NEOGENE VOLCANISM AND HOLOCENE EARTHQUAKES IN THE TANLU FAULT ZONE, EASTERN CHINA

In this paper, we study the relationship between Neogene volcanism and Holocene earthquakes in the Tanlu fault, eastern China, We find that fault segments through which Neogene and Quaternary magma have extrude d do not show Holocene slip because they are covered by unfaulted basa lts. In contrast, fault branches that are away from the Quaternary vol canic centers display Quaternary faulting and are responsible for eart hquakes as recorded both historically and geomorphologically, Therefor e, magma intrusion appears to modify fault activity in two different w ays: (1) within the faults, the cooling magma serves as a cohesion or barrier, welding the faults so that they become stronger in resisting slip; (2) beneath the faults, the upwelling magma promotes slip of fau lts above the magma body, and hence generation of earthquakes. Physica lly, the first case results from contraction of the cooling magma, whi ch causes a relative increase in fault-normal stress so that the fault failure resistance is enhanced. The second case results from the upwa rd dynamic force and the heat brought in by the magma body, both of wh ich cause the effective fault-normal stress to decrease so that the fa ult failure resistance is reduced. That could explain why earthquakes occur on faults bypassing the volcanic centers as typified in the Yish u fault zone and in the regions where heat flows are relatively high a s shown in the Bohai Bay area.