Hypocenter Locations Research Articles

Upper crustal structure across the Xiaojiang fault system revealed by ambient noise tomography from a dense short-period array

The Central Yunnan block (CYB) plays a key role in accommodating the crustal tectonic deformation of the southeastern Tibet, but its exact mechanism is poorly understood. Here we obtained a fine S-wave velocity structure of the upper crust across the Xiaojiang fault system in CYB, using the ambient noise tomography with a linear dense short-period array. The result shows a strong S-wave velocity variation along the profile, which is consistent with geomorphology and hypocenters occurred in this area. The velocity variations along the profile correspond well to the major faults seen on the surface and are consistent with the transition from surface erosion, to ductile basement and to rigid basement in the greater depth. Combining the upper crustal structure from our image with the location of hypocenters, we propose that most faults of Xiaojiang fault system cut through the upper crust steeply and control the tectonic deformation of the upper crust in the CYB.

The micro‐seismicity of Co. Donegal (Ireland): Defining baseline seismicity in a region of slow lithospheric deformation

AbstractA catalogue of precisely located micro‐seismicity is fundamental for investigating seismicity and rock physical properties in active tectonic and volcanic regions and for the definition of a ‘baseline’ seismicity, required for a safe future exploitation of georesource areas. In this study, we produce the first manually revised catalogue of micro‐seismicity for Co. Donegal region (Ireland), an area of about 50K M2 of on‐going deformation, aimed at localizing natural micro‐seismic events occurred between 2012 and 2015. We develop a stochastic method based on a Markov chain Monte Carlo (McMC) sampling approach to compute earthquake hypocentral location parameters. Our results indicates that micro‐seismicity is present with magnitudes lower than 2 (the highest magnitude is 2.8).The recorded seismicity is almost clustered along previously mapped NE‐SW trending, steeply dipping faults and confined within the upper crust (focal depth less than 10 km). We also recorded anthropogenic seismicity mostly related to quarries' activity in the study area.

Effect of Geometry and Number of Seismic Stations on Micro-Earthquake (MEQ) Hypocenters in Geothermal Fields

Micro-earthquake (MEQ) distribution describes subsurface conditions that can contribute to monitoring the dynamics of geothermal reservoirs. Thus, the distribution of MEQ hypocenter locations with high accuracy becomes extremely important. Experiments were conducted with 3 variations of geometry and number of seismic stations, while Geiger and Coupled Velocity-Hypocenter methods were used to determine the location of MEQ. Experimental results show that in determining the location of the MEQ, the geometry and number of seismic stations played an important role. Increasing the number of stations with relatively long distances can result in less accurate locations of MEQ, error and bias in determining the location of MEQ will be greater when the azimuth gap value is greater. This is shown by the distribution of MEQ that are more spread out in variations 4A and 4B (4 seismic stations) compared to the distribution of MEQ hypocenters using data from 8 seismic stations. The azimuth gap variations of stations 4A and 4B are 283° and 267°, and 8 stations have a value of 222°. The large value of the azimuth gap is due to the distribution of stations only on one side so that there are horizontal angles that are not covered by seismic stations.

Overview of the tectonic and seismicity in Continental Borderland (offshore) Baja California, México

