3D S-wave Velocity Model Research Articles

Unraveling Precambrian cratonic roots beneath South America: A contribution from surface wave tomography

We examine some aspects of the tectonic evolution of Precambrian cratonic roots beneath South America based on lithospheric distribution of shear-wave velocities. We derive our model by inverting 26,984 fundamental mode Rayleigh wave group velocity dispersion curves, at periods of 9–180 s. We first regionalize our measurements and then invert the result for a 3D S-wave velocity model extending to 200 km depth. Fast velocities beneath the Amazonian and São Francisco cratons, and beneath buried cratonic units in the Parnaíba and Paraná basins, are long wavelength features consistent with previous tomography studies. For the Amazonian craton at 150 km depth, we find an increase of velocities with province age, except for the Maroni-Itacaiúnas province, where we hypothesize that K'Mudku intraplate tectono-thermal events at the middle-late Mesoproterozoic and emplacement of a large igneous province following the breakup of Pangea could have altered at least partially its lithosphere. Our results are consistent with a São Francisco paleocontinent whose borders extend beyond the surface limits of the present São Francisco craton into the neighboring Araçuaí and Brasília belts. Based on slow shear-wave velocities in the upper mantle beneath the Borborema province, consistent with lithospheric thinning, we argue that a possible cratonic root of the São Francisco Paleocontinent beneath this province has likely been eroded away. This analysis is further corroborated by tectonic events that led to the alteration of the Borborema mantle, including hydration in the Paleoproterozoic, rifting in the early-middle Tonian, reworking during Neoproterozoic Brasiliano events, and lithospheric stretching during the breakup of Pangea. Finally, we also image a fast shear-wave velocity structure in the region of the Río de la Plata craton, consistent with magnetotelluric studies.

Ambient noise tomography of El Hierro island (Canary Islands)

El Hierro island is one of the most active islands in the Canary Islands from a volcanological point of view. This is the reason why the imaging of the internal crustal structure is of huge importance. The geophysical exploration methods employed on El Hierro Island, such as gravimetry and seismic tomography, allowed obtaining the high-resolution characterization of the crust’s deep part. However, these methods did not yield significant information about the surface and the shallower part of the crust. To gain a deeper insight into the shallow geological structure of El Hierro island, we employed Ambient Noise Tomography to construct a 3D S-wave velocity model. Our investigation revealed the presence of seven significant seismic velocity anomalies, partly identified by previous studies. We identified two high-velocity anomalies located in the eastern and western parts of the island at a depth between 0 and 3 km below sea level (b.s.l.). We interpreted these anomalies as dense intrusive complexes of dikes, possibly linked to the Tanganasoga volcano and the formation of the Tiñor edifice. Additionally, we observed two high-velocity anomalies in the northern and southern parts of the island at a depth between 3 and 4 km b.s.l., which we related to the accumulation of solidified igneous rocks. On the other hand, a low-velocity anomaly was observed in the Golfo valley, between 0 and 0.5 km b.s.l., and we interpreted it as megalandslide deposits. This anomaly was evidenced for the first time in the present study. Finally, two low-velocity anomalies were observed in the southern part of the island at different depths, between 0–0.5 km b.s.l. and 0–2 km b.s.l. These were interpreted as fractures generated by Quaternary volcanism along the SSE Rift. Also, one of them was evidenced for the first time in this study, corresponding to the zone of the fractures produced during the Quaternary volcanism. This study has allowed us to gain a more detailed understanding of the shallow geological structure of the island. Even if most of the anomalies had been evidenced previously, we could observe the existence of two low-velocity zones in the shallow crust that have not been observed before.

