Local Earthquakes Research Articles

Crustal structure beneath the Alcock Seamount in the Andaman Sea

Alcock and Sewell seamounts are important geomorphic features in the Andaman Sea due to their association with the tectonic history of the Sunda subduction zone. Despite the availability of global geophysical data, the nature of crust beneath these seamounts remains debated due to the paucity of local geophysical data. Therefore, a passive ocean bottom seismometer (OBS) experiment was conducted by deploying broadband OBSs in the Andaman region from 25th December 2013 to 10th May 2014. Based on the local earthquakes recorded by OBS, we attempted to analyse the waveforms of the events using the state-of-the-art transfer function, and estimated the crustal velocities beneath the Alcock seamount. The inferred P wave velocity increases from 5.7 km/s to 7.3 km/s at a depth of 9 km and from 7.3 km/s to 8.1 km/s at a depth of 15 km. We validated the upper velocity model using an active OBS record obtained from a concurrent multichannel seismic survey acquired onboard RV Samudra Ratnakar. Further, we carried out traveltime streamer tomography by downward continuing the shot gathers of MCS data and obtaining high-resolution velocity-depth models over two shallow ridges on the Alcock Seamount. Tomography-derived velocity-depth model matches with that estimated from the active and passive OBS records. The presence of high subsurface velocities beneath the Alcock Seamount suggests the occurrence of extensive volcanoclastic deposits, possibly formed as a result of the magmatic intrusion. Higher velocities for deep crustal layers suggest an oceanic or island arc crust beneath the Alcock Seamount, which has probably evolved due to backarc volcanism in the Andaman Sea.

Fiber Optic Acoustic Sensing to Understand and Affect the Rhythm of the Cities: Proof-of-Concept to Create Data-Driven Urban Mobility Models

In the framework of massive sensing and smart sustainable cities, this work presents an urban distributed acoustic sensing testbed in the vicinity of the School of Technology and Telecommunication Engineering of the University of Granada, Spain. After positioning the sensing technology and the state of the art of similar existing approaches, the results of the monitoring experiment are described. Details of the sensing scenario, basic types of events automatically distinguishable, initial noise removal actions and frequency and signal complexity analysis are provided. The experiment, used as a proof-of-concept, shows the enormous potential of the sensing technology to generate data-driven urban mobility models. In order to support this fact, examples of preliminary density of traffic analysis and average speed calculation for buses, cars and pedestrians in the testbed’s neighborhood are exposed, together with the accidental presence of a local earthquake. Challenges, benefits and future research directions of this sensing technology are pointed out.

Monitoring seismicity in the southern Sichuan Basin using a machine learning workflow

Monitoring seismicity in real time provides significant benefits for timely earthquake warning and analyses. In this study, we propose an automatic workflow based on machine learning (ML) to monitor seismicity in the southern Sichuan Basin of China. This workflow includes coherent event detection, phase picking, and earthquake location using three-component data from a seismic network. By combining PhaseNet, we develop an ML-based earthquake location model called PhaseLoc, to conduct real-time monitoring of the local seismicity. The approach allows us to use synthetic samples covering the entire study area to train PhaseLoc, addressing the problems of insufficient data samples, imbalanced data distribution, and unreliable labels when training with observed data. We apply the trained model to observed data recorded in the southern Sichuan Basin, China, between September 2018 and March 2019. The results show that the average differences in latitude, longitude, and depth are 5.7 ​km, 6.1 ​km, and 2 ​km, respectively, compared to the reference catalog. PhaseLoc combines all available phase information to make fast and reliable predictions, even if only a few phases are detected and picked. The proposed workflow may help real-time seismic monitoring in other regions as well.

