Underground Velocity Structure Research Articles

Seismic hazard prediction of the Hunhe Fault in the Shen-Fu New District

Earthquake prevention and disaster mitigation are crucial aspects of social welfare that significantly impact national public security. This paper presents a seismic risk assessment and hazard prediction of the Hunhe Fault in the Shengyang-Fushun (Shen-Fu) New District. The target area is at risk of seismic damage due to two major branch ruptures, namely, F9 and F1; these ruptures have the potential to generate maximum earthquakes with a magnitude of 6.0 in the next 50 to 100 years. A three-dimensional underground velocity structure and asperity source model were established for the target faults. Subsequently, a hybrid technique combining deterministic and empirical approaches was employed to simulate the broadband strong ground motion of the target region in anticipation of the occurrence of expected scenario earthquakes. The distributions of peak ground acceleration (PGA), peak ground velocity (PGV) and peak ground displacement (PGD) for the area are provided, and the results indicate that densely populated urban areas could experience PGA values close to 280 cm/s2 along the fault traces. This study provides a reliable basis for engineering construction and urban planning in the Shen-Fu New District.

Study of 3-D velocity structure characteristics in Dadu river Houziyan reservoir area at different impoundment stages

High-resolution three-dimensional VP, VS and VP/VS images in the Houziyan Reservoir Area were obtained by using Fast Marching Tomography Package (FMTOMO) with the travel time data from 6330 seismic events monitored by the Houziyan Reservoir Seismic Network. This analysis yielded the 3-D velocity structure, including longitudinal wave velocity (VP), shear wave velocity (VS), and the ratio of longitudinal and shear wave velocity (VP/VS) at different impoundment stages. The data changes at various impoundment times, depths of sections, and directions of profiles were analyzed to obtain these results. The final findings demonstrate the following results: 1) Through tomographic analysis, it was determined that the underground velocity structure in the Houziyan reservoir area was anisotropic before impoundment. 2) The area of high wave velocity increases in stage 1, stage 3, and stage 4. The area of low wave velocity increases in stage 2, especially in depth, indicating significant changes in the underground velocity structure at different impoundment stages. 3) Compared to the changes in underground velocity structures in other reservoirs after impoundment, the Houziyan reservoir exhibited a unique pattern. 4) In general, the underground velocity structure displayed an overall increasing trend after impoundment. However, it also exhibited instances of decreasing velocity, reflecting continuous dynamic adjustments to the underground velocity structure after impoundment. These conclusions highlight the impact of impoundment in the reservoir area on the underground velocity structure and provide scientific theoretical support for seismic risk assessment following impoundment in the reservoir area.

Joint inversion method of multipoint ambient noise horizontal-to-vertical spectral ratio for 3-D velocity structure of local site and its application

SUMMARY The diffusion field theory has been widely used to interpret ambient noise wave fields. Based on this theory, 1-D subsurface velocity structure inversion method is developed. However, few studies have referred to the noise horizontal to vertical (NHV) spectral ratio inversion of 3-D subsurface velocity structures, and almost no effective 3-D NHV inversion tools have been developed. To develop a useful tool for obtaining 3-D soil layer velocity structures, we combined the NHV forward calculation formula derived from diffusion field theory with the guided Monte Carlo algorithm and then extended the single-point NHV inversion to multipoint joint inversion through a joint objective function. Subsequently, a new 3-D soil layer velocity structure inversion method was proposed. Subsequently, a synthetic 2-D case was used to verify the proposed method. Finally, the proposed method was applied to the Xiangtang Array in Tangshan, China, to identify the 3-D velocity structures of the site based on noise observations. The results show that the proposed multipoint joint 3-D inversion method is effective for identifying 3-D underground velocity structures.

