Journal of Natural Disaster Science, Volume 24, Number 1, 2002, pp1-13

Determination of Subsurface Structure of Tottori Plain Using Microtremors
and Gravity Anomaly
Tatsuya NOGUCHI and Ryohei NISHIDA
Department of Civil Engineering, Tottori University(Received for 3. Apr., 2002)

ABSTRACTMicrotremor observations and a gravity survey were made to determine the subsurface structures of the Tottori
Plain. This area was severely damaged during the 1943 Tottori earthquake (M7.2), damage being concentrated in
the plain. Microtremors were recorded by seismic arrays (8 sites) and 3-component single-site observation (417points). Gravity data were obtained at about 400 points. The microtremor data were analyzed by the spatial auto
correlation (SPAC) method and the horizontal-to-vertical spectral ratio (H/V). The subsurface structures were
determined by 1) S-wave velocity structure models obtained at the array observation sites, 2) a 3D bedrock configuration based on H/V and the residual gravity anomaly, 3) 2D density structures for three cross sections
obtained using the gravity anomaly based on a depths of S-wave velocity structures. A shallow bedrock area
extends over a belt-like zone along the coast. This bedrock suddenly deepens from the eastern mountainous area
to the plain. The depth to the Vs=3500m/s layer ranges from a 300m minimum to an 800m maximum.

1. INTRODUCTION
The Tottori plain in the Sanin area of Japan, the target area of
this study, was severely damaged by the 1943 Tottori earthquake(M7.2). The severely damaged area was concentrated on the
Tottori plain (Architectural Institute of Japan, 1944), whereas there
was less damage in the western and southern parts of Tottori City.
This concentration of damage is thought to have been caused by
the characteristics of the earthquake ground motion affected by the
local subsurface structure of the Tottori plain.
The subsurface cross section (Tottori Subsurface Institutes,1991) for which borehole data on the sedimentary layer had beencollected was investigated. A contour map (Akagi et al., 1993) of
the bedrock of the diluvium and alluvium had already beenobtained. In addition, PS logging (exploration of the sedimentarylayer) and elastic wave exploration of the mountainous area (e.g.,
Oyo corporation, 1996) has been done. At several points in therock layer structure beneath the plain (stratum of Neogene system
or Paleogene system), gravity anomal was found by a gravity survey. No detailed density structure however has been determined
and PS logging and elastic wave exploration have been done only
on limited points.
There are two ways to determine subsurface structures. One
is to visually explore the geologic profile by means of bore-holedata The other is to make a geophysical exploration to obtain physical parameters such as elastic wave velocity and investigating
density. Effective methods for obtaining elastic wave velocity and
2D or 3D bedrock structures are microtremor exploration and agravity survey. The use of microtremors is divisible into three
types: array observation using several seismometers, 3-component
observation at a single site (single-site observation), and 3-compo-

nent observation at soil and rock sites. Array observation records
are analyzed by the spatial auto correlation (SPAC) or F-K
method. The S-wave velocity structure can be determined fromarray records by these methods. Single-site observation records
can be analyzed by means of the horizontal-to-vertical spectral
ratio (H/V). Soil and rock site observation records can be analyzed
by means of the horizontal- (soil site) - to - horizontal (rock site)
spectral ratio.
Array observations have been made at many sites in order todetermine the S-wave structure to a depth of 2 or 3km. (e.g.,
Okada et al., 1990; Yamanaka et al., 1995; Miyakoshi et al., 1994;
Matsushima et al., 1990). The predominant period of the surface
layers and depth to the bedrock have been determined at several
sites by a H/V (e.g., Ohmachi et al., 1994; Tokimatsu et al., 1997;
Ishida et al., 1996). Gravity exploration can determine a density
structure by use of Bouguer gravity anomaly. The 3D configuration of bedrock from several meters to several kilometers and adensity structure model have been obtained by this method (e.g.,
Kikuchi, 1982; Komazawa, 1984; Makino, 1997). In those explorations, the subsurface structures were determined on the basis ofdata being in the same period band as that of the estimation ofstrong motion by means of the elastic wave theory. Microtremor
exploration and gravity observations therefore are applicable fordetermining the subsurface structure in this the Tottori plain area.
The purpose of our study was to determine the subsurface
structures of the Tottori plain by the use of microtremors and gravity observation records.
2.

