Temporary strong-motion observation network for Wenchuan aftershocks and site classification

The Ms = 8.0 (MW = 7.9) Wenchuan earthquake of May 12, 2008 was the largest destructive event on the Chinese mainland since the 1976 Tangshan earthquake. Thousands of aftershocks occurred in the following months. To improve the performance of the National Strong-Motion Observation Network System (NSMONS) in China, a temporary strong-motion observation network was quickly set up along the Longmenshan Fault in the most heavily damaged areas. We briefly reviewed the history and the advances in temporary strong-motion observation in China where temporary networks have been proven to be a very effective way of accumulating strong-motion records from large aftershocks. Then the case of the Wenchuan earthquake was described together with the process of compiling, standardizing and disseminating the collected data. A total of 3250 records were collected from 92 temporary stations for 949 aftershocks between May 13 and October 10, 2008. The largest peak ground acceleration (PGA) is 966.5 cm/s2 from an MS = 4.2 earthquake on August 10, 2008. Some general statistics, such as the number of recorded events with various magnitudes, PGA and epicentral distance, were analyzed and compared with the data from permanent stations. These results show that the temporary network can effectively compensate for the large instrument spacing of the permanent network to record near-field ground motion. Since no borehole data are available for those temporary stations, their site classifications are inferred using a horizontal-to-vertical spectral ratio (HVSR) technique based on the observed strong-motion records. A scheme of six site classes is proposed to improve the reliability of the HVSR technique. As a result, the site classes of 66 temporary stations are recommended finally.