1984年9月14日,日本本州中部发生了M6.9的“长野县西部地震”(日本气象厅命名),有感范围甚广。此次地震是日本近年来仅次于北海道浦河地震(1982年3月21日,M7.1)的内陆大震。就长野县一带而言,则是明治时代以来发生过的最大地震(1918年11月曾发生过 M6.5的地震,长野烈度为日本气象厅烈度标度的4度)。长野县临近日本中部地区最大的活断层——阿寺断层,为了监视该断层的活动,从1978年开始把这一地区指定为日本全国8个地震特定观测地区之一,并布设了 On September 14, 1984, M6.9 “Western Nagano Prefecture Earthquake” (named after Japan Meteorological Agency) occurred in central Honshu, Japan, with a wide range of sensations. The earthquake was the second largest inland earthquake in Japan in recent years after the Puhe earthquake in Hokkaido (M7.1, March 21, 1982). In the case of Nagano Prefecture, it was the largest earthquake ever since the Meiji era (an M6.5 earthquake occurred in November 1918 and the intensity of Nagano is 4 degrees on the intensity scale of Japan Meteorological Agency). Nagano Prefecture, the largest active fault in Japan’s central region, the A-Masu fault, has been designated as one of eight seismic-specific observational areas in Japan since 1978 in order to monitor the activity of the fault.