Maximum Acceleration Recording from Lushan Earthquake on April 20, 2013

The Lushan earthquake on April 20, 2013 is another destructive event after the Wenchuan earthquake in 2008. More than 100 strong motion stations were triggered in the main shock. The 51BXD station located in Baoxing Country recorded a peak ground acceleration (PGA) of more than 1g, that is 1.026g (-1 005.3 gal) in the EW direction. It has been the first time that more than 1g is recorded at free field site in China and it should be a milestone event in Chinese strong motion observation history. However, in the released official Lushan intensity map, the around area was classified to intensity Ⅶ, which raises an interesting question why such a great PGA leads to a low intensity. During the post-earthquake field survey, it was found that this station was located on a steep slope, not a real "free-field" station, so the recording could be affected by the topography. By comparing with 3 other nearby stations with the next generation attenuation models, it is shown that the earthquake response spectrum less than 0.2 s was amplified significantly. To support this evidence, a background noise test was performed around this area, and predominant period and horizontal acceleration amplification factor were calculated with the traditional spectral ratio method. The calculation results clearly show that from the bottom of the valley to the top, the predominant frequency increases and the amplification factors also raise with the predominant frequency increasing. In addition, the response spectrum of 51BXD recording was calculated and compared with some others from the Wenchuan earthquake to interpret why there were so slight damages to its around buildings. The preliminary conclusion is that the topography is the root cause for 1g peak ground acceleration recording in the Lushan earthquake.