July 1, 2020.
Earthquake is natural phenomena which cause of loss in life and properties. Although compared to the other country, Thailand has less seismicity and risk from earthquakes. It cannot be reassured because the fundamental knowledge of active faults in our country is still insufficient. The largest earthquake (ML 6.3, by TMD) in Thailand modern history occur at Phayao Fault Zone on the 5th of May 2014 in the fault that was believed to be less active. This earthquake sequence surprised Thai geoscientists because such a large earthquake was expected to take place on Mae Chan Fault Zone, which shows more seismogenic potential .
The project aims to investigate the structure and mechanism of active faults in Thailand. We currently conduct an investigation of the fault zone than recently has high seismicity such as Phayao Fault Zone and Western Fault Zones. The most massive fault in Thailand, Mae Chan Fault Zone, which potentially hazardous, is a major focus in our study.
The Mae Chan Fault Zone
The most interesting active fault in Thailand is Mae Chan Fault Zone (MCFZ), which geological evidence  shows that it is the longest (and probably most active) fault in Thailand. The MCFZ shows a clear-cutting trace that can be defined from a satellite image. Potentially, this longest fault is then capable of producing a larger earthquake in Thailand. However, according to the earthquake catalog of the Thailand Meteorological Department (TMD), MCFZ shows only a small number of “observed earthquakes”. This contradiction is leading to two possible explanations. First, there might be a large number of earthquakes that cannot be detected by the national seismic network. The MCFZ is located near to the border of the country where the number of seismometers is small, and the coverage is inadequate. The number of observed earthquakes is then significantly lower than the actual behavior of the fault zone. Second, MCFZ has small seismicity, and its mechanical process might be complicated than the general Andersonian faulting mechanic. For example, the particular characteristic of faults, such as curvature or crustal fluid (e.g., Phayao Fault Zone), might play a significant role in the earthquakes' mechanism.
The MCFZ investigation was initialized in 2018 with the survey trip looking for suitable sites for seismometers and magnetometers installation. We plan to install seven broadband seismometers to cover the area of MCFZ with a station spacing of around 20 km (Figure 1). Now, four stations are operating in the MCFZ area, and three stations will be installed this year (relocated from Nan province). This dense seismic network will significantly increase the earthquake detectability in the area . The great azimuthal coverage will be capable of the investigation of earthquake mechanisms . The crustal imaging via receiver function technique [Ammon] will be applied to investigate the structure of the fault. One of our master students is developing the algorithm for the inverse problem of receiver function .
The 3D structure of MCFZ will be investigated via the Magnetotelluric (MT) method. Because our goal is to examine the deep crustal structure of the fault zone, the long period electromagnetic data is then required for the analysis. Twenty-four hours long of the data were acquired for each MT site. Spending several trips and several months on the field, we successfully survey 42 MT stations covering the area of MCFZ (Figure 1). The resistivity model of MCFZ will be constructed via the inversion of MT data in the next stage. The variation of resistivity will be inferred as the mechanical properties of the fault zone. For example, the high conductivity body is observed in Mae Lao Segment of Phayao Fault Zone (Figure 2). This conductive body is interpreted to be crustal fluid, which reduced the effective friction along the fault plane . The fault zone structure from MT inversion will be corporately analyzed with the mechanism of the fault observed from the seismic investigation.
Fig. 1 Location of MT site (black triangle) and seismic station (red diamond) for Mae Chan Fault Zone investigation.
Fig. 2 3D resistivity structure of Phayao Fault Zone that inverted from MT inversion . Red area indicates the low resistivity structure (ML) which juxtaposing to the location of earthquake epicenters. The low resistivity is interpreted to be crustal fluid that reduced the effective friction of fault plane.
Post-seismic investigation, Wang Nhau earthquakes, Lampang, 14th March 2019
Although, MCFZ is our main study since the beginning of the project. We also investigate other active faults that show a seismic activity in the present time. On the 14th March 2019, earthquake of magnitude (ML) 4.1 occurred at Wang Nhau district, Lampang province. Several clear foreshocks were observed ahead of the mainshock for this earthquake sequence. According to the active fault map of the Department of Mineral Resource, these earthquakes were released by the activity of the southern part of Phayao Fault Zone. However, the location of epicenters falls in the middle of the western and eastern segments. The structure difference between the two segments is then important to understand the generating process of this earthquake sequence. We deploy 20 MT sites (Figure 3) to investigate the local structure of this fault segment. The resistivity structure and seismicity will be linked and compared with the Northern segment of PYFZ.
Fig. 3 Post-seismic MT array survey for M4.1 14 March 2019 Wang Nhau, Lampang earthquake. Dense MT sites were deployed around the area of epicenters, southern part of Phayao Fault Zone. Circles indicate background seismicity of the area which mainly accommodated by Mae Lao 2014 earthquake sequence.
Laos Earthquake, ML 6.4 21 November 2019
On the 21st November 2019, earthquake magnitude (ML) 6.4 took place in Laos at a distance only around 10 km from the Thailand border. The propagation of the seismic wave from this event can be felt even in Bangkok. The earthquake took place on the location where the fault trace is complicated and contains less information. Considering its impact, the location of the earthquake is just about 30 km to the newly operate Xaiyaburi Dam. The earthquake also causes damage to Thai’s houses and trigger landslides in districts near the Thai-Laos border. The fault that generates this earthquake is also expected to connect to the major active faults in Thailand. To observe the mechanism and characteristic of aftershock, we arrange a rapid team to install three broadband seismometers in Nan province along the border within 24 hours after the mainshock.
Fig. 4 Three temporary stations for aftershocks investigation of Laos M6.4 earthquake.
Assoc. Prof. Dr. Weerachai Siripunvaraporn, Dr.Songkun Boonchaisuk, Dr. Puwis Amatykul and Dr. Sutthipong Noisagool
Geophysics Research Group, Department of Physics, Faculty of Science, Mahidol University
 Fenton, C. H., Churusiri, P., and Wood, S. H., 2003, “Recent Paleoseismic Investigations in Northern and Western Thailand”, Annals of Geophysics, Vol. 46, N. 5., 957 – 997.
 Bondar, I., and McLaughlin, K. L., 2009, “A New Ground Truth Data Set for Seismic Studies”, Seismological Research Letters, Vol. 80, p. 465-472.
 Noisagool, S., Boonchaisuk, S., Pornsopin, P., and Siripunvaraporn, W., 2016, “The Regional Moment Tensor of the 5 May 2014 Chiang Rai Earthquake (Mw = 6.5) and Its Aftershocks and Implication of Stress and Instability of Phayao Fault Zone”, Journal of Asian Earth Science, Vol. 127, P. 231-245.
 Sarun Trinakoon, Puwis Amatykul, Weerachai Siripunvaraporn, Sutthipong Noisagool, 2019, “Comparing Performance of Multi-frequencies Occam’ s Receiver Function Inversion to Standard Linearized Receiver Function Inversion”, Journal of Physics: Conference Series.
 Boonchaisuk, S., Amatayakul, P., Rung-Arunwan, T., Vachiratianchai, C., Noisagool, S., Siripunvaraporn, W., (2017), “3-D Magnetotelluric Imaging of the Phayao Fault Zone, Northern Thailand: Evidence for Fluid in the Source Region of the 2014 Chiang Rai Earthquake”, Journal of Asian Earth Science, Vol 127, p. 210-221.
Assoc. Prof. Dr. Weerachai Siripunvaraporn
Geophysics Research Group, Department of Physics, Faculty of Science, Mahidol University, Bangkok - 10400, Thailand