Previous investigations have shown that the seismic response of slopes during the Wenchuan earthquake was highly variable. The present study tries to give an answer to the question： Which are the main factors affecti...Previous investigations have shown that the seismic response of slopes during the Wenchuan earthquake was highly variable. The present study tries to give an answer to the question： Which are the main factors affecting the seismic response degree of slopes？ With the support of the China Geological Survey Bureau, we set 3 monitoring sections in Jiulong slope, Mianzhu city, China with the aim to record the site response of the slope during the affershoeks of the Wenehuan earthquake. After the Wenchuan earthquake, which happened on 12 May 2008, 30 aftershocks have been recorded in these monitoring points. We analyzed 11 records, with magnitudes ranging from ML = 4.6 to ML = 3.1. The amplification factors of the horizontal compound PGA and 3D compound PGA have been determined for the 3 points at different elevations on the slope. Results showed that the dynamic response of the slope on the earthquake was controlled by factors such as topography and the thickness of the Quaternary overburden.展开更多
Based on Gutenberg-Richter＇s relation,Bath＇s law,Omori＇s law and Well＇s relation of rupture scale,this paper forecasts the temporal decay,total number,possible area and greatest magnitude of strong aftershocks（gr...Based on Gutenberg-Richter＇s relation,Bath＇s law,Omori＇s law and Well＇s relation of rupture scale,this paper forecasts the temporal decay,total number,possible area and greatest magnitude of strong aftershocks（greater than or equal to M6.0） of the MS8.0 Wenchuan earthquake by using the magnitude and statistical parameters of earthquakes in California area of USA.The number of strong aftershocks,the parameters of Gutenberg-Richter＇s relation and the modified form of Omori＇s law are validated based on the relocation data of aftershock sequence of the MS8.0 Wenchuan earthquake.Moreover,the spatio-temporal characteristics and wave energy release of the strong aftershocks（M≥6.0） are analyzed.The result shows that strong aftershocks may occur at the end of local drop and sharp drop on the wave energy release curve.展开更多
Shear wave splitting has been measured from analyzing the three-component digital seismograms recorded at Guiquan station after the 1985 Ms6 1 Luquan earthquake in Yunnan Province. The variations in parameters ofshear...Shear wave splitting has been measured from analyzing the three-component digital seismograms recorded at Guiquan station after the 1985 Ms6 1 Luquan earthquake in Yunnan Province. The variations in parameters ofshear wave splitting with time for over 100 aftershocks have two periods, the local stress Period and the regionalstress period. In the local stress period, there exist two vertical, paralell crack sets intersecting at about (50-60°), both affect on the propagation of S-waves, and the local stress is slightly stronger than the regional stress.With the activity of aftershock going down and the local stress dying away, it is returned to the state of the regional stress in the focal area. The polarizations of the fast split S-wave and their period variations are identicalwith the azimuths and changes of the principal compressive stress axis of focal stress field inferred independentlyfrom earthquake mechanisms, hense, it is interpreted that the shear wave splitting is the effects of anisotropy ofEDA cracks controlled by stress field. The time delay of the slow split S-wave, except the difference betweenthe two periods shows in some examples that it increases in a few hours before an event and decreases in a fewdays after an event on the individual background of period.展开更多
The displacement field generated by aftershocks in Landers earthquake fault zone and its adjacent areas is calculated in this study. The result is compared with the displacement field of the main shock calculated by c...The displacement field generated by aftershocks in Landers earthquake fault zone and its adjacent areas is calculated in this study. The result is compared with the displacement field of the main shock calculated by co-seismic slip model of Wald and Heaton (1994). The result shows that the direction of displacement generated by aftershocks in Landers seismic fault plane and its adjacent areas is consistent with that generated by main shock. The rupture of aftershock is generally inherited from main shock. The displacement generated by aftershocks is up to an order of centimeter and can be measured by GPS sites nearby. So when we use geodetic data measured after earthquake to study the geophysical problems such as crustal viscosity structure, afterslip distribution, etc., only the displacement field generated by aftershocks considered, can uncertainty be reduced to minimum and realistic result be obtained.展开更多
Jiuzhaigou is a world-heritage site located in the plateau area of Northwest Sichuan Province,China.Serious slope failures in the epicentral area were triggered by the Ms7.0 Jiuzhaigou earthquake occurred on August 8,...Jiuzhaigou is a world-heritage site located in the plateau area of Northwest Sichuan Province,China.Serious slope failures in the epicentral area were triggered by the Ms7.0 Jiuzhaigou earthquake occurred on August 8,2017.The source areas of the hazards are usually concentrated near ridge crests,revealingthe possible occurrence of ground motion amplification phenomena.To explore the role of the amplification of ground motions in the formation of earthquake-triggered slope failures,two seismometers were installed,on the next day after the main shock,at the bottom of the slopeof Jubao Mountain near the seismogenic fault.The two monitoring sites are located at elevations of 1414 m(J1)and 1551 m(J2,the top of the mountain).Five aftershocks were recorded by the monitoring instruments.We compared the mean levels of the peak ground acceleration(PGA)observed at different