Focal depths and mechanisms of Tohoku-Oki aftershocks from teleseismic P wave modeling

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.

Numerical simulation of dynamic Coulomb stress changes induced by M6.5 earthquake in Wuding, Yunnan and its relationship with aftershocks

Based on the discrete wavenumber method, we calculate the fields of dynamic Coulomb rupture stress changes and static stress changes caused by M6.5 earthquake in Wuding, and study their relationship with the subsequent after- shocks. The results show that the spatial distribution patterns of the positive region of dynamic stress peak value and static stress peak value are similarly asymmetric, which are basically identical with distribution features of aftershock. The dynamic stress peak value and the static stress in the positive region are more than 0.1 MPa and 0.01 MPa of the triggering threshold, respectively, which indicates that the dynamic and static stresses are helpful for the occurrence of aftershock. This suggests that both influences of dynamic and static stresses should be con- sidered other than only either of them when studying aftershock triggering in near field.

Magnitude and distance distribution of strong aftershocks in Sichuan-Yunnan region

研究了川滇地区1966年以来12次主震震级Ms≥6.5的地震序列中，主震与强余震（本文定义为所有Ms≥5的余震）震级差分布特征和强余震与主震距离的分布特征。结果表明，强余震与主震震级差服从截断的指数分布，据此推导出了强余震与主震震级差的概率密度函数；强余震距离分布的优势范围是距主震10-39km，且强余震与主震震中的距离服从正态分布。

Models of wave duration and event fre-quency of explosion aftershocks

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.

Study of the correlations between main shocks and aftershocks and aftershock synthesis method

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.

Spatio-temporal characteristics of strong aftershocks of the M_S8.0 Wenchuan earthquake

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.

Variations in shear wave splitting during aftershocks of the Luquan earthquake in Yunnan Province

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.

Spatio-temporal characteristics of aftershocks and seismogenic structure of the 2011 Mw9.0 Tohoku earthquake,Japan

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.

Slope Seismic Response Monitoring on the Aftershocks of the Wenchuan Earthquake in the Mianzhu Section

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.

Study on displacement field generated by aftershocks in Landers earthquake fault zone and its adjacent areas

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.

High-resolution seismicity imaging and early aftershock migration of the 2023 Kahramanmara?(SE Türkiye)M_W7.9&7.8 earthquake doublet

We build a high-resolution early aftershock catalog for the 2023 SE Türkiye seismic sequence with PALM,a seamless workflow that sequentially performs phase picking,association,location,and matched filter for continuous data.The catalog contains 29,519 well-located events in the two mainshocks rupture region during 2023-02-01–2023-02-28,which significantly improves the detection completeness and relocation precision compared to the public routine catalog.Employing the new PALM catalog,we analyze the structure of the seismogenic fault system.We find that the Eastern Anatolian Fault(EAF)that generated the first M_(W)7.9 mainshock is overall near-vertical,whereas complexities are revealed in a small-scale,such as subparallel subfaults,unmapped branches,and stepovers.The seismicity on EAF is shallow(<15 km)and concentrated in depth distribution,indicating a clear lock-creep transition.In contrast,the SürgüFault(SF)that is responsible for the second M_(W)7.8 mainshock is shovel-shaped for the nucleation segment and has overall low dip angles(~40°–80°).Aftershocks on the SF distribute in a broad range of depth,extending down to~35 km.We also analyze the temporal behavior of seismicity,discovering no immediate foreshocks within~5 days preceding the first mainshock,and no seismic activity on the SF before the second mainshock.

Depth and region dependence of b-value for micro-aftershocks of the May 12th,2008 Wenchuan earthquake and its tectonic implications

Micro-aftershocks with magnitude range of 1.5--4 around the Wenchuan earthquake epicenter, the southern part of the Longmenshan fault zone, exhibit good frequency-magnitude linear relationships, thus enabling b-value analysis. The average b-value for micro-aflershocks of M1.5-4 from July to December of 2008 in our local study region is about 0.88, similar to the b-value for all aftershocks ofM3.0-5.5 from May, 2008 to May, 2009 along the whole Longmenshan fault zone. The similarity between the local and regional b-values possibly indicates that the southern part of the Longmenshan fault zone has similar seismogenic environment to the whole Longmenshan fault zone. Alternatively, it may also imply that b-values derived from all events without consideration of structural variation can not discriminate local-scale tectonic information. The present study shows that the b-value for the Wenchuan earthquake micro-aftershocks varies with different regions. The b-value in southwest of the Yingxiu town is higher than that in the northeast of the Yingxiu town. The high b-value in the southwest part where the Wenchuan earthquake main shock hypocenter located indicates that the current stress around the hypocenter region is much lower than its surrounding area. The b-values are also dependent on depth. At shallow depths of 〈5 km, the b-values are very small （-0.4）, possibly being related to strong wave attenuation or strong heterogeneity in shallow layers with high content of porosity and fractures. At depths of-5-11 km, where most aftershocks concentrated, the b-values become as high as -0.9-1.0. At the depth below -11 km, the b-values decrease with the depth increasing, being consistent with increasing tectonic homogeneity and increasing stress with depth.

Effect of Aftershocks on Assessment of Seismic Hazard for Hebei Seismic Zone,China

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.

Seismic response of cracking features in Jubao Mountain during the aftershocks of Jiuzhaigou Ms7.0 earthquake

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.

Models of wave duration and event frequency of explosion aftershocks

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.

Comparison of the spatial distribution of ground motion between main shocks and strong aftershocks

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.

Large aftershocks triggering by Coulomb failure stress following the 2001 MS=8.1 great Kunlun earthquake

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.

Method discussion for quick response grey prediction of stronger aftershocks of an earthquake sequence

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.

The relationships between earthquakes and positions of the sun and moon（Ⅱ）——Sometemporalcharacteristicsoftheaftershocksequencesofstrongearthquakes

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.

Some Foreshocks and Aftershocks Associated with the Moiyabana Earthquake of 2017 in Botswana

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.