Evaluation of fault stability and seismic potential for Hutubi underground gas storage due to seasonal injection and extraction

The Hutubi gas field was put into production in 1998 and then converted into an underground gas storage (UGS) facility in 2013, and since then a cluster of earthquakes associated with seasonal injection and extraction activities have been recorded nearby. To evaluate the fault stability and seismic potential, we established a pseudo-3D geomechanical model to simulate the process of seasonal injection and extraction. Reservoir pore pressures from 1998 to 2019 were obtained through multiphase reservoir simulation and validated by history matching the field injection and production data. We then imported pore pressures into the geomechanical model to simulate the poroelastic perturbation on faults for over 20 years. The fidelity of this model was validated by comparing the simulated surface deformation with global positioning system (GPS) measured data. We used Coulomb failure stress (CFS) as the indicator for the likelihood of fault slippage. The simulation results show that the location of the induced earthquake cluster was within the positive Coulomb stress perturbation (ΔCFS) area, in which fault slippage was promoted. In addition, ΔCFS at the earthquake location kept increasing after the injection began. These findings could explain the induced earthquakes with the Coulomb failure stress theory. Furthermore, we conducted a parameter sensitivity study on the dominant factors such as the maximum operating pressure (MOP), frictional coefficient, and dip angle of the pre-existing fault. The results indicate that the magnitude of ΔCFS caused by seasonal injection and extraction decreases with distance; MOPs are constrained to 32.9, 36.2, and 39.5 MPa according to different ΔCFS thresholds; the critical dip angle ranges are 0–20° and 80°–100°; and strengthening the fault friction can either increase or decrease the seismic potential. This study can help determine the MOP for Hutubi underground gas storage (HTB UGS) and provide a framework for simulating the potential causes of induced seismicity for other sites.

Major California faults are smooth across multiple scales at seismogenic depth

Surface traces of earthquake faults are complex and segmented on multiple scales. At seismogenic depth the detailed geometry of faults and earthquake rupture is mainly constrained by earthquake locations. Standard earthquake locations are usually too diffuse to constrain multi-scale fault geometry, while differential-timing relocation mainly improves finest scale precision. NLL-SSST-coherence, an enhanced, absolute-timing earthquake location procedure, iteratively generates traveltime corrections to improve multi-scale precision and uses waveform similarity to improve fine-scale precision. Here we apply NLL-SSST-coherence to large-earthquake sequences and background seismicity along strike-slip faults in California. Our relocated seismicity at seismogenic depth along major fault segments and around large-earthquake ruptures often defines smooth, planar or arcuate, near-vertical surfaces across the sub-km to 10’s of km scales. These results show that multi-scale smooth fault segments are characteristic of major, strike-slip fault zones and may be essential to large earthquake rupture. Our results suggest that smoothness and curvature of faults influences earthquake initiation, rupture, rupture direction and arrest, and can define earthquake hazard. The results corroborate that surface traces of strike-slip fault zones reflect complex, shallow deformation and not directly simpler, main slip surfaces at depth, and support use of planar or smoothly curved faults for modeling primary earthquake rupture.

A Disturbance Frequency Index in Earthquake Forecast Using Radio Occultation Data

Earthquake forecasting is the process of forecasting the time, location, and magnitude of an earthquake, hoping to gain some time to prepare to reduce the disasters caused by earthquakes. In this paper, the possible relationship between the maximum electron density, the corresponding critical frequency, and the occurrence of earthquakes is explored by means of radio occultation data based on mechanism analysis and actual earthquake-nearby data. A new disturbance frequency index is proposed in this paper as a novel method to help forecast earthquakes. Forecasting of the location and timing of earthquakes is based on the connection between proven new frequency distributions and earthquakes. The effectiveness of this index is verified by backtracking observation around the 2022 Ya’an earthquake. Using this index, occultation data can forecast the occurrence of earthquakes five days ahead of detection, which can help break the bottleneck in earthquake forecasting.