The seismic activity in the Continental Borderland, offshore Baja California, presented a unique opportunity to study a region that is not typically known for high seismic activity, in comparison to the nearby Mexicali Valley. In 2022 and the beginning of 2023, this region experienced unusual activity. Some events caused concern among the population, specifically, two earthquakes occurred near the city of Ensenada on April 15th and July 25th, 2022, with magnitudes of Mw 4.2 and 4.5, respectively. A third event, with Mw6.1, occurred near San Quintin, Baja California, on November 22nd, 2022, and is associated with the San Isidro Fault. This event, surpassing the typical magnitude observed in the region based on historical seismicity, was felt across a significant stretch of the Pacific coast, spanning from Guerrero Negro to Tijuana, México. In June of the same year, a magnitude 4.2 event occurred in the same fault. This fault is a relay for the Santa Catalina Fault in its southeastern sector. The hypocentral locations of the earthquakes varied in depth from 5 to 20 km, mostly occurring between 5 and 20 km. Magnitudes ranged from 1.5 to 6.1. Additionally, continuous offshore seismic activity was observed between Punta Colonet and the town of San Quintin in November–December 2022 and May 2023. The hypocentral locations of these events ranged from depths of 2–18 km, with the majority occurring between 3 and 10 km. Magnitudes ranged from 1.5 to 4.5. This activity appears to be in the form of swarms, as previously reported by other authors. However, the seismic activity observed in May 2023 displays a northeast-southeast trend, with focal mechanisms exhibiting similar strike angles. This leads us to suggest that this seismic activity is associated with the Colonet Fault, which can be observed in the continental region of the coast. Focal mechanisms of major earthquakes indicated strike-slip motion, aligning with the dominant deformation pattern controlled by major faults in the region. A concentration of activity near the coastline becomes apparent upon analyzing seismic activity recorded in various catalogs from agencies such as ISC, RESNOM, USGS, NCEDC, and SCEC. While no reports of moderate inverse mechanism earthquakes have been documented in the Baja California offshore region, they occur in California. Notably, no seismic activity was reported in the area where the Mw6.1 earthquake occurred between 1930 and 2021, suggesting the reactivation of old faults along the continental border, similar to what occurs in the sector of California. One possibility is that the recurrence intervals of these earthquakes are very long, which would explain the lack of historical records. Previous authors have highlighted the potential for large complex ruptures involving combinations of strike-slip and dip-slip faults, particularly exemplified by the 2010 Mw7.1 El Mayor- Cucapah event in northern Baja California. Seismic activity in the region concentrates along the El Descanso, Coronado-Banks, Santa Catalina, Bahía Soledad, and San Isidro faults. This observation is supported by gravimetric and magnetic anomalies found in various structures that have not been previously interpreted, including the newly reported La Chorera fault, based on bathymetry data. Most of the mentioned faults occur in the gravimetric and magnetic anomalies, however, many other structures have not yet been described.

Simulation‐based characterization of the variability of earthquake risk to buildings in the near‐field

AbstractRecent advancements in high performance computing platforms and computational workflow for regional‐scale simulations are enabling unprecedented modeling of fault‐to‐structure earthquake processes. Regional simulations resolving ground motions at frequencies relevant to engineered systems are becoming computationally viable and provide a new capability to improve understanding of the geographical distribution and intensity of risk to buildings and critical infrastructure. As computational capabilities advance, it is essential to move beyond illustrative single rupture realizations for scenario earthquake events towards the development of a full suite of rupture realizations that appropriately characterize the range of risk to building systems. The work described in this article investigates the application of a suite of fault rupture realizations with the objective of assessing near‐fault, site‐specific seismic demand variability for building structures. A representative high‐performance regional‐scale computational model is utilized to execute ground motion and building response simulations based on 18 kinematic rupture realizations of an M7 strike‐slip scenario earthquake. The fault rupture models for the scenario earthquake are created by systematically perturbing the hypocenter location and stochastically generating rupture parameters (slip, rise time, rake angle) to represent a breadth of ground motion intensities resulting from the spatial and temporal variabilities of an earthquake rupture process. The resulting seismic demand variability for three‐story (short period) and forty‐story (long period) steel moment‐resisting frame buildings is characterized in terms of the median and distribution of peak inter‐story drift ratio for a range of near‐fault sites. The full suite of 18 fault rupture realizations and approximately 280,000 nonlinear dynamic building simulations indicate that the three‐story building undergoes higher median seismic demand and significantly greater variability of demand at a given site than the forty‐story building, which has important implications for the level of certainty in predicting building performance during an earthquake. The simulations performed provide deeper insight into the relationship between fault rupture parameterization and building response, which is essential information for developing a representative suite of rupture realizations for specific earthquake scenarios.

Relocation of Earthquake Hypocenter Using Double-Difference Method in Western Part of Sumatera

The western part of Sumatra has a fairly complex tectonic setting where there is a convergent meeting between the Indo-Australian plate and the Eurasian plate. These plates form a subduction zone and also the Sumatran Fault. The subduction zone and the Sumatran Fault cause many earthquakes in western Sumatra. This study will identify the location of the earthquake hypocenter, which is quite accurately obtained by relocating the hypocenter and making a cross-section in order to analyze the subduction zone and the Sumatran Fault. Hypocenter relocation can be done using the double difference (DD) method using the hypoDD program. This study used data on the arrival time of P and S waves from BMKG in the range of January 2016 to December 2020. The results of the relocation indicate that contains several earthquakes in groups with an average depth of fewer than 60 km that is close to the main source of the earthquake, namely the subduction zone and the Sumatran Fault, so it can be said that this area has a high level of seismicity.

