Three-dimensional P-wave Velocity Structure Research Articles

Crustal P-wave velocity structure and earthquake distribution in the Jiaodong Peninsula, China

The Jiaodong Peninsula of eastern China is densely populated and prone to frequent moderate-strong earthquakes that are mostly distributed along a group of northeastern oriented Mesozoic-Cenozoic faults. An improved understanding of the seismic velocity structure in the area is of great significance for the roles that the faults played in the formation and evolution of the various tectonic features and for the assessment of future earthquake risks. By utilizing earthquake records of the Shandong Provincial Seismic Network for the period from January 2013 to January 2020, this study simultaneously re-locates earthquakes and determines the three-dimensional P-wave velocity structure beneath the Jiaodong Peninsula using regional P-wave travel times by applying the double-difference tomography method. After the relocation, the travel time residuals are reduced by an order of magnitude, and the epicenters are more concentrated and demonstrate a closer spatial relationship with known active faults. The resulting P-wave velocity structure at different horizon depths indicates lateral heterogeneities in the study area. Clear differences in the characteristics of velocity anomalies are observed between the Jiaobei Uplift, Jiaolai Basin and Sulu ultra-high pressure metamorphic belt. The anomalies are mostly NE oriented, which is consistent with the strike of the regional faults and may suggest structural control of the faults to the geological configurations. Earthquakes generally occurred along the edges of high or low velocity regions, and areas accommodating the intersections of the Penglai-Weihai, Penglai-Qixia and Mouping-Jimo fault zones have the greatest potential for future damaging earthquakes in the area.

Upper crustal velocity and seismogenic environment of the <em>M</em>7.0 Jiuzhaigou earthquake region in Sichuan, China

On August 8, 2017, a magnitude 7.0 earthquake occurred in Jiuzhaigou County, Sichuan Province, China. The deep seismogenic environment and potential seismic risk in the eastern margin of Tibetan Plateau have once again attracted the close attention of seismologists and scholars at home and abroad. The post-earthquake scientific investigation could not identify noticeable surface rupture zones in the affected area; the complex tectonic background and the reason(s) for the frequent seismicity in the Jiuzhaigou earthquake region are unclear. In order to reveal the characteristics of the deep medium and the seismogenic environment of the <i>M</i>7.0 Jiuzhaigou earthquake region, and to interpret the tectonic background and genesis of the seismicity comprehensively, in this paper, we have reviewed all available observation data recorded by the regional digital seismic networks and large-scale, dense mobile seismic array (China Array) for the northern section of the North–South Seismic Belt around Jiuzhaigou earthquake region. Using double-difference seismic tomography method to invert the three-dimensional P-wave velocity structure characteristics of the upper crust around the Jiuzhaigou earthquake region, we have analyzed and discussed such scientific questions as the relationship between the velocity structure characteristics and seismicity in the Jiuzhaigou earthquake region, its deep tectonic environment, and the ongoing seismic risk in this region. We report that: the P-wave velocity structure of the upper crust around the Jiuzhaigoug earthquake region exhibits obvious lateral inhomogeneity; the distribution characteristics of the shallow P-wave velocity structure are closely related to surface geological structure and formation lithology; the <i>M</i>7.0 Jiuzhaigou earthquake sequence is closely related to the velocity structure of the upper crust; the mainshock of the <i>M</i>7.0 earthquake occurred in the upper crust; the inhomogeneous variation of the velocity structure of the Jiuzhaigou earthquake area and its surrounding medium appears to be the deep structural factor controlling the spatial distribution of the mainshock and its sequence. The 3D P-wave velocity structure also suggests that the crustal low-velocity layer of northeastern SGB (Songpan–Garzê Block) stretches into MSM (Minshan Mountain), and migrates to the northeast, but the tendency to emerge as a shallow layer is impeded by the high-velocity zone of Nanping Nappe tectonics and the Bikou Block. Our results reveal an uneven distribution of high- and low-velocity structures around the Tazang segment of the East Kunlun fault zone. Given that the rupture caused by the Jiuzhaigou earthquake has enhanced the stress fields at both ends of the seismogenic fault, it is very important to stay vigilant to possible seismic hazards in the large seismic gap at the Maqu–Maqên segment of the East Kunlun fault zone.

