Chiang Mai J. Sci. 2016; 43(6) 1279

Chiang Mai J. Sci. 2016; 43(6) : 1279-1291http://epg.science.cmu.ac.th/ejournal/Contributed Paper

Characterization of Khlong Marui Fault Zone UsingSeismic Reflection and Shear-wave Velocity Profiles:Case Study in Khiriratnikhom District, Surat ThaniProvince, Southern ThailandSawasdee Yordkayhun* [a, b], Preeya Sreesuwan [a, b], Kamhaeng Wattanasen [a, b]

[a] Department of Physics, Faculty of Science, Prince of Songkla University, Hat Yai, Songkhla,

90112, Thailand.

[b] Geophysics Research Center, Prince of Songkla University, Hat Yai, Songkhla, 90112, Thailand.

* Author for correspondence; e-mail: sawasdee.y@psu.ac.th

Received: 29 February 2016

Accepted: 8 July 2016

ABSTRACT

Detailed fault mapping and characterization are important for seismic hazardassessment. The Khlong Marui Fault Zone (KMFZ) is a major active strike-slip fault system insouthern Thailand. It extends in a southwest-northeast direction from Phuket towards SuratThani province. Although the general fault system can be identified from surface observations,investigation of the fault zone in Surat Thani province is challenging because the surfaceexpression is not obvious and thick sediments cover the area. Therefore, shallow seismicreflection profiles were acquired in the Khiriratnikhom district, Surat Thani province.The aims of this study were to characterize the subsurface geological structures in the vicinityof the fault zone. For the seismic data analysis, conventional data processing such as dataediting, static correction and frequency filtering are effective in enhancing signal to noise ratioof stacked section. However, detailed geological information at shallow levels in the subsurfaceare not well imaged due to the effects of data acquisition and processing. To address thislimitation, seismic reflection and shear wave velocity (Vs) profiles were obtained frommultichannel analysis of surface waves (MASW) and are jointly interpreted. The results showa sequence of subsurface boundaries extending from the surface to a depth of about 250 m.The variations in seismic velocities and vertical offset of the main horizon are the fault signatureobserved on seismic sections and in the shear wave velocity fields. The results coincide wellwith the fault strike obtained from a previous geophysical interpretation. This finding suggeststhe possibility of ongoing activity of the KMFZ.

Keywords: seismic reflection, shear wave velocity, MASW, Khlong Marui fault zone,Surat Thani

1. INTRODUCTION

During the past decade, increasingdamage and loss of lives associated with

earthquakes has been recorded, especially inand around urban areas and in vicinity of

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weak zones within major fault systems [1, 2].Even if the earthquake epicenter is far awayfrom the area, the geological characteristicsbeneath the fault zone can play an importantrole in seismic wave amplification [3].Therefore, availability of subsurfacegeologic information is critical for long-termseismic hazard assessments and for futuredevelopment plans in the region. Generally,geophysical investigations using seismicreflection survey have been applied to imagesubsurface geological structures, especiallyto detect and characterize the hidden faultsunder the fault zone [4, 5, 6].

Tectonically, Thailand is considered tobe a low seismicity region since it is faraway from plate boundaries. However,historical and instrumental studies haverecorded a number of moderate earthquakeevents since 1950 and most have occurredalong known fault zones [7, 8, 9, 10]. About14 fault zones in Thailand have beenidentified as active faults and two of thesefault zones are situated in southern Thailand:the Ranong Fault Zone (RFZ) and theKhlong Marui Fault Zone (KMFZ) [11].The KMFZ was the main target for theintegrated geophysical study of this faultzone project, which was initiated in 2011.Evidence, previously gathered fromgeological and geophysical data at a pilotstudy performed in the Vibhavadee district,Surat Thani province [12], suggests thatthere are a number of buried faults existingalong the proposed fault segments. However,no clear evidence of the major fault zonewere observed in the pilot study. As a partof the project, the regional trend of faultstrike identified from remote sensing,seismic reflection, airborne radiometric andgeomagnetic data [13] revealed that the

