In this paper changes in focal mechanisms, parameters of wave spectra, and stress drops for the Ms=5.0 foreshock and Ms=6.0 mainshock in February 2001 in Yajiang County, Sichuan, and seismicity in epicentral region are studied. Comparison of focal mechanisms for the Yajiang earthquakes with distribution patterns of aftershocks, the nodal plane I, striking in the direction of NEN, of the Yajiang M=5.0 event is chosen as the faulting plane; the nodal plane II, striking in the direction of WNW, of the M=6.0 event as the faulting plane. The strikes of the two faulting planes are nearly perpendicular to each other. The level of stress drops in the epicentral region before the occurrence of the M=6.0 earthquake increases, which is consistent with increase of seismicity in the epicentral region. The rate decay of the Yajiang earthquake sequence, changes in wave spectra for foreshocks and aftershocks, and focal mechanisms are complex.

We restore the seismic source spectrums of 1012 earthquakes (2.0 ≤ ML ≤ 5.0) in the mid‐northern part of Sichuan‐Yunnan seismic block between January 1, 2009 and December 31, 2015, then calculate the source parameters (e.g., seismic moments M0, focal scale r and stress drop Δσ) and fit the calibration relationship between these parameters. Based on the regional seismic tectonic background, the distribution of active faults and seismicity, the study area is divided into four statistical units. For each unit the stress distribution characteristics, change of stress drop with location, correlation between the stress‐strain loading and the dynamic process of regional deformation, are discussed respectively. The results show that seismic moments M0 are consistent with the magnitude‐moment relation that lgM0 = 0.92ML + 10.46. The relationship between stress drop and magnitude is consistent with the result gained by Nuttli that intraplate earthquake follows the ISD model, with a statistical relationship lgΔσ = 0.31 lgM0 – 3.92. Seismic source stress drop results show the following, (1) The stress at the end of the Jinshajiang fault is low, the overall sliding rate of the fault unit is high, and strong earthquake activity is very rare. In the fault belt consisting of three secondary faults, stress‐strain loading deceases gradually from northwest to southeast along Litang fault, the northwest section which is relatively locked is more likely to accumulate strain than southeast section. (2) Stress drop of Xianshuihe fault zone is divided by Kangding, the southern section is low and northern section is high. Southern section (Kangding‐Shimian) is difficult to accumulate higher strain in the short term, but in northern section (Garzê‐Kangding), moderate and strong earthquakes have not filled the gaps of seismic moment release, there is still a high stress accumulation in partial section. (3) High stress‐drop events were concentrated on Anninghe‐Zemuhe fault zone, strain accumulation of this unit is strong, and stress level is the highest, earthquake risk is high. (4) On Lijiang‐Xiaojinhe fault zone, stress drop characteristics of different magnitude earthquakes are not the same, which is related to complex tectonic setting, the specific reasons still need to be discussed deeply. Stress background in the Muli area is low and may be affected by the local tectonic environment. The study also shows that, (1) Stress drops display a systematic change with different faults and locations, high stress‐drop events occur mostly in the fault intersection area. Faults without locking condition and mainly creeping are mainly characterized by low stress drop. (2) Contrasting to what is commonly thought that “strike‐slip faults are not easy to accumulate stress”, Xianshuihe fault zone and Anninghe‐Zemuhe fault zone all exhibit high stress levels, which may be due to that the magnitude and intensity of medium‐strong earthquakes are not enough to release the accumulated energy. On the other hand, in the complex dynamics of regional d...

Identifying the locations of potential landslide areas is fundamental for disaster prevention and mitigation. Considering the close relationship between landslides and geomorphic evolution, a new approach is proposed to predict which areas are prone to landslides based on the analysis of topographic features in the area affected by the 2014 Ludian, Yunnan, China, Ms6.5 earthquake. In our method, the expected slope angle (ES), which is related to the local relief in a grid cell, is defined to describe the terrain features within grid cells. Based on the relationship between the expected slope angle and the average slope, the cells in the study region can be classified into 3 different terrains. Our study shows that large landslides triggered by the Ludian earthquake are prone to occur in regions where the expected slope angle and average slope angle show obvious differences, which means that the region has sharp changes in relief and slope. In comparison with the coseismic landslides triggered by the 2017 Jiuzhaigou, Sichuan, China, Ms7.0 earthquakes, preliminary analyses infer that the distribution of coseismic landslides follows the general mass transportation features of a special region and that their occurrences are the kind of simultaneous adjustment of excess geomorphicity of hill slopes to a steady state.

As a case study of spatial and temporal variations in earthquake stress drops between the 2008 Ms 8.0 Wenchuan and 2013 Ms 7.0 Lushan earthquakes, we computed 1828 stress-drop values for earthquakes with magnitudes 1.7 ≤ M L ≤ 5.0 during an eight-year time span before and after major earthquakes. We divide the study area into three subregions (the southern segment of the Longmen Shan fault zone; the southwestern junction of the Longmen Shan and Sichuan Basin; and the southwestern margin of the Sichuan Basin) and calculate individual event stress drops in each. The results show that regions of alternating high and low stress drop are found on either side of the southwestern segment of the Longmen Shan fault zone. During the two-year period after the 2008 Ms 8.0 Wenchuan earthquake, the stress state of the southern Longmen Shan fault shows no significant change. A marked increase in stress level appears about 18 months before the 2013 Ms 7.0 Lushan earthquake near the Lushan hypocenter zone. Two months after the Ms 7.0 event, the stress drops suddenly attenuate, with significantly less seismic energy release per event. We find that changes in the patterns of high and low stress drop values are consistent with the process of stress accumulation or transfer from the pre-mainshock to postmainshock periods. The results indicate that major earthquakes are the dominant cause of temporal and spatial evolution in stress levels. Stress drop variations show obvious temporal and spatial patterns that may suggests subtle changes in the character of stress fields on faults and spatial variations related to local intense compression and tectonic effects.

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