Dense Seismic Network Research Articles

Isotropic and radially anisotropic S-velocity structure beneath the Arabian plate inferred from surface wave tomography

We estimated Rayleigh- and Love-wave group-velocity dispersion curves using dense seismic networks in Saudi Arabia and nearby countries. We merged our data with group- and phase-velocity dispersion curves from previous studies to produce a new isotropic and radially anisotropic S-velocity model for the Arabian plate and nearby regions. Strong negative radial anisotropy (VSH < VSV) is observed in the crustal depth beneath the southern Red Sea, but weak positive radial anisotropy (VSH > VSV) and relatively thick lithosphere are found beneath the northern Red Sea, implying different tectonics between the two regions. We found low-velocity anomalies beneath Cenozoic volcanism in the Arabian Peninsula, but corresponding radial anisotropic perturbations are observed differently in spatiotemporal regimes depending on source origin. From the strong positive radial anisotropy at asthenospheric depth, which correlates with a circular pattern of azimuthal anisotropy from shear-wave splitting, we infer toroidal mantle flow around regions of thick lithosphere including an eastern part of the Arabian platform, the Mesopotamian Plain, and the Zagros belt. On the contrary, weak radial anisotropy is found beneath the thin lithosphere of the Arabian shield and in the thick lithosphere of the Mesopotamian, which may imply vertical flow due to asthenospheric upwelling and cratonic growth, respectively.

Seismic imaging of reservoir heterogeneity using a network with high station density at The Geysers geothermal reservoir, CA, USA

We analyze results of high-resolution seismic imaging at The Geysers geothermal reservoir in northern California, USA, using a dense seismic network to image the spatial heterogeneity of the reservoir structure and flow paths. The project uses 92 seismic stations spaced at approximately 500 m over a 5 km × 5 km study area. Microseismic data for more than 17,000 earthquakes have been acquired over a period of 13 months and automatically processed for P- and S-wave phase arrival times. The data are subsequently inverted using a joint inversion approach to image the spatial heterogeneity of the reservoir including the 3D P- and S-wave velocity structure and [Formula: see text]/[Formula: see text] ratio, and to locate earthquake hypocenters. The resulting tomographic images are appraised by integration into The Geysers’ 3D reservoir model and by a spatial correlation to the injection and production wells. Spatial correlation of P-wave velocity images to water injection and steam production wells reveal higher velocities below injection wells, due to higher water saturation, and lower velocities in the vicinity of steam-producing wells, due to the presence of steam in the surrounding reservoir rocks. The spatial correlation of [Formula: see text]/[Formula: see text] to steam in the reservoir indicates decreased estimates in the vicinity of steam production wells. In contrast, the [Formula: see text]/[Formula: see text] ratio reveals high values in the reservoir for regions near water injection wells and along the potential flow path of water through the reservoir. The estimates of shear modulus indicate high values in a region of competent graywacke, which is known for a lack of fractures and steam production, and low values in a region that is dominated by water flow, suggesting that fractured rock created a pathway for the water through the reservoir. The heterogeneity observed in the S-wave velocities indicates a compartmentalized reservoir, which correlates spatially with fault projections in the 3D reservoir model.

Extreme seismic anisotropy indicates shallow accumulation of magmatic sills beneath Yellowstone caldera

Understanding the distribution and mobility of crystal mushes within modern magmatic systems is crucial to volcanic hazard assessments as distinct pockets of mobile magma may become interconnected and lead to melt accumulation on shorter time scales than magma that is broadly distributed in a homogeneous mush. Here, we reveal that Yellowstone's upper-crustal magma reservoir in the top 20 km is heterogeneous in both melt concentration and texture. We exploit ambient noise in an unprecedented dense temporary seismic network to jointly constrain vertically- and horizontally-polarized shear wave speeds to create enhanced 3D isotropic and anisotropic shear velocity models. Our models show an exceptionally low-velocity (>20% reduction) layer 4–7 km beneath the surface, situated near the top of the reservoir previously-imaged by earthquake P-wave tomography. The presence of strong radial anisotropy (20%) within this layer indicates the uppermost portion of the modern Yellowstone reservoir is organized as a sill complex, with up to 28% of melt fraction in horizontally-elongated volumes at depths where rhyolite was commonly stored before past eruptions. The findings demonstrate that high-resolution anisotropic imaging through a dense seismic network can constrain both magma distribution and reservoir texture, which are important to understand the evolution and hazard assessment of volcanic systems like Yellowstone.

