Chiang Mai J. Sci. 2016; 43(6) 1269

Chiang Mai J. Sci. 2016; 43(6) : 1269-1278 http://epg.science.cmu.ac.th/ejournal/ Contributed Paper

Shear Wave Velocity Estimation of the Near-surface Sediments of and Vicinity, for Seismic Site Characterization Aomboon Naksawee [a], Koichi Hayashi [b] and Passakorn Pananont* [a] [a] SEIS-SCOPE, Department of Earth Sciences, Kasetsart University, 50 Ngamwongwan Road, Latyao, Chatuchak, 10900 Bangkok, Thailand. [b] OYO Corporation/Geometrics, 2190 Fortune Drive, San Jose, CA, 95131, USA. * Author for correspondence; e-mail: [email protected]; [email protected]

Received: 29 February 2016 Accepted: 8 July 2016

ABSTRACT Bangkok and vicinity is located in the Lower Central Plain of Thailand in a region characterized by thick Quaternary basin fills. This region contains the largest city of Thailand (Bangkok) with more than 10 million people and has been affected by distant, moderate to large earthquakes. The average shear wave velocity of the top 30 m of the subsurface (Vs30 ) is an important parameter for seismic site characterization as it can affect the level of ground shaking during an earthquake and thus affect the level of potential damages.

In this study, the Vs30 is estimated for Bangkok and vicinity using both passive and active of surface waves methods. The Vs30 of the shallow sediments from 206 sites in Bangkok and vicinity varies from 86 m/s to 207 m/s, which are classified as the soil type Class E and Class D; based on the soil type classification of the U.S. National Earthquake Hazards Reduction Program (NEHRP). In general, the southwestern part of the Lower Central

Plain in Samut Prakan and Chachoengsao has lower Vs30 (Vs30 < 120 m/s) compared to that in the western and northern parts of the study area (Vs30 > 160 m/s). Vs30 maps are generated and they can be used for seismic microzonation, land use planning, and development of Bangkok and vicinity, helping to increase the earthquake preparedness of the city in the future.

Keywords: site classification, seismic microzonation, MASW, Vs30, Bangkok, lower central plain, Thailand

1. INTRODUCTION Local geological conditions (aka. site event, in Mexico; the 1989 Loma Prieta and effects or site classification) can generate the 1994 Northridge events in USA; the amplification of the ground motion and 1995 Kobe event in Japan; and the 2010 increase damage to structures during an Christchurch event in New Zealand have earthquake [1]. Many recent destructive clearly showed that local soil conditions earthquakes such as the 1985 Michoacan can have a significant influence on the 1270 Chiang Mai J. Sci. 2016; 43(6)

ground motion and on the damage pattern includes Bangkok and the vicinity covering [2-6]. In general, the softer the sediment, , , Samut Prakan, the higher amplification of the ground and parts of Nakhon Pathom, Nakhon motion. Average shear wave velocity of the Nayok, Samut Sakhon, Chachoengsao and sediment is known to correlate well with Phra Nakhon Si Ayutthaya provinces. the ground motion during an earthquake [7]. Average shear wave velocity over a depth of