Potential for rupture before eruption at Campi Flegrei caldera, Southern Italy

Volcanoes reawakening after long repose must rupture the crust before magma can erupt. Rupture is preceded by repeatable variations in the rate of seismicity with ground movement, which trace the amount of applied stress that is released by local earthquakes. A rupturing sequence has been developing across four episodes of ground uplift at Italy’s Campi Flegrei caldera: in 1950-1952, 1969-1972, 1982-1984 and since 2004. We predicted in 2016 that the approach to rupture would continue after an additional uplift of 30-40 cm at the location of largest movement. We have updated our analysis with new data on changes in the numbers of local earthquakes with amounts of ground movement. Here we show that subsequent events have confirmed our prediction and that the unrest has been changing the structure of Campi Flegrei’s crust. The results provide new constraints for evaluating the volcano’s potential to erupt or to subside without eruption.

Seismo-Hydrogeodynamic Effects in Groundwater Pressure Changes: A Case Study of the YuZ-5 Well on the Kamchatka Peninsula

Seismo-hydrogeodynamic effects (SHGEs) in groundwater level (pressure) variations in a range of periods from minutes to hours and days during local and teleseismic earthquakes were considered based on the data of precision observations in a deep piezometric well located in a seismically active region. With the use of the tidal analysis and frequency dependence of the barometric response of the water level, a static confined response of groundwater pressure in a range of periods from hours to the first tens of days was established. The annual water level trend was characterized by the seasonal function of a hydrostatic head change in the well. In the groundwater pressure, changes were detected due to several types of seismo-hydrogeodynamic effects: 1—the coseismic fluctuations during the first tens of seconds and minutes after the arrival of seismic waves from the earthquakes with magnitudes of 5.3–9.1 at epicentral distances of 80–700 km; 2—the supposed hydrogeodynamic precursors of the two strongest events; 3—the four types of variations under the vibration impact of seismic waves from Mw = 6.8–9.1 earthquakes at epicentral distances of 80–14,600 km. The dependence of the distinguished types of SHGEs on the earthquake parameters, the intensity of the seismic impact in the well area and the amplitude-frequency composition of seismic waves were considered.

Mapping of the seismic tsunami intensity along the coast of Martinique, Lesser Antilles

The tsunami hazard in Martinique due to local, regional and distant seismicity is investigated. Former deterministic scenarios are processed. They were proposed elsewhere based on historical records and the analysis of the morpho-tectonics of the area, i.e., two local subduction scenarios of magnitudes Mw 7.5 and Mw 8.0 representative of the local 1839 earthquake, the regional Mw 7.2–7.5, 1867 Virgin Island event, along with the distant Mw ∼8.5, 1755 Lisbon earthquake. The tsunami runup and currents are simulated and analyzed. Then, a recent tsunami intensity index is applied and discussed. The modeling of the regional 1867 scenario shows negligible impact on the coast of Martinique which could explain the lack of direct observations in Martinique. Despite a difference of one unit magnitude between the local Mw 7.5 and the distant Mw ∼8.5 earthquakes, it is found that their tsunami signature for Martinique is nearly equivalent. The local Mw 8.0 event thus controls the tsunami hazard in the island.In the case of this tsunami, whose source is at the east, the western side of the island is not impacted because of wave refraction. Eastward, the coral reef barrier also inhibits tsunami impacts in some sites. Interestingly, it is shown that most of the potentially inundated areas are mangroves that are already subject to daily tidal flooding and seasonal cyclonic activity. They represent an efficient protection against tsunamis. Four occupied locations are however exposed to severe inundation (horizontal penetration of hundreds of meters): Pointe Marin, Le Vauclin, Le Robert and, for the largest, Baie de la Trinité.In terms of current hazard, we have applied two simple procedures to identify sheltered areas for watercrafts: an integrated one, that is easier to use but less accurate, and a direct one, derived from the mapping, which is more complex to implement by end-users but of better precision and efficiency. A large tsunami intensity is obtained at the 4 sites exposed to inundation, possibly yielding severe damages on land and at sea, and which would be extreme at Baie de la Trinité. Even a distant tsunami could impact that site although to a lower extent compared to a strong local earthquake.

