Neotectonics of the Southeastern Segment of the Phrae Fault System, Phrae Basin, Northern Thailand

Abstract

This thesis is composed of four main objectives. These objectives aim firstly, to identify neotectonic evidences which relate to displacement of the southeastern segment of the Phrae fault system; secondly, to analyze stress axis orientation acting on the study area and nearby; thirdly, to determine age of fault-related sediments and to recognize past movement events of the fault segment; fourthly, to classify activeness characteristic of the fault segment. The thesis study includes six steps of methodology, viz. planning and preparation, field reconnaissance, remote-sensing and aerial photographic interpretations, field investigation, laboratory experiment, and discussion and conclusion steps. Based on remote-sensing interpretation, field investigation, seismic profiles, and focal mechanism data, the southeastern segment of the Phrae fault system is a potentially active fault, located in SE margin of the Phrae basin, lain in NNE-trend with approximately 20 km-long. The west-dipping major fault is characterized as basin-bounded fault. The minor fault is found in high terrace, roughly lying parallel to the major fault showing antithetic east dipping to the major one. Tectonic geomorphological evidences along the fault segment including a shutter ridge, triangular facets and offset stream channels together with field evidences of normal faulting along the minor fault, thermoluminescence dating, and focal mechanism data, indicate that the fault segment is undergoing sinistral movement with small component of normal with maximum slip rate of 0.06 mm/yr. These evidences are consistent with the study on contemporary stress axis orientation in this area which reveals roughly E-W trend and N-S trend of extensional and compressional axes, respectively. Two paleoearthquake events with large magnitude (Mw ~7) were taken place in the study area. The first event was occurred between 0.9 Ma and 1.1 Ma and the second event was between 0.05 Ma and 0.17 Ma. Finally, recurrence estimation of large earthquakes generated by the fault segment is 0.9 Ma. Additionally, small earthquakes have been detected by Chiang Mai seismic station throughout the years, also indicate present-day active tectonism in this area.