Seismic source zones for Thailand was first introduced by Nutalaya et al. (1985), based upon both seismological and geological evidences. Subsequently, Shrestha (1990) identified 9 active faults in the country on the basis of only seismological data analysis. Recently Department of Mineral Resources put an enormous effort to produce the active fault map of Thailand (Fig. 1) with the co-operative research studies of Chulalongkorn University (Thailand) and Akita University (Japan) on the bases of all the relevant geotectonic, digitally - enhanced satellite-borne image, geochronological, historical and seismological data along with the earlier published works (e.g., Hinthong, 1995; Nutalaya et al., 1985). Apart from the above - mentioned information applied, the other significant data are from the results of morphotectonic trench log and geophysical investigations. This commences with remote sensing interpretation to determine significant lineaments, follows with Quaternary field mapping of landforms, and then identifying morphotectonic features and styles – including primary, as fault scarp degradation and secondary, such as offset and beheaded streams, sag ponds, shutter and pressure ridges. Then ground geophysic (resistivity and GPR) surveys are carried out to confirm the existence of the interpreted faults. Detailed geodetic survey has been performed in detail to locate the appropriate site for exploratory trenching. Detailed stratigraphic logging has been done prior to sampling for Quaternary dating have been made. Based upon these consecutive sequences of work, we can delineate the Fault Zone (FZ), which is herein defined as linear or elongate zone of seismicity commonly classified by neotectonic movement and coincident with major tectonic structures. As displayed in Fig. 1., Thailand consists of 15 active FZ. Followings are a summary of the faults that have been studied.
1) The 155 km-long, E-trending Mae Chan FZ indicates the latest movement of ca. 1.5 ka, based on TL and C-14 dating data (Kosuwan et al., 1999). Offset stream of about 600 m has been observed and this indicates a left-lateral movement of the fault. Recent activity is also marked by the presence of a few hot springs located along fault lineaments. In this region, earthquake events with intensity of VII or greater are reported for year 460, 534, and 1715 A.D (Nutalaya et al., 1985). It is quite likely that these historical earthquake events were reported to be responsible for inundation of the ancient town of Wiang Yonok situated on the easternmost part of this fault. The recorded largest tremor of a body-wave magnitude (Mb) 4.0 took place on 1 September, 1978. Seismological data indicate that earthquakes of lower magnitudes also occurred along this fault. About 10 earth tremor of magnitude greater than Mb 3.0 were recorded since 1978, and three of these are over Mb 4.5.
2) The Mae Tha FZ forms a curiously arcuate trace appoximately 140 km-long, roughly NW-trending to the east of the Chiang Mai basin. The fault plane has a moderate dipping angle to west and northwest. Along the northern part of its fault trace it sharply truncates the Mae Kuang River, with the offset of about 4.5 km in the right lateral slip. Small earthquakes with mostly less than Mb 3.0 and shallow depth occurred abundantly in the northwestern part of the fault. However, our geomorphologic investigations show poorly-defined morphotectonic features during Quaternary. Hot spring locations are mainly in the southern part of the fault. No dating data have been done along this fault yet.
3) The arcuate, NE-trending Thoen FZ is approximately 140 km-long. Quartz concentrates from fault - related colluvium yielded displacement ages of ca. 4.0, 3.0, and 2.0 ka using TL dating approach and from charcoal samples using C-14 method also. During the past two decades, more than two dozen of earthquakes with the magnitudes of Mb 3.0 to 5.0 were detected nearby this fault to some extent. The fault is situated more or less in the Phrae and Long basins. The latest maximum tremors at the eastern side of the Phrae Basin and Thoen fault were recorded on 22 and 23 December 1980 with the magnitudes of Mb 4.0 and 3.7, respectively. Our recent morphotectonic and remote-sensing investigations reveal that the former is considered to be relatively older than the western side of the Phrae basis where the Phrae fault (Fenton et al., 1997) is situated. Afterwards until 1983, a few quakes with magnitudes ranging from Mb 2.5 to 4.0 were instrumentally recorded. Most hot springs are discovered in the southern part of the Thoen Fault. Our geomorphological evidence advocates the left lateral movement.