locations,and investigated the directional variations inthe shaking energy by analyzing the polar diagrams of the Arias intensity(Ia).Then,in order to identify the directional resonance phenomenonandtheir frequencies and amplification coefficients,we examined the horizontal-to-vertical spectral ratio(HVSR)and the standard spectral ratio(SSR).Polar diagrams of theArias intensity(Ia)indicated that the site response of Jubao Mountain showed a pronounced directivity(in theEW direction)with shaking maxima near the hill top oriented orthogonally to the elongation of the relief.We observed anobvious resonance phenomenonat site J2 at relatively low frequencies(2.5-9 Hz)and very weak spectral amplifications at site J1 at high frequencies(5-15 Hz),which suggested that the predominant frequency of monitoring site J2 was obviously attenuated and that the difference in the spectra was related to the influences of the local-scale site conditions of the whole mountain.The results of spectral ratio analyses(HVSR and SSR)showed that the direction of resonance was concentrated around an EW orientation,and the amplification factors near the hill top were larger than 2.It suggests that geologic factors also play a significant role in the anisotropic amplifications affecting the tops of slopes besides the topographic effects.展开更多
The great Kunlun earthquake occurred on Nov. 14, 2001 in Qinghai Province, China. Five large aftershocks with magnitude larger than 5.0 occurred near the Kunlun fault after main shock. Calculations of the change in Co...The great Kunlun earthquake occurred on Nov. 14, 2001 in Qinghai Province, China. Five large aftershocks with magnitude larger than 5.0 occurred near the Kunlun fault after main shock. Calculations of the change in Coulomb failure stress reveal that 4 of 5 large aftershocks occurred in areas with Dsf >0 (10-2~10-1 MPa) and one aftershock occurred in an area with Dsf =-0.56 MPa. It is concluded that the permanent fault displacement due to the main shock is the main cause of activity of large aftershocks, but not the whole cause.展开更多
Aftershocks of the 2011 Tohoku-Oki great earthquake have a wide range of focal depths and fault plane mechanisms. We constrain the focal depths and focal mechanisms of 69 aftershocks with Mw 〉 5.4 by modeling the wav...Aftershocks of the 2011 Tohoku-Oki great earthquake have a wide range of focal depths and fault plane mechanisms. We constrain the focal depths and focal mechanisms of 69 aftershocks with Mw 〉 5.4 by modeling the waveforms of teleseismic P and its trailing near-surface reflections pP and sP. We find that the ＂thrust events＂ are within 10 krn from the plate interface. The dip angles of these thrust events increase with depth from ~ 5~ to ~ 25~. The ＂non-thrust events＂ vary from 60 km above to 40 km below the plate interface. Normal and strike-slip events within the overriding plate point to redistribution of stress following the primary great earthquake; however, due to the spatially variable stress change in the Tohoku-Oki earthquake, an understanding of how the mainshock affected the stresses that led to the aftershocks requires accurate knowledge of the aftershock location.展开更多
Two earthquakes occurred on 11 August 2012 in East Azerbaijan province, NW Iran with Mw 6.3 at 12:23:15.9 and Mw 6.1;11 minutes after the first shock at 12:34:34.8 GMT time. In this study, we concentrated on the prope...Two earthquakes occurred on 11 August 2012 in East Azerbaijan province, NW Iran with Mw 6.3 at 12:23:15.9 and Mw 6.1;11 minutes after the first shock at 12:34:34.8 GMT time. In this study, we concentrated on the properties of the aftershock sequences in a duration of 7.5-month time-period data after the main shocks, recorded by Broadband Iranian Network (BIN). We obtained p, c and K parameters with the maximum likelihood method and the occurrence rate was modeled by the modified Omori formula. Besides, we showed that aftershocks of the aftershock in 7 Nov. 2012 (M = 5.6) also decay according to the modified Omori law. We investigated the spatial and temporal variations of b-values before and after the twin earthquakes with Mw 6.3 and Mw 6.1 approximately 20 km around North Tabriz Fault (NTF) in the region from 1996 to 2013 using Iranian Seismological Center (IRSC). We believed that a research of the past 17 years has clearly established that spatial b-value changes and seismicity rate are highly significant and meaningful. Moreover, our research led to a warning for a probable strong motion along NTF for the coming couple of years.展开更多
On the 3rd of April 2017, an earthquake of moment magnitude 6.5 occurred near Moiyabana in central Botswana. This paper is aimed at studying the spatial distribution of the foreshock and aftershock sequences associate...On the 3rd of April 2017, an earthquake of moment magnitude 6.5 occurred near Moiyabana in central Botswana. This paper is aimed at studying the spatial distribution of the foreshock and aftershock sequences associated with the Moiyabana earthquake. The foreshocks and aftershocks data used were from the Botswana Geoscience Institute (BGI) and the Seisan software was used to analyze the foreshock and aftershock events. The analyses revealed nine epicenter locations of foreshocks which are spread out across the country and most of them are located in the central and southern parts of Botswana, while the aftershocks are clustered around the mainshock. Although five of the nine foreshocks occurred far from the boundaries of major tectonic units, the other four occurred near key features such as the Zoetfontein, Lecha and Chobe faults. The spatial distribution of aftershocks indicates that the stress released by the mainshock, re-activated the planes of weakness in the vicinity of the mainshock and farther away from the mainshock. Hence, this affected the Zoetfontein fault, the boundary between Passarge basin and Magondi belt and the boundary between the Kaapvaal craton and Limpopo mobile belt. The aftershocks also show a northwest-southeast trend, which probably indicates the rupture plane;and mainly lie within the Limpopo mobile belt that is sandwiched between the Kaapvaal craton to the south and Zimbabwe craton to the north. Furthermore, the aftershocks concentration to the south reveals a close relation in demarcating the boundary of the Kaapvaal craton and the Limpopo mobile belt.展开更多
