10th Asian Regional Conference of IAEG (2015)

Characteristics of seismic motions and pore pressure response in the filled slopes
Issei DOI(1) and Toshitaka KAMAI(1)(1) Disaster Prevention Research Institute, Kyoto University, Japan
E-mail: doi.issei.5e@kyoto-u.ac.jp
Abstract
Landslides in the filled slopes due to strong seismic motion frequently caused
damages to the constructions in Japan (e.g. Kamai, 2011a). Seismic observations are urgent
in order to understand the generation mechanism of large shaking. Seismic observations areconducted in the filled slope in Tokyo, Japan (Kamai, 2011b). A three-component
seismometer and a barometer are settled on the ground in the filled slope, while porepressure is measured in the borehole at the depth of 4.8 m (ground water level is at ~ 2 mdepth). Another seismometer is settled on the sediment ground, 1 km apart from the filledslope site, for comparison. The seismograms are continuously recorded with the samplinginterval 5 ms, while barometric and pore pressures are recorded every 10 ms. We analyzed
waveform records of 24 earthquakes whose magnitudes and epicentral distances ranged
3.5-6.5 and 20-300 km, respectively. The peak ground acceleration (PGA) was ranging
3-170 gal, which was a few times larger in the filled slope than that on the ground for all theanalyzed earthquakes. Vertical component of S-waves was so much amplified that it almost
equaled to those in horizontal components. From spectral analysis, this is found to be due tothe amplification of the waveform components of 5-8 Hz and around 20 Hz. Transientchange in pore pressure was also observed according to the arrival of the seismic waves.
The level of pore pressure increased when large acceleration due to S-wave’s arrival wasobserved, which indicated that large shear waves made the soil plastic. Then, the level ofpore pressure gradually decreased but never returned to the original level. It is indicated thatthe soil condition may be changed into another state due to the strong seismic motion.

Keywords: filled slope, strong motion, pore pressure

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in the valley which was made by dissecting the loam
plateau. We presented here characteristics of strong
motions and pore pressure behaviors in the filledslopes through such observations.

Introduction

Japan experiences lots of huge earthquakes.
Landslides in the filled slopes due to strong seismic
motions were often reported. For example,Midorigaoka district, Sendai city suffered from hugedamages on houses and residential areas due to 2011
off Pacific Tohoku earthquake (Kamai, 2011a; 2014).
It is reported that Nankai earthquake or Kanto
earthquake is going to take place in the near futurewith high possibility (The headquarters forearthquake research promotion, 2013; 2014). It isurgent to elucidate the generation mechanism oflandslides in the filled slopes.
In order to estimate the stability of the slope
during earthquake shaking, it is important to know
the role of the ground water as well as thecharacteristics of the seismic strong motions. Kamai(2011b) conducted multi observations of groundmotions and pore pressure in the filled slope inMeguro ward, Tokyo prefecture. This slope was filled2.

Data

We have observation sites in the Fusuma-choPark, which is located in the filled slope in Meguroward,Tokyoprefecture.
Highsensitivity
seismometers (KVS300) with the natural frequency 2Hz as well as a barometer were installed on theground. Pore pressure is also measured at depth of 4.8m, which almost corresponds to the bottom of thefilled slope. The ground water level is 2-3 m in depth,
which means that approximately half of the filledslope is saturated. The drilling log in the Fusuma-cho
Park is shown in Figure 1. For comparison, we haveanother seismometer on a sediment ground at
Ryugenji Temple, which is located about 1 km awayfrom Fusuma-cho Park. Ryugenji Temple is located1

10th Asian Regional Conference of IAEG (2015)
was 4.8. The spectrums of 5 seconds records forP-parts, S-parts and coda parts (after the times twice
larger than S-time) are displayed in Figure 4.
There are several remarkable features in thewaveforms on the filled slope. First, we observed
amplifications on the filled slope in every component
as well as in every type of seismic waves. Second, theamplification was so large in the updown component
for S-waves that its amplitude almost equaled tothose in horizontal ones. This is considered to bemainly due to the amplifications for 5-8 Hz andaround 20 Hz from spectral analysis. Lastly, the levelof the pore pressure increased by 0.02-0.03 kPa,
when the large S-waves arrived.

Figure 1: Vertical geological profile in the filledslope at Fusuma-cho Park.

