Investigation Seismic Response in Phitsanulok Province

Abstract

Phitsanulok, located in the lower northern region of Thailand, has historically shown no hidden fault lines. However, on June 29, 2023, an unexpected earthquake with a magnitude of 4.5 occurred, with its epicenter just 5 km beneath Phitsanulok. Scientists believe the quake may have been triggered by a previously undetected fault in the area. Despite this, seismic data is limited, and much of the available data is static, consisting of Atterberg limits, unit weight, soil classification, and standard penetration test (SPT) results. Although SPT data has its limitations, it is the only consistently recorded field parameter and can provide estimation of soil cohesion and friction angle. Additional seismic data was sourced from related studies, with waveform data generously provided by the Earthquake Observation Division of the Meteorological Department. The soil profile in the study area displays two main characteristics: one where clay and sand layers alternate, and another where a sand layer lies at the bottom, overlain by clay. Analysis of three borehole logs indicates that the amplitude of acceleration ranges from two to three times of the original acceleration, as determined by the finite element method. While many engineers in Thailand favor the pseudo-static approach, this study aims to investigate the behavior of the soil profile in the Phitsanulok area during an earthquake event, focusing on aspects such as liquefaction potential, shear failure, and soil amplification capacity. For practical purposes, the horizontal coefficient used to represent earthquake forces in the pseudo-static method could be 0.07 for earthquake magnitude of 4.5.