Citation:
Nepal, N., T. Hayashida, T. Yokoi (2021), Geophysical prospecting using microtremors toestimate 1-D shallow shear wave velocity profiles in Thimphu, Bhutan, Synopsis of IISEE-GRIPS Master’s
Thesis.

GEOPHYSICAL PROSPECTING USINGMICROTREMORS TO ESTIMATE 1-D SHALLOWSHEAR WAVE VELOCITY PROFILES IN THIMPHU,
BHUTAN
Nityam Nepal1,2Supervisor:Takumi HAYASHIDA3, Toshiaki YOKOI3
ABSTRACTMicrotremor measurements were conducted at 25 sites in Thimphu to obtain 1-D shallow shear wave
velocity (Vs) profiles. The Vs profiles were obtained from the inversion of the phase velocities using
the microtremor Spatial Autocorrelation method. The averaged Vs values in the upper 30 m obtained at
16 sites are categorized under “class C (dense soil and soft rock)” of the NEHRP classification. The
horizontal-to-vertical spectral ratios of the observed microtremors were also obtained and compared tothe theoretical ones calculated from the inverted Vs profiles. Almost all sites showed a flat characteristicof the spectral ratios between 1 Hz and 15 Hz. The computed site amplifications showed most sites in
Thimphu have predominant frequencies above 4 Hz.
We also performed microtremor array surveys at six sites in South Ibaraki, Japan, using the same method
and sensors used in Thimphu to corroborate the applied methods and to understand the effects ofgeology/geomorphology on the results obtained. The results obtained in Thimphu and South Ibaraki
showed clear distinctions in the Vs profiles and predominant frequencies. Thimphu City, located on
mountainous terrain, showed higher values of Vs30 and dominant frequencies than most sites in South
Ibaraki, which is located on a deep sedimentary basin.
Keywords: Microtremors, Thimphu, SPAC method, Vs30, H/V spectral ratio.
1. INTRODUCTION
The seismicity of Bhutan is low in normal times, but the recent paleoseismic studies revealed theexistence of a significant event with a magnitude of 8±0.5 in the 18th century (Le Roux Mallouf et al.,2016). Since then, the only noticeable event was the 2009 Mongar earthquake (Mw6.1) in the east, whichcaused local damage and claimed 12 lives. Significant events located outside the territory, such as the2011 Sikkim earthquake (Mw6.9), have caused considerable damage in the country (Diehl et al., 2017).
Bhutan falls under the “category Zone V (highest risk zone)” in the Indian seismic zonation system,
indicating a Peak Ground Acceleration value of 0.36g. However, this estimation is much lower than the
recent hazard results (Stevens et al., 2020). Most of the buildings in Bhutan consist of traditional styled
rammed earth and stone masonry houses, and these structures do not follow any building code.
Generally, significant seismic risks arise from the probable collapse of buildings and otherstructures. For this reason, Bhutan faces high vulnerability in terms of both hazard and risk. Seismic
hazards need to be understood well to eliminate risks as far as possible. Seismic wave amplification
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Earthquake and Geophysics Division, Department of Geology and Mines, Bhutan.
IISEE-GRIPS Master’s course student.
3International Institute of Seismology and Earthquake Engineering, Building Research Institute.
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occurs depending on local geological conditions (e.g., thickness and physical properties of subsurface
sedimentary soils), and effects of amplification at the subsurface soil are different from site to site andare controlled by the geological condition. Studying local site effects and understanding the subsurface
profiles is essential for understanding the impact of subsurface geology on the strong ground motion forearthquake hazard assessment and risk mitigation.
Geophysical exploration using microtremors (ambient seismic noise) is a reliable andstraightforward technique to understand subsurface structures and deduce soil characteristics. In thisstudy, we will use the microtremor array data measured in the city of Thimphu to construct 1-D
subsurface Vs profiles. The study also uses microtremor array data measured in South Ibaraki, where
geological information, PS loggings and Vs30 information are available, to validate the techniquesapplied and confirm the results. The geological and geomorphological difference of Thimphu and South
Ibaraki will also be helpful to understand the effects of different environments on microtremors.
2. METHODOLOGY
2.1. SPAC method
The primary method used in this study is the Spatial Autocorrelation (SPAC) method, which uses
microtremor array recordings to estimate the characteristics of the subsurface Vs profile from theRayleigh-wave phase velocities. The technique was first proposed by Aki (1957, 1965) and completed
by Okada (2003).
The coherence function between sensor pairs is derived based on a stochastic approach.
This approach assumes that: 1) Microtremor propagates as a sum of the individual sources and reaches
the array as a plane wave. For this assumption, the sources of microtremors are located distant from thearray; 2) Microtremors propagate uniformly and continuously from all directions, and the distributionof signals from source to sensors is uniform (Aki, 1957). Based on the assumptions, the SPAC coefficient
between a sensor pair can be linked by fitting it to the Bessel function of the first kind of zero order.
𝜌

