Journal of Asian Earth Sciences 127 (2016) 231–245

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Journal of Asian Earth Sciences

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The regional moment tensor of the 5 May 2014 Chiang Rai earthquake(Mw = 6.5), Northern Thailand, with its aftershocks and its implication tothe stress and the instability of the Phayao Fault Zone

http://dx.doi.org/10.1016/j.jseaes.2016.06.0081367-9120/� 2016 Elsevier Ltd. All rights reserved.

⇑ Corresponding author at: Department of Physics, Faculty of Science, MahidolUniversity, 272 Rama 6 Road, Rachatawee, Bangkok, Thailand.

E-mail address: wsiripun@gmail.com (W. Siripunvaraporn).

Sutthipong Noisagool a, Songkhun Boonchaisuk c, Patinya Pornsopin d, Weerachai Siripunvaraporn a,b,⇑aDepartment of Physics, Faculty of Science, Mahidol University, 272 Rama 6 Road, Rachatawee, Bangkok, Thailandb ThEP Center, Commission on Higher Education, 328, Si Ayutthaya Road, Rachatawee, Bangkok, ThailandcGeoscience Program, Mahidol University, Kanchanaburi Campus, Saiyok, Kanchanaburi, Thailandd Seismological Bureau, Thai Meteorological Department, 4353 Sukumvit Road, Bangna, Bangkok, Thailand

a r t i c l e i n f o a b s t r a c t

Article history:Received 17 December 2015Received in revised form 6 June 2016Accepted 13 June 2016Available online 14 June 2016

Keywords:Northern ThailandFocal mechanismRegional moment tensorStress orientation

On 5 May 2014, the largest earthquake in Thailand modern history occurred in Northern Thailand withover a thousand aftershocks. Most of the epicenters are located within the transition area of the Mae Laosegment (north) and Pan segment (central) of the Phayao Fault Zone (PFZ). Good quality data from allevents (ML > 4) are only available for the seismic stations closer to the epicenters (<500 km). The regionalmoment tensor (RMT) inversion was applied to derive a sequence of thirty focal mechanisms, momentmagnitudes and source depths generated along the PFZ. Our studies reveal that 24 events are strike – slipwith normal (transtensional), four are strike – slip with thrust (transpressional), and two are reverse. Themain shock has an Mw of 6.5, slightly larger than previously estimated (ML 6.3) while Mw of the after-shocks is mostly lower than ML. This suggests that a regional magnitude calibration is necessary. Thehypocenter depths of most events are around 11 km, not as shallow as estimated earlier. In addition, astress inversion was applied to these 30 focal mechanisms to determine the stresses of the region, theMohr’s diagram, and the principal fault planes. The retrieved maximum stress direction (N18E) is inagreement with other studies. One of the derived principal fault plane with a strike of N48E is in goodagreement with that of the Mae Lao segment. Both estimated shape ratio and plunges led us to concludethat this area has a uniaxial horizontal compression in NNE-SSWwith small WNW-ESE extension, similarto the interpretation of Tingay et al. (2010). Based on the Mohr’s diagram of fault plane solutions, we pro-vide geophysical evidence which reveals that the high shear stress Mae Lao segment is likely to slip firstproducing the main shock on 5 May 2014. The energy transfer between the segments has then led tomany aftershocks with mixed mechanisms. At the end, we re-visited the analysis of the former largestearthquake in Northern Thailand in the past decades, the 11 September 1994 event. Its focal mechanismwas re-calculated based on the available P-wave polarities. The strike – slip motion should be the mech-anism of the earthquake, not the normal motion as originally believed.

� 2016 Elsevier Ltd. All rights reserved.

1. Introduction

On 5 May 2014 at 11:08 UTC, a ML 6.3 earthquake (ThailandMeteorological Department; TMD); Mw 6.2 (U.S. Geological Survey(USGS) moment magnitude) occurred in Pan District, Chiang Raiprovince in the northern part of Thailand (Fig. 1). The main shockwas felt throughout the northern cities in Thailand, particularlyin Chiang Rai City which is 31 km from the epicenter and has a

population of 230,000. The main shock was followed by >1000aftershocks (with ML > 2). The earthquake intensity was recordedat VIII Mercalli 10 km around the epicenter (Wiwegwin andKosuwan, 2014). The earthquake resulted in only one casualty,but damaged >9000 residences and a few main roads (DMR,2014a; TMD, 2014). Liquefaction was observed in many areas aswell (DMR, 2014b). Because Thailand is located away from theplate boundary, seismicity in Thailand is low in terms of the num-ber and magnitude of earthquakes compared to neighboring coun-tries such as Myanmar or Indonesia. The 5 May main shock istherefore registered as the largest earthquake in Thailand afterthe installation of the first seismometer in Thailand since 1974.

Fig. 1. (a) A map of northern Thailand with related fault zones and seismic stations (green triangles) with names. The study area where the main event occurred on 5 May2014 is enclosed in a rectangle and expanded in detail in (b). Background seismicity in neighboring region are from NEIC (1980–2007) and from TMD (2007 – 04/05/2014),and focal mechanisms of large earthquakes from ISC (2012). The focal mechanism of the main event from ISC catalog is shown in green along with the Mw 5.2 earthquakewhich occurred on 11 September 1994 in red. The fault zones where the 5 May 2014 earthquake occurred is the Phayao Fault Zone (PFZ) is marked with red lines. The PFZ isdivided into three segments. The name of each one is given alongside of the segment in which MLS is the Mae Lao segment, PS the Pan segment and MJS the Mae Jai segment.MCFZ is the Mae Chan Fault Zone. Note that the epicenter of the 11 September 1994 event was located near Mae Jai segment by ISC (2012), however, it was estimated to belocated closer to Mae Lao segment in USGS catalog with the same focal mechanism. (For interpretation of the references to color in this figure legend, the reader is referred tothe web version of this article.)

