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 Ch...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.展开更多
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 i...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-prone area.展开更多
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(Yangji...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.展开更多
A cluster of earthquakes occurred in the Taiwan Shoal region on the outer rise of the Manila Trench. Although most were of small to medium magnitudes, one strong earthquake occurred on September 16, 1994. Several prev...A cluster of earthquakes occurred in the Taiwan Shoal region on the outer rise of the Manila Trench. Although most were of small to medium magnitudes, one strong earthquake occurred on September 16, 1994. Several previous studies have provided important information to progress our understanding of this single earthquake. However, little is currently known about the earthquake cluster, and it is necessary to investigate the deep crustal structure of the Taiwan Shoal region to understand the mechanisms involved in controlling and generating it. This study presents a two-dimensional seismic tomographic image of the crustal structure along the OBS2012 profile based on ocean-bottom seismograph(OBS) data, which exhibits a high-velocity anomaly flanked by low-velocity anomalies in the upper crust beneath the Taiwan Shoal. In this study, 765 earthquakes(Richter magnitude ML > 1.5) occurring between 1991 and 2015 were studied and analyses of earthquake epicenters, regional faults, and the crustal structure provides an improved understanding of the nature of active tectonics in this region. Results of analyses indicate firstly that the high-velocity area represents major asperities that correspond to the location of the earthquake cluster and where stress is concentrated. It is also depicted that the earthquake cluster was influenced by fault interactions. However, the September 1994 earthquake occurred independently of these seismic activities and was associated with reactivation of a preexisting fault. It is also determined that slab pull is resisted by the exposed precollision accretionary prism, and the resistive force is causing accumulation of inplane compressive-stress. This may trigger a future damaging earthquake in the Taiwan Shoal region.展开更多
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 format...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.展开更多
Onshore-offshore seismic experiments were carried out for the first time in northern South China Sea using large volume airgun sources at sea and seismic stations on land. The experimental results indicate that seismi...Onshore-offshore seismic experiments were carried out for the first time in northern South China Sea using large volume airgun sources at sea and seismic stations on land. The experimental results indicate that seismic signals from the new airgun array of R/V Shiyan 2 can be detected as far as 255 km. The signal effective area reaches nearly 50000 km2, which covers Hong Kong and Pearl River Delta. Compared with the old airgun array, the signal amplitude, propagation distance and effective area of the new airgun array have been increased notably, which demonstrates that the upgrade of the airgun source was successful. Comparisons with previous experimental results in other regions show that the shooting effect of the new airgun array is similar to those best airgun sources in the world. Especially, it is a new breakthrough in using the permanent seismic stations onshore to record long distance airgun signals offshore, which has great significance to the realization of the "seismic radar" concept and the 3D seismic surveys of crustal structure in coastal areas.展开更多
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 estima...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 stress concentration and strain energy accumulation, and presents the deep dynamic conditions for the earthquake’s pregnancy and occurrence.展开更多
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 phases.This...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 phases.This paper analyzes the characteristics of later PmP arrival times from earthquakes with different focal depths.The results show that PmP arrival time differences from earthquakes at a range of focal depths are gradually lowered with increasing offset.We 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 km.Where 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 motions.These were used to investigate the role of PmP phases in crustal tomography beneath an active volcanic region.Results of the detailed resolution analysis show that the addition of PmP data can improve significantly the resolution of the lower crustal structure in tomographic images.After 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 imaged.These results suggest that the PmP phase has an important role in detailed crustal tomography.展开更多
基金supported by the National Natural Science Foundation of China(Nos.41506046,41376060,41706054)the Opening Foundation of Key Laboratory of Ocean and Marginal Sea Geology,CAS(No.MSGL15-05)+1 种基金WPOS(No.XDA11030102-02)the Strategic Priority Research Program of the Chinese Academy of Sciences(No.XDA13010101)
文摘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.
基金supported by the Strategic Priority Research Program of Chinese Academy of Sciences (No. XDA13010101)the National Natural Science Foundation of China (Nos. 91328 206, 41576041, 41506046)+1 种基金 the Natural Science Foundation of Guangdong Province (No. 2017A030311015) Special Project of Guangdong Province
文摘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-prone area.
基金supported by the Strategic Priority Research Program of Chinese Academy of Sciences (No. XDA3010101)the National Natural Science Foundation of China (Nos. 913 28206, 41576041, 41506046)+1 种基金the Natural Science Foundation of Guangdong Province of China (No. 2017A0303 11015)the Special Project of Guangdong Province of China
文摘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.