GEOLOGICAL FEATURES

The topography (Akagi et al.,1993) of the Tottori plain is that

KEY WORDS: subsurface structure, Tottori plain, microtremors, Bouguer gravity anomaly

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T. NOGUCHI AND R. NISHIDA

of sand dunes expanding along the coast, a delta area that occupies
the main part of the plain, an alluvial fan in the southern part of theplain , and a valley plain. In the north, there is an extensive coastal
sand dune, and Mt. Kyushou is located southeast of Tottori City.
A geological map is shown in Figure 1. The Cretaceous Cenozoic stratigraphy is shown in Table 1. The surface rocks arecomprised of the Tertiary system, and the Mesozoic volcanic granite rocks (Akagi et al., 1993). The Mesozoic volcanic are distributed in the southern of Mt. Kyushou on the southwest edge of theplain. The granite rocks occur along the south edge of Koyama
Pond, in the main part of Mt. Kyushou and in the mountainous
area that stretches northwest from Mt. Kyushou. The Neogene
system stratum is composed pyroclastic and sedimentary rocks.
Pyroclastic rocks that consist of tuff and basalt are distributed
along the southern edge of Koyama Pond and in the southwest of
the plain. Conglomerate and sandstone are widely distributed in
the southern part of the plain and mudstone occurs in the southeast.

3. OBSERVATION
3.1 Microtremor observations
Array observations at 8 sites and 3-component single-site
observations at 410 points were made to observe microtremors in
the Tottori plain. Locations of the observation points are shown in
Figure 2 and the observation system in Figure 3. Observations
were made at 417 points at night from October 1998 to October
2001. The array observation system consisted of four vertical seismometers, a direct current amplifier, and a data-recorder. Here,the array with a 3 - 70m radius is called the S-array and that with a
200 - 500m radius the L-array. Natural periods (T0) of the seismometers were 1s for the S-arrays and 8s for the L-arrays. An
overview of array observations is given in Table 2. A 3-compo-

Table 2.

Overview of the observation system and setting. A:
Simultaneous observation system using a cable, seismometer of T0=1s, for the S-array. B: System using a
GPS timing receiver, seismometer of T0=8s, for the Larray.

Table 1. Quaternary stratigraphy of the Tottori plain(Akagi et al., 1993).

Fig. 1

Geological map of the Tottori plain (Akagi et al., 1993). ●: PS logging sites. Line segments represent thesubsurface sections in Figure 5.

DETERMINATION OF SUBSURFACE STRUCTURE OF TOTTORI PLAIN USING MICROTREMORS AND GRAVITY ANOMALY

Fig. 2

Locations of microtremor observation sites. Dots: single-site observation sites, and stars: array observation sites.

Fig. 3Array observation system (1); S-array system ①, L-array system ②.
Single-site observation system (2).

nent seismometer (T0=5s) was used in the single site observation
system.
3.2 Gravity observations
This information was used in the gravity analysis. ① 417
points at 500 m interval (Nishida et al., 2001), ② 197 points at 200- 300m intervals in central Tottori City (Nakagawa et. al., 1993),③ 192 points at 50 - 200m intervals in the Yoshioka hot springarea (Tottori Prefecture, 1999), ④ 43 points in the mountainous
area surrounding the plain (Komazawa et al., 2000), ⑤ 296 points
around the Yoshioka - Shikano fault area and its peripheral mountains. A Racoste Lon Berg's G type gravimeter was used. The precision of the elevation measurements was kept within 1m.