Recent seismicity in the Messina area, southern Italy, and comparison to the local geology and tectonics

We have investigated the seismicity occurred during 2000-2021 in the area of Messina, the town which suffered greatest loss of human lives over the territory devastated by the magnitude 7.1 earthquake of December 28, 1908. We have found that most of recent activity was located beneath the historical centre of the town, in and near the very peculiar sickle-shaped harbor zone which prompted the Greek colonizers in the VII Century B.C. to give Messina the old name of “Zancle”(“Sickle” in the ancient Greek language). Extracting from the whole dataset (consisting of hundred earthquakes of maximum magnitude 3.8) the data relative to a small sequence of 28 events concentrated in a few days at the end of 2013, and performing high-quality Bayesian hypocenter locations of these events, we have found very clear epi-hypocentral trends suitable for comparison with the local structural scenario. The joint analysis of seismic, geological and geomorphological data including morphobathymetric curves of the sea bottom in the study area, has brought us to propose that the small sequence in question (and probably most of activity recorded during the whole 22-years period) may have been generated by internal dynamics of a local horst/graben system, the position of which (i) appears to correspond to one of the minor horsts documented in the Messina Strait basin area and (ii) is very close to the upper edge of the 1908 earthquake blind source reported in the Database of Individual Seismogenic Sources of the Italian National Institute of Geophysics and Volcanology.

A Preliminary Tomography Inversion Study on the Palu Koro Fault, Central Sulawesi Using BMKG Seismic Network

The Palu Koro Fault, which frequently causes solid and destructive earthquakes, is the most exciting aspect of Central Sulawesi’s tectonics. We used local earthquake data from the area in this preliminary study. Our preliminary investigation entails analyzing local seismic data and estimating the velocity structure and hypocenter location using a tomographic approach that employs P-wave arrival times from the Agency for Meteorology, Climatology, and Geophysics earthquake observation network from 2010 to 2019. We use a 1-D initial velocity structural model in this tomographic inversion method, and the results are evaluated using the checkerboard test, derivative weight sum, and ray hit count. To the west and east of the Palu Koro Fault area, preliminary tomographic inversion results show a relatively high seismic velocity. These areas are interpreted as West Sulawesi Mandala and East Sulawesi Mandala Poso Fault and Wekuli Fault. The low-velocity anomaly is found near the Palu Koro Fault. Because of the high-speed thrusting, the relocated hypocenters also clustered around the Palu Koro Fault area. In the following activity, we will determine the speed structure of the S wave, which is expected to provide better geological information in the interpretation.

Impact of Hypocenter Location on Rupture Extent and Ground Motion: A Case Study of Southern Cascadia

AbstractThe Cascadia subduction zone has well‐documented geological records of megathrust earthquakes with the latest ∼Mw 9 rupture occurring in CE 1700. The paleoseismic observations suggest that southern Cascadia is mature for future earthquakes since the last event. Consequently, it is crucial to investigate the potential rupture scenarios. Although existing interseismic locking models share similar moment deficits, whether future earthquakes would be margin‐wide or segmented, as suggested by paleoseismic records, remains unknown. Accordingly, we aim to investigate: (a) possible rupture segmentation patterns, (b) whether south Cascadia can host margin‐wide ruptures, and (c) whether the existing locking models suggest similar future rupture scenarios. Assuming constant effective normal stress, we estimate the stress distribution constrained by the locking models (i.e., Gamma model from Schmalzle et al. (2014, https://doi.org/10.1002/2013GC005172), the preferred model from S. Li et al. (2018, https://doi.org/10.1029/2018JB015620), and the best‐fit model from Lindsey et al. (2021, https://doi.org/10.1038/s41561-021-00736-x)) from static calculation and discover they lead to different stress distributions, indicating distinct seismic potentials. Our dynamic rupture scenarios show that the south can generate both segmented ruptures (>Mw 7.3–8.4) and margin‐wide ruptures (>Mw 8.6) depending on the hypocenter location. The along‐strike rupture length appears to coincide with rupture lengths hypothesized from paleoseismology, and the Schmalzle model produces reasonable coastal subsidence amplitudes from the CE 1700 event. Therefore, we propose that three high‐slip trench‐breaching patches are sufficient for reproducing historical subsidence records. Our results can be further applied in seismic and tsunami hazard assessment, and serve as a comparison for non‐trench‐breaching scenarios.