Wet mantle transition zone beneath the Indian shield: Constraints from P-to-S receiver functions analysis

Indian plate passed over four plumes on its way toward the Eurasian plate. These interactions with plumes certainly affected the upper mantle and crust of the Indian plate as manifested by its thinned lithosphere and relatively low shear velocities both in the crust and upper mantle, but the depth extent of these effects of plume-lithosphere interaction remains ambiguous. In this study, we investigate the mantle transition zone beneath the Indian shield using P receiver functions computed at 23 stations covering the entirety of the Indian shield to investigate the depth extent of the imprints of the plume-lithosphere interaction as well as to study the lateral variations of transition zone beneath a stable intraplate setting like the Indian shield. Our results show good agreement with the results of previous studies as well as with the tomographic models in terms of the average apparent depths of the 410 and 660 discontinuities and the transition zone thickness. However, unlike previous studies, we find compelling evidence of a persistent mid-transition zone discontinuity beneath all the stations and a low-velocity layer beneath some regions. We also investigated the frequency dependence of amplitudes of receiver functions and found that most of the stations showed a strong dependence of amplitudes on frequency. Based on the evidence from our investigation, we demarcate the regions potentially containing relatively more weight percentage of water inside the otherwise considered as a ‘dry’ mantle transition zone. These regions should be further investigated in detail by a dense seismic network and a realistic 3-D velocity model.

Tracking supercritical geothermal fluid distribution from continuous seismic monitoring

Continuous seismic monitoring could play a pivotal role in deep geothermal energy exploration. We monitored seismicity near geothermal production areas of the Kuju volcanic complex with a dense seismic network and automated event detection. Most events were shallow (less than 3 km below sea level) and distributed along a boundary between regions of high and low resistivity and S-wave velocity, interpreted as a lithological boundary or related fracture zone. Deeper events located on top of subvertical conductors may reflect fracturing associated with magmatic fluid intrusion. A correlation may exist between seismicity and heavy rainfall three days prior to increased pore pressure in pre-existing fractures. Our findings support the presence of supercritical geothermal fluids and demonstrate the importance of continuous seismic monitoring in supercritical geothermal energy exploration.

Development of SLO KARST Near Fault Observatory site in SW Slovenia

The project RI-SI-EPOS (Research Infrastructure-Slovenia-European Plate Observing System) provided new scientific equipment for solid Earth science related to geology, seismology, geodesy and karstology. Karst research infrastructure is primarily used at the SLO KARST NFO (Near Fault Observatory) developing site. The area covers ~ 2600 km2 and is one of the most seismically active areas in SW Slovenia. It consists mostly of karstified Mesozoic carbonate rocks and partly of non-karstified Eocene flysch. The landscape has numerous karst features including caves, poljes, dolines, uvalas, karst springs, ponors and periodic karst lakes, with typical karst underground water drainage. Site development commenced with the installation of seven temporary seismic stations in the area, along with other geoscience equipment (gravimeter, 3D laser terrestrial scanner, GNSS antenna, TM72 extensometers, spectrometer for methane and drone), deployed in 2020. With the new dense seismic network, locations and other seismic parameters are already determined more reliably than in the past, contributing to a better understanding of active tectonic deformations at the junction between the seismically active Friuli region (Italy) and the Zagreb–Petrinja area (Croatia). The new solid Earth Geoscience research infrastructure is already collecting a big amount of data following the FAIR principles (making data Findable, Accessible, Interoperable and Reusable) aiming to be included in the national and international research databases. Here, the first results from the newly operating SLO KARST NFO temporary seismic network (May 2020–June 2021) are discussed.

