Relationship between the earthquake sequences of Tangshan and Xingtai and the three dimensional velocity structure──Discussion on predicting strong earthquakes of swarm-type

In this paper, based on the results of tomographic image of Tangshan and Xingtai areas, the relations between thecharacteristics of the two strong earthquake sequences and their three-dimensional velocity structures are studied.The research results indicate that:① Mosaic distribution of low-velocity bodies and high-velocity bodies, especially the existence of high-velocity bodies with large size in crust are the common basis of development of thetwo earthquake sequences. ② Scale, depth, and heterogeneity of high-velocity and low-velocity bodies are theimportant factors to effect the characteristic of earthquake sequences. ③ The depth of the high-velocity body inTangshan area is less than that in Xingtai area, which is the principal reason why the dominant focal depth and thebiggest focal depth of Tangshan earthquake sequence are less than Xingtai's. ④ The depth of the high-velocitybodies in Ninghe area is more than that in Tangshan-Luanxian area, which lead to the biggest magnitude and epicentral intensity are lower. These results could be helpful for predicting the main shock of strong swarm-typeearthquakes and later strong aftershocks.

Dynamic subduction process of local plate revealed by Ibaraki earthquake sequence of 1982 in Japan

The kinematics and dynamics of plate tectonics are frontal subjects in geosciences and the strong earthquake occurred along the plate boundary result directly from plate movement. By analyzing Ibaraki earthquake sequence, it has been found that the focal fault plane shows a special image of grading expansion along the direction of strike and adjustment along the dip direction respectively. With the consideration of strike, dip and slip directions of focal mechanism, we have confirmed that Ibaraki earthquake belongs to a thrust fault earthquake occurred under the Japan Trench. The cause of the earthquake sequence is discussed in the paper. The study on the temporal-spatial distribution of the earthquake sequence with a time-scale between the year-scale spatial geodetic data and the second-scale moment tensor of the strong earthquake has indicated the dynamic process of Pacific Plate subduction under the Eurasia Plate. According to the average slip distance of earthquake and the velocity of plate movement, it is predicted that a strong earthquake might occur in recent years.

Spatiotemporal evolution of the 2021 M_(S)6.4 Yangbi earthquake sequence

Based on the seismic phase reports of the Yangbi area from January 1 to June 25,2021,and the waveform data of M≥4 earthquakes,we obtained the relocation results and focal mechanism solutions of the M_(S)6.4 Yangbi earthquake sequence using the HypoDD and CAP methods.Based on our results,our main conclusions are as follows:(1)the M_(S)6.4 Yangbi earthquake sequence is a typical foreshock-mainshock-aftershock sequence.The fore-shocks of the first two stages have the obvious fronts of migration and their migration rate increased gradually.There was no apparent front of migration during the third stage,and the occurrence of the mainshock was related to stress triggering from a M5.3 foreshock.We tentatively speculate that the rupture pattern of the Yangbi earthquake sequence conforms to the cascading-rupture model;and(2)the main fault of the M_(S)6.4 Yangbi earthquake sequence is a NW-trending right-lateral strike-slip fault.As time progressed,a minor conjugate aftershock belt formed at the northwest end of this fault,and a dendritic branching structure emerged in the southern fault segment,showing a complex seismogenic fault structure.We suggested that the fault of the Yangbi earthquake sequence may be a young sub-fault of the Weixi-Weishan fault.

The Characteristic Analysis and Seismic Triggering Study of the M6.2 and M6.1 Dayao Earthquake Sequences in 2003