Shear wave velocity structure of the upper crust in north Xiaojiang fault zone in SE Tibet via short-period ambient noise dense seismic array

The Xiaojiang Fault Zone (XJF) is an important boundary fault on the southeast border of the Tibetan Plateau, with a complex subterranean structure and strong tectonic activity. In the present study, 252 seismograph stations were deployed in the northern section of the XJF to collect ambient noise data in a span of 35 days. The minimum resolution attained was 2 km × 2 km to develop a high-precision underground velocity structure model. The velocity model helped in revealing that the low-velocity distribution originated below the XJF. It was also found that the fault, which is the primary channel for magma to rise, might be the cause of the upper crustal deformation of the XJF. The Dongchuan mining area is located to the west of the XJF, which has a low-velocity structure. However, the high-velocity structure appeared on both sides of the ore body. It is speculated that the ore was formed by magma intrusion. The intrusion serves as a heat source for the mining area and promotes deposit formation.

On research of dispersion characteristics of multi-component surface waves from traffic-induced seismic ambient noise

As one of the commonly used sources in passive surface wave exploration, traffic-induced seismic ambient noise is widely used in urban underground velocity structure detection at present. However, characteristics of the dispersion curve extracted from such kind of ambient noise lack systematical research, especially for the multi-component surface wave. Due to the complex character of the real-world underneath structures, dispersion characteristics concluded from real-world traffic-induced seismic ambient noise that is hard to verify. The numerical simulation technique is a good choice for systematical research of characteristics of dispersion curves extracted from multi-component traffic-induced seismic ambient noise. Most of the numerical examples that appeared in the present literature, however, are based on the dispersion-curve-based numerical simulation technique. Hence, most of these can only simulate the vertical component. In this paper, the two-dimensional finite difference method is used to simulate multi-component traffic seismic ambient noise. The characteristics of original traffic noise are analyzed, and the effect of trace intervals on surface wave imaging of different types of traffic-induced noise is studied. Rayleigh wave dispersion energy image is extracted by the passive surface-wave frequency wavenumber transformed method. The differences in Rayleigh-wave dispersion images in traffic noise with different components are analyzed and compared. The results show that Rayleigh wave dispersion energy images appearing different characteristics in different components. The surface wave dispersion information presented by different types of traffic noise is frequency-dependent. The trace spacing also strongly affect the surface-wave dispersion energy images extracted from different types of traffic-induced noise. The numerical results are significant for guiding the traffic-induced seismic noise data acquisition and processing.

Shallow shear wave velocity structure of the Dongshan sag area using surface wave data in a deep reflection profile of the Yuanmou area of Yunnan province, China

Deep reflection seismic data contain abundant surface wave information for which the velocity structure in the near-surface shallow underground can be obtained using multichannel analysis of surface waves (MASW). This paper uses the MASW method to process deep reflection data in the Yuanmou area in Yunnan province, China. The MASW method uses the phase velocity spectral transformation of Rayleigh waves to obtain its dispersion curve. The initial model for different lithology classifications is established to invert the underground velocity structure. This study obtains a two-dimensional shear wave velocity structure profile to investigate the shear wave velocity structure. Generally, the study results have shown that the shear wave velocity structure obtained by the MASW method corresponds well with the surface conditions and accurately reflects faults' changes, folds, and lithology. Moreover, the formation sand content calculation shows a large difference in Dongshan sag's formation sand content. The sand contents of Cretaceous and Middle Jurassic strata are over 30% similar, and the sandstone content of underlying Early Jurassic strata is mostly 30% lower. The stratum buried depth of Early Jurassic strata is between 0– 800 m. The dip angle of LZJF at this location is 90°. The west side of the LZJF is based on Proterozoic uplift, and the Quaternary sedimentary thickness exceeds 300 m above the uplift stratum. Whereas on the east side of the LZJF, the stratum of the Dongshan sag section is mainly a synclinal structure.