A multiscale magma system beneath the Tengchong volcano in western Yunnan revealed by ambient noise tomography

SUMMARY The western Yunnan is located in the SE Tibetan Plateau, and is characterized by the active Tengchong volcano (TCV), complex crust–mantle coupling and intense earthquakes. To elucidate tectonism in the western Yunnan, we construct a 3-D S-wave velocity model to 80 km depth via ambient noise tomography using dense seismic stations. Our model shows significant low-velocity anomalies at different depths in the crust and uppermost mantle. Compared with the results of previous regional tomography, we image low-velocity anomalies consistent with a large-scale source of partial melts in the uppermost mantle beneath the Tengchong and Baoshan blocks, rather than just below the Tengchong block. Our results also reveal a magma chamber extending from the shallow subsurface to the lower crust beneath the TCV, which is fed by the mantle source. Based on these findings, we propose that the mantle source and crustal magma chamber form a multiscale magma system. Moreover, the mantle source is potentially resulted from asthenospheric upwelling, which is related to the subduction of the Indian slab. In addition, our model shows that the 1976 M7.4 and M7.3 Longling earthquakes occurred near a magma chamber. Thus, fluids from the magma chamber likely reduced the frictional coefficient on the seismogenic fault and caused the Longling earthquakes.

Two Unconnected Low-Velocity Zones in the Eastern Boundary of the Sichuan-Yunnan Block Revealed with High-Resolution Ambient Noise Tomography

The Anninghe-Zemuhe-Xiaojiang fault zone (AZXFZ) is an important boundary fault zone on the southeastern margin of the Tibetan Plateau, with frequent strong earthquakes. Previous studies have imaged widespread low-velocity zones in this area. However, there are still many disputes on the connectivity and genesis of the low-velocity zones. In this study, we obtain the Rayleigh wave phase velocity dispersion curves at 4–25-s periods using observations from 378 broadband stations located near the AZXFZ. The new 3D S-wave velocity model has a lateral resolution of about 30 km in 0–35-km depth and is obtained by direct inversion of surface wave dispersion data. The new results clearly image two low-velocity zones and a high-velocity zone in the middle crust of the study region. The low-velocity zone on the western side of the Lijiang-Xiaojinhe fault is related to the eastward flow of crustal material and the movement of the left lateral strike-slip faults in the Tibetan Plateau, while the low-velocity anomaly distributed along the Daliangshan fault and Xiaojiang fault is the superimposition effect of shear heating of the faults and upwelling of mantle material. The uplift of Gongga Shan is a combination of the continuous accumulation of crustal material in the middle and lower crust of the southeastern margin of the Tibetan Plateau as well as the bending and compression of the Sichuan-Yunnan Block and the Xianshuihe-Anninghe fault zone.

Multi-Mode Surface Wave Tomography of a Water-Rich Layer of the Jizhong Depression Using Beamforming at a Dense Array

Urban structure imaging using noise-based techniques has rapidly developed in recent years. Given the complexity of the cross-correlation function in high-frequency signals, here, the beamforming (BF) method was used to analyze one data set taken from a dense array in the Jizhong Depression and obtain multi-mode dispersion curves. Multi-mode surface waves improved inversion stability, reduced non-uniqueness, and yielded a one-dimensional shear wave (S-wave) velocity model. Interpolation yielded a high-resolution three-dimensional (3D) S-wave velocity model for the study area. The model shows that velocity gradually changed in the horizontal direction and greatly increased in the vertical direction, which is largely consistent with changes in the sedimentary environment related to the continuous subsidence of the Jizhong Depression since the Quaternary. A low-velocity anomaly at a depth of ~300–400 m was revealed and determined to be caused by either a deep-buried ancient river course or low-lying area. This study demonstrates the potential of the BF method for processing dense array data sets of urban exploration. The high-resolution 3D S-wave velocity model provides a new reference for studying the Quaternary structure of the Jizhong Depression, as well as groundwater resources, urban infrastructure, and underground spaces.

Ambient noise tomography for a high-resolution 3D S-wave velocity model of the Kinki Region, Southwestern Japan, using dense seismic array data