3

4. ANALYSIS
4.1 Analysis of microtremor data
Portions of 40.96s or 81.92s without artificial noise wereselected from the array observation records. The dispersion curveof the phase velocity was calculated by the SPAC method (Aki,1957). Spectra were smoothed by use of a Parzen window with a
0.3Hz band for the S-array and a 0.1Hz one for the L-array.
Dispersion curves for the phase velocity are shown in Figure 4. A
dispersion curve was obtained for each array radius (dots) and thetheoretical curve (solid line) was obtained for the final model.
Portions of 20.48s without artificial noise were selected fromthe 3-component microtremor records. H/V was calculated by use

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T. NOGUCHI AND R. NISHIDA

Fig. 4Phase velocity dispersion curves at array observation sites. Dots show thephase velocity, solid lines the theoretical curves obtained from the final
subsurface structure model.

of using the averaged Fourier spectra of the selected data.
Horizontal components were composed. Spectra were smoothedby means of a Parzen window with a 0.3Hz band .
4.2 Analysis of gravity data
Terrain correction was made with a topographic 50m meshdigital data for ①,②,③,⑤ data and a 250m mesh for ④ to obtain
the Bouguer gravity anomaly. Density measurement results forrock samples (NEDO, 1977) and CVUR method (Komazawa,
1995) were used to determine a suitable density, and an automatic
2D analysis method (Komazawa, 1995) to determine a 2D density
structure. This method uses the automatic forwarding methods of
Talwani et al. (1959). The parameters required are density differences between surface and bedrock layers of two homogeneous
layers and a control point for bedrock depth.

5. DETERMINATION OF THE SUBSURFACESTRUCTURE
5.1 Determination of the S-wave velocity structure
The parameters (number of layers, density, P-wave velocity;[Vp] S-wave velocity; [Vs]) used in the subsurface structure models were selected on the basis of the geological features and subsurface cross section (Tottori Subsurface Institute, 1991). Density
was obtained by referring to a previous report (Society of
Exploration Geophysicists of Japan, 1988), and Vp was found by
the formula (Kitsunezaki et al., 1990),
Vp = 1.11Vs + 1290 (m/s)(1)

To obtain the parameters of the sedimentary layers, a subsurface cross section (Figure 5) (Tottori Subsurface Institute, 1991)
around the array observation points and PS logging results for theDETERMINATION OF SUBSURFACE STRUCTURE OF TOTTORI PLAIN USING MICROTREMORS AND GRAVITY ANOMALY

Table 3. PS Logging profiles (Oyo Corporation, 1996).
Location of these points are shown in Figure 1.

locations shown in Figure 1 (Table 3) (Oyo Corporation, 1996)
were used. The velocity of the uppermost layer was decided suchthat it fits the minimum phase velocity of the observation value.
From the geological background, the rock layer is thought tobe divided mainly into granite rock of the Neogene, and Paleogene
systems and Mesozoic Volcanic rocks (Table 1). The Tottori
Green Tuff Research Group (1989) reported that the Neogene system stratum does not have clear boundaries and is an unconformity. Accordingly, the velocity value is considered to increase gradually in the deep part of the layer. We further divide the Neogene
system into three strata based on the degree of consolidation; ①slight consolidation, ② medium consolidation and ③ marked consolidation layers. Moreover to determine the velocity of layers weassumed that the ④ granite of the Paleogene or Mesozoic Volcanic
rock layer is the lowest stratum. In the rock layer, the velocity
contrast is greatest at the boundary between layers ③ and ④. For
Table 4.

Fig. 5

5

Parameters for determination of the S-wave velocity
structural model.

Subsurface cross sections around the array observation. Locations of sections areshown in figure 1 (Tottori Subsurface Institute, 1991).

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T. NOGUCHI AND R. NISHIDA

S-wave velocity values of the layers assumed from the geologicalclassification, ① Vs=700m/s, ② 1000m/s, ③ 1500m/s, ④
3500m/s were determined using experiential values (Miyakoshi,

Table 5.

Final subsurface structure models at the array observation sites.