Three-Dimensional P-wave Velocity Structure of the Zhuxi Ore Deposit, South China Revealed by Control-Source First-Arrival Tomography

The Zhuxi ore deposit, located in Jiangxi province, South China, is the largest tungsten reserve in the world. To better understand the geological structure and distribution of orebodies, we conducted a high resolution three-dimensional P-wave velocity tomography of the uppermost 0.5 km beneath the Zhuxi ore deposit and adjacent area. Our velocity model was derived from 761,653 P-wave first arrivals from 998 control-source shots, recorded by a dense array. As the first 3D P-wave velocity structure of the Zhuxi ore deposit, our model agrees with local topographic and tectonic structures and shows depth-dependent velocity similar to laboratory measurements. The Carboniferous formations hosting the proven orebodies are imaged as high velocities. The high-velocity anomalies extend to a larger area beyond the proven orebodies, and the locations of high–low velocity boundaries are in accordance with the boundaries between the Neoproterozoic formation and the Carboniferous–Triassic formation. Seismic tomography reveals that high-velocity anomalies are closely related to the mineralized areas. Our results are helpful for further evaluating the total reserves and suggest that seismic tomography can be a useful tool for mineral exploration.

Three-Dimensional P-Wave Velocity Structure of Tehran from Local Micro-Earthquake Tomography

Tehran region is located at the southern foothills of central Alborz Mountains which is one of the most seismically active regions due to the convergence between the Arabian and the Eurasian plates. In this study, 3-D crustal velocity structure of Tehran region was investigated using first P-wave arrival times of events with magnitudes less than 4. Data used was recorded by the stations from Tehran Disaster Management and Mitigation Organization, Iranian Seismological Center and International Institute of Earthquake Engineering and Seismology from 2004 to 2018. In order to obtain the crustal velocity structure of Tehran, iterative, damped least-squares method was used to solve/obtain earthquake parameters and velocity structures, simultaneously. Final results show three tectonic characteristics in the study area including high velocity anomalies in northern and eastern Tehran Mountains, and low velocity anomaly in triangular Tehran basin, so that the separation of the eastern Tehran Mountains and Tehran basin is especially sharp in cross-section profiles. Moreover, some low velocity anomalies are appeared around major faults in the study area (e.g. North Tehran Fault). The tomographic maps also show that the Tehran region contains of an upper crust layer with the thickness of 6 km which is located over middle crust layer bedrock with constant velocity up to 15 km. This upper crust layer consists of a sedimentary layer (~ 3 km thickness) overlaid a basement layer. Moreover, the thick and thin upper crust layers are separated by North Tehran and Kosar/Ghasr-Firooz faultsystems from the Tehran basin, respectively.

D6 日光周辺域の 3 次元 P 波速度構造 : 1993 日光周辺域合同地震観測・広域テレメター観測で得られたやや遠地地震を用いて

D6 日光周辺域の 3 次元 P 波速度構造 : 1993 日光周辺域合同地震観測・広域テレメター観測で得られたやや遠地地震を用いて

B04 桜島火山の3次元P波速度構造と震源再決定(火山の地震,口頭発表)

B04 桜島火山の3次元P波速度構造と震源再決定(火山の地震,口頭発表)

A07 遠地地震を用いた霧島火山群の三次元 P 波速度構造

A07 遠地地震を用いた霧島火山群の三次元 P 波速度構造

Crustal structure of the Betic–Rif system, western Mediterranean, from local earthquake tomography

