Full length article3-D magnetotelluric imaging of the Phayao Fault Zone, Northern Thailand: Evidence for saline fluid in the source region of the 2014 Chiang Rai earthquake
Graphical abstract
Introduction
Most of the seismicity in Thailand is localized in the north as there are many fault zones that cut through the region (Fig. 1a). The number and magnitude of earthquakes is generally lower than neighboring countries like Myanmar. Nevertheless, an unexpected moderate earthquake occurred on 5 May 2014 in Pan District, Chiang Rai province, northern Thailand (Fig. 1a and b) with thousands of aftershocks (Fig. 1b). The local magnitude (Ml) of the main shock was reported as 6.3 by the Thai Meteorological Department (TMD) with a hypocenter depth of 7 km, but later regional moment tensor inversion studies by Noisagool et al. (2016) suggested Mw 6.5 with a depth of 14 km. The main event was the largest earthquake in Thailand since the installation of the seismometer in 1974. The epicenters of the main shock and aftershocks were mostly in the Phayao Fault Zone (PFZ, Fig. 1b) which runs from south to north and is clearly separated into three major segments (Fig. 1b; Uttamo et al., 2003, DMR, 2007, DMR, 2014). The two segments involved with the recent events are the left–lateral strike–slip Mae Lao Segment (MLS) and the right lateral strike–slip Pan Segment (PS) (Fig. 1b). Interestingly, most of the epicenters are in the transition area between the MLS and the PS (Fig. 1b). The focal mechanisms (Noisagool et al., 2016) of most events (Ml > 4) indicate strike–slip motions in agreement with the characteristics of these fault segments.
It is interesting that the second largest recent earthquake in northern Thailand (Mw 5.2, 15 km depth, on 11 September 1994; USGS) also occurred in the PFZ (Fig. 1b). The focal mechanism of this earthquake was the normal motion (ISC, 2012; Fig. 1b). However, Noisagool et al. (2016) pointed out that its focal mechanism should be similar to the main event of the 5 May 2014. In fact, its epicenter is still in doubt as in 1994 there were no seismometers installed nearby. Since the PFZ is classified as a tentatively active fault (Charusiri et al., 1999) due to the lack of the data, even with the 1994 event (M5) and a few M4 events between 1994 and 2014 in this area, no scientists expected the PFZ to have enough energy to rupture and produce a Mw 6.5 earthquake so soon. This might be because of its very slow slip rate of about 0.1 mm/yr (Fenton et al., 2003), and relatively short length (45 km). In addition, Noisagool et al. (2016) has suggested a very crucial point about the regional stress and the fault plane instability revealed from the inversion of derived focal mechanisms from the 5 May 2014 earthquake sequences. They found that the Mae Lao Segment (MLS) has a higher shear stress than the other segments of the PFZ. With the stress from the north (≈N18E; Noisagool et al., 2016), the MLS is likely to be the fault that produced the main shock. Its movement has induced the Pan Segment (PS) which has a lower shear stress to slip. Noisagool et al. (2016) showed that there were mixtures of all three kinds of focal mechanisms: strike–slip, normal, and thrust motions. These mixed mechanisms result from the motions of both the MLS and PS and the structural heterogeneities between them.
In contrast to the PFZ, the east-trending Mae Chan Fault Zone (MCFZ, Fig. 1a) in the north of the PFZ has a slightly faster slip rate and is longer. It was classified as an active fault in the study of Charusiri et al. (1999). Most of the scientists then believed that the MCFZ would be the fault that produces large events based on its paleo-seismicity (Nutalaya et al., 1985, Charusiri et al., 2000, Fenton et al., 2003). This has led to many geological and geophysical studies on the MCFZ (e.g., Wood, 1995, Wood, 2001, Hinthong, 1995, Rymer et al., 1997, Kosuwan et al., 1998, Kosuwan et al., 2000, Kosuwan et al., 2003, Kosuwan and Lumjuan, 1999, Fenton et al., 2003, Wood et al., 2004, Wood et al., 2015, Phodee et al., 2014) in the past decades and little attention on the PFZ. In the end, only a few M4 earthquakes occurred in the MCFZ since the installation of first seismometer. With the accumulated tectonic stress coming from the north to both faults, this leads to the intriguing question of why the PFZ slipped instead of the MCFZ. Answers probably lie in the characteristics of these two faults, e.g. the fault plane, the frictional coefficient, and the interconnected fluid in the fault.