𝑟, 𝜔

,
||

|

|

𝑑𝜃 ≅ 𝐽0

𝜔𝑟
𝑐𝜔

(1)

where 𝜌 𝑟, 𝜔 is the coherence function between sensors A and B, 𝑆 𝑟, 𝜔 is the cross-spectrumof the sensor pair, 𝐺 𝜔 are the power spectrum of the microtremor, and 𝑟, 𝜔, and 𝑐 𝜔 are thesensor-to-sensor distance, the angular frequency, and the phase velocity, respectively. After we obtain
the dispersion curves of Rayleigh-wave, we apply a heuristic inversion technique to estimate 1D
subsurface Vs profiles. An initial velocity model consisting of horizontal layers is assumed and eachlayer thickness and Vs value are determined. After the inversion we obtain a Vs structure model which
optimally fits the observed phase velocities.
2.2. H/V spectral ratio (HVSR) method
The horizontal-to-vertical (H/V) spectral ratio of microtremors is calculated using three-component
microtremor data at a single station. This method was first developed by Nakamura (1989), which
described that the spectral ratios of the horizontal to vertical component corresponds to the site
amplification characteristics. There are discussions regarding the physical interpretation of the H/Vratios, and recent works show that the H/V represents the predominant frequency of the subsurface soil
(Pierre-Yves et al., 2008). The H/V spectral ratio can be defined as(2)𝑓where 𝑃 indicates the power spectrum.

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3. DATAThe Thimphu city expands in the N-S direction along the valley and is divided by Wang River that runs
from north to south. The populated area is entirely located within the valley deeply carved by Wang
River, with the settlements built on the valley slopes.
The microtremor array measurements werecarried out in the northern region of
Thimphu at 25 sites (Figure 1). For mostmeasurements, a triangular array with four
sensors was applied. We measured
microtremors using arrays with radii of 5 m
and 10 m. At sites where space was restricted
for the 10 m array measurement, linear
arrays consisting of two sensors were
deployed. The regional geological map of
Bhutan categorizes all arrays to be on theLower metasedimentary unit of the Greater
Himalayan
Zone(NeoproterozoicCambrian) (Long et al., 2011), which mainly
consists of metasedimentary rocks such as
quartzite, biotite-muscovite-garnet schist,and paragneiss (Long and McQuarrie, 2010).
A detailed geological map of the area is not
Figure 1. Location of microtremor arrayavailable at present.
sites in Thimphu.
South Ibaraki Prefecture, Japan, is
situated in the northeastern corner of the Kanto
Basin, the largest sedimentary basin in Japan. The
maximum
depthofsediments
reaches
approximately several hundred meters to 1~ km in
the region. Mt. Tsukuba lies to the north of the basin,
whose geology is mainly comprised of mainly
gabbro and granite rocks. The sedimentary soil
thickness gradually decreases from south to North,and recorded ground motions show areal variationsof seismic wave amplification. We took
microtremor measurements at six different sites inSouth Ibaraki (Figure 2). The six sites were
distributed on different geomorphological units.
Except for site MT (Mt. Tsukuba), which was
Figure 2. Location of microtremor array
located on a hillside at an elevation of 170 m abovesites in South Ibaraki.
sea level, the other five sites, IS, KZ, KA, SH, andTH were located on a low-lying sedimentary basin. Strong-motion observation stations operated by
National Research Institute for Earth Science and Disaster Resilience (NIED) (KiK-net: sites KA, MT,
KZ; K-net: sites SH, IS) or Japan Meteorological Agency (JMA) (site TH) were installed at all sites.
PS-logging data are available at four sites; IS, KA, MT, and SH. The calculated Vs30 values and thePS-logging data for these sites will be used to corroborate the results of observed values obtained fromthe array measurements and validate the microtremor survey technique applied in this study. The
measurements were taken using four array radii: 5 m, 10 m, 15 m, and 20 m.