基金supported by the Strategic Priority Research Program of Chinese Academy of Sciences (No. XDA13010101)the National Natural Science Foundation of China (Nos. 91328206, 41676042, 41376060, and 41506046)
文摘A cluster of earthquakes occurred in the Taiwan Shoal region on the outer rise of the Manila Trench. Although most were of small to medium magnitudes, one strong earthquake occurred on September 16, 1994. Several previous studies have provided important information to progress our understanding of this single earthquake. However, little is currently known about the earthquake cluster, and it is necessary to investigate the deep crustal structure of the Taiwan Shoal region to understand the mechanisms involved in controlling and generating it. This study presents a two-dimensional seismic tomographic image of the crustal structure along the OBS2012 profile based on ocean-bottom seismograph(OBS) data, which exhibits a high-velocity anomaly flanked by low-velocity anomalies in the upper crust beneath the Taiwan Shoal. In this study, 765 earthquakes(Richter magnitude ML > 1.5) occurring between 1991 and 2015 were studied and analyses of earthquake epicenters, regional faults, and the crustal structure provides an improved understanding of the nature of active tectonics in this region. Results of analyses indicate firstly that the high-velocity area represents major asperities that correspond to the location of the earthquake cluster and where stress is concentrated. It is also depicted that the earthquake cluster was influenced by fault interactions. However, the September 1994 earthquake occurred independently of these seismic activities and was associated with reactivation of a preexisting fault. It is also determined that slab pull is resisted by the exposed precollision accretionary prism, and the resistive force is causing accumulation of inplane compressive-stress. This may trigger a future damaging earthquake in the Taiwan Shoal region.
基金supported by the Chinese Academy of Sciences (Grant no.KZCX3-SW-234)the National Natural Science Foundation of China (Grant no.40476025)the Science and Technology Program of Guangdong Province (Grant no.2004B33201002).
文摘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.
基金Supported by Chinese Academy of Sciences (Grant No. KZCX3-SW-234)National Natural Science Foundation of China (Grant Nos. 40674051, 40574019 and 40276015)+1 种基金Guangdong Natural Science Foundation (Grant No. 021557)South China Sea Institute of Oceanology (Grant No. LYQY200302)
文摘Onshore-offshore seismic experiments were carried out for the first time in northern South China Sea using large volume airgun sources at sea and seismic stations on land. The experimental results indicate that seismic signals from the new airgun array of R/V Shiyan 2 can be detected as far as 255 km. The signal effective area reaches nearly 50000 km2, which covers Hong Kong and Pearl River Delta. Compared with the old airgun array, the signal amplitude, propagation distance and effective area of the new airgun array have been increased notably, which demonstrates that the upgrade of the airgun source was successful. Comparisons with previous experimental results in other regions show that the shooting effect of the new airgun array is similar to those best airgun sources in the world. Especially, it is a new breakthrough in using the permanent seismic stations onshore to record long distance airgun signals offshore, which has great significance to the realization of the "seismic radar" concept and the 3D seismic surveys of crustal structure in coastal areas.
基金Acknowledgements The authors would like to thank Prof. Feng Xuanmin of Earthquake Administration of Guangdong Province for providing materials of the earthquake comprehensive list in Guangdong Province. This work was supported by the Chinese Academy of Sciences （Grant No. KZCX3-SW-234）, the National Natural Science Foundation of China （Grant No. 40476025） and the Science and Technology Program of Guangdong Province （Grant No. 2004B33201002）.
文摘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 stress concentration and strain energy accumulation, and presents the deep dynamic conditions for the earthquake’s pregnancy and occurrence.
基金supported by National Natural Science Foundation of China (Grant Nos. U0933006, 41006028)South China Sea Institute of Oceanology,Chinese Academy of Sciences (Grant No. SQ200911)Chinese Academy of Sciences (Grant No. 1731005933)
文摘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 phases.This paper analyzes the characteristics of later PmP arrival times from earthquakes with different focal depths.The results show that PmP arrival time differences from earthquakes at a range of focal depths are gradually lowered with increasing offset.We 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 km.Where 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 motions.These were used to investigate the role of PmP phases in crustal tomography beneath an active volcanic region.Results of the detailed resolution analysis show that the addition of PmP data can improve significantly the resolution of the lower crustal structure in tomographic images.After 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 imaged.These results suggest that the PmP phase has an important role in detailed crustal tomography.