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The epicenters of the main event and its aftershocks weremostly located in the Phayao Fault Zone (PFZ; Fig. 1b). Accordingto Uttamo et al. (2003), DMR (2007) and DMR (2014a), the PFZcan be divided into three major segments: the Mae Lao segment(the NE-SW-trending left lateral strike-slip faults) in the north,the Pan segment (the almost N-S-trending right lateral strike-slipfaults) in the middle, and the Mae Jai segment (the NNW-SSE-trending normal faults) in the south. According to the TMD, all epi-centers of the main event and aftershocks were located in the tran-sition area between the Mae Lao and Pan segments (Fig. 1b;Wiwegwin and Kosuwan, 2014). Earlier studies (e.g., USGS, CMT,GEOFON, Wiwegwin and Kosuwan, 2014) concluded that the focalmechanism of the main shock was the strike – slip motion (Fig. 1b).Geological features of the region are oriented according to thedirection of these fault segments. In the south of the transitionzone, most rocks align parallel to the Pan segment, while they ori-ent to the NE – SW along the Mae Lao segment in the north. Westof the fault zones, rock is mostly batholith granites, while sedimen-tary rock lying on top of the volcanic rock can be seen in the east.

Both surface geological and paleoseismic studies indicated thatthe PFZ can accumulate enough energy to generate a moderateearthquake (Bott et al., 1997; Fenton et al., 2003; Pananont, 2009;and Pailoplee et al., 2009). Evidence was the past largest northernThailand earthquake with a magnitude of 5.2 (USGS) occurred on11 September 1994 near the Mae Jai segment (Fig. 1b) with a nor-mal slip motion focal mechanism (ISC, 2012). Since then there wereonly a fewM4 earthquakes occurred along the PFZ segments. How-ever, not many scientists expected the PFZ to slip enough to pro-

duce this ML 6.3 earthquake within this century as its fault lengthis relatively short (Pailoplee et al., 2013). In the past few decades,most of the geophysical and geological research was shifted tothe northern fault zone, the Mae Chan Fault Zone (MCFZ inFig. 1b) as most scientists believed that an earthquake larger thanM6 was most likely to occur along the MCFZ (Charusiri et al.,1999) since it is longer and has a higher slip rate (Fenton et al.,2003; Pailoplee et al., 2009). Till present day, a few earthquakesoccurred on the MCFZ have a magnitude less than 5.

Since there are not many moderate to large earthquakesoccurred in the region, and also, the lack of the regional seismome-ters in the past, much information about the fault zones and themaximum horizontal stress in northern Thailand was then missed.After the great Sumatra earthquakes in 2004, many broadbandseismic stations were installed by the TMD throughout Thailand.The installation has led to a new era of earthquake seismology inThailand in which the local earthquakes will be analyzed andresearched. Therefore, after the 5 May 2014 earthquakes,Wiwegwin and Kosuwan (2014) reported the focal mechanismsof the main event and the aftershocks based on the pattern ofthe first P-wave motions. Most of their focal solutions are strike-slip and only a few show normal motions. However, accuracyand reliability of the P-wave motion requires an azimuthal cover-age of the seismic stations and good quality data. Since most of theseismic networks in northern Thailand (Fig. 1a) are located to thesouth of the epicenters with only one station (CRAI) located northof the events, and good quality data for most of the aftershocks areonly available for the stations closer to the events (Fig. 1a), the

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focal mechanisms based on the P-wave motion technique mayhave large deviations.

In this independent study, we provide another set of focalmechanisms from both main shock and aftershocks of the 5 May2014 event. However, our technique was based on the regionalmoment tensor (RMT) inversion technique which is the most suit-able method for calculating the focal mechanism solutions as evenwhen the seismic stations are not well-distributed as in our case oreven only one station is available (see example in Dreger andHelmberger, 1993; Walter, 1993; Kim et al., 2000; and Stichet al., 2003). The RMT technique is also better at retrieving theearthquake source parameters of even a small earthquake suchas the aftershocks in our case (e.g., Fan and Wallace, 1991;Dreger and Helmberger, 1993; Orgulu and Aktar, 2001; Xu et al.,2010; Herrmann et al., 2011a,b; Whidden and Pankow, 2012;Herman et al., 2014; Görgün, 2014; Görgün and Görgün, 2015).

With many of these advantages, in this paper, we applied theRMT inversion technique using the code of Herrmann (2013) tothe seismic data set obtained from the seismic stations shown inFig. 1a generated from the main event and its ML > 4.0 aftershocks.This results in thirty focal mechanisms describing the orientationsof the fault plane solutions, moment magnitudes, and sourcedepths along with the stress orientations of the P-axis and T-axis.In addition, as the stress in the northern Thailand is a key to under-stand the regional tectonic and kinematic, particularly the Himala-yan extrusion into the South East Asia (Molnar and Tapponnier,1975; England and Molnar, 2005; Morley et al., 2004; Charusiriand Pum-Im, 2009; Tingay et al., 2010), a stress inversion is there-fore performed on these 30 focal mechanisms using the STRESSIN-VERSE package (Vavrycuk, 2014) yielding the stress axes, theinstability fault plans from each focal mechanism, and the twoprincipal axes. The derived stress information was then linked tosupport the tectonic studies of Tingay et al. (2010). After the earth-quake, based on the epicenters which mostly located in the transi-tion area between the Mae Lao segment and the Pan segment(Fig. 1b), local geologists questioned whether it was the Mae Laosegment or the Pan segment that slipped first producing the mainshock. Here, we provide geophysical evidence based on the Mohr’sdiagram to reveal that the Mae Lao segment is highly likely to ini-tiate the main event from the compression coming from the north.In addition, we also re-visited the 11 September 1994 event to val-idate that the mechanism should be shown as the strike – slipmotion, rather than the normal motion as originally believed.

2. Methodology, data analysis, and results

With a limitation of data qualities in this study, the grid searchinversion of shear dislocation source (or double-couple momenttensor) was proved to be better than the full moment tensor inver-sion when signals are noisy or station number is limited(Herrmann et al., 2011a,b). Here, we, therefore, applied the gridsearch inversion (Herrmann, 2013; Herrmann et al., 2011a,b) togenerate the moment tensor characterized by the strike, dip, andrake of the fault planes. Fig. 2 summarizes the three major stepsof the RMT inversion to produce the focal mechanism of eachearthquake event. The first step (described in detail in Section 2.1)is to compute a series of Green’s functions for difference sourceepicentral distances and depths. The Green’s function will be usedfor later steps for all events. Then, for each event, the regionalwaveforms with the same set of frequency as the pre-computedGreen functions are prepared (Section 2.2) for the last step, the gridsearch inversion and the final solution selection (Section 2.3). Inaddition, to assure that the slight shift of the epicenters in ourstudy does not affect the final solution, a sensitivity analysis wasthen conducted.

