An earthquake measuring 6.4 occurred in Menyuan County, Haibei Prefecture, Qinghai Province (101.62°E, 37.68°N) at 01:13a.m. on January 21, 2016 (hereafter referred to as the Menyuan earthquake), the focal depth of this earthquake was about 10km (http://www.csi.ac.cn/). The earthquake occurred in the vicinity of the Lenglongling fault zone, where the geological structure is complex and tectonic movement and historical seismicity are strong (He Wengui et al., 2010; He Wengui et al., 2000; Ma Baoqi et al., 2008). Therefore, it is necessary to study the crustal deformation state before the earthquake. The project "China Continental Tectonic Environment Monitoring Network" provides important basic data for crustal deformation observation (Zhan Wei et al., 2011). In this paper, through analyzing the inter-station baseline time series and strain time series near the epicenter with the help of GPS reference station time series data provided by the First Monitoring and Application Center of China Earthquake Administration, the crustal deformation characteristics before the earthquake are discussed, and results provide basic data for the study of the seismogenic process and earthquake mechanism.1 THE INTER-STATION BASELINE TIME SERIES ANALYSIS
Since the GPS baseline time series can weaken the influence of system error and common mode noise, the baseline time series is more sensitive than the single station coordinate time series to reflect the change of the surrounding tectonic environment (Zhang Fengshuang, 2011), the inter-station baseline time series reflecting the relative movement between stations is less affected by the frame of reference (Jiang Zaisen et al., 2009; Ma Haiping et al., 2013). Based on the analysis of the GPS baselines near the epicenter of the Menyuan earthquake, it is found that the baselines with precursory anomalies before the Menyuan earthquake are QHGC-GSGL, QHME-GSGL, NMAY-QHME, as shown in Fig. 1 (The adjustment was made to the antenna of GPS reference station QHGC in December 2015, and the time was close to the earthquake time, so it does not affect the analysis of the long time anomaly before the earthquake).
The baselines QHGC-GSGL and QHME-GSGL across the Lenglongling fault zone were in a steady shortening state from 2010 to 2014, and the movement trend of the baselines was shifted from 2014 to early 2016 to a state in which the shortening was gradually slowed down, and the baselines returned to a normal movement state after the earthquake. The baseline NMAY-QHME across the northern margin of Qilian Mountains fault zone and the Longshoushan southern marginal fault zone was in a stable shortening stage from 2011 to the end of 2014, with a movement rate of about 2.8mm/a. The baseline has undergone a turning point since the end of 2014, and the baseline was in a relatively locked state since the end of 2014 until the occurrence of the Menyuan earthquake, with a displacement deficit of about 2mm. It indicates that the stress energy accumulation continued during this period. The baseline was restored to normal motion track after the earthquake. The QHGC-GSGL, QHME-GSGL, NMAY-QHME baselines are in a movement trend of transition after 2014, and the movement after 2014 presents the trend of slowed down compressional movement, this shows that during this period, the stress field energy was in a state of continuous accumulation.2 STRAIN TIME SERIES ANAYSIS OF DEFORMATION UNITS NEAR THE EPICENTER
The inter-station baseline reflects the crustal deformation along the direction of connection, and the combination of more than three GPS continuous stations is enough to calculate the geometric deformation parameters of an area. The analysis of the time series of these deformation parameters can be helpful to understand the deformation properties and strength of the region, and study its dynamic deformation characteristics (Jiang Zaisen et al., 2007; Zhu Shuang et al., 2015; Ma Haiping et al., 2016). The strain deformation parameters are not affected by the reference datum and can more reliably reflect the deformation properties and the degree of strain accumulation. In order to further discuss the dynamic changes of the tectonic setting of the epicentral area before the Menyuan earthquake, in this paper, the time series of the continuous deformation parameters of three GPS continuous stations, namely GSML (Minle, Gansu), GSGL (Gulang, Gansu) and QHGC (Gangcha, Qinghai) around the earthquake epicenter are calculated, and the results are shown in Fig. 3, where (a) is the time series of the surface strain Δ, (b) is the time series of the linear strain in the EW direction εe; the negative value indicates the compression state, the positive value represents the extensional background, and then the dynamic process of the crustal deformation near the epicenter before the earthquake is revealed.
Fig. 3 shows that the change of EW linear strain εe and surface strain is not linearly cumulative, as represented by a faster to slower accumulation process. The EW linear strain εe showed a deviation from the original compression rate in 2014, the compression rate decreased from the previous trend of -1.06 × 10-8/a to -0.31×10-8/a. This reflects the slowdown in the rate of strain accumulation in the Menyuan seismogenic region since 2014. The accumulation of plane strain shows the trend of compression, the accumulation rate was faster before 2014 and tends to be flat after 2014, and its compression rate decreased from -0.96×10-8/a to -0.29×10-8/a. This reflects that the contraction rate of the area is gradually weakening, that is, there are signs of deformation deficits in the earthquake area, indicating that the region accumulated a high degree of strain energy before the earthquake in Menyuan. Therefore, there was an abnormal background change in the area before the earthquake.3 CONCLUSION
(1) The baseline measurement results indicate that the movement trend of the two baselines across the Lenglongling fault zone shifted in the period from 2014 to early 2016 and entered a state in which the shortening movement was relatively slowed down. Baselines across the northern margin of the Qilian Mountains fault zone and Longshoushan southern marginal fault zone were shifted at the end of 2014, which was in a relatively locked state a year before the Menyuan earthquake. The movement trend of the three baselines turned after 2014, reflecting that the compression motion slowed down, and during this period, the stress field energy was in a certain accumulation state.
(2) The GPS strain rate of the deformation units shows that the EW linear strain and surface strain rate has weakened gradually since 2014, suggesting that there are obvious deformation deficits existing in the epicentral area in nearly two years, which indicates that the region had accumulated a high degree of strain energy before the earthquake in Menyuan. Therefore, there was a significant background anomaly change before the earthquake.
ACKNOWLEDGMENTS: The authors are grateful to the First Monitoring and Application Center of China Earthquake Administration for providing the time series data of GPS reference stations.
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