4) The approximately 115 km-long, NE-trending Uttaradit FZ shows rather distinct and straight scarps along the Nam Nan River near Nam Pat District. Satellite image and field data indicate that the fault is likely to extend to northern Lao PDR, where few earthquakes with magnitude about Mw 3.0 were recorded. The fault forms a prominent northwest-facing escarpment, the base of which is marked by a very sharp bedrock-alluvium contact, and crossing young alluvial deposit. No dating data is confirmed for the Quaternary movement, however Fenton et al. (1997) estimate the maximum earthquake of about Mw 7.0 using surface rupture length method (Well and Coppersmith, 1994).
5) The approximately 85 km-long, N-trending Pua FZ is a west-dipping normal fault. The northern segment of the fault is marked by a very prominent linear escarpment. The middle part of the fault is characterized by a well developed wine-glass canyon at Nam Khun valley, Ban Santisuk village. Landsat TM imagery shows the fault continuing as a very prominent lineament crossing recent alluvial deposits. Earthquakes with the magnitude of Mb 7.0 occurred very close to the fault during 1934 and 1935 in northern Lao PDR and Thailand, respectively (Bott et al., 1997). The short-term and long-term slip rates are estimated ca. 0.6 and 0.06 mm/yr, respectively, and the maximum earthquake is about Mw 7.25 (Fenton et al., 1997). Our preliminary TL dating data confirm the movement age of the Pua FZ in the Quaternary period.
6) The 45 km-long, NNW-trending Phayao FZ exhibits a sharp scarp clearly observed from the Landsat image suggesting Quaternary movement with eastward down-thrown side to the Payao Basin, possibly related to the so-called Kwan Phayao Lake. This lake is interpreted representing the large sag pond – an internal fault wedge basin which forms branches in the bifurcating dextral fault as a result of slip movement. The 1994 Pan earthquake with the magnitude about Mb 6.0 at the northern extension of the fault provides the good evidence on a fault – generating earthquake. Based on focal mechanism, Bott et al. (1997) considered that the similarly oriented fault is located possibly within the Payao basin.
7) The Mae Yom FZ has been mapped as a steeply dipping NE-striking fault that extends for a distance of approximately 25 km. The fault indicates the latest movement of ca. 5.0 ka, based on preliminary TL and C-14 dating data. Offset of stream about 70 m of tributary of Mae Yom river indicats that left-lateral strike-slip fault movement with the paleoearthquake magnitude of about 6.5 in the Ritcher scale.
8) The 65 km-long, NE-trending Mae Ing FZ clearly indicated by gological and Landsat image investigations. The fault plane dips toward NW. Afterwards until 1983, many time quakes with magnitudes ranging from ML 2.8 to 4.0 were instrumentally recorded. Two tremors occurred on 23 August 1999 and 7 July 1990 with ML 4.0 and 3.8, respectively.
9) The almost 215 km-long, straight and N-trending Mae Hong Son FZ clearly indicated by gological and Landsat image investigations. The fault is a sharp lineament with normal movements. Although no large earthquakes are recorded in Thailand, further to the north in Myanmar, there have been several small to large frequent earthquakes including that of the Mw 5.1 on 1 March 1989. However, at present no dating data have been done for the paloeseismic investigation.
10) The discontinuous, 250 km-long, NW-trending Moei (or Mae Ping ) FZ has its several spray fault segments, which are clearly observed from both Landsat and NOAA satellite images. This FZ which passes the Upper Chao Phraya Basin is very likely to extend to Cambodia and may have caused the development of the Ton le Sab Lake, the other fault-wedge basin formed by the bifurcating dextral movement. The major, W-WNW branch is observed cross-cutting Holocene sediments in Tak (Bhongaraya and Thiramongkol, 1998). Shrestha (1987) noted that an earthquake on 23 September 1933, with no record of intensity and magnitude, was felt along this fault for duration of about 1 minute. An earthquake of Mb 5.6 occurred nearby the fault on 17 February 1975. A fault plane solution indicates a principal component of right-lateral movement (Le Dain et al., 1984). Our recent preliminary results on morphotectonic, geophysical and TL dating investigations of this fault in east-central part of Thailand (Nakhon Nayok) reveal several movements along this fault in the Late Quaternary period.