Estimating the average shear wave velocity 30 m from the surface, Vs30 is determined, of the sediments can provide key information using the combined active and passive of the site condition. The average shear wave surface wave method at 206 sites throughout velocity of the top 30 m (Vs30) of the area is the study area. Seismic site classification widely used to estimate the potential maps of Bangkok and vicinity, based on amplification of the ground motion at a these Vs30 results, and using the NEHRP site during an earthquake, and it is used in standard for earthquake hazard assessment, several building codes [8-11]. The distribution are also presented. of the Vs30 has been used to generate local and regional maps of site categories based 2. GEOLOGICAL SETTING on the National Earthquake Hazards Program Bangkok, Nonthaburi, Pathum Thani, (NEHRP) [12, 13]. Samut Prakan and parts of Nakhon The study area, Bangkok and vicinity, is Pathom, Nakhon Nayok, Samut Sakhon, located in the Lower Central Plain of Chachoengsao and Phra Nakhon Si Ayutthaya Thailand. The paleoenvironment of this comprise the largest community in Thailand, region is characterized by Pleistocene to containing more than 10 million people Holocene fluvial and coastal plain deposits and covering more than 6,000 km2. Located of the Gulf of Thailand. in the Lower Central Plain of Thailand, is generally considered to be a relatively Bangkok and its vicinity are characterized stable continental region, characterized by by thick Pleistocene fluvial sediments of very low seismicity; there have been no the Chao Phraya basin (sand, silt and clay). reports of seismicity within a 100 km radius A study of land subsidence monitoring of Bangkok. However, there have been in Bangkok and surrounding areas [14] several cases of the minor to moderate summarizes the geology of the Lower shaking of the tall buildings in Bangkok Central Plain as Quaternary fluvial deposits, during moderate to large earthquakes consisting of unconsolidated to semi-hard occurring elsewhere in and around Thailand sediments, , i.e. gravels, sand, silt and clay, [14-16]. The soft sediment in Bangkok is accumulated in the basin. The basement is thought to amplify the ground motion of Precambrian to Jurassic age, and crops during an earthquake [17]. out along the edges of the Central Plain. The main objective of this paper is The fluvial deposits underlie relatively thin to determine site classifications for the (0-21 m) soft marine Bangkok clay and Pleistocene fluvial sediments and Holocene deltaic sediments, as a result of the coastal marine deposits located in Bangkok and environment, especially the last sea level vicinity in central Thailand, located between rise of the Gulf of Thailand during Holocene latitudes 13.508°N and 14.248°N and [18-20], as shown in Figure 1. The topography longitudes 100.254°E and 101.013°E of the study area is relatively flat with a very (approximately 6,000 km2). The study area low topographic gradient along the 80 km Chiang Mai J. Sci. 2016; 43(6) 1271

N-S transect, from less than 10 m above Thailand. There are no bedrock exposures MSL in the north down to sea level in the in the study area. south at the shoreline of the Gulf of

Sedimentary and Metamorphic Rocks Fluvial deposits: gravel, sand, silt, and clay ofchannel, river bank, and flood basin. Coastal wave-doinated deposits: sand and gravelly sand of beach ridge, barrier and dune. Coastal tide-dominated deposits: sand and gravelly sand of beach ridge, barrier, and dune. Terrace deposits: gravel, sand, silt, clay, and laterite. Sandstone, argillaceous limestone, shale, and chert. Orthogneiss and paragneiss,banded, augen; amphibolite schist, quartz-mica schist, quartz-kyanite schist, silimanite-mica schist; quartzite; marble; calc-silicatea; migmatite and pegmatite. Igneous Rocks Biotite granite, tourmaline granite, granodiorite, biotite-muscoving granite, muscovite-tourmaline granite, and biotite-tourmaline granite. Rhyolite, andesite, ash-flow tuff, volcanic breccia, rhyolitic tuff and andesitic tuff.

Figure 1. Geologic setting of the study area in the Lower Central Plain of Thailand. All of the study area is covered with the Quaternary sediments with no rock outcrops. (modified from DMR, 2007). 1272 Chiang Mai J. Sci. 2016; 43(6)

3. METHODOLOGY

The average shear wave velocity of the Geophone position → Active Spacing 1 m., 24 channels Sampling rate = 1 ms top 30 m (Vs30) of the sediments in Bangkok Record length = 0.25 s Geophone position → Passive and vicinity was estimated by using active Spacing 15 m., 10 channels Sampling rate = 2 ms and passive surface wave methods [21, 22]. Record length = 20 s

