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In this work, we propose a geodetic model for the seismic sequence, with doublet earthquakes, that occurred in Bandar Abbas, Iran, in November 2021. A dataset of Sentinel-1 images, processed using the InSAR (Interferometric Synthetic Aperture Radar) technique, was employed to identify the surface deformation caused by the major events of the sequence and to constrain their geometry and kinematics using seismological constraints. A Coulomb stress transfer analysis was also applied to investigate the sequence’s structural evolution in space and time. A linear inversion of the InSAR data provided a non-uniform distribution of slip over the fault planes. We also performed an accurate relocation of foreshocks and aftershocks recorded by locally established seismographs, thereby allowing us to determine the compressional tectonic stress regime affecting the crustal volume. Despite the very short time span of the sequence, our results clearly suggest that distinct blind structures that were previously unknown or only suspected were the causative faults. The first Mw 6.0 earthquake occurred on an NNE-dipping, intermediate-angle, reverse-oblique plane, while the Mw 6.4 earthquake occurred on almost horizontal or very low-angle (SSE-dipping) reverse segments with top-to-the-south kinematics. The former, which cut through and displaced the Pan-African pre-Palaeozoic basement, indicates a thick-skinned tectonic style, while the latter rupture(s), which occurred within the Palaeozoic–Cenozoic sedimentary succession and likely exploited the stratigraphic mechanical discontinuities, clearly depicts a thin-skinned style.

The microseismic activity and subsurface structures are critical for assessing potential seismic hazards and improving mining safety. However, due to the limitations of the seismic stations in the Laohutai coal mine area, the characteristics of the complex microseismic activity and subsurface structure remain debated and poorly understood. In this study, we deployed a new linear dense seismic array in the Laohutai coal mine, and detected 324 new microseismic events by applying the Match and Locate method. The double-difference relocation results show that the microseismic events are mainly distributed around the sub-faults of the Laohutai coal mine at depths of ~ 0.2–1.5 km and gradually migrate toward the coal mine over time. In addition, we constructed ambient noise tomography to further investigate the subsurface structure of the coal mine. The tomographic results indicate that the shear-wave velocity structure is well-correlated with the subsurface tectonics. The microseismic activities are more likely to occur near the high-velocity zone after a minor stress disturbance, suggesting that the microseismic activities may be associated with the characteristic of the subsurface structure. The relationship between microseismic events and the subsurface structures contributes to the understanding of seismic behavior in coal mines and provides valuable insights into mine exploration. Graphical Abstract

The 2020 Jepara earthquake (Mw 6.7) represents a significant deep-focus seismic event within the subducting Indo-Australian Plate beneath Java Island. This study investigates its characteristics using automatic moment tensor inversion and double-difference relocation based on seismic data from the Meteorology, Climatology, and Geophysics Agency (BMKG). A total of 1,899 earthquakes recorded between 2010 and 2022 were analyzed, focusing on events with a depth of up to 600 km. Moment tensor analysis revealed a normal faulting mechanism with two nodal planes: the first with a strike of 310º, dip of 59º, and rake of -68º, and the second with a strike of 92º, dip of 37º, and rake of -121º. The centroid depth was refined to 544 km, and the variance reduction (VR) exceeded 60%, which classifies the results as high-quality (Category A). Approximately 85% of the hypocenters were successfully relocated, improving root mean square (RMS) travel times to less than 1 second. The earthquake is associated with slab dehydration and the polymorphic phase transition of olivine to spinel under high pressure and temperature. These processes contribute to changes in fault mechanisms and the structural variation of the mantle transition zone. The findings enhance understanding of the deep-focus earthquakes beneath Java and can significantly contribute to future earthquake mitigation in Indonesia.

