Hypocenter Data Research Articles

Seismogenic Fault Model for the 2021 Ms 6.4 Yangbi, China, Earthquake, Constraints from Multisource Data

Abstract Deciphering a comprehensive 3D fault model for the regions with moderate-to-strong earthquakes is crucial for understanding earthquake triggering mechanisms and assessing future seismic hazards. On 21 May 2021, a massive Ms 6.4 earthquake occurred in Yangbi, Dali City, China, near the northern Red River fault zone. Despite numerous studies conducted over the past two years, the seismogenic fault of this earthquake remains a topic of controversy. In this article, we refine the workflow for 3D construction of fault surfaces from Riesner et al. (2017) and used it for the Yangbi earthquake. We constructed a seismogenic fault model for the Yangbi earthquake and Caoping fault from the collected multisource data. One utilizes a combination of focal mechanisms and relocated hypocenters, whereas the other combines geological and geophysical data from the study area. Upon analyzing these two fault models and the relocated hypocenter data, we propose that the seismogenic fault in the Yangbi earthquake is an undiscovered blind fault or a secondary blind fault of the Weixi–Qiaohou fault, rather than the surface-emerging Caoping fault.

Delineating geological structure utilizing integration of remote sensing and gravity data: a study from Halmahera, North Molucca, Indonesia

Halmahera Island is the result of the interaction between the Indo-Australian Plate, the Molucca Sea Plate, and the Philippine Plate, which gave rise to many active geological structures that are vulnerable to seismic activity around the island. The complexity of this geological structure makes Halmahera Island very interesting to study. This study aims to identify geological structures by integrating remote sensing and gravity satellite data. Surface lineament analysis using the remote sensing method was carried out based on Sentinel-1A imagery data. The gravity method uses GGMPlus satellite data to clarify the continuity of geological structures that cannot be clearly mapped on the surface. Lineament analysis on gravity data uses techniques such as fast sigmoid edge detection (FSED) and Euler deconvolution. The results of lineament interpretation based on integrating remote sensing and gravity satellite data show that the NE-SW structure controls the northern and northeastern arms of Halmahera.
 In contrast, the southern arm is dominated by the NW-SE structure. The Euler depth estimation shows that the Halmahera area contributes to having geological structures at various depths. Deep structures reach 4 km, while shallow structures are found at depths of up to 2 km. Earthquake hypocenter data strengthen the interpretation of this geological structure. This comprehensive study yields an excellent correlation between gravity and remote sensing techniques in describing the general structural framework of the area. The new finding is an NE-SW trending geological structure on the northern Halmahera arm, which may be caused by two different tectonics first, the subduction of the Molucca Sea Plate with the Halmahera Plate in the west. Second, the strike-slip movement is trending NE-SW, which cuts the northern and northeastern arms due to the rotational movement of the thrust fault with the Philippine Plate to the west.

The Seismic Activity of the Nasudake Volcano Using Hypocenter Data (1998–2022) from the National Research Institute for Earth Science and Disaster Resilience

The seismic activity of the Nasudake volcano is summarized based on hypocenter data from the National Research Institute for Earth Science and Disaster Resilience (NIED). The observation period spans 24 years, from October 1998 to December 2022, and the number of earthquakes during the period is 12,064. Of these, 80.1% occurred between depths of 0.7 and 2.6 km; 94% registered between -1.6 and 0.9 on the Richter scale. The lower limit of the detectable magnitude (magnitude of completeness, Mc) for the transition of the volcano-observation system was examined and found to be -0.5 for Mc before 2005 and -1.4 for Mc after 2012. Focusing on zones where seismic activity was concentrated, we divided the active zones into four areas. The greatest concentration of seismic activity, 76.4% of the total number of earthquakes, was between Chausu-dake and Asahi-dake. The depth of the earthquakes in this active area was shallower than 2 km, and most were micro-earthquakes (1 ≤ M < 3) or smaller. Earthquakes have been occurring regularly for 24 years. This active area includes the Chausu-dake lava dome formed by activities in 1408–1410 and two craters where phreatic eruptions have occurred since the Meiji era (1868–1912). Therefore, in considering volcano disaster-prevention measures, the relationship between this seismic area and volcanic activity should continue to be closely monitored.