Seismic events are very complex spatial-temporal phenomena. Seismic catalogues, reporting information about spatial-temporal distribution of the main shocks, are nowadays available for many seismic areas in the world,...Seismic events are very complex spatial-temporal phenomena. Seismic catalogues, reporting information about spatial-temporal distribution of the main shocks, are nowadays available for many seismic areas in the world, very often major events mark the beginning of a series of earthquakes （aflershocks） whose frequency and energy are meanly decreasing in time down to the background level of activity. Azerbaijan is one of the most active segments of the Alpine-Himalayan seismic belt and marks the junction between the African-Arabian and Indian plate to the south, and Eurasian plate to the north. The cluster of earthquakes that struck near Varzeghan-Ahar was centered near the Gosha-Dagh fault, but preliminary data suggested that the fault was not responsible for the temblor. On the late afternoon of Saturday, August 11, 2012, the northwest of Iran was shaken by two of the strong earthquakes in Iranian history. First was hit by Mw （moment magnitude scale） = 6.4 Richter at local time 16：54 （12：23 GMT （Greenwich Mean Time））, and about 11 min later, an Mw = 6.3 struck 10 km to the west. The spatial-temporal clustering of micro earthquakes （aftershocks） near Varzeghan, is parameterized by means of a generalized passion model. The region has known faults but numerous smaller or deeply buried faults remain undated, according to the Geological Survey of Iran.展开更多
The M8.0 Wenchuan earthquake occurred on the Longmenshan fault zone. Based on field investigation of the surface rupture and focal mechanism study of the aftershocks, we discuss the geological relationship of the main...The M8.0 Wenchuan earthquake occurred on the Longmenshan fault zone. Based on field investigation of the surface rupture and focal mechanism study of the aftershocks, we discuss the geological relationship of the main, secondary and triggered ruptures. The main rupture is about 200km long and can be divided into the south part and the north part. The south part consists of two parallel fault zones characterized by reverse faulting, with several parallel secondary ruptures on the hanging wall of the main fault, and the north part is a single main fault zone characterized by lateral strike-slip and reverse faulting. Compared to a 300km long aftershock distribution, the surface rupture only occupies 200km, and the remaining lOOkm on the northeast of the main rupture was triggered by aftershocks. Study on the ruptures of this earthquake will be useful for studying the earthquake risk evolution on the Longmenshan fault system.展开更多
We use the slowness-azimuth station correction( SASC) method to improve the location accuracy of the Wenchuan aftershocks recorded by the Nagqu and Hotan seismic arrays.The results show that the standard deviations of...We use the slowness-azimuth station correction( SASC) method to improve the location accuracy of the Wenchuan aftershocks recorded by the Nagqu and Hotan seismic arrays.The results show that the standard deviations of back-azimuth and slowness errors of Wenchuan aftershocks recorded by the Nagqu array decreased by 32% and 58%respectively after correction. The decrease is 38% and 71% for the Hotan array. After the correction,the location accuracy of all Wenchuan aftershocks recorded by the Nagqu array is improved. For the Hotan array,the accuracy is improved in the slowness estimation for 78% of aftershocks and in back-azimuth estimation for all aftershocks.展开更多
By calculating the azimuth, phase angle and change rate of tidal force, and contrasting their rupture types, we find that in 6 of 11 reverse faulting earthquakes, the angles between the direction of the horizontal tid...By calculating the azimuth, phase angle and change rate of tidal force, and contrasting their rupture types, we find that in 6 of 11 reverse faulting earthquakes, the angles between the direction of the horizontal tidal force and compressive principal stress are within 33°, the vertical phase angles of all 8 strike-slip earthquakes are in the upward peak section and the vertical phase angles of two extensional normal faulting earthquakes are in the downward peak section. According to the above statistics, the mechanism of tidal force triggering of the Wenchnan Ms8. 0 Earthquake and its strong aftershocks is discussed.展开更多
To consider the infl uence of aftershocks in engineering design, the correlations between main shocks and aftershocks should be examined, and an aftershock simulation method with main shock ground motions needs to be ...To consider the infl uence of aftershocks in engineering design, the correlations between main shocks and aftershocks should be examined, and an aftershock simulation method with main shock ground motions needs to be developed. In this study, the data on the sequences of main shock–aftershock ground motions and other related parameters were collected. Using these data, correlations between the magnitude, frequency, duration and energy of the main shock–aftershock ground motions were investigated. The results showed that the magnitude of the aftershock can be larger than that of the main shock. The shapes of the Fourier amplitude spectra of main shocks and aftershocks were similar;however, the predominant frequency and high-frequency components of the aftershock tended to be larger. Considering the magnitude diff erence between the main shock and the aftershock, the correlation of durations was explored. Additionally, a new concept, the duration ratio, was defi ned to describe the concentration of seismic energy release, and main shock energy was strongly positively correlated with the energy attenuated during the main shock–aftershock sequence. Finally, based on these results regarding correlation, an aftershock synthesis using recorded main shock ground motions was constructed with the trigonometric series method for seismic design, and some examples are given to analyze the rationality of this synthetic method.展开更多