Hypocenter relocation of the Mw 5.9 eastern Manggarai earthquake 2022 and its aftershocks based on BMKG seismic network

The moderate earthquake (Mw 5.9) struck eastern Manggarai and its surroundings on February 21, 2022, at 12:36:00 UTC. Although this earthquake was classified as a moderate earthquake, it produced a significant number of aftershocks. The aftershocks could be observed as the post-seismic activity, which was still well recorded until March 31, 2022. The hypocenter location of the aftershocks is updated by applying the double-difference method. The aftershock distribution is striking in west-east orientation and dipping to the south, emphasizing a geometry of faults. The strike and dip directions are consistent with the focal mechanism determined by the Indonesian Agency for Meteorology, Climatology, and Geophysics (BMKG). Most aftershocks after the relocation are distributed at a depth of < 25 km; the depth of the mainshock is 21.5 km. The aftershock distribution in this study shows that the earthquake sequence propagates toward the up-dip direction. The aftershock relocation also reveals that most aftershocks are situated slightly off the main fault plane in the complex splay faults. Therefore, we conjecture that the co-seismic activity may trigger the aftershocks. In addition, this study also shows that the fault system consists of several fault segments. Our study benefits earthquake disaster mitigation, especially in mapping the segment faults of the Flores backarc thrust.

Relocation of the 8 January 2022 Menyuan (Ms 6.9), China, earthquake sequence: A conjugated type of rupturing event with a seismic gap in the Qilian-Haiyuan fault in northeast Tibet

On 8 January 2022, an earthquake with Ms 6.9 struck Menyuan County, Qinghai Province, China, which provides an opportunity to investigate the seismotectonics of the Qilian-Haiyuan fault in northeast Tibet. We collect seismic observation data from China Seismic Network Center from 8 January to 6 February 2022 around the 2022 Menyuan mainshock. The double-difference relocation algorithm is applied to relocate 904 earthquakes, and 820 high-precision hypocenter locations are obtained. The mainshock epicenter was located at 10.2 km depth in the Tuolaishan fault. The aftershock sequence extended towards nearly E-W along the Tuolaishan fault and NW-SE along the Lenglongling fault, parts of the Qilian-Haiyuan fault, but there were fewer aftershocks at the beginning of the rupture near the intersection between the Lenglongling and Tuolaishan faults, suggesting a conjugated type of rupturing with a seismic gap. Integrating with focal mechanism solutions of mainshock and one large aftershock, we suggest that the rupture is a left-lateral strike-slip event with a high dip. The stress change around the faults intersection. The 1986 Ms 6.4 earthquake may trigger the 2016 Ms 6.4 earthquake, and then the extrusion stress could transfer to around the 2022 Ms 6.9 earthquake source area due to the Indo-Eurasian collision. Previous tomographic models revealed low-velocity anomalies under the Menyuan earthquakes, reflecting that fluids could decrease friction coefficient and normal stress on the fault planes and further promote the generation of these earthquakes. These fluids could be contained in the lower crustal flow and mantle upwelling flow under northeast Tibet.

Seismicity Analysis and Velocity Structure of Two-phase Geothermal Field in Southern Bandung

Over a duration of six months, we conducted a microearthquake analysis to characterize the subsurface conditions in the two-phase geothermal field located in Southern Bandung. Throughout the study, a total of 754 microearthquake (MEQ) data were recorded from at least 15 monitoring stations. After selection process of the dataset, 362 events were selected based on their azimuthal gap angle being less than 180°. Initially, the earthquake locations were determined using the Geiger method. Subsequently, we updated the hypocenter locations through simultaneous inversion, incorporating 1D velocity structure and 3D tomographic inversion. Our analysis revealed three primary seismicity clusters, which likely correspond to the injection and production activities within the geothermal field. The southern cluster aligns with the injection wells, extending from a depth of 1 km to 8 km, suggesting that the MEQ events were induced by injection activity. In the production area, a higher concentration of events is observed, densely distributed between depths of 1 km and 3 km. We suspect that the third cluster is associated with the development activity of a steam-dominated geothermal field located east of the main field. Notably, low Vp/Vs values near the surface are detected and exhibit increased thickness towards the north. These characteristics are interpreted as indicative of the steam zone, as the anomaly's location aligns closely with the production area. The thickening of the low Vp/Vs zone towards the north suggests the presence of steam as the impact of fluid extraction in the region, leading to a decline in pressure.