New insights on Moho depth and regional lithospheric structure of central Oklahoma based on receiver function analysis from dense seismic networks

Knowledge of the midcontinent crustal structure of North America is crucial for understanding the evolutionary history of the ancient North American craton as Laurentia grew through accretion ∼1.5 Ga to 1 Ga. Although Oklahoma has been recognized as a tectonically stable region since the Phanerozoic, its crustal structure records the earlier formation of the Mazatzal and southern Granite-Rhyolite provinces 1.6 Ga to 1.4 Ga. We present results from teleseismic receiver function analysis applied to 221 events recorded on 169 broadband stations in central Oklahoma. Our findings include a Moho depth map of central Oklahoma based on stacked and depth-converted teleseismic P receiver functions. The results are interpreted together with gravimetric and magnetic datasets and a recently established seismic velocity model of the crust.The Moho map shows a generally flat crust-mantle boundary in central Oklahoma with an average depth of 43.5 km while we observe a sudden thickening on the crust of the northwestern part of Oklahoma where the Moho deepens to over 50 km depth. We also find a Mid-lithosphere discontinuity at the upper-most mantle in north-central Oklahoma, presented as a negative phase deepening southeastward from 60 km to 80 km. We further observe an intracrustal discontinuity at the Nemaha uplift and Anadarko shelf regions in a depth range of 17–30 km. The hypothesis of the Mid-continent Rift (MCR) extending into Oklahoma is examined in terms of the crustal structure and Moho depth variation revealed by receiver functions. We do not find evidence of Moho structure or lower-crustal underplay characteristics similar to what has been discovered in the northern part of MCR, but the intracrustal discontinuity that deepens towards the hypothesized MCR region suggests upper-crustal volcanics potentially caused by the extended expansion regime of the failed rift near the south-most termination.

Local Earthquake Seismic Tomography Reveals the Link Between Crustal Structure and Volcanism in Tenerife (Canary Islands)

AbstractVolcanic activity on Tenerife Island is extremely diverse. Three radial rift zones are characterized by cinder cones from basaltic fissure eruptions. A triple junction in central Tenerife exhibits a complex of merged, predominantly phonolitic, stratovolcanoes. The Las Cañadas caldera and widespread ignimbrite deposits reveal high explosive potential. We investigated the crustal and upper mantle structure beneath Tenerife using local earthquake data recorded by two dense seismic networks on the island. For our tomographic inversion, we selected &gt;130,000 P‐ and S‐wave arrivals from ∼6,300 events that occurred during seismic unrests in 2004–2005 and 2017–2021. Synthetic tests confirmed that we could robustly resolve seismic velocity structures to ∼20 km depth. In the upper crust (down to ∼7 km) beneath central Tenerife, a prominent high‐velocity anomaly represents the rigid core of the volcanic complex; at greater depths, a strong low‐velocity anomaly reveals abrupt crustal thickening. Vp and Vs contour lines of 5.2 and 2.85 km/s, respectively, reveal Moho depth variation; crustal thickness beneath Las Cañadas reaches ∼17 km, whereas that beneath other parts of Tenerife is ∼10 km. An anomaly at ∼5 km beneath the caldera with low Vp, low Vs, and high Vp/Vs might be associated with a major phonolitic magma reservoir. Similar anomalies at ∼ sea level may represent shallow magma sources responsible for recent eruptions. Seismicity occurs in a columnar area of high Vp, high Vs, and low Vp/Vs, and may represent hydrothermal fluid migration through brittle media. Based on our results, we constructed a conceptual model of volcanic activity on Tenerife.

Evidence of structural segmentation of the Uttarakhand Himalaya and its implications for earthquake hazard

The earthquake hazard associated with the Main Himalayan Thrust (MHT) is a critical issue for India and its neighbouring countries in the north. We used data from a dense seismic network in Uttarakhand, India, to model the lateral variations in the depths of MHT (2–6% drop in Vs at 12–21 km depths), Moho (a sharp increase in Vs (by ~ 0.5–0.7 km/s) at 39–50 km depths) and lithosphere (a marked decrease in Vs(~ 1–3%) at 136–178 km depths), across the Himalayan collisional front. Our joint inversion of radial PRFs and group velocity dispersion data of Rayleigh waves detects three NNE trending transverse lithospheric blocks segmenting the lithosphere in Uttarakhand Himalaya, which spatially correlate well with the northward extension of the Delhi -Haridwar Indian basement ridge, an inferred tectonic boundary and great boundary fault, respectively. Our radial receiver function imaging detects highly deformed and segmented crustal and lithospheric structures associated with three mapped transverse lithospheric blocks, suggesting a reduction in rupture lengths of future earthquakes, thereby, reducing earthquake hazards in Uttarakhand.