2.1. Green’s function and velocity model

The first step of the RMT inversion is to prepare a series of thepre-computed Green’s functions (Fig. 2), which can be generatedfrom the regional velocity structure within the earthquake sourcearea. Since there are no seismic stations close to the epicenters(see Fig. 1a), a velocity model from the CRUST 2.0 model (Bassinet al., 2000) was extracted at the location of the epicenters andwas used to generate the Green’s function needed for the othersteps of the RMT inversion (Fig. 2). However, the focal mechanismof the main event obtained in this way was not consistent with theresults obtained from the USGS, CMT and GEOFON. The disagree-ment is caused by the CRUST 2.0 model not giving the correctvelocity model of the region. The CRUST 2.0 model has the depthof Moho at 43 km which is 10 km more than the depth obtainedin Noisagool et al. (2014). A ‘‘better” velocity model in the regionof the epicenters is therefore necessary.

The two stations closer to the main event epicenter and itsaftershocks are the CHTO and CMMT stations (Fig. 1b). Both areinstalled at the same location in Chiang Mai city about 123 kmfrom the epicenter but CHTO is operated by IRIS and CMMT is oper-ated by TMD. Both of them gave very high quality of data. Wetherefore construct the 1-D velocity models beneath these CHTOand CMMT stations based on the tele-seismic receiver functionmethod (Langston, 1979). Although CHTO and CMMT are about123 km from the earthquake epicenters, the velocity modelbeneath the station should be similar to that of the epicenter area,unlike the CRUST 2.0 model, and therefore more preferable for thecalculation of the Green’s functions.

The receiver functions at both stations were taken from ourprevious studies of crustal thickness and Poisson’s ratio (seeNoisagool et al., 2014). To construct the final 1-D velocity modelbeneath these two stations, an initial velocity model is generatedby shortening the velocity model from the CRUST 2.0 model(Bassin et al., 2000) so that its crustal thickness (at a depth of43 km) matches our earlier studies at a depth of 32 km (seeNoisagool et al., 2014) as shown by the black line in Fig. 3. We alsorandomly varied our starting initial velocity model within a rangeof ±0.4 km/s at different depths within the crust, and within arange of ±0.2 km/s in the uppermost mantle before graduallydecreasing it to no variation at about a depth of 100 km. This pro-duced a total of 100 initial models. They were run separately in theinversion using a code by Herrmann and Ammon (2004) to gener-ate 100 inverted velocity models. These models were then com-bined to produce the probability distribution velocity modelswhere the mean velocity values at each depth were selected asour final model (Fig. 3a). Our velocity models at both CMMT andCHTO stations are also in good agreement with the models derivedby Hu et al. (2008) and Bai et al. (2010) as shown in Fig. 3a. Thepredicted receiver function generated from our inverted velocitymodel fits reasonably well within a range of the observed receiverfunctions (Fig. 3b).

As we have more receiver functions at the CMMT station than atthe CHTO, we used the final velocity model beneath the CMMT sta-tion (Fig. 3) for the calculation of the Green’s function. In this study,the Green’s function of the three fundamental faults (seeHerrmann and Wang, 1985; Jost and Herrmann, 1989; Minsonand Dreger, 2008) is calculated from the wavenumber integrationmethod (Haskell, 1963, 1964; Herrmann, 2013). A series of theGreen’s functions were calculated at various source – receiver dis-tances with different grid sizes: 1 km grid size for the epicentraldistance between 0 and 200 km, 5 km between 201 and 500 kmand 10 km between 500 and 1000 km. In addition, source depthswere increased every 0.5 km to a depth of 30 km. These pre-computed Green’s functions are stored into a large database.During the grid search inversion, the necessary Green’s function

Fig. 2. A flow chart displaying the three major steps of the RMT inversion: (2.1) Green’s function preparation (2.2) waveform preparation and (2.3) grid search inversion andfinal solution selection.

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is then extracted from the database to reduce the computationalcost of re-constructing them.

2.2. Waveform preparation and data processing

There are many seismic broadband stations installed through-out Thailand by the TMD and Mahidol Network. However, onlythe seismic stations (shown in Fig. 1a) near the epicenters wereable to make good quality recording of the 46 large aftershocks(ML > 4) between 5 May and 15 August 2014. Since the crustalthickness varies from one terrane to another (see Noisagool et al.,2014), to minimize the errors that might be due to the velocitystructure difference between that of the receiver stations and thatof the CMMT station, the data to be used for the regional momenttensor inversion is limited to within 500 km of the epicenters(Fig. 1a).

To prepare the waveform data for the grid search inversion inthe next step, for each event, the digital seismograms were decon-volved into the ground velocity, had the instrument response

removed, had the trend and mean of the seismograms discarded,and the seismograms at �20 s prior to the P-wave arrival and150 s after the P-wave arrival were cut. During these steps, thewaveform data was also manually reviewed to discard the bad sig-nal. The arrival time was calculated from the velocity modelbeneath the CMMT station. The data was then rotated into the ver-tical, radial and transverse components and then resampled at0.2 s.

Since earthquakes with different magnitudes can yield differentdominant frequencies, different filter bands can provide differentfocal mechanism solutions (Barth and Wenzel, 2010; Herrmannet al., 2011b). A small event often has higher amplitude in the highfrequency band, while a larger event yields a lower frequencyband. To cover ranges of frequencies, and to guarantee that boththe Green’s function and the regional waveforms have the samefrequency bands, we applied four sets of bandpass filters to theseismograms: F1 = 0.02–0.05, F2 = 0.02–0.065, F3 = 0.02–0.075,and F4 = 0.02–0.1 Hz. The filtered waveforms will be used duringthe grid search inversion.