The high-resolution hypocenter locations of the mainshocks on July 21 （M6.2） and October 16, 2003 （M6.1） and their aftershock sequences are determined in Dayao, Yunnan by using a double-difference earthquake location algorithm. The results show that the epicenters of the two mainshocks are very close to each other and the distribution of the aftershock sequence appears to be very linear. The distribution of the earthquake sequence is very consistent with the focal mechanism, and both mainshocks are of nearly vertical right-lateral fault. Unlike most other double earthquakes in the Yunmm area, the aftershock distribution of the M6.2 and M6.1 Dayao earthquakes does not appear to be a conjugated distribution but to be in a line, and there are some stacks in the two earthquake sequences. It can be inferred that they are all controlled by the same fault. The distribution of aftershocks is asymmetrical with respect to the mainshock location and appears to be unilateral. The aftershocks of the M6.2 mainshock centralize in the northwest of M6.2 earthquake and the aftershocks of the M6.1 earthquake are in the southeast of the mainshock, moreover, the M6.1 earthquake appears to be another rupture on the southeastern extensiou of the same fault as the M6.2 earthquake. The results of Coulomb failure static stress changes △σf show that the earthquake on July 21 （M6.2） apparently triggered the earthquake on October 16 （M6.1）, the two mainshocks have stress triggering to their off-fault aftershocks to different extents, and the M6.5 earthquake that occurred in Yao＇an in 2000 also triggered the occurrence of the two Dayao earthquakes.

Earthquake-generating Structure Revealed by 1999 Sichuan Mianzhu Earthquake Sequence with M_S=5.0

Through the accurate determination of hypocenter location, the measurement of the main focal mechanism solutions and the analyses of time dependent processes for Sichuan Mianzhu earthquake sequence with M S=5 0 occurred along the middle segment of Longmenshan fault zone on November 30, 1999, the distribution of focal depth section, the stress release and seismicity features are given to reveal the earthquake-generating structure of the earthquake sequence. The obtained results show the activity features for this sequence as follows: ① There is no obvious foreshock, the accumulated strain energy releases in nearly pulse way, fluctuation of the seismic activity level is not quite evident during the duration of the sequence, and the statistical relation between the large events and the small events is less compatible with the traditional G-R estimation; ② The epicenters of the earthquake sequence are not distributed on the main Longmenshan piedmont faults emerging out ground, but near the hidden Longmenshan piedmont faults. The direction of epicenter distribution is not very coincident with the fault strikes. The microscopic epicenters are relatively far from the macroscopic epicenters. The focal depths range from 5km to 16km; ③ The fractures of focal fault plane with the NE strike appear out the strike-slip displacement with a few overthrust components under the major principal compressive stress of NNE direction. From these, we consider it should be paid more attentions to the underground hidden faults near the ground fault zone on a large scale for their generating earthquake risk.

Study on Focal Mechanism Changing Characteristics of the 2013 Zogang-Markam Ms6. 1 Earthquake Sequence

The M_S6. 1 earthquake was a foreshock-mainshock-aftershock type which occurred in the boundary region between Zogang and Markam counties on August 12,2013. Within 9hours before the main shock seven earthquakes of greater than M_L2. 0 occurred,with a maximum of M_L4. 7. In this paper,the earthquake focal mechanism changing process of the Zogang-Markam M_S6. 1 earthquake sequence is studied by calculating the correlation coefficient of body wave spectral amplitudes,and the result shows that the correlation coefficients of spectral amplitude of foreshocks present high value fluctuation with an average value of 0. 86,which shows that the focal mechanism of foreshocks are similar;and the correlation coefficients of spectral amplitude of aftershocks present low value,which shows that the possibility of a large earthquake is not high after a time.

Variation of stress during the rupture process of the 1995 M_L=4.1 Shacheng, Hebei, China, earthquake sequence

According to the rupture dynamics of earthquakes, variations of the apparent stress and the difference between the static stress drop and the dynamic stress drop during the rupture of earthquakes are analyzed for the July 20, 1995 ML=4.1 Shacheng, Hebei, China, earthquake sequence. Results obtained show that the apparent stress for main-shock is about 5 MPa, and the average apparent stress for aftershocks 0.047 MPa. During the rupture of the main-shock, the dynamic stress drop is approximately 1.6 times greater than the static stress drop with the difference of nearly 2.7 MPa. The dynamic stress drop is less than the static stress drop for all aftershocks with the average difference of -0.75 MPa. Therefore, when the mainshock occurs the final stress on the focal fault is higher than the dynamic frictional stress, corresponding to that the fault is abruptly locked. When the aftershocks occur the final stress on the focal fault is lower than the dynamic frictional stress, corresponding to that the fault overshoots. It can be seen from the above results that there could be some differences in the physic processes between the mainshock and the aftershocks.