Monitoring storm evolution using a high-density seismic network

Data acquired by a dense seismic network deployed in the Cerdanya basin (Eastern Pyrenees) is used to track the temporal and spatial evolution of meteorological events such as rainfall episodes or thunderstorms. Comparing seismic and meteorological data, we show that for frequencies above 40 Hz, the dominant source of seismic noise is rainfall and hence the amplitude of the seismic data can be used as a proxy of rainfall. The interstation distance of 1.5 km provides an unprecedented spatial resolution of the evolution of rainfall episodes along the basin. Two specific episodes, one dominated by stratiform rain and the second one dominated by convective rain, are analyzed in detail, using high resolution disdrometer data from a meteorological site near one of the seismic instruments. Seismic amplitude variations follow a similar evolution to radar reflectivity values, but in some stratiform precipitation cases, it differs from the radar-derived precipitation estimates in this region of abrupt topography, where radar may suffer antenna beam blockage. Hence, we demonstrate the added value of seismic data to complement other sources of information such as rain-gauge or weather radar observations to describe the evolution of ground-level rainfall fields at high spatial and temporal resolution. The seismic power and the rainfall intensity have an exponential relationship and the periods with larger seismic power are coincident. The time intervals with rain drops diameters exceeding 3.5 mm do not result in increased seismic amplitudes, suggesting that there is a threshold value from which seismic data are no longer proportional to the size of the drops. Thunderstorms can be identified by the recording of the sonic waves generated by thunders, with. Single thunders detected to distances of a few tens of kilometers. As the propagation of these acoustic waves is expected to be strongly affected by parameters as air humidity, temperature variations or wind, the seismic data could provide an excellent tool to investigate atmospheric properties variations during thunderstorms.

Preliminary Study on the Duration of High-Frequency Seismic Waves in Northern Taiwan from Deep Regional Earthquakes

The high-frequency seismic waves are usually generated by shallow earthquakes and is observed at a location near the earthquake source. Seismic energy tends to attenuate as it propagates through the Earth’s surface and interior; however, this is not the case at the subduction zone. A regional earthquake that occurred within the subducting slab was observed to have high-frequency energy, although it is from the deep (i.e. &gt; 100 km) and with moderate magnitude (i.e. Mw 5). This phenomenon resulted from the slab effect on focusing the earthquake signal or known as the guided wave. Taiwan has a complicated tectonic feature, where Taipei city, its capital, sits above the oblique subduction of Philippine Sea Plate (PSP), i.e. the southern Ryukyu subduction zone, enhancing its exposure to strong shaking resulting from the deep regional earthquakes. In this study, we use six regional earthquakes at the southern Ryukyu subduction zone recorded by the Formosa Array (FM Array). The FM Array is a dense seismic network with a total of 140 stations located in the northern part of Taiwan, with the distance between stations approximately 5 kilometers. With high-resolution data, we could identify the distribution of the guided wave phenomenon through body wave dispersion of deep regional earthquake signals, where only stations above 10 – 20 km from the plate interface showed dispersion. An Hp/Lp ratio with a 5-second moving window is applied to the dispersed signal. The results indicate that the high-frequency signal has a longer duration at stations that sit 10 to 20 km above the plate interface. This study suggests that the slab effect will produce an anomalous seismic intensity at the subduction region and should be considered in the Ground Motion Prediction Equation (GMPE) so that the damaged to buildings resulting from strong shaking could be reduced.

The topography of the 660-km discontinuity beneath the Kuril-Kamchatka: Implication for morphology and dynamics of the northwestern Pacific slab

The 660-km discontinuity (660) plays a potentially important role in deep slab dynamics and mantle convection. Increasing numbers of seismic observations suggest controversial morphologies of the Pacific slab beneath the Kuril-Kamchatka, highlighting the poorly understood interaction of the slab and mantle discontinuities. Here we collect near-source S-to-P converted waves from a large dataset with several dense seismic networks and systematically image the topography of the 660 around the Pacific slab beneath the Kuril-Kamchatka. We conduct detailed comparisons of the 660 depths and seismicity along some vertical cross sections. In comparison with the discontinuity depth in the IASP91 model, the 660 exhibits broad depressions up to 32–63 km with apparent downward deflections beneath the Kamchatka Peninsula and northern Kuril (region I), supporting slab penetration into the lower mantle; in contrast, the 660 depressions beneath southern Kuril (region II) are less than 21–28 km with a relatively flat configuration, implying a stagnating slab with possible hot entrained mantle materials and/or partial melts below it. We interpret these regional variations in the 660 topography as reflecting local low-temperature anomalies due to different slab morphologies associated with contrasting subduction modes. We suggest compound effects of pressure-driven mantle flow and trench retreat for inducing the inferred subduction mode change of the Pacific slab from region I to region II. Our results can provide more insight into subduction dynamics in the northwestern Pacific.

