A significant seismic swarm occurred in Yigong, Bomi, Tibet, in July and August of 2020. 25 earthquakes with M_L≥4.0 occurred during about 30 days and the magnitude of the maximum earthquake reached M_L4.9(hereinafter referred to as the Bomi swarm). The proportion of large to small earthquakes in Bomi swarm is unbalanced, the number of earthquakes with larger magnitude is somewhat higher, and the proportionality coefficient, b value, of Gutenberg-Richter relationship is about 0.3, obviously smaller than the average b value of 1.0 of the whole seismic sequence. The seismicity of Bomi swarm has two dense stages, one is from July 19 to August 1 and another is from August 8 to 18, few earthquakes occurred between these two stages. For spatial distribution of earthquakes, the main areas of earthquake distribution in these two stages are almost overlapped. However, comparing with the previous stage, the southern boundary of the dense distribution of earthquakes in the latter stage has an extending trend to SE direction. The focal mechanism and the centroid depths of 20 earthquakes with M_L≥4.0 have been calculated by CAP method. Results show that the centroid depths are shallow, most of them are distributed in the range of 3~4km. Viewing from the focal mechanism, taken July 27, 28 as the time boundary, the focal mechanisms before that time are mainly thrust with strike-slip component, the strike directions of nodal planes are inconsistent. After that time, the focal mechanism shows a good consistency with near EW-trending tensile rupture.

The retroactive statistical results on historical earthquake catalogue have shown that earthquakes in Bomi region mostly occurred during July and August, indicating the obvious seasonal characteristics, and earthquakes mainly concentrated in a very small area(about 15km×20km)in space. The magnitude of maximum earthquake in each year is generally stable in the range of M_L4.5~5.0, the annual average seismic energy release is roughly equivalent to one earthquake with M_L4.9. It should be pointed out that swarms or significant earthquakes do not occur every year. During a total of 51 years from 1970 to 2020, significant swarms or earthquakes with M_L≥4.0 occurred only in 18 years, accounting for about 35% of total time period.

The correlation between seasonal meteorological factors and the seismicity in Bomi region is studied in this paper and the results show that there is a close but very complex relationship between them. Generally, the seismicity in Bomi region is closely related to the rainfall intensity and precipitation process in the first half of the year. The swarms mainly occurred during the periods with the peak precipitation, and generally followed the end of the first significant precipitation process in the year. The contrastive analysis shows that the strength of the seismicity is qualitatively proportional to the starting time of precipitation above designated scale, the days of precipitation above designated scale during the first half year, as well as the increasing rate of precipitation from April to June. Specificly, the earlier the starting time of precipitation above designated scale, the more the number of days with precipitation above designated scale in the first half of the year, the longer the time interval from the starting of the precipitation above designated scale to the seismicity, the higher the increasing rate of the monthly average precipitation from April to June, and the more the expected rainfall in June, the higher the seismicity level of this year will be.

Bomi swarm is located to the north of Jiali fault zone and obviously off the Jiali fault zone. The seismicity in Bomi region is not the result of the fault activity of the Jiali fault zone, nor is related to the aftershock activity of Milin M6.9 earthquake in 2017, which occurred about 44km south of Jiali fault zone, since there is no obvious tectonic correlation among of them. Viewing from the geographical terrain, the seismicity in Bomi region mainly concentrated in the middle part of the NE-trending Lequ Zangbo River and its branches on both sides. Due to the lower terrain, it becomes an area for fast convergence of water from surrounding regions in the summer, which provides the basic conditions for fluid-triggered earthquakes in July and August every year. The lithology in the earthquake densely distributed area is mainly quartz sandstone and siltstone with relatively higher permeability, which is convenient for fluid penetration and leads to the pore pressure increasing in shallow crustal medium, thus, is liable to trigger seismicity. The local area with dense earthquake distribution in Bomi region is truncated and confined by several faults. The faults may act as a “water-retaining wall”, which has a certain confining effect on water infiltration and diffusion. On the other side, the faults, especially for normal faults, have better fluid conductivity, which is convenient for fluid infiltrating rapidly. Under the action of both the gravity and load pressure of the surface water, the fluid infiltrates rapidly along the fracture zone and the sandstone-like rock medium with good permeability, resulting in the rapid increase of the pore pressure in the underground cracks, faults and porous medium, therefore leading to the decrease of the strength for faults or cracks, and consequently triggering the seismicity. Considering the contribution of accumulated precipitation, groundwater level change, as well as warming and snowmelt to surface water level uplift in the first half of the year, the temporal variation of pore pressure at different depths are simulated by the numerical methods under the simplified conditions. The simulation results support the mechanism explanation on seismicity in Bomi region proposed in the paper.