3.1 Active and Passive Surface Wave Methods Active and passive surface wave methods were performed at 206 sites in Bangkok and vicinity. A Geometrics’ 24-channel Figure 2. Schematic representation of active Geode seismograph with 4.5 Hz vertical- (linear) and passive (triangular) surface wave component geophones was used for both surveys. The data from both methods are then the active and passive surveys. In the active combined during the data processing step. survey [23, 24], 24 geophones with a geophone spacing of 1 m were used to The recorded seismic waves from active produce a 23 m long survey line. For these and passive methods were analyzed using a active surveys, impact energy was created phase-shift method (multi-channel analysis by striking a metal plate at the end of the line, of surface waves, MASW) and a spatial 5 m offset from the first geophone, with a auto-correlation (SPAC) method respectively 20 lbs ( 10 kg) sledgehammer to generate to generate phase velocity images in the seismic waves (Figure 2). The sampling frequency domain from surface waves, such interval was 1 ms with a record length of as Rayleigh wave [23, 26, 27]. Then dispersion 0.25 s. The typical penetration depth of curves were created from the phase velocity the active method is about 5-15 m, depending images. Dispersion curves from both active on the survey geometry, allowing the and passive surface wave methods at the determination of the shear wave velocity same site are then combined to generated of the shallow subsurface. In order to reach an integrated dispersion curve of the site. the depth of 30 m, a passive method A 1-D inversion using a non-linear least (aka. microtremor array measurement: MAM) square method was then applied to the is also introduced in this work. The passive dispersion curves to generate a 1-D method consists of 10 channels of 4.5 Hz shear wave velocity model of the shallow vertical component geophones with different subsurface. An example of the results of geophone spacing lying in a triangular the active and passive surface wave methods shape with a base of 30 m (Figure 2). The is shown in Figure 3. sampling interval used is 2 ms. The ambient noise is record for 20 s with 30 records at 3.2 NEHRP Site Classification each site. SeisImager/SW software of The National Earthquake Hazards Geometrics [25] was used to process and Program (NEHRP) characterizes the site analyze both the active and passive surface condition for evaluating the effect of the wave data. subsurface on ground motion during an Chiang Mai J. Sci. 2016; 43(6) 1273

30 earthquake by using the average shear Vs30 = di Σ n ( ) wave velocity of the top 30 m of the i = 1 Vi subsurface (Vs30) (Table 1: [10, 28]); the Vs30 where vi and di denote shear wave can be obtained using the following velocity (m/s) and thickness of the ith layer in equation: a total of n layers in the top 30 m of the subsurface.

Figure 3. Active and passive surface wave data processing step. Shot records from both active and passive surveys (a and b, respectively) and phase velocity images of active and passive surface wave methods (c and d) are converted to dispersion curves (e and f). Then the dispersion curves of both surveys are combined (g). Eventually the 1D shear wave velocity model are inverted from the combined dispersion curve (h). 1274 Chiang Mai J. Sci. 2016; 43(6)

Table 1. Soil classification according to IBC 2006 (ICC, 2006).

Site class Soil profile name Shear wave velocity, Vs30 (m/s)

A Hard rock Vs30 > 1,500 ≤ B Rock 760 < Vs30 1,500 ≤ C Very dense soil and soft rock 360 < Vs30 760 ≤ D Stiff soil profile 180 < Vs30 360

E Soft soil profile Vs30 < 180

4. RESULTS AND DISCUSSIONS classification (Figure 4). Except in the very 4.1 Shear Wave Velocity Distribution and northern end and the western edge of the Seismic Site Classification study area, most of the shallow subsurface

The average shear wave velocity of the has Vs30 less than 180 m/s, which is top 30 m Vs30 deduced from the combined categorized as soil type Class E (soft soil) active and passive surface wave methods and capable of amplifying ground motions for 206 sites in the study area indicates that during an earthquake. The shear wave the shallow sediments in Bangkok and velocity models estimated from the combined vicinity have relatively low Vs30: 86-207 m/s, passive and active surface wave method which are classified as the soil type Class E are shown in Figure 5. and Class D based on the NEHRP soil-type

Figure 4. A map of average shear wave velocities of the top 30 m (Vs30) distribution in the

