Spatio-temporal variations of shallow seismic velocity changes in Salton Sea Geothermal Field,California in response to large regional earthquakes and long-term geothermal activities

We measure spatio-temporal variations of seismic velocity changes in Salton Sea Geothermal Field,California based on cross correlations of daily seismic traces recorded by a borehole seismic network from December 2007 to January 2014.We find clear co-seismic velocity reductions during the 2010 M 7.2 El Mayor–Cucapah,Mexico earthquake at~100 km further south,followed by long-term recoveries.The co-seismic reductions are larger with longer post-seismic recoveries in higher frequency bands,indicating that material damage and healing process mostly occurred in the shallow depth.In addition,the co-seismic velocity reductions are larger for ray paths outside the active fluid injection/extraction regions.The ray paths inside injection/extraction regions are associated with smaller co-seismic reductions,but subtle long-term velocity increases.We also build 3D transient water flow models based on monthly injection/extraction rates,and find correlations between several water flow parameters and co-seismic velocity reductions.We interpret the relative lack of co-seismic velocity changes within the geothermal region as unclogging of fracture network due to persistent fluid flows of geothermal production.The long-term velocity increase is likely associated with the ground water depletion and subsidence due to net production.

Seismic Velocity Structure and Composition of the Continental Crust of Eastern China

On the basis of a one-by-one latitude-longitude grid three-dimensional seismic velocity model, the crustal P-wave velocity structure in eastern China (105-125°E and 18-41°N) is obtained, and a set of geotherms for each grid is established for P-T correction on P-wave velocities. The average depths of sub-crustal layers and their average P-wave velocities of 18 tectonic units in eastern China are exhibited. Our result presents a 32-34 km thick crust beneath eastern China, which is thinner than previous studies, with an average velocity of 6.54 km/s, corresponding to a 5 kg/m3 variation in crustal mean density. The thicker upper but thinner middle and lower crust results in a lower average seismic velocity of eastern China. An intermediate crustal composition with a SiO2 content of 59.7 wt% has been estimated. However, there exists a significant lateral variation in the crustal structures among the tectonic units of eastern China. The structure and composition features of some regions in eastern China indicate that extension has played an important role in the continental crust evolution of eastern China.

On Quadratic Wasserstein Metric with Squaring Scaling for Seismic Velocity Inversion

The quadraticWasserstein metric has shown its power in comparing prob-ability densities.It is successfully applied in waveform inversion by generating ob-jective functions robust to cycle skipping and insensitive to data noise.As an alter-native approach that converts seismic signals to probability densities,the squaring scaling method has good convexity and thus is worth exploring.In this work,we apply the quadratic Wasserstein metric with squaring scaling to regional seismic to-mography.However,there may be interference between different seismic phases in a broad time window.The squaring scaling distorts the signal by magnifying the unbalance of the mass of different seismic phases and also breaks the linear super-position property.As a result,illegal mass transportation between different seismic phases will occur when comparing signals using the quadratic Wasserstein metric.Furthermore,it gives inaccurate Fr´echet derivative,which in turn affects the inver-sion results.By combining the prior seismic knowledge of clear seismic phase sep-aration and carefully designing the normalization method,we overcome the above problems.Therefore,we develop a robust and efficient inversion method based on optimal transport theory to reveal subsurface velocity structures.Several numerical experiments are conducted to verify our method.

Forward prediction for tunnel geology and classification of surrounding rock based on seismic wave velocity layered tomography

Excavation under complex geological conditions requires effective and accurate geological forward-prospecting to detect the unfavorable geological structure and estimate the classification of surround-ing rock in front of the tunnel face.In this work,a forward-prediction method for tunnel geology and classification of surrounding rock is developed based on seismic wave velocity layered tomography.In particular,for the problem of strong multi-solution of wave velocity inversion caused by few ray paths in the narrow space of the tunnel,a layered inversion based on regularization is proposed.By reducing the inversion area of each iteration step and applying straight-line interface assumption,the convergence and accuracy of wave velocity inversion are effectively improved.Furthermore,a surrounding rock classification network based on autoencoder is constructed.The mapping relationship between wave velocity and classification of surrounding rock is established with density,Poisson’s ratio and elastic modulus as links.Two numerical examples with geological conditions similar to that in the field tunnel and a field case study in an urban subway tunnel verify the potential of the proposed method for practical application.