Special Issue on NIED Frontier Research on Science and Technology for Disaster Risk Reduction and Resilience 2023

We are very pleased to publish this Special Issue, NIED Frontier Research on Science and Technology for Disaster Risk Reduction and Resilience 2023, which includes three papers and one tutorial. The first paper, authored by Dhakal and Kunugi, presents the orientation changes that earthquake movements have caused in the accelerometers of a seafloor observation network for earthquakes and tsunamis along the Japan Trench (S-net). This study is expected to contribute to more accurate analyses of S-net data. The second and third papers are reports of research done on volcanoes. The paper by Tanada and Ueda presents the seismic activity of the Nasudake volcano through the use of NIED hypocenter data gathered from 1998 to 2022. Tanada and Ueda state that earthquakes have regularly occurred near the Chausudake lava dome and two craters and that, from the volcano disaster-prevention point of view, it is important to monitor the relationship between this seismic area and volcanic activity. The third paper, by Miyagi et al., reports on an experiment aimed at grasping the movement of climbers and sharing the level of disaster threat with climber-related organizations in real time. Their online visualization system for the movement of climbers helps to provide solutions to problems related to volcanic disaster prevention not only during disaster events but also in the periods between. The last paper is a tutorial by Hanashima and Usuda on “SIP4D-ZIP,” which is a standard for synthesizing and sharing common information from multiple organizations. This tutorial explains how to realize a versatile and automatic mutual data sharing and usage system for various kinds of necessary disaster information. In conjunction with the fourth 7-year mid/long term plan period 2016-2022, NIED has published seven special issues since 2017, including this 2023 issue. We believe that the articles in these seven issues can contribute to the advancement of science and technology for disaster risk reduction and resilience.

Homogenization of magnitudes of the ISC Bulletin

SUMMARY We implemented an automatic procedure to download the hypocentral data of the online Bulletin of the International Seismological Centre (ISC) in order to produce in near real-time a homogeneous catalogue of the Global and EuroMediterranean instrumental seismicity to be used for forecasting experiments and other statistical analyses. For the interval covered by the reviewed ISC Bulletin, we adopt the ISC locations and convert the surface wave magnitude (Ms) and short-period body-wave magnitude (mb) as computed by the ISC to moment magnitude (Mw), using empirical relations. We merge the so obtained proxies with real Mw provided by global and EuroMediterranean moment tensor catalogues. For the most recent time interval (about 2 yr) for which the reviewed ISC Bulletin is not available, we do the same but using the preferred (prime) location provided by the ISC Bulletin and converting to Mw the Ms and mb provided by some authoritative agencies. For computing magnitude conversion equations, we use curvilinear relations defined in a previous work and the chi-square regression method that accounts for the uncertainties of both x and y variables.

Spatial–Temporal Distribution and Focal Mechanisms for the Shallow Earthquakes in the Rongchang-Luxian Region in Southern Sichuan Basin, China

AbstractAn Ms 6.0 earthquake struck the Luxian County in southern Sichuan basin, China, on 15 September 2021, which was the largest local event thus far and caused heavy casualties and serious property losses. Using data of 107 temporary and six permanent stations from five months before to four and a half months after the time of the mainshock in the Rongchang-Luxian region, we utilize the machine learning-based workflow for phase picking, earthquake detection and relocation, and obtain precise hypocenter data for 22,806 events. We found that earthquakes increase significantly since July 2021, showing north-northeast-, northeast-, east-northeast- and east-southeast-directed seismic strips. Almost all events are located within the sedimentary layers at 1–7 km depth above the crystalline basement, and more than half of them occurred in Silurian and Ordovician formations. The Ms 6.0 Luxian earthquake sequence occurred in an area where no clear seismicity was observed before the mainshock. On map view, the aftershock zone shows a convex polygon with an overall strike of 112°. The focal depths of aftershocks southwest to the mainshock are distributed in a narrower range of 4–7 km and extend to a wider range of 2–7 km on the northeast side. It is speculated that the seismogenic fault is a blind thrust fault dipping to the southwest at 2–7 km depth. The focal mechanisms of 13 ML 3.0+ earthquakes show either pure reverse faulting or reverse faulting dominated mechanisms with a strike-slip component. The centroid depths fall in the range of 1.95–6.75 km. The maximum and middle principal stress axes are nearly horizontal; the azimuth of the maximum horizontal stress is about 120° and the stress shape ratio is 0.78. These results provide an important reference for reunderstanding the local seismic risk and for detailed studies on the seismological aspects about the unusual Luxian earthquake in the future.