Application of full waveform inversion method in underground pipeline detection

Urban underground pipelines are an important part of today’s urban infrastructure. A comprehensive and systematic grasp of the current status of urban underground pipelines can provide people with basic information on urban planning, construction and management, and help people better develop and use cities and their underground space. Full waveform inversion is a well-known method that could reconstruct the underground velocity structures with high resolution. In this paper, we test the ability of full waveform inversion method in underground pipeline detection. The numerical test shows that we could use full waveform inversion to detect the regular urban underground pipelines in different sizes and depths.

Application of pseudo-spectral full waveform inversion in underground pipeline detection

An accurate description of urban underground pipelines can effectively avoid blind construction and damage to underground pipelines during urban planning, design, construction, and management. Full waveform inversion could reconstruct the underground velocity structures with high resolution and has been widely used in urban engineering geophysical prospecting. The pseudo-spectral method combines the efficiency and accuracy of seismic forward modeling. Using the pseudo-spectral method for full waveform inversion can effectively improve this method’s detection accuracy for urban pipelines. In this paper, we test the pseudo-spectral full waveform inversion method in underground pipeline detection. The synthetic test shows that compared with conventional full wave inversion, the pseudo-spectral method is easier to converge and has higher accuracy. The method has great potential to be used to detect the urban underground pipelines.

Improvement of the objective function in the velocity structure inversion based on horizontal-to-vertical spectral ratio of earthquake ground motions

SUMMARYGiven the improvements that have been made in the forward calculations of seismic noise horizontal-to-vertical spectral ratios (NHVSRs) or earthquake ground motion HVSRs (EHVSRs), a number of HVSR inversion methods have been proposed to identify underground velocity structures. Compared with the studies on NHVSR inversion, the research on the EHVSR-based inversion methods is relatively rare. In this paper, to make full use of the widely available and constantly accumulating strong-motion observation data, we propose an S-wave HVSR inversion method based on diffuse-field approximation. Herein, the S-wave components of earthquake ground motion recordings are considered as data source. Improvements to the objective function has been achieved in this work. An objective function with the slope term is introduced. The new objective function can mitigate the multisolution phenomenon encountered when working with HVSR curves with multipeaks. Then, a synthetic case is used to show the verification of the proposed method and this method has been applied to invert underground velocity structures for six KiK-net stations based on earthquake observations. The results show that the proposed S-wave EHVSR inversion method is effective for identifying underground velocity structures.

Cause of destructive strong ground motion within 1\u20132\xa0s in Mukawa town during the 2018 Mw 6.6 Hokkaido eastern Iburi earthquake

We investigated the cause of destructive ground motion during the 2018 Hokkaido eastern Iburi earthquake. We conducted strong motion observations of aftershocks and microtremors and the surface wave method in the damaged areas of the town of Mukawa, Hokkaido prefecture, Japan. The ground accelerations were continuously recorded during a period of approximately 3 months after the main shock on September 6, 2018. The heavily damaged buildings were mainly situated around the strong motion station (HKD126) in Mukawa town. Such concentration of damage can be explained by the strong power that was observed in the 1–2 s period of the response spectrum at this station. We estimated the S-wave velocity profiles of this station site and a temporary station site that was installed on a nearby hill. The estimated S-wave velocity, which was inverted from phase velocity structures with the microtremor array and the surface wave method observations explained the difference in the SH-wave amplification characteristics between the two sites. An analysis of HKD126 and the temporarily observed records clearly indicates the strong effects of the local geological conditions on the heavily damaged area of Mukawa. The strong ground motion power generated during the main shock in Mukawa for 1–2 s period was mainly amplified by this shallow underground velocity structure.