In order to realize the rapid and efficient identification of earthquakes, blasting and collapse events, this paper applies the Convolutional Neural Network(CNN)in deep learning technology to design a deep learning training module based on single station waveform recording of single event and a real-time test module based on multiple stations waveform recording of single event.
On the basis of ensuring that the data is comprehensive, objective and original, the three-component waveforms of the first five stations that recorded the P-wave arrival time of each event are input, and the current mainstream convolutional neural network structures are used for learning test. The four main convolutional neural network structures of AlexNet, VGG16, VGG19 and GoogLeNet are used for learning training, and the learning effects of different network structures are compared and analyzed. The results show that in the training process of various convolutional neural network structures, the accuracy rate and the cost function curve of the training set and the test set of each network are basically the same. The accuracy rate increases gradually with the increase of the training times and exceeds 90%, and finally stabilizes around a certain value. The cost function curve decreases rapidly with the increase of the training times, and eventually the stability does not change near a relatively small value. At the same time, over-fitting occurred in all convolutional neural network structures during training, except for AlexNet. In the end, the cost function of each type of structural training set and test set is finally lower than 0.194, and the recognition accuracy of each type of structure for training sets and test sets is over 93%. Among them, the recognition accuracy of AlexNet network structure is the highest, the accuracy of the training set of AlexNet network structure is as high as 100%, the test set is 98.51%, and no overfitting occurred; the accuracy of VGG16 and VGG19 network structure comes second, and the recognition accuracy of GoogLeNet network structure is relatively low, and the trend curves of the accuracy and cost function in training and test set of each network in the training process are basically the same. Subsequently, in order to test the event discrimination efficiency of the CNN in deep learning in the real-time operation of the digital seismic network, we select the trained AlexNet convolutional neural network to perform event type determination test based on the waveform recording of multiple stations of a single event. The final result shows that the types of a total of 89 events are accurately identified in the 110 events with M ≥0.7 recorded by Shandong seismic network, and the accuracy rate is about 80.9%. Among them, the accuracy rate of natural earthquake is about 74.6%, that of explosion is about 90.9%, and that of collapse is 100%. The recognition accuracy of collapse and explosion events is relatively high, and it basically reaches or exceeds the recognition accuracy of manual determination in the daily work of the seismic network. The accuracy of natural earthquake identification is relatively low. Among the 18 misidentified natural earthquakes, up to 13 events were judged as blasting or difficult to identify due to distortion of waveforms recorded by some stations(They are determined to be explosion and earthquake each by the records of two of the five stations). If sloughing off the recognition type error events caused by waveform distortion due to the background noise interference that overwhelms the real event waveform or waveform drift, the recognition accuracy of earthquake will become 91.4%, and the recognition accuracy of all events will increase from 80.9%to 91.7%, which is basically equivalent to the recognition accuracy of manual judgment in the daily work of the seismic network. This indicates that deep learning can quickly and efficiently realize the type identification of earthquake, blasting and collapse events.