Monitoring of velocity changes based on seismic ambient noise: A brief review and perspective

Over the past two decades,the development of the ambient noise cross-correlation technology has spawned the exploration of underground structures.In addition,ambient noise-based monitoring has emerged because of the feasibility of reconstructing the continuous Green’s functions.Investigating the physical properties of a subsurface medium by tracking changes in seismic wave velocity that do not depend on the occurrence of earthquakes or the continuity of artificial sources dramatically increases the possibility of researching the evolution of crustal deformation.In this article,we outline some state-of-the-art techniques for noise-based monitoring,including moving-window cross-spectral analysis,the stretching method,dynamic time wrapping,wavelet cross-spectrum analysis,and a combination of these measurement methods,with either a Bayesian least-squares inversion or the Bayesian Markov chain Monte Carlo method.We briefly state the principles underlying the different methods and their pros and cons.By elaborating on some typical noisebased monitoring applications,we show how this technique can be widely applied in different scenarios and adapted to multiples scales.We list classical applications,such as following earthquake-related co-and postseismic velocity changes,forecasting volcanic eruptions,and tracking external environmental forcing-generated transient changes.By monitoring cases having different targets at different scales,we point out the applicability of this technology for disaster prediction and early warning of small-scale reservoirs,landslides,and so forth.Finally,we conclude with some possible developments of noise-based monitoring at present and summarize some prospective research directions.To improve the temporal and spatial resolution of passive-source noise monitoring,we propose integrating different methods and seismic sources.Further interdisciplinary collaboration is indispensable for comprehensively interpreting the observed changes.

Crustal velocity structure of central Gansu Province from regional seismic waveform inversion using firework algorithm

The firework algorithm（FWA） is a novel swarm intelligence-based method recently proposed for the optimization of multi-parameter, nonlinear functions. Numerical waveform inversion experiments using a synthetic model show that the FWA performs well in both solution quality and efficiency. We apply the FWA in this study to crustal velocity structure inversion using regional seismic waveform data of central Gansu on the northeastern margin of the Qinghai-Tibet plateau. Seismograms recorded from the moment magnitude（MW） 5.4 Minxian earthquake enable obtaining an average crustal velocity model for this region. We initially carried out a series of FWA robustness tests in regional waveform inversion at the same earthquake and station positions across the study region,inverting two velocity structure models, with and without a low-velocity crustal layer; the accuracy of our average inversion results and their standard deviations reveal the advantages of the FWA for the inversion of regional seismic waveforms. We applied the FWA across our study area using three component waveform data recorded by nine broadband permanent seismic stations with epicentral distances ranging between 146 and 437 km. These inversion results show that the average thickness of the crust in this region is 46.75 km, while thicknesses of the sedimentary layer, and the upper, middle, and lower crust are 3.15,15.69, 13.08, and 14.83 km, respectively. Results also show that the P-wave velocities of these layers and the upper mantle are 4.47, 6.07, 6.12, 6.87, and 8.18 km/s,respectively.

Two-step interface and velocity inversion—— Study o e of the Tangshan seismic region

This paper studies the computation method of two step inversion of interface and velocity in a region. The 3 D interface is described by a segmented incomplete polynomial; while the reconstruction of 3 D velocity is accomplished by the principle of least squares in functional space. The computation is carried out in two steps. The first step is to inverse the shape of 3 D interface; while the second step is to do 3 D velocity inversion by distributing the remaining residual errors of travel time in accordance with their weights. The data of seismic sounding in the Tangshan Luanxian seismic region are processed, from which the 3 D structural form in depth of the Tangshan seismic region and the 3 D velocity distribution in the crust below the Tangshan Luanxian seismic region are obtained. The result shows that the deep 3 D structure in the Tangshan seismic region trends NE on the whole and the structure sandwiched between the NE trending Fengtai Yejituo fault and the NE trending Tangshan fault is an uplifted zone of the Moho. In the 3 D velocity structure of middle lower crust below that region, there is an obvious belt of low velocity anomaly to exist along the NE trending Tangshan fault, the position of which tallies with that of the Tangshan seismicity belt. The larger block of low velocity anomaly near Shaheyi corresponds to a denser earthquake distribution. In that region, there is an NW trending belt of high velocity anomaly, probably a buried fault zone. The lower crust below the epicentral region of the Tangshan M S=7.8 earthquake is a place where the NE trending belt of low velocity anomaly meets the NW trending belt of high velocity anomaly. The two sets of structures had played an important role in controlling the preparation and occurrence of the M S=7.8 Tangshan earthquake.