Application of a simplified calculation for full-wave microtremor H/V spectral ratio based on the diffuse field approximation to identify underground velocity structures

Under the diffuse field approximation, the full-wave (FW) microtremor H/V spectral ratio (H/V) is modeled as the square root of the ratio of the sum of imaginary parts of the Green’s function of the horizontal components to that of the vertical one. For a given layered medium, the FW H/V can be well approximated with only surface waves (SW) H/V of the “cap-layered” medium which consists of the given layered medium and a new larger velocity half-space (cap layer) at large depth. Because the contribution of surface waves can be simply obtained by the residue theorem, the computation of SW H/V of cap-layered medium is faster than that of FW H/V evaluated by discrete wavenumber method and contour integration method. The simplified computation of SW H/V was then applied to identify the underground velocity structures at six KiK-net strong-motion stations. The inverted underground velocity structures were used to evaluate FW H/Vs which were consistent with the SW H/Vs of corresponding cap-layered media. The previous study on surface waves H/Vs proposed with the distributed surface sources assumption and a fixed Rayleigh-to-Love waves amplitude ratio for horizontal motions showed a good agreement with the SW H/Vs of our study. The consistency between observed and theoretical spectral ratios, such as the earthquake motions of H/V spectral ratio and spectral ratio of horizontal motions between surface and bottom of borehole, indicated that the underground velocity structures identified from SW H/V of cap-layered medium were well resolved by the new method.

Building damage concentrated in Longtoushan town during the 2014 Ms. 6.5 Ludian earthquake, Yunnan, China: examination of cause and implications based on ground motion and vulnerability analyses

Though the 2014 Ludian Earthquake had only a moderate magnitude (Ms 6.5), high-level ground motions of almost 1g occurred at Longtoushan Town (seismic station 53LLT), which located near the intersection of a conjugate-shaped seismogenic fault. The building damages on the pluvial fan and the river terrace at Longtoushan was clearly different. In order to examine the generation of the large acceleration at 53LLT, the focal mechanisms and the rupture processes of the conjugate-shaped seismogenic fault were determined. We found that there were two continuous impulsive waves in the records of 53LLT that were generated from two different faults, the Baogunao fault and the Xiaohe fault, respectively. Site effects on the pluvial fan and the river terrace at Longtoushan Town and their relations to different building damages were examined. We found that the predominant period at the pluvial fan was about 0.25 s, close to the fundamental natural period of multi-story confined masonry buildings. Ground motions on the pluvial fan and the river terrace were simulated through convolving synthesized bedrock motions with the transfer functions, which were analyzed using the one-dimensional underground velocity structures identified from H/V spectral ratios of ambient noise. Building collapse ratios (CRs) are estimated based on the vulnerability function of the 2008 Wenchuan Earthquake and are compared with the observed values. We found that the observed building CRs on the pluvial fan are much higher than the estimated values. High-level ground shaking that is far beyond the design level was a reason for serious building damage.

Sparse blind deconvolution based low-frequency seismic data reconstruction for multiscale full waveform inversion

Full waveform inversion (FWI) reconstructs the underground velocity structures by minimizing the data residual between calculated wavefields and observed wavefileds. The conventional FWI usually uses some local optimization algorithms which lead to a strong dependency on initial velocity model. The objective function corresponding to low-frequency data components has less local minima. Reconstructing low-frequency information from recorded seismic data and using it in FWI can reduce cycle-skipping and thus weaken the dependency of inversion process on initial model. In this paper, based on the conventional frequency down-shifting method, we propose a sparse blind deconvolution-convolution low-frequency data reconstruction method, which can simultaneously update the wavelet and reconstruct the low-frequency components. First, we extract the subsurface reflection impulse responses (SRIR) by solving a L1 norm sparse constraint problem using the Fast Iterative Shrinkage-Thresholding Algorithm (FISTA). Then we test the accuracy of our algorithm, and discuss the effect of wavelet error and noises on the reconstruction result.When the wavelet is inaccurate, we update the amplitude of wavelet by alternately inverting L1 norm constraint and Tikhonov regularization problems, and correct the time-shift error by cross-correlating the direct waves. After that we can get the accurate wavelet and SRIR simultaneously. Then using the reconstructed data successively as observed data, combining it with dynamic random sources and layer-stripping methods, we propose a new strategy for the fast multiscale FWI. We test our method by numerical examples in several cases including blended acquisition cases. The results show that it has good anti-noise property and it can reconstruct valid low-frequency components when the observed data lacking low-frequency information. The example using inaccurate wavelet shows that the blind deconvolution-convolution algorithm is able to obtain accurate wavelet and low-frequency data simultaneously.