Research interest in the Kinki region, southwestern Japan, has been aroused by the frequent occurrence of microearthquake activity that do not always coincide with documented active fault locations. Previous studies in the Kinki region focused mainly on deep, large-scale structures and could not efficiently resolve fine-scale (~ 10 km) shallow crustal structures. Hence, characterization of the upper crustal structure of this region at an improved spatial resolution is required. From the cross-correlation of the vertical components of the ambient seismic noise data recorded by a densely distributed seismic array, we estimated Rayleigh wave phase velocities using a frequency domain method. Then, we applied a direct surface wave tomographic method for the measured phase velocity dispersion data to obtain a 3D S-wave velocity model of the Kinki region. The estimated velocity model reveals a NE–SW trending low-velocity structure coinciding with the Niigata–Kobe Tectonic Zone (NKTZ) and the active Biwako-seigan Fault Zone (BSFZ). Also, we identified fine-scale low-velocity structures coinciding with known active faults on the eastern side of the NKTZ, as well as sets of low-velocity structures across the Tanba region. Furthermore, sedimentary basins manifest as low-velocity zones extending to depths ranging from ~ 1.5 to 2 km, correlating with those reported in previous studies. Our results therefore contribute towards fundamental understanding of earthquake faulting as well as tectonic boundary and will be useful for hazard assessment and disaster mitigation.Graphical abstract

3D S-Wave Velocity Model of the Crust and Upper Mantle beneath the Sea of Okhotsk and the Kamchatka Peninsula

Abstract A 3D S-wave velocity model (from 0 to 350 km depth) is determined for the region of the Sea of Okhotsk and the Kamchatka peninsula, through Rayleigh wave analysis applied to the traces of 278 earthquakes registered by 12 seismic stations, both located within (and nearby) of the study area. This model reveals the principal geological and tectonics features present in the study area, e.g., the presence of two lower-crust hot plumes located at the northwest of the Sea of Okhotsk, which are shown as two zones of low S-wave velocity (from 20 to 30 km depth). Also, a conspicuous low S-wave velocity zone is determined at the southwest of the Sea of Okhotsk (from 35 to 60 km depth), which can be matched up with a high conductivity layer previously determined from 30 to 65 km depth. For the Kamchatka peninsula, low S-velocities are determined beneath the volcanic belt from the upper crust (~5 km-depth) down to a depth of ~60 for the southern part, and down to a depth of ~140 km for the northern part. This low S-wave velocity pattern is enlarged in size at the northwest (north of ~55°N), following the location of the Kliuchevskoi and Sheveluch volcanoes, which confirms that these volcanoes must be a part of the same subduction-induced volcanic process. The present model shows that the subducting Pacific slab terminates near to the Aleutian-Kamchatka junction, i.e., no relict slab underlies the extinct northern Kamchatka volcanic arc. This model shows that this slab shoals towards north, and there exists a gap associated with the loss of this slab beneath Sheveluch and Kliuchevskoi volcanoes. The low S-wave velocity pattern determined at northwest of the slab edge confirms the presence of the asthenospheric flow, which would pass through this gap to the northwest around the north slab edge. Finally, the present model shows the precise location and detailed structure of the asthenosphere, which is a new result that has not been determined in other previous studies.

S-Wave Velocity Structure of the Crust and Upper Mantle beneath the North China Craton Determined by Joint Inversion of Rayleigh-Wave Phase Velocity and Z/H Ratio

Abstract The North China craton (NCC) is one of the oldest craton in the world. Since the Cenozoic, the NCC has undergone severe lithospheric thinning, accompanied by extensive crustal deformations and volcanic activities. To better understand the mechanisms of the crustal and lithospheric deformations and intraplate volcanisms, we construct a high-resolution 3D S-wave velocity model for the NCC by jointly inverting Rayleigh-wave phase velocity dispersion and Z/H ratio measurements. Across the NCC, our model reveals significant lateral variations. In the shallow crust, prominent low-velocity anomalies associated with thick sediments are resolved in the Bohai Bay basin, and the Ordos basin and its surrounding grabens. Meanwhile, our model also shows that sediments are thin or even missing in the southeast of the Ordos basin and other orogens in the study area. The sedimentary structures of the Bohai Bay basin and the surrounding grabens of the Ordos basin may be the superficial response to the subduction of the Pacific plate and the northeastward push of the Tibetan plateau, respectively. Under the Datong volcano, our model reveals an integrated low-velocity anomalies from the mid-to-lower crust to the mantle, which verifies that the Datong volcano has a deep origin. Besides, the low-velocity zone beneath the eastern NCC (ENCC) connects to the low-velocity anomalies under the Datong volcano in the upper mantle. Combining the previous studies, we further propose that the asthenospheric upwelling of the Datong volcano and the subduction of the Pacific plate may jointly contribute to the lithospheric thinning of the ENCC.

