Study of the electromechanical coupling process before the 2020 AD 6.4 earthquake Yutian, China

local stress environment

In Figure 6, the spatial distribution of LURR anomalies in three different time periods before the Yutian earthquake was calculated to detect those regions with high pressure build-up. Numbers were derived by LURR assessment of the Benioff strain of earthquakes Ms ≤ 4.0 (data from the China Earthquake Networks Center catalog). The time window is 0.5 years and the spatial window has a radius of 80 km which slips at 0.25° in both latitude and longitude. The internal friction coefficient used to evaluate the CFS is 0.4 and a LURR value greater than 1.0 is plotted at the central location using Matlab’s two-dimensional line interpolation.

Figure 6

Spatial distribution of the LURR anomaly in Xinjiang before the Yutian earthquake. The computed time window is shown in both maps, the LURR value is given by the color code, and the red point in (c) is the Yutian earthquake. The three figures were generated using Matlab R2016b (https://www.mathworks.com/products/matlab.html ; accessed September 27, 2022).

The results showed that there was no significant anomaly in Yutian region in the second half of 2019 (Fig. 6a), but the anomaly increased significantly in March 2020. It is clear that the spatial distribution of the LURR anomaly was expanding and the amplitude was also large. improved, especially in the area around the future epicenter where LURR values ​​exceeded 4 (Fig. 6b). Since then, the LURR values ​​in this region started to weaken before the Yutian earthquake (Fig. 6c).

In order to demonstrate that stress accumulation in the Yutian earthquake leads to the change of LURR, we constructed a profile. We generated a GPS profile along the direction of the Ashikule fault using GPS data from 1999 to 2019 to verify the motion characteristics of the fault zone extending at the southwest end of the Altun Tagh fault zone (Fig. 7a). The calculated data were obtained from the First Monitoring Center of the Chinese Earthquake Administration. The velocity field was calculated using a semi-observed array analysis, GNSS (Global Navigation Satellite System) in MIT (GAMIT)/Global Kalman filter (GLOBK) 27,28,29 from the GPS velocity field associated with mainland China. The results show that the GPS velocity field trends are within 200 km in the south and 300 km in the north of the profile in NNE and NNW or N, respectively (Fig. 7b). The average speeds on both sides of the clip also differ greatly. The NW-SE tensile condition of the northeast direction ruptures prevails, slip rates on the north and south sides of the fault are approximately 9 and 16 mm/a, respectively, possibly due to northward subduction extrusion from the South India Plate, and the obstruction of the Tarim massif in the north, The western Kunlun Fault is extruded.

Figure 7

GPS velocity fields before earthquakes in the Ashikule fault zone (a) and the N 50° E velocity profile (b). The east-north velocity components were designated as positive; The white line in the top panel on the left is ASF (Ashikule Error). Figure 7a was generated using General Mapping Tools (GMT) 4 (https://www.generic-mapping-tools.org/download/ , accessed 27 September 2022).

geodetic data

This change in the stress state of the crust computed by LURR can also be recorded by nearby geodetic data at the same time periods (Fig. 8). Observation of vertical pendulum tilt of Yutian Seismic station is located in a cave in Aqiang Town, Yutian Province, Xinjiang Uygur Autonomous Region (Fig. 2) with geographic coordinates of 36.4°N, 81.9°E and an altitude of 1200m. The site consists of igneous carbonate rocks and marbles. The observation site is located at the intersection of the eastern side of the western Kunlun fold belt and the Tarim Basin. The Pulu fault, which was active in the early Quaternary, was distributed in a northeast direction, about 8 km north of the station. Approximately 40 km to the east of the station is the Alton Tag fault zone, which is north-eastward and active in the Holocene. Approximately 47 km to the south is the Karacax fault zone, which turns to the northeast, is active in the Holocene. The station was built in October 2009, inside a room in the cave. The cave is 30 m long and powered by solar energy. Official monitoring began in November 2016.

Figure 8

Vertical pendulum tilt scale charts recorded at the Yutian Observatory: (a) Blue and red curves indicate the daily NS and EW data for the vertical pendulum tilt scale, respectively. The green vertical line is the amount and time of the Yutian earthquake; The orange box shows the period of anomalous annual variation (b) the annual variation of north-south direction from January 1, 2017 to June 26, 2020; (c) Annual variation of electricity and water trend from January 1, 2017 to June 26, 2020. The change in the shift of the NS component (b) in Agust 2019 resulted from the man-made shift. The yellow dotted line (b) represents the previous trend movement from Agust and December. The yellow dotted line of the EW component (c) represents the likely normal annual trend at the end of 2019. The horizontal axis unit is time, “T/a” and “T/m” mean “time/year” and “time/month” on straight.

As shown in the daily average curve (Fig. 8a), the EW component observed with the tilt of the Yutian vertical pendulum was typically in decline E from mid-September to mid-April of the following year. The time of the E-dip phase was from November 27, 2019 to April 14, 2020, the same period as in previous years; However, the annual variance in this period is significantly reduced compared to the same periods in previous years (Fig. 8c). There was no environmental intervention at the site during the observation period. Comparative statistics show that the annual variance from 2019 to 2020 is 0.264 (Table 2), which is much smaller than the annual variance during the same period in previous years, and therefore, there is an anomalous annual variance. The Yutian Ms 6.4 earthquake occurred 73 days after the end of the anomaly (Fig. 8a).