On January 19, 2020, an M_S6.4 earthquake occurred in Jiashi county. This earthquake located in the intersection of the three tectonic systems of South Tianshan, Tarim Basin and West Kunlun-Pamir. From 1997~2003 a group of strong earthquake swarms with M_S≥6.0 occurred in this area, which constitute an extremely rare Jiashi strong earthquake swarm in mainland China. Based on the digital waveforms of Xinjiang Seismic Network, the best double-couple focal mechanisms of the main shock, foreshock and some aftershocks with M_S≥3.6 were determined by CAP method, the Jiashi M_S6.4 earthquake sequence was relocated by multi-step locating method. We analyzed the characteristics of focal depth, focal mechanisms and source rupture to determine the seismogenic structure. The nodal plane parameters of the best double-couple focal mechanism by CAP method are: strike 190°, dip 32° and rake 31° for nodal plane Ⅰ, and strike 74°, dip 73° and rake 118° for nodal plane Ⅱ; the centroid depth is 12.1km. The focal mechanism of main shock is thrust type, but the M_S5.4 foreshock is a strike-slip event with a focal depth of 17.1km, and the focal mechanism parameters are: strike 83°, dip 78°, rake 173° for nodal plane I and strike 174°, dip 83°, rake 12° for nodal plane Ⅱ. The foreshock and mainshock are very close in space, but the rupture types are quite different, which shows the complexity of the seismogenic structure. The relocated sequence shows two dominant distribution directions, namely, the near EW direction and the near SN direction. Most of the aftershocks in the sequence are distributed in the EW direction, parallel to the strike of the Kepingtage nappe structure. The M_S5.4 foreshock and the M_S6.4 mainshock are both located near the dominant distribution in the near NS direction, and have a certain spatial distance from the distribution of aftershocks in the near EW direction. This feature may reflect that the mainshock and subsequent aftershocks are located on different fault zones. Combined with the geological structural background near the source area, it is inferred that the seismogenic structure of the M_S5.4 foreshock is a strike-slip fault L₀ with a high dip angle in NNW direction, and the basic information of the seismogenic fault L₀ may be: strike NNW(about 175°), the fault plane is nearly upright, and the fault depth can reach about 15km. L₀ may be a branch fault of the NNW-directed seismogenic structural system of the Jiashi earthquake swarm from 1997 to 1998. Since most of the aftershocks distributed on the east side of the Fault L₀, we judge that L₀ and related faults may have a certain control effect on the distribution of aftershocks. According to the location of the main shock, the spatial distribution of aftershocks and the occurrence characteristics of the fault in the source area, it is inferred that the seismogenic structure of the Jiashi M_S6.4 mainshock is a NS-directed gentle-dipping fracture. The main shock caused the simultaneous activity of the Kepingtage nappe structure, resulting in a dense distribution of aftershocks with a certain distance from itself.

After a large earthquake, more seismic activities are observed in the focal region and its adjacent area. The obvious increased earthquakes are called the aftershocks. Generally speaking, aftershock sequence gradually weakens and sometimes has ups and downs. The time when the aftershock activity begins to be confused with background seismic activity is known as the aftershock activity duration. Aftershock sequence is one of the enduring research fields in seismology. Aftershocks accord with two important statistical relationships, one is the G-R relationship describing the relation between the magnitude and frequency, the other is the modified Omori formula describing the characteristics of aftershock decay with time. On this basis, a number of studies from different angles explain the mechanism of aftershock activity. From the perspective of the medium heterogeneity, it is universally accepted that aftershock is a result of further rupture of residual asperities. From the perspective of stress, these models, e.g. rate-state dependence, subcritical crack growth, creep or afterslip and so on, think that the fault stress change caused by mainshock is the main cause for aftershock. But other researchers, by studying real aftershock observations, think that the fault stress change caused by mainshock is not the main cause or has very weak control over the aftershocks. Pore pressure diffusion caused by mainshock fault slip is also considered as an important incentive for aftershocks. There is a relationship between the frequency of aftershocks and pore pressure changes. Dry rock pressurized in physical experiment can produce acoustic emission sequence similar to mainshock-aftershock sequence type earthquake. Though fluid plays an important role in aftershock activities, it is not the essential element for aftershock. Overall, there is no single model which can fully explain the phenomenon of aftershock activity.
Assuming the rupture of the residual asperities inside the mainshock rupture plane randomly leads to the aftershocks, the size of the residual asperities conforms to fractal distribution, and the rupture or instability strength of the residual asperities accords with the lognormal distribution. Taking the postseismic stress relaxation as the mechanical load, the loading stress attenuates according to negative exponential law. Taking the Coulomb failure as the judgment criterion of the instability, combining the mechanical interactions among the residual asperities, the artificial aftershock sequence, including occurring time, location and magnitude, is simulated under different conditions. The agreement between output and the actual statistical characteristics of aftershock activities is detected by G-R relationship and modified Omori formula as a basis for further adjustments to the model parameters. On this basis, the influences of the medium viscosity properties on aftershock activities have been discussed.
The results show that viscosity coefficient of rheological properties of the lower part of the lithosphere has an important effect on the duration of aftershock activity. The viscosity coefficient of the lower part of the lithosphere controls the duration of the aftershock activity, the lower the viscosity coefficient, the sooner the stress relaxation of the lower lithosphere, and the faster the loading rate to the upper part of the lithosphere, the shorter the duration of the aftershock activity. On the contrary, the higher the viscosity coefficient, the slower the loading rate to the upper part of the lithosphere, and the longer the duration of the aftershock activity. This simulation conclusion is consistent with the observed result. The viscosity coefficient as one of the important lithosphere physical parameters controls the decay rate of aftershock activity. Under this model conditions, p value, the decay rate of modified Omori law, changes with the viscosity coefficients in a negative exponential function. The relationship that the viscosity coefficient is lower and the decay of aftershock sequence is faster provides a reference for the study of the main influence factors of aftershock decay. The relationship corresponds to the observation that the decay rate of the aftershock sequence shows a good positive correlation. The b value of the G-R relationship of aftershock sequence characterizes the ratio relationship of large to small earthquakes. The modeling studies suggest that the G-R relationship of the aftershock sequence is irrelevant with the viscosity coefficient, but mainly controlled by the size distribution of the residual asperities. In another word, it is mostly correlative to the heterogeneity of tectonics and medium.