Fig. 3. (a) Mean velocity models from the receiver function inversion beneath theCHTO (green line) and CMMT (red line) stations from this study and others (blueand dark blue). CHTO and CMMT seismometers are located on the same place. Theblack line shows the initial model modified from the CRUST 2.0 model byshortening it to fit with crustal thickness obtained from the receiver functionstudy (Noisagool et al., 2014). (b) The observed and the predicted receiver functiontime series. The predicted receiver function is drawn from the 67� source distance.The total fit between the observed and predicted RFs is about 83%. (For interpre-tation of the references to color in this figure legend, the reader is referred to theweb version of this article.)

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2.3. Grid search inversion and final solution selection

2.3.1. Grid search inversionIn this step (Fig. 2), a waveform grid search algorithm in Com-

puter Program in Seismology (Herrmann and Ammon, 2004;Herrmann et al., 2011a,b) was applied to find a fault plane solutionthat can produce a predicted waveformmatched with the observedseismogram. A grid search for the best-fit solution runs with a 5�increment of strike [0, 360), dip [0, 90], and rake [180, �180) ofsource parameters and a 0.5 km increment of source depth in therange from 0 to 30 km. As with Herrmann et al. (2011a), a weight-ing factor with a reference distance of 200 km was also applied to

the inversion to down-weight the data along with a time shift of15 s for the inversion to adjust the arrival time. Both of thesetwo processes are needed to avoid the improper sensor responses,the uncertainty of the epicenters for the case of short source-receiver distance, the crustal heterogeneity along the ray path fromthe source to the receiver locations for the case of large source-receiver distance, and the small discrepancy between the source– receiver distance and the pre-computed Green’s function grids(e.g., Xu et al., 2010; Herrmann et al., 2011a,b; Herman et al.,2014; Cubuk et al., 2014).

For each event from each bandpass filter, the grid search algo-rithm provides a series of focal mechanisms and depths with dif-ferent fitting values defined by the dot product of observed (o)and predicted data (p) as fit ¼ o�p

ffiffiffiffiffi

o�opffiffiffiffiffi

p�pp (Herrmann et al., 2011a).

Examples of these focal mechanisms are shown in Fig. 4 (forthe main event) and Fig. 5 (for one of the aftershocks with ML4.7 which occurred on 6 May 2014 at 12:42:12 UTC). Since wehave four filter bands, each event therefore has four series of focalmechanisms and depths (Figs. 4 and 5 as examples) whereas onlyone best-fit solution and depth must be selected to represent theevent.

2.3.2. Final solution selection to represent the event and qualitycontrol

A final selection of all the available focal mechanisms anddepths is based on the criteria used in Herman et al. (2014), i.e.,considering both fitting and the stability of the focal mechanismat different depths. Herman et al. (2014) classified the solutionsinto three types: type A where the fit shows a clear high peakand its focal mechanism is uniform at all depths, type B wherethe fit does not show a clear peak but its focal mechanism is uni-form at all depths, and type C where there is no clear peak forthe fit and its focal mechanism varies with depth. As withHerman et al. (2014), we control the quality of the result by keep-ing all type A results and discarded all type C results. When no typeA result is available, type B results are considered. With these cri-teria, the focal mechanism solutions from all filter bands of themain event are categorized as type A (Fig. 4 and first event inFig. 6). The final solution from the main event is chosen from bandF2 as it gives the best fit. For the aftershock event (Fig. 5 and eventnumber 18 in Fig. 6), only the result from band F4 is type B whilethe other bands are type A. In this case, the result from band F3shows the best fit and will be selected as a final solution.

From 5 May to 15 August 2014, there were a total of 47 eventswith magnitude ML > 4. However, a few events right after the mainshock were totally dominated by the main shock surface wave.Thus, only 43 events were left for the analysis with the waveformpreparation and data processing, and the grid search inversion. Aseries of best-fit solutions from four different bandpass filters plot-ted in the same column of each event is shown in Fig. 6 as a func-tion of time that the earthquakes occurred. According to ourcriteria above, the final solutions of each event were picked anddenoted by blue squares (Fig. 6). Due to the poor fit in the analysis,13 events were then discarded leaving just 30 events. The finalsolutions of these 30 events are plotted at the locations of the epi-centers shown in Fig. 7 and the details are given in Table 1.

2.3.3. Validation of the final solution due to the uncertainty of theepicenters

After the main shock, USGS, CMT, EMSC, TMD estimated themain shock epicenter based on their available data and techniques.These epicenters differ by up to 0.1� of latitude and longitude asshown in Fig. 8a. As an accurate epicenter position is crucial forthe RMT inversion, its mis-location may affect the focal mechanismsolution. To assure that our final solution does not change

Fig. 4. Best fitting focal mechanisms versus depths for different filter bands indicated with F1, F2, F3 and F4 for the main event on 5 May 2014. Final focal mechanisms andearthquake source information for each filter are listed in each figure along with the observed (blue) and predicted (red) seismograms. (For interpretation of the references tocolor in this figure legend, the reader is referred to the web version of this article.)

236 S. Noisagool et al. / Journal of Asian Earth Sciences 127 (2016) 231–245

according to the uncertainty of the epicenter and can still be usedto represent the event, we therefore conduct the sensitivity analy-sis of the epicenters.

The epicenters are perturbed within ±0.1� around the TMD’sepicenter with an increment/decrement of 0.01� resulting in 121epicenters. All were used in the grid search inversion to generate

121 final focal mechanisms. Two example results are shown inFig. 8. Fig. 8a shows that the focal mechanism solutions of the mainevent generated with different epicenters within a range of ±0.1�yield the same result (FMS in Fig. 8 were resampled to 0.02� foran easier view), i.e. strike – slip motion. A similar experimentwas applied to one of the aftershocks (event number 18 in Figs. 6

Fig. 5. Best fitting focal mechanisms versus depths for different filter bands indicated with F1, F2, F3 and F4 for the aftershock on 6 May 2014. Final focal mechanisms andearthquake source information for each filter are listed in each figure along with the observed (blue) and predicted (red) seismograms. (For interpretation of the references tocolor in this figure legend, the reader is referred to the web version of this article.)

S. Noisagool et al. / Journal of Asian Earth Sciences 127 (2016) 231–245 237

and 7 and Table 1) and obtained similar results as shown in Fig. 8b.Our sensitivity studies therefore confirm that adding the smallvariations within a range of ±0.1� to the epicenters in this regionwill not affect the final results from the RMT inversion. This is inagreement with Xu et al. (2010), Herrmann et al. (2011a,b),Herman et al. (2014) and Cubuk et al. (2014).

