On the Forecasts of the Lushan Earthquake in 2013

A magnitude 7.0 earthquake occurred at the Lushan County, Ya'an City, Sichuan Province, China in April 20, 2013. We had various forecasts of the time and location of this earthquake. In order to advance development of geosciences and improve accuracies of forecasts of earthquakes, which, should help to minimize damages from earthquakes, we summarize these forecasts in this paper. After a magnitude 9 earthquake occurred off the northeastern coast of Japan on March 11, 2011, we used the method of commensurability to analyze the earthquakes in recent years in the world and found that the dates of these earthquakes are all at commensurable points. According to our analyses dates (in units of years from the A.D.) of earthquakes in the Sichuan-Yunnan block follow a law of commensurability as 2.44K+b with K an integer numbering between 0 and 5. From this law we predicted an earthquake to occur at the time 2013. 24 A.D. in the block, which is consistent with the Ya'an (Lushan) earthquake. We further checked a map of gravity anomalies from satellite measurements, which actually reflects the distribution of crust-density anomalies. From the map we found that there were two highly abnormal regions of local abrupt changes of gravity gradients on the west side of the Ya'an and at the Wenchuan County, respectively. The two regions have common characteristics. The 2008 Wenchuan earthquake only released the energy and stress in the northeastern section of the Longmenshan fault zone. In the southwestern section of the fault zone, particularly in the regions of gravity anomalies there, the energy and stress became more concentrated due to the Wenchuan earthquake. Prior to the Ya'an earthquake one of us (Zeng Zuoxun) had noticed the pattern of gravity anomalies and repeatedly pointed out that the next high-magnitude earthquake in the Sichuan Basin after the Wenchuan earthquake was about to happen between Ya'an and Kangding. It is unfortunate that the studies of commensurability and gravity anomalies were carried out by different authors without communications about the studies. If the two studies were combined before the Ya'an earthquake, a short-term forecast of the earthquake could have been realized. This once again shows that earthquake forecasts need to take synthetic approaches.