The neglected Istanbul earthquakes in the North Anatolian Shear Zone: tectonic implications and broad-band ground motion simulations for a future moderate event

SUMMARYİstanbul (Marmara Region, NW Turkey) is one of the megacities in the world and suffered from destructive earthquakes on the North Anatolian Fault, a member of the North Anatolian Shear Zone, throughout history. The 1999 Kocaeli and Düzce earthquakes emphasize the earthquake potential of the fault, crossing the Sea of Marmara, and the importance of seismic hazards in the region. The studies in the last 20 yr have concentrated on the main fault and its future destructive earthquake potential. In this study, unlike the previous ones, we focus on the two main topics about the earthquakes not interested previously in İstanbul: (1) Investigating recent earthquake activity masked by the blasts in the metropolitan area and its tectonic implications, (2) revealing their effects in İstanbul utilizing numerical ground motion simulations for a future moderate event (Mw 5). First, the 386 earthquakes from 2006 to 2016 are relocated with the double-difference method using the dense seismic network operated in the same period. The source mechanisms of the events (ML ≥ 3), including the most recent 2021 Kartal–İstanbul earthquake (ML 4.1), are determined. In addition to the analysis of the recent seismic activity, the location of the two moderate and pre-instrumental-period İstanbul earthquakes, which occurred in 1923 (Mw 5.5) and 1929 (Mw 5.1), are revised. Using the relocated epicentres outside of the principal deformation zone and the fault plane solutions, the roles of the earthquakes in the stress regime of the Marmara region are explained. The epicentres on the Cenozoic or Palaeozoic formation in the Istanbul–Zonguldak Zone are interpreted as the re-activation of the palaeo-structures under the recent tectonic stresses, and their fault plane solutions agree with the synthetic/antithetic shears of a transtensional regime corresponding to the right lateral strike-slip system with mainly N–S extension in the Marmara Region. In the second part, we investigate the effects of moderate scenario events (Mw 5) considering the current earthquake epicentres in the İstanbul metropolitan area, using characterized earthquake source model and 1-D velocity structure verified with the broad-band (0.1–10 Hz) numerical ground motion simulation of the 2021 Kartal–İstanbul earthquake. The simulated PGAs agree with the ground-motion prediction equations for short epicentral distances (&amp;lt;30 km). Furthermore, according to the empirical relation for Turkish earthquakes, the maximum PGA value of the synthetic models (∼0.3 g) corresponds to the felt intensity of MMI IX. The simulated spectral accelerations for the Mw 5 earthquake scenarios may exceed the design spectrum between 0.2 and 0.6 s given in the Turkish Building Earthquake Code (2018). In addition, certain models also generate spectral accelerations close to the design-level spectrum between 0.4 and 1 s, leading to resonance phenomena. The results indicate that a moderate event (Mw 5) in the İstanbul metropolitan area is capable of damage potential for the mid-rise buildings (4–10 stories) because of the site condition with resonance phenomena and poor construction quality.

Deep long period seismicity preceding and during the 2021 Fagradalsfjall eruption, Iceland

We use a dense seismic network on the Reykjanes Peninsula, Iceland, to image a group of earthquakes at 10–12 km depth, 2 km north-east of 2021 Fagradalsfjall eruption site. These deep earthquakes have a lower frequency content compared to earthquakes located in the upper, brittle crust and are similar to deep long period (DLP) seismicity observed at other volcanoes in Iceland and around the world. We observed several swarms of DLP earthquakes between the start of the study period (June 2020) and the initiation of the 3-week-long dyke intrusion that preceded the eruption in March 2021. During the eruption, DLP earthquake swarms returned 1 km SW of their original location during periods when the discharge rate or fountaining style of the eruption changed. The DLP seismicity is therefore likely to be linked to the magma plumbing system beneath Fagradalsfjall. However, the DLP seismicity occurred ~ 5 km shallower than where petrological modelling places the near-Moho magma storage region in which the Fagradalsfjall lava was stored. We suggest that the DLP seismicity was triggered by the exsolution of CO2-rich fluids or the movement of magma at a barrier to the transport of melt in the lower crust. Increased flux through the magma plumbing system during the eruption likely adds to the complexity of the melt migration process, thus causing further DLP seismicity, despite a contemporaneous magma channel to the surface.