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4. RESULTS AND DISCUSSION
4.1. South Ibaraki
In this study, data processing of the SPAC method is carried out using software SPAC2021 developedby Dr. Toshiaki Yokoi (2021), and horizontal-to-vertical spectral ratio analysis was carried out using
software Prochv developed by Dr. Takumi Hayashida (2021).
The observed phase velocities at South Ibaraki sites were compared with the theoreticalones computed using available PS-logging information (see Figure 3). The PS-logging values wereavailable down to 20 m depth at sites IS and SH, 400 m at site KA, and down to 92 m at site MT. The
comparisons between the observed and the theoretical phase velocities dispersion curves are similar atsites IS, KA, and SH in the higher frequency side, representing the phase velocities in shallow depthsoils (Figure 3). In the lower frequency side, the theoretical curves show discrepancies; the observed
phase velocities are much higher than the theoretical ones, possibly due to a lack of deeper structural
information in the logging data. At site MT the dispersion curve shows similarity in the lower frequency
side but discrepancies in the higher frequency side. This could be due to the soft velocity layers of the
PS logging (~100 m/s) for the top 2 m layer. All sites show rough correlations between the observed
and theoretical phase velocities.

Figure 3. Comparison of the observed phase velocities obtained from SPAC analysis and the theoreticalphase velocities obtained from PS-logging data.
The 1-D Vs profiles obtained at the five sites in the basin (IS, KA, KZ, SH, and TH)
represent the dispersive characteristics of the Rayleigh-wave phase velocities. The Vs profiles obtained
from the microtremors were compared to the theoretical ones derived from PS-logging data. Both data
show good correlations. These results suggest that the inversion process was efficient, and the obtained
results were reliable.
Table 1. Vs30 values obtained from observed data and PS
logging.
Site ID Vs
Phase
profiles velocities
IS
285
293

SPAC
coefficients299

PS logging

KA

304

317

301

No logging

KZ

274

274

209

301

SH

671

667

702

692

MT

222

226

221

206

TH

305

370

329

No logging

246

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The Vs30 values were
calculated at all sites using various
methods such as CEN (2004), Vs30
valuesfrom
SPAC
coefficients(Hayashida and Yokoi, 2021), andLamda – 40 m method (Konno andKataoka, 2000). The Vs30 values
obtained from the observed data werealso compared to the theoretical values
obtained from PS-logging data (Table 1).
All the Vs30 values show goodcorrelations with those of the estimatedones from Vs profiles. According to theNational Earthquake Hazards Reduction Program (NEHRP) site classification, MT falls under class C(soft rock) while the rest of the sites fall under class D (stiff soil).
The predominant frequencies of H/V and site amplifications were also calculated. The
theoretical site amplifications computed from the Vs profiles at three sites were compared to theamplification models provided by the National Institute for Land and Infrastructure Management(NILIM). For all the sites, the predominant frequencies of the theoretical site amplifications are roughly
similar to the frequency of the dominant peaks obtained around 1 Hz in the NILIM models. Theoretical
H/V obtained from PS logging and Vs profiles were compared to the observed H/V. All values showed
a good correlation for all sites and have almost flat characteristics except for site MT. However, thepredominant frequencies of site amplification did not match those of H/V.
4.2. Thimphu
The phase velocities obtained for most sites in Thimphu are lower than South Ibaraki. The frequency
ranges and the phase velocities of the dispersion curves varied among sites in Thimphu. Phase velocities
below 5 Hz are observed only at three sites. In the lower frequency side, the phase velocities have highvariations among the sites.
Vs models in Thimphu are obtained at 22 sites out of 25 sites. The average maximum
depth obtained for all sites is about 24 m. The depth reached 30 m at only eight sites.
The Vs30 values were also computed in Thimphu for 22 sites using the same methods inSouth Ibaraki. Following the National Earthquakes Hazards Reduction Program (NEHRP) classification,
six sites in Thimphu are categorized under class D (stiff soil) and 16 sites are classified under class C(dense soil/soft rock).
The theoretical H/V ratios for most sites in Thimphu have flat characteristics. At site
TSP016, the theoretical ratios have a dominant peak at 6.6 Hz, whereas the observed H/V ratio has adominant peak at 5.5 Hz. Site TSP022 has a clear dominant peak for the theoretical ratios at 8.5 Hz, but
in contrast the observed ratios have a peak at 1 Hz. The predominant frequencies of site amplification
had considerable variation (2-12 Hz) among the sites in Thimphu. This might suggest large range
heterogeneity of the shallow subsurface layers in the area. Conducting microtremor measurements withlarger array might result in deeper Vs profiles. However, due to constraints in the availability of theopen surface in the area, the use of larger arrays is not possible. At most of the sites, the predominantpeaks have higher values than those observed at sites in South Ibaraki. Similar to the results in South
Ibaraki, the dominant frequencies of site amplification did not match those of H/V at sites in Thimphu
either.
4.3. Limitations
The power spectral densities, which indicate microtremor amplitude levels, hold the information of theavailable frequency range for the analysis. The power spectral densities for the sites in Thimphu werecompared to the power spectral densities of the sites in South Ibaraki. The population of Thimphu city
is almost 1/10 of South Ibaraki. South Ibaraki is a neighboring region of the capital city, Tokyo, andtraffics can be the primary source for the generation of ambient vibrations. Unlike Japan, Bhutan does
not use railway transport; trains are also a major source of tremors that are detectable even up to 100 km
away (e.g., Brenguier et al., 2019). Therefore, the source of ambient vibrations is much weak in Thimphu.
The lower limit of the phase velocity dispersion curves was set at frequencies for which
the SPAC coefficient curves reached their peaks at all sites. This limited the range of phase velocities inthe dispersion curves at around 5 Hz. As a result, information about the deep layers could not be obtainedat most sites. Using larger arrays, the information of the deeper parts may be obtained. However, as theavailability of space is restricted in the current study area, larger arrays could not be observed in thisstudy.