3. Discussion

3.1. Types of motions of focal mechanism solutions

Fig. 7 shows the focal mechanisms plotted at the epicenters ofthe main shock and its 29 aftershocks (ML > 4). They are all located

Fig. 6. Main shock and aftershocks are listed as a function of time that the earthquake occurred. In each event, four inverted focal mechanism solutions (FMS) from filter F1 toF4 are plotted from bottom to top, respectively. Grey-scale as a background (and color bar) indicates the value of the fitting solution of each focal mechanism. Quality of thefitting curve based on the criteria used in Herman et al. (2014) is shown with different colors: type A in green, type B in yellow, and type C in red. The final solutions aremostly selected from type A with some from type B and plotted on top of each event in blue. Some of the events have poor data and fitting and therefore cannot yield a FMS.Events with FMS are numbered from 1 to 30. The same numbering of events is used in Table 1 and Fig. 7. (For interpretation of the references to color in this figure legend, thereader is referred to the web version of this article.)

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in the ‘‘transition” zone between the Pan segment and the Mae Laosegment. To classify the type of motions of these thirty focal mech-anism solutions, the same criteria as were used in Zoback (1992)were applied and were shaded with different colors for differentmechanisms (Fig. 7). For the main event, the focal mechanism isstrike – slip in agreement with USGS, CMT, GEOFON andWiwegwin and Kosuwan (2014). The two nodal planes for themain event show that their strikes can correspond in either tothe orientations of the strike of the Pan segment in NNW – SSEor that of the Mae Lao segment in NNE – SSW (Fig. 7).

For the focal mechanism solutions of the aftershocks, 23 eventsindicate the strike-slip motion with 4 events revealing normal slipand 2 events showing reverse motions (Fig. 7). The normal andthrust faulting indicates that the vertical movements are parts ofthis transition zone. The 23 strike – slip events in Fig. 7 can alsobe further classified. Following the Zoback (1992) criteria, nineteenof them are categorized as strike – slip with minor normal faulting(transtensional strike – slip faulting) distributed on both Pan andMae Lao segments. Only four of them are strike – slip with thrustfaulting (transpressional strike – slip faulting) and these onlyoccurred on the eastern side of the Pan segments (Fig. 7). However,there is no indication of the transpressional faulting (thrust withstrike – slip) and the transtentional faulting (normal with strike– slip). These results indicate that the PFZ is dominated by theNNE-SSW compression with small amount of heterogeneities lead-ing to the difference in focal mechanism of some small events (e.g.,Rivera and Kanamori, 2002).

3.2. The moment magnitudes (Mw) and hypocenter depths

There are two significant differences between the resultsobtained from our RMT technique and those from the P-wavemotion pattern. First, the hypocenter depths from the TMD catalogand the depths from our RMT results show large discrepancies(Fig. 9a). The TMD depths are mostly less than 10 km with only afew deeper than 20 km, while our estimated depths are mostly

around 11 km. The large discrepancy can physically provide somemeaning as many researchers used the information obtained fromthe first motion technique to describe the initial stage of fault rup-ture of the earthquake, while those derived from the RMT inversionrepresent the mechanism of the main earthquake rupture process(Scott and Kanamori, 1985; Kubo et al., 2002; Rhie and Kim, 2010;Husen and Hardebeck, 2010). However, here it is skeptical to inter-pret it as the results may have some error due to the poor stationdistribution and the different in velocity model used.

Another interesting fact obtained from our study is that, for themain event, our result of Mw 6.5 is larger than the ML 6.3 esti-mated by TMD (Table 1, Fig. 9b). This is in agreement with otherstudies which show that for an event having magnitude >6, ML isoften saturated and therefore yields a lower value than Mw (e.g.,Scott and Kanamori, 1985; Kanamori et al., 1993; Kubo et al.,2002; Clinton et al., 2006; and among many examples). As Mw isoften considered as the best scale for larger magnitude events,the main event on 5 May 2014 should be registered with a magni-tude of Mw 6.5. Nevertheless, our Mw 6.5 is slightly higher thanMww 6.1 estimated by USGS and Mwc 6.2 by GCMT (Dziewonskiet al., 1981; Ekström et al., 2012). The slight difference might comefrom the different assumptions of source used in the inversionalgorithms. The RMT algorithm (Herrmann, 2013; Herrmannet al., 2011a,b) used in this study is applied under the assumptionof only the double-couple source, while that used in the USGS con-tains both double couple source and non-double couple source(15% for our main event as reported in the USGS catalog). The slightdifference could also be the results of using different stationcoverage.

In contrast to the main event, most of the aftershocks show alower Mw than ML as shown in Fig. 9b and Table 1. The relation-ships between Mw and ML in Fig. 9b show that, for smaller earth-quakes, ML is about 0.5 higher than Mw. There are twoexplanations for the differences. Seismic stations used to estimateML of TMD might be different from those used to estimate Mw inour study. Secondly, a calibration of ML estimated in Thailand

Fig. 7. The final focal mechanisms of the main shock and its 29 aftershocks. Events are colored according to the mechanism following Zoback (1992). See Fig. 6 and Table 1 forthe event numbers.

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might be necessary, as it has never been conducted before, in orderto fit with the local geology. This is similar to what was conductedin other regions prior to a direct comparison with Mw before usingthe comparison for the analysis of earthquake hazards and tecton-ics of the region (Margaris and Papazachos, 1999; Wu et al., 2005;Kim and Park, 2005; Shoja-Taheri et al., 2007; Bobbio et al., 2009;Ristau, 2009; Hutton et al., 2010; Baruah et al., 2012). To correct forML in Thailand, the process requires the crustal velocity structureand the local scale seismic attenuation study in Thailand (e.g.,Pailoplee, 2014).

3.3. Stress orientation in Northern Thailand

Based on the focal mechanisms from a series of 5 May 2014earthquake in this area (Fig. 7; Table 1), to obtain the stress direc-tion in Northern Thailand, one quick and easy way is to convertthem to the orientations of the P-axis and T-axis. The P- and T-axes from each event are shown in Fig. 10 along with the localaverage. Fig. 10 shows that the local P-axis is on average alignedin the NNE – SSW direction (or N15E), whereas the local T-axis ison average in the NW – SE direction. Due to the uncertainty ofthe fault-plane orientation and the fault frictional strength, thestress derivation based on the focal mechanism solution (givenas Quality C in World Stress Map), in fact, can deviate from theactual stress (Heidbach et al., 2008).