Empirical Fragility Curves of Buildings in Northern Miyagi Prefecture During the 2011 off the Pacific Coast of Tohoku Earthquake

In this study, empirical fragility curves expressed in terms of relationship between damage ratio indices of buildings and ground motion indices were developed in northern Miyagi prefecture located in near-field areas during the 2011 off the Pacific coast of Tohoku Earthquake. The ground motion indices were evaluated from observed ground motions at strong-motion stations and estimated at sites at which no strong-motion accelerometers were deployed during the mainshock. The ground motions at the non-instrumental sites were estimated using the empirical Green’s function method based on bedrock motions inverted from observed records on surfaces from small events that occurred inside the source fault, transfer functions due to underground velocity structures identified from microtremor H/V spectral ratios, and a short-period source model of the mainshock. The findings indicated that the empirical fragility curves as functions of Japan Meteorological Agency (JMA) instrumental seismic intensity during the 2011 Tohoku Earthquake almost corresponded to those during the 1995 Kobe Earthquake and the seven disastrous earthquakes that occurred between 2003 and 2008. However, the empirical fragility curves as functions of peak ground velocity were the lowest. A possible reason for this is that the response spectra of the ground motions in the period ranging from 1.0 s to 1.5 s were small during the 2011 Tohoku Earthquake. Another reason could be the seismic resistant capacities of buildings in the studied districts involved during the 2011 Tohoku Earthquake exceeded those in the cities affected during the 1995 Kobe Earthquake.

Relationship between Building Damage Ratios and Ground Motion Characteristics during the 2011 Tohoku Earthquake

Fragility curves, the relationships between three kinds of building damage ratios, i.e., total collapse ratio (TCR), collapse ratio (CR) and damage ratio (DR), as well as ground motion characteristics, i.e., peak ground acceleration (PGA), peak ground velocity (PGV), Japan Meteorological Agency instrumental seismic intensity (IJMA) and spectral intensity (SI), were studied for the 2011 Tohoku Earthquake. It was found that the DR was related better to ground motion characteristics than the TCR and CR during this earthquake, and the correlation between the DR and any one of the PGV, IJMA or SI indices was almost the same. The DR during the 2011 Tohoku Earthquake reached almost the same level as that during the Kobe Earthquake for the same level of ground motions. There were some districts where the DRs did not always correspond to reasonable levels of ground motion characteristics. One reason for this was considered to be the representativeness of the ground motions over each district. Thus, one method of estimating ground motions at damaged sites was proposed by using the underground velocity structures identified from the H/V spectral ratios of microtremors, after we confirmed the consistency of the H/ V spectral ratios between earthquakes and microtremors. It is expected to be used for the construction of fragility curves with greater accuracy.

Source-Rupture Process of the 2007 Noto Hanto, Japan, Earthquake Estimated by the Joint Inversion of Strong Motion and GPS Data

