At 14:28 on May 12, 2008 (Beijing and Sichuan Time), a great earthquake occurred in Sichuan province, China, with a surface wave magnitude of 8.0. The earthquake epicenter was located in Wenchuan County at latitude 31.021?N and longitude 103.367?E, and a focal depth of 14 km. The associated fault rupture is mainly thrust with a strike-slip component on the Longmenshan fault belt. However, the source mechanism is quite complex. The rupture process is mainly unilateral with the main rupture spreading to the northeast about 300 km from the epicenter The post-earthquake geological studies found a 200 km long surface rupture along the central branch of the Longmenshan fault belt (i.e Yingxiu- Beichuan fault) and a rupture of 60 km along the front branch (i.e. Guanxian-Jiangyou fault). As of August 29, 2008, a total of 261 aftershocks with magnitude larger than 4.0 have occurred. Among them, 31 aftershocks were larger than 5.0, and 8 aftershocks larger than 6.0. The largest aftershock occurred on May 25 with a magnitude of 6.4.
The China Digital Strong Motion Observation Network was completed in March 2008 after nearly five years of construction and trial operation, resulting in a broad distribution of observation stations in China and intensive distribution in some local areas. During the Wenchuanearthquake of May 12, strong-motion records were obtained from about 460 permanent ground stations and three arrays for topographical effect and structural response observation. After the main shock, 59 mobile instruments were quickly deployed in the hard hit areas to record ground motions generated by strong aftershocks.
A total of about 1,400 components of ground motions from the main shock were recorded, and as of 1 August 2008, over 20,000 components from strong aftershocks have been recorded. For the mainshock records, there are more than 500 components with peak acceleration larger than 10 gal (cm/sec/sec), 200 larger than 50 gal, 115 larger than 100 gal, 42 larger than 200 gal, 16 larger than 400 gal, and 7 larger than 600 gal, and no records over 1000 gal.
The largest peak ground acceleration from the main shock is 958 gal recorded at Wolong station in Wenchuan County, Sichuan. Recorded peak accelerations and preliminary peak velocity values from some key stations along the fault are listed below:
Wolong station in Wenchuan County: Epicentral distance = 19 km; Distance to Yingxiu-Beichuan fault = 23 km PGA (gal) - 957.7 (EW), 652.9 (NS), 948.1 (UD) PGV (cm/s) - 51.5 (EW), 41.7 (NS), 30.4 (UD)
Bajiao station in Shifang City: Epicentral distance = 67 km; Distance to Yingxiu-Beichuan fault = 10 km PGA (gal) – 556.2 (EW), 581.6 (NS), 633.1 (UD) PGV (cm/s) – 62.7 (EW), 89.8 (NS), 49.6 (UD)
Qingping station in Mianzhu City: Epicentral distance = 88 km; Distance to Yingxiu-Beichuan fault = 3 km PGA (gal) – 824.1 (EW), 802.7 (NS), 622.9 (UD) PGV (cm/s) – 133.0 (EW), 65.3 (NS), 39.6 (UD)
Zengjia station in Guanyuan City: Epicentral distance = 314 km; Distance to Yingxiu-Beichuan fault = 86 km PGA (gal) – 424.5 (EW), 410.5 (NS), 183.3 (UD) PGV (cm/s) – 44.0 (EW), 25.8 (NS), 24.6 (UD)
Preliminary analysis of these strong-motion records shows the following characteristics of ground motion from the earthquake, especially for near-fault ground motion:
(1) Large peak accelerations are recorded from stations located along the fault, and the distance to the rupturing fault, rather then the epicentral distance, clearly controls the ground motion attenuation.
(2) Peak accelerations at stations in the fault rupture propagation direction are relatively large, consistent with the fault rupture propagation or directivity effect.
(3) In the near-fault records, peak acceleration in the EW direction is in general larger than that in the NS direction.
(4) Peak accelerations at stations on hanging wall of the thrust fault are generally larger than those from stations on footwall, which shows hanging wall effect on ground motion.
(5) The area with large accelerations seems to be relatively larger near the northeast segment of the fault than the southwest segment of the fault.
(6) For some near-fault records peak acceleration is larger in the vertical direction than in one or two horizontal directions.
(7) Large velocity pulses of the ground motion appear in some near-fault records. Peak ground accelerations for both horizontal components from the main shock versus the distance to the fault were compared with other ground motion attenuation relationships. The peak horizontal ground accelerations of the Wenchuan Earthquake decrease much more slowly than those from other attenuation relationships, especially in the fault distance ranging from 100 to 300 km. Some recorded peak values are about 100 gal even at the fault distance of about 400 km. The peak ground acceleration is highly varied in the fault distance ranging from 100 to 500 km.