We have determined the three-dimensional P-wave velocity structure of the crust and uppermost mantle beneath the Iberia–Africa collision zone using local earthquake tomography. We have inverted arrival times of first-arriving P phases listed in the bulletins of the Instituto Geográfico Nacional and phases picked by us on permanent stations of other regional networks and on temporary broadband stations deployed in the frame of the TOPO-IBERIA project. In total we have used 38,624 P-wave arrival times from 2362 local events recorded at 120 seismic stations. The most remarkable result is the imaging of a large high velocity body following approximately the western Alboran coastline, with a horizontal dimension of at least 200km and extending in depth from the surface down to 24km. This body, not imaged previously with this extent using seismic tomography, coincides with surface exposures of subcontinental mantle (peridotites) in Iberia and Africa and with a belt of positive gravity anomalies. We have also found a marked contrast in the seismic velocities of the middle and lower crust of the Alboran basin, coinciding with the location of the Trans-Alboran Shear Zone. We attribute this contrast to widespread magmatic intrusions in the eastern part of the basin, resulting in higher P-wave velocities than in the west. This contrast would also explain the different orientation of the Trans-Alboran Shear zone with respect to the surface features and faults in the Alboran basin. We also image thick crust beneath the Betics and Rif, accompanied by downgoing lithosphere of the Iberian foreland and Gulf of Cadiz beneath the Betic–Rif–Alboran system.

Joint Inversion of Body-Wave Arrival Times and Surface-Wave Dispersion for Three-Dimensional Seismic Structure Around SAFOD

We incorporate body-wave arrival time and surface-wave dispersion data into a joint inversion for three-dimensional P-wave and S-wave velocity structure of the crust surrounding the site of the San Andreas Fault Observatory at Depth. The contributions of the two data types to the inversion are controlled by the relative weighting of the respective equations. We find that the trade-off between fitting the two data types, controlled by the weighting, defines a clear optimal solution. Varying the weighting away from the optimal point leads to sharp increases in misfit for one data type with only modest reduction in misfit for the other data type. All the acceptable solutions yield structures with similar primary features, but the smaller-scale features change substantially. When there is a lower relative weight on the surface-wave data, it appears that the solution over-fits the body-wave data, leading to a relatively rough V s model, whereas for the optimal weighting, we obtain a relatively smooth model that is able to fit both the body-wave and surface-wave observations adequately.

Three‐Dimensional P‐Wave Velocity Structure Around the Xiaojiang Fault Zone and Its Tectonic Implications

AbstractAs the southeastern margin of the Tibetan Plateau, the Xiaojiang fault zone plays an important role in the lateral escape of Tibetan Plateau materials. In this work, the 3‐D P‐wave velocity structure of the Xiaojiang fault zone and its surrounding areas was imaged by using travel time data from permanent seismic stations and a dense temporal seismic array. The results show that in the upper and middle crust, the Xiaojiang fault zone is mainly of low velocity, while high velocity anomalies exist to its east. In the middle and lower crust, a low velocity anomaly is shown in the central portion of the Xiaojiang fault zone, and high velocity anomalies exist both in the northern and southern areas. The northern high velocity anomaly near Panzhihua extends from lower crust to the surface, and the southern one in the lower crust extends to the upper mantle. We consider that the low velocity anomaly in the central portion of the Xiaojiang fault zone is caused by high temperature derived from upper mantle. The high anomalies in the northern area are most likely related to the late Paleozoic mantle plume activity which caused a large number of basic and ultrabasic mantle materials intruding into the crust. These high‐velocity anomalies, with greater mechanical strength, play a certain impediment to southward escaping of Tibetan Plateau materials, and lead to the rapid uplift of the northern sub‐block of the Sichuan‐Yunnan active block. The north‐dipping high velocity anomaly in the southern area is a further barrier for southward extrusion of the Sichuan‐Yunnan active block, leading to strong deformation and intense seismicity within the low‐velocity area in the overlaying upper and middle crust. Meantime, this high‐velocity block contributes to stabilization of the upper mantle, which makes mantle materials of its southern region less affected by the southward escape of Tibetan Plateau materials.