The Tohoku megathrust earthquake, which occurred on March 11, 2011 and had an epicenter that was 70 km east of Tohoku, Japan, resulted in an estimated ten′s of billions of dollars in damage and a death toll of more t...The Tohoku megathrust earthquake, which occurred on March 11, 2011 and had an epicenter that was 70 km east of Tohoku, Japan, resulted in an estimated ten′s of billions of dollars in damage and a death toll of more than 15 thousand lives, yet few studies have documented key spatio-temporal seismogenic characteristics. Specifically, the temporal decay of aftershock activity, the number of strong aftershocks (with magnitudes greater than or equal to 7.0), the magnitude of the greatest aftershock, and area of possible aftershocks. Forecasted results from this study are based on Gutenberg-Richter’s relation, Bath’s law, Omori’s law, and Well’s relation of rupture scale utilizing the magnitude and statistical parameters of earthquakes in USA and China (Landers, Northridge, Hector Mine, San Simeon and Wenchuan earthquakes). The number of strong aftershocks, the parameters of Gutenberg-Richter’s relation, and the modified form of Omori’s law are confirmed based on the aftershock sequence data from the Mw9.0 Tohoku earthquake. Moreover, for a large earthquake, the seismogenic structure could be a fault, a fault system, or an intersection of several faults. The seismogenic structure of the earthquake suggests that the event occurred on a thrust fault near the Japan trench within the overriding plate that subsequently triggered three or more active faults producing large aftershocks.展开更多
In this paper, the earthquake hazard parameters (λ m,b and M max) from the maximum likelihood method for the raw catalogue and declustered catalogues have been used to discuss the effect of the aftershocks on the ear...In this paper, the earthquake hazard parameters (λ m,b and M max) from the maximum likelihood method for the raw catalogue and declustered catalogues have been used to discuss the effect of the aftershocks on the earthquake hazard estimation. The declustered catalogues have been compiled from the raw catalogues by deleting the aftershocks in different time interval after main shock according to the criteria for the aftershock activity period. As an example, Hebei seismic zone is taken to show the effect of the aftershock on the earthquake hazard assessment because three strong earthquakes with the aftershocks occurred from 1966 to 1976. The results have been shown that the effect of the aftershocks on the parameters λ m,b is significant. The difference between the clustered and declustered catalogues has reached in seismic activity rate and recurrence period, at most, 45% and 90%, respectively. But the difference in M max is smaller. Based on this, the suggestion that the aftershocks should be omitted in the estimation of the earthquake hazard could be made, but how long the aftershocks activity duration is still left to the future research.展开更多
The contained underground explosion (CUE) usually generates huge number of aftershocks. This kind of after-shocks induced by three CUEs was investigated in the paper. The conclusions show that the duration of aftersho...The contained underground explosion (CUE) usually generates huge number of aftershocks. This kind of after-shocks induced by three CUEs was investigated in the paper. The conclusions show that the duration of aftershock waveforms are rather short, 70 percent of them range from 2 to 7; the occurrences of the aftershocks conform to negative power function, which has the power of -1.6. The aftershock sequence attenuates a little bit faster, with power of -1.0, within two weeks of post-explosions. During the early stage of post-explosions the aftershocks show up in a cluster, however, they usually show up individually during the late stage of post-explosions. The number of aftershocks generated by the compatible explosions differs by several times because of different me-dium and geological structure; within one month after an explosion with Richater magnitude of 5.5, the number of aftershocks attenuates to the background. Hereafter there are still tiny numbers of aftershocks.展开更多
Using the ground motion attenuation relation, we calculated and compared the effective peak acceleration （EPA） generated by main shocks and their strong aftershocks of 21 earthquake sequences with Ms≥7 occurred in ...Using the ground motion attenuation relation, we calculated and compared the effective peak acceleration （EPA） generated by main shocks and their strong aftershocks of 21 earthquake sequences with Ms≥7 occurred in Chinese mainland and offing of China during 1966-2002. The result shows that EPA of strong aftershocks usually exceed that of main shock for 76.2% earthquake sequences and EPA of more than 50% strong aftershocks are greatly larger than that of main shocks in large area, which suggests that it is necessary to take damage produced by strong aftershock into account in the probabilistic seismic hazard analysis and the seismic design.展开更多
In this paper, we take occurrence process of early strong aftershocks of a main after shock type′s earthquake sequence as a complex grey system, and introduce predicting method for its stronger aftershocks by grey p...In this paper, we take occurrence process of early strong aftershocks of a main after shock type′s earthquake sequence as a complex grey system, and introduce predicting method for its stronger aftershocks by grey predicting theory. Through inspection prediction for 1998 Zhangbei M S=6.2 earthquake sequence, it shows that the grey predicting method maybe has active significance for the investigation of quick response prediction problems of stronger aftershocks of an earthquake sequence.展开更多