To determine the local stress, these 30 focal mechanisms (Fig. 7;Table 1) were applied with the iterative joint inversion method forstress and fault plan solution via a STRESSINVERSE package ofVavrycuk (2014). The inversion algorithm is a modified versionof the Michael’s method (Michael, 1984 and 1987) by applyingthe fault instability constraint (Lund and Slunga, 1999). In the earlyiteration, the algorithm follows Michael’s method without anyconstraints to find the stress orientation. Then, Michael’s methodis applied again but with the fault instability constraint to findthe stress axis and unstable fault plan for each focal mechanism.The technique is iteratively repeated until the stress convergesto the desired value. The joint inversion was run with 100 realiza-tions of random noises in order to estimate the uncertainty of theoutputs. To determine the fault friction, a friction ranging from0.2 to 1 with an increment of 0.25 was applied in the inversionfor fault instability calculation.

The retrieved principal stress direction is shown in Fig. 11aalong with the confidence regions. The r1, r2 and r3 stress axesare (azimuth/plunge): 197.9�/3.2�, 99.4�/69� and 289.1�/20.7�,respectively (Fig. 11a). The shape ratio (Vavrycuk, 2014), R =(r1 – r2)/(r1 – r3) is 0.92 where its distribution is shown inFig. 11b. The fault friction is 0.7 which indicates an intermediateto high strength Phayao Fault Zone (Scholz, 2002). The derivedmaximum stress (r1; Figs. 11a and 12) is around N18E which issimilar to the direction of the P-axis derived from the focal

Table 1Final results of 30 events from regional moment tensor inversion derived in Figs. 6 and 7.

No. Date (2014) Time (GMT) Lat Lon ML H (km) TMD Mw H (km) RMT Nodal Plane 1 Nodal Plane 2 Fit

Str Dip Rk Str Dip Rk

1 05/05 11:08:42 19.69 99.69 6.3 7.0 6.5 14.0 260 90 �5 350 85 �180 0.672 05/05 12:20:57 19.86 99.68 5.2 0 4.6 12.0 255 85 0 345 90 �175 0.603 05/05 13:04:45 19.67 99.74 4.5 7.0 3.8 14.0 245 70 15 150 75 160 0.724 05/05 13:18:02 19.70 99.73 4.3 7.0 4.0 22.5 30 40 85 215 50 95 0.545 05/05 13:34:29 19.69 99.65 4.7 6.0 4.4 11.0 80 90 0 170 90 180 0.646 05/05 14:26:52 19.67 99.65 4.4 7.0 3.8 13.0 230 75 25 135 65 165 0.787 05/05 16:03:06 19.67 99.72 4.0 0 3.6 15.0 35 70 �30 135 60 �160 0.618 05/05 16:07:25 19.60 99.62 4.5 0 3.9 13.5 35 50 �70 185 45 �110 0.789 05/05 16:20:17 19.69 99.69 4.5 5.0 4.3 10.0 260 90 0 350 90 180 0.45

10 05/05 17:35:28 19.72 99.71 4.8 4.0 3.9 11.5 175 55 20 75 75 145 0.6911 05/05 18:02:33 19.64 99.73 4.1 0 3.6 11.5 255 70 10 160 80 160 0.6012 05/05 19:12:04 19.82 99.71 4.7 6.0 4.3 10 265 85 �5 355 85 �175 0.5213 05/05 20:05:25 19.61 99.62 4.4 10.0 3.9 11.5 25 65 �10 120 80 �155 0.8314 05/05 21:17:05 19.65 99.66 5.1 23.0 4.6 12.0 75 90 25 345 65 180 0.6015 05/05 23:04:55 19.75 99.62 5.2 7.0 5.1 11.0 260 80 10 170 80 170 0.7316 06/05 00:50:16 19.73 99.69 5.9 20.0 5.5 12.0 80 85 10 350 80 175 0.6617 06/05 00:58:19 19.70 99.53 5.6 2.0 5.6 13.0 260 80 15 170 75 170 0.7218 06/05 12:42:12 19.63 99.62 4.7 4.0 4.3 9.0 265 85 0 355 90 �175 0.6519 06/05 13:47:01 19.75 99.7 4.7 6.0 4.4 12.0 80 90 0 170 90 180 0.6120 06/05 14:50:12 19.74 99.59 4.9 1.0 4.3 8.5 250 85 10 160 80 175 0.7121 06/05 15:57:32 19.67 99.67 4.6 9.0 4.1 14.5 115 20 �25 230 81 �110 0.4822 06/05 18:29:39 19.73 99.62 4.2 12.0 4.1 1.0 35 45 �95 220 45 �85 0.6923 06/05 20:52:26 19.70 99.59 4.8 7.0 4.3 12.0 175 90 0 265 90 180 0.3624 08/05 20:43:38 19.69 99.63 4.7 2.0 4.4 1.0 40 50 90 220 40 90 0.7925 12/05 11:05:29 19.8 99.72 5.0 8.0 4.7 11.0 75 90 10 345 80 180 0.6226 14/05 17:23:57 19.57 99.67 4.4 9.0 4.5 10.0 265 85 5 175 85 175 0.5627 16/05 04:31:34 19.66 99.63 4.8 1.0 4.3 7.0 255 85 �5 345 85 �175 0.6128 21/05 10:19:49 19.64 99.7 4.1 7.0 4.1 8.0 270 85 �5 0 85 �175 0.8129 26/06 07:29:36 19.71 99.67 4.6 9.0 4.1 11.5 30 70 �10 125 80 �160 0.7930 15/07 13:30:53 19.7 99.7 4.3 9.0 3.9 15.5 60 80 �45 160 45 �165 0.77

Fig. 8. A series of focal mechanisms obtained by perturbing the epicenters within ±0.1� around the TMD’s epicenter with an increment/decrement of 0.02� for (a) the mainevent and (b) the aftershock on 6 May 2014. Focal mechanisms are colored based on the Fit value. Epicenters of the main event (Fig. 8a) from USGS, TMD, GCMT and EMSC arealso displayed.