Ambient noise tomography of Misti volcano, Peru

To better understand the recent internal structure of Misti volcano, we determined a 3D S-wave velocity model applying Ambient Noise Tomography (ANT). We used data from 23 broadband and short-period seismic stations temporarily installed at Misti volcano between March and December 2011. This dataset allowed us to obtain empirical Green's functions by cross-correlating seismic ambient noise signals. Then, we retrieved 104 dispersion curves using the frequency-time analysis (FTAN) and, through a non-linear multiscale inversion, we obtained nine 2-D Rayleigh waves group velocity maps for periods in the range 0.7 s - 2 s. Finally, we carried out the depth inversion through a Bayesian transdimensional inversion to obtain a 3-D S-wave velocity model down to 3 km depth. Our study highlights five relevant seismic velocity anomalies. We observed the presence of three high-velocity zones located in the west-northwest, southwest and southeast parts of the crater, that could be related to intrusive bodies possibly associated with the formation of Misti volcano. We also observed two low-velocity anomalies in the volcano's western and central parts, which coincide with previous studies' findings and are related to fractured and weakened materials associated with the external caldera collapse and recent eruption episodes.

High-Resolution Crustal S-wave Velocity Model and Moho Geometry Beneath the Southeastern Alps: New Insights From the SWATH-D Experiment

We compiled a dataset of continuous recordings from the temporary and permanent seismic networks to compute the high-resolution 3D S-wave velocity model of the Southeastern Alps, the western part of the external Dinarides, and the Friuli and Venetian plains through ambient noise tomography. Part of the dataset is recorded by the SWATH-D temporary network and permanent networks in Italy, Austria, Slovenia and Croatia between October 2017 and July 2018. We computed 4050 vertical component cross-correlations to obtain the empirical Rayleigh wave Green’s functions. The dataset is complemented by adopting 1804 high-quality correlograms from other studies. The fast-marching method for 2D surface wave tomography is applied to the phase velocity dispersion curves in the 2–30 s period band. The resulting local dispersion curves are inverted for 1D S-wave velocity profiles using the non-perturbational and perturbational inversion methods. We assembled the 1D S-wave velocity profiles into a pseudo-3D S-wave velocity model from the surface down to 60 km depth. A range of iso-velocities, representing the crystalline basement depth and the crustal thickness, are determined. We found the average depth over the 2.8–3.0 and 4.1–4.3 km/s iso-velocity ranges to be reasonable representations of the crystalline basement and Moho depths, respectively. The basement depth map shows that the shallower crystalline basement beneath the Schio-Vicenza fault highlights the boundary between the deeper Venetian and Friuli plains to the east and the Po-plain to the west. The estimated Moho depth map displays a thickened crust along the boundary between the Friuli plain and the external Dinarides. It also reveals a N-S narrow corridor of crustal thinning to the east of the junction of Giudicarie and Periadriatic lines, which was not reported by other seismic imaging studies. This corridor of shallower Moho is located beneath the surface outcrop of the Permian magmatic rocks and seems to be connected to the continuation of the Permian magmatism to the deep-seated crust. We compared the shallow crustal velocities and the hypocentral location of the earthquakes in the Southern foothills of the Alps. It revealed that the seismicity mainly occurs in the S-wave velocity range between ∼3.1 and ∼3.6 km/s.

Surface wave tomography using 3D active-source seismic data

Surface wave tomography (SWT) is a powerful and well-established technique to retrieve 3D shear-wave (S-wave) velocity models at the regional scale from earthquakes and seismic noise measurements. We have applied SWT to 3D active-source data, in which higher modes and heterogeneous spatial sampling make phase extraction challenging. First, synthetic traveltimes calculated on a dense, regular-spaced station array are used to test the performance of three different tomography algorithms (linearized inversion, Markov chain Monte Carlo [MCMC], and eikonal tomography). The tests suggest that the lowest misfit to the input model is achieved with the MCMC algorithm, at the cost of a much longer computational time. Then, real phases were extracted from a 3D exploration data set at different frequencies. This operation included an automated procedure to isolate the fundamental mode from higher order modes, phase unwrapping in two dimensions, and the estimation of the zero-offset phase. These phases are used to compute traveltimes between each source-receiver couple, which are input into the previously tested tomography algorithms. The resulting phase-velocity maps show good correspondence, highlighting the same geologic structures for all three methods. Finally, individual dispersion curves obtained by the superposition of phase-velocity maps at different frequencies are depth inverted to retrieve a 3D S-wave velocity model.