Exploratory Data Analysis of Earthquake Moment Tensor Catalog in Japan Using Non-linear Graph-Based Dimensionality Reduction

The catalog of moment tensor solutions, which contains the information on spatial location, origin time and fault mechanism type of earthquakes, has been interpreted by researchers based on a wealth of past knowledge. However, the long-term routine analyses have led to the accumulation of a huge amount of data in the moment tensor catalog, and it is worth considering moving away from the artisanal approach. In this study, using dimensionality reduction of unsupervised machine learning, we performed exploratory data analysis of the moment tensor catalog in Japan to objectively obtain comprehensive images of seismic activity and to acquire knowledge on the spatial and temporal characteristics of the earthquake mechanism. Source parameters of the moment tensor catalog in Japan, spatial location (latitude, longitude, and focal depth) and source-mechanism diagram information were embedded in two-dimensional space via a non-linear graph-based dimensionality-reduction method, Uniform Manifold Approximation and Projection. On the embedding map, earthquakes in eastern and western Japan are distributed separately and are further embedded to reflect their characteristic fault mechanism and focal depth in each region. The similarity degree of the earthquakes can be obtained as the distance on the embedding map. This study demonstrates that the data visualization using dimensionality reduction is useful for intuitively and objectively understanding the regional characteristics of earthquake mechanisms. The embedding map can also be employed to visualize temporal changes in regional seismic activity and to perform a similarity search with a past event.

High-precision seismic relocation and three-dimensional shallow crustal structure in the southern Sichuan basin

Since 2014, rates of earthquakes have increased rapidly in the Weiyuan and Changning regions of the southern Sichuan basin, where the shale gas and salt mine have been operated. We perform double-difference seismic tomography using P- and S-wave arrival times from 44,759 events recorded by the permanent seismic networks and two local networks. Data augmentation is offered by a temporary broadband array deployed recently in the Weiyuan region. Relocated seismicity is concentrated at depths of 2–8 km, and mainly occurs in and around hydraulic fracturing well pads or salt wells. Some moderate earthquakes occur relatively deeper. The distribution of M 3.0+ earthquakes clearly reveal several pre-existing faults in the Weiyuan region. Our retrieved three-dimensional shallow crustal models show that although the Vp and Vs values at earthquake locations vary between the Weiyuan and Changning shale gas demonstration zones, the Vp/Vs ratios are coherently low. Such low Vp/Vs values of the source regions suggest the presence of gas-saturated cracks with relatively large aspect ratios under high pore pressure, rather than a great number of liquid-filled cracks.

Classification of Earthquake Damage of Buildings in Songyuan Area Based on Image Recognition

Since the emergence of human beings on the earth, various disasters have been accompanied. Among many natural disasters, the earthquake is undoubtedly one of the most threatening disasters. This project uses Res Net-50 model for deep learning and image recognition of building structural damage. Through the program to assess the local earthquake damage, given the feasible standards to facilitate a unified understanding of the earthquake situation, thereby improving the efficiency of disaster relief. Through experiments, the accuracy of the training set of the two classifications finally reached about 89.3 %, and the prediction accuracy of the test set finally reached about 71.4 %, Through the identification of post-earthquake building damage in Songyuan area, it can be learned that the accuracy of the software identification binary classification task is 73.21 %. Experiments show that taking photos can be used to predict the damage level of buildings in a certain area, and seismic damage identification can provide basis and support for post-disaster rescue and reconstruction and economic loss assessment.

On the seismic response and earthquake-triggered failures of subaqueous slopes in Swiss lakes