This paper deals with the distributive characteristics of the occurrence time of earthquakes with respect to the aftershock sequences of strong earthquakes. The distribution of lunar and solar local hour angles at the...This paper deals with the distributive characteristics of the occurrence time of earthquakes with respect to the aftershock sequences of strong earthquakes. The distribution of lunar and solar local hour angles at the time of commencement of moderate and strong aftershocks indicates that the time of commencement of moderate and strong aftershocks is modulated by the positions of the sun and moon and then the earthquake restrained time zones exists also. In this paper the differences of earthquake restrained time zones between the preshock sequences and the aftershock sequences are compared, and the possible mechanism is analyzed preliminarily. And the possible maximum scope of accuracy in predicting the occurrence time of an earthquake is determined as well.展开更多
基金supported by National Natural Science Foundation of China (Grant No.41072231)China Geological Survey Bureau (Grant Nos 1212010914010 and 1212011220154)Program for Changjiang Scholars and Innovative Research Team in University" (Grant No. IRT0812)
文摘Previous investigations have shown that the seismic response of slopes during the Wenchuan earthquake was highly variable. The present study tries to give an answer to the question： Which are the main factors affecting the seismic response degree of slopes？ With the support of the China Geological Survey Bureau, we set 3 monitoring sections in Jiulong slope, Mianzhu city, China with the aim to record the site response of the slope during the affershoeks of the Wenehuan earthquake. After the Wenchuan earthquake, which happened on 12 May 2008, 30 aftershocks have been recorded in these monitoring points. We analyzed 11 records, with magnitudes ranging from ML = 4.6 to ML = 3.1. The amplification factors of the horizontal compound PGA and 3D compound PGA have been determined for the 3 points at different elevations on the slope. Results showed that the dynamic response of the slope on the earthquake was controlled by factors such as topography and the thickness of the Quaternary overburden.
基金supported by Basic Science and Research Fund for Chinese Commonweal Institutes under grant No. 2008B07the National Natural Science Foundation of China under grant No. 90715042+1 种基金Special Research Project of Earth-quake Engineering under grant No. 200808008National Science and Technology Support Plan under grant No. 2006BAC13B02
文摘Based on Gutenberg-Richter＇s relation,Bath＇s law,Omori＇s law and Well＇s relation of rupture scale,this paper forecasts the temporal decay,total number,possible area and greatest magnitude of strong aftershocks（greater than or equal to M6.0） of the MS8.0 Wenchuan earthquake by using the magnitude and statistical parameters of earthquakes in California area of USA.The number of strong aftershocks,the parameters of Gutenberg-Richter＇s relation and the modified form of Omori＇s law are validated based on the relocation data of aftershock sequence of the MS8.0 Wenchuan earthquake.Moreover,the spatio-temporal characteristics and wave energy release of the strong aftershocks（M≥6.0） are analyzed.The result shows that strong aftershocks may occur at the end of local drop and sharp drop on the wave energy release curve.
文摘Shear wave splitting has been measured from analyzing the three-component digital seismograms recorded at Guiquan station after the 1985 Ms6 1 Luquan earthquake in Yunnan Province. The variations in parameters ofshear wave splitting with time for over 100 aftershocks have two periods, the local stress Period and the regionalstress period. In the local stress period, there exist two vertical, paralell crack sets intersecting at about (50-60°), both affect on the propagation of S-waves, and the local stress is slightly stronger than the regional stress.With the activity of aftershock going down and the local stress dying away, it is returned to the state of the regional stress in the focal area. The polarizations of the fast split S-wave and their period variations are identicalwith the azimuths and changes of the principal compressive stress axis of focal stress field inferred independentlyfrom earthquake mechanisms, hense, it is interpreted that the shear wave splitting is the effects of anisotropy ofEDA cracks controlled by stress field. The time delay of the slow split S-wave, except the difference betweenthe two periods shows in some examples that it increases in a few hours before an event and decreases in a fewdays after an event on the individual background of period.
基金National Natural Science Foundation of China (40374012) and National Basic Key Project (2002CCA04500).
文摘The displacement field generated by aftershocks in Landers earthquake fault zone and its adjacent areas is calculated in this study. The result is compared with the displacement field of the main shock calculated by co-seismic slip model of Wald and Heaton (1994). The result shows that the direction of displacement generated by aftershocks in Landers seismic fault plane and its adjacent areas is consistent with that generated by main shock. The rupture of aftershock is generally inherited from main shock. The displacement generated by aftershocks is up to an order of centimeter and can be measured by GPS sites nearby. So when we use geodetic data measured after earthquake to study the geophysical problems such as crustal viscosity structure, afterslip distribution, etc., only the displacement field generated by aftershocks considered, can uncertainty be reduced to minimum and realistic result be obtained.