Rupture Directivity of the 2021 ML 2.2 Gwangyang, Korea, Microearthquake: Toward Resolving High-Resolution Rupture Process of a Small Earthquake

Abstract Moderate-to-large earthquakes (M&amp;gt;6) frequently show clear rupture directivity. Recent studies revealed that a substantial percentage of small-sized earthquakes (M&amp;lt;∼5) display rupture directivity as well, owing to enhanced seismic monitoring. Is rupture directivity a common feature for earthquakes of all sizes? In this study, we investigated the rupture directivity of the 27 August 2021 ML 2.2 Gwangyang microearthquake that occurred at the southern tip of the Korean Peninsula by utilizing data from a recently expanded dense seismic network. The mainshock had two foreshocks and three aftershocks, which enabled us to define the fault plane from precise relative event locations and a well-constrained focal mechanism. Analysis of apparent source time functions obtained with the empirical Green’s function technique reveals that the mainshock ruptured unilaterally toward the east. A detailed analysis of the source pulse leaving the focal sphere in 3D indicates that rupture directivity is consistent with the fault geometry and slip direction. Our study demonstrates that we can resolve rupture directivity of unilaterally rupturing microearthquakes (M&amp;lt;∼3) given adequate azimuthal coverage, and that this will lead to an improved understanding of the seismogenic processes in regions of low seismicity worldwide.

High‐Resolution Imaging of Fault Zone Structure Along the Creeping Section of the Haiyuan Fault, NE Tibet, From Data Recorded by Dense Seismic Arrays

AbstractHigh‐resolution imaging of fault zone structures is essential for understanding earthquake physics and fault mechanics. As a major left‐lateral strike‐slip fault in northeastern Tibet, fine structures of the damage zone in the creeping (Laohushan) section of the Haiyuan fault remain unclear. To resolve geometry and velocity reduction of the damage zone, we deployed a dense temporary network of 110 seismic stations around the creeping section of the Haiyuan fault. Travel time delays from teleseismic P arrivals suggest an approximately 1‐km‐wide low‐velocity zone, likely illuminating a broader damage zone around the Haiyuan fault. A catalog is constructed for local earthquakes based on phase picks identified from a machine learning technique. The amplification of waveforms from these local events and waveform modeling of fault zone trapped waves indicate a narrower inner damage zone with depth‐dependent width (ranging from 150 to 50 m) that extends to a depth of approximately 4 km. These values are generally consistent with those found on other noncreeping faults in California, suggesting that these damage zone properties are not affected by fault slip behaviors at shallow depth. In addition, a clear bi‐material velocity contrast across the fault is revealed by the analysis of teleseismic P arrivals. Assuming the contrast extends to a depth of 15 km, we find that P‐wave velocity is approximately 5% slower in the crustal block north of the fault. Our study shows that a temporary dense seismic network is effective in illuminating cross‐fault velocity contrast and fault geometry.

High‐Resolution Monitoring of Controlled Water Table Variations From Dense Seismic‐Noise Acquisitions

AbstractWater‐resource management has become a major global issue in a world threatened by climate change. High‐resolution geophysical methods may be of valuable help in monitoring the water masses, both in space and time. Passive seismic interferometry takes advantage of ambient seismic noise to recover the variations in seismic wave velocity induced by changes in groundwater. We present hereafter the time and space monitoring of an hydraulic dome artificially formed to prevent biological and chemical pollutants from entering the exploitation field of Crépieux‐Charmy (Lyon, France). We use a dense seismic network to passively monitor the water table changes induced by infilling of an infiltration basin at the water supply facility for the two million inhabitants of the Lyon Metropolis (France). We assess the hourly seismic velocity variations over 19 days, during which two filling and drainage cycles were performed. The use of a dense three‐component seismic network allows fine characterization of the seismic wavefield, and offers the possibility to include the analysis of 4,851 raypaths in a robust inversion algorithm based on ray theory. The velocity variations are mapped with high resolution. They are directly related to the water table variations and to residual water saturation changes within the unsaturated zone. This seismic experiment highlights the three‐dimensional (3D) implementation and evolution of a hydraulic dome under the infiltration basin. This dynamic information helps in the understanding and modeling of water flows between the water table and a river, which represents a fundamental issue for discussions on the effectiveness of the barrier.