Table 2 Statistics of the annual variance of the electronic warfare component of the vertical pendulum inclination.

Anomalous changes in the Yutian vertical pendulum near the epicenter of the Yutian earthquake two months before the earthquake reflect the instability and rupture of the fault zone in this region. Previous studies showed that the Altun Tagh, Kunlun, Karakax fault zones and other strike-slip faults were formed in the northern boundary of the Qinghai-Tibet Plateau due to the eastward movement of the Qinghai-Tibet Plateau block and Tarim obstruction. Basin 30,31,32. The GPS velocity of the Alton Tag fault gradually decreases from west to east. The GPS velocity of the western part is relatively small, its pressure is the largest, and the surface deformation occurs mainly in the south. However, in the north, due to the stable Tarim massif, there are few variants of the surface shape. As shown in Figure 7b, the southern side of the Yutian earthquake seismic fault is moving eastward at a speed of 3 mm/a. Approximately two months before the earthquake, an anomaly deformation was observed in the northwest direction of the earthquake showing that the Alton Tag and Karakax faults show eastward movement, the rate of movement of the first being higher than the second. The instability and laxity of the fault junction occurred 2 months before the earthquake due to the long-range drag motion and continuous pressure build-up at the fault junction, and the Karakax fault temporarily escaped from the eastward extrusion of the Qinghai-Tibet Plateau. The westward movement occurred under the influence of the Tarim Basin, resulting in the tensile properties of the Ashikol fault. When the tensile stress buildup exceeded the critical rupture state, the Yutian Ms 6.4 earthquake occurred in 2020.

Relationship between Benioff strain and LURR PV

LURR models can be used to detect differences in the crustal stress state in the source region prior to an earthquake. The LURR is evaluated in this paper based on the law of dynamic evolution of the constitutive relationship of rocks. All geophysical quantities that can reflect the mean instability process in the seismic zone can be considered as response quantities. The LURR theory is based on the mechanism of tidal stress excitation. Since tidal stress is much less than tectonic stress, it can only cause earthquakes and not trigger them. When tectonic stress is low, slight changes in tidal stress are difficult to produce, which means that the center of the crust is in a stable state, and thus, LURR <1. ومع ذلك ، عندما يتراكم الإجهاد التكتوني إلى مستوى عالٍ ، فإن أي زيادة طفيفة الإجهاد ، مثل إجهاد المد والجزر ، قد يؤدي إلى حدوث زلازل صغيرة ، مما يؤدي إلى اختلافات في إطلاق سلالة Benioff بين مرحلتي التحميل والتفريغ ، وبالتالي ، LURR> 1. The method of calculating the Benioff strain response based on LURR has better continuity compared to the geophysical monitoring data. Small changes in LURR fluctuation in the time curve can accurately determine the critical state before the earthquake, when the rocky medium is in an elastic phase (steady state) and the expansion is full in the seismic zone.

The LURR of the geoelectric field is based on the hypothesis that the rock volume expansion caused by cracks generated in the critical state of the earthquake can alter the geoelectric field near the focal region. In particular, no new cracks were created in the solid medium during the loading and unloading process, the number of cracks remained unchanged (Fig. 9a), and fluid transmission in the rock mass was not affected, therefore, the change in flow voltage was very small (Fig. 9b) . In addition, the change in the LURR time curve for the geoelectric field and the LURR value was close to 1.0. When the stress accumulation reaches a high level, the rocks enter the yield phase (instability state), and according to the Kaiser effect, the number of cracks generated in the rock experiment in this phase was much higher than that in the degassing phase (Fig. 1a). 9 c). In the loading stage, the expansion of the rock and the generation of micro-cracks lead to changes in fluid transport, which leads to the possibility of leaching (Fig. 9d). Therefore, the generated local electric field will cause the value of LURR to gradually increase and deviate from the constant value 135,36.

Figure 9

The number of ruptures and the potential difference in cases of wear and tear during the loading and unloading process; (a) and (b) changes in fracture number and flow potential in the elastic phase, respectively; (c) and (d) are changes in the fraction number and flow potential in the expanded substance phase, respectively.

Our calculation results show that LURR has a very large predictive value. As shown in Figure 5, the LURR anomaly of the Benioff strain near the epicenter began to rise rapidly in October 2019 and reached the peak in March 2020. At this time, the LURR anomaly in the Hotan geoelectric field, 290 km from the epicenter also shifted in Simultaneously, but not significantly. Then, the rally began rapidly around December 2019, peaking in one to three months before the main shock occurred. This phenomenon indicates that the change in the electric field is related to the propagation of fractures and fluid infiltration during earthquake preparation. Anomalies in the electric field often occur in the late phase of earthquakes, and therefore, the peak value of the electric field anomalies may be later than the LURR value of the Benioff strain as the response quantity.

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