Study on the Moment Magnitude of Small and Moderate Earthquakes Located in the Inner Mongolia Region

Based on digital seismic waveform data from Inner Mongolia Digital Seismic Network, the source spectrum parameters of 182 small and moderate earthquakes from January, 2009 to September, 2016 are derived, and the seismic moment M₀ and moment magnitude M_W of the earthquakes are calculated. The M_L-M_W relationship and the relationship between stress drop and magnitude are obtained using the linear regression method. It is clear that incorporating the moment magnitude into the seismic quick report catalog and the official earthquake catalog can enrich earthquake observation report content, thus providing better service for earthquake emergency and earthquake scientific research.     

The Study on Seismicity and Relocation of the July 3,2015 Pishan MS6.5 Earthquake

The July 3, 2015 Pishan M_S6.5 earthquake occurred in the intersection area of the Tarim block and West Kunlun block where the moderate-strong earthquakes have become active in recent years. This paper has studied the seismicity parameters of the earthquake sequences such as the b-value in the Pishan region and its vicinity. In addition, we also relocated the aftershocks of the Pishan M_S6.5 earthquake using the seismic phase report by the double-difference method. The temporal and spatial variation characteristics of the Pishan earthquake sequence in the rupture zone are analyzed. The study is of great significance in the seismic hazard assessment in this region.     

A Preliminary Analysis of the Earthquake Disaster of Buildings during Two Destructive Earthquakes in Xinjiang

This paper introduces the characteristics of earthquake disasters in the 2013 Urumqi M_S5.1 and the 2015 Pishan M_S6.5 earthquakes, which are directly beneath the cities. Based on the discussion on regional tectonic background, site conditions, seismic fortification level of buildings, we preliminarily analyzed the causes of earthquake disaster for buildings caused by the two earthquakes.Finally,we give some advice on earthquake resistance for residential buildings and earthquake damage prevention in Xinjiang.     

Effects of the Nepal MS8.1 Earthquake in 2015 on Seismic Activity in the Qinghai-Tibetan Plateau

We obtained the displacement and deformation caused by the 2015 Nepal M_S8.1 earthquake adopting the finite element method, and analyzed the displacement and deformation characteristics and effect of three large earthquakes on seismic activity in the Qinghai-Tibetan block. Our primary results suggest southward movement of the Qinghai-Tibetan block is caused by a large earthquake occurring on thrust fault in the Himalayan zone, the displacement direction is reverse to the background displacement. The occurrence of these large earthquakes will result in stress unloading and earthquake activity will be weakened in stress unloading areas. Through the simulation results, we can detect the distribution area of stress loading and unloading caused by large earthquakes. Simultaneously, it provides a fundamental evidence for determination of earthquake activity trend.     

Application on Anomaly Detection of Geoelectric Field Based on Deep Learning

The deep learning method has made nurnerials achievements regarding anomaly detection in the field of time series. We introduce the speech production model in the field of artificial intelligence, changing the convolution layer of the general convolution neural network to the residual element structure by adding identity mapping, and expanding the receptive domain of the model by using the dilated causal convolution. Based on the dilated causal convolution network and the method of log probability density function, the anomalous events are detected according to the anomaly scores. The validity of the method is verified by the simulation data, which is applied to the actual observed data on the observation staion of Pingliang geoeletric field in Gansu Province. The results show that one month before the Wenchuan M_S8.0, Lushan M_S7.0 and Minxian-Zhangxian M_S6.6 earthquakes, the daily cumulative error of log probability density of the predicted results in Pingliang Station suddenly decreases, which is consistent with the actual earthquake anomalies in a certain time range. After analyzing the combined factors including the spatial electromagnetic environment and the variation of micro fissures before the earthquake, we explain the possible causes of the anomalies in the geoelectric field of before the earthquake. The successful application of deep learning in observed data of the geoelectric field may behefit for improving the ultilization rate both the data and the efficiency of detection. Besides, it may provide technical support for more seismic research.     

