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Tomographic inversion of OBS converted shear waves:case study of profile EW6 in the Dongsha area

Acta Oceanologica Sinica 2024年第8期43卷 13-25页

作者： Genggeng Wen Kuiyuan Wan Shaohong Xia Xiuwei Ye huilong xu Chaoyan Fan Jinghe Cao Shunshan xu CEA Key Laboratory of Earthquake Monitoring and Disaster Mitigation Technology Guangdong Earthquake AgencyGuangzhou 510070China Guangdong Science and Technology Collaborative Innovation Center for Earthquake Prevention and Disaster Mitigation Guangzhou 510070China MOE Key Laboratory of Disaster Forecast and Control in Engineering-Urban Earthquake Safety Laboratory Guangzhou 510070China CAS Key Laboratory of Ocean and Marginal Sea Geology South China Sea Institute of OceanologyChinese Academy of SciencesGuangzhou 510301China Southern Marine Science and Engineering Guangdong Laboratory(Guangzhou) Guangzhou 511458China 6 Centro de Geociencias Universidad Nacional Autónoma de MéxicoQuerétaroC.P.76230Mexico

Studies of converted S-wave data recorded on the ocean bottom seismometer(OBS)allow for the estimation of crustal S-wave velocity,from which is further derived the Vp/Vs ratio to constrain the crustal lithology and geophysical *** a precise S-wave velocity model is important for deep structural research,and inversion of converted S-waves provides a potential ***,the inversion of the converted S-wave remains a weakness because of the complexity of the seismic ray path and the inconsistent conversion *** this study,we introduced two travel time correction methods for the S-wave velocity inversion and imaged different S-wave velocity structures in accordance with the corresponding corrected S-wave phases using seismic data of profile EW6 in the northeastern South China Sea(SCS).The two inversion models show a similar trend in velocities,and the velocity difference is<0.15 km/s(mostly in the range of 0–0.1 km/s),indicating the accuracy of the two travel time correction methods and the reliability of the inversion *** to simulations of seismic ray tracing based on different models,the velocity of sediments is the primary influencing factor in ray tracing for S-wave *** the sedimentary layer has high velocities,the near offset crustal S-wave refractions cannot be *** contrast,the ray tracing of Moho S-wave reflections was not significantly impacted by the velocity of the *** two travel time correction methods have their own advantages,and the application of different approaches is based on additional *** works provide an important reference for future improvements in converted S-wave research.

关键词： converted S-wave S-wave velocity structure inversion ocean bottom seismometer northeastern South China Sea

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Tomographic inversion of OBS converted shear waves:case study of profile EW6 in the Dongsha area

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Acta Oceanologica Sinica 2024年第08期 13-25页

作者： Genggeng Wen Kuiyuan Wan Shaohong Xia Xiuwei Ye huilong xu Chaoyan Fan Jinghe Cao Shunshan xu CEA Key Laboratory of Earthquake Monitoring and Disaster Mitigation Technology Guangdong Earthquake Agency Guangdong Science and Technology Collaborative Innovation Center for Earthquake Prevention and Disaster Mitigation MOE Key Laboratory of Disaster Forecast and Control in Engineering-Urban Earthquake Safety Laboratory CAS Key Laboratory of Ocean and Marginal Sea Geology South China Sea Institute of Oceanology Chinese Academy of Sciences Southern Marine Science and Engineering Guangdong Laboratory (Guangzhou) Centro de Geociencias Universidad Nacional Autónoma de México

Studies of converted S-wave data recorded on the ocean bottom seismometer(OBS) allow for the estimation of crustal S-wave velocity, from which is further derived the Vp/Vs ratio to constrain the crustal lithology and geophysical properties. Constructing a precise S-wave velocity model is important for deep structural research,and inversion of converted S-waves provides a potential solution. However, the inversion of the converted S-wave remains a weakness because of the complexity of the seismic ray path and the inconsistent conversion *** this study, we introduced two travel time correction methods for the S-wave velocity inversion and imaged different S-wave velocity structures in accordance with the corresponding corrected S-wave phases using seismic data of profile EW6 in the northeastern South China Sea(SCS). The two inversion models show a similar trend in velocities, and the velocity difference is <0.15 km/s(mostly in the range of 0–0.1 km/s), indicating the accuracy of the two travel time correction methods and the reliability of the inversion results. According to simulations of seismic ray tracing based on different models, the velocity of sediments is the primary influencing factor in ray tracing for S-wave phases. If the sedimentary layer has high velocities, the near offset crustal S-wave refractions cannot be traced. In contrast, the ray tracing of Moho S-wave reflections was not significantly impacted by the velocity of the sediments. The two travel time correction methods have their own advantages, and the application of different approaches is based on additional requirements. These works provide an important reference for future improvements in converted S-wave research.