S-wave velocity structure beneath Changbaishan volcano inferred from receiver function

The S wave velocity structure in Changbaishan volcanic region was obtained from teleseismic receiver function modeling. The results show that there exist distinct low velocity layers in crust in volcano area. Beneath WQD station near to the Tianchi caldera the low velocity layer at 8 km depth is 20 km thick with the lowest S-wave velocity about 2.2 km/s At EDO station located 50 km north of Tianchi caldera, no obvious crustal low velocity layer is detected. In the volcanic region, the thickness of crustal low velocity layer is greater and the lowest velocity is more obvious with the distance shorter to the caldera. It indicates the existence of the high temperature material or magma reservoir in crust near the Tianchi caldera. The receiver functions and inversion result from different back azimuths at CBS permanent seismic station show that the thickness of near surface low velocity layer and Moho depth change with directions. The near surface low velocity layer is obviously thicker in south direction. The Moho depth shows slight uplifting in the direction of the caldera located. We con- sider that the special near surface velocity structure is the main cause of relatively lower prominent frequency of volcanic earthquake waveforms recorded by CBS station. The slight uplifting of Moho beneath Tianchi caldera indicates there is a material exchanging channel between upper mantle and magma reservoir in crust.

Composition and Seismic Properties of the Oceanic Lithosphere: A Synthesis of Ophiolites and Core Samples of the IODP

Our knowledge of the oceanic lithosphere largely comes from analogy with ophiolite complexes and the direct scientific drilling of the present-day oceanic crust(e.g.,Christensen and Salisbury,1975,1989;Smith and Vine,1989;Dilek and Furnes,2011,2014).In this study,we summarized previous experimental results on seismic properties of oceanic lower crust and upper mantle according to different tectonic settings.The results are used to highlight the compositional heterogeneity and the nature of the oceanic Moho.Observation in different ophiolites reveal an ideal oceanic lithosphere profile with ideal petrologic units and seismic units(Dilek and Furnes,2011,2014).The lithospheric mantle beneath ocean basins is composed of tectonized peridotites,which include layered lherzolites and harzburgites and lenses of dunites with chromitites and nearly correspond to the seismic Layer 4.The overlying layered gabbros and mafic sheeted dike complex equal to the seismic Layer 3,as a result of crystallization from a magma chamber.The transitional unit between the former two petrologic units consists of layered ultramafic and mafic rocks,corresponds to the petrological Moho.The seismic Layer 2 and 1 are well defined by pillow lavas and massive flows,and the overlying abyssal sediments,respectively.Compared these results with the refraction seismic profiles,the oceanic crust and upper mantle show different composition and structure.The Pwave velocities of the Layer 3 gabbros varies from 6.7 to 7.0 km s-1 and have low velocity gradients of<0.1 km s-1.Although the gradual increase of P-and S-wave velocities with depth can be attributed to the increasing proportion of mafic minerals from the top to the bottom,prehnite-pumpellyite facies alteration of basalts,greenschist-faces metamorphism to epidote-amphibolite facies metamorphism of gabbros will decrease the velocities of the Layer 2 and Layer 3(Christensen and Salisbury,1975,1989),because the P-wave velocities of chlorite and hornblende are 6.00 and 7.00 km s-1,respectively,lower than those of plagioclase and pyroxene,respectively(Carlson,2004).In addition,local velocity anomalies near the petrologic Moho can be related serpentinization of ultramafic rocks(Salisbury and Christensen,1978;Carlson et al.,2009).In the Layer 4,the characteristic P-wave velocities of the upper mantle should fall in the range of 7.8 to 8.2 km s-1.Poisson’s ratios of chrysotile and lizardite,which are stable in oceanic crustal environments according to the phase diagram,is 0.267 and 0.359,respectively,higher than those of olivine and pyroxene(Wang et al.,2013).Serpentinization will significantly decreased velocities and densities of peridotites and is the main reason for the variation of the Moho reflectivity beneath oceans.

