Detecting temporal changes in fault zone properties at seismogenic depth have been a long-sought goal in the seismological community for many decades. Recent studies based on waveform analysis of repeating earthquakes...Detecting temporal changes in fault zone properties at seismogenic depth have been a long-sought goal in the seismological community for many decades. Recent studies based on waveform analysis of repeating earthquakes have found clear temporal changes in the shallow crust and around active fault zones associated with the occurrences of large nearby and teleseismic earthquakes. However, repeating earthquakes only occur in certain locations and their occurrence times cannot be controlled, which may result in inadequate sampling of the interested regions or time periods. Recent developments in passive imaging via auto- and cross-correlation of ambient seismic wavefields （e.g., seismic noise, earthquake coda waves） provide an ideal source for continuous monitoring of temporal changes around active fault zones. Here we conduct a systematic search of temporal changes along the Parkfield section of the San Andreas fault by cross-correlating relatively high-frequency （0.4-1.3 Hz） ambient noise signals recorded by 10 borehole stations in the High Resolution Seismic Network. After using stretch/compressed method to measure the delay time and the decorrelation-index between the daily noise cross-correlation functions （NCCFs）, we find clear temporal changes in the median seismic velocity and decorrelation-index associated with the 2004 M6.0 Parkfield earthquake. We also apply the same procedure to the seismic data around five regional/teleseismic events that have triggered non-volcanic tremor in the same region, but failed to find any clear temporal changes in the daily NCCFs. The fact that our current technique can detect temporal changes from the nearby but not regional and teleseismic events, suggests that temporal changes associated with distance sources are very subtle or localized so that they could not be detected within the resolution of the current technique （-0.2%）.展开更多
A devastating MW 7.7 earthquake struck near Mandalay,Myanmar,on March 28,2025,causing extensive damage and casualties across Myanmar and neighboring regions.The 2025 event occurred in a well-recognized seismic gap alo...A devastating MW 7.7 earthquake struck near Mandalay,Myanmar,on March 28,2025,causing extensive damage and casualties across Myanmar and neighboring regions.The 2025 event occurred in a well-recognized seismic gap along the Sagaing Fault.Here we focus on the mainshock rupture properties based on backprojection of teleseismic P waves and early aftershock locations,analysis of near-field seismic recordings for the mainshock initiation,and remotely triggered seismicity following the MW 7.7 mainshock.We find that the~500 km mainshock rupture can be revealed by both rapid back-projection of teleseismic P waves from multiple broadband arrays and early aftershock locations within about 3 h from the Thai Meteorological Department(TMD)catalog.The rupture speed went supershear in the southward propagation after the initial bilateral subshear ruptures,as expected for large strike-slip earthquakes of such sizes.Clear fault zone head waves that are reflected along a bimaterial fault interface are observed at the only near-fault station GE.NPW on the slower side about 2.6 km away from the Sagaing fault,consistent with the preferred direction of a supershear rupture propagating to the south.In addition,aftershocks from the regional TMD catalog appear to be located mostly to the east of the mainshock rupture.While we cannot completely rule out mis-locations from the one-sided station distribution,these off-fault seismicity could also be explained by reactivations of subsidiary faults within the Shan Plateau,or an eastward dipping of the mainshock rupture plane.Although no immediate foreshocks were found from several nearby stations,we identify one sub-event with magnitude~6 at the beginning of the mainshock with a slightly different focal mechanism about 20–30 km south of the hypocenter determined by the United States Geological Survey(USGS).The mainshock also occurred when the tidal stresses reached its maximum on the right-lateral strike-slip fault,likely indicating that the timing of the mainshock is modulated by the solid earth tides.We find a significant increase of seismic activity near the Thailand/Myanmar border,in multiple(geothermally active)regions of Yunnan province in Southwest China,as well as the Xingfengjian reservoir in the Guangdong province in South China.Because static stress changes from the mainshock are small but negative near the Thailand/Myanmar border,the occurrence of microseismicity in this and other regions can be mainly explained by remote triggering from dynamic stress changes of the mainshock rupture.Our analyses demonstrate the importance of rapid analysis on openly available seismic data and catalog to better understand the rupture properties and triggered seismicity following large earthquakes.展开更多