Lower Central Plain of Thailand. Most of the study area has the Vs30 less than 180 m/s (Soil type Class E). The lowest Vs30 (Vs30 < 120 m/s) is located in the southwestern corner of the study area in Samut Prakan and Chachoengsao provinces. Chiang Mai J. Sci. 2016; 43(6) 1275

Figure 5. Representative 1-D shear wave velocity functions in the study area: (The locations of each sites are indicated with green rectangles in Figure 4.) 1. Sao Thong Klang Temple, , . 2. Klong Kun Ya School, Bang Bo district, Samut Prakan province. 3. Darun Na Uem Mosque, , Bangkok 4. Klong Mai School, Muang district, Samut Prakan province. 5. Sang Ma Nee Temple, Nong Sua district, . 6. Soon Tha Ree Tham Phi Ka Ram Temple, , . 7. Bua Su Wan Pra Dit Temple, , Pathum Thani province. 8. Lat Sai Temple, Wang Noi districts, Phra Nakhon Si Ayutthaya province. 1276 Chiang Mai J. Sci. 2016; 43(6)

To obtain clearer distribution of the SUMMARY AND CONCLUSIONS

Vs30 in the study area, the Vs30 are subdivided Site classifications are an important with more refined boundary values as component of seismic microzonation and follows: (1) Vs30 <120 m/s (2) Vs30 between seismic hazard analysis. In this paper,

120-139 m/s (3) Vs30 between 140-159 m/s average shear wave velocities (Vs30) were

(4) Vs30 between 160-179 m/s and (5) estimated using active and passive surface

Vs30 >180 m/s and the revised map of wave surveys at 206 sites in Bangkok and

Vs30 is generated (Figure 4). The areas that the vicinity in the Lower Central Plain of have lowest Vs30 (Vs30 < 120 m/s) are located Thailand and a Vs30 map of the study area in the southeastern corner of the study were generated. The Vs30 in the study area area in Samut Prakan and Chachoengsao is relatively low and varies from 86-207 m/s, provinces where a land subsidence problem which are classified as the soil type has been prominent in the past [29]. Class E and Class D based on NEHRP site

The Vs30 increases towards the northwest classification. This finding corresponds of the study area in Phathum Thani and well with the thick Pleistocene fluvial the southwest Phra Nakhon Si Ayutthaya. sediments and Holocene marine sediments.

The area that has highest Vs30 (Vs30 > 180 More than 95% of the study area in the m/s) is in southern Phra Nakhon Si Ayutthaya Lower Central Plain of Thailand has a Vs30 near the northernmost paleoshoreline during less than 180 m/s (Class E). This can have the Holocene, suggesting a near-surface the biggest amplification potential and can covering of rather thin marine clays [30]. greatly affect ground shaking during an The southern part of the study area also earthquake. shows relatively low Vs30 (Vs30 less than 160 Overall, the southwestern part of the m/s) in Samut Prakan and Samut Sakhon Lower Central Plain in Samut Prakan provinces where the southern end the region and Chachoengsao has lower Vs30 (Vs30 < abuts the Gulf of Thailand. 120 m/s) compared to those of the western

As the study area occupies about the and northern part of the study area (Vs30 > same boundary as the Lower Central Plain, 160 m/s) which can reflect the thickness it is worth noting that, as the Vs30 of the soft of the marine clay deposit on the clay is very low [31], it can be seen that paleotopography during the Holocene. area that has the lower Vs30 could imply a The southern part of the Lower Central thicker layer of the soft clay in the subsurface. Plain also has a relatively lower Vs30 than Therefore the area where the marine clay has the northern part as it may have a thicker thickest accumulation is in the southwestern marine clay sediment layer as the topography corner of the Lower Central Plain, not in the gradually slopes down to the Gulf of center as previously thought. This finding Thailand to the south. The results of this could reflect the paleotopography of the study are very important for preliminary Lower Central Plain during the Holocene: site evaluations for city development and lower in the southwestern corner and higher urban planning in Bangkok and vicinity in towards the west and the north. terms of seismic effects. However additional Chiang Mai J. Sci. 2016; 43(6) 1277

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