基金financially supported by the National Natural Science Foundation of China(Grand Nos.41877235,1704243)the Funds for Creative Research Groups of China(Grant No.41521002)the Independent Fund of State Key Laboratory of Geohazard Prevention and Geoenvironment Protection(Grant No.SKLGP2015Z001)
文摘Jiuzhaigou is a world-heritage site located in the plateau area of Northwest Sichuan Province,China.Serious slope failures in the epicentral area were triggered by the Ms7.0 Jiuzhaigou earthquake occurred on August 8,2017.The source areas of the hazards are usually concentrated near ridge crests,revealingthe possible occurrence of ground motion amplification phenomena.To explore the role of the amplification of ground motions in the formation of earthquake-triggered slope failures,two seismometers were installed,on the next day after the main shock,at the bottom of the slopeof Jubao Mountain near the seismogenic fault.The two monitoring sites are located at elevations of 1414 m(J1)and 1551 m(J2,the top of the mountain).Five aftershocks were recorded by the monitoring instruments.We compared the mean levels of the peak ground acceleration(PGA)observed at different locations,and investigated the directional variations inthe shaking energy by analyzing the polar diagrams of the Arias intensity(Ia).Then,in order to identify the directional resonance phenomenonandtheir frequencies and amplification coefficients,we examined the horizontal-to-vertical spectral ratio(HVSR)and the standard spectral ratio(SSR).Polar diagrams of theArias intensity(Ia)indicated that the site response of Jubao Mountain showed a pronounced directivity(in theEW direction)with shaking maxima near the hill top oriented orthogonally to the elongation of the relief.We observed anobvious resonance phenomenonat site J2 at relatively low frequencies(2.5-9 Hz)and very weak spectral amplifications at site J1 at high frequencies(5-15 Hz),which suggested that the predominant frequency of monitoring site J2 was obviously attenuated and that the difference in the spectra was related to the influences of the local-scale site conditions of the whole mountain.The results of spectral ratio analyses(HVSR and SSR)showed that the direction of resonance was concentrated around an EW orientation,and the amplification factors near the hill top were larger than 2.It suggests that geologic factors also play a significant role in the anisotropic amplifications affecting the tops of slopes besides the topographic effects.
基金The State Sciences and Technology Key Project During the Tenth Five-year Plan (2001BA601B02-02-04) China-Greece International Cooperative Research Project on Seismology and Chinese Joint Seismological Science Foundation (103073).
文摘The great Kunlun earthquake occurred on Nov. 14, 2001 in Qinghai Province, China. Five large aftershocks with magnitude larger than 5.0 occurred near the Kunlun fault after main shock. Calculations of the change in Coulomb failure stress reveal that 4 of 5 large aftershocks occurred in areas with Dsf >0 (10-2~10-1 MPa) and one aftershock occurred in an area with Dsf =-0.56 MPa. It is concluded that the permanent fault displacement due to the main shock is the main cause of activity of large aftershocks, but not the whole cause.
基金funded by the grants of National Natural Science Foundation of China (41274086) to LB and JR, and a University of Michigan Rackham Merit Fellowship to LML
文摘Aftershocks of the 2011 Tohoku-Oki great earthquake have a wide range of focal depths and fault plane mechanisms. We constrain the focal depths and focal mechanisms of 69 aftershocks with Mw 〉 5.4 by modeling the waveforms of teleseismic P and its trailing near-surface reflections pP and sP. We find that the ＂thrust events＂ are within 10 krn from the plate interface. The dip angles of these thrust events increase with depth from ~ 5~ to ~ 25~. The ＂non-thrust events＂ vary from 60 km above to 40 km below the plate interface. Normal and strike-slip events within the overriding plate point to redistribution of stress following the primary great earthquake; however, due to the spatially variable stress change in the Tohoku-Oki earthquake, an understanding of how the mainshock affected the stresses that led to the aftershocks requires accurate knowledge of the aftershock location.
文摘Two earthquakes occurred on 11 August 2012 in East Azerbaijan province, NW Iran with Mw 6.3 at 12:23:15.9 and Mw 6.1;11 minutes after the first shock at 12:34:34.8 GMT time. In this study, we concentrated on the properties of the aftershock sequences in a duration of 7.5-month time-period data after the main shocks, recorded by Broadband Iranian Network (BIN). We obtained p, c and K parameters with the maximum likelihood method and the occurrence rate was modeled by the modified Omori formula. Besides, we showed that aftershocks of the aftershock in 7 Nov. 2012 (M = 5.6) also decay according to the modified Omori law. We investigated the spatial and temporal variations of b-values before and after the twin earthquakes with Mw 6.3 and Mw 6.1 approximately 20 km around North Tabriz Fault (NTF) in the region from 1996 to 2013 using Iranian Seismological Center (IRSC). We believed that a research of the past 17 years has clearly established that spatial b-value changes and seismicity rate are highly significant and meaningful. Moreover, our research led to a warning for a probable strong motion along NTF for the coming couple of years.