1994). Parameters of the subsurface structure models are shown in
Table 4.
By restricting Vs and Vp densities to the given value andchanging only the layer thickness, a subsurface model could beestablished by trial and error. The final subsurface structure wasobtained by these processes (Table 5).
Based on the I-I’ section in the north to south direction andthe II-II’ section in the east to west direction , columnar sections of
the S-wave velocity structures at 7 observation sites are shown in
Figure 6. Minimum and maximum depths to the Vs=3500m/s
layer respectively of about 300m and 800m were obtained. In the
I-I’ section, the tendency is to gradually become deeper from GNT
to KAR and suddenly to become shallow at TTA. In the II-II’ section, the trend is to deepen from TTD on the west to JHK.
5.2 Bedrock structure determined by H/V
We tried to determined bedrock depth from the peak period of
the observed microtremors, H/V, at the central array point. To
determine bedrock depth, the bedrock layer that responds to the
H/V peak period must be ascertained. Rayleigh wave are predicted
to prevail in microtremors, and H/V to express the ellipsoidal
trace. The peak period of H/V is assumed to change with the depthof the Vs=500 - 3500m/s layers. Based on the models in Table 5,
five bedrock model patterns were assumed, in which the Vs=500,
700, 1000, 1500 and 3500m/s layers were defined as the respective
lowermost layers (Figure 7). We calculated H/V of the Rayleighwave fundamental mode in each pattern and compared the H/Vs ofthese Rayleigh waves (H/V-R) and the H/Vs of the microtremors
observed at the center of array (H/V-M) (Figure 8). On the basisof this comparison, the relation between the H/V peak period andbedrock depth can be found by investigating the pattern that best
fits the peak periods obtained by calculation and the H/V of the
microtremors.
The features of this comparison fall into two main classifications: ① (KAR, YNG, JHK, NIK, GNT and SHB) and ② (TTA
and TTD).
First, for group ①, clear peaks in the H/V-Rs of four patterns,
in which Vs = 500, 700, 100, and 1500m/s layers are the lowest,
are shown in the period of 0.5 - 1.0s and these value correspond to
the H/V-M peak. The H/V-R of the pattern with the lowermost
layer of Vs=3500m/s has two peaks in all the regions except SHB,
and the short period peak fits the peak of H/V-M. In the SHB,
there is only one H/V-R peak which corresponds to the H/V-M
peak.

Fig. 6

Density measurements of rock samples, collected in the mountainous area of the eastern region (NEDO, 1977).

Fig. 7

Subsurface model assumed when the Rayleigh wave fundamental
mode is calculated (Figure 9).

DETERMINATION OF SUBSURFACE STRUCTURE OF TOTTORI PLAIN USING MICROTREMORS AND GRAVITY ANOMALY

Fig. 87Comparison of the H/V of the Rayleigh wave fundamental mode (H/V-R) and the H/V of microtremors (H/V-M)
at central points of the array sites. We assumed that the lowermost layers are Vs=500, 700, 1000, 1500, 3500m/s
layers for the model in the Table 5. Bold solid lines denote H/V-M.

Next, for group ②, H/V-Rs of the four patterns, in which
Vs=700, 1000, 1500, and 3500m/s layers are the lowermost, have aclear peak in the period of 0.5s - 1.0s in TTA and in the period of
0.7s - 2.0s in TTD. The H/V-R peak periods of these four patterns
differ, those of the Vs=1000m/s and Vs=1500m/s patterns corresponding to the peak period of H/V-M. In this case, the discrepancy of H/V-R and H/V-M peak periods occurs on the short periodside in the Vs=700m/s pattern and on the long period side in the
Vs=3500m/s one. Accordingly, the case in which the peak periodof H/V-R and H/V-M overlap in both groups ① and ② is theVs=1000 or 1500m/s pattern.
Assuming that the peak period of H/V-M indicates the same
bedrock depth in the Tottori plain, the H/V-M peak period corresponds to the bedrock depth to Vs=1000 or 1500m/s layer. Ingroup ① regions, however because the peak period of H/V-R in all
the patterns fits the H/V-M peak period, bedrock depth can not be
determined from this comparison. A much more detailed investigation is required in order to regard H/V-M as an indication of
bedrock depth. Such an investigation is described in Section 6.