Principle and Technique of Calculating Seismic Velocities from Lattice Preferred Orientation (LPO) Data of Minerals

Deducing seismic velocity anisotropy from LPO (lattice preferred orientation) data of deformed rock samples, a significant link between geology and seismology in geoscience researches, has extensive and promising application. In this paper, the principle and programming technique are highlighted to illustrate the comprehensive automatization procedure. In computer algorithm, two aspects of the programs commonly used in the world are improved: (1) the addition of a pre processing subroutine makes the straightforward calculation realizable from universal stage readings to geographic azimuths and thus to solution of all crystal axes, even for non orthorhombic a and c axes in monoclinic systems. (2) the simplification and perfection of programming can be implemented in the light of Visual Basic (VB) resource. The algorithm has been verified with results from practical data of omphacite samples.

A drilling technology guided by well-seismic information integration

The predictions by drilling-related mechanical and geological models are in some degree inaccurate due to non-unique solution of seismic velocity model.To address this problem,a new drilling technology guided by well-seismic information integration is proposed which consists of seismic velocity update of drilled formations,seismic velocity prediction of the formation ahead of drilling bit,and the prediction of geological feature and drilling geological environmental factors ahead of bit.In this technology,real information(velocity,formation and depth)behind the drilling bit and local pre-stack seismic data around the wellbore being drilled are used to correct the primitive seismic velocity field for a re-migration of seismic data and to update geological features and drilling geological environmental factors ahead of the drilling bit.Field application shows that this technology can describe and predict the geological features,drilling geological environmental factors and complex drilling problems ahead of the bit timely and improve the prediction efficiency and accuracy greatly.These new updated results are able to provide scientific basis for optimizing drilling decisions.

Effects of porosity on seismic velocities, elastic moduli and Poisson's ratios of solid materials and rocks

The generalized mixture rule(GMR) is used to provide a unified framework for describing Young’s(E),shear(G) and bulk(K) moduli, Lame parameter(l), and P- and S-wave velocities(Vpand Vs) as a function of porosity in various isotropic materials such as metals, ceramics and rocks. The characteristic J values of the GMR for E, G, K and l of each material are systematically different and display consistent correlations with the Poisson’s ratio of the nonporous material(v0). For the materials dominated by corner-shaped pores, the fixed point at which the effective Poisson’s ratio(n) remains constant is at v0=0.2, and J(G) > J(E) > J(K) > J(l) and J(G) < J(E) < J(K) < J(l) for materials with v0> 0.2 and v0< 0.2, respectively.J(Vs) > J(Vp) and J(Vs) < J(Vp) for the materials with v0> 0.2 and v0< 0.2, respectively. The effective n increases, decreases and remains unchanged with increasing porosity for the materials with v0< 0.2,v0> 0.2 and v0=0.2, respectively. For natural rocks containing thin-disk-shaped pores parallel to mineral cleavages, grain boundaries and foliation, however, the n fixed point decreases nonlinearly with decreasing pore aspect ratio(a: width/length). With increasing depth or pressure, cracks with smaller a values are progressively closed, making the n fixed point rise and finally reach to the point at v0=0.2.

Integrative method in lithofacies characteristics and 3D velocity volume of the Permian igneous rocks in H area, Tarim Basin

This paper introduces horizon control, seismic control, logging control and facies control methods through the application of the least squares fitting of logging curves, seismic inversion and facies-controlled techniques. Based on the microgeology and thin section analyses, the lithology, lithofacies and periods of the Permian igneous rocks are described in detail. The seismic inversion and facies-controlled techniques were used to find the distribution characteristics of the igneous rocks and the 3D velocity volume. The least squares fitting of the logging curves overcome the problem that the work area is short of density logging data. Through analysis of thin sections, the lithofacies can be classified into eruption airfall subfacies, eruption pyroclastic flow subfacies and eruption facies.