The formation of the Dabashan orocline, central China: Insights from high-resolution 3D crustal shear-wave velocity structure

The Dabashan orocline is a large thrust belt in central China. Determination of its high-resolution three-dimensional (3D) crustal shear-wave (S-wave) velocity structure can enhance our understanding of the intracontinental evolution of the South China plate and North China plate. In this study, we estimated the Rayleigh wave phase and group velocities at periods of 5–50 s, P-wave receiver functions, and Ps and SsPmp travel times from 63 permanent stations and 29 portable stations; we also constructed a 3D S-wave velocity model and crustal thickness and Vp/Vs ratio maps of the Dabashan orocline and adjacent areas. The crustal thickness and Vp/Vs ratio maps show prominent thick crust (50–55 km) and high Vp/Vs ratios (1.8–1.85), suggesting a mafic lower crust in the Dabashan orocline. The 3D S-wave velocity structure shows northeast-dipping low-velocity anomalies in the upper crust of the Dabashan orocline, and high velocities extended from near the surface to the lower crust in the Hannan-Micang and Shennong-Huangling domes, suggesting deep roots of the two domes. We propose that the two domes acted as rigid indenters and rotated clockwise together with the South China plate, penetrating into the Qinling-Dabie orogenic belt. Weak sediments in the upper crust of the Dabashan orocline were compressed and blocked by the two domes and the rigid Qinling-Dabie orogenic belt, resulting in vertical accretion of the sediments and crust and subsequent formation of the northeast-dipping low-velocity anomalies and the thick crust beneath the Dabashan orocline.

3D wave-equation dispersion inversion of Rayleigh waves

The 2D wave-equation dispersion (WD) inversion method is extended to 3D wave-equation dispersion inversion of surface waves for the shear-velocity distribution. The objective function of 3D WD is the frequency summation of the squared wavenumber [Formula: see text] differences along each azimuth angle of the fundamental or higher modes of Rayleigh waves in each shot gather. The S-wave velocity model is updated by the weighted zero-lag crosscorrelation between the weighted source-side wavefield and the back-projected receiver-side wavefield for each azimuth angle. A multiscale 3D WD strategy is provided, which starts from the pseudo-1D S-velocity model, which is then used to get the 2D WD tomogram, which in turn is used as the starting model for 3D WD. The synthetic and field data examples demonstrate that 3D WD can accurately reconstruct the 3D S-wave velocity model of a laterally heterogeneous medium and has much less of a tendency to getting stuck in a local minimum compared with full-waveform inversion.

High-resolution 3D crustal S-wave velocity structure of the Middle-Lower Yangtze River Metallogenic Belt and implications for its deep geodynamic setting