Distribution of the peak periods was examined to obtain therelative configuration of the bedrock over the entire plain. A contour map of the peak periods is shown in Figure 9. The 3D
bedrock configuration can be roughly obtained from this figure. A
shallow bedrock area spreads out over a belt-like zone in the
coastal area and bedrock suddenly becomes deep from the eastern
mountainous area, including Mt. Kyushou, to the plain around
JHK. The deepest area is the basin-like zone around KAR, TTD,YNG, and JHK.
5.3

Determination of the bedrock structure by the Bouguer
gravity anomaly
Suitable assumed densities of the surface layers should be
used when making a density estimation with the Bouguer anomaly.
In this study, the average density of the bedrock was assumed inorder to survey the bedrock configuration of the plain. The surface
density of the mountainous area around the plain therefore indicates bedrock. In addition, suitable density for the surface layer ofthe mountainous area around the plain was determined. The

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T. NOGUCHI AND R. NISHIDA

Fig. 9

Contour map of the H/V peak period. The contour interval is 0.1s. Dots shows single-site observation sites, stars
array observation sites.

Fig. 10Density estimations by the CVUR method. Calculated results for (1); the mountainous area of the eastern regionand (2); the mountainous area of the western region. The density when the curve takes minimum is the mostsuitable.

respective calculated density values at two places in east and west
were 2.38 and 2.42g/cm3, which were used for the edge of the plain
in a CVUR method (Figure 10). Density measurement results forthe rock samples from the east area gives an average dry density
for all the samples of 2.59g/cm3 and an average density for the
Neogene system of 2.50g/cm3 (Table 6) (NEDO, 1977). The
Neogene system area is widely distributed throughout the plain(Figure 1). The averaged surface layer density is expected to beless than the value for the rocks because the surface rocks have
weathered due to water penetration. As a result, a suitable density
for the mountainous area is determined to be 2.50〜2.40g/cm3.
The bedrock configuration was determined using a Bouguer anomaly with an assumed density of 2.40g/cm3.
A filter technique that removes the deep structure effect fromthe original Bouguer gravity anomaly must be applied to obtain theeffect of the subsurface structure. According to Nozaki (1997),

DETERMINATION OF SUBSURFACE STRUCTURE OF TOTTORI PLAIN USING MICROTREMORS AND GRAVITY ANOMALY

exploration depth is about 1/10 the typical survey line length in theinvestigation range. Based on Nozaki (1997), exploration depth
should be considered to be about 1 km because the objective area
is about 10km from south to north. In order to determine subsurface structures shallower than 1 km, band-path filtering was usedon the original Bouguer gravity anomaly.
To remove the structure effect of the part deeper than 1km, aband pass filter that combines two upward continuation filters of
50 and 1000m was used. A contour map of the residual Bouguer
gravity anomaly is shown in Figure 11. High and low gravity
anomaly areas are present near the coast and inland. The gravity
anomaly suddenly changes from the mountainous area around Mt.
Kyushou to the plain. The low gravity anomaly area is located on
the southern side of the plain and the north side of Koyama Pond.

Table 6.

Density measurements of rock samples, collected in themountainous area of the eastern region (NEDO, 1977).

Fig. 119

5.4 Quantitave density structure analysis
The density structure was automatically calculated from theobserved gravity anomaly. In addition, depth to the bedrock layer
in the 2-layer model was restricted to the outcrop point and that of
S-wave velocity structural models in the array observation (Table
7). Sections of the 2-dimensional models were set so as not tointersect obliquely with or parallel the anomaly. As gravity anomalies are strongly affected by underlying structures , depth errors in
the derived structural models in the area where the 3-dimensional
structure effect is large are 10〜20% (Komazawa private message).

Table 7.

Conditions of control points for analysis of the 2D density structural models. “Distance” means the distancefrom the left edge in Figure 12. “Depth” means thedepth to the Vs=1500m/s layer.

Contour map of the residual Bouguer anomaly. The contour interval is 0.2mGal, assumed density is 2.4g/cm3.
Band-pass filtering combined two upward-continuation filters, one continued to a height of 50m, the other to
1000m. Triangles show the array observation sites. Lines A-A’ B-B’ and C-C’ mark the location of the 2D density structure model in Figure 12.