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5. CONCLUSIONS
Microtremor measurements were conducted at 25 sites in Thimphu city and six sites in South Ibaraki.
The observed phase velocity dispersion curves, Vs profiles, Vs30, H/V, and site amplifications obtainedfor South Ibaraki are well compared to theoretical ones. This implies that the results obtained in Thimphu
using the same methods are acceptable. The effects of different geology and geomorphology on
microtremors are also clearly shown by the results obtained. For sites located on the sedimentary basin,
reliable phase velocities are possibly obtained in the lower frequency side (~1 Hz) due to deeper
sediments. For site MT and the sites in Thimphu, reliable phase velocities were obtained only in thehigher frequency range (> 4 Hz), and the obtained Vs profiles were shallow in depth. This implies thatthe predominant frequencies for Thimphu sites could not be well justified, and a more detailed study ofthe geology and geomorphology of the area might help corroborate the results obtained. The observed
phase velocities in Thimphu ranged from ~180 to 930 m/s in the frequency range of 4 Hz to 30 Hz. In
South Ibaraki, a wide range of phase velocities were obtained from 130 to ~2500 m/s in the same
frequency range. The observed values in South Ibaraki vary widely owing to the variability in thegeomorphology of the sites. According to the NEHRP classification, six sites in Thimphu were locatedon stiff soil (class D), and 16 sites were located on soft rock (class C).
The dominant frequencies of site amplification could also be well explained. The
dominant frequencies of site amplification at sites located on the basin in South Ibaraki have dominantpeaks below 2 Hz. Site MT had the dominant peak at ~8 Hz. At sites in Thimphu, the dominant peaks
were observed within a wide range frequency (2–12 Hz), and hence the correlation of the dominantfrequencies among the sites in Thimphu could not be determined. The shallow depth of the Vs profiles
obtained in this study might not properly constrain the characteristics of Thimphu. The observed andtheoretical H/V ratios of most sites in Thimphu had almost flat features. They do not match thepredominant frequency of site amplification.
ACKNOWLEDGEMENTS
This research was conducted as the individual study of the training course “Seismology, Earthquake
Engineering and Tsunami Disaster Mitigation” by IISEE/BRI, JICA, and GRIPS. My sincere thanks
goes to my colleagues at the Department of Geology and Mines for their support, especially to mysupervisor Dr. Dowchu Drukpa and colleagues Mr. Phuntsho Pelgay, Mr. Karma Namgay, Mr. Sonam
Tshering, and Mr. Lungten Chedup, without whom the microtremor measurements in Thimphu would
not be possible. I would also like to express my thankfulness to the local governments of Tsukuba City,
Tsuchiura City, Kasumigaura City, Ishioka City, and Shimotsuma City for their cooperation andunderstanding for our survey and Dr. Yokoi and Ms. Monica Olivar for their help during the microtremor
survey in South Ibaraki. I also want to thank all the professors and staff at IISEE, all the lecturers, GRIPS,
and the staff at JICA for their support during the individual study period.
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Aki, K., 1965.
Brenguier, F., Boué, P., Ben-Zion, Y., Vernon, F., Johnson, C. W., Mordret, A., Coutant, O., Share, PE., Beaucé, E., Hollis, D., Lecocq, T., 2019
CEN, 2004.
Diehl, T., Singer, J., Hetényi, G., Grujic, D., Clinton, J., Giardini, D., & Kissling, E., 2017.
Hayashida, T. & Yokoi, T., 2021.
Konno, K. & Kataoka, S., 2000.
Nakamura, Y., 1989.
Okada, H., 2003.
Sanchez-Sesma, F. J., Rodríguez, M., Iturrarán-Viveros, U., Luzón, F., Campillo, M., Margerin, L.,
García-Jerez, A., Suarez, M., Santoyo, M. A., Rodríguez-Castellanos, A., 2011.
Stevens, Victoria & Risi, Raffaele & Le Roux-Mallouf, R.., Drukpa, D., & Hetényi, G., 2020.

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