Analysis on the Characteristics of the Kalpin MS5.3 Earthquake Sequence of December 1, 2013 and Anomalies before the Earthquake

This paper introduces the basic parameters, focal mechanism solutions and earthquake sequence characteristics of the Kalpin M_S5.3 earthquake sequence of December 1, 2013, and analyzed seismic activity before the earthquake, the adjacent tectonic features and the precursory anomaly at fixed points within a range of 200km. Research indicates:(1) The earthquake occurred on Kalpin fault, the source rupture type is thrust faulting with sinistral strike-slip component. (2) The earthquake sequence is mainshock-aftershock type, with the aftershock distribution attenuating quickly and trending NE. (3) Abnormal seismic activity before the earthquake was characterized by seismically nesting quiescence of M_S2.0-4.0 earthquakes, seismic quiescence of M_S4.0 earthquakes and seismic belts of M_S3.0 earthquakes in the Kalpin block, abnormal enhancement zone of moderate earthquakes on Puchang fault and seismological parameters. (4) Anomalies of precursory observation data at fixed stations are mainly characterized by mutation. Apart from the borehole tiltmeter in Halajun, the spatial distribution of other abnormal precursors showed a phenomenon of migration from the near field to far field and from the epicenter to the peripheries.     

Review of the Research of the 2011 MW9.0 Tohoku-Oki Earthquake

On March 11, 2011, a M_W9.0 earthquake occurred in the Japan Trench, causing tremendous casualties, and attracting extensive concern. Based on the results of related research, this paper analyzes the observations, phenomena and understandings of the earthquake from varied aspects, and obtains four main conclusions. (1) The earthquake, occurring in the subduction zone in the Japan Trench located in the northwest boundary of the pacific plate has two zones of concentrated coseismic slip at different depths, and the slip in the deep zone is relatively small. Though there have been many M7.0 historical earthquakes, slips in the shallow zone are large, but there have been few historical strong earthquakes. (2) Constrained by GPS data, the study of fault movement shows that fault movement in the Japan Trench has a background of widely distributed stability and locking (the locking zone is equivalent that of coseismic rupture zone). Perturbation occurred after the 2008 M8.0 Hokkaido earthquake, several M7.0 events had after slips larger than the coseismic slip, and two obvious slow slip events were recorded in 2008 and 2011. Eventually, the March 9, 2011 M7.0 foreshock and the March 11, 2011 M_W9.0 mainshock occurred. The pre-earthquake changing of the fault movement in the Japan Trench is quite clear. (3) Traditional precursory observation show no obvious anomaly, possibly due to monitoring reason. Anomaly before earthquake consists of high stress state in focal zone reflected by some seismic activity parameters, short period anomaly in regional ground motion, etc. (4) The analysis of physical property in focal zone aroused more scientific issues, for example, is there obvious difference between physical property in focal zone and its vicinity? Does frictional property of fault determine seismogenic ability and rupture process? Whether pre-earthquake fault movement include pre-slips? Could deep fluid affect fault movement in focal zone? Experience is the best teacher, and authors hope this paper could be a modest spur to induce others in basic research in earthquake forecast and prediction.     

The NE Directed Seismicity Belt in Tibet after the MS8.1 Nepal Earthquake and Its Predictive Significance

After the 2015 M_S8.1 Nepal earthquake, a strong and moderate seismicity belt has formed in Tibet gradually spreading along the northeast direction. In this paper, we attempt to summarize the features and investigate the primary mechanism of this behavior of seismic activity, using a 2-D finite element numerical model with tectonic dynamic settings and GPS horizontal displacements as the constraints. In addition, compared with the NE-trending seismicity belt triggered by the 1996 Xiatongmoin earthquake, we discuss the future earthquake hazard in and around Tibet. Our results show that:the NE-directed seismicity belt is the response of enhanced loading on the anisotropic Qinghai-Tibetan plateau from the Indian plate and earthquake thrusting. Also, this possibly implies that a forthcoming strong earthquake may fill in the gaps in the NE-directed seismicity belt or enhance the seismic hazard in the eastern (the north-south seismic zone) and western (Tianshan tectonic region) parts near the NE-directed belt.     

