THE USE OF LOW COST SEISMIC ANDMICROTREMOR SURVEY TECHNIQUES TO
DETERMINE SHEAR WAVE VELOCITY STRUCTURE
Karleen Black
MEE10503

Supervisor:

Toshiaki YOKOI

ABSTRACTWe seek to identify reliable and inexpensive geotechnical survey methods to determine shear wave
velocity (Vs) structure, for use in site effect studies and building codes of earthquake proned
developing countries such as Jamaica. Two microtremor survey techniques spatial autocorrelation(SPAC) and centerless circular array (CCA) along with seismic survey technique multichannel
analysis of surface waves (MASW) were identified as suitable candidates for testing.
For our test site we selected Yoshino Park, Ibaraki Prefecture, Japan, which sufferedliquefaction damage during “The 2011 Off the Pacific Tohoku Earthquake”. As our study site hadliquefaction damage, the liquefaction potential was estimated using the factor of safety (Fs) method
and the Vs from our velocity structure results. Both the liquefaction potential (Fs: 0.077-0.32) and theprobability of liquefaction occurrence were very high (0.9 to 1.0).
These exploration methods and Fs-method for estimating liquefaction potential would beaffordable and applicable in Jamaica.
Keywords: microtremors, velocity structure, dispersion curve, liquefaction.

1. INTRODUCTION
Jamaica has experienced numerous earthquakes in its 300 year recorded history. This is attributable toits location within the boundary zone of the North American and Caribbean plates. The two mostdamaging tremors which destroyed the existing capitals (Port Royal 1692, Kingston 1907), also cause
damage from liquefaction, mass movement and tsunami. Budgetary constraints over the years haveresulted in insufficient geotechnical data for comprehensive earthquake risk assessment. The purposeof this study is to learn cost effective microtremor survey techniques suitable for densely populated
urban areas in order to determine shear wave velocity structure for the coastal plains such as Kingston.

2. METHODOLOGY
Three potentially viable methods are common midpoint–cross correlation multichannel analysis ofsurface waves (CMPCC-MASW), spatial autocorrelation (SPAC) and centerless circular array (CCA).
2.1. CMPCC-MASW
MASW was proposed by Park et al. (1999) and Xia et al. (1999) using the integration transformationthat directly converts time-distance domain waveform data into an image of phase velocity versusUniversity of the West Indies, Earthquake Unit, Jamaica.
International Institute for Seismology , Building Research Institute.



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frequency. Long receiver arrays are essential to precisely determine phase velocities at low frequencies(Park et al. 1999), but decrease the lateral resolution of the survey. This trade-off relation between highlateral resolution and accuracy of phase velocity was overcome by CMPCC-MASW proposed by
Hayashi and Suzuki (2004). With each
Receivers
1
2
3
4
5
shot, a shot gather cross-correlation is
calculated for all trace pairs. Trace pairs
Spacing 1
with common midpoints (cmps) aregrouped together. Other cross-correlations
Spacing 2
for minor cmps with equal spacing
Spacing 3(CMPCC gathers) are stacked and orderedwith respect to spacing as, only the phase
Spacing 4
difference between 2 traces is stored.
Fast Fourier Transform is applied forconversion to frequency domain, with
Phase velocity curve
integration over spacing with respect toFigure 1. Receivers 1-5 and 2-4 are shot gathers with a common
apparent phase velocity.
midpoint. 1-4 and 2-5 are CMPCC gathers at spacing 3.
(1)
where c(), ω and x are the phase velocity, the frequency and spacing, respectively. The maximum
amplitude at each frequency gives the phase velocity.
2.2. SPAC
The SPAC method of extracting phase velocities from microtremors is based on theory proposed by Aki(1957, 1965). The SPAC coefficient is defined as the azimuthal average of the coherence between thevertical component records of a central sensor with those of each sensor on the array circumference(Figure 4), and may coincide to a known function shown in the fourth member Eq. (2).