Performance Evaluation of AE Sensors Installed Like Hydrophones in Adaptive Monitoring Networks During a Decametre-Scale Hydraulic Stimulation Experiment

AbstractIn the framework of the STIMTEC and STIMTEC-X hydraulic stimulation experiments at the Reiche Zeche mine, Freiberg (Germany), we installed acoustic emission (AE) sensors for the recording of picoseismicity both conventionally using pneumatic coupling and experimentally like a hydrophone, i.e. the sensors were placed in the borehole without a further coupling system or cementing. We investigate performance measures of the hydrophone-like acoustic emission (HAE) sensors such as frequency bandwidth, sensitivity, first motion polarity, coupling and placement quality to assess the sensor’s applicability in adaptive monitoring networks. HAE sensors can be paired with hydraulic equipment, especially with the double packer probe used for stimulation at the decametre scale because the monitored frequency content differs from injection-related noise. This offers a unique opportunity to improve the network geometry and consequently the quality of a seismic catalogue. We analyse the sensor characteristics using active ultrasonic transmission measurements from boreholes with different orientations in the rock volume, noise measurements preceding active centre punch hits in the access galleries and passive recordings of induced acoustic emission events. HAE sensors placed in water-filled boreholes show good sensitivity performance even without optimal coupling to the crystalline rock for recording distances up to 17 m. The HAE sensors record the wavefield adequately for first-arrival identification, polarity picking and amplitude characteristics but are less suitable for detecting S-waves. Due to the borehole geometry HAE sensors record waves with incidence angles from the side, resulting in opposite polarity compared to side-view AE sensors as observed in the field and lab. We discuss the advantages of adaptive monitoring networks with HAE sensors being optimally placed for each stimulation interval configuration anew to improve seismic event detection and quality of event hypocentre locations during hydraulic stimulations. We show that we are able to significantly reduce the azimuthal gap, halve the location uncertainties and improve the network coverage for the purpose of focal mechanism estimations.

Seismic activity at the eastern Pyrenean termination

Although the seismic activity at the eastern Pyrenees is nowadays moderate and sparse, with events usually not exceeding magnitudes 4.5, this area has been affected in the past by the most destructive event occurred in the Pyrenees, reaching an intensity of IX, whose seismogenic structure is not well understood and still under debate. In order to progress in the knowledge of these structures, we present here the results derived from a 14 months-long broad-band seismic deployment focused on the Cerdanya Basin, but encompassing the eastern termination of the Pyrenees. The dense station coverage has allowed us to obtain accurate hypocentral locations, as well as up to 23 focal mechanisms from local low-magnitude earthquakes. In addition to a relatively sparse seismicity, several clusters of seismic events located in well-defined, small areas and depth ranges have been identified. The results show a few low-magnitude seismic events located in the southern limit of the Cerdanya Basin that could be related to oblique secondary faults within the footwall of the Têt Fault, the major tectonic structure in the area. Our data shows also that the Capcir Fault has associated seismicity, with some of the events located out of the fault plane, perhaps on secondary fault branches. To the east, a cluster of low-magnitude events is detected in the epicentral area of two relatively large earthquakes occurred recently. Further west, the Maladeta Massif has a sustained seismic activity, although its origin does not seem to be related to the most relevant structure in the area, the North Maladeta Fault. Regarding focal mechanisms, most of them show normal fault solutions with nodal planes NW-SE oriented, which agree with the extensional regime perpendicular to the axis of the chain derived from the seismic and geodetic data.

Dynamic Rupture Models, Fault Interaction and Ground Motion Simulations for the Segmented Húsavík‐Flatey Fault Zone, Northern Iceland