基金funded in part by the National Science Foundation of United States under grants EAR-0710959 and EAR-0956051support of U.S. Air Force Research Laboratory under grant FA8718-07-186 C-0005 and Dr. Peter Gerstoft
文摘Detecting temporal changes in fault zone properties at seismogenic depth have been a long-sought goal in the seismological community for many decades. Recent studies based on waveform analysis of repeating earthquakes have found clear temporal changes in the shallow crust and around active fault zones associated with the occurrences of large nearby and teleseismic earthquakes. However, repeating earthquakes only occur in certain locations and their occurrence times cannot be controlled, which may result in inadequate sampling of the interested regions or time periods. Recent developments in passive imaging via auto- and cross-correlation of ambient seismic wavefields （e.g., seismic noise, earthquake coda waves） provide an ideal source for continuous monitoring of temporal changes around active fault zones. Here we conduct a systematic search of temporal changes along the Parkfield section of the San Andreas fault by cross-correlating relatively high-frequency （0.4-1.3 Hz） ambient noise signals recorded by 10 borehole stations in the High Resolution Seismic Network. After using stretch/compressed method to measure the delay time and the decorrelation-index between the daily noise cross-correlation functions （NCCFs）, we find clear temporal changes in the median seismic velocity and decorrelation-index associated with the 2004 M6.0 Parkfield earthquake. We also apply the same procedure to the seismic data around five regional/teleseismic events that have triggered non-volcanic tremor in the same region, but failed to find any clear temporal changes in the daily NCCFs. The fact that our current technique can detect temporal changes from the nearby but not regional and teleseismic events, suggests that temporal changes associated with distance sources are very subtle or localized so that they could not be detected within the resolution of the current technique （-0.2%）.
基金supported by National Science Foundation Grant RISE-2425889supported by Earthquake Prediction Open Fund,China Earthquake Administration(Grant No.XH25006D)supported by the International Partnership Program of Chinese Academy of Sciences(Grant No.164GJHZ2023006MI).
文摘A devastating MW 7.7 earthquake struck near Mandalay,Myanmar,on March 28,2025,causing extensive damage and casualties across Myanmar and neighboring regions.The 2025 event occurred in a well-recognized seismic gap along the Sagaing Fault.Here we focus on the mainshock rupture properties based on backprojection of teleseismic P waves and early aftershock locations,analysis of near-field seismic recordings for the mainshock initiation,and remotely triggered seismicity following the MW 7.7 mainshock.We find that the~500 km mainshock rupture can be revealed by both rapid back-projection of teleseismic P waves from multiple broadband arrays and early aftershock locations within about 3 h from the Thai Meteorological Department(TMD)catalog.The rupture speed went supershear in the southward propagation after the initial bilateral subshear ruptures,as expected for large strike-slip earthquakes of such sizes.Clear fault zone head waves that are reflected along a bimaterial fault interface are observed at the only near-fault station GE.NPW on the slower side about 2.6 km away from the Sagaing fault,consistent with the preferred direction of a supershear rupture propagating to the south.In addition,aftershocks from the regional TMD catalog appear to be located mostly to the east of the mainshock rupture.While we cannot completely rule out mis-locations from the one-sided station distribution,these off-fault seismicity could also be explained by reactivations of subsidiary faults within the Shan Plateau,or an eastward dipping of the mainshock rupture plane.Although no immediate foreshocks were found from several nearby stations,we identify one sub-event with magnitude~6 at the beginning of the mainshock with a slightly different focal mechanism about 20–30 km south of the hypocenter determined by the United States Geological Survey(USGS).The mainshock also occurred when the tidal stresses reached its maximum on the right-lateral strike-slip fault,likely indicating that the timing of the mainshock is modulated by the solid earth tides.We find a significant increase of seismic activity near the Thailand/Myanmar border,in multiple(geothermally active)regions of Yunnan province in Southwest China,as well as the Xingfengjian reservoir in the Guangdong province in South China.Because static stress changes from the mainshock are small but negative near the Thailand/Myanmar border,the occurrence of microseismicity in this and other regions can be mainly explained by remote triggering from dynamic stress changes of the mainshock rupture.Our analyses demonstrate the importance of rapid analysis on openly available seismic data and catalog to better understand the rupture properties and triggered seismicity following large earthquakes.