(2)
where r and θ are inter-station distance and azimuth between two observation points xA and xB ,respectively;  the azimuth of incidence for incoming plane waves; k the wave number. The second
member denotes the theoretical core of SPAC whereas the third one is the practical way to calculate
SPAC coefficient  (r,) from microtremor records, where J0 is the zero order Bessel function of thefirst kind. E[ ] denotes the ensemble average over the time; CA,B() the cross spectra of the records
obtained at xA and xB. The wavenumber k() is estimated by fitting J0(kr) with the observed SPAC
coefficient at various inter-station distance r for each frequency. The phase velocity c()=k() is
calculated for each frequency.
2.3. CCA
Center-less circular array (CCA) method developed by Cho et al. (2004) uses a spectral representation
which may be considered a general case to SPAC. Similarly the vertical component of microtremor
records are used to determine the phase velocities of Rayleigh waves from sensors located on a circle,
but with none at its centre. The CCA coefficient is defined as follows,2(3)where r and k represent the sensor spacing and the wavenumber, respectively, whereas J0 and J1 denotethe zero and first order Bessel functions of the first kind, respectively. G0 and G1 are the power spectral
densities. The wavenumber k() is estimated by fitting {J0(kr)/J1(kr)}2 with the observed CCA
coefficient at various inter-station distance r for each frequency. The phase velocity c()=k() is
calculated for each frequency.
3. MEASUREMENT
3.1. Experimental Site
Yoshino Park (36.074982N, 140.000737E) is an oxbow lake along the Kokai River, in Joso City, Ibaraki
Prefecture, which is used for recreational fishing (Figure 2). Strong ground shaking during “The 2011
Off the Pacific Coast of Tohoku Earthquake” (magnitude 9.0) caused localized liquefaction within thepark.
Multiple arrays were deployed (Figure 3). Two mutually orthogonal, stationary linear
MASW arrays (MASW1, MASW2) of 24 sensors with inter-sensor spacing of 1 meter; one triangular
SPAC array of sevensensors, with sides of 30
meters; and one hexagonal
CCA array with maximum
inter-station distance of 3.0
meters. The undamaged
side of the lake had one
triangular SPAC array (7sensors) with sides of 50
meters and a large
triangular SPAC array (3sensors’) with sides 90
FigureFigure 2.
2. Location
Location map
map ofof survey
survey site.
site.
meters in length spanningthe lake from undamaged side to the island. For MASW the shot points were moved along the arrayfrom -0.5m to 23.5m at 1.0m intervals. This symmetry allowed the records to be processed in bothforward and reverse order.
3.2. Data Processing and Analysis
All array data were individually processed with aseries FORTRAN programs, following a four-stepprocedure involving. (1) multiplexing and or
resampling and along with digital anti-alias
filtering; (2) Data processing mainly composed ofcross-correlating the records; (3) Determination ofRayleigh wave phase velocity; (4) Inversion todetermine shear wave velocity structure.
Averaged dispersion curves were3

Figure 3. Map of arrays deployed at Yoshino Park.

calculated for MASW (forward and reverse) and SPAC.

4. RESULTS AND DISCUSSION
4.1. Dispersion Curves and Velocity Structure
The dispersion curve for CCA showing much variation in the 6-8Hz frequency range (Figure 4 (a)) anddue unresolved problems with the data no further analysis was done. All three SPAC arrays showed
good agreement, with a strong peak at 3Hz and a weaker peak at 8Hz, MARRAY also has a distinct peak

400

Shear Wave Velocity (m/s)
0

MASW
SPAC
CCA_.86m

300

100

200

300

400

500

0.0

Depth (m)Phase Velocity (m/s)

Vs (Yoshino Park)

Dispersion Curves
Yoshino Park

200

5.0
10.0

100
15.0
0

20.0
0.0

5.0

10.0

15.0

20.0
25.0

Frequency (Hz)

(a)

(b)

400
300
Depth (m)Phase Velocity (m/s)

500

Line1: 2D Shear Wave Velocity Structure

Dispersion Curves: SPAC+MASW(2D)Line 1

200
100
0
0

5

(c)

10

15

20

Frequency (Hz)(d)