SUMMARY Seismically triggered subaqueous mass movements in lakes may generate tsunamis that can cause significant damage on the shore. In this study, we assess the seismic response and stability of subaqueous slopes in Swiss lakes based on recorded seismological data, historical and geological information and geotechnical surveys. We performed seismic investigations at multiple locations in Lake Lucerne using Ocean Bottom Seismometers (OBS). For these locations, we derived ground-motion amplification functions from local and regional earthquakes and horizontal-to-vertical spectral ratios (H/V) from the earthquake and ambient vibration recordings. The results show (1) very high amplification levels, often exceeding values of 50–100 in the frequency range between 1 and 10 Hz, (2) the fundamental frequency of resonance in the range of 0.5–3.5 Hz and (3) laterally variable site response even for closely located stations. We sought also the signatures of non-linear site response in the H/V curves or ground-motion amplification functions but found only weak indicative effects and no clear evidence. This is most likely due to the low levels of ground motion recorded during the OBS campaigns. We conducted back analyses of historical earthquakes in Switzerland with available documental and/or geological evidence of induced (tsunamigenic) subaqueous slope failures in Swiss lakes. The data set of historical events was complemented with a selection of instrumentally recorded earthquakes in Switzerland. For the analyses, we selected multiple sites in Swiss lakes which failed in the past or are prone to failure in the future. We modelled the ground motion at these locations assuming Swiss standard reference rock conditions (vs30 = 1105 m s−1). The modelled ground motion intensity measures (IM) included peak ground acceleration (PGA), peak ground velocity (PGV) and pseudospectral acceleration (PSA) at 0.3, 1 and 2 s. We estimated the minimum ground motion and macroseismic intensity at reference rock conditions required to trigger the failures of subaqueous slopes. In addition, we defined a threshold for the seismic triggering of such failures in terms of moment magnitude (Mw) and epicentral distance (Re) as: $$\begin{eqnarray} M_{\rm w}=2.891+1.904\log_{10}(R_e+5.166)\: {\rm for}\: R_e\ge 3.7\: {\rm km}. \end{eqnarray}$$ Our results are consistent with previous studies based on worldwide observations. Furthermore, we related the modelled ground motions to the Swiss seismic hazard products and estimated the return period of critical ground shaking responsible for triggering subaqueous slope failures (with potential for tsunami generation) to be in the range of 36–224 yr. Finally, based on previously collected geotechnical data (in situ Cone Penetration Testing and laboratory sediment analysis), we determined the most likely values of the seismic coefficient k to be used with the ground motion IMs modelled at reference rock conditions in infinite slope stability analyses to estimate the factor of safety (FS). For PGA, we found a k = 1; for PGV, k = 2; for PSA0.3s, k = 0.6; for PSA1s, k = 2 and for PSA2s, k = 5.5. These estimates are conservative and affected by the trade-off between the thickness of unconsolidated sediments and the slope angle. Thus, we recommend applying them to slopes with a low-to-moderate gradient (<15°) and sediment thickness of more than 2 m.

LOTOS code for the local earthquake tomography in the Bucaramanga nest, Colombia

LOTOS code for the local earthquake tomography in the Bucaramanga nest, Colombia

Lateral variations in the signature of earthquake‐generated deposits in Lake Iznik,NWTurkey

AbstractUsing lake‐sediment cores to document past seismicity requires a comprehensive understanding of possible lateral variations in depositional processes. This study aims to reveal the lateral variations in earthquake‐induced event deposits throughout Lake Iznik, a large lake located on the middle strand of the North Anatolian Fault. Based on stratigraphic, sedimentological and geochemical analyses of 14 sediment cores from two subbasins across the lake, five different types of event deposits (T1–T5) were identified and characterised. One event deposit type (T5) is restricted to a delta mouth, characterised by the occurrence of authigenic Fe‐Mn carbonates and interpreted to result from flood events. The four other types of event deposits are characterised by their synchronicity between cores and their age consistency with historical earthquakes and are interpreted to be likely generated by earthquakes. The locally prominent 1065 CE historical earthquake that ruptured the sub‐lacustrine Iznik Fault produced at least three different types of event deposits. One deposit type (T2) is only observed for this very local earthquake, implying that the type of event deposit might also depend on ground‐motion parameters. At the lake scale, the occurrence of various event deposits depends on the flow distance from the source of sediment destabilisations to the coring site.