The Middle-Lower Yangtze River Metallogenic Belt (MLYMB) is an important mineral resource region in China. High-resolution crustal models can provide crucial constraints to understand the ore-forming processes and geodynamic setting in this region. Using ambient seismic noise from 107 permanent and 82 portable stations in the MLYMB and the adjacent area, we present a new high-resolution 3D S-wave velocity model of this region. We first extract 5–50 s Rayleigh wave phase velocity dispersion data by calculating ambient noise cross-correlation functions (CFs) and then use the surface wave direct inversion method to invert the mixed path travel times for the 3D S-wave velocity structure. Checkerboard tests show that the horizontal resolution of the 3D S-wave velocity model is approximately 0.5°–1.0° and that the vertical resolution decreases with increasing noise and depth. Our high-resolution 3D S-wave velocity model reveals: (1) A V-shaped high-velocity zone (HVZ) is located in the lower crust and the uppermost mantle in the study region. The western branch of the HVZ passes through the Jianghan Basin, the Qinling-Dabie orogenic belt and the Nanxiang Basin. The eastern branch, which almost completely covers the main body of the MLYMB, is located near the Tanlu Fault. The low-velocity anomalies between the western and eastern branches are located in the area of the Qinling-Dabie orogenic belt. (2) High-velocity uplifts (HVUs) are common in the crust of the MLYMB, especially in the areas near the Tanlu Fault, the Changjiang Fault and the Yangxin-Changzhou Fault. The intensities of the HVUs gradually weaken from west to east. The V-shaped HVZ in the lower crust and uppermost mantle and the HVUs in the middle and lower crust likely represent cooled mantle intrusive rocks. During the Yanshanian period, fault systems formed in the MLYMB due to the convergence between the South China Plate and the North China Plate, the multiple-direction drifting of the Paleo-Pacific Plate and its subduction beneath the Eurasian Plate. The dehydration of the cold oceanic crust led to partial melting in the upper mantle. Temperature differences caused strong convection of the upper mantle material that underplated the lower crust and rose to near the surface along the deep fault systems. After mixing with the crustal materials, mineralization processes, such as assimilation and fractional crystallization, occurred in the MLYMB.

Estimating S-wave velocities from 3D 9-component shallow seismic data using local Rayleigh-wave dispersion curves – A field study

Surface waves are widely used to delineate near-surface structures. Most of the surface-wave studies focus on 2D and single-component data, how to use the surface-wave method on 3D multi-component data has not been studied. In this paper, we perform a 3D 9-component (9C) shallow-seismic survey in Rheinstetten, Germany. We transform the 9C data from the cartesian coordinates to the ray-based coordinates and separate Love and Rayleigh waves in the transformed data. The data is windowed into groups constituted by 16 receiver points, and we perform dispersion analysis in each windowed data individually. Rayleigh-wave energy acquired by four different components in each windowed data is stacked in the frequency-velocity domain to improve the accuracy of the dispersion image, and the Rayleigh-wave dispersion curve is picked along the energy peak in the stacked dispersion image. We estimate a total of 242 Rayleigh-wave dispersion curves from the 3D 9C data, which are then inverted to build a 3D S-wave velocity model. The 3D S-wave velocity model shows the existence as well as the spatial variation of the main geological feature, namely a refilled trench from the 18th century. We compare 2D sections of the 3D model to the GPR profiles acquired in the same survey area, which proves the relatively high accuracy of the structures in the estimated model. We also compare the observed seismic waveforms to the synthetic waveforms simulated by using the 3D model, which also proves that the estimated model is fairly reliable. This study provides a cost-efficient way to use surface waves in 3D multi-component data to image the near-surface S-wave velocity model.

Continental lithospheric subduction and intermediate-depth seismicity: Constraints from S-wave velocity structures in the Pamir and Hindu Kush

The Pamir has experienced more intense deformation and shortening than Tibet, although it has a similar history of terrane accretion. Subduction as a primary way to accommodate lithospheric shortening beneath the Pamir has induced the intermediate-depth seismicity, which is rare in Tibet. Here we construct a 3D S-wave velocity model of the lithosphere beneath the Pamir by surface wave tomography using data of the TIPAGE (Tien Shan–Pamir Geodynamic program) and other seismic networks in the area. We imaged a large-scale low velocity anomaly in the crust at 20–50 km depth in the Pamir overlain by a high velocity anomaly at a depth shallower than 15 km. The high velocity anomalies colocate with exposed gneiss domes, which may imply a similar history of crustal deformation, partial melting and exhumation in the hinterland, as has occurred in the Himalaya/Tibet system. At mantle depths, where the intermediate-depth earthquakes are located, a low velocity zone is clearly observed extending to about 180 km and 150 km depth in the Hindu Kush and eastern Pamir, respectively. Moreover, the geometry of the low-velocity anomaly suggests that lower crustal material has been pulled down into the mantle by the subducting Asian and Indian lithospheric mantle beneath the Pamir and Hindu Kush, respectively. Metamorphic processes in the subducting lower crust may cause the intermediate-depth seismicity down to 150–180 km depth beneath the Pamir and Hindu Kush. We inverted focal mechanisms in the seismic zone for the stress field. Differences in the stress field between the upper and lower parts of the Indian slab imply that subduction and detachment of the Indian lithosphere might cause intense seismicity associated with the thermal shear instability in the deep Hindu Kush.