Distance (m)Figure 4. (a) Dispersion curves for MASW, SPAC and CCA. (b) Vs structure from combined dispersion
curve (SPAC+MASW). (c) MASW 2D dispersion curve (0-5Hz, 1D SPAC). (d) 2D Vs structure using (c).

at 6Hz. As the dispersion curves for SPAC (0Hz to 5Hz) and MASW (5 Hz to 20 Hz) showed a similar
trend and were in better agreement with each other compared to CCA results, their combined dispersion
curve was used to determine shear wave (Vs) velocity structure (Figure 4 (c)). Dispersion curves for
2D MASW were again combined with 1D SPAC results to show lateral variation of Vs at the site
(Figure 4. (c and d)). Figure 4 (d) shows a well layered subsurface structure.

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5. ESTIMATION OF LIQUEFACTION POTENTIAL USING FS-METHOD AND
EXPLORATION RESULTS
The method of Andrus et al. (2003, 2004) is applied for the estimation of liquefaction potential. The
factor of safety (Fs) is defined by the ratio of Cyclic Resistance Ratio (CRR) to Cyclic Stress Ratio(CSR),.

(4)

Liquefaction is predicted to occur when Fs is less than or equal to 1.0, whereas when Fs isbigger than 1.0, liquefaction is predicted not to occur. The method chosen for estimation ofliquefaction potential is the Factor of Safety method (Fs-method). The shear wave velocity results
obtained in this study along with acceleration data from KIK-Net station IBRH10 (NIED) are used.
The probability of liquefaction PL is required for making risk-based design decision. Juang et al.
(2002) proposed an approximated Fs-PL relation as follows,(5)

Depth (m)Using a model constructed with 1m layers and the calculated boundary layers from V s structure to adepth of 10 meters, Fs and PL were determined for each layer. Figure 5 shows a summary of theseresults. From surface to 10
meters depth, our results show
Evaluation of Liquefaction Potentialconsistently low values for CRR
0
0.2
0.4
0.6
0.8
1
(0.062-0.099), whereas CSR
values are consistently higher
0CSR(0.194-0.380). This results invery low values for Fs ranging
CRR
5
from 0.32 to 0.077. SinceFsliquefaction is predicted toPL10occur at Fs less than 1, theselow Fs values categorizes
Figure 5. Calculated CSR, CRR, Fs and PL values for each
Yoshino Park as a high risk
1m layer to a depth of 10m. PL is always close to 1,liquefaction
site.
The
indicating a high probability for liquefaction occurrence.
probability of liquefaction (PL)is found to be in the range
0.943 (layer 1) to 1.0 (layer 6) which makes Yoshino Park a site where liquefaction is most likely tooccur.

6. CONCLUSIONSPAC and MASW are shown to give good estimates of Rayleigh wave dispersion and show a similardispersion trend across their overlap frequency (circa 5Hz). Thus, their dispersion curves can easily
be combined to find Vs structure, with MASW having the advantage of showing lateral variation.
CCA is also a possible candidate for determining shallow structure, but further study is necessary.
The non-invasiveness and relative low cost for deployment make these survey methods suitable
candidates for geotechnical exploration in developing countries such as Jamaica. Fs-method using Vs,
can be a reliable tool for liquefaction potential estimation provided the velocity structure is sound andthere exists good data on soil properties and lithology to create a reasonable subsurface layer model.

5In the final assessment, these exploration methods would be affordable in Jamaica as thegreatest cost would be the initial capital outlay for acquiring equipment. Further, many of the sensors
needed have already being acquired. As, all equipment can fit into the trunk of a car there is also no
additional transportation cost. Although not all existing borehole data went to basement, there isenough data to create reasonable soil layer models. Thus, Fs-method using Vs to estimateliquefaction potential would be affordable as no further equipment or tests would be required.

AKNOWLEDGEMENT
I wish to thank Haruhiko Suzuki, Hideki Saito and Hiroto Nakagawa of OYO Corporation for theirtime and assistance during field observation.

REFERENCES
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Aki, K., 1965, A note on the use of microseisms in determining the shallow structure of the earth’s
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