Geochemical precursory characteristics of soil gas Rn, Hg, H2, and CO2 related to the 2019 Xiahe Ms5.7 earthquake across the northern margin of West Qinling fault zone, Central China

The Xiahe Ms5.7 earthquake occurred in Xiahe county, Gannan prefecture, China (35.10°N, 102.69°E) on October 28, 2019, with a source depth of 10 km. This study investigates the spatial and temporal evolution characteristics of cross-fault soil gas concentrations prior to the Xiahe Ms5.7 earthquake by analyzing Rn, Hg, H2, and CO2 data collected from 11 profiles across the northern margin of the West Qinling fault zone from 2016 to 2019. The spatial distribution of these gases showed varying trends, with Rn concentration intensity decreasing from the Wushan segment to the east and west sections, while Hg, H2, and CO2 all broke the trend in the West Qinling fault zone's northern margin. The soil gas concentration intensity demonstrated a significant response to the Xiahe Ms5.7 earthquake, particularly in the west Ganjia sections. By integrating the seismogenic model and numerical simulation results, we explored the physical mechanism underlying these abnormal trends. Our findings suggest that the continuous decline characteristic of fault gas could be a valuable indicator of fracture tectonic activity, while an upward trend after continuous decline may signal a medium and short-term seismogenic event in the source area. These results provide a foundation for improved tracking of earthquake location and timing in a fault zone through cross-fault soil gas methods, highlighting the importance of enhancing deep fluid flow monitoring and seismogenic model research in fault zones.

Seismic anisotropy in the upper crust around the north segment of Xiaojiang faults in the SE margin of Tibetan Plateau

The Xiaojiang faults located in the SE margin of the Tibetan Plateau is a fault system of left-lateral strike-slip, striking NS, between the 2nd-order Sichuan-Yunnan block and the 1st-order South China block. The Xiaojiang faults and the surrounding areas are characterized by strong tectonic movements and intense seismic activities. Using seismic data from January 2013 to November 2020 recorded at the stations of the temporary QiaoJia seismic Array (QJ Array), deployed by the Institute of Geophysics, China Earthquake Administration, this study investigates the upper crustal anisotropy by the shear-wave splitting analysis on small local earthquakes, discusses the deformation patterns in the upper crust in the north segment of the Xiaojiang faults, evaluates the stress distribution in the study area, and analyzes its relationship with the regional tectonic structure. Adopting the data processing technique of shear-wave splitting, a total of 875 effective records were obtained at 50 stations. The mean direction of polarizations of fast shear-wave (PFS) is 162° ± 44° in the study areaand the mean normalized time-delay is 4.96 ± 2.38 ms/km. Based on the spatial distribution of the PFS and the regional geologic structure, the study area is divided into two zones: the zone N and the zone S. The PFS in the zone N is scattered, but the dominant PFS direction is in NNW, which is consistent with the direction of the regional maximum principal compressive stress. In the zone N, there are a few smaller local areas (i.e., subzones A, B, C, and D) in which the orientations of the PFS are quite different from the surrounding area. In the zone S, the dominant directions at most stations are in nearly NS, consistent with the strike of the Xiaojiang fault. It reveals the detailed spatial distribution of seismic anisotropy in the upper crust, as well as in situ principal compressive stress, indicating the influence of the regional stress, the complex tectonic environment, and maybe also the impact of the South China block. It also reveals that there also might be an upper-crust scale of tectonic line at near 26°20′N under Xiaojiang faults, which coincides with the north-south tectonic boundary in the lithospheric anisotropy.

