Lihua Fang

S wave velocity structure in southwest China from surface wave tomography and receiver functions

Using the surface wave records of 504 teleseismic events at 50 temporary and 92 permanent seismic stations in southwest China, we obtain phase velocity maps at 10, 15, 25, 40, 60, and 75 s at a grid spacing of 0.5° × 0.5° from the interstation correlation method and surface wave tomography. We also obtain the S wave velocity structures beneath three profiles using the joint inversion of receiver functions and surface waves. At short periods (10 and 15 s), high-velocity zones (HVZs) are found in the Panzhihua-Emeishan region, the Sichuan basin and the Weixi-Lijiang region surrounding the low-velocity zones (LVZs) from Songpan-Ganzi block to the east of Lijiang, where the elevations are significantly higher. At long periods (40, 60, and 75 s), HVZs are found in the Weixi-Lijiang region, the Panzhihua-Chuxiong basin,and the Kunming-Tonghai region, which form a belt in the center part of the study area. The fast polarization directions on both sides of the belt defined by the shear wave splitting of teleseismic SKS waves vary significantly and indicate that the flow of material from the plateau is blocked in two different depth intervals and leads to different horizontal extents. The long-period maps and the structures along the three profiles show that LVZs are present in the upper mantle beneath rapidly slipping fault zones, such as the Anninghe-Zemuhe-Xiaojiang fault zone, the Red River fault zone, and the Xiaojinhe fault zone, implying that these faults penetrate deep into the mantle.

Relocation of the 2012 M s 6.6 Xinjiang Xinyuan earthquake sequence

At 05:07 AM on June 30, 2012 (Beijing time), an Ms6.6 earthquake (hereafter referred to as Xinyuan earthquake) occurred at the junction of Xinyuan County, Ili Kazak Autonomous Prefecture and Hejing County, Bayinguoleng Mongol Autonomous Prefecture in Xinjiang Uygur Autonomous Region. The location of the main shock determined by China Earthquake Network Center is 43.4°N, 84.8°E and focal depth is 7 km. By the end of 08:00 AM on July 23, 501 aftershocks were recorded, of which 9 aftershocks with magnitude larger than ML4.0, and the largest magnitude of the aftershocks is ML4.9. The earthquake affected a large area, such as Yili, Urumqi, Shihezi, Karamay and so on. According to the statistics from Department of Civil Affairs, Xinjiang Uygur Autonomous Region, 155 thousand people were affected, with a direct economic loss of nearly 2 billion yuan (RMB). Xinyuan earthquake occurred in the northern Tianshan fault zone near the Kashi River fault. Seismic activity in the Kashi River fault is strong. More than three earthquakes with magnitude larger than 7 in the past 200 years have occurred near the Kashi River fault. These strong earthquakes are March 8, 1812 Nilka M8.0 earthquake, December 23, 1906 Manas M7.7 earthquake, March 10, 1944 Xinyuan M7.2 earthquake (Figure 1). We quickly relocated the aftershock sequences after the earthquake. Our new results provide important reference information for the study of seismogenic structure, the analysis of the spatial and temporal characteristics of the aftershock activity, and the determination of aftershock activity trend.

Relocation of the Yushu M S 7.1 earthquake and its aftershocks in 2010 from HypoDD

After the Yushu MS7.1 earthquake on April 14, 2010, a large number of aftershocks were recorded by the surrounding permanent network and temporary seismic stations. Due to the distribution of stations, knowledge about velocity structure, the reliability of seismic phases, and so on, the location result from conventional method is usually of low precision, from which it is difficult to recognize the spatial and temporal distribution and the trends of aftershock activity. In this paper, by using teleseismic waveforms recorded by permanent station, the seismic velocity structure beneath the vicinity is obtained from receiver function stacking and inversion methods. And the Yushu earthquake sequences are relocated from seismic phase data by HypoDD. The results show that the Yushu MS7.1 earthquake occurred at 13 km depth; the aftershock sequences were distributed mainly in the NWW along the Garzê-Yushu fault, and most aftershocks were concentrated in a 100 km length and 5–20 km depth. Combined with the velocity structure, it can be inferred that the earthquake mainly destroys the high-velocity layer of the upper crust. In the west of the seismic fault near (33.3°N, 96.2°E), the aftershock sequences were distributed like a straight column, suggesting there was a comminuted break from 25km depth to the ground.

Preliminary analysis on the source properties and seismogenic structure of the 2017 M s 7.0 Jiuzhaigou earthquake

At GMT time 13:19, August 8, 2017, an Ms7.0 earthquake struck the Jiuzhaigou region in Sichuan Province, China, causing severe damages and casualties. To investigate the source properties, seismogenic structures, and seismic hazards, we systematically analyzed the tectonic environment, crustal velocity structure in the source region, source parameters and rupture process, Coulomb failure stress changes, and 3-D features of the rupture plane of the Jiuzhaigou earthquake. Our results indicate the following: (1) The Jiuzhaigou earthquake occurred on an unmarked fault belonging to the transition zone of the east Kunlun fault system and is located northwest of the Huya fault. (2) Both the mainshock and aftershock rupture zones are located in a region where crustal seismic velocity changes dramatically. Southeast to the source region, shear wave velocity at the middle to lower crust is significantly low, but it rapidly increases northeastward and lies close to the background velocity across the rupture fault. (3) The aftershock zone is narrow and distributes along the northwest-southeast trend, and most aftershocks occur within a depth range of 5–20 km. (4) The focal mechanism of the Jiuzhaigou earthquake indicates a left-lateral strike-slip fault, with strike, dip, and rake angles of 152°, 74° and 8°, respectively. The hypocenter depth measures 20 km, whereas the centroid depth is about 6 km. The co-seismic rupture mainly concentrates at depths of 3–13 km, with a moment magnitude (Mw) of 6.5. (5) The co-seismic rupture also strengthens the Coulomb failure stress at the two ends of the rupture fault and the east segment of the Tazang fault. Aftershocks relocation results together with geological surveys indicate that the causative fault is a near vertical fault with notable spatial variations: dip angle varies within 66°–89° from northwest to southeast and the average dip angle measures ~84°. The results of this work are of fundamental importance for further studies on the source characteristics, tectonic environment, and seismic hazard evaluation of the Jiuzhaigou earthquake.

Crustal thickness and Poisson's ratio in southwest China based on data from dense seismic arrays

Crustal thickness and Poissons ratio are important parameters to characterize regional isostasy state and material composition or state. Using the teleseismic waveform data from 141 permanent stations and 785 temporary stations in southwest China, we obtain the crustal thickness and average Poissons ratio by the H-κ stacking of receiver functions. In the west (the SE Tibetan plateau and the Yunnan-Burma-Thailand block) and southeast (the Cathaysian block and southern Yangtze craton) of the study region, there are high correlation coefficients for the crustal thicknesses between what we obtain from the receiver functions and what we calculate from the Airy isostasy model, indicating that a state of isostasy can be achieved at the crust-mantle boundary beneath these two regions. In the northeast (northern Yangtze craton), the correlation coefficient is lower, indicating that the effect of the lithosphere needs to be considered for a regional isostasy. Intermediate Poissons ratios (0.26≤σ≤0.28) are found beneath the northern Panzhihua-Emeishan region. Combing the high velocity features from previous study, we speculate that it may be related to the Emeishan large igneous province. High Poissons ratios (σ>0.28)- are found beneath the SE Tibetan plateau and the nearby strike-slip faults, such as the Anninghe-Zemuhe fault and the northern Xiaojiang fault. Combing the low velocity zones from previous study, we speculate that there may be partially melted and lower crustal flow.