The occurrence of the largest earthquake in Thailand history on the PFZ is a good reason to conduct geological and geophysical investigations to probe the characteristic of the PFZ. One of the geophysical methods conducted worldwide is the magnetotelluric (MT) method. A magnetotelluric survey yields the resistivity structure which can be used to image faults and to study the fault properties and characteristics and its relation with earthquakes (Bedrosian et al., 2002, Becken and Ritter, 2012, Rao et al., 2004, Patro et al., 2005, Wannamaker et al., 2014; among many others). In many regions around the world, MT results reveal that high conductivity anomalies are often associated with the faults and seismicity of the region. In the San Andrea Fault, seismicity along each fault segment was linked to resistivity anomalies probed by MT surveys (Bedrosian et al., 2002, Bedrosian et al., 2004, Unsworth and Bedrosian, 2004a, Unsworth and Bedrosian, 2004b, Becken and Ritter, 2012). For the creeping segment of the fault, the high conductivity zone was observed within the fault indicating the presence of saline fluid. In contrast, in the locked fault segment, there was no high conductivity zone within the fractured fault. In the tectonic studies of the Central Indian Tectonic Zone, many conductors were found at different depths, e.g., from upper crust to mid-crust, and/or from mid- to lower crust, in association with the faults where seismicity also took place (e.g., Rao et al., 2004, Patro et al., 2005, Naganjaneyulu and Santosh, 2010, Abdul Azeez et al., 2013). Their interpretations of the low resistivity bodies were mainly based on either the mafic magmatic underplating and/or the presence of the aqueous fluid either in the fractures or intruding from the deep. In the Cascadia subduction system, USA (Wannamaker et al., 2014), the relatively high resistivity anomalies were interpreted as a lack of fluid and sediment indicating a plate locking zone, while the low resistivity anomalies were associated with the no plate lock zone implying with the presence of shallow sediment, and the fluid at great depth. In an arc-continent collision of Taiwan, Bertrand et al. (2012) found conductive zones at great depth and these have some relationship with the crustal seismicity. They interpreted the low resistivity zones as the interconnected saline fluid. A deep crustal conductor associated with the fluid was also observed in the New Zealand Southern Alps (Wannamaker et al., 2002) and in Japan (Mitsuhata et al., 2001, Ogawa and Honkura, 2004). The low resistivity anomalies were also found to be related to the low velocity zone in the lower crust beneath the Tangshan area indicating the existence of fluids which help to weaken the upper and middle crustal seismogenic layers and so cause large earthquakes (Wang et al., 2013). Similar scenarios were also found in the 1995 Kobe earthquake, Japan (Zhao et al., 1996), the 2001 Bhuj earthquake, India (Mishra and Zhao, 2003), the 2000 western Tottori earthquake, western Japan (Zhao et al., 2004) and the 2008 Iwate-Miyagi inland earthquake, Japan (Ichihara et al., 2011), among many others. With the correlation of low velocity and low resistivity, one of the explanations for the earthquake occurrence in these regions is the crustal heterogeneities, rather than just the stress condition alone (Zhao et al., 1996, Zhao et al., 2004, Mishra and Zhao, 2003, Wang et al., 2013)
Many of these studies have shown the importance of MT method for studying fault zones and earthquake mechanisms. Here, to study the characteristic of the PFZ, we conducted a 3-D MT survey covering the PFZ with more stations in the region where the Pan Segment intersects the Mae Lao Segment which is the area where most of the epicenters were located. In the absence of any geological and geophysical studies, our resistivity model obtained from the MT survey is therefore the first providing detailed information in the region around the PFZ.
Section snippets
Regional geologic framework of the Phayao Fault Zone and Northern Thailand
The tectonic setting and the regional geologic map around the Phayao Fault Zone (PFZ) are shown in Fig. 2a and b, respectively. According to the tectono-stratigraphic zone (Metcalfe, 2013, Morley et al., 2011, DMR, 2014), the tectonics of the northern Thailand is divided into the Inthanon zone on the west (west of the Mae Tha Fault Zone, MTFZ, in Fig. 2a), the north – south trend Sukhothai fold belt in the center which covers most of our study area (Fig. 2a), and the Indochina terrane on the
Magnetotelluric survey and the three-dimensional inversion
In November 2015, 29 MT stations were deployed covering a 140 km × 140 km area (Fig. 1b and Fig. 2) to ensure that the MT technique can “image” the near surface down to the lower crust. Uniform coverage of the stations is impossible as some of the land was not accessible due to lack of navigable terrain and/or permission from the owner. Most of the MT sites were above sedimentary rocks (Fig. 2b) in rice and corn farms. Only a few sites were placed over the granitic rocks. The main interest of this
Interpretation and discussion
As stated earlier, there are two main reasons why we prefer the inverted model in Fig. 5, Fig. 6 as our final model. First, it can produce the responses that fit the observed data with an RMS of 1.97 (Fig. 3 and Fig. 4). Second, the surface geology (Fig. 7a) corresponds closely with the final inverted model at 50 m depth (Fig. 7b). West of the PFZ, the Triassic granite rock (Tgr) corresponds well with high resistivity bodies (>100 Ω m). Most of the Quaternary alluvial deposit (Qa; Fig. 7a) within
Conclusion
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A total of 31 magnetotelluric (MT) stations were deployed within a 140 km × 140 km area to investigate the Phayao Fault Zone (PFZ) rupture area in northern Thailand.
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We found conductive anomalies beneath the Mae Lao Segment (MLS) and partially beneath the Pan Segment (PS) from a depth of 4 km to mid-crust where both MLS and PS are part of the PFZ. They were interpreted as saline fluid rich zones.
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The saline fluid rich zones strongly support our assumption of previous studies (Noisagool et al., 2016)
Acknowledgement
This research has been supported by the Development and Promotion of Science and Technology Research Grant 037/2557, the Thailand Center of Excellence in Physics (ThEP), and the Thailand Research Fund (RSA5780010). We would like to thank Dr. Michael Allen for editing the English of this manuscript, and the two reviewers who provide many good comments to improve the manuscript.
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