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Estimation of Displacement and Extension due to Reverse Drag of Normal Faults: Forward Method

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International Journal of Geosciences 2024年第1期15卷 25-39页

作者： Shunshan xu Angel Francisco Nieto-Samaniego huilong xu Susana Alicia Alaniz-Álvarez Universidad Nacional Autónoma de México Centro de Geociencias Apartado Postal 1-742 Querétaro Qro. México South China Sea Institute of Oceanology Chinese Academy of Sciences Guangzhou China

In the case of reverse drag of normal faulting, the displacement and horizontal extension are determined based on the established equations for the three mechanisms: rigid body, vertical shear and inclined shear. There are three sub-cases of basal detachment for the rigid body model: horizontal detachment, antithetic detachment and synthetic detachment. For the rigid body model, the established equations indicate that the total displacement on the synthetic base (Dt2) is the largest, that on the horizontal base (Dt1) is moderate, and that on the antithetic base (Dt3) is the smallest. On the other hand, the value of (Dt1) is larger than the displacement for the vertical shear (Dt4). The value of (Dt1) is larger than or less than the displacement for the inclined shear (Dt5) depending on the original fault dip δ0, bedding angle θ, and the angle of shear direction β. For all original parameters, the value of Dt5 is less than the value of Dt4. Also, by comparing three rotation mechanisms, we find that the inclined shear produces largest extension, the rigid body model with horizontal detachment produces the smallest extension, and the vertical shear model produces moderate extension.

关键词： Fault Rotation Fault Drag Fault Displacement Extension Forward Model

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Estimation of Displacement and Extension due to Reverse Drag of Normal Faults:Forward Method

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International Journal of Geosciences（地球科学国际期刊） 2024年第1期

作者： Shunshan xu Angel Francisco Nieto-Samaniego huilong xu Susana Alicia Alaniz-Álvarez

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Characteristics of PmP phases from earthquakes and their role in crustal tomography:An active volcanic area example,northeastern Japan

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Science China Earth Sciences 2011年第5期54卷 640-646页

作者： XIA ShaoHong QIU xueLin xu huilong ZHAO MingHui SHI XiaoBin Chinese Academy of Sciences Key Laboratory of Marginal Sea Geology South China Sea Institute of Oceanology Chinese Academy of Sciences Guangzhou 510301 China

Later earthquake-sourced PmP phases have the potential to significantly improve ray coverage and resolution of crustal tomography methods,as their trajectories are quite different from those of shallower P *** paper analyzes the characteristics of later PmP arrival times from earthquakes with different focal *** results show that PmP arrival time differences from earthquakes at a range of focal depths are gradually lowered with increasing *** found that where the first recorded P-wave phase was the intra-crustal refraction phase(Pg),the differences in arrival time between Pg and PmP phases decreased with increasing focal depth at an offset of less than 120 *** the first P-wave phase is the upper mantle refraction phase(Pn),the difference in arrival times between Pn and PmP phases became larger with an increase in focal depth at an offset of more than 150 km.A total of 394 PmP phases and 3356 first P phases were picked from seismograms in the active volcanic area of northeastern Japan,according to the characteristics of calculated arrival times,amplitudes and particle *** were used to investigate the role of PmP phases in crustal tomography beneath an active volcanic *** of the detailed resolution analysis show that the addition of PmP data can improve significantly the resolution of the lower crustal structure in tomographic *** the PmP data were included in the tomographic inversion,the path of upwelling magma,along which a series of low-frequency microearthquakes is clearly distributed,was better *** results suggest that the PmP phase has an important role in detailed crustal tomography.

关键词： PmP phase crustal structure tomography earthquake active volcanoes

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Offshore Fault Geometrics in the Pearl River Estuary,Southeastern China:Evidence from Seismic Reflection Data

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Journal of Ocean University of China 2018年第4期17卷 799-810页

作者： CAO Jinghe XIA Shaohong SUN Jinlong ZHAO Fang WAN Kuiyuan xu huilong CAS Key Laboratory of Ocean and Marginal Sea Geology South China Sea Institute of OceanologyChinese Academy of SciencesCruangzhou 510301China University of Chinese Academy of Sciences Beijing 100049China

The Pearl River Estuary(PRE) is located at the onshore-offshore transition zone between South China and South China Sea Basin, and it is of great significant value in discussing tectonic relationships between South China block and South China Sea block and seismic activities along the offshore active faults in PRE. However, the researches on geometric characteristics of offshore faults in this area are extremely lacking. To investigate the offshore fault distribution and their geometric features in the PRE in greater detail, we acquired thirteen seismic reflection profiles in 2015. Combining the analysis of the seismic reflection and free-air gravity anomaly data, this paper revealed the location, continuity, and geometry of the littoral fault zone and other offshore faults in PRE. The littoral fault zone is composed of the major Dangan Islands fault and several parallel, high-angle, normal faults, which mainly trend northeast to northeast-to-east and dip to the southeast with large displacements. The fault zone is divided into three different segments by the northwest-trending faults. Moreover, the basement depth around Dangan Islands is very shallow, while it suddenly increases along the islands westward and southward. These has resulted in the islands and neighboring areas becoming the places where the stress accumulates easily. The seismogenic pattern of this area is closely related to the comprehensive effect of intersecting faults together with the low velocity layer.