Using Grid Search and Guided Random Search (Simulated Annealing) Methods in Determining the Earthquake Hypocenter in the Majalengka Region, West Java on November 11, 2021

Tectonic earthquakes are generally caused by the release of energy produced by a pressure from a moving slab. This study aimed to test the use of grid search methods and guided random search in determining the hypocenter position of the earthquake in the case of the Majalengka regional earthquake, West Java which occurred on November 11, 2021. In this study used earthquake source data with the number of 5 stations, are COCO, KAPI, PALK, MBWA, and NWAO which can be accessed on the IRIS. From the results of determining the earthquake hypocenter using the grid search method, the position of the earthquake hypocenter is obtained with a latitude position of 6.596861oS and for a longitude position of 108.2871°E with a depth of 200 km, while from the processing of the simulated annealing method, the latitude position is 6.501645°S and longitude 108.2252°E with depth of 172.1 km. Meanwhile, if we compare the position of the earthquake hypocenter according to IRIS, are the latitude position of 6.6027°S and for the longitude position of 108.105°E with a depth of 164.41 km, because the hypocenter data from IRIS has been revised by a seismologist so that it is used as a reference, the results that are closest to the position according to IRIS are the results of the simulated annealing method.

Initial Result of P Wave Tomography Model in Sunda-Banda Arc Transition using FMTOMO

Our study area is located near island Sumbawa, Sumba, Flores, West Timor, Indonesia and East Timor, popularly known as Sunda-Banda arc transition zone. The tectonic setting is mainly controlled by the movement of the oceanic lithosphere Indo-Australian plate subducting the Eurasian plate and Northward migration of Australian continental lithosphere into western Banda-arc in the region of Flores, Sumba and Timor island. We tried to image velocity structure beneath these regions using regional events and tomography inversion model. We collected 5 years of regional events from the Indonesian Agency of Meteorology, Climatology and Geophysics. In total, we reserved 3186 events recorded on 29 stations. For data processing, we used fast marching method as ray tracing between sources and receiver. We then employed subspace inversion as the tomography procedure to estimate the best velocity model representing the tectonic model in the region. Hypocenter data distribution is concentrated on shallow parts of the region and along the Benioff zone down to a maximum depth of 400 km. One of challenge of this study is that although events are abundance, the stations used are mostly located onshore and does not extend in the south-north direction that leads us to under determined problem in the inversion process. However, checker-board models show most our target area can be retrieved to its initial model with sign of smearing effects shown start from a depth of 50 km. After six iteration and optimized selection of damping and smoothing parameters, we observed low velocity anomaly under Bali, Lombok, Sumba, East Nusa Tenggara at shallow depth that may be related with volcanic activity. Deeper low anomaly can also be seen that may be related with partial melting process. A band of fast velocity is clearly seen that goes deepen to the north depicting subducting slabs own to a depth of 300 km. We also observed a possible of fast velocity in the northern part of our stations at shallow depth that we believe may represent the back arc thrust.