文摘On the 3rd of April 2017, an earthquake of moment magnitude 6.5 occurred near Moiyabana in central Botswana. This paper is aimed at studying the spatial distribution of the foreshock and aftershock sequences associated with the Moiyabana earthquake. The foreshocks and aftershocks data used were from the Botswana Geoscience Institute (BGI) and the Seisan software was used to analyze the foreshock and aftershock events. The analyses revealed nine epicenter locations of foreshocks which are spread out across the country and most of them are located in the central and southern parts of Botswana, while the aftershocks are clustered around the mainshock. Although five of the nine foreshocks occurred far from the boundaries of major tectonic units, the other four occurred near key features such as the Zoetfontein, Lecha and Chobe faults. The spatial distribution of aftershocks indicates that the stress released by the mainshock, re-activated the planes of weakness in the vicinity of the mainshock and farther away from the mainshock. Hence, this affected the Zoetfontein fault, the boundary between Passarge basin and Magondi belt and the boundary between the Kaapvaal craton and Limpopo mobile belt. The aftershocks also show a northwest-southeast trend, which probably indicates the rupture plane;and mainly lie within the Limpopo mobile belt that is sandwiched between the Kaapvaal craton to the south and Zimbabwe craton to the north. Furthermore, the aftershocks concentration to the south reveals a close relation in demarcating the boundary of the Kaapvaal craton and the Limpopo mobile belt.
文摘Seismic events are very complex spatial-temporal phenomena. Seismic catalogues, reporting information about spatial-temporal distribution of the main shocks, are nowadays available for many seismic areas in the world, very often major events mark the beginning of a series of earthquakes （aflershocks） whose frequency and energy are meanly decreasing in time down to the background level of activity. Azerbaijan is one of the most active segments of the Alpine-Himalayan seismic belt and marks the junction between the African-Arabian and Indian plate to the south, and Eurasian plate to the north. The cluster of earthquakes that struck near Varzeghan-Ahar was centered near the Gosha-Dagh fault, but preliminary data suggested that the fault was not responsible for the temblor. On the late afternoon of Saturday, August 11, 2012, the northwest of Iran was shaken by two of the strong earthquakes in Iranian history. First was hit by Mw （moment magnitude scale） = 6.4 Richter at local time 16：54 （12：23 GMT （Greenwich Mean Time））, and about 11 min later, an Mw = 6.3 struck 10 km to the west. The spatial-temporal clustering of micro earthquakes （aftershocks） near Varzeghan, is parameterized by means of a generalized passion model. The region has known faults but numerous smaller or deeply buried faults remain undated, according to the Geological Survey of Iran.
基金sponsored by the Special Earthquake Research Program(20070851)National Key Basic Research Development Planning grogram(2004CB418401)+1 种基金Basic Science Research Professional of Institute of Crustal Dynamics (2008)National Science and Technology Support Program(2006BAC13B01 -0202),China
文摘The M8.0 Wenchuan earthquake occurred on the Longmenshan fault zone. Based on field investigation of the surface rupture and focal mechanism study of the aftershocks, we discuss the geological relationship of the main, secondary and triggered ruptures. The main rupture is about 200km long and can be divided into the south part and the north part. The south part consists of two parallel fault zones characterized by reverse faulting, with several parallel secondary ruptures on the hanging wall of the main fault, and the north part is a single main fault zone characterized by lateral strike-slip and reverse faulting. Compared to a 300km long aftershock distribution, the surface rupture only occupies 200km, and the remaining lOOkm on the northeast of the main rupture was triggered by aftershocks. Study on the ruptures of this earthquake will be useful for studying the earthquake risk evolution on the Longmenshan fault system.
基金sponsored by the Basic Scientific Research Special Program of Institute of Geophysics,China Earthquake Administration(DQJB08819)
文摘We use the slowness-azimuth station correction( SASC) method to improve the location accuracy of the Wenchuan aftershocks recorded by the Nagqu and Hotan seismic arrays.The results show that the standard deviations of back-azimuth and slowness errors of Wenchuan aftershocks recorded by the Nagqu array decreased by 32% and 58%respectively after correction. The decrease is 38% and 71% for the Hotan array. After the correction,the location accuracy of all Wenchuan aftershocks recorded by the Nagqu array is improved. For the Hotan array,the accuracy is improved in the slowness estimation for 78% of aftershocks and in back-azimuth estimation for all aftershocks.
基金sponsored under the Seismic Situation Tracing Directional Work Project in Contract System in 2009 (20090203012009010107),China
文摘By calculating the azimuth, phase angle and change rate of tidal force, and contrasting their rupture types, we find that in 6 of 11 reverse faulting earthquakes, the angles between the direction of the horizontal tidal force and compressive principal stress are within 33°, the vertical phase angles of all 8 strike-slip earthquakes are in the upward peak section and the vertical phase angles of two extensional normal faulting earthquakes are in the downward peak section. According to the above statistics, the mechanism of tidal force triggering of the Wenchnan Ms8. 0 Earthquake and its strong aftershocks is discussed.
基金National Nature Science Foundation of China under Grant No.51778183the National Science Foundation for Distinguished Young Scholars of China under Grant No.51525802
文摘To consider the infl uence of aftershocks in engineering design, the correlations between main shocks and aftershocks should be examined, and an aftershock simulation method with main shock ground motions needs to be developed. In this study, the data on the sequences of main shock–aftershock ground motions and other related parameters were collected. Using these data, correlations between the magnitude, frequency, duration and energy of the main shock–aftershock ground motions were investigated. The results showed that the magnitude of the aftershock can be larger than that of the main shock. The shapes of the Fourier amplitude spectra of main shocks and aftershocks were similar;however, the predominant frequency and high-frequency components of the aftershock tended to be larger. Considering the magnitude diff erence between the main shock and the aftershock, the correlation of durations was explored. Additionally, a new concept, the duration ratio, was defi ned to describe the concentration of seismic energy release, and main shock energy was strongly positively correlated with the energy attenuated during the main shock–aftershock sequence. Finally, based on these results regarding correlation, an aftershock synthesis using recorded main shock ground motions was constructed with the trigonometric series method for seismic design, and some examples are given to analyze the rationality of this synthetic method.