Micro-seismic events detection and its tectonic implications in Northeastern Hainan Province

In this paper, we combine the U-net-based phase picking method (PhaseNet) with Graphics Processing Unit-Based Match and Locate technology (GPU-M&L) and a deep-learning-based seismic signal de-noising method (DeepDenoiser) as a workflow for automatically extracting micro-seismic information from continuous raw seismic data. PhaseNet is first used to detect missed seismic phases by scanning through the 5-year continuous waveform data recorded at five broad-band stations in Hainan province. Then Rapid Earthquake Association and Location method (REAL), VELEST program (1-D inversion of velocities and hypocenter locating) and HypoDD (a double-difference locating method) are applied to associate seismic phases with events and to locate, respectively. This initially established catalogue can be served as the template for the following match-filter work. We choose events with a high signal-to-noise ratio (SNR) as templates and apply GPU-M&L to detect more small earthquakes which are difficult to pick by routine methods due to the low SNR. Then, a deep learning-based noise reduction technique named DeepDenoiser is applied to extract seismic signal from noise to provide a better picking of arrival time and then to improve the relocation effects. Finally, we use HypoDD to relocate these events with P- and S- wave arrival times picked by PhaseNet. Compared with the five events listed in the China Earthquake Networks Center routine catalogue, in this study, we detect and locate 977 earthquakes by following the above procedure. Our relocation results illustrate quite a complex distribution pattern of events due to the complicated fault system in the northeastern part of Hainan Province.

Properties of the Petrinja (Croatia) earthquake sequence of 2020–2021 – Results of seismological research for the first six months of activity

We present locations of almost 14,000 events from the first six months of the Petrinja (Croatia) earthquakes sequence (mainshock 29 December 2020, Mw 6.4). The catalogue is estimated complete for ML ≥ 1.20. Initially sparse local seismograph network was densified a week after the mainshock, which reduced uncertainties of focal locations by about 75%. Source-specific station corrections were used in an iterative location scheme. The hypocentres were located with 30 different sets of velocity models and program control parameters in order to gain insight into the epistemic uncertainty of each earthquake location. The bulk of epicentres are located close to, and to the SE of the causative, dextral strike-slip Petrinja fault, but considerable activity has been triggered on smaller faults, up to 25 km away. About a half of the 78 first-motion polarity focal mechanism solutions (FMS) from the full first year of activity were found to have mechanisms similar to the foreshock, the mainshock and the largest of aftershocks, while the rest indicated almost pure reverse faulting. Strike-slip mechanisms occurred more often close to the main fault. The P-axes of FMS for aftershocks were found to be on the average rotated by 16° clockwise with respect to the P-axis of the mainshock, regardless of the style of faulting. The spatial distribution of aftershocks coincides very well with the areas of positive Coulomb stress change caused by the mainshock rupture. The inferred cut-off seismogenic depth in the area (7–10 km), estimated on the basis of published values of the geothermal gradient and the assumption of predominantly granitic upper crust, is considerably shallower than the depth of the deepest reliably located aftershocks (16–18 km). This fact may be explained by considering ophiolite-dominated crust, and by increased strain rate during the sequence, which raises the ability of crustal rocks for brittle failure.

Semi-automated template matching and machine-learning based analysis of the August 2020 Castelsaraceno microearthquake sequence (southern Italy)

The accurate characterization of microearthquake sequences allows seismologists to better understand the physical processes involved in earthquake nucleation and rupture propagation and to gain insights on fault geometry at depth. Standard procedures for seismic sequences analysis are based on manual detection and phase-picking, requiring a huge amount of work from expert seismologists, particularly in the case of microseismic events. Here we show how the investigation of a low-magnitude foreshock-mainshock-aftershock sequence, occurred in August 2020 close to Castelsaraceno village (southern Italy), greatly benefited from the application of a semi-automated template matching and machine-learning based workflow. The phase-picking was automatically performed through a deep-learning algorithm on 202 microearthquakes detected between July and October 2020, followed by an automatic multi-step absolute and relative earthquake location procedure. The 72 relocated events of the seismic sequence were clustered in time (7–12 August) and in a narrow range of depths (10–12 km). The Ml 2.1 foreshock doublet and the Ml 2.9 mainshock identified a persistent asperity. The joint analysis of aftershocks distribution, the mainshock focal mechanism, and the geology of the study area suggest the occurrence of the sequence along a NNE-SSW left-lateral, transtensional fault in the brittle portion of the crystalline basement.