KMFZ may pass through Surat Thaniprovince from Phanom, Bantakun,Khiriratnikhom, Punpin and Thachangdistrict to the Gulf of Thailand (Figure 1).Although there is evidence of tectonicactivity associated with the KMFZ, thegeological structure and characteristics ofthe fault zone are still unclear. Therefore,an extensive study of fault characterizationsbeneath the variable thickness Quaternarysediment is incorporated into the recentstudy. Six seismic reflection profiles, ofabout 2-3 km each, were surveyed roughlyperpendicular to the fault strike in theKhiriratnikhom district, Surat Thani province.Among them, 3 lines were used to confirmthe existence of a fault segment that hasbeen outlined by the Department ofMineral Resources (DMR) and the other 3lines were used to detect and characterizesections of the fault zone that have beenidentified by previous geophysical data [13].Although seismic reflection methodsprovide an image of subsurface geologicalstructures, their shallow information is ofteninadequate due to the effect of acquisitiongeometry and data processing. Thus, shearwave velocity (Vs) profiles derived from theMASW methods were partly combinedfor gaining near surface information.The advantages of the MASW methods arethat they provide superior resolution toP-wave methods in soft soil and take intoaccount any velocity inversion [14, 15].In this study, after briefly describing thegeology at the study sites, we explain howdata were acquired and processed. Resultsand interpretation of the seismic sectionsand Vs profiles at all survey lines will beillustrated.

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2. GEOLOGICAL SETTING

Since the end of Mesozoic era, tectonicmovements in conjunction with collisionsof Indo-Australian, Eurasian and WestPacific plates formed the structurallycomplex areas, such as Gulf of Thailandand Andaman Sea [16, 17]. After thecompletion of clockwise rotations ofcrustal blocks, the KMFZ was developed[18]. The KMFZ is considered to be anactive strike-slip fault that cuts acrossPeninsula Thailand from Phuket Island inthe southwest towards Surat Thani Provincein the northeast (Figure 1). The strike of thefault zone can be traced for a distanceexceeding 150 km and 10 km width, andcomprised of about 10 segments [19, 20].

Khiriratnikhom district lies in thecentral part of Surat Thani provincewhere the Tapee River runs in a NE-SWdirection and is surrounded by N-S trendingmountainous ranges (Figure 1). The basementis represented by rocks of Carboniferous-Permian period found in western mountainareas, composed of limestone, mudstone,shale, sandstone and siltstone. Permianlimestone occurs in the middle part and

contain Permian fossils. The siltstones areyellow-brown in color, thinly bedded andcontain carbonaceous layers. Triassic-Jurassicsedimentary rocks and Triassic-Cretaceoussedimentary rocks are distributed in thesouthern region. Both sedimentary rockunits are composed of sandstone, siltstone,limestone lenses, and conglomerate. In thenorthern part, Triassic-Jurassic graniticrocks are dominated by batholiths andplutons. A Quaternary sedimentary basinformed in the vicinity of the main river.This sedimentary fill is represented byfluvial (Qa) and terrace (Qt) deposit [21].The terrace deposits (Qt) consists mainly ofgravel, clay, and coarse grain and poorlysorted of sand layers. The fluvial deposit (Qa)are composed of an alternating sequence ofsilty clay and sand layers.

3. MATERIALS AND METHODS

3.1 Theoretical BackgroundReflection seismology can determine

possible changes in subsurface elasticproperties by measuring the two-way traveltime of seismic waves propagated from asurface seismic source into the subsurface

Figure 1. Geological map showing KMFZ distributions and the study area. Zooming panelshows 6 survey lines (KR1-KR6). Red line marked the KMFZ proposed by DMR and blackline marked the KMFZ proposed by previous geophysical data.

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and reflected back to the surface. At theinterface between layers with contrastingacoustic impedance, the reflection signal isgoverned by Zoeppritz equation [22] and atnormal incident it is simply described bythe reflection coefficient (R).

R = (1)

Where ρ1 and ρ

2 are the density of

medium 1 and medium 2, while V1 and V

2

are the wave velocity of medium 1 andmedium 2, respectively.