AbstractThe Húsavík‐Flatey Fault Zone (HFFZ) is the largest strike‐slip fault in Iceland and poses a high seismic risk to coastal communities. To investigate physics‐based constraints on earthquake hazards, we construct three fault system models of varying geometric complexity and model 79 3‐D multi‐fault dynamic rupture scenarios in the HFFZ. By assuming a simple regional prestress and varying hypocenter locations, we analyze the rupture dynamics, fault interactions, and the associated ground motions up to 2.5 Hz. All models account for regional seismotectonics, topo‐bathymetry, 3‐D subsurface velocity, viscoelastic attenuation, and off‐fault plasticity, and we explore the effect of fault roughness. The rupture scenarios obey earthquake scaling relations and predict magnitudes comparable to those of historical events. We show how fault system geometry and segmentation, hypocenter location, and prestress can affect the potential for rupture cascading, leading to varying slip distributions across different portions of the fault system. Our earthquake scenarios yield spatially heterogeneous near‐field ground motions modulated by geometric complexities, topography, and rupture directivity, particularly in the near‐field. The average ground motion attenuation characteristics of dynamic rupture scenarios of comparable magnitudes and mean stress drop are independent of variations in source complexity, magnitude‐consistent and in good agreement with the latest regional empirical ground motion models. However, physics‐based ground motion variability changes considerably with fault‐distance and increases for unilateral compared to bilateral ruptures. Systematic variations in physics‐based near‐fault ground motions provide important insights into the mechanics and potential earthquake hazard of large strike‐slip fault systems, such as the HFFZ.

Observation site selection for physical model parameter estimation towards process-driven seismic wavefield reconstruction

SUMMARYThe ‘big’ seismic data not only acquired by seismometers but also acquired by vibrometers installed in buildings and infrastructure and accelerometers installed in smartphones will be certainly utilized for seismic research in the near future. Since it is impractical to utilize all the seismic big data in terms of the computational cost, methods which can select observation sites depending on the purpose are indispensable. We propose an observation site selection method for the accurate reconstruction of the seismic wavefield by process-driven approaches. The proposed method selects observation sites suitable for accurately estimating physical model parameters such as subsurface structures and source information to be input into a numerical simulation of the seismic wavefield. The seismic wavefield is reconstructed by the numerical simulation using the parameters estimated based on the observed signals at only observation sites selected by the proposed method. The observation site selection in the proposed method is based on the sensitivity of each observation site candidate to the physical model parameters; the matrix corresponding to the sensitivity is constructed by approximately calculating the derivatives based on the simulations, and then, observation sites are selected by evaluating the quantity of the sensitivity matrix based on the D-optimality criterion proposed in the optimal design of experiments. In this study, physical knowledge on the sensitivity to the parameters such as seismic velocity, layer thickness, and hypocentre location was obtained by investigating the characteristics of the sensitivity matrix. Furthermore, the effectiveness of the proposed method was shown by verifying the accuracy of seismic wavefield reconstruction using the observation sites selected by the proposed method.

The Rocks That Did Not Fall: A Multidisciplinary Analysis of Near‐Source Ground Motions From an Active Normal Fault

AbstractOn 8 July 2021 a M6.0 normal faulting earthquake rocked the community of Walker and the surrounding region near the California‐Nevada border. In the 1990s, field surveys of nearby Meadowcliff Canyon identified numerous precarious rocks deemed likely to topple in the event of strong shaking. Despite their proximity (∼6 km) to the 2021 earthquake, the precarious rocks still remain standing. In this work, we combine advanced source and ground motion characterization techniques to help unravel this mystery. High‐precision hypocentral locations reveal a clear north/south‐striking, east‐dipping rupture plane along the southern extension of the Slinkard Valley fault. The mainshock nucleated near the base of the fault, triggering thousands of aftershocks. Bayesian source spectral analyses indicate that the mainshock had a moderately‐high stress drop (∼17 MPa), and that aftershocks with deeper hypocenters have higher stress drops. Peak Ground Acceleration (PGA) recordings at regional stations agree well with existing ground motion models, predicting PGA of ∼0.3 g in Meadowcliff Canyon, a level sufficient to topple precarious rocks based on PGA‐derived stability criteria. We demonstrate that despite these large ground accelerations, the pulse duration in Meadowcliff Canyon is too short to supply the impulse necessary to damage these features, observations which support the application of dynamic toppling models that account for the joint effects of pulse amplitude and duration when assessing rock fragility. This study provides a unique vantage point from which to interpret rarely‐observed strong‐motion recordings from close to an active normal fault, one of many that dominate hazard along the eastern Sierra.