A study on the focal parameters of Shidian M=5.9 earthquake sequences in 2001

On the basis of about 300 earthquake wave forms observed in the Shidian MS=5.9 sequences on April 12, 2001 recorded in Kunming Digital Seismic Network, the spectra of shear wave have been used to estimate the focal parameters of these earthquake sequences. The results show that within the magnitude range of 1.5~5.3, the seis-mic moments are 1010~1016 N?m, the corner frequencies are 0.2~0.8 Hz, radii of the focal rupture are 200~2 500 m and the stress drops are 0.1×105~20×105Pa. Through the statistical analyses of variation of corner frequency fc and stress drop ?σ with time, it is discovered that the average corner frequency of the foreshock sequences is obviously lower than that of the aftershock sequences. Contrarily, the average stress drops ?σ of the foreshock sequences are clearly higher than that of the aftershocks. It is considered that these variation characteristics of av-erage corner frequency and stress drops before and after the main shock have index significance to the precursory information before a strong earthquake. The higher stress drops for the foreshock sequences show that the higher shear stresses have been stored in the area of main shock. After the main shock, most of the stresses have been released, so the aftershock sequences show a rupture process of lower stresses.

Characteristics of earthquake sequences in the Kuril Islands

A total of 6 earthquake sequences that occurred in the Kuril Islands region in 1964-1976 has been collected and their characteristics have been analyzed. When these sequences are judged from the spatial relation between their source distribution and the subduction zone, all of them belong to plate boundary earthquake sequences. The parameters of 5 ones among these sequences, which are located just in the subduction zone, are in fair agreement. For these 5 sequences, the major axis of epicenter distribution area is longer; the ratio of major axis to minor axis is high; the focal depths have a greater range; the directions of dip are consistent with the direction of subduction; the dip angles are of moderate value. It is estimated that the only exception is related to the low accuracy of data in the early stage.

The fracture characteristic of the 1989 Datong-Yanggao earthquake sequence

This paper, first, analyzed the seismic activity patterns of the Datong-Yanggao earthquake sequence by 4 stages,finding that three times earthquakes of Mis were located in a same NNE seismically active zone and two times located in 2 NWW seismically active zones. Then the focal-mechanism solutions of 12 earthquakes of M≥4 and 114 earthquakes of M≥1. 0 in various zones were obtained, a tectonic analysis was made for these data, the obtained fracture characteristic of this sequence is: the conjugate fracture combination with the NNE right lateral faults as its trunk and the two NWW left lateral faults as its branches. Finally, it is pointed out that there is an obvious difference between the seismic fracture and the tensile dip-slip tectonic activity of the main body of the seismic region.

The environment shear stress field for the 1976 Tangshan earthquake sequence

The environment shear stress of Tangshan main earthquake and 38 great aftershocks have been calculated by the acceleration data of Tangshan earthquake sequence. The environment shear stress for 52 smaller aftershocks from July of 1982 to July of 1984 have also been calculated by use of the digital data of the Sino-American cooperation recorded by the instrumental arrays in Tangshan. The results represent that the environment shear stress τ0 values have a weak dependence on the seismic moment, only the small and moderate earthquakes will be able to occur in the region with smaller τ0 value and the large earthquakes are only in the region with greater τ0 value. The peak acceleration, velocity and displacement will be larger for the earthquakes occurred in the region with greater τ0 value, Therefore, the measurement of environment shear stress τ0 value for the significant region will play an important role in earthquske prediction and engineering shock-proof. The environment shear stress values for the great aftershocks occurred in the two ends of the main fault are often higher than that for the main shock. This case may represent the stress concentration in the two ends of the fault. This phenomenon provides the references for the place where the great aftershock will occur.