Multichannel Analysis of SurfaceWaves (MASW) is a non-intrusive, fast andcost-effective geophysical method recentlydeveloped for Vs determination andincreasingly used in earthquake andgeotechnical engineering studies [23].The MASW method utilizes dispersioncharacteristic of Rayleigh waves (groundroll) as the crucial property to estimate theshear wave velocities. Ground roll are oftenobserved in the conventional seismicreflection/refraction data, especially whenlow natural-frequency geophones are used.Consequently, the same dataset can beanalysed for seismic reflection and MASWmethods. The propagation velocity of shearwave in an elastic medium is given byequation 2 [22]:

Vs = (2)

Where μ is shear modulus and ρ isdensity of the medium. Dynamic elasticproperties of soil including shear modulusderived from seismic velocities and densityare importance for site investigation andconstruction purposes.

3.2 Data AcquisitionsSix seismic ref lection survey lines

namely KR1 to KR6 were acquired in a

northwest to southeast direction androughly perpendicular to the two proposedfault segments associated with the KMFZ(Figure 1). The total lengths of the surveylines range from approximately 2-3 km.As mentioned earlier, survey lines KR1, KR2and KR3 were used to detect the fault zoneproposed by previous geophysical studies,whereas KR4, KR5 and KR6 were used toverify the fault location proposed by DMR.The field surveys were difficult because oflimited access to some areas, such as in thevicinity of the Tapee River and the urbanarea. In particular, we were not able toacquire data across the proposed fault byDMR and the south-eastern part of thesurvey lines was skipped. Therefore, surveylines were selected along the agriculturalroads and relatively flat topography to avoidthe extremely noisy conditions from trafficand the urbanized region. Off-end source/receiver geometry was used in conjunctionwith 24 geophones at 5 m spacing (Figure 2aand 2c). The natural frequency of thevertical geophones used is 14 Hz. A walkawaynoise test performed in the area reveals thata 30 m minimum offset appear to beoptimum recording window. For the seismicsource, 10-15 hits of 5 kg sledgehammer onsteel plate provided sufficient signal-to-noiseratio of seismic energy. Source spacingwas set at 5 m intervals, providing 12 foldscoverage every 2.5 m CMP in the subsurfaceusing roll along movement. The data wererecorded by a 24-channel GeometricSmartSeis seismograph using a recordlength and sampling interval of 1024 ms and0.5 ms, respectively. Recording such a longseismic trace allows us to extract the Vsprofile by analysis of the surface waves(Figure 2b). Recording parameters of theseismic profile are shown in Table 1.

ρ2V

2 - ρ

1V

1

ρ2V

2 + ρ

1V

1

√μρ

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Figure 2. a) Details of data acquisition geometry and parameters. b) Surface wave characteristicsand dispersion. c) Field surveys.

Table 1. Data recording information.

3.3 Data ProcessingSeismic reflection data processing was

performed using the Globe Claritas Version5.5 software [24]. The processing flowapplied to the seismic data is summarizedin Table 2. Converting raw data from SEG2to SEGY format is done as pre-processingstep. An example of raw shot gathers isshown in Figure 3. Clear reflector eventscan be seen in the upper part while a low

signal-to-noise ratio due to the contaminationof ground roll is observed in the lower part.The field geometry files were edited andassigned into the raw data followed byroutine editing of the dead and noisy traces.Refraction static corrections were appliedto the data to compensate the effect ofnear-surface low velocity layer [25]. Automaticgain control (AGC) with 150 ms sliding timewindow was applied to balance the trace. Byinspection of the power spectra (Figure 3),the low frequency band from 15 to 40 Hzrepresents the low frequency and strongamplitudes of ground roll, while backgroundnoise dominates at frequencies higher than150 Hz. Therefore, band-pass filter of30-150 Hz appears to enhance the usefulsignal frequencies as shown in Figure 2b.After CDP sorting, stacking velocitiesfunctions in the range from 1000-3000 m/swere picked based on an integrated analysisof constant velocity stack and semblanceplots. The velocities were updated twice andused for normal moveout (NMO) correction.