Filling the gap in a double seismic zone: Intraslab seismicity in Northern Chile

Double seismic zones (DSZs) of intermediate-depth intraslab seismicity are observed in many subduction zones around the globe, and have been related to dehydration reactions in the downgoing crust and mantle lithosphere. These reactions occur at, to first order, constant temperatures, which explains the observed linear arrangements of seismicity that appear to follow isotherms of thermal models.Intermediate-depth seismicity in Northern Chile, however, exhibits a pattern of intraslab seismicity that substantially deviates from a classical DSZ. Whereas two parallel seismicity planes are present in the updip part of the slab, these abruptly change into a 25–30 km thick, homogeneously seismogenic volume at a depth of ∼80–100 km. Seismicity rate and moment release significantly increase in this depth interval. In order to understand which processes evoke this configuration and what distinguishes the Northern Chile subduction zone from more conventional subduction zone settings (e.g. Japan), we performed a detailed seismological investigation of slab seismicity in Northern Chile using data from the IPOC permanent network. We determined >600 moment tensors of intraslab earthquakes, processed and evaluated location uncertainties for 8 years of high-resolution earthquake hypocenter data, and performed statistical analysis of the different seismicity populations.We observe that earthquakes both in the highly active cluster and the DSZ above exhibit consistently downdip extensive source mechanisms that align with the dip angle and direction of the slab. This implies strong slab pull, which is also evident from slab steepening outlined by hypocenters towards the downdip termination of the highly active cluster. Moreover, events in the cluster show a very weak aftershock productivity and a high background event rate, which leads to a temporal distribution of seismicity that is close to a purely random process. We find that the position of the highly seismogenic volume spatially coincides with: 1) the disappearance of the velocity contrast between oceanic crust and the underlying mantle in receiver function images, 2) the transition from the “cold nose” (i.e. the stagnant part) to the hot part of the mantle wedge, as evidenced by seismic attenuation images, and 3) with an increase of the slab dip angle. Based on these different pieces of evidence, we speculate that high tensile stresses and heat input from above could lead to a sudden burst of kinetically delayed metamorphic reactions there that then enables the observed increased seismicity rates. Since these reactions have overall a negative volume change that leads to slab densification and hence further increases slab pull, the spatial pattern of seismicity we observe could result from a runaway-type process, which would explain its abrupt start and high moment release rates.

Regional Seismic Activity after 2012 M8.6 Sumatra Earthquake

The Sumatra subduction zone, one of the most active plate tectonic margins in the world, is characterised by the Indo-Australia Plate subducting beneath the Sunda plate and Andaman micro plate, causing seismic activity along the plate boundary. There had been five major earthquakes of magnitude greater than 8.0 in this region from 2004 to 2014. Three of them are dip-slip and rest of the two is strike-slip type events. Regional earthquake activity after the occurrence of those five events was analyzed. Hypocentral data obtained from the Data Management Center at the Incorporated Research Institutions for Seismology for the period from January 2000 to December 2014 of magnitude 3.0 were used for the analysis. A statistical analysis was carried out to know whether earthquake activity has increased after the major five events and the analysis was carried out both qualitatively and quantitatively. The results of the analysis show that the number of earthquakes in the region has increased considerably after the occurrence of April, 2012 magnitude 8.6 and 8.2 strike-slip events. Further results show that there is no change in the regional earthquake activity after the occurrence of other three major dip-slip type events. Present study results reasonably agree with the results obtained by the other studies carried out with different methods. In the context of Sri Lanka, strike-slip type focal mechanism of the 2012 two major events may be the reason for increasing of activity in the region, especially in Eastern part of Sri Lanka near Maduruoya, Highland-Wijayan boundary and Wadinagala area of Ampara District. Keywords: seismology, Sumatra subduction zone, regional seismicity, earthquake, Sri Lanka

Subsurface faults inferred from reflection seismic, earthquakes, and sedimentological relationships: Implications for induced seismicity in Alberta, Canada

Abstract Given the recent induced seismic activity in Alberta, identification of subsurface faults and areas of structural complexity has become increasingly important in improving our understanding of the controls on induced seismic events. Using a 3D geological model supplemented with 2D and 3D reflection seismic data, several basement-bounded and basement-rooted faults, which extend upward and into the Devonian strata, are identified in areas coincident with increased seismic activity. The presence of faults in the study area was confirmed through a statistically significant correlation of high-quality seismic event data from historical and recent (1970–2016) induced earthquakes in Alberta to the edge of the Devonian-aged Swan Hills platform. Along with the identification of faults using reflection seismic and earthquake hypocenter data, a review of the role of pre-existing structure on depositional patterns that has been previously used to infer deep structure in this area, is also presented in this study. Several pre-existing extensional and/or transtensional style faults in the deeper strata and shallow basement of the study area are consistent with structure influencing the sedimentation of the overlying stratigraphy. Therefore, a better understanding of genetic fault-reef associations and the relationship to overlying strata may aid in identification of fault locations, style, and orientation. Considering the advancement of proper avoidance strategies during the planning stages of unconventional resource development or storage, this paper demonstrates the use of geological knowledge and relationships to identify areas comprising faults that may be prone to reactivation.