关键词： offshore active faults littoral fault zone Pearl River Estuary earthquake seismic reflection data

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Diagenesis and Evolution of the Holocene Coquinite from the Haishan Island,Eastern Guangdong,China

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Acta Geologica Sinica(English Edition) 2009年第1期83卷 70-80页

作者： SUN Jinlong xu huilong QIU xuelin ZHAN Wenhuan LI Yamin CAS Key Laboratory of Marginal Sea Geology South China Sea Institute of Oceanology the Chinese Academy of Sciences Guangzhou Guangdong 510301 China Graduate University of the Chinese Academy of Sciences Beijing 100049 China

The coastal hard rock with a thickness of over 5 m and a distribution area of nearly 200 ha in the Haishan Island, south China, has long drawn researchers＇ attention. However, there were controversies over its formation and classification, and these controversies in turn lead to the dispute of sea level changes and coastal uplift-subsidence of this area. To investigate its diagenesis and evolution, petrographic analysis, elemental geochemistry, isotopic analysis, and radiocarbon dating were used in the present study. Radiocarbon dating indicates that the deposition of the Haishan Coquinite commenced in the mid Holocene and lasted to -600 a B.P. Petrographic analysis shows that the Haishan Coquinite is cemented by low-Mg calcite, indicating that the cementation occurred in a meteoric environment. The elemental geochemistry and isotopic values demonstrate that the coquinite suffered strong leaching, which was thought to be responsible for the meteoric cementation of the coquinite. According to these results, the diagenesis of the coquinite is revealed： deposition of the Haishan Coquinite commenced in the mid Holocene in a shoal environment, initial cementation occurred and cement may be high-Mg calcite or aragonite; latterly the coquinite exposed to meteoric environment as a result of lowering of relative sea level, and the cement altered to low-Mg, which took the morphologies of bladed calcite rim and equant spar. A four-stage evolution model is proposed：（1） deposition stage, （2） initial cementation, （3） exposure to and cementation in meteoric environment, and （4） erosion stage. The published reports indicate that the hard rock should be designated as coquinite. Based on these studies, mid-Holocene sea level in this area was discussed, and the Haishan Island was proposed to uplift with a rate of ～5 mm/a in the last -600 a.

关键词： beachrock coquinite diagenesis evolution model mid Holocene sea level

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Characteristics of the crustal structure and hypocentral tectonics in the epicentral area of Nan＇ao earthquake （M7.5）, the northeastern South China Sea

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Chinese Science Bulletin 2006年第B12期51卷 95-106页

作者： xu huilong QIU xuelin ZHAO Minghui SUN Jinlong ZHU Junjiang Key Laboratory of Marginal Sea Geology South China Sea Institute of Oceanology Chinese Academy of Sciences Guangzhou 510301 China Faculty of Earth Resources China University of Ge osciences Wuhan 430074 China

1918 Nan’ao earthquake (M7.5) occurred in the northeast coastal areas of Guangdong Province. With the seismogeological survey of the epicentral area and history materials analyses, the earthquake epicenter was estimated to locate in the intersection part of the Binhai fault zone (Littoral) and Huanggangshui fault, which strikes NEE and NW, respectively. The activities of the NEE-striking thrust fault and NW-striking extensional fault that were attributed to 1918 Nan’ao earthquake occurred in the Dongshan Island of the epicentral area; they reflected the focal stress field with compression in NW-SE direction and extension in NE-SW direction. The isoseismal contour of seismic intensity X shows a shape of ‘X’ composed of two mutually overlapping ellipses with two axes striking NEE and NW, respectively, and such shape implies that the occurrence of this earthquake is controlled by a pair of conjugate seismotectonic faults constituted by the NEE-striking Binhai fault zone and the NW-striking Huanggangshui fault. The Binhai fault zone is a dominant seismogenic structure, and the NW-striking Huanggangshui fault is the subdominant one. The onshoreoffshore deep seismic profile that crossed the epicentral area and was perpendicular to the strike of the Binhai fault zone was obtained. According to the analyses of the seismic data, the Binhai fault zone is defined as a low velocity zone with SE dip-slip in thecrustal structure section. The Binhai fault zone is a boundary fault between the South China subplate and South China Sea subplate. The crust structure on the northwest side of Binhai fault zone is a normal continental crust with a thickness of 30 km, and the one on the southeast side of the fault zone is a thinning continental crust with a thickness of 25―28 km. The Binhai fault zone is an important seismogenic fault and also is an earthquake-controlling fault. The intersection part between the Binhai fault zone and the low velocity zone of upper crust is advantageous to

