correlates with duration—is essential for SSDS development. We advise taking multiple cores on gentle slopes to study SSDS—additional to basin cores—to resolve small magnitude local earthquakes and relative differences in frequency content of past events.

Electromagnetic ULF Fields from an Underground Seismic Source on the Earth’s Surface and in the Ionosphere

A theoretical formalism has been developed to calculate the electromagnetic fields generated in the atmosphere–ionosphere system by a finitelength underground horizontal current source. A numerical model with a realistic profile of the ionosphere in a vertical geomagnetic field has been designed based on this theory. It is shown that the apparent impedance of the electromagnetic field created by an underground source on the Earth’s surface is one order of magnitude higher than the Earth’s impedance, which can be used to discriminate perturbations from seismogenic sources. The presented results of numerical modeling allow us to relate perturbations created by a large-scale underground source in the Earth surface magnetic field and in the electric field in the ionosphere. Based on these model estimates it is concluded that many of the ULF electric field perturbations detected in satellite data before earthquakes cannot be attributed to direct emission from seismogenic sources.

Modelling seismic ground motion and its uncertainty in different tectonic contexts: challenges and application to the 2020 European Seismic Hazard Model (ESHM20)

Abstract. Current practice in strong ground motion modelling for probabilistic seismic hazard analysis (PSHA) requires the identification and calibration of empirical models appropriate to the tectonic regimes within the region of application, along with quantification of both their aleatory and epistemic uncertainties. For the development of the 2020 European Seismic Hazard Model (ESHM20) a novel approach for ground motion characterisation was adopted based on the concept of a regionalised scaled-backbone model, wherein a single appropriate ground motion model (GMM) is identified for use in PSHA, to which adjustments or scaling factors are then applied to account for epistemic uncertainty in the underlying seismological properties of the region of interest. While the theory and development of the regionalised scaled-backbone GMM concept have been discussed in earlier publications, implementation in the final ESHM20 required further refinements to the shallow-seismicity GMM in three regions, which were undertaken considering new data and insights gained from the feedback provided by experts in several regions of Europe: France, Portugal and Iceland. Exploration of the geophysical characteristics of these regions and analysis of additional ground motion records prompted recalibrations of the GMM logic tree and/or modifications to the proposed regionalisation. These modifications illustrate how the ESHM20 GMM logic tree can still be refined and adapted to different regions based on new ground motion data and/or expert judgement, without diverging from the proposed regionalised scaled-backbone GMM framework. In addition to the regions of crustal seismicity, the scaled-backbone approach needed to be adapted to earthquakes occurring in Europe's subduction zones and to the Vrancea deep seismogenic source region. Using a novel fuzzy methodology to classify earthquakes according to different seismic regimes within the subduction system, we compare ground motion records from non-crustal earthquakes to existing subduction GMMs and identify a suitable-backbone GMM for application to subduction and deep seismic sources in Europe. The observed ground motion records from moderate- and small-magnitude earthquakes allow us to calibrate the anelastic attenuation of the backbone GMM specifically for the eastern Mediterranean region. Epistemic uncertainty is then calibrated based on the global variability in source and attenuation characteristics of subduction GMMs. With the ESHM20 now completed, we reflect on the lessons learned from implementing this new approach in regional-scale PSHA and highlight where we hope to see new developments and improvements to the characterisation of ground motion in future generations of the European Seismic Hazard Model.