10

T. NOGUCHI AND R. NISHIDAFirst, the A-A’ section (Figure 12 (1)) whose both edges arelocated at outcrop points (border between the mountainous area
and a sedimentary layer) was determined. The Society of
Exploration Geophysicists of Japan (1988) has reported the relationship between the elastic wave velocity and densities of soils
and rocks in each area of Japan. Accordly, the soil density that
corresponds to a sedimentary layer of a Vp=1500 - 2000m/s is 1.8- 2.0g/cm3, and the soft rock density that corresponds to a Neogene
system of Vp=2000 - 3000m/s is 1.9 - 2.1g/cm3. We consider thatthe target area should show the same relationship for P-wave
velocity and density. The density of the surface layer, including
the sedimentary layer and Neogene system, therefore was assumed
to be 2.0g/cm3. The density difference between the surface andbedrock layers was assumed to be 0.4g/cm3, and the assumed density of the original Bouguer gravity anomaly without filtering was
set at 2.4g/cm3. The upper parts in Figure 12 (1), (2), (3) show the
Bouguer gravity anomaly value where the measured and calculated
values overlap. The lower parts in Figure 12 (1), (2), (3) give thecalculated density structure. According to Figure 12 (1), maxi-

Fig. 12

mum depth to the bedrock layer is about 350m. The subsurface
configuration slopes mildly from the west edge (left side of the figure) to the deepest part, then changing suddenly from the deepest
part to the outcrop point.
The B-B’ and C-C’ section next were determined. These sections are passing through sites the array observation sites where Swave velocity structure was obtained. In the A-A’ section, NIK is
the near the deepest part . In the S-wave structural model at NIK,
the depth to the Vs=1500m/s layer is the value (358m) nearest thedepth for the deepest part of section A-A’ (about 350m). Thesesections were next determined based on the depth to the Vs=1500m/s layer for the restricted points. Results are shown in Figure 12(2) and (3). The Bouguer anomaly for the calculated values fitsthat for the observed one. Accordingly, the density structure
model is considered to be the proper one. The B-B’ section showsa sudden change from the north edge (left side of the figure) to thedeepest part; a mild slope from the deepest part to the center; and aflat configuration from the center to the south edge. The C-C’ section shows a flat configuration from the west edge (left side of the2D density structure models. The upper part shows the Bouguer anomaly value where the measured and calculated values. Arrows show the locations of the control points in the models. Conditions of the control points of
the A-A’, B-B’, and C-C’ sections are shown in Table 7.

DETERMINATION OF SUBSURFACE STRUCTURE OF TOTTORI PLAIN USING MICROTREMORS AND GRAVITY ANOMALY

Fig. 13

Overlay map of Figure 9 and 11. Open circles: the H/V peak period. The larger thecircle, the longer the period.

Fig. 14

Contour map of the geological boundary between Quaternary and Neogene in the
Tottori Plain (Akagi et al., 1993). The contour interval: 10m.

11

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T. NOGUCHI AND R. NISHIDA

figure) to the center; a mild slope from the center to the deepest
part; and a sudden change from the outcrop points on the subsurface on the east edge to the deepest part.
6.

