Coulomb stress changes due to the 2021 MS7.4 Maduo Earthquake and expected seismicity rate changes in the surroundings

On May 22 nd,2021,an MS7.4 earthquake occurred near the Maduo county of the Qinghai Province,China,within the Bayan Har Block.Seismic activities have been intense in this block,thus whether the Maduo Earthquake will bring subsequent seismic hazards to its surrounding regions raises wide concerns.In this paper,we first calculated the Coulomb failure stress changes caused by the Maduo Earthquake on nearby faults,and estimated how much these faults are brought closer or further from their next failures based on their stressing rates.Next,we combined the Coulomb failure stress changes with the rate-state frictional law to estimate the seismicity rate in the study region in the next decade.A declustered catalogue before the Maduo Earthquake was adopted to calculate background seismicity rate,and rate-state parameters are constrained by fault slip rates.Our results show that the Maduo Earthquake increases stress accumulations in the northwestern portion of the Qingshuihe fault(0.02 MPa at maximum),the two ends of the Kunlun Mountain Pass-Jiangcuo fault(0.01 MPa at maximum),and the northwestern portion of the Maduo-Gande fault(on average~0.09 MPa),and seismicity rates are expected to increase near these faults.What is especially worth noting is the seismic hazard in the region extending from the eastern end of the Kunlun Mountain Pass-Jiangcuo fault to the Maqin-Maqu seismic gap on the Eastern Kunlun fault,which is calculated to have experienced a maximum stress increase of 0.67 MPa after the Maduo Earthquake.On the other hand,stress accumulations are reduced in the southern end of the Elashan fault,the Eastern Kunlun fault segment to the west of Maduo,and the northwestern portion of the Dari fault.Seismic hazards are expected to be low in these regions.For the study region as a whole,the probability of an M≥6 earthquake taking place in the next decade is estimated to be 59%,about twice the value calculated for the time period before the Maduo Earthquake.

Methods for Estimating Mean Annual Rate of Earthquakes in Moderate and Low Seismicity Regions

Two kinds of methods for determining seismic parameters are presented, that is, the potential seismic source zoning method and grid-spatially smoothing method. The Gaussian smoothing method and the modified Gaussian smoothing method are described in detail, and a comprehensive analysis of the advantages and disadvantages of these methods is made. Then, we take centrai China as the study region, and use the Gaussian smoothing method and potential seismic source zoning method to build seismic models to calculate the mean annual seismic rate. Seismic hazard is calculated using the probabilistic seismic hazard analysis method to construct the ground motion acceleration zoning maps. The differences between the maps and these models are discussed and the causes are investigated. The results show that the spatial smoothing method is suitable for estimating the seismic hazard over the moderate and low seismicity regions or the hazard caused by background seismicity; while the potential seismic source zoning method is suitable for estimating the seismic hazard in well-defined seismotectonics. Combining the spatial smoothing method and the potential seismic source zoning method with an integrated account of the seismicity and known seismotectonics is a feasible approach to estimate the seismic hazard in moderate and low seismicity regions.

The gravity and isostatic Moho in North China Craton and their implications to seismicity

The Bouguer gravity is the combination of field sources in different depths. Based on the multi-scale analysis of the Bouguer gravity,we can get the gravity anomaly caused by the Moho undulation. This study presents the various orders of approximation of gravity anomaly in North China Craton（NCC）,the possible source depths with radial logarithmic power spectrum,and the relationship between the deep structure and gravity anomaly. Furthermore,we discuss the isostatic compensation about the Moho depth from gravity and deep seismic sounding profiles（DSS）. The results show that：（1） the fourth approximation could have resulted from the Moho undulation,（2） in contrast to the isostatic Moho,the inverted gravity Moho and the DSS Moho show that most of NCC has been isostatically compensated,and（3） the isostatic compensation rate has some close relation to the seismicity.