240 S. Noisagool et al. / Journal of Asian Earth Sciences 127 (2016) 231–245

mechanisms (Fig. 10). Our stress direction from the stress inver-sion is also consistent with the alignment of the stress providedin World Stress Map (Heidbach et al., 2008) and the strain modelof Simons et al. (2007) as shown in Fig. 12. Fault plane solutionof each event is also plotted in the Mohr’s diagram (Fig. 11c).Fig. 11d shows the derived two principal fault (PF) planes

(Vavrycuk, 2011) where one has strike 47.8, dip 70.1, and rake12.8, and another has strike 349.8, dip 73.3, and rake 173.1. Thefirst PF plane (�N48E strike) is in agreement with the Mae Lao seg-ment as it has a strike orientations approximately N30E – N50E(Fig. 1b), while the second PF plane (�N10W) is incompatible withthe Pan segments with has strike around N5E – N13E (Fig. 1b).

Fig. 9. (a) Hypocenter depth from our study versus that from TMD estimation. (b) Mw estimated from our study versus ML calculated from TMD.

Fig. 10. Orientation of (a) the P-axis and (b) the T-axis derived from the focal mechanisms of Fig. 7.

S. Noisagool et al. / Journal of Asian Earth Sciences 127 (2016) 231–245 241

Based on the relative values of the principle stress and their plungeangle, it indicates that this area has a uniaxial compression(r2 ffi r3) in NNE-SSW with small WNW-ESE extension. Our resultis similar to the explanation of Charusiri and Pum-Im (2009) whointerpret that the Cenozoic tectonic evolution of Thailand was con-trolled by the N-S compression and the E-W extension. However,our result provides a well-defined angle where the stress wasapplied.

Prior to this study, there is no direct stress measurement avail-able in the northern Thailand. However, to study stress orientationin Thailand’s basins, Tingay et al. (2010) managed to infer thestress in Northern Thailand through the available focal mecha-nisms and borehole data scattering around the region both in Thai-

land and neighboring countries. The fan-shaped stress pattern isshown as dashed line in Fig. 12 which is in agreement with thestrain data from the GPS measurement (Simons et al., 2007) andalso the strain estimation from P-axis in the focal mechanismsfrom the World Stress Map (Heidbach et al., 2008). Their estima-tion of the maximum horizontal stress in Northern Thailand isaround NNE-SSW, consistent with our P-axis (Fig. 10). Tingayet al. (2010) then concluded that present day deformation inNorthern Thailand is dominated by the stress transfer from theHimalayan intrusion (Wang et al., 2011). Since our stress measure-ment of the principal stress (N18E) does not significant differ fromtheir estimation, our result therefore provide a great evidence thatstrongly support the study of Tingay et al. (2010) about the present

Fig. 11. (a) Inverted principle stress axes and uncertainty estimation from 100 realization with 10� deviation. The r1, r2 and r3 stress axes are (azimuth/plunge): 197.9�/3.2�,99.4�/69� and 289.1�/20.7�, respectively. (b) Shape ratio R = (r1 – r2)/(r1 – r3) distribution. (c) Full Mohr’s diagram and the fault plane solution (plus sign) for each focalmechanism. (d) Two principle fault planes (PF1 and PF2). Arrows indicate direction of strike for each PF.

242 S. Noisagool et al. / Journal of Asian Earth Sciences 127 (2016) 231–245

day deformation in Northern Thailand. The work of Tingay et al.(2010) is just one example demonstrating the important of thestress orientation in Northern Thailand.

3.4. The instability of the Phayao Fault Zone

Since most epicenters from these earthquakes occurred in thetransition area between Mae Lao Segment and Pan Segment(Fig. 1b), there was a debate among Thai geologists whether theright lateral strike – slip Pan segment or the left lateral strike – slipMae Lao segment initiates the slip to produce the main shockbefore both faults moved to generate the aftershocks. In this sec-tion, based on the information obtained from the stress inversion,we can provide geophysical evidence to this debate.

Based on the Mohr – Coulomb failure criterion, plotting thefault plane solution in the Mohr’s diagram (Fig. 11c) shows twodistinct shear stress zones: the high shear stress zone is linkedwith a high 2h angle and the low shear stress zone with a low 2hangle as indicated in Fig. 11c. As stress axis is almost uniaxial witha small plunge, the h angle can be estimated from the differencebetween the angles of the fault strike (/) with respect to the max-imum compressional axis (r1), i.e. h = / – r1. Any faults that havethe 2h angle falls within the high shear stress zone have a higher

chance to slip if external stress is applied to the area. Based onour earlier estimations, the strike of the Mae Lao covers fromN30E to N50E (Fig. 1b), while ranges from N5E – N13E for Pan seg-ment (Fig. 1b). Since r1 axis is about N18E, the difference of bothterms for Mae Lao segment would yield 2h � 24�–64�, which fitsentirely well with the high shear stress zone. Similarly, for thePan segment, the difference yields 2h � 10�–26 which match thelow shear stress zone. Since Mae Lao segment has a higher shearstress than the Pan segment, if external compression is applied, itis likely to initiate the slip when the shear stress exceeds the faultsliding criteria (Byerlee, 1978). The Pan segment which has a lowershear stress can then be activated by a local stress transferbetween both segments during the earthquake activity.

Fig. 7 also supports our hypothesis that the Mae Lao segment islikely to slip first. Since earlier result suggested that the compres-sion is originated from the north, it then causes the left-lateralmovement of the Mae Lao segment. This is evidently shown bythe first two events (main shock and large aftershock; event No.1 and 2 in Fig. 7) as both epicenters are closer to the Mae Lao seg-ment. After that, small magnitude events (ML < 5) occurred in thetransition area in which the focal mechanisms were mixed withnormal, strike-slip and thrust motions. These small events mayindicate the transfer of the energy from the Mae Lao segment to

Fig. 12. The local stress field derived from the STRESSINVERSE code (Vavrycuk, 2014) based on the main shock on 5 May 2015 and its aftershocks (Fig. 11a). The fan shaperegional stress estimated by Tingay et al. (2010) is shown in dashed black lines. The strain measurements (Simons et al., 2007) are shown with arrow heads. The stressesderived from the focal mechanisms provided by World Stress Map (Heidbach et al., 2008) are shown in green and red based on their mechanism (SS: Strike-Slip fault and TF:Thrust fault), and by ISC catalog are shown in violet. The stress derived from the borehole is shown in dark blue by WSM (Heidbach et al., 2008).