Study of the Change in Wave Velocity Ratio before and after Two Strong Earthquakes Using Airgun Source Data

Using the signals excited by the large-volume airgun source at the Binchuan transmitting seismic station from January to June, 2016, arrival-time data was acquired at four stations near the epicenter of the Eryuan M_S4.5 and M_S4.0 earthquakes on February 8, 2016, as well as the epicenter of the Yunlong M_S5.0 and Eryuan M_S4.6 earthquakes on May 18, 2016 through the waveform cross-correlation technique. The wave velocity ratio of the four stations was calculated using the single-station method. At the same time, the b-value and the focal mechanism consistency parameters of the study area were also calculated. The results show that:(1) the wave velocity ratio of each station experienced a process of decline-recovery-fast rise before the two strong earthquakes, and a significant quasi-synchronous rapid rise occurred within 3-12 days before the earthquake; (2) the timing of the rapid rise of the wave velocity ratio of the four stations before the Yunlong M_S5.0 and Eryuan M_S4.6 earthquakes were related to the epicentral distance. The station which observed the earliest increase in rapid rise is the farthest one from the epicenter, and the station where the rapid rise appeared in the latest is closest to the epicenter; (3) the form of change of the wave velocity ratio before the earthquake was different between stations located at different directions in the epicenter area; (4) the b-value and the focal mechanism consistency parameter which is commonly used to characterize the stress level both showed a downward trend before the two strong earthquakes, and were consistent with the change in the wave velocity ratio. According to the preliminary analysis, the wave velocity ratio obtained by using airgun source can better reflect the change in the stress state of the underground medium.     

Using Airgun Source Signals to Study Regional Wave Velocity Changes before and after the Yunlong MS5.0 and Yangbi MS5.1 Earthquakes

On the basis of the airgun source signals recorded by the stations from January, 2016 to June, 2017, we use cross-correlation detection technology to obtain the characteristics of the stable phase travel time change of each station. We used the Yunlong M_S5.0 and Yangbi M_S5.1 earthquakes as samples. According to regional characteristics, 13 stations with high signal-to-noise ratios and complete data were selected (including 3 fixed stations and 10 active source stations). They are divided into four regions, and on the basis of the GNSS baseline data, the characteristics of regional wave velocity changes before and after the earthquake are analyzed. The results show that the station phase travel time change and the regional stress characteristics represented by the GNSS baseline data have good correlation in the short-term. Due to different degrees of regional stress, there are differences in the travel time changes of different stations in the four regions. Before the Yunlong M_S5.0 and Yangbi M_S5.1 earthquakes, with regional stress adjustment, there is an upward trend in the travel time changes of related stations in the adjacent areas of up to 0.02s. The difference is that there are differences in the time nodes and duration of the travel time anomalies, and there is a reverse descent process after the Yangbi M_S5.1 earthquake. There are different degrees of travel time fluctuations in the relevant stations before and after the two earthquakes, but the fluctuation range before and after the earthquake was small. Compared with the water level change of the reservoir, the adjustment of the regional stress is more likely to have a substantial impact on the travel time changes of the relevant stations.     

Stress changes on major faults caused by 2013 Lushan earthquake and its relationship with 2008 Wenchuan earthquake

On April 20, 2013, an Ms7.0 earthquake occurred in Ya’an-Lushan region, Sichuan Province, China, killing and injuring morethan one thousand people. Therefore, it is critical to outline the areas with potential aftershocks before reconstruction andre-settlement as to avoid future disasters. Based on the elastic dislocation theory and multi-layered lithospheric model, we calculate the co-and post-seismic stress changes caused by the Wenchuan and Lushan earthquakes to discuss the relationshipbetween Mw7.9 Wenchuan earthquake and Ms7.0 Lushan earthquake, the influences on the distribution of aftershock caused bythe Lushan earthquake, and the stress changes on major faults in this region. It is shown that the Coulomb failure stress increment on the hypocenter of Lushan earthquake caused by the Wenchuan earthquake is about 0.0037-0.0113 MPa. And the possible maximum value (0.0113 MPa) is larger than the threshold of stress triggering. Therefore, the occurrence of Lushanearthquake is probably effectively promoted by the Wenchuan earthquake. The aftershock distribution is well explained by theco-seismic stress changes of Lushan earthquake. By the two ends of the rupture of Lushan earthquake with increased Coulombfailure stress, a lack of aftershock recordings indicates the high seismic hazard. The stress accumulation and correspondingseismic hazard on the Kangding-Dafu segment of the Xinshuihe fault, the Beichuan-Yingxiu fault, the Pengxian-Guanxianfault, and the Ya’an fault are further increased by the Lushan earthquake and post-seismic process of Wenchuan earthquake.     

