การสำรวจธรณีฟิสิกส์เพื่อตรวจสอบรอยเลื่อนศักยภาพมีพลังใต้ผิวดินใน อำเภอสันทราย จังหวัดเชียงใหม่

Abstract

This study applied geophysical survey including gravity, electrical resistivity, and seismic reflection methods to locate potentially active subsurface faults and associated geologic structure in the vicinity of the epicenters of significant earthquakes occurred in San Sai District, Chiang Mai Province. This prospective fault system is oriented in NNW-SSE direction across San Sai District area in the eastern part of the Chiang Mai Basin. The area is covered by Quaternary and Tertiary sediment comprising gravel, sand, silt, clay and laterite. The underlying basement has been reported to be the Carboniferous clastic sedimentary rocks including sandstone and shale. Two geophysical survey lines with 1 and 1.2 km long traversing the prospect fault trend line were conducted. Gravity data were acquired at every 30 m station. The Bouguer gravity profiles show smooth increasing gravity values from western to eastern direction and do not represent dominant gravity anomaly with related to proposed fault locations. However, the gravity results represent the west dipping direction of the rock basement. Electrical resistivity surveys were acquired using a dipole-dipole electrode configuration with 10 m electrode spacing while seismic reflection data were collected using 10 m geophone spacing and 5 m shot spacing. The electrical resistivity and seismic reflection results were integrated for fault interpretation together with previous study data. The interpretation presents two normal fault systems, the first cutting the upper part of the sections and the second the lower part. A few faults have offsets across an inferred upper layer and appear to continue upward to the surface. These particular faults may be appropriate candidates for being the active faults related to the significant earthquake activities in the area. This potentially active fault system should be studied in more details to confirm its geometry, orientation and lateral extent.