基金supported by the National Natural Science Foundation of China (No. 51278474)Special Research Project of Earthquake Engineering (No. 201108003)International Science and Technology Cooperation Program of China (No. 2011DFA21460)
文摘The Tohoku megathrust earthquake, which occurred on March 11, 2011 and had an epicenter that was 70 km east of Tohoku, Japan, resulted in an estimated ten′s of billions of dollars in damage and a death toll of more than 15 thousand lives, yet few studies have documented key spatio-temporal seismogenic characteristics. Specifically, the temporal decay of aftershock activity, the number of strong aftershocks (with magnitudes greater than or equal to 7.0), the magnitude of the greatest aftershock, and area of possible aftershocks. Forecasted results from this study are based on Gutenberg-Richter’s relation, Bath’s law, Omori’s law, and Well’s relation of rupture scale utilizing the magnitude and statistical parameters of earthquakes in USA and China (Landers, Northridge, Hector Mine, San Simeon and Wenchuan earthquakes). The number of strong aftershocks, the parameters of Gutenberg-Richter’s relation, and the modified form of Omori’s law are confirmed based on the aftershock sequence data from the Mw9.0 Tohoku earthquake. Moreover, for a large earthquake, the seismogenic structure could be a fault, a fault system, or an intersection of several faults. The seismogenic structure of the earthquake suggests that the event occurred on a thrust fault near the Japan trench within the overriding plate that subsequently triggered three or more active faults producing large aftershocks.
文摘In this paper, the earthquake hazard parameters (λ m,b and M max) from the maximum likelihood method for the raw catalogue and declustered catalogues have been used to discuss the effect of the aftershocks on the earthquake hazard estimation. The declustered catalogues have been compiled from the raw catalogues by deleting the aftershocks in different time interval after main shock according to the criteria for the aftershock activity period. As an example, Hebei seismic zone is taken to show the effect of the aftershock on the earthquake hazard assessment because three strong earthquakes with the aftershocks occurred from 1966 to 1976. The results have been shown that the effect of the aftershocks on the parameters λ m,b is significant. The difference between the clustered and declustered catalogues has reached in seismic activity rate and recurrence period, at most, 45% and 90%, respectively. But the difference in M max is smaller. Based on this, the suggestion that the aftershocks should be omitted in the estimation of the earthquake hazard could be made, but how long the aftershocks activity duration is still left to the future research.
文摘The contained underground explosion (CUE) usually generates huge number of aftershocks. This kind of after-shocks induced by three CUEs was investigated in the paper. The conclusions show that the duration of aftershock waveforms are rather short, 70 percent of them range from 2 to 7; the occurrences of the aftershocks conform to negative power function, which has the power of -1.6. The aftershock sequence attenuates a little bit faster, with power of -1.0, within two weeks of post-explosions. During the early stage of post-explosions the aftershocks show up in a cluster, however, they usually show up individually during the late stage of post-explosions. The number of aftershocks generated by the compatible explosions differs by several times because of different me-dium and geological structure; within one month after an explosion with Richater magnitude of 5.5, the number of aftershocks attenuates to the background. Hereafter there are still tiny numbers of aftershocks.
基金Department of Science and Technology Important International Cooperate Subject (2004DFA06000)Joint seis-mological Science Foundation of China (604025).
文摘Using the ground motion attenuation relation, we calculated and compared the effective peak acceleration （EPA） generated by main shocks and their strong aftershocks of 21 earthquake sequences with Ms≥7 occurred in Chinese mainland and offing of China during 1966-2002. The result shows that EPA of strong aftershocks usually exceed that of main shock for 76.2% earthquake sequences and EPA of more than 50% strong aftershocks are greatly larger than that of main shocks in large area, which suggests that it is necessary to take damage produced by strong aftershock into account in the probabilistic seismic hazard analysis and the seismic design.
文摘In this paper, we take occurrence process of early strong aftershocks of a main after shock type′s earthquake sequence as a complex grey system, and introduce predicting method for its stronger aftershocks by grey predicting theory. Through inspection prediction for 1998 Zhangbei M S=6.2 earthquake sequence, it shows that the grey predicting method maybe has active significance for the investigation of quick response prediction problems of stronger aftershocks of an earthquake sequence.
文摘This paper deals with the distributive characteristics of the occurrence time of earthquakes with respect to the aftershock sequences of strong earthquakes. The distribution of lunar and solar local hour angles at the time of commencement of moderate and strong aftershocks indicates that the time of commencement of moderate and strong aftershocks is modulated by the positions of the sun and moon and then the earthquake restrained time zones exists also. In this paper the differences of earthquake restrained time zones between the preshock sequences and the aftershock sequences are compared, and the possible mechanism is analyzed preliminarily. And the possible maximum scope of accuracy in predicting the occurrence time of an earthquake is determined as well.