The Arkalochori village in central Crete was hit by a large earthquake (Mw = 6.0) on 27 September 2021, causing casualties, injuries, and severe damage to the infrastructure. Due to the absence of apparent surface rupture and the initial focal mechanism solution of the seismic event, we initiated complementary, multi-disciplinary research by combining seismological and remote sensing data processing, followed by extensive field validation. Detailed geological mapping, fault surface measuring accompanied with tectonic analysis, fault photorealistic model creation by unmanned aerial system data processing, post-seismic surface deformation analysis by DInSAR image interpretation coupled with accurately relocated epicenters recorded by locally established seismographs have been carried out. The combination of the results obtained from these techniques led to the determination of the contemporary tectonic stress regime that caused the earthquake in central Crete, which was found compatible with extensional processes parallel to the Hellenic arc.

The occurrence of faulting in subducting plates is a major process that changes the mechanical properties of the subducting lithosphere and carries surface materials into mantle wedges. Two ocean-bottom seismometer networks deployed on the frontal accretionary wedge of the northern Manila trench in 2005 and on the outer slope of the trench in 2006 were used to detect earthquakes in the subducting plate. All available P and S manually picked phases and the waveforms of 16 short-period, three-component stations were used. Relocation was performed using the double-difference method with differential times derived from the phase-picked data. Two intraplate earthquake sequences of small-to-moderate magnitudes in the northern Manila subduction system were investigated in this study. The results revealed distinct fault planes, but a contrasting seismogeny over the northern Manila Trench. The seismicity in the frontal wedge (as measured in 2005) was mainly contributed by a fluid overpressure sequence, whereas that in the incoming plate (as measured in 2006) was contributed by the aftershocks of an extensional faulting sequence. The obtained seismic velocity models and Vp/Vs ratios revealed that the overpressure was likely caused by high pore-fluid pressure within the shallow subduction zone. By using the near-field waveform inversion algorithm, we determined focal mechanism solutions for a few relatively large earthquakes. Through the use of data obtained from global seismic observations, we determined that stress transfer may be responsible for the seismic activity in the study area during the period of 2005–2006. In late 2005, the plate interface in the frontal wedge area was unlocked by the overpressure effect due to a thrusting-dominant sequence. This event changed the stress regime across the Manila Trench and triggered a normal fault extension at the outer trench slope in mid-2006. However, in the present study, a hybrid focal mechanism solution indicating reverse and strike–slip mechanisms was implemented, and it revealed that the plate interface locked again in late 2006.

We here show relocated hypocenters within a seismic sequence with normal faults in the northern part of Ibaraki Prefecture, triggered after the M 9.0 2011 off the Pacific coast of Tohoku Earthquake. Depth-sections of the hypocenters from the center of the northern Ibaraki region to the north show an earthquake alignment dipping westwards at 40° to 50° at depths shallower than 10 km. On the other hand, hypocenters from the center to the south show a cross-cutting geometry consisting of conjugate westward- and eastward-dipping planes at the same depths. The dip angles of the hypocenter alignments are roughly consistent with the nodal planes of focal mechanisms of large normal earthquakes, and exhibit optimal-orientations in terms of the frictional failure criterion. Furthermore, comparison of the focal mechanisms recorded before and after the 2011 Tohoku Earthquake suggests that the stress field abruptly changed from horizontal compression to extension in the study area. The most plausible explanation of the drastic stress changes is a significant reduction in trench-normal compressive stress compared with reduction in trench-parallel stress accompanying large horizontal extensional deformation within the overlying plate.

We investigate an earthquake sequence involving an Mw = 4.6 mainshock on 2 December 2020, followed by a seismic swarm in July–October 2021 near Thiva, Central Greece, to identify the activated structures and understand its triggering mechanisms. For this purpose, we employ double-difference relocation to construct a high-resolution earthquake catalogue and examine in detail the distribution of hypocenters and the spatiotemporal evolution of the sequence. Furthermore, we apply instrumental and imaging geodesy to map the local deformation and identify long-term trends or anomalies that could have contributed to stress loading. The 2021 seismic swarm was hosted on a system of conjugate normal faults, including the eastward extension of the Yliki fault, with the main activated structures trending WNW–ESE and dipping south. No pre- or coseismic deformation could be associated with the 2021 swarm, while Coulomb stress transfer due to the Mw = 4.6 mainshock of December 2020 was found to be insufficient to trigger its nucleation. However, the evolution of the swarm is related to stress triggering by its major events and facilitated by pore-fluid pressure diffusion. The re-evaluated seismic history of the area reveals its potential to generate destructive Mw = 6.0 earthquakes; therefore, the continued monitoring of its microseismicity is considered important.