Fracture Characters and Seismogenic Fault of the Yajiang Earthquake Sequence with M_S6.0 in Sichuan in 2001

The Yajiang earthquake sequence in 2001, with the major events of M S 5.1 on Feb. 14 and of M S 6.0 on Feb.23, are significant events in the Sichuan region during the last 13 years. Eighty-eight earthquakes in the sequence with at least 5 distinct onset parameters for each recorded by the Sichuan Seismic Network in the period of Jan. 1 through June 30, 2001 were chosen for this study. The events are relocated and the focal mechanism is derived from P-wave onsets for 13 events with relatively larger magnitudes. The focal depth of all earthquakes fall between a range of 2km to 16km, with dominant distribution between 9km to 11km. The foreshocks, the M S5.1 earthquake and the M S6.0 earthquake and their aftershocks are all located close to the Zihe fault and the dominant epicentral distribution is in NW direction, identical to that of the fault. The fracture surface of the focal mechanism is determined in accordance to the mass transfer orientation in the recent earth deformation field in the Yajiang region. The P axes of the principal compressive stress in focal mechanism solutions of the 13 events show bigger vertical components, and the horizontal projection trending SE. The earthquakes are of left-lateral, strike-slip normal, and normal strike-slip types. The rupture surface of most earthquakes strike NW-SE, dipping SW. Based on the above information, we conclude that the Zihe fault that crosses the earthquake area, striking NW and dipping SW, is the seismogenic fault for the Yajiang earthquake sequence.

Accurate Location of the Yao'an Earthquake Sequence and the Yongsheng Earthquake Sequence

The Yao’an M_S6.5 earthquake occurred on Jan. 15, 2000 and the Yongsheng M_S6.0 earthquake occurred on Oct. 27, 2001 in Yunnan Province, China. They are both located in the middle of the Dian block. Their epicenters are close to each other, the tectonic and strain characters of the earthquakes were similar, and there were many aftershocks after the two main shocks. In order to further study the spatial-temporal distributions and fault rupture characters of the main shocks and aftershocks, the latter are located using the Geiger earthquake location algorithm (Geiger) and the double difference earthquake location algorithm (DD) based on the seismic phase data of the two earthquake sequences. They were recorded by two Near Source Digital Seismic Networks (YNSSN and YSNSSN) deployed by the Yunnan Seismological Bureau (YNSB). Then, two main shock parameters were relocated using DD based on the data of larger magnitude aftershocks and the two main shocks that were recorded by the Kunming Regional Digital Seismic Network (KMSN). Combining the spatial-temporal distributions of the two earthquake sequences, the tectonic and strain characters of earthquakes, the rupture processes of the two aftershock sequences along faults are analyzed and discussed contrastively.

Analysis on Double-difference Earthquake Location and the Seismicity Pattern of the Yangjiang Earthquake Sequences

The locations of about 400 earthquakes in Yangjiang, Guangdong Province are determined using the double, difference earthquake location algorithm （DDA）. The seismicity pattern becomes concentrated from discrete grids. The rupture characteristics of the Yangjiang earthquake sequence show a conjugated distribution in NW and NE directions. The major distribution trends NE and dips NE with an angle of 30^o and a length of 30km,and the minor distribution trends NW and dips SE with an angle of 30^o and a length of 20km. The focal depth is 5km - 15km. The distribution of the Enping earthquake sequence,which is not far from Yangjiang,is NW-trending. The relationship between hypocenter distribution and geological structure is discussed.

The Precursory Significance of Cumulative Slip of Repeating Earthquake Sequences Prior to Moderately Strong Earthquakes A Case Study of Four Remarkable Earthquake Sequences of Haicheng- Xiuyan

The Haicheng-Xiuyan region is an earthquake-prone area in Liaoning Province where earthquake sequences frequently occur and is regarded as the regional seismic window. In this area we found many earthquake events with the highest waveform similarity in the records of the same station from some remarkable seismic sequences,namely repeating earthquake sequences. In principle,rupture areas of the repeating events overlap with each other and are most closely located. Therefore these events may reflect the seismic process near the earthquake fault. In this paper, we identified four remarkable earthquake sequences of Haicheng-Xiuyan by waveform cross-correlation. The result shows that the cumulative slip of repeating earthquakes is related to moderately strong earthquakes,among which the Xiuyan M_S5. 4 foreshock sequence has the strongest and most apparent pre-shock accelerating-like slip behavior.