A study on off-fault aftershock pattern at N-Adria microplate

The spatial features of the aftershock sequences triggered by three moderate magnitude events with coda-duration magnitudes 4.1, 5.1 and 5.6, which occurred in Northeastern Italy and Western Slovenia, were investigated. The fractal dimension and the orientations of the planar features fitting the hypocentral data have been inferred. The spatial organization is articulated through two temporal phases. The first phase is characterized by the decreasing of the fractal dimension and by vertically oriented planes fitting the hypocentral foci. The second phase is marked by an increase of the fractal dimension and by the activation of different planes, with more widespread orientation. The aftershock temporal distribution is analysed with a model based on a static fatigue process. The process is favoured by the decrease of the overburden pressure, the sharp variations of the mechanical properties of the medium and the unclamping effect resulting from positive normal stress changes caused by the mainshock stress step.

Statistical Estimation of the Seismicity Level Change around Hokkaido, Northern Japan

The temporal seismicity change in two seismically active zones around Hokkaido, northern Japan was investigated using the statistical estimate of the seismicity level (SESL’09) procedure. Hypocenter data provided by the Japan Meteorological Agency from 1960 to 2013 were analyzed. The seismicity of two geographically different zones, formed by Pacific Plate subduction and Amurian Plate convergence, showed different statistical characteristics. Low cross-correlation values between the two zones also suggest independent seismic processes for each area. However, an anomalously high cross-correlation period was identified from 1996 to 2000, with a time lag of 8 weeks. A 6-month seismic quiescence period before the strongest Hokkaido Toho-Oki Earthquake (4 October 1994, Mj 8.2) was observed on the Pacific side.

Development of support system for estimation of earthquake fault plane with hypocenter data

Development of support system for estimation of earthquake fault plane with hypocenter data

The deep structure of south-central Taiwan illuminated by seismic tomography and earthquake hypocenter data

The Taiwan mountain belt is generally thought to develop above a through-going basal thrust confined to within the sedimentary cover of the Eurasian continental margin. Surface geology, magnetotelluric, earthquake hypocenter, and seismic tomography data suggest, however, that crustal levels below this basal thrust are also currently being involved in the deformation. Here, we combine seismic tomography and earthquake hypocenter data to investigate the deformation that is taking place at depth beneath south-central Taiwan. In this paper, we define the basement as any pre-Eocene rifting rocks, and use a P-wave velocity of 5.2km/s as a reference for the interface between these rocks and their sedimentary cover. We found that beneath the Coastal Plain and the Western Foothills clustering of hypocenters near the basement-cover interface suggests that this interface is acting as a detachment. This detachment is located below the basal thrust proposed from surface geology for this part of the mountain belt. Inherited basement faults appear to determine the geometry of this detachment, and their inversion in the Alishan area result in the development of a basement uplift and a lateral structure in the thrust system above them. However, across the Shuilikeng and the Chaochou faults, earthquake hypocenters define steeply dipping clusters that extend to greater than 20km depth, above which higher velocity basement rocks are uplifted beneath the Hsuehshan and Central ranges. We interpret these clusters to form a deeply penetrating, east-dipping ramp that joins westward with the detachment at the basement-cover interface. It is not possible to define a basal thrust to the east, beneath the Central Range.

Quantitative description of induced seismic activity before and after the 2011 Tohoku‐Oki earthquake by nonstationary ETAS models

The epidemic‐type aftershock sequence (ETAS) model is extended for application to nonstationary seismic activity, including transient swarm activity or seismicity anomalies, in a seismogenic region. The time‐dependent rates of both background seismicity and aftershock productivity in the ETAS model are optimally estimated from hypocenter data. These rates can provide quantitative evidence for abrupt or gradual changes in shear stress and/or fault strength due to aseismic transient causes such as triggering by remote earthquakes, slow slips, or fluid intrusions within the region. This extended model is applied to data sets from several seismic events including swarms that were induced by the M9.0 Tohoku‐Oki earthquake of 2011.