Holocene surface-rupturing paleo-earthquakes along the Kachchh Mainland Fault: shaping the seismic landscape of Kachchh, Western India

This study explores the seismotectonics of Kachchh in western India, a region with a low-to-moderate strain rate and a history of significant earthquakes, notably the 1819, Mw 7.8 Allah Bund, and the 2001, Mw 7.6 Bhuj. Despite its substantial seismic risk, comprehensive studies on Kachchh’s seismogenic sources are scarce. This is attributed to the concealed nature of active structures, hindering definitive age constraints in paleoseismological research. Our research comprises a detailed paleoseismic analysis of the north-verging, reverse Jhura Fault underlying the Jhura anticline, a segment of the Kachchh Mainland Fault. This fault segment shows evidence of surface-rupturing earthquakes in the area south of the Great Rann of Kachchh. The investigation reveals three paleoseismic events: Event I before 9.72 ka B.P., Event II between 8.63–8.20 ka B.P., and Event III between 6.20–6.09 ka B.P. The elapsed time since the last event on this fault is > 8000 years, suggesting that the area is exposed to a significant earthquake hazard. This highlights the need for more precise characterization of individual seismogenic sources for future earthquake preparedness.

Impact of coseismic uplifting on relative sea level change in the Southern Adriatic during the past 4.500 years – New evidence from Dubrovnik epicentral area based on analysis of algal rims and tidal notches

The Southern Adriatic is recognized as one of the most seismically active regions of the Central Mediterranean. It hosted the strongest historically known and instrumentally recorded earthquakes in Croatia and Montenegro. We conducted a detailed study of algal rims and tidal notches along the Dubrovnik–Konavle coastal area, aimed to contribute in reconstruction of relative sea level (RSL) change, palaeoearthquakes and neotectonics during the last 4500 years. The RSL reconstruction based on high-resolution geochronology showed that the spatial variability of RSL change is largely controlled by local tectonics, leading generally to an alternation of periods of RSL rise and rapid falls caused by coseismic uplift events. During the interseismic periods RSL rose at rates from 1.03 to 0.7 mm/yr before AD, followed by a slowdown to ~0.43 mm/yr, and reaching ~1.3 mm/yr after 1850s. The only exception relates to a period of slow RSL drop probably related to the 3.2 ka cold event. Rapid RSL falls caused by coseismic uplift indicate the occurrence of several large-scale events during the 4th–6th centuries AD, the 800–1360 cal AD period, and in 1395, with hints of earlier events between 1395 cal BC and 565 cal AD. These instances correspond to either unknown or insufficiently documented earthquakes. Conversely, the earthquakes in 1520 and 1667 have been clearly identified. The most compelling evidence of coseismic uplift is associated with the 1667 Dubrovnik earthquake, with an estimated uplift of 40–60 ± 15 cm documented along 40 km of coastline. Analyses of multiple sites with the same high-resolution marker show that Holocene tectonic deformations vary along different fault segments with minimum uplift rates ranging from ~0.3 to ~0.9 mm/yr over ~1.5 ka. The main seismogenic source responsible for coseismic uplifts is attributed to the Dalmatian unit basal thrust and its NE-dipping splays at the Adria-Dinarides boundary.

Active Faults, Kinematics, and Seismotectonic Evolution during Tajogaite Eruption 2021 (La Palma, Canary Islands, Spain)

During the 2021 La Palma strombolian and fissure eruption, two faults were identified that controlled the spatial distribution of earthquake hypocenters and effusive eruptive vents. One of these faults has a NW-SE trend (Tazacorte Fault: TZF) and the other one shows an ENE-WSW trend (Mazo Fault: MZF). Previous works on fault structural analysis in La Palma indicated that the eruption zone was compatible with an extensional tectonic strain ellipsoid which activated normal-strike-slip directional faults at the confluence of TZF and MZF. These fractures were activated during the 2021 Tajogaite eruption, determining the NW-SE and WSW-ENE spatial distribution of vents. Both faults were mapped in real time during the volcanic eruption from fieldwork and remote sensing imagery (aerial drone images). We have collected more than 300 fracture data associated with the effusive vents and post-eruption seismic creep. Since the affected area was densely inhabited, most of these fractures affect houses and infrastructures. Some of the houses affected by the TZF were damaged 9 months after the eruption, although they were not damaged during the eruption. Surprisingly, these houses already had repairs made to the same fractures since 1980, giving information of previous fault creep movement. During the 2021 Tajogaite eruption, shallow seismicity was spatially related to both faults, suggesting a seismic behavior instead of the precedent creep movement. However, the lack of seismicity after the eruption indicates that the faults went back to creep aseismic behavior, similarly to 1980. The mapping and monitoring of these faults (TZF and MZF) is relevant bearing in mind that they have been active since 1980 and the post-eruptive phase of the 2021 volcanic eruption, which has to be included in the land use planning in areas affected by the volcanic eruption and creep movement. Furthermore, both faults could act as seismogenic sources triggering volcanic earthquakes with potential high macroseismic intensities and mass movements. The data presented here show the importance of having this type of study before the onset of the eruption, thus allowing a better interpretation of seismic data during volcanic unrest.