关键词：南海东北部 1918年南澳地震滨海断裂带地壳结构震源构造震中区

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Spatial Variations of b-Values in the Coastal Area of Guangdong

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Journal of Ocean University of China 2018年第1期17卷 177-185页

作者： ZHOU Pengxiang XIA Shaohong SUN Jinlong CAO Jinghe xu huilong ZHAO Fang CHEN Chuanxu CAS Key Laboratory of Ocean and Marginal Sea Geology South ChinaSea Institute of OceanologyChinese Academy of Sciences College of Earth Sciences University of Chinese Academy of Sciences Institute of Deep-Sea Science and Engineering Chinese Academy of Sciences

We used earthquake catalogs recorded by Guangdong Seismological Network from 2008 to 2014 to resolve the spatial variations of b-values in the coastal area of Guangdong, particularly in three key research areas(Yangjiang, Heyuan, and offshore Nanao Island) with strong seismicity. Our results revealed that b-values exhibited significant spatial variations, and zones with low b-values could indicate the most likely seismogenic area of large earthquakes. We observed three clear low b-value patches in the offshore Nanao Island. We found a distinct high b-value peak at the depth of 11 km and two minimum peaks at about 14 and 7–8 km in the Yangjiang area. The overall b-values generally decrease with depth in the Heyuan area. The spatial variations of b-values reflect tectonic anomalies; that is, the ‘low-high-low' distribution of b-values in the offshore Nanao Island and the Yangjiang area may indicate the anomaly of the crustal structure with a weak layer. The b-values of reservoir-induced seismicity are obviously lower than that induced by tectonism. This finding indicates that the reservoir area is generally at high stress state under the condition of high pore pressure. We inferred that large earthquakes might be prone to occur at 10–12 km depth in the offshore Nanao Island, at 12–15 km depth in the Yangjiang area, and at the lower part of the seismic activity zone in the Heyuan area. Moreover, the upstream area of the Xinfengjiang reservoir is the most likely area of future large earthquakes in the Heyuan area.

关键词： b-values earthquake Guangdong seismogenic area stress

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Seismogenic Structures of the 2006 ML4.0 Dangan Island Earthquake Offshore Hong Kong

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Journal of Ocean University of China 2018年第1期17卷 169-176页

作者： XIA Shaohong CAO Jinghe SUN Jinlong LV Jinshui xu huilong ZHANG Xiang WAN Kuiyuan FAN Chaoyan ZHOU Pengxiang CAS Key Laboratory of Ocean and Marginal Sea Geology South China Sea Institute of OceanologyChinese Academy of SciencesGuangzhou 510301China Seismological Bureau of Guangdong Province Guangzhou 510070China University of Chinese Academy of Sciences Beijing 100049China

The northern margin of the South China Sea, as a typical extensional continental margin, has relatively strong intraplate seismicity. Compared with the active zones of Nanao Island, Yangjiang, and Heyuan, seismicity in the Pearl River Estuary is relatively low. However, a ML4.0 earthquake in 2006 occurred near Dangan Island(DI) offshore Hong Kong, and this site was adjacent to the source of the historical M5.8 earthquake in 1874. To reveal the seismogenic mechanism of intraplate earthquakes in DI, we systematically analyzed the structural characteristics in the source area of the 2006 DI earthquake using integrated 24-channel seismic profiles, onshore–offshore wide-angle seismic tomography, and natural earthquake parameters. We ascertained the locations of NW-and NE-trending faults in the DI sea and found that the NE-trending DI fault mainly dipped southeast at a high angle and cut through the crust with an obvious low-velocity anomaly. The NW-trending fault dipped southwest with a similar high angle. The 2006 DI earthquake was adjacent to the intersection of the NE-and NW-trending faults, which suggested that the intersection of the two faults with different strikes could provide a favorable condition for the generation and triggering of intraplate earthquakes. Crustal velocity model showed that the high-velocity anomaly was imaged in the west of DI, but a distinct entity with low-velocity anomaly in the upper crust and high-velocity anomaly in the lower crust was found in the south of DI. Both the 1874 and 2006 DI earthquakes occurred along the edge of the distinct entity. Two vertical cross-sections nearly perpendicular to the strikes of the intersecting faults revealed good spatial correlations between the 2006 DI earthquake and the low to high speed transition in the distinct entity. This result indicated that the transitional zone might be a weakly structural body that can store strain energy and release it as a brittle failure, resulting in an earthquake-pron

关键词： Dangan Island intraplate seismicity South China Sea crustal velocity model seismogenic structure

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