Constraints on the three-dimensional thermal structure of the lower crust in the Japanese Islands

We propose a method for modeling a three-dimensional thermal structure with a particular focus on the lower crust. Our method enables high-resolution modeling without heat flow data, but with earthquake hypocenter data in the crust and seismic attenuation data in the upper mantle. In our method, the temperature at the bottom of the seismogenic layer is estimated under the assumption that the bottom depth corresponds to a brittle-ductile transition zone. Next, the temperature beneath the Moho discontinuity is estimated using seismic attenuation data of the mantle and a few point temperature data inferred from mantle xenoliths. Finally, the temperatures between the two depths are linearly interpolated. Attempts to construct an actual model for the Japanese Islands are also described.

Montserrat geothermal system: A 3D conceptual model

AbstractWe constructed a three‐dimensional conceptual model of a geothermal system on the Caribbean island of Montserrat. The model was generated using magnetotelluric resistivity data, earthquake hypocenter data, and a three‐dimensional P wave velocity model, all plotted using a shared geographical reference. The results of the study suggest a high‐temperature fracture‐controlled geothermal system at the intersection of two faults in the SW of the island. We also present a “prospectivity index” map that represents a proxy of the spatial variation in harvestable heat flux at 1500 m depth. The index is the product of relative permeability around modeled faults and a proxy for the subsurface temperature calculated using P wave velocity anomalies.

Thermal structure and intermediate-depth seismicity in the Tohoku-Hokkaido subduction zones

Abstract. The cause of intermediate-depth (>40 km) seismicity in subduction zones is not well understood. The viability of proposed mechanisms, which include dehydration embrittlement, shear instabilities and the presence of fluids in general, depends significantly on local conditions, including pressure, temperature and composition. The well-instrumented and well-studied subduction zone below Northern Japan (Tohoku and Hokkaido) provides an excellent testing ground to study the conditions under which intermediate-depth seismicity occurs. This study combines new finite element models that predict the dynamics and thermal structure of the Japan subduction system with a high-precision hypocenter data base. The upper plane of seismicity is principally contained in the crustal portion of the subducting slab and appears to thin and deepen within the crust at depths >80 km. The disappearance of seismicity overlaps in most of the region with the predicted phase change of blueschist to hydrous eclogite, which forms a major dehydration front in the crust. The correlation between the thermally predicted blueschist-out boundary and the disappearance of seismicity breaks down in the transition from the northern Japan to Kurile arc below western Hokkaido. Adjusted models that take into account the seismically imaged modified upper mantle structure in this region fail to adequately recover the correlation that is seen below Tohoku and eastern Hokkaido. We conclude that the thermal structure below Western Hokkaido is significantly affected by time-dependent, 3-D dynamics of the slab. This study generally supports the role of fluids in the generation of intermediate-depth seismicity.

Geological features of a collision zone marker: The Antique Ophiolite Complex (Western Panay, Philippines)

The Antique Ophiolite Complex exposed along the western side of Panay Island, central Philippines was derived from the Jurassic to Cretaceous proto-South China Sea oceanic leading edge of the Palawan microcontinental block. The subduction and ultimate closure of this ocean basin resulted in the emplacement and exposure of this lithospheric fragment along the collisional boundary of the microcontinental block and the oceanic- to island arc-affiliated Philippine mobile belt. The ophiolite complex has volcanic rocks having normal- to transitional mid-ocean ridge basalt (MORB) to island arc tholeiitic (IAT) geochemistry consistent with the transitional MORB–IAT characteristics of its peridotites. The chromitites manifest subduction signature suggestive of the involvement of water in its generation. All of these would be consistent with generation in a supra-subduction zone environment, specifically in a subduction-related marginal ocean basin. The collision of the Palawan microcontinental block with the Philippine mobile belt along western Panay resulted, aside from ophiolite emplacement, into arc curvature, island rotation, serpentinite diapirism and thrusting along the forearc side. The offshore bathymetric expression of the microcontinental block along the collision zone shows the leading edge of this oceanic bathymetric high to have spread laterally. This is indicative of its being buoyant resulting to non-subduction as supported by available earthquake hypocenter data.