Apparent Stress Changes of the 2010 Luanxian Earthquake Sequence

On March 6,2010,an earthquake of M L4. 5 took place in Luanxian,Hebei Province,with plenty of foreshocks and aftershocks. From December 2009 to March 2010,a series of M L≥ 2. 5 earthquakes were recorded by the digital seismic network of the capital region,which were selected to calculate the apparent stress in this region. The results show that,firstly,a high value anomaly of apparent stress appeared before the M L4. 5 and peak value appeared on the main shock, which then decreased after the ML4. 5 earthquake. The apparent stress of the main shock is much greater than that of most aftershocks,the sequence type is considered as a main shock-aftershock. Secondly,the size of apparent stress perfectly reflects the state of the stress field in the hypocenter region,and we can discuss seismic sequence properties through the changing process of apparent stress,in combination with the traditional methods to identify a sequence more accurately. Finally,in the case of magnitude less than or equal to M L3. 3,correlation between magnitude and apparent stress is positive.

A Shear-wave Splitting Study of the Yingjiang M_S6.1 Earthquake Sequence Occurring on May 30,2014

Earthquakes of M_S5. 6 and M_S6.1 occurred in Yingjiang,Yunnan on May 24 and May 30,2014 respectively. In this paper,we use the waveform data recorded by mobile seismic stations( KAC) which were set up in the source area after the Yingjiang MS5. 6earthquake on May 24,2014 to study the shear-wave splitting characteristics of Yingjiang M_S6.1 earthquake sequence with the SAM method. The result shows that predominant polarization of fast shear-waves before the M_S6.1 earthquake is consistent with the direction of regional principal compressive stress,and predominant polarization of fast shear-waves before the M_S6.1 earthquake show better consistency and smaller dispersion compared to after the M_S6.1 earthquake,and there may be a deflection for the fast shearwaves predominant polarization between the M_S6.1 earthquake sequence and foreshock sequence. We found that the time-delays generally exhibited a lower level before the M_S6.1earthquake and a relatively higher value after the M_S6.1 earthquake.

Research on Stress Drops and the Focal Mechanisms of the Xinyuan- Hejing ML6.8 Earthquake Sequences

Based on the digital waveform data recorded by Xinjiang Digital Seismic Network for the Xinyuan-Hejing M_L6.8 earthquake sequences of June 30,2012,this paper analyzes the stress drops of earthquake sequences and the correlation coefficients of focal mechanisms significant for strong aftershocks.Firstly,the source parameters of the Xinyuan-Hejing M_L6.8 earthquake sequences are obtained by applying the spectrum analysis and the Brunes source model.Then,the correlation coefficients of spectral amplitudes are calculated using the low-frequency spectral amplitude recorded by the same station for the different events.Finally,based on the results of the correlation coefficients of spectral amplitudes,the events with similar focal mechanisms are grouped using the clustering method.The results show that:(1)The stress drop values show a steady trend in the aftershock sequence calm period and the stress drop values show a rise-fall in strong aftershocks.(2)The moving average correlation coefficient of amplitude spectrum begins to spread after the main shock.It shows that the correlation decreases between the main shock and the aftershocks in mechanisms.(3)The results of focal mechanism groups show that the earthquake sequences are mainly strike slips.The stress distribution of the main pressure axis is nearly NS,which is the same as the structural stress field.(4)The magnitude and mechanism show that there is an agreement before the strong aftershock,which shows that the regional stress field is enhanced.

Study on the Cause of Complex Spatial Distribution of the Tangshan Earthquake Sequence

By analyzing higher-accuracy location data of the Tangshan earthquake sequence,a clear distribution pattern of three aftershock belts in the NE,NWW,and NW directions of has been obtained.The analysis reveals three rupture planes of strong events of MS7.8,MS7.1 and MS6.9 in the sequence.It indicates that the complex pattern is closely related to the earthquake source,and the NE-,NWW-and NW-trending regional fault zones,which have been revealed by the research of the pre-seismicity anomaly.In summary,the source is located in the junction of the three fault zones,and the rupture planes of the three strong events located in the source can be regarded as the locked segments on the three fault zones.On these grounds,the paper explains the complexity of the source and epicentral distribution of aftershocks.