A High‐Resolution Seismic Catalog for the Southern Apennines (Italy) Built Through Template‐Matching

AbstractThe incompleteness of earthquake catalogs is a well‐known issue caused by our technical limitation in detecting the small‐to very small‐magnitude seismicity falling near or below the background seismic noise. The detection of small‐magnitude events is fundamental for improving our knowledge of geometry and kinematics of seismogenic sources and the spatio‐temporal characteristics of seismicity, thus leading to better models for seismic hazard. Template‐matching (TM) is a powerful technique that, based on similarity measure (cross‐correlation) of seismic waveforms, allows to detect hidden earthquakes that are similar to known events (called templates). The high computational effort often limits such technique to small areas and for short time frames (less than 1 year). In this work, we present the first application of template‐matching at regional scale for the Italian Peninsula, focusing on the Southern Apennines. We use about 3,600 high‐quality events as templates, scanning 6‐year long continuous recordings (2009–2014), at more than 180 stations of the INGV network. About 20,000 new events are found, showing a comparable quality to the template catalog in terms of hypocentral solution, reaching a decrease of the magnitude of completeness of about one unit. To highlight the improved quality of the TM catalog, we report two main examples regarding the Sannio‐Matese area, where TM allowed us to unravel relevant details on the spatio‐temporal distribution of the local seismicity.

The Berkovići (BIH) ML = 6.0 earthquake sequence of 22 April 2022 – Seismological and seismotectonic analyses

In the southeast of Bosnia and Herzegovina, the Berkovići earthquake sequence started with the mainshock on 22 April 2022 21:07 UTC at focal depth 22 km with magnitude ML = 6.0 (Mw = 5.7). Our preliminary estimation of the mainshock's maximum intensity is VII EMS for Berkovići where 29% of buildings were damaged. We analysed the first nine months of this sequence, 22 April 2022–22 January 2023. The earthquakes were located using a guided grid-search algorithm with source-specific station corrections as a mean of solutions for 54 combinations of velocity models and program control parameters. The analysis of aleatory variation and epistemic uncertainty showed that they are very dependent on the station coverage, especially for focal depth. The event catalogue consists of 7217 earthquakes and can be considered complete for ML ≥ 1.3. Focal depths (15–30 km) are considerably larger than average for the Dinarides, but consistent within the zone of mid-crustal events where the earthquakes occurred. Focal mechanisms were determined with the first-motion polarity method for eight earthquakes: five of them, including the mainshock, were due to reverse faulting on faults striking in the Dinaric direction, with the preferred main fault gently dipping to the northeast. However, three events were due to normal faulting, unexpected for this area. We constructed a regional seismotectonic cross-section to delineate a potential seismogenic source of the mainshock, and it suggests that the mainshock occurred on the NE-dipping blind ramp of the basal thrust of the Dalmatian tectonic unit. Moreover, another NE-dipping and blind ramp of this basal thrust could be responsible for the Ston–Slano 1996 earthquake, located to the SW of the Berkovići mainshock hypocentre at the horizontal distance of c. 35 km.