Collisional tectonics between the Eurasian and Philippine Sea plates from tomography evidences in Southeast China

Abstract The upper mantle structure of Southeast China is important for us to understand the deformation and mantle dynamics process associated with the interaction between the Eurasian plate and Philippine Sea (PHS) slab. We determined a detailed three-dimensional P-wave velocity (Vp) structure of the crust and upper mantle down to 400 km depth beneath Southeast China by applying teleseismic tomography to 6869 high-quality P-wave arrival times. The data were collected very carefully from the original seismograms of 635 teleseismic events recorded by 65 broadband stations deployed in Southeast China. Our images show that the high-Vp PHS slab subducts toward the north along the Ryukyu trench at the latitude of about 24°N and extends down to 350 km depth and even more. High-Vp anomalies are imaged in the upper mantle under central and southern Taiwan, which represent the subducted Eurasian plate. Break-off Eurasian plate at a big angle subducting eastward is revealed under central Taiwan at depths from the upper mantle to 400 km. While continuous Eurasian plate under South Taiwan is mainly imaged from the Moho down to 400 km depth, a torn mantle window within the Eurasian continent beneath central and northern Taiwan created by the northward motion of the Philippine Sea plate is the upwelling path of the asthenosphere. The tomographic images also show the low-Vp anomalies spread widely under the coastal areas of Mainland China and Taiwan Strait. The structure of the crust and upper mantle suggests that the mountain building process in the central part of Taiwan is mainly attributed to the subduction–collision tectonics at the boundary between the Eurasian continental lithosphere and the subducting oceanic lithosphere of the PHS slab.

Deep structure of the central uplift belt in the Qiangtang terrane Tibetan Plateaufrom teleseismic P-wave tomography

We have determined a detailed three-dimensional P-wave velocity structure of the crust and upper mantle down to a depth of 200 km beneath the central uplift belt of Qiangtang by applying Fast Marching Teleseismic Tomography (FMTT) to 9532 high-quality P-wave arrival times. The travel time data were determined by careful interpretations of original seismograms of 506 teleseismic events recorded by a network of 54 broadband stations deployed in the Qiangtang terrane on the Tibetan Plateau during September 2008 and November 2010. Our tomographic imaging beneath the central uplift belt of Qiangtang shows a deep boundary rupture structure for which the lithospheric velocity structure is significantly different on the northern and southern sides. Tomographic images along 33癗 and 33.5癗 show that lithospheric structures beneath the sides of the central uplift belt have different scales. The crust at the sides of the central uplift belt exhibits E-W trending high-velocity anomalies. However very low velocities are found within the crust beneath the northern Qiangtang basin overlying and high-velocity anomalies in the lithospheric mantle. Imaging along north-south profiles (88.5癊 and 89.5癊) shows that the central uplift of Qiangtang is a significant tectonic boundary fault zone.

Faulting mechanism of the Campania–Lucania 1980 earthquake, Italy, from high-resolution, 3D velocity structure, aftershock relocation, fault-plane solutions, and post-seismic deformation modeling

This study performs a detailed reconstruction of the rupture mechanism of the 1980 Campania–Lucania (southern Italy) earthquake. This is achieved by relocation of the main event through computation of fault-plane solutions of the aftershocks, P-wave velocity inversion, and analysis of post-seismic ground deformation, which provide an overall picture of the faulting mechanism. All of these data are in favor of a complex rupture mechanism, as already identified by many studies, which consists of three separate events. The present study defines a graben-like rupture, with the first event rupturing a (>20-km-long) segment of a large, high-angle, NE-dipping, SE-NW-striking, normal fault. The two successive ruptures occurred separately, the first along the southern segment, and the second along the northern segment, of a complementary SW-dipping, normal fault. This mechanism is well evidenced by the revised location of the hypocenter of the main event, and the location of the aftershocks and their fault-plane solutions, as well as by the underlying three-dimensional P-wave velocity structure. The model proposed by Amoruso et al. [2005a] that was based on the inversion of co-seismic vertical displacement data is confirmed by the present analyses, as it satisfies all of the available experimental observations, and better constrains the location and fault-plane solutions of the aftershocks, the velocity discontinuities, and the rupture observations at the surface. This conclusion is also supported by analyses of the post-seismic data.