Development of an engineering bedrock map beneath Jakarta based on microtremor array measurements

Abstract Jakarta has been selected as a study area for seismic microzonation by considering its population and its infrastructure growth, seismicity and geological setting. One of the important factors in seismic hazard analysis is the characteristics of the ground overlying bedrock, and, as yet, this has not been studied for Jakarta. This study was intended to estimate the depth to bedrock by applying microtremor array exploration. The phase velocity of microtremors was estimated by the spatial autocorrelation (SPAC) method, whereas the S-wave velocity profile was estimated by inversion using a genetic algorithm. Microtremor array analysis has been conducted at 55 sites that covered the whole of Jakarta. The result of one-dimensional (1D) S-wave velocity profile estimation indicates that the engineering bedrock depth has pronounced differences between northern (>700 m) and southern Jakarta (about 300 m). A 3D S-wave velocity model was constructed from 1D profiles resulting from a second inversion in which the depth of constant-velocity layers, with velocities of 500, 700 and 900 m s −1 , was determined. The constructed 3D velocity structures show a bedrock morphology that has a depth range of 350–725 m, with depths increasing towards northern Jakarta. The implication of differences in bedrock depth is that estimated seismic amplification for the thicker sediment will be higher.

Ambient seismic noise tomography reveals a hidden caldera and its relation to the Tarutung pull-apart basin at the Sumatran Fault Zone, Indonesia

We analyzed the noise recordings of a short-period seismic network to derive a shallow crustal S-wave velocity model at the Sumatra Fault in Northern Sumatra, Indonesia. By correlating the noise of 40 seismic stations' recording for 9months, we could recover Rayleigh waves from vertical component recordings with sufficient signal-to-noise ratio. Group velocities of the Rayleigh waves could be determined in the period range from 0.71 to 4.4s. These group velocities were used to invert for 2D group velocity maps at specific periods. Finally, the derived group velocity maps were inverted for a 3D S-wave velocity model. This model shows a region of a strong velocity decrease off the Great Sumatran Fault Zone, at the northeastern margin of the young Tarutung pull-apart basin. This observed low velocity block coincides with a caldera-like morphological feature which is interpreted as the surface expression of a hidden volcanic caldera. Considering the surface manifestations of geothermal activity around this anomaly, we conclude that the caldera is still acting as a heat source. On the other hand, the weak morphological expression at the surface indicates a certain age of the caldera which might be older than the Tarutung pull-apart basin. The findings provide important constraints on general concepts for the formation of pull-apart basins along the Sumatran fault and their relation to volcanism.

表層部分に注目した地震動シミュレーションのための関東平野の3次元S波速度深部地盤モデル

表層部分に注目した地震動シミュレーションのための関東平野の3次元S波速度深部地盤モデル

Evaluation of proxies for seismic site conditions in large urban areas: The example of Santiago de Chile

Characterizing the local site response in large cities is an important step towards seismic hazard assessment. To this regard, single station seismic noise measurements were carried out at 146 sites in the northern part of Santiago de Chile. This extensive survey allowed the fundamental resonance frequency of the sedimentary cover, derived from horizontal-to-vertical (H/V) spectral ratios, to be mapped. By inverting the spectral ratios under the constraint of the thickness of the sedimentary cover, known from previous gravimetric measurements, local S-wave velocity profiles have been retrieved. After interpolation between the individual profiles, the resulting high resolution 3D S-wave velocity model allows the entire area, as well as deeper parts of the basin, to be represented in great detail. Since one lithology shows a great scatter in the velocity values only a very general correlation between S-wave velocity in the uppermost 30 m ( v s 30 ) and local geology is found. Local S-wave velocity profiles can serve as a key factor in seismic hazard assessment, since they allow an estimate of the amplification potential of the sedimentary cover. Mapping the intensity distribution of the 27 February 2010 Maule, Chile, event ( Mw = 8.8) the results indicate that local amplification of the ground motion might partially explain the damage distribution and encourage the use of the low cost seismic noise techniques for the study of seismic site effects.