RecordingparametersRecording systemSourceGeophonesShort intervalReceiver intervalOffsetChannelsSampling intervalRecord length

Details

Geometric SmartSeis5 kg SledgehammerVertical, 14 Hz5 m5 m30 m24, off-end0.5 ms1.024 s

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The 70% NMO stretch mute was used toeliminate refraction energy and preserveshallow reflections. The stack sections wereproduced after stacking is performed for all

CDP traces. Finally, the time sections wereconverted to depth sections using the intervalvelocity field.

Table 2. Processing steps for seismic reflection data.

Step

1. Data import

2. Setup of filed geometry

3. Editing

4. Elevation statics and Refraction statics(Field statics)

5. Band-pass filtering

6. Automatic gain control (AGC)

7. CMP sorting

8. Velocity analysis

9. Normal moveout (NMO)

10. Stack

11. Time to depth conversion

Descriptions and Parameters

SEG2 to SEGY conversion

Assign input shot locations and receiverlocations into headers

Kill bad traces and fix polarity reversals

Calculate static corrections based on nearsurface models and elevations

Minimum phase Butterworth filteringf = 15, 30, 150, 240 Hz, Designamplitude = 0,1,1,0Adjust amplitude using 150ms sliding window

Sort data by common midpoint number

Integrate analysis of constant stacked velocitypanels and semblance plots

Apply stacking velocity function including70% stretch mute

Convert to depth section using intervalvelocity

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For the MASW method, data wereprocessed using the SurfSeis version 3software developed by the Kansas GeologicalSurvey, USA. The processing utilized aniterative inversion method to convert thepicked dispersion curve into a 1D S-wavevelocity model. First, the SEG2 format, rawdata were converted to the software inputformat. Then, noisy traces were removedand high cut filtering was applied to removehigh frequency reflection energy andambient noise. A shot gather in time-space(t-x) domain was transformed into the phasevelocity-frequency (f-v) domain using a 2Dtransformation (Figure 4). Dispersion curves

were extracted by picking the phase velocityat different frequency values. By setting upan initial model based on a dispersion curveand adjusting the model parameter (Vs) withthe objective of minimizing the error betweenthe calculated and picked dispersion curve,a 1D velocity model placed at the center ofthe geophone spread is archived (Figure 4).To account for the non-uniqueness of thesolution found in the inversion process,the optimum models were selected basedon tracking RMS error and consideringgeological information. Finally, 1D Vs profileswere interpolated along the survey line togenerate a 2D Vs section.

Figure 3. Example of shot gather showing (a) raw data and their power spectra, (b) theresults after some processing steps were applied, including editing, static corrections, filteringand AGC.

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4. RESULTS AND DISCUSSIONS

The results are interpreted based onintegration of the seismic sections, 2D Vssections, geology and lithology informationfrom available shallow boreholes near thearea. The lack of deep boreholes and thelow resolution of previous geophysicalstudies have made structural interpretationdifficult. In this study, the estimated verticalresolution for the seismic section is about8 m based on the one-quarter wavelengthcriteria and using the 60 Hz dominantfrequency and average velocity of 2000 m/s.Faulting is indicated based on the coherencyloss of some strong continuous reflections,the abrupt change in dip angle of reflections,and presence of diffraction events [22].

Interpreted time sections and depthsections and 2D Vs sections for KR1-KR3survey lines are illustrated in Figure 5 and 6.The first coherent horizon in the seismicsections is observed at approximately30-100 m depth, and it appears to be

down-dipping from northwest towardsoutheast (marked as yellow solid line inFigure 5). This is probably the base ofQuaternary sediments or transition zonebetween Quaternary and Permian unit. Thestructural setting of this horizon appearsto be karst topography. Below this horizon aseismic pattern of discontinuous and variouslydipping reflectors is visible to about 250 mdepth, corresponding to the highly fracturedrocks at the fault zone in the sequences ofPermian limestone unit (Figure 5 and 6).Clear evidence of limestone can be seen inthe borehole, mountain and outcrop nearthe survey lines (Figure 7). The fault planeappears on the seismic section as a normalfault in certain area that dips to the east andwest with steep angle of about 70-90 degrees.Relatively small vertical offset observedin some part of the main horizon maycharacterize the strike-slip faults deformwith small amounts of transtension, whereas30-50 m vertical offset is interpreted to

Figure 4. Example of raw shot gather (a), picked dispersion curve (b) and 1D Vs modelfrom MASW analysis (c).