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the Pan segment. Interestingly, the moderate magnitude earth-quakes (events 14, 15, 16 and 17) with ML > 5 occurred consecu-tively in the middle of the sequences but with just strike – slipmotions implying that either the Mae Lao or Pan segments slipped.It then followed by a series of small magnitude events (ML < 5)with mixed focal mechanisms to release the energy in the transi-tion area and make fault adjustment. Following the magnitude –energy relation of Gutenberg and Richter (1956), these 30 events

Fig. 13. (a) Moment tensor solution of the 11 September 1994 event with Mw 5.2 (after Ifocal mechanism solutions with least errors derived from the FOCMEC code of Snoke et amain shock.

provide coverage of about 95% energy released by faults duringthe study period. Our results from the stress inversion are there-fore unlikely to significantly alter from those of Fig. 11.

3.5. A re-visit of the 11 September 1994 event

Prior to the 5 May 2014 event, one of the largest earthquake inNorthern Thailand in the past decades occurred on 11 September

SC, 2012). The thin black line indicates the double – couple solution. (b) The possiblel. (1984) from the polarity data of (a). The thick grey line is solution of 5 May 2014

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1994 (Fig. 1b) with Mw 5.2. It was characterized by GCMT Harvard(Dziewonski et al., 1981) as a normal slip with a compression in E –W directions (Fig. 13a). However, the focal mechanism solution ofGCMT showed large portion of non-double couple source whichmight be caused by the quality of data and the global station cov-erage. In 1994, the only local seismometer available in Thailandwas CHTO station in Chiang Mai (Fig. 1a). It is therefore impossibleto re-process the waveform data using the recent techniques aswhat we conducted here for the 5 May 2014 events.

To re-analysis the 11 September 1994 event, the P-wave polar-ities based on Fig. 13a were extracted from ISC data based (ISC,2012) and re-computed using FOCMEC program (Snoke et al.,1984). A set of ‘‘possible” focal mechanism solutions with leasterrors is shown in Fig. 13b. None of them shows pure normal slipmechanism similar to the early solution of the GCMT catalog. Mostof them indicates that the earthquake mechanism that generatedthe 1994 event is the strike – slip motion (Fig. 13b). Furthermore,by placing the focal mechanism of the 5 May 2014 event (Figs. 6and 7) on top of the P-wave polarities of the 1994 event, surpris-ingly, the polarities matched well with compression and dilationregimes (Fig. 13b). Thus, we can conclude that the earthquakemechanism for the 1994 event should be strike – slip motion,instead of normal motion.

4. Conclusion

On 5 May 2014, the largest earthquake in Thailand modern his-tory occurred on the Phayao Fault Zone (PFZ) in Northern Thailandwith an ML 6.3. The PFZ can be divided into the Mae Lao segment(north), Pan segment (central) and Mae Jai segment (south). Mostof the epicenters are located within the transition area of theMae Lao segment and Pan segment. Good quality waveform data(ML > 4) from 5 May 2014 to 15 August 2014 were available forthe seismometers installed in the Northern Thailand by the ThaiMeteorology Department (TMD). The regional moment tensor(RMT) inversion was then applied to these data to generate a seriesof 30 focal mechanism solutions covering 95% of energy releasedfrom the PFZ. Twenty-three of these 30 events are generated fromthe strike – slip motions, four from the normal slip, and two fromthe reverse motions. The computed moment magnitude (Mw) isslightly higher than an ML 6.3 estimated by the TMD, an Mww6.1of USGS, and Mwc 6.2 of Harvard GCMT. The different mightbe due to the assumptions of source used in the inversion algo-rithms, different station coverage, or different velocity model toestimate the Green’s function. As Mw is considered as a betterscale than ML, the main event on 5 May 2014 should be registeredwith an Mw 6.5, which is still the largest earthquake in Thailandmodern history. In contrast to the main event, most of our Mwaftershocks are lower than those of ML estimated from TMD. Thishas led us to suggest a calibration of ML estimated in Thailand tofit with the Thai geology. In addition, the hypocenter depths fromour calculation of most earthquakes are around 11 km, while thoseestimated earlier were much shallower.

The actual stress of the region was also determined with theSTRESSINVERSE program (Vavrycuk, 2014) from these 30 focalmechanisms. The inverted principal stress direction (N18E) is inagreement with the alignment of the stress provided by the WorldStress Map (Heidbach et al., 2008), the strain model (Simons et al.,2007), and the fan-shaped stress pattern from Tingay et al. (2010).One of the principal fault planes with a strike N47.8E is in agree-ment with the strike directions of the Mae Lao segment. Sincer2 ffi r3 and low plunge of stress axis, it indicates that this areashould has a uniaxial compression in NNE-SSW with smallWNW-ESE extension, also in agreement with Tingay et al. (2010).By plotting the fault plane solution in the Mohr’s diagram, the high

shear stress zone is linked with the Mae Lao segment, while thelow shear stress zone fits with the Pan segment. This informationindicates that, if external stress is applied from the north, theMae Lao segment is likely to slip first to produce the main shock.The energy transfer between the faults then generates the after-shocks occurred on both faults. We also re-visited the 11 Septem-ber 1994 event, and shows that its early calculated focalmechanism which is a normal motion might not be accurate, andshould be replaced with the strike – slip motion, similar to the 5May 2014 event.

Acknowledgement

This research has been supported by the Thailand Center ofExcellence in Physics (ThEP), the Royal Global Jubilee Program(RGJ:PHD/0100/2556), and the Development and Promotion ofScience and Technology Research Grant 037/2557. We would liketo thank the Thailand Meteorology Department (TMD) for support-ing the earthquake data for our study. We would also like to thankDr. Robert Herrmann for the Computer Program in Seismology(CPS) and his prompt support for users, and Dr. Michael Allen forediting the English of this manuscript.

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