Historical pattern and behavior of earthquake ruptures along the eastern boundary of the Sichuan-Yunnan faulted-block, southwestern China

The eastern boundary of the Sichuan-Yunnan faulted-block in southwestern China, which contains four major fault zones named Xianshuihe, Anninghe, Zemuhe, and Xiaojiang, behaves as a huge left-lateral strike-slip active fault system, where the most frequent seismicity in continental China occurs. To uncover the history of earthquake ruptures of this fault system, we developed an empirical relation between rupture extent and seismic intensity distribution based on data of those earthquakes whose rupture extents and intensity distributions are well known. Through analyzing various types of data, including distributions of damage or intensity of historical earthquakes as well as surface ruptures and aftershock zones of modern earthquakes, we determined systematically the locations and spatial extents of ruptures for 36 main earthquakes, and built up a spatial-temporal pattern of the rupture history of the fault system for the last several hundred years. The spatial-temporal rupture pattern contains information of multi-cycle and broad-scale ruptures, from which we find that for most fault segments, scales of segment-ruptures are time- or cycle-variable and only in two or three cases do scales of segment-ruptures in successive cycles seem to be characteristic. In infrequent cases, several adjacent and connecting fault units may rupture simultaneously, resulting in cascading ruptures. Triggering of ruptures is common along the fault system but differs in various periods even on a single fault zone. Triggered ruptures may not occur on fault-segments adjacent to a preceding rupture of major earthquake but on other segments at some distance away from the preceding rupture, implying that the rupture history determines whether a segment can be triggered or not. Recurrence intervals of major segment-ruptures are longer on the Anninghe and Zemuhe fault zones and on the southernmost segment of the Xiaojiang fault zone than that on the Xianshuihe fault zone and the northern and middle segments of the Xiaojiang fault zone, probably due to the partitioning of slip along major and secondary faults and their complicated fault geometry. Four seismic gaps along the fault system are recognized, where major earthquakes have been absent for a long time.