Thermal structure of the southern Caribbean and northwestern South America: implications for seismogenesis

Abstract. The seismogenesis of rocks is mainly affected by their mineral composition and in situ conditions (temperature and state of stress). Diverse laboratory experiments have explored the frictional behaviour of the rocks and rock-forming minerals most common in the crust and uppermost mantle. However, it is debated how to “upscale” these results to the lithosphere. In particular, most earthquakes in the crust nucleate down to the crustal seismogenic depth (CSD), which is a proxy for the maximum depth of crustal earthquake ruptures in seismic hazard assessments. In this study we propose a workflow to upscale and validate those laboratory experiments to natural geological conditions relevant for crustal and upper-mantle rocks. We used the southern Caribbean and northwestern South America as a case study to explore the three-dimensional spatial variation of the CSD (mapped as D90, the 90 % percentile of hypocentral depths) and the temperatures at which crustal earthquakes likely occur. A 3D steady-state thermal field was computed for the region with a finite-element scheme using the software GOLEM, considering the uppermost 75 km of a previously published 3D data-integrative lithospheric configuration, lithology-constrained thermal parameters, and appropriate upper and lower boundary conditions. The model was validated using additional, independent measurements of downhole temperatures and heat flow. We found that the majority of crustal earthquakes nucleate at temperatures less than 350 ∘C, in agreement with frictional experiments of typical crustal rocks. A few outliers with larger hypocentral temperatures evidence nucleation conditions consistent with the seismogenic window of olivine-rich rocks, and can be due to either uncertainties in the Moho depths and/or in the earthquake hypocentres or the presence of ultramafic rocks within different crustal blocks and allochthonous terranes accreted to this complex margin. Moreover, the spatial distribution of crustal seismicity in the region correlates with the geothermal gradient, with no crustal earthquakes occurring in domains with low thermal gradient. Finally, we find that the largest earthquake recorded in the region (Mw=7.1, Murindó sequence, in 1992) nucleated close to the CSD, highlighting the importance of considering this lower-stability transition for seismogenesis when characterizing the depth of seismogenic sources in hazard assessments. The approach presented in this study goes beyond a statistical approach in that the local heterogeneity of physical properties is considered in our simulations and additionally validated by the observed depth distribution of earthquakes. The coherence of the calculated hypocentral temperatures with those expected from laboratory measurements provides additional support to our modelling workflow. This approach can be applied to other tectonic settings worldwide, and it could be further refined as new, high-quality hypocentral locations and heat flow and temperature observations become available.

Capturing Directivity in Probabilistic Seismic Hazard Analysis for New Zealand: Challenges, Implications, and a Machine Learning Approach for Implementation

ABSTRACT The proximity of fast-slipping crustal faults to urban areas may result in pulse-like ground motions from rupture directivity, which can contribute to increased levels of damage even for engineered structures. Systematic modeling of directivity within probabilistic seismic hazard analysis (PSHA) remains challenging to implement at the regional scale, despite the availability of directivity models in the literature. In the process of developing the 2022 National Seismic Hazard Model for New Zealand (2022 NSHM), we explored the feasibility and impact of modeling directivity for PSHA at a national scale using the previous generation 2010 NSHM. The results of this analysis allowed us to quantify the impact of directivity on the resulting seismic hazard maps for New Zealand and gain insights into the factors that contribute to the expected increases (and decreases) in ground-motion level. For the 2022 NSHM, the earthquake rupture forecast (ERF) seismogenic source models introduced enormous challenges for directivity modeling due to the abundance of large multisegment or multifault ruptures with complex geometries. To overcome these challenges, we applied a machine learning-based strategy to “overfit” an artificial neural network to capture the distributions of directivity amplification and its variability for each unique rupture in the earthquake rupture forecast. This produces a compact representation of the spatial fields of amplification that are computationally efficient to generate within a complete PSHA calculation for the 2022 NSHM. This flexible and reproducible framework facilitates the implementation of directivity in PSHA at a regional scale for complex ERF source models and opens up the possibility of more complex characterization of epistemic uncertainties for near-source ground motion in practice.