In order to know to what degree can elastic wave velocity be influenced by effective stress changes in rock medium at the bottom of reservoir, and the quantitative relationship between changes of elastic wave velocity, pore pressure and effective stress during the process of reservoir water body load-unloading and water infiltration, pre-works have been done on the geological structures and hydrogeologic conditions in the Zipingpu reservoir, Sichuan Province, and its adjacent areas in detail, and a mathematical model was deduced which could describe rock-mass distortion and stability influenced by liquid seepage in porous rock media during the process of reservoir water body load-unloading and water infiltration. On this basis the paper makes a further study to establish a quantitative mathematical model to describe the relationship between elastic wave velocity and effective stress, and also taking Zipingpu reservoir as an example, calcutates the dynamic changes of pore pressure, additional elastic effective stress, and elastic wave velocity using finite element method. The simulation results show that: the change in incremental value of elastic wave velocity during the process of reservoir water body load-unloading and water infiltration is not obvious, the maximum amplitude is only about ±0.013 km/s, and changes mainly concentrate in local areas above 5km depth under the reservoir bottom. The change in incremental value of elastic wave velocity is associated with changes of additional effective stress, similar to reservoir water-level fluctuation in curve shapes. Incremental value of elastic wave is related to location, depth, and additional effective stress of different observation points. The velocity change of P-wave is greater than S-wave at the same observation point. The time sequence changes of elastic wave velocity obtained by this paper are generally similar to the relative velocity variations of the Zipingpu reservoir region obtained by other scholars using seismic ambient noise method, but the variation range is slightly different.

Focusing on decay and generation of the aftershock activity,the latest theoretical advancements and applications on the modified Omori law,the ETAS model and the BASS model have been introduced in this paper. Up to now,the modified Omori law is the best formula for fitting the temporal decay of aftershock activity,which has been used to describe the decay characteristics of aftershock sequences quantitatively. Essentially,the modified Omori law is a typical statistical model on observing phenomena,but it has been used widely in practice owing to its less parameters,simple calculation,and higher capability of describing the general features of sequence decay. ETAS model,which takes into account the generation of the high order aftershocks with statistical self-similar characteristics,is a physical expansion of the modified Omori law,and the generation intensity of high order aftershocks is concerned to be related to the magnitude of the father earthquake. Since considering the problems of high order aftershock generation,ETAS model not only has a large improvement in study field of aftershock sequences,but also has many other applications,such as detection of relative quiescence,removing of the cluster aftershock activity,assessment of background seismicity,detection of seismic activity triggered by external factors and so on. BASS model follows the modified Bath's law,while the ETAS model follows the similar rate with the magnitude of the father earthquake,this is the largest difference between ETAS model and BASS model. So,the BASS model is a completely self-similar theoretical model compared with the ETAS,but only few papers concerning its application.