Study on the Anisotropy of the Underground Medium in Hohhot

In this paper, the data of earthquake events of magnitude M_S6.0 and above produced in Hohhot Seismic Station from 2008 to 2015 and the data of M_L ≥ 1.0 seismic events from 2015 to 2016 in Horinger Seismic Station and the surrounding mobile stations in southern Hohhot are selected. Using Splitlab and SAM software, the spin-correlation method, the least-energy method and the cross-correlation coefficient method are used to analyze the teleseismic and near-seismic phases (SKS, S). The results of this study are in good agreement with the results previously obtained by other researchers. The study of teleseismic SKS splitting reflects the characteristics of the anisotropy of the upper mantle beneath Hohhot, that is, the anisotropy of the upper mantle shows NW, which reflects "fossil" Anisotropy, mainly in the continental structure of stable units and preserves the history of mantle deformation information. The crustal anisotropy reflected by the near-earthquake S-wave splitting study is similar to that of the active fault zone, trending NE as a whole and is consistent with the tectonic stress field background of the northeastern margin of Ordos block.     

Comparative Study of Changes in Stress-drop of the Jinggu MS6.6 and Ludian MS6.5 Earthquake Sequences

The earthquake stress-drop values of two sequences were accurately calculated after taking away the effects due to regional earthquake anelastic attenuation and station site response, using waveform data and seismic phase data of sequences of the Jinggu M_S6.6, and Ludian M_S6.5 earthquakes in Yunnan. These results show that the stress drop with magnitude increases within the scope of this study of magnitude. After eliminating the influence of the magnitude, the average value of stress-drop in the Jinggu sequence is higher than that of the Ludian sequence at the same magnitude range. This may be related to the stress state in different regions. In terms of the changes of time and space of stress-drop, before M_S5.8 strong aftershock, the stress-drop is "slowing down-turning up-keeping a high value" after the mainshock, meanwhile, almost all of the abnormally high stress drop value is distributed around the M_S5.8 strong aftershock, showing that the stress environment in the region was increasing after the mainshock. And after the M_S5.9 strong aftershock, stress-drop rapidly declines to a relatively stable state, meanwhile, the high value of stress-drop is distributed around the strong aftershock, showing that the regional tectonic stress gets more fully release, its stress environment begins to rapidly decrease. For the Ludian sequence without a strong aftershock occurring, the average value of stress drop is lower than that of the Jinggu earthquake sequence at the same magnitude range, while at the same time, the stress-drop of the aftershock sequence almost hasn''t changed much. In the time after the mainshock, combined with the release characteristics of the main energy, the stress in the region is excessively released, the subsequent stress in the region gradually returns to normal. This may be the reason why the activity of Ludian aftershocks significantly was weaker and subsequently there were no strong aftershocks occurred.     

Moment Magnitude and Its Calculation

In this paper, we give a brief introduction to the proposal and development history of the earthquake magnitude concept. Moment magnitude M_W is the best physical quantity for measuring earthquakes. Compared with other magnitude scales used traditionally, moment magnitude is not saturated for all earthquakes, regardless of big and small earthquakes, deep and shallow earthquakes, far field and near field seismic data, geodetic and geological data, moment magnitude can be measured, and can be connected with well-known magnitude scales such as surface wave magnitude M_S. Moment magnitude is a uniform magnitude scale, which is suitable for statistics with wide magnitude range. Moment magnitude is the preferred magnitude selected by the International Seismological community, and it is preferred by the departments responsible for publishing seismic information to the public.Moment magnitude is a uniform magnitude scale, which is suitable for statistics with wide magnitude range. Moment magnitude is a preferred magnitude for international seismology, it is preferred by the agency responsible for providing information about earthquakes to the public. We provide all formulas used in the calculation of moment magnitude, and the calculation steps in detail. We also analyzed some problems and rules to solve these problems by using different formulas and numerical value calculation steps.     