Low velocity zone of upper mantle and its effect on PdSwr phase related to 670 kmdiscontinuity

This paper studied some properties of PdSwr phase related to 670 km discontinuities in detail, and theoretically processed a preliminary analysis to this phase. We discussed the relationships between the incident angle ih of PdSwr phase with its path, epicentral distance, travel-time and relative amplitude due to low velocity zone (LVZ) of upper mantle, and preliminarily pointed out the main characters of PdSwr phase recorded in seismogram. The PdSwr phase is concentrated in range of 13.5~96.5. When epicentral distance is greater than 33, the start point of PdSwr phase is relatively well distinguishable and could thus be determined more easily. When the epicentral distance is between 13.5 to 33, the triplication of PdSwr's travel-time curve could be slightly distinguished due to the low velocity zone and 220 km seismic velocity discontirluity of upper mantle. The relevant observed PdSwr phase should be in a more complex pattern and it should be more difficult to determine its start point

Rheology of continental lithosphere and seismic anisotropy

Rheology of rocks controls the deformation of the Earth at various space-time scales,which is crucial to understand the tectonic evolution of continental lithosphere.Researches of rock rheology are mainly conducted via high-pressure and hightemperature rheological experiments and multi-scale observations and measurements of naturally deformed rocks.At present,a large amount of data from such kinds of studies have been accumulated.This paper first provides an up-to-date comprehensive review of the rheological mechanisms,fabric types and seismic properties of the main rock-forming minerals at different depths of continental lithosphere,including olivine,orthopyroxene,clinopyroxene,amphibole,plagioclase,quartz and mica.Then,progress in high-pressure and high-temperature experiments and natural deformation observations is introduced,mainly regarding the rheological strength and behavior,seismic velocity and anisotropy of lithospheric mantle peridotite,eclogite,mafic granulite,amphibolite and felsic rocks.Finally,by taking the Tibetan Plateau as an example,the application of rock rheology for quantitative interpretation of seismic anisotropy data is discussed.The combination of mineral deformation fabrics and seismic anisotropy is expected to make an important breakthrough in understanding the rheological properties and structure of continental lithosphere.

Shallow Structure of the Crust in the Sulu-Dabie Region,China and its Seismotectonic Implication

The P-wave velocity structure in the shallow crust is investigated in and around the Sulu-Dabie region by using seismic reflection data for deep soundings in 48 survey profiles and from rock velocity determinations.The observed velocity distributions show obvious heterogeneities in this region.The low velocity anomalies are observed mainly in the west of the Dabie region and the East Sea regions.The high velocity anomalies emerge in the shallow crust of the Sulu and Dabie orogeny.These high-velocity anomalies can be attributed to the ultra-high pressure metamorphosed(UHPM)rock formed by exhumation motion of mantle materials during the orogeny.The high-velocity anomalies in the different shallow layers beneath the Sulu region are located to the northeast of the Tan-Lu fault.The high-velocity anomalies beneath the Dabie region are located southwest of the Tan-Lu fault.Such a distribution pattern of velocity anomaly zones may reveal historical motion of a left-lateral strike-slip for the Tan-Lu fault,which differs from the result of a right-lateral strike-slip motion regime known from modern seismology,indicating a more complex tectonic motion along the Tan-Lu fault.

Morphology and possible origins of the Perm anomaly in the lowermost mantle of Earth

We have constrained a small-scale,dome-shaped low-velocity structure near the core-mantle boundary(CMB)of Earth beneath Perm(the Perm anomaly)using travel-time analysis and three-dimensional(3-D)forward waveform modeling of seismic data sampling of the mantle.The best-fitting dome-shaped model centers at 60.0°E,50.5°N,and has a height of 400 km and a radius that increases from 200 km at the top to 450 km at the CMB.Its velocity reduction varies from 0%at the top to–3.0%at 240km above the CMB to–3.5%at the CMB.A surrounding 240-km-thick high-velocity D''structure has also been detected.The Perm anomaly may represent a stable smallscale chemical pile in the lowermost mantle,although the hypothesis of a developing mantle plume cannot be ruled out.

