The focal mechanisms of 32 earthquakes with magnitude of M_S≥3.0, which were recorded by the Guangdong Digital Seismographic Network, are calculated by using P-wave polarity and amplitude ratio data, and those of 83 ...The focal mechanisms of 32 earthquakes with magnitude of M_S≥3.0, which were recorded by the Guangdong Digital Seismographic Network, are calculated by using P-wave polarity and amplitude ratio data, and those of 83 earthquakes of Guangdong and its adjacent areas derived by previous researchers are sorted out. On the basis of this, the characteristics of focal mechanism in Guangdong and its adjacent areas are summarized. Meanwhile the focal mechanisms of three earthquake areas, i.e., Yangjiang, Xinfengjiang and Nanao, are discussed respectively. Results show clear regional feature in the function and nature of the stress, and earthquakes are mostly of strike-slip associated with dip-slip component. Earthquakes in the Xinfengjiang Reservoir area are all reservoir-induced ones, so they are only of strike-slip and normal faulting types. In the Yangjiang area, some thrust earthquakes occurred. The dominant orientation of principal compressive stress axis changes from NS in Hainan, Gaungxi to NWW in Guangdong and finally near EW in the coastal areas of Guangdong and Fujian Provinces.展开更多
The focal mechanisms of 62 moderate-small earthquakes since 1980 in the ＂ Huoshan seismic window＂ region are calculated with the method developed recently by Snoke, combining the use of the first motion of P, SV and...The focal mechanisms of 62 moderate-small earthquakes since 1980 in the ＂ Huoshan seismic window＂ region are calculated with the method developed recently by Snoke, combining the use of the first motion of P, SV and SH waves with their amplitude ratios. Based on these abundant focal mechanisms, the mean tectonic stress field in the ＂Huoshan seismic window＂ region is inverted with the average stress tensor method, and the result shows that the ＂Huoshan seismic window＂ region is horizontally compressed in the near EW direction and horizontally dilated in the near NS direction, which is in accord with statistical results of focal mechanism parameters. We estimate the difference （also referred to as consistency parameter 0） between the force axis direction of the focal mechanism solution and the mean stress tensor, then further analyze the variation characteristics of 0 versus time, and the relationship with moderately strong earthquakes in the east China region. The result indicates that 0 in the ＂ Huoshan seismic window＂ region is in good correspondence with moderately strong earthquakes in the East China region. When 0 is lower than the mean value, corresponding moderately strong earthquakes may occur in the East China region.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
文摘The focal mechanisms of 32 earthquakes with magnitude of M_S≥3.0, which were recorded by the Guangdong Digital Seismographic Network, are calculated by using P-wave polarity and amplitude ratio data, and those of 83 earthquakes of Guangdong and its adjacent areas derived by previous researchers are sorted out. On the basis of this, the characteristics of focal mechanism in Guangdong and its adjacent areas are summarized. Meanwhile the focal mechanisms of three earthquake areas, i.e., Yangjiang, Xinfengjiang and Nanao, are discussed respectively. Results show clear regional feature in the function and nature of the stress, and earthquakes are mostly of strike-slip associated with dip-slip component. Earthquakes in the Xinfengjiang Reservoir area are all reservoir-induced ones, so they are only of strike-slip and normal faulting types. In the Yangjiang area, some thrust earthquakes occurred. The dominant orientation of principal compressive stress axis changes from NS in Hainan, Gaungxi to NWW in Guangdong and finally near EW in the coastal areas of Guangdong and Fujian Provinces.
基金funded by the Youth Program of Earthquake Scientific Research Fund of Anhui Province(20120704)Contract Subject of Earthquake Administration of Anhui Province(201210)
文摘The focal mechanisms of 62 moderate-small earthquakes since 1980 in the ＂ Huoshan seismic window＂ region are calculated with the method developed recently by Snoke, combining the use of the first motion of P, SV and SH waves with their amplitude ratios. Based on these abundant focal mechanisms, the mean tectonic stress field in the ＂Huoshan seismic window＂ region is inverted with the average stress tensor method, and the result shows that the ＂Huoshan seismic window＂ region is horizontally compressed in the near EW direction and horizontally dilated in the near NS direction, which is in accord with statistical results of focal mechanism parameters. We estimate the difference （also referred to as consistency parameter 0） between the force axis direction of the focal mechanism solution and the mean stress tensor, then further analyze the variation characteristics of 0 versus time, and the relationship with moderately strong earthquakes in the east China region. The result indicates that 0 in the ＂ Huoshan seismic window＂ region is in good correspondence with moderately strong earthquakes in the East China region. When 0 is lower than the mean value, corresponding moderately strong earthquakes may occur in the East China region.
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.