Background Seismicity and Its Application to Seismic Hazard Assessment in the North China Region

The historical earthquake activity is intense in the North China region. However, no middle-sized earthquakes have occurred in the last decades in the region since the MS6.2 earthquake in the Zhangbei region in 1998. The quiescence of moderate and strong earthquakes is quite prominent in North China. In this paper, we use small earthquake records in 1970~2009 to study background seismic activity in the North China region. The spatial distributions of seismic parameters are presented, including b-value, the maximum magnitude and annual occurrence probability of earthquakes of M≥6.0. Our results show regions with low b-value that include the Yuncheng region in the Shanxi rift, the Suqian region located in the Tancheng-Lujiang fault zone and the Shijiazhuang region in the Taihangshan block. Our analysis on the synthetic spatial pattern of seismicity indicate that seismicity in the North China region is mainly affected by the regional dynamic factors of deep structures.     

Determination of Focal Depths of the MS5.8 Alxa Left Banner, Inner Mongolia Earthquake Sequence

Using the double-difference earthquake location algorithm, the deterministic method (PTD method) and the CAP seismic moment tensor inversion method, the paper selects the primary waveform data of 78 earthquakes recorded by the "China Earthquake Science Array Probe Project in the Northern Part of North South Seismic Belt", the "China Earthquake Scientific Exploration Array Data Center" of Institute of Geophysics, China Earthquake Administration, and the Inner Mongolia Digital Seismic Network to calculate the focal depths of the mainshock and the seismic sequence of the M_S5.8 Alxa Left Banner earthquake in Inner Mongolia. The results show that the focal depth of the main shock is 20.6km, determined by the double-difference earthquake location method, 18.1km by the PTD method, and 19.2km by the CAP method. The focal depth of the earthquake sequence calculated by the double-difference location method is larger. The deterministic method (PTD method) and double-difference location method are the methods that fit the tectonic characteristics of the seismic source area, and the CAP method is suitable for larger earthquakes.     

Short-term Earthquake Prediction in the Sichuan-Yunnan Region Using the Method of Modulated Earthquakes

By scanning modulated or un-modulated earthquakes spatio-temporally in the region of Sichuan-Yunnan,short-term non-stationary seismic precursory patterns were extracted with significant difference and the characteristic of non-stationary short-term seismic anomalies were analyzed as well as prediction efficiency of modulated small earthquakes before a strong earthquake. Besides,small earthquake modulation ratios near the region of the epicenter were calculated and sorted by time. The results indicated that there were significant effects using the modulated earthquake method to predict earthquakes greater than MS6. 0 in a short time. Before the MS8. 0 Wenchuan earthquake,there were obvious short-term precursory seismicity gap patterns of modulated small earthquakes.     

Spatial Correlation Anomalies of the Diurnal Variation of the Geomagnetic Vertical Component before the Yingjiang MS6.1 and Ludian MS6.5 Earthquakes

Significant anomalies were observed at the geomagnetic stations in the southwest region of China before the Yingjiang M_S6.1 earthquake and the Ludian M_S6.5 earthquake in 2014. We processed the geomagnetic vertical component diurnal variation data by the spatial correlation method. The results show that during the period from April 1 to May 20, 2014,there existed quasi-synchronous decrease changes in the coefficient curves between the five geomagnetic stations of Guiyang,Hechi,Nanshan,Muli,Yongning and Xinyi and Hongshan stations.Furthermore,there was a high gradient zone in the normalized correlation coefficient contour map with background values removed. The epicenters of the Yingjiang M_S6.1 earthquake and the Ludian M_S6.5 earthquake are located in the gradient zone or near the gradient zone.     

