Statistical properties of reported earthquake precursors show apparent focal mechanism dependence. Intensity of anomaly is described by the ′anomaly ratio′ as defined by the number of stations/items reporting anoma...Statistical properties of reported earthquake precursors show apparent focal mechanism dependence. Intensity of anomaly is described by the ′anomaly ratio′ as defined by the number of stations/items reporting anomalies before the target earthquake over the number of stations/items in operation around the target earthquake. Variation of the ′anomaly ratio′ with the magnitude of the target earthquake was studied for dip-slip earthquakes all over China, strike-slip earthquakes in eastern China, and strike-slip earthquakes in western China, respectively. It is observed that for strike-slip earthquakes, the ′anomaly ratio′ increases linearly with the magnitude of the target earthquake, while earthquakes in eastern China and western China have different slopes. For dip-slip earthquakes, however, the ′anomaly ratio′ has no statistically significant change with the magnitude of the target earthquake. Limited data imply that the ′anomaly ratio′ seems proportional to the apparent stress of the target earthquake. The result might be heuristic for the analysis of candidate earthquake precursors.展开更多
An earthquake with magnitude 6.6 occurred on November 16,1983 in the Kaoiki,Hawii.The hierarchical clustering analyses were made by stages for the focal mechanism solutions of 74 earthquakes with M≥3.0 in this area f...An earthquake with magnitude 6.6 occurred on November 16,1983 in the Kaoiki,Hawii.The hierarchical clustering analyses were made by stages for the focal mechanism solutions of 74 earthquakes with M≥3.0 in this area from 1977 to 1985,it was found that each type of focal mechanism is roughly with invariable percentage,and that the strike-slip events including the mainshock,which did not appear in other stages,occurred in the stage around M=6.6 mainshock.The dominant types in the other stages were depressed in the period around the mainshock. This is a believable precursor phenomena.orientation of the principal axes of stress field was inversed by fitting slip vector based on the focal mechanisms in the different periods.The results show that the stress field around the main shock with M=6.6 was more stable,its maximum and minimum principal axes is nearly horizontal in EW and NS direction respectively,the middle one is nearly vertical.The middle principal axis of stress field in the other periods appeared to be horizontal and the corresponding seismic rupture horizontally slip with a small dip.The significant difference between the stress field of source region in the period around the strong shock and that in the other periods shows that the seismogeny and occurrence of the strong shock were controlled by the variation of stress field.The characteristic environment factors of Kaoiki region and its effects were discussed.展开更多
We carried out a simultaneous study of ground-based magnetic field and lower ionospheric anomalies during major earthquakes occurring around Japan in 2010 and 2012. Ultra Low Frequency (ULF) geomagnetic field waveform...We carried out a simultaneous study of ground-based magnetic field and lower ionospheric anomalies during major earthquakes occurring around Japan in 2010 and 2012. Ultra Low Frequency (ULF) geomagnetic field waveforms of Esashi station and Very Low Frequency (VLF) Japanese transmitter (JJY) electric signal amplitude received in Moshiri station Hokkaido during nighttime (22:00-02:00 LT) were used to minimize the local interference. Twenty earthquakes having magnitude greater than 5.5 were considered for the data analysis for two years. Nighttime amplitude fluctuations and polarization from the received VLF transmitter signal amplitude and ULF magnetic field respectively were calculated to identify anomalous signatures in relation to every earthquake. We found most earthquakes analyzed indicating VLF amplitude anomalies simultaneously occurred with ULF magnetic field anomalies within a week prior to the earthquakes. Stronger anomalies were observed for larger magnitude and shallower earthquakes. Focal mechanism of earthquakes was also examined to identify the effectiveness of generating anomaly. Both VLF and ULF anomalies were observed for reverse fault type earthquakes occurring under the strong pressure in the crust. Obtained results may indicate the common anomaly source both for VLF and ULF in the lithosphere and are consistent with currently proposed Lithosphere-Atmosphere-Ionosphere (LAI) coupling scenarios during the earthquake preparation period.展开更多
基金National Natural Science Foundation of China (40274013) and MOST Project (2001BA601B02).
文摘Statistical properties of reported earthquake precursors show apparent focal mechanism dependence. Intensity of anomaly is described by the ′anomaly ratio′ as defined by the number of stations/items reporting anomalies before the target earthquake over the number of stations/items in operation around the target earthquake. Variation of the ′anomaly ratio′ with the magnitude of the target earthquake was studied for dip-slip earthquakes all over China, strike-slip earthquakes in eastern China, and strike-slip earthquakes in western China, respectively. It is observed that for strike-slip earthquakes, the ′anomaly ratio′ increases linearly with the magnitude of the target earthquake, while earthquakes in eastern China and western China have different slopes. For dip-slip earthquakes, however, the ′anomaly ratio′ has no statistically significant change with the magnitude of the target earthquake. Limited data imply that the ′anomaly ratio′ seems proportional to the apparent stress of the target earthquake. The result might be heuristic for the analysis of candidate earthquake precursors.
文摘An earthquake with magnitude 6.6 occurred on November 16,1983 in the Kaoiki,Hawii.The hierarchical clustering analyses were made by stages for the focal mechanism solutions of 74 earthquakes with M≥3.0 in this area from 1977 to 1985,it was found that each type of focal mechanism is roughly with invariable percentage,and that the strike-slip events including the mainshock,which did not appear in other stages,occurred in the stage around M=6.6 mainshock.The dominant types in the other stages were depressed in the period around the mainshock. This is a believable precursor phenomena.orientation of the principal axes of stress field was inversed by fitting slip vector based on the focal mechanisms in the different periods.The results show that the stress field around the main shock with M=6.6 was more stable,its maximum and minimum principal axes is nearly horizontal in EW and NS direction respectively,the middle one is nearly vertical.The middle principal axis of stress field in the other periods appeared to be horizontal and the corresponding seismic rupture horizontally slip with a small dip.The significant difference between the stress field of source region in the period around the strong shock and that in the other periods shows that the seismogeny and occurrence of the strong shock were controlled by the variation of stress field.The characteristic environment factors of Kaoiki region and its effects were discussed.
文摘We carried out a simultaneous study of ground-based magnetic field and lower ionospheric anomalies during major earthquakes occurring around Japan in 2010 and 2012. Ultra Low Frequency (ULF) geomagnetic field waveforms of Esashi station and Very Low Frequency (VLF) Japanese transmitter (JJY) electric signal amplitude received in Moshiri station Hokkaido during nighttime (22:00-02:00 LT) were used to minimize the local interference. Twenty earthquakes having magnitude greater than 5.5 were considered for the data analysis for two years. Nighttime amplitude fluctuations and polarization from the received VLF transmitter signal amplitude and ULF magnetic field respectively were calculated to identify anomalous signatures in relation to every earthquake. We found most earthquakes analyzed indicating VLF amplitude anomalies simultaneously occurred with ULF magnetic field anomalies within a week prior to the earthquakes. Stronger anomalies were observed for larger magnitude and shallower earthquakes. Focal mechanism of earthquakes was also examined to identify the effectiveness of generating anomaly. Both VLF and ULF anomalies were observed for reverse fault type earthquakes occurring under the strong pressure in the crust. Obtained results may indicate the common anomaly source both for VLF and ULF in the lithosphere and are consistent with currently proposed Lithosphere-Atmosphere-Ionosphere (LAI) coupling scenarios during the earthquake preparation period.