On April 20, 2013, the Lushan M~7.0 earthquake struck at the southern part of the Longmenshan fault in the eastern Tibetan Plateau, China. The shear-wave splitting in the crust indicates a connection between the direction of the principal crustal com- pressive stress and the fault orientation in the Longmenshan fault zone. Our relocation analysis of the aftershocks of the Lushan earthquake shows a gap between the location of the rupture zone of the Lushan Ms7.0 earthquake and that of the rup- ture zone of the Wenchuan MsS.0 earthquake. We believe that stress levels in the crust at the rupture gap and its vicinity should be monitored in the immediate future. We suggest using controlled source borehole measurements for this purpose.

Reservoirs are an important component in geothermal systems. Hot fluids accumulate in reservoirs, which consist of thick layers of permeable rock. Micro-earthquake methods can be used to identify the permeable zone in a geothermal system. In particular, observations of micro-earthquake hypocenters provide a promising technique to detect such permeable zones. Determinations of the hypocenters are performed using the single-event determination method. Micro-earthquake hypocenter relocations are performed to obtain more accurate locations and to reduce errors that occur because of inaccurate velocity models. The relocation method that is used in this study is double-difference relocation, which is the most efficient and fastest method available and is capable of generating small errors with no necessity of station corrections. In this study, we use data recordings by 18 stations of the micro-earthquake activity in the geothermal Field R from June to October 2012. The processing data start from raw time series data. The distributions of the relocated hypocenters were then matched to the supporting magnetotelluric and geologic data. The result of this study shows that there is a significant amount of seismic activity on the southern part of Mount R. The distribution of micro-earthquakes forms a cluster and a northwest-southeast structure pattern. The distribution of the hypocenters can be interpreted as the permeable zone beneath the surface, with the northwest- southeast fault pattern as the hydrogeology controller of the geothermal system in the geothermal Field R.

The seismicity rate markedly increased in the northeastern Noto Peninsula of Ishikawa Prefecture around the end of 2020, with an M w 6.2 event on 5 May, 2023, followed by many aftershocks. Previous earthquake relocation studies have detected upward migration of microearthquakes via multiple faults and clusters, suggesting the involvement of crustal fluids in this sequence. Since some active faults exist near the source region, there was concern that this sequence could lead to a larger earthquake; it became a reality with the M w 7.5 earthquake on 1 January 2024. This study aims to develop an algorithm to precisely relocate microearthquake hypocenters in quasi-real time for better monitoring. A fine view of seismicity requires relative relocation methods such as the double-difference (DD) method with numerous arrival time difference data precisely measured by the waveform correlation analysis. However, the standard DD method has the disadvantage of huge computational costs when data increase, making it unsuitable for real-time monitoring in such situations. We developed a quasi-real-time algorithm that relocates only new earthquakes using the DD method each time a new time window of data is added. The major improvement is that our method incorporates a traditional simple relative relocation and preserves constraints between different time windows; the relative locations of new events are constrained from reference events that were already relocated in the previous time windows. We tested a daily relocation algorithm on 11,546 events from 19 June, 2022, to 31 May, 2023, in the Noto Peninsula earthquake sequence. We found that our modification substantially reduced artificial hypocenter offsets between different time windows and succeeded in resolving the fine fault structures from the cloud-like distribution of initial hypocenters. If we do not impose constraints between different windows, the relocated hypocenters are scattered and do not show fine planar structures. Our algorithm greatly reduces the computational cost, allowing for quasi-real-time earthquake relocation and monitoring. We hope this algorithm will help monitor the spatio-temporal distribution of future earthquake sequences. Graphical Abstract