DISCUSSIONFirst, we reconsider the Rayleigh wave H/V (H/V-R) in TTA
and TTD. As stated previously, the peak periods of the Rayleighwave H/V in the 5 patterns have different values in the ① group(TTA and TTD). In contrast, 4 patterns, except the one having thelowest layer of Vs=3500m/s have almost same values in the other② group (KAR, YNG, JHK, NIK, GNT, and SHB). We investigated the cause of this difference. Table 5 shows there is a difference between the ① and ② groups for the in Vs=100 - 300m/s layers. The ① group has a thick Vs=300m/s layer whereas the ②group has only a Vs=100 - 200m/s layer. Accordingly, the averagevelocity value of the layer above the Vs=500m/s layer in the ①group is large, compared with that in ② group; i.e., the velocity
contrast between Vs-500m/s layer and upper layers is small in the① group as compared with that in the ② group. The H/V peak
can not be distinguished when the velocity contrast is small(Ohmach et al., 1994). Figure 8 shows that in the ① group, the
H/V-R can not be distinguished clearly in the pattern with a lowermost layer of Vs=500m/s, though it is clear in the other patterns.
In ② group, however, the pattern having a lowermost layer of
Vs=500m/s shows a clear peak as do the other patterns. The velocity structure of the sedimentary layer therefore affects the peak
period of the Rayleigh wave H/V.
The peak period distribution of the H/V (Figure 9) was compared with the residual Bouguer gravity anomaly (Figure 11). Two
figures are considered to show the 3D relative bedrock configuration. To better visualize this relationship , Figure 9 overlaid with
Figure 11 is shown as Figure 13. Open circles in that figure show
the H/V peak period. Larger circles represent longer periods. This
figure shows that the enlarging tendency of the open circles corresponds to the decreasing tendency of the residual gravity anomalies from SHB to the seashore and from Mt. Kyushou to the plain.
These tendencies mean the deepening one of bedrock depth. From
SHB to the south side of the plain, the contrary tendency for smaller open circles corresponds to the tendency for a decrease in theresidual gravity anomaly. This means that there is a contrary tendency in the south area of the plain on the basis of the bedrock
depth. The cause of the low gravity anomaly is thought that
bedrock density is not constant, either; bedrock density is low.
The geological map in Figure 1 show that the pyroclastics and conglomerates are distributed on the southern side of the mountainarea, whereas the granitic and rhyolitic rocks are distributed
throughout the area around Koyama Pond and Mt.Kyushou.
Those rock densities are lower than the granitic and rhyolitic rockdensities, therefore, the southern side is considered to be a low
gravity anomaly area because bedrock density is low.
The peak period distribution of H/V (Figure 9) next was compared with findings of a previous study (Akagi et al., 1993). The
contour of the geological boundary between the Neogene andQuaternary is shown in Figure 14 (Akagi et al., 1993). The H/Vpeak period distribution in Figure 9 is the depth distribution from
Vs=1000m/s to Vs=1500m/s layer, as stated in 5.2. Figure 14
shows a contour map determined from bore-hole data from theboundary between the sediment and bedrock layers. It is considered to correspond to the depth distribution to the Vs=700m/s
layer. This comparison between Figures 9 and 14 is in the strict
sense, a comparison of the depth distribution in different layers.
Because as the S-wave velocity structure in Figure 6 indicates thatthe depth change to the Vs=700 m/s layer shows almost the same
trend as that to the Vs=1500m/s layer, we believe there is no problem concerning the discussion of relative bedrock depth. Around
Mt. Kyushou on the eastern side of the Sendai River and on the
coastal area, the sudden change in the configurations of the contour
are almost completely overlapped in each figure. Around TTD and
TTA northeast part of Koyama pond, there is a the deep area in
Figure 9, which it is not shown in Figure 14.
7. CONCLUSION
Microtremor observations and a gravity survey were carriedout, and the subsurface structure of the Tottori plain determined.
The following conclusions were made:(1) The array observations of microtremors at 8 sites gave 6 8 layers of S-wave velocity structure, the Vs=100 - 3500m/s layers. At the 8 sites, bedrock depths to the Vs=3500m/s layer range
from a 300m minimum to an 800m maximum.
(2) Comparison with the calculated H/V-R by used of thestructural model suggested that the H/V-M peak period shows
bedrock depth from the Vs=1000 to 1500m/s layers, but the relationship between H/V and bedrock depth requires a more thoroughinvestigation.
(3) The 3D bedrock configuration could be roughly determined from the counter maps of the H/V peak period and residual
gravity anomaly. The structure where the bedrock suddenly
becomes deep along the coast and in the eastern peripheral area ofthe plain was also determined.
(4) Three bedrock structure cross sections were establishedbased on 2D analysis with the gravity anomaly. In that analysis, adensity difference of 0.4g/cm3 was used, and the bedrock depth tothe Vs=1500m/s layer of the S-wave velocity structure were usedas the control points. Maximum bedrock depth was found about
400m.
ACKNOWLEDGMENTS
We thank Dr. H. Morikawa, (Tokyo Institute of Technology),
Dr. K. Miyakoshi and Dr. B. Zhao, (Geo-Research Institute), and
Dr. J. Akamatsu, (Kyoto University) for their helpful advice andassistance. We also are grateful to Dr. M. Komazawa (National
Institute of Advanced Industrial science and Technology) who provided the gravity data, analysis program, and helpful guidance.
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