Seismicity of Tehri Area of the Himalayan Region, India

A database for the seismicity of the Tehri region (29.5˚N - 31.5˚N and 77.5˚E - 79.5˚E) from November 1, 1853, to March 31, 1989, has been prepared using a Compatible Personal Computer System. The seismicity database is complete for events with mb ≥ 4.5 only since 1963. It is inferred that the general seismicity of the area is considerably low, which is associated with four main tectonic features identified based on the spatial distribution of events in the area. Earthquakes in the Tehri area is of shallow focus, and maximum seismic activity is confined in the region beyond 60 km east and northwest of Tehri. The cumulative Number of Events as a Function of Time (CNET) for the period from 1963 to 1988 has indicated that precursory swarms do not precede the medium-sized earthquakes of the Tehri area. However, the CNET curves for total events and those with mb ≥ 4.6 have indicated a sharp 2-fold seismicity rate increase from 1986 compared to the preceding period. The October 20, 1991 (IST) earthquake of mb = 6.5 of Uttarkashi is believed to be associated with this seismicity rate change. The continuous increasing trend of the CNET curve before 1986 has been attributed to the detection changes.

Application of the predictable model ofregional time-magnitude to North and Southwest China region

In this paper, the method which can combine different seismic data with the different precision and completeness, even the palaeo-earthquake data, has been applied to estimate the yearly seismic moment rate in the seismic region. Based on this, the predictable model of regional time-magnitude has been used in North China and Southwest China. The normal correlation between the time interval of the events and the magnitude of the last strong earthquake shows that the model is suitable. The value of the parameter c is less than the average value of 0.33 that is obtained from the events occurred in the plate boundary in the world. It is explained that the correlativity between the recurrence interval of the earthquake and the magnitude of the last strong event is not obvious. It is shown that the continental earthquakes in China are different from that occurred in the plate boundary and the recurrence model for the continental events are different from the one for the plate boundary events. Finally the seismic risk analysis based on this model for North China and Southwest China is given in this paper.

Application of Markovian models for non-ergodic and non-stationary earthquake times series for the identification of seismic patterns and future projections

The current earthquake forecast algorithms are not free of shortcomings due to inherent limitations.Especially,the requirement of stationarity in the evaluation of earthquake time series as a prerequisite,significantly limits the use of forecast algorithms to areas where stationary data is not available.Another shortcoming of forecast algorithms is the ergodicity assumption,which states that certain characteristics of seismicity are spatially invariant.In this study,a new earthquake forecast approach is introduced for the locations where stationary data are not available.For this purpose,the spatial activity rate density for each spatial unit is evaluated as a parameter of a Markov chain.The temporal pattern is identified by setting the states at certain spatial activity rate densities.By using the transition patterns between the states,1-and 5-year forecasts were computed.The method is suggested as an alternative and complementary to the existing methods by proposing a solution to the issues of ergodicity and stationarity assumptions at the same time.

Recent geodynamics of major strike-slip zones

The subject of this study is strike-slip fault zones, where temporal variations of accumulation in strike-slip deformation complicate the standard process of deformation accu- mulation and release during strong earthquakes. These temporal variations are expressed in the E1 Ghab segment of the Dead Sea Transform zone （DST, Eastern Mediterranean） and in the Talas-Fergana fault zone （Central Asia）. According to Global Positioning System （GPS） data, the strike-slip deformations within these zones are not now accumulating or are accumulating at a rate that is significantly less than their average rate during the Holocene and Quaternary or the Pliocene-Quaternary. Simultaneously, weak transverse shortening has been measured in both zones by GPS. In both of these zones, strong earthquakes have not registered within the XX century, yet epochs of intensified seismicity （strong earthquakes） took place throughout history. In the southern and central parts of the E1 Ghab zone, there is evidence of 30 strong historical earthquakes of Ms ≥ 5.7; however, no instrumental earthquakes of Ms 〉 5 have been identified. The temporal distribution of seismic energy released by these earthquakes demonstrates a 350 ± 50-year cycle. Values for the seismic energies released during the peak phases of these cycles are approximated by a sinusoid that suggests the possibility of a 〉1800-year cycle （＂hyper-cycle＂）, which began around the 3rd century, reached its maximum in the 12th century, and has continued until now. A combination of geological, archaeoseismological, and geodetic data show that the rate of sinistral strike-slip deformation varied in the fault zone, probably in conformity with the variation of seismicity during the ＂hyper-cycle.＂ In the Talas-Fergana fault zone, trenching and 14C dating that was correlated with right lateral offsets, gave a possible preliminary estimate of the average rates of the Late Holocene strike slip of about 10 mm per year, with a decrease in the SE direction to 4 mm-4.5 mm per year. These studies also showed that the slip in the Talas-Fergana fault zone was realized mainly during strong earthquakes. New trenching and 14C dating of paleoearthquake records identified the epoch of seismicity intensification dating to the XIV-XVII centuries. These paleoearthquakes could produce a total dextral slip at several meters. Therefore, consid- eration of these epochs was necessary to determine a calculated average slip rate during the Late Holocene.