Abstract The source-rupture process during the 2007 Noto Hanto earthquake, which was a reverse-faulting event located near the western coast of the Noto peninsula in central Japan, was estimated by the kinematic linear waveform inversion method. The waveform inversion was conducted by jointly using velocity waveforms recorded at 12 near-source strong-motion stations and static horizontal displacements recorded at 19 Global Positioning System (GPS) stations. In order to implement reliable Green’s functions into the inversion, a one-dimensional underground velocity structure for each individual strong-motion station was modeled through waveform modeling of an aftershock. The waveform inversion result showed the following image of the source-rupture process. The larger slip concentrated in the vicinity of the hypocenter, and it extended to the shallow portion of the source fault with a maximum slip of 5.1 m. The whole source process was characterized by the total seismic moment of 1.57×10 19 N · m ( M w 6.7) with the rupture duration of approximately 9 s. The obtained source model explained observed strong ground motions and GPS-measured static horizontal displacements well. The slip distribution correlated well with the spatiotemporal aftershock distribution, and the northeast end of the rupture was blocked by the resistive structure in the crust. From these facts, the 2007 Noto Hanto earthquake could be concluded as reactivation of preexisting seafloor active faults, F14 and F15, which were reported by the previous geophysical and geological surveys off the shore of the Noto peninsula.

Validation of spatial autocorrelation (SPAC) method with L-shape array in Jyoso City, Japan

This study is aimed to validate the efficiency of an L-shape array for SPAC method using microtremor in estimating the shear wave velocity (V,) structure. The experiment for validation was conducted in the Toyota Community Baseball Ground, Jyoso City, Ibaraki Prefecture, Japan in March 2009 with an equilateral triangle array with side length of 40 m and in June 2010 with an equilateral triangle array with side length of50 m, together with an L-shape array of the similar size. Multichannel Analysis of Surface Waves (MASW) was also performed simultaneously in June 20I 0. In the same lot PS-logging data are available from nearby LBRH IO station of the K.IK-NET (NTED, Japan) that shows soft sediment of about 20 m thick with V, of 110 m/s in the geological column of the site.
 The comparison of the determined phase velocity and that calculated from PS logging data shows close matching of two sets of curves separately. One is between PS logging and the triangle array (40 m), and the other is between the triangle array (50 m) and the L-shape array (50 m). Former two are of the almost same place whereas other two arrays are deployed about 200 m away from the other set. Some discrepancy between two sets is shown. This seems due to lateral variation of underground velocity structure which is consistent with the result of MASW.
 Based on the results of analysis one can say that a L-shape array can be applied to estimate shear wave velocity for shallow depth so it can be layout in urban areas to determine phase velocity information from microtremors. Therefore, it may be feasible to apply it in the Kathmandu Valley, Nepal that is based upon the soft soil with high possibility of liquefaction or earthquake hazard.

Basin and crustal velocity structure models for the simulation of strong ground motions in the Kinki area, Japan

We constructed a prototype of the basin and crustal structure model for the Kinki area, southwest of Japan, for the simulation of strong ground motions of hypothetical crustal and subduction earthquakes. We collected results of the deep seismic velocity profiles obtained by the reflection experiments and seismic imaging results, which were conducted in the Kinki area. The obtained profiles give underground velocity structures of the crust, from the surface to the subducting slab. We also gather the basin velocity structure information of the Osaka, Kyoto, Nara, and Ohmi basins. To examine the applicability of the constructed velocity structure model to the ground motion simulation, we simulated waveforms of an intermediate size event occurred near the source area of the hypothetical subduction earthquakes. Simulated ground motions using the basin and crustal velocity structure model are fairly well reproducing the observations at most of stations, and the constructed basin and crustal velocity structure model is applicable for the long-period ground motion simulations.

2004年9月5日紀伊半島沖・東海道沖の地震による地震動

This article reviews studies on ground motion characteristics during the 2004 earthquake sequence off the Kii peninsula and the Tokai district occurred on September 5. Long-period ground motions are observed at stations in the large sedimentary basins such as Kanto, Nobi, and Osaka. Typical predominant periods of ground motions at each basin are approximately 8-10s for Kanto basin, 3s for Nobi, and 6s for Osaka, respectively, which corresponds to thickness of sedimentary layers of each basin. Long-period ground motion modelings for the event were done by several authors for confirming and improving underground velocity structure models. The reliable underground structure model confirmed by this waveform modeling shall be used for strong ground motion simulation of the anticipated Tokai, Tonankai and Nankai earthquakes.