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Figure 5. Time sections and depth sections with interpretation of survey line KR1 (a and b),KR2 (c and d), and KR3 (e and f), respectively. Main horizon is marked as yellow line, whilepossible fault is marked as red line.

represent the fault throw of major faultsystem. In addition, one of the main criteriaused in identifying strike-slip faults in seismicsections are complex flower structures [26].This feature is characterized by fan-like, rathersteep faults converge at depth into a singleand sub-vertical fault. Although the seismicenergy was limited and the deeper faults werenot imaged in the sections, evidences of partlyflower structures in line KR1-KR3 (Figure 5)may indicate the strike-slip movement. Thisobservation confirmed the fault strike derived

from previous geomagnetic interpretation [12]and indicates that tectonic activity along thefault zone may be complicated. The locationof the fault plane is also coincident with theabrupt change in the shallow Vs field (Figure6). This suggests that faults possibly affectthe shallow subsurface in this area. By visualinspection of the Vs fields, the internalstructures of Quaternary unit itself can bedivided into 2 layers, where the cover layeris characterized by low velocity of about200-500 m/s with 10-20 m thickness.

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Figure 6. Zooming panel for the depth sections of survey line KR1 (a), KR2 (b) and KR3 (c),overlain by their Vs sections. Note that the depth sections are displayed with vertical exaggerationof 2.

Figure 7. (a) An available borehole information near the survey lines. (b) Limestone mountainand fractured outcrop near the survey lines.

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Figure 8. Time sections and depth sections with interpretation of survey line KR4 (a and b),KR5 (c and d), and KR6 (e and f), respectively. Main horizon is marked as yellow line, whilepossible fault is marked as red line.

In the northern Khiriratnikhom district,there is a clear flat horizon that followsmost of the KR4-KR6 survey lines at about20-30 m depth which could be consistentwith the top of Carboniferous-Permian units.One of the main uncertainties in thestructural interpretation is that there is noclear evidence of buried fault associated

with major fault zone observed beneaththese survey lines (Figure 8). However, it isinteresting to note that a prominent undulationin the main horizon is clearly seen in themiddle of KR6 survey line. Based on geologyand available boreholes, this event couldpotentially be a granite intrusion in the area.

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5. CONCLUSIONS

A total of six seismic reflection profileswere acquired in Khiriratnikhom district,Surat Thani province with the aim ofcharactering the subsurface associated withthe KMFZ. The main finding can be drawnbased on integrated analyses of the seismicreflection and Vs sections obtained fromthe MASW methods. A small discrete offsetof the main horizon, weak and terminatedreflection as well as abrupt changes in Vs inthe shallow subsurface are evidence for theburied faults beneath the three seismicprofiles. This agrees with the fault strike thathas been proposed by previous geophysicaldata. However, no clear evidence of the faultis visible in the other three seismic reflectionprofiles located in the northern part of thestudy area. Apart from fault zone, graniticrock may extrude to the near surface inthis region. This study together with theinformation from trenching, earthquakeand tectonic information will allow betterunderstanding the seismic hazard assessmentof the area.

ACKNOWLEDGEMENTS

This work was supported by thegovernment budget of Prince of SongklaUniversity, contract no. SCI580324S. Also,acknowledge to the software and researchfacilities support from Geophysics ResearchCenter, Prince of Songkla University (PSU).Seismic equipment was supported by theInternational Program in Physical Science(IPPS), Uppsala University, Sweden. Fieldwork crews from Department of Physics,PSU are thanked for their working hard indata acquisition.

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