Aftershock activity ofMay 12 M8.0 W enchuan Earthquake sequence shows an obvious segmen-ted feature,most of the large aftershocks are d istributed in the south and north parts of aftershock zone.Thrusting is dom inant in the south partwith a small strike slip component.The aftershock activ-ity decays gradually and presents the sequence feature ofmainshock-aftershock pattern.The north part is the end ing area of the mainshock fracture,where the strike slipping is dom inant and shows an obvi-ous swarm feature,it therefore became the major area for large aftershocks.Modulation of earth tide on aftershock activity is remarkable,most of large aftershocks occur during the period of flood and neap tide.The time period around 16:00 is the dom inant occurring time for large aftershocks.Thep-val-ue,a parameter ofmod ified Omori formula,increases gradually with time,and reaches about 1 in the final.Based on the previous study,the sequence patterns,magnitude ofmaximum aftershock,as well as the duration of aftershock activity have been d iscussed.The primary results also show that the mag-nitude d ifference between mainshock and the maximum aftershock is proportional to the rupture size of the mainshock for huge earthquakes of about M8.0.This means that,when the magnitudes of earth-quakes are nearly the same,large rupture size corresponds to the sufficient energy release.

Statistic analysis has been made on the characteristics of the contemporary tectonic stress field of East China based on 143 sets of single focal mechanical solutions of moderate and small earthquakes and on 17 sets of composite focal mechanism solutions. Results show that the stress field of East China exhibits a NNE (about 80?) oriented principal compression and NNW (about 350?) principal tension, and the principal stresses are mainly horizontal or near-horizontal. In the context of basically consistent orientation and action mode of stress field, there are some variations in the stress field among different seismotectonic regions, which are possibly related to the distribution of major active faults in the respective regions. This indicates that fault slip may be dominated or affected by existing tectonics. The feature of earthquake fracture and the orientation of fault slip of contemporary earthquakes in East China have been analyzed with data of focal mechanism solutions of contemporary moderate and small events, and of the orientations of major axis for innermost isoseismal of historical and contemporary events and so on. It indicates that the earthquake fractures of East China extend at large in NE and NW directions, and sometimes with NNE, NEE, NWW, or near EW directions. The movements of earthquake faults are mainly of strike-slip or near strike-slip with oblique-slip component. There exist some regional variations in the movement of earthquake faults. The historical moderate and strong events in East China exhibit mainly a NE trending slip, while the modern events show the NW-SE trending slip on the land and NE and NW trend slip in the sea area.

This paper discusses the mechanism for seismic quiescence before large earthquakes based on experimental results of acoustic emission during rock deformation. Under the condition of biaxial compression and constant loading point velocity,samples simulating discontinuous faults,including compressional en-echelon faults,fault with macro-asperity,and model-Ⅲ shear fault,show relative quiescence of acoustic emission before slip instability,which is characterized by obvious decrease of occurrence rate and release of strain energy of acoustic emission events. The analysis indicates that the predominant deformation mode during this stage is creep sliding along fault zone,especially along the newborn fault segment in the discontinuous area,causing decrease of differential stress in the sample. The reason for occurrence of this stage is that a relatively uniform distribution of resistance along a fault zone is necessary for its slip instability,but uneven surface of newborn fault segment does not meet this condition,so the creep sliding is needed for resistance uniformization along the fault zone. Because there exist only very small asperities along the fault zone and the stress relaxation occurs in the whole sample during this stage,acoustic emission activity becomes weak. Based on the experimental results,it is suggested that the process of creep sliding and resistance-uniformization along fault zone is one possible mechanism for seismic quiescence before large earthquake.