Seismogenesis of the 2021 Mw 7.1 earthquake sequence near the northeastern Japan revealed by double-difference seismic tomography

Recently, a Mw 7.1 earthquake struck the Pacific coast of northeastern Japan on 2021/02/13. To investigate the cause of this earthquake and its aftershocks, we applied a Vp/Vs model-consistency constrained double-difference (DD) seismic tomography method to the earthquake arrival time data from the dense seismic networks in Japan to image the velocity structures at the hypocenter areas. Compared to the conventional DD tomography, the new method can determine the Vp/Vs model with high resolution and precision. The reliability of the seismic tomography results is testified by checkerboard resolution test. The velocity profiles show that the oceanic crust and uppermost part of the oceanic mantle are characterized by low Vs and high Vp/Vs due to hydration reaction, while the lower oceanic mantle is associated with high Vs and low Vp/Vs anomalies that are likely caused by serpentine dehydration. The mainshock of the Mw 7.1 earthquake sequence is located at the boundary between the low velocity layer and underlying high velocity zone. The aftershocks are mainly distributed within the low velocity layer. The rupture plane inferred from the aftershock distribution and mainshock focal mechanism solution has penetrated ∼10 km into the slab and extended ∼50 km along a direction nearly parallel to the trench. These observations have suggested that the Mw 7.1 earthquake sequence occurs as a result of reactivation of a pre-existing fault likely created at the outer-rise before subduction. The association of the mainshock with the velocity anomaly edge suggests that this earthquake sequence is probably caused by dehydration of the mantle serpentines, which increases the pore pressure on the fault. Besides, the variation of rock mechanics at the hypocenter area of the mainshock also facilitates the nucleation of the whole earthquake sequence.

Combined Large-NSeismic Arrays and DAS Fiber Optic Cables across the Hengill Geothermal Field, Iceland

AbstractFrom June to August 2021, we deployed a dense seismic nodal network across the Hengill geothermal area in southwest Iceland to image and characterize faults and high-temperature zones at high resolution. The nodal network comprised 498 geophone nodes spread across the northern Nesjavellir and southern Hverahlíð geothermal fields and was complemented by an existing permanent and temporary backbone seismic network of a total of 44 short-period and broadband stations. In addition, we recorded distributed acoustic sensing data along two fiber optic telecommunication cables near the Nesjavellir geothermal power plant with commercial interrogators. During the time of deployment, a vibroseis survey took place around the Nesjavellir power plant. Here, we describe the network and the recorded datasets. Furthermore, we show some initial results that indicate a high data quality and highlight the potential of the seismic records for various follow up studies, such as high-resolution event location to delineate faults and body- and surface-wave tomographies to image the subsurface velocity structure in great detail.

The Investigation of Shallow Structures at the Meishan Fault Zone with Ambient Noise Tomography Using a Dense Array Data

Seismic monitoring relies on seismography. However, the high cost of seismic equipment has presented a challenge to increasing the density of seismic networks in previous decades. Due to the large station spacing and inferior coverage of stations, this situation has led to a loss of detail in many research results. Along with the improvement of technology, the problem of increasing the density of seismographic observations is no longer an impossible issue. This makes it feasible to deploy a dense seismic network for monitoring earthquakes. This study deployed a linear dense array across the Meishan Fault in west-southern Taiwan for the purpose of analyzing the shallow fault zone structure. While the 1906 Meishan earthquake occurred in a period when historic records were available, the surficial geology surveys of the Meishan Fault are challenging because farming and construction engineering have obscured the outcrop. Early surveys of the Meishan Fault were mainly seismic surveys. In recent decades, over thirty profiles have been completed. However, the reflection seismic records had poor signal-to-noise ratios because the Meishan Fault is buried under thick sediments. Thus, the shallow structure of the Meishan Fault is still not known in detail. This study applied double-beamforming tomography to a dense seismic array to obtain high-resolution images of the Meishan Fault zone. The result shows that there is a south-dipping interface near the fault trace as indicated by the Central Geological Survey of Taiwan. In addition, we observed velocity transitions of perturbation profiles that may be caused by a branch fault, the Chentsoliao Fault. This study demonstrates that the ambient noise double beamforming method is an effective tool for imaging the detailed shallow structure along with the dense seismic array.