Strong earthquake clustering around the eastern Tibetan Plateau after the 2008 MW7.9 Wenchuan earthquake

After the 2008 M_W7.9 Wenchuan earthquake, the eastern Tibetan Plateau experienced a series of M_W6.0 earthquakes, including the 2013 M_W6.6 Lushan, 2014 M_W6.1 Kangding and 2017 M_W6.5 Jiuzhaigou events. Based on available constraints, we build a three-dimensional viscoelastic finite element model to calculate Coulomb failure stress caused by these strong earthquakes. In this model, the geometry and slip vector of the initial rupture zone of each earthquake are used to better evaluate the earthquake-related stress projection. Considering reasonable ranges of viscosities for the crust and upper mantle in different tectonic units, numerical results show that after the Wenchuan earthquake, the coseismic Coulomb failure stress change at the hypocenters of the subsequent earthquakes increased to approximately +0.012–+0.040, +0.01–+0.03, and+0.008–+0.015 MPa, respectively. With viscoelastic relaxation of the lower crust and upper mantle, the Coulomb failure stress change at the hypocenters of these earthquakes accumulated to about +0.014–+0.042, +0.016–+0.036, and +0.003–+0.007 MPa just before their occurrence. This suggests that the Wenchuan earthquake indeed triggered or hastened the occurrence of the Lushan, Kangding and Jiuzhaigou events, supporting that strong earthquake clustering around the eastern Tibetan Plateau could be related to stress interaction between the seismogenic faults. Besides, ～94% and ～6% of the stress increase around(and before the occurrence of) the Kangding earthquake were contributed by the Wenchuan event and the Lushan event, respectively; the positive Coulomb failure stress change at the Jiuzhaigou earthquake hypocenter was related to coseismic slip partitioning of the Wenchuan earthquake. This means that stress interaction among the earthquakes could be controlled by the combined effect of stress of the previous events and by the complexity of earthquake ruptures. Thus, in researches on the earthquake-triggering mechanism, special attentions should be paid on both details of the rupture model and multiple factors of previous earthquakes.     

Enhancement of Seismicity Recorded by the Qiaojia Array before the 2013 Lushan and 2014 Ludian Earthquakes

A seismic array of twenty four seismometers(Qiaojia array) operated by the Institute of Geophysics,China Earthquake Administration was situated along the Zemuhe fault and the north end of Xiaojiang fault,which is a part of the east boundary of the Chuan-Dian( Sichuan-Yunnan) rhombus crustal mass. The Qiaojia array started operation at the end of February,2012. Since then the April 20,2013 Lushan MS7. 0 earthquake and the August 3,2014 Ludian MS6. 5 earthquake have occurred in the vicinity of the Qiaojia array. The earthquake catalogue recorded by the Qiaojia array since March,2012 is used in this study. The temporal variation of the earthquake count before the Lushan event and the Ludian event is analyzed. The results are as follows:(1) A very clear gradually increasing variation of the count of M ≥ 2. 0 earthquakes within the region,where all earthquakes recorded by the Qiaojia array coverage can be found before the Lushan event and the Ludian event,and the increasing range and duration of the count before the Ludian event are both larger than those before the Lushan event.(2) In the region covered by the Qiaojia array,for earthquakes with depth h ≤10 km a rising process of the count was manifested before both events,along with a nearly same duration of about five and a half months,and for earthquakes with depths h 10 km a rising-dropping process of the count was manifest before the Lushan event,before which the rising-dropping process appeared again. The variation of the count for earthquakes with depth h ≤ 10 km is the reverse of that for earthquakes with depth h 10 km.(3) Within and near the region covered by the Qiaojia array,the variation of the count manifests a rising process for earthquakes with depth h ≤ 10 km or h 10 km before the Lushan event and only for earthquakes with depth h ≤ 10 km before the Ludian event. The variation of the count manifests a weakening process for earthquakes with depth h 10 km before the Lushan event. It is shown from the above results that the seismicity within and near the regioncovered by the Qiaojia array showed a steeply rising change before both the Lushan event and the Ludian event. This phenomenon could be revelatory to understanding the process of seismicity development.