P-wave tomographic images beneath southeastern Tibet: Investigating the mechanism of the 2008 Wenchuan earthquake

We used 71670 P-wave arrival times from 3594 earthquakes recorded by the Sichuan and Yunnan seismic networks to determine the three-dimensional P-wave velocity structure in the crust and uppermost mantle beneath the southeastern Tibetan Plateau. Our results show that prominent low P-wave velocity (low-Vp) anomalies exist in the mid- to lower crust of the Songpan-Ganze and Sichuan-Yunnan blocks. In contrast, a high P-wave velocity (high-Vp) anomaly is resolved in the middle and lower crust beneath the Sichuan Basin. Our tomographic results provide seismic evidence for a dynamic model of lower crustal flow. Ongoing lower crustal flow beneath the central and eastern Tibetan Plateau abuts against the mechanically strong Sichuan Basin resulting in accumulated strain in the Longmen Shan region. When a critical accumulation of strain energy was reached, its sudden release led to the occurrence of 2008 Wenchuan earthquake. Pronounced low-Vp anomalies are observed in the uppermost mantle in the region south of ∼26°N. Combining these results with shear-wave splitting investigations, we suggest that the flow of asthenospheric material has impacted the velocity structure of the uppermost mantle and caused the thinning of the southwestern Yangtze Craton.

Tracing the Indian lithospheric mantle beneath central Tibetan Plateau using teleseismic tomography

We determined a detailed three-dimensional P-wave velocity structure of the crust and upper mantle down to 400 km depth beneath central Tibet by applying teleseismic tomography to 28,146 high-quality P-wave arrival times. The data were collected very carefully from the original seismograms of 1625 teleseismic events recorded by 131 broadband stations of five portable seismic networks deployed in central Tibet. Our results show that the Indian lithospheric mantle has subducted beneath central Tibet and its frontier has passed through the Bangong-Nujiang Suture and extended northward beneath the Qiangtang Terrane at 34° north latitude. A prominent low-velocity anomaly is revealed within the Tethyan Himalayan Sequences close to the Yarlung-Zangbo Suture which is the boundary between the Tethyan Himalayan Sequences and the Lhasa Terrane. The subduction of the Indian lithospheric mantle has caused the east–west extension in central Tibet. The Indian slab beneath the region has not sutured with the Asian lithospheric mantle in the north. The obtained tomographic images can explain many geological and geophysical features such as strong seismic anisotropy in the upper mantle and K-rich and ultra-potassic lavas in northern Tibet.

Three-dimensional P-Wave Velocity Structure and the Explosion Field at the Shallow Part of Suwanosejima Volcano

Three-dimensional P-Wave Velocity Structure and the Explosion Field at the Shallow Part of Suwanosejima Volcano

Seismic structure and origin of active intraplate volcanoes in Northeast Asia

Three-dimensional P-wave velocity structure beneath the Changbai and other intraplate volcanic areas in Northeast Asia is determined by inverting 1378 high-quality P-wave arrival times from 186 teleseismic events recorded by 61 broadband seismic stations. Low-velocity (low-V) anomalies are revealed beneath the Changbai, Longgan, Xianjindao volcanoes. High-velocity (high-V) anomalies are found in the mantle transition zone, where deep-focus earthquakes under Hunchun occur at depths of 500–600 km. The high-V anomaly reflects the deep subduction of the Pacific slab under NE Asia which may have contributed to the formation of the Changbai, Longgang, Xianjindao and Jingpohu intraplate volcanoes. A low-V anomaly is also revealed in the mantle transition zone, which may have a close relationship with the occurrence of deep earthquakes under the Hunchun area. Our results support the Big Mantle Wedge (BMW) model by Zhao et al. [Zhao, D., Lei, J., Tang, Y., 2004. Origin of the Changbai volcano in northeast China: evidence from seismic tomography, Chin. Sci. Bull. 49, 1401–1408; Zhao, D., Maruyama, S., Omori, S., 2007. Mantle dynamics of western Pacific and East Asia: insight from seismic tomography and mineral physics. Gondwana Res. 11, 120–131.] who proposed that the intraplate volcanoes in NE Asia are caused by the back-arc magmatism associated with the deep dehydration process of the subducting slab and convective circulation process in the BMW above the stagnant Pacific slab.

