Geosciences Journal Vol. 19, No. 4, p. 731  740, December 2015 DOI 10.1007/s12303-015-0011-8 ⓒ The Association of Korean Geoscience Societies and Springer 2015 Probabilistic analysis of the seismic activity and hazard in northern Thailand ABSTRACT: The seismic activity and hazard level in northern Thai- land, including at the Mae Moh Coal Mine (MMCM), were clarified. For the probability of exceedance (POE), Chiang Mai, Lamphun, and Lampang provinces have a 70–90% and 20–40% POE of a M W -5.0 and M W -6.0 earthquake, respectively, in the next 50 y. In the case of a M W -7.0 earthquake, the POE is less than 10% in the whole study area. Regarding the probabilistic seismic hazard anal- ysis, the ground shaking maps indicated that the southeastern part of the MMCM and northwestern part of Phayao provinces were high hazard areas, with an earthquake ground shaking of around 0.28–0.32g and 0.18–0.24g for a POE of 2% and 10%, respectively, in the next 50 y. In addition, the probability maps revealed that these high hazard areas showed a 60–80% and 30–40% POE of a modified Mercalli intensity (MMI) level III and IV, respectively, in the next 50 y. The low hazard areas of Chiang Mai and Uttaradit provinces had a ground shaking level for a 2% and 10% POE in the next 50 y of around 0.16 and 0.08, respectively, with a POE of a MMI level III or IV of less than 20% each. Key words: earthquake, probability, seismic hazard analysis, Mae Moh Coal Mine, northern Thailand 1. INTRODUCTION As a result of the Indian-Eurasian Plate collision, the Main- land Southeast Asia is dominated by a number of areal seismic source zones (Pailoplee and Choowong, 2013) and seismogenic fault zones (Pailoplee et al., 2009) (Fig. 1a). Tectonically, northern Thailand and the neighboring areas (black square in Fig. 1a) were subjected to a high tension producing a large number of intermontain basins (Morley et al., 2001). Among these, the Mae Moh Basin in Lampang province is the most economically significant region due to the presence of the largest coal mine in Thailand, the Mae Moh Coal Mine (MMCM) (Figs. 1b and c; Benammi et al., 2002). However, due to the present-day tectonic activities, the faults bounding half-grabens and grabens in most basins of northern Thailand are still seismogenically active (Fig. 1b). As a result, earthquakes have been generated continuously over time in the northern part of Thailand (Fig. 1c). For instance, a M W -7.0 earthquake occurred on March 24 th , 2011 at the Nam Ma Fault Zone (no. 1 in Fig. 1c). Thereafter, there was a cluster of shallow crustal earthquakes in December 2011 (2.5–3.0 m b ) and in March 2013 (2.4–3.4 m b ) that were generated within a 30 km-radius from the MMCM (no. 2 in Fig. 1c). These small but quite close events presented a hazard risk to some infrastructures and created panic among the local people at Lampang province, including at the MMCM. In addition, the latest hazardous earthquake event (M W 6.2) on May 5 th 2014 (no. 3 in Fig. 1c) confirms that northern Thailand, including the MMCM, is not quiescence from seismic hazard. Although the MMCM is an open pit mine and so may not be affected as seriously by earthquake ground shaking as underground mines, there are some short-storey power plants and operation centers that may show resonance with the short-period seismic waves generated from such local earth- quakes. Therefore, a detailed assessment of the seismic activity and hazards in northern Thailand, including the MMCM area, are required in order to provide an effective mitigation plan for any forthcoming earthquakes. 2. SEISMIC SOURCES The study area (black square in Figs. 1b and c) is occupied by at least six seismic source zones (Pailoplee and Choowong, 2013) and 36 possible active fault zones (Pailoplee et al., 2009). From the regional tectonic setting controlling the style of fault movement and rate of fault slip, four unique seismogenic zones were classified as follows. (1) North-South Strike-slip Fault. The 1,200 km-long north to south orientated Sagaing Fault Zone in Myanmar (no. 13 in Fig. 1b) moves with a high velocity of 23 mm/y (Bertrand and Rangin, 2003). An earthquake of magnitude 8.0 was generated by this fault on May 23 th 1912 (Brown, 1914). (2) Northwest-Southeast Strike-slip Faults. These north- west to southeast orientated fault zones are delineated along the border between eastern Myanmar and western Thailand, such as the Three Pagoda, Sri Sawath, Pan Luang, Pa Pun and Moei-Tongyi Fault Zones (No. 5, 8, 18, 19 and 22 in Fig. 1b). Based on morphological analyses and paleoseis- mological investigations, the rate of fault slip of these fault zones has been estimated at around 0.73–2.00 mm/y (Fen- ton et al., 2003; Charusiri et al., 2004). An earthquake of M W -5.6 occurred at the Moei-Tongyi Fault Zone on Feb- ruary 17 th 1975. (3) North-South Normal Faults. These fault zones are the faults bounding the intermontain basins and are mainly Santi Pailoplee* Punya Charusiri } Earthquake and Tectonic Geology Research Unit (EATGRU), Department of Geology, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand *Corresponding author: Pailoplee.S@gmail.com