Characteristics of regional crustal deformation before 2016 Menyuan Ms6.4 earthquake

On January 21, 2016, a strong earthquake with a magnitude of Ms6.4 happened at Menyuan, Qinghai Province of China. In almost the same place, there was another strong earthquake happened in 1986, with similar magnitude and focal mechanism. In this paper, we analyze the characteristics of regional crustal deformation before the 2016 Menyuan Ms6.4 earth- quake by using the data from 10 continuous Global Positioning System （GPS） stations and 74 campaign-mode GPS stations within 200 km of this event： （a） Based on the velocity field from over ten years GPS observations, a regional strain rate field is calculated. The results indicate that the crustal strain rate and seismic moment accumulation rate of the Qilian- Haiyuan active fault, which is the seismogenic tectonics of the event, are significantly higher than the surrounding regions. In a 20 km~ 20 km area around the seismogenic region, the maximum and minimum principal strain rates are 21.5 nanostrain/a （NW-SE extension） and -46.6 nanostrain/a （NE-SW compression）, respectively, and the seismic moment accumulation rates is 17.4 Nm/a. The direction of principal compression is consistent with the focal mechanism of this event. （b） Based on the position time series of the continuous GPS stations for a time-span of about 6 years before the event, we calculate the strain time series. The results show that the dilatation of the seismogenic region is continuously reduced with a ＂non-linear＂ trend since 2010, which means the seismogenic region has been in a state of compression. However, about 2-3 months before the event, both the dilatation and maximum shear strain show significant inverse trends. These abnormal changes of crustal deformation may reflect the non-linear adjustment of the stress-strain accumulation of the seismogenic region, when the accumulation is approaching the critical value of rupture.

Study on the correlativity of earthquake's impact on cities

Introduction With rapid development and advancement of economy and society, lots of city groups or city belts with ex-tra-large cities as their centers have been formed in China. The regions these city groups lie in usually havewell-developed economy, dense population, and are regional politics and culture centers. Some groups lie in theregions with high level of earthquake activity, such as the Surrounding Capital City Group with the centers of Bei-jing and Tianjin. Once a large earthquake occurs, its influence will spread to very extensive region and its disasterwill be tremendous too. So earthquake resistance and disaster mitigation of city group will be very significant issue.The cities in a group have close distance with each other; they can carry out unified preparation for disaster as onewhole and reduce the heavy load of single city before an earthquake, and have an advantage of prompt mutual-aidafter an earthquake because of close distance. It is especially significant to mitigate the lose of lives. One importantprecondition is that all the cities in one group cannot be exposed to the same level of destroy during one earth-quake. So the division of city group in the region with dense cities distribution shall be very significant to theemergent mutual-aid in early time after a large earthquake. For this goal, the characteristics and correlativity ofearthquake′s impact on cities in one group need to clearly be considered. The cities with similar features and strongcorrelativity of historical earthquake influence have large chance to suffer same level destroy during the futurestrong earthquake and are disadvantage to provide mutual-aid and shall not be divided into one group. ……

Research on the Seismic Activity of the Lower Yangtze River-South Yellow Sea Seismic Zone

The Lower Yangtze River-South Yellow Sea Seismic Zone,located at the southeast of the Northern China Seismic Zone, characterized by moderate-strong earthquakes, is an intensive earthquake zone,which is controlled by a series of faults within the Lower Yangtze River-South Yellow Sea Seismic Zone. This article counts and calculates the bvalue,V 4 and energy density value of medium-small earthquakes by taking full advantage of the latest data from regional seismic stations,reviews data of historical earthquakes and seismic structure,and discusses the relationship between spatial distribution of the b-value, historical strong earthquakes and spatial distribution of energy density of medium-small earthquakes,and further investigates the seismic activity of the Lower Yangtze River- South Yellow Sea Seismic Zone. This article obtains seismic activity parameters of the Lower Yangtze River-South Yellow Sea Seismic Zone as calculation parameters for probabilistic seismic hazard analysis,and discusses the trend of this seismic zone in the next one hundred years and deduces the potential seismic hazard region within this seismic zone,which provides references and methods for long-term prediction on seismic activity. The research results are significant to seismic zoning, seismic safety evaluation of engineering sites and long-term prediction of seismic activity.