P-wave tomography of the mantle beneath the South Pacific Superswell revealed by joint ocean floor and islands broadband seismic experiments

Three-dimensional P-wave velocity structure of the mantle beneath the South Pacific Superswell is determined through passive broadband seismic experiments on the ocean floor and islands between 2003 and 2005. We collected approximately 1500 relative times of long-period teleseismic P-waves by using a waveform cross-correlation. We analyzed this data set with relative time tomography to depths of 2000 km. The resultant structure shows lateral heterogeneity of approximately ±2%, in which a distinct low velocity region is found beneath the center of the Superswell at a depth of 1600 km. At 1200 km depth, an elongated low velocity region is found beneath the Society to Pitcairn hotspots. At 800 km depth, two linear low velocity regions are located beneath Tuamotu and Austral islands. Isolated low velocity regions are identified beneath the Society, Marquesas, and Macdonald hotspots at 400 km depth. Our new tomographic images reveal that the large low velocity region rooted in the deep lower mantle is split into two sheets at 1200 km depth and these terminate at approximately 800 km depth. This feature appears to be consistent with the characteristics of a thermo-chemical pile or dome.

Seismic imaging of the crust and upper mantle beneath the North China Craton

We determined three-dimensional (3D) P-wave velocity structure down to 600 km depth beneath the North China Craton using 149,054 arrival times from 7940 local and regional earthquakes, and 193,085 data from 12,657 teleseismic events, which were recorded by 585 seismic stations in our study region. Our tomographic images show some new features. Prominent low-velocity (low-V) and high-velocity (high-V) anomalies are imaged beneath the North China Basin, Trans-North China Orogen, and the cratonic Ordos Block. A lithospheric root of over 250 km thick is imaged clearly beneath the Ordos block. A high-V anomaly which may represent the subducting Pacific slab is imaged in the mantle transition zone beneath the eastern edge of the study area. A few other high-V zones are also imaged in the deep portion of the upper mantle, which may reflect the stagnant Pacific slab and detached Archaean continental lithosphere. Two prominent low-V anomalies are imaged beneath the North China Basin, which may reflect asthenospheric upwelling associated with the deep subduction of the Farallon and Pacific plates since late Mesozoic. We consider that the lithospheric thinning of the eastern part of the North China Craton was caused by the long-term replacement, metamorphism, and chemical and thermal erosion of the ancient lithosphere by the hot asthenosphere.

Three-dimensional P-wave velocity structure of the crust beneath Hainan Island and its adjacent regions, China

Using over 3 500 first P arrival times recorded by nine digital seismic stations from Hainan Digital Seismic Network during 1999∼2005, a 3-D P-wave velocity model of the crust under Hainan Island and adjacent regions has been determined. The results show that the pattern of velocity anomalies in the shallower upper crust is somewhat associated with the surface geological tectonics in the region. A relative low-velocity anomaly appears north of the Wangwu-Wenjiao fault zone and a relative high-velocity anomaly appears south of the Wangwu-Wenjiao fault zone, corresponding to the depressed areas in north Hainan Island, where many volcanoes are frequently active and geothermal values are relatively higher, and the uplifted and stable regions in central and south of the Hainan Island. In the middle and lower crust velocities are relatively lower in east Hainan than those in west Hainan, possibly suggesting the existence of the upwelling of hot materials from the mantle in east Hainan. The pattern of velocity anomalies also indicates that NW faults, i.e., the Puqian-Qinglan fault, may be shallower, while the E-W Wangwu-Wenjiao fault may be deeper, which perhaps extends down to Moho depth or deeper.