Automatic estimation of traveltime parameters in VTI mediausing similarity-weighted clustering

Compared with hyperbolic velocity estimation methods,nonhyperbolic methods(such as shifted hyperbola)are better choices for large offsets or vertical transverse isotropy(VTI)media.Since local seismic event slope contains subsurface information,they can be used to estimate zero-offset two-way traveltime and normal moveout velocity.The traditional velocity estimation methods require a great deal of manual work and are also prone to human error.In order to estimate the traveltime parameters for VTI media automatically,in this paper,we propose to use predictive painting and similarity-weighted clustering to obtain traveltime parameters.The predictive painting is used to estimate zero-offset two-way traveltime,and the shifted-hyperbola traveltime equation is used to obtain velocity and anisotropy attributes.We first map local slopes to zero-offset two-way traveltime and moveout-parameters domain and then use similarity-weightedk-means clustering to find the maximum likelihood anisotropy parameters of the main subsurface structures.In order to demonstrate that,we apply the similarity-weighted clustering method to synthetic and field data examples and the results are of higher accuracy when compared to the ones obtained using multiparameter semblance-based method.From estimation error section,it can be seen that the estimation error of multiparameter semblance-based method is about 3-5 times that of the proposed method.

Lithospheric structure in NW of Africa:Case of the Moroccan Atlas Mountains

This study presents the outcomes of the local earthquake tomography applied in the Moroccan Atlas domains. A seismic data collected by 36 seismic and a linearized inversion technics are used for determination of local velocity structure.The interpretation of tomography images results emphasizes a new and detailed lithosphere structure： a remaining subducted zone beneath the Souss Basin located from 20-to 45-km depth dipping to the North is detected and interpreted as a body that marks the border between the Moroccan Anti-Atlas and the Meseta-Atlas domains.A subduction zones is detected in the SW of the High Atlas, beneath the Hercynian Tichka massif from 10 to 50-km inclined away from Anti Atlas and in the eastern part of Anti Atlas, dipping northward from Jbel Ougnat at 15e40 km.The junction of the western and middle High Atlas is depicted by two high velocity blocks subducting from 10 to 50 km depth. The first is dipping SW beneath the High Atlas and the second is dipping SE beneath the Ouarzazate Basin.In the northern part of the southwestern High Atlas, a high velocity body dipping towards the north beneath the Essaouira Basin from 15 to 45 km depth.In northeastern part of the High Atlas in the Mougeur zone, a high velocity body is detected from 10 to 45 km depth, dipping to the Se E beneath the eastern High Atlas.The negative lithospheric anomalies found in the upper and in the lower crust are interpreted as a hot asthenospheric material upwelling from deep and gradually replacing the part of crust detached in the High Atlas. The occurrence magmatic activities in these regions testify the existence of a remaining subduction process. This paper argues the implication of these deep structures in the evolution of the Moroccan Atlas Mountain.

Upper crustal structure under Jingtai–Hezuo profile in Northeastern Tibet from topography-dependent eikonal traveltime tomography

The Northeastern Tibetan plateau records Caledonian Qilian orogeny and Cenozoic reactivation by continental collision between the Indian and Asian plates. In order to provide the constraint on the Qilian orogenic mechanism and the expansion of the plateau,wide-angle seismic data was acquired along a 430 km-long profile between Jingtai and Hezuo. There is strong height variation along the profile,which is dealt by topography flattening scheme in our crustal velocity structure reconstruction. We herein present the upper crustal P-wave velocity structure model resulting from the interpretation of first arrival dataset from topography-dependent eikonal traveltime tomography. With topography flattening scheme to process real topography along the profile,the evenness of ray coverage times of the image area（upper crust）is improved,which provides upper crustal velocity model comparable to the classic traveltime tomography（with model expansion scheme to process irregular surface）. The upper crustal velocity model shows zoning character which matcheswith the tectonic division of the Qaidam-Kunlun-West Qinling belt,the Central and Northern Qilian,and the Alax blocks along the profile. The resultant upper crustal P-wave velocity model is expected to provide important base for linkage between the mapped surface geology and deep structure or geodynamics in Northeastern Tibet.