Great earthquakes of M≥8 in the mainland of China and their evolution

Abstract By utilizing the information about earthquakes recorded during the long Chinese history, the relationship between twelve great earthquakes of M ⩾ 8 and the smaller earthquakes in vast areas both before and after the occurrences of these twelve great earthquakes was analyzed. The evolution process of a sequence of earthquakes with an M ⩾ 8 event as the mainstay, from the preparatory phase to their occurrences, is studied. It is found that the evolution process of an earthquake sequence consists of four phases: a relatively quiescent period, earthquake activities before the mainshock, mainshock activities and aftershock activities. Such a sequence of earthquakes, with one M ⩾ 8 event, is the result of earthquake development during a sufficiently long period of time and over a vast region. While the preparatory area of an M ∼ 8 earthquake is usually a large active fault zone or a tectonic zone, the preparatory area of an M ∼ 8 1 2 earthquake may be several times larger, consisting of several large active fault zones or tectonic zones. China and its continental vicinity may be divided into five such large seismic regions related to the development of M ∼ 8 1 2 earthquakes, which may be further divided into over twenty small seismic zones related to the development of M ∼ 8 earthquakes. The duration of preparatory time for M ⩾ 8 earthquakes differs in different places. Near the boundaries of plates, the duration is generally 40–50 years, while in intracontinental places the duration may be over 100 to 300–400 years or even longer. Chinese intracontinental M ⩾ 8 earthquakes are also related to the activity of large earthquakes along the boundaries with surrounding plates. The preparatory process of an M ⩾ 8 earthquake is not only controlled by the tectonic conditions nearby, but may also be influenced by plate movements all over the globe.