Fault geometry of M6-class normal-faulting earthquakes in the outer trench slope of Japan Trench from ocean bottom seismograph observations

Since the 2011 Mw 9.0 Tohoku-oki earthquake, intra-plate normal-faulting earthquakes, including several M7-class earthquakes, have occurred in the outer trench slope area from the trench to the outer rise along the Japan Trench. Concerns regarding large earthquakes and associated tsunamis have also arisen. Based on aftershock distributions, several outer trench slope normal-faulting earthquakes (hereinafter referred to as outer-rise earthquakes) are likely related to the rupture of multiple faults. However, few observations have clearly shown how multiple faults act during outer-rise earthquakes. During the ocean bottom seismograph (OBS) observations in the outer trench slope area of the central Japan Trench from September 2017 to July 2018, three M6-class normal-faulting earthquakes (Mw 6.2 on September 20, Mw 6.2 on October 06, and Mw 6.0 on November 12) occurred around the OBS network. The near-field OBS observations provided detailed information on hypocenter locations and focal mechanisms of the mainshocks and aftershocks, including immediately after the mainshocks. We investigated the fault configurations of normal-faulting earthquakes based on OBS observations. During the September 2017 earthquake, the mainshock ruptured high-angle normal faults with a dip angle of 65°. Off-fault aftershock activities that were not directly related to the mainshock rupture and could be explained by the stress changes caused by the mainshock were confirmed. However, hypocenter distributions and focal mechanisms of the main and aftershocks of the October and November 2017 earthquakes suggest that the mainshock ruptured multiple faults with various dipping directions, angles, and strike orientations. The complicated fault geometry should be considered a possible fault model for large outer-rise earthquakes and related tsunamis.

Preliminary Estimation of Geological Structure Based on Relocated Hypocenter of Microearthquake: Case Study at “X” Geothermal Field

Generally, analysis of geological structure in the geothermal field is carried out using remote sensing methods, geological mapping, and gravity analysis. These methods can provide the distribution of geological structure on the surface but cannot provide information on whether the structure is active or not. This study aims to utilize improved hypocenter location which can be used to delineate the distribution of active faults. This is useful for obtain a better interpretation of geological structures in geothermal areas. Hypocenter locations are determined by picking the arrival time of the P-wave and S-wave. This method sometimes leads to uncertainty of hypocenter location due to subjectivity of picking arrival time. The double-difference method is applied with additional input pairing events from waveform cross-correlation (WCC) to overcome the uncertainty of hypocenter location. The result shows 3 microearthquake clusters formed in the western, central, and eastern parts of the “X” Geothermal Field. The cluster formed in the western area shows that there are two orientations of conjugated hypocenter distribution with the orientation of NW-SE and NE-SW. These hypocenters reveal the presence of geological structures which provide permeability pathways for fluids rising in the western part of the geothermal field. Multiplet-clustering analysis in the middle cluster which coincides with the reservoir zone shows two different sub-clusters indicating the possibility of two different sources mechanism working in the area. By integrating all the analysis and hypocenter distribution, these methods provide a better interpretation of structure’s distribution in the “X” Geothermal Field.

Long-term array observation by ocean bottom seismometers at the Chile Triple Junction

Seafloor seismic observations were conducted twice in the vicinity of the Chile Triple Junction (CTJ) in order to investigate the crustal activities associated with the subduction of the hot ridge. Herein, we present the details of the most recent seismic observation for the two-year period between January 2019 and January 2021. Furthermore, the hypocenter location, magnitude, and focal mechanisms of the local earthquakes were revealed by analyzing the data from both deployments, including the one conducted between 2009 and 2010. In total, more than 2100 local earthquakes were detected during the two observation periods. In both observations, earthquakes were found to have actively occurred along the Chile Ridge and the Darwin Fracture Zone. The magnitudes of these earthquakes range from −0.3 to 5.0. From the events cataloged during the time period 2019 to 2021, a clear seismicity gap is observed at 46.4°S with the predominant faulting type differing across this divide. North of the seismicity gap, normal faulting earthquakes periodically occur along the Chile Ridge, thereby indicating continuous ridge opening. By contrast, the earthquakes to the south of the seismicity gap, where the Chile Ridge has already been subducted, occurred intermittently and are dominated by reverse faulting. The latter earthquakes are associated with the Antarctic plate subduction. We propose that a local transform fault, with E-W strike direction, exists 10 km north of the seismicity gap, based on the hypocenter locations and focal mechanisms of three M > 4 earthquakes. This local transform fault was probably formed by the effect of the active ridge subduction.© 2017 Elsevier Inc. All rights reserved.