11) The 215 km-long, arcuate NW-trending Sri Sawat FZ hosts several spray faults. Though several hundred shallow-depth quakes were reported along this FZ, and ground cracks and landslides were encountered in a NW-SE direction within the epicentral area, the Si Sawat FZ is regarded as the source of these reservoir-induced earthquakes (RIS) by Klaipongpan et al. (1991) and Chung and Liu (1992). Two large shallow-depth tremors took place on 15 and 22 April 1983, with the Mb 5.3 and 5.9, respectively, and were followed by at least 140 aftershocks throughout that year. Shrestha (1987) notes that the Sri Sawat FZ exhibits a dextral movement in Quaternary period. But Klaipongpan et al. (1991) emphasized, based on the long-term seismicity study as well as the focal mechanism for the 1983 RIS, that the east-trending reverse fault at the southern end of the fault is considered to account for the surface ruptures where few hot springs are closely located. Our results on geophysical and TL dating data of the fault-related sediments and fault gouge material strongly confirm the latest movement in the early Holocene epoch.
12) The NW-trending Three Pagoda FZ joins the Sagiang FZ of Myanmar. Its length in Thai portion is about 200 km. It occupies the Quae Noi and Mae Klong River channel. The moderate earthquake of Mb 5.8 was reported close to this FZ during 11 January 1960. Two other tremors occurred on 22 January and 11 July 1985 with Mb 4.5 and 3.9, respectively. However, these quakes are regarded to have occurred as a result of RIS (Hetrakul et al., 1991). Later minor quakes, as evidenced by the swarm-type spatial and temporal distributions, show mostly the dextral motion along the NE-trending conjugate fault set. TL-, ESR, and C-14 dating data of samples collected from fault – related colluviums nearby this FZ, yield the movement age of 2.0 ka.
13) The Tha Khaek FZ lies in the channel of Mae Khong River, which has been mapped as a steeply dipping NW-striking fault that extends for a distance of about 140 km. This FZ is clearly indicated by Landsat image investigations. Satellite image indicate that the fault is extend to central Lao PDR, where two earthquakes were recorded on 22 January 1997 with ML 4.2 and on 11 June 1988 with ML 4.1.
14) The Ranong FZ is one of the northeast- to north-northeast-trending faults that traverse the Peninsular Thailand. The Ranong FZ extends from near Prachup Khirikhan on The Gulf of Thailand coast southwest towards Ranong on the Andaman Sea. The total fault length is approxmately 220 km. The FZ, which follows the channel of the Kraburi River, has its subsidiary faults cutting Late Cretaceous granites (Charusiri, 1989) and Cenozoic sediments. However, a few hot springs was found near and along the southern end of the fault, implying that the fault significantly provides conduits for the geothermal field. The hotsprings in Myanmar and Vietnam are always spatially associated with several active faults. Landsat information indicates that the Ranong Bay is largely controlled by the fault movement. In addition, data from seismic stratigraphy in Andaman Sea indicate a well – defined Ranong FZ cross-cutting Cenozoic sediments almost to the top horizons. Sinistral movement along this FZ is reported by Garson et al. (1975) as having a total horizontal slip of 20 km. Shrestha (1990) noted that an earthquake of Mb 5.6 happened along this fault during 30 September 1978. Our recent TL dating data confirm the movement along the Ranong FZ occurring in Late Quaternary time.
15) The 130 km-long, Klong Marui FZ which cuts across the Phang Nga Bay and Ban Don Bay, follows the Klong Marui channel and mainly pass Tertiary granites (Charusiri, 1989) and Cenozoic sediments. The FZ may have occurred in Middle Tertiary (Charusiri, 1989) with at least 150 km-long total sinistral displacement (Garson et al., 1975). Hot springs are mostly concentrated at the southern portion of the fault. Recent studies indicate that the northern part of Khlong Marui fault is probably related to opening of the Gulf of Thailand and is therefore a basin-bounding normal fault rather than a major strike-slip fault. At present there is no geochronological information to confirm the young movement in Late Quaternary period. A few minor earthquakes were recorded in Phang Nga and south of Phuket provinces.
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