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on the loam plateau.
Seismic motions were recorded every 5 ms whilebarometer and pore pressure were every 10 ms. The
resolutions were as high as 24 bit. The data wereacquired from October, 2013.
The locations of observation sites and epicentersused in this analysis were shown on the map in Figure2. The depth and magnitudes were ranging from 10
km to 400 km, and from 3.5 to 6.5, respectively.
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Amplification of the updown component forS-waves

In Figure 5, we arranged the seismic waveforms
and pore pressure records at Fusuma-cho Park forsome of the events shown in Figure 2. At a glance,
we found, for all the events, that the amplitudes in theupdown component equaled to those in the horizontal,
as was the case with the event on Nov. 29, 2013
(Section 3). In Figure 6, the spectral ratios of updown
components for S-waves on the filled slope to thoseat the sediment ground were shown for all the events
analyzed. All the spectral ratios have peaks
remarkably at 5-8 Hz and slightly at around 20 Hz.
This indicated that such amplification take place
regardless of the back-azimuths or magnitudes of theevents. Therefore, shallow portion of the filled slope
can be related to such amplification.
The ground water level was estimated to be at
depth of 2-3 m. Boring data show that alluvium has aSeismic waveform and pore pressure records
for the event on Nov. 29, 2013.

In Figure 3, we show seismic waveform records
at Fusuma-cho Park and Ryugenji Temple, together
with the pore pressure records at Fusuma-cho Park,
accompanied with the event located 42 km eastward
at depth of 69 km on Nov. 29, 2013. The magnitude

Figure 2: Map showing the observation sites (a triangle) and epicenters used in this analysis (stars). The seismic
event on Nov. 29 is shown by a white circle.

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10th Asian Regional Conference of IAEG (2015)

Figure 3: Seismic waveform and pore pressure records for the event on Nov. 29, 2013.

Figure 4: Spectrum of the seismic waveform records onthe filled slope (red) and on the sediment ground(blue) for the event on Nov. 29, 2013.

Figure 5: Seismic waveform and pore pressure records onthe filled slope for several events whose epicenters
are shown in Figure 2. The maximum acceleration
was less than 10 gal in the second and third eventsfrom the bottom.

thickness of 4-5 m beneath the fills of 4-5 m
thickness. Taking the 1/4 wavelength law into
account, the two peaks in the spectrum may derivefrom the converted waves from S to P on the top of
the ground water and at the bottom of alluvium.
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According to Figure 5, the long-term increase inpore pressure accompanied with S-wave arrival wasobserved when the maximum acceleration was over
~10 gal. Kamai (2011b) already found thisphenomenon and interpreted that the soil became
plastic to be partly collapsed. We did not observe
such a trend in pore pressure for P-wave arrival,

Long-term pore pressure response to strong
ground motions

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ratio10th Asian Regional Conference of IAEG (2015)

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Freq (Hz)Figure 7: Relationship between barometric pressure andpore pressure during one hour in which the event on
Nov. 29 was included. The color shows the time
periods: before shaking (blue), during shaking(black), and after shaking (red).

Figure 6: Spectral ratios of updown components for
S-waves on the filled slope to those at the sedimentground were shown for all the events analyzed.

though the maximum acceleration for event 1 exceeds
40 gal. This can be interpreted that the partial
collapse of soil took place in accompany with shear
waves and not with compressional waves.
6. Relationship between pore pressure and airpressureAcknowledgements
The authors are grateful to the Japan
Meteorological Agency for consulting the earthquake
catalog. This work was supported by JSPS
KAKENHI Grant Number 26282110. We used
General Mapping Tool (Wessel and Smith, 1991) for
drawing figures.

In general, air pressure increase lowers the groundwater level, resulting in the decrease of values of porepressure. In Figure 7, we show the relationshipbetween air pressure and pore pressure during one
hour in which the event on Nov. 29 was included.
We can see a linear trend of the relationshipbetween air pressure and pore pressure, however thetrend is different before (blue) and after (red) themainshock shaking. This also implies that the soil
partially collapsed to resultantly change the ratio withwhich the soil and the ground water support thesediments above.
7.

References
The headquarters for earthquake research promotion.
(2013): Long-term evaluation of seismic activity
along Nankai Trough (2nd version).
The headquarters for earthquake research promotion.
(2014): Long-term evaluation of seismic activity
along Sagami Trough (2nd version).
Kamai, T. (2011a): Landslides in residential slope
induced by the 2011 off the Pacific coast of Tohoku
Earthquake, J. Jpn. Soc. Nat. Disas. Sci., 3-, 2,193-197.
Kamai, T. (2011b): Seismic response of valley fills tostrong earthquakes on residential region in Tokyo,
48, 6, J. Landslide Soc. Jpn., 48, 6, 30-39.
Kamai, T. (2014): Landslides in urban residential
slopes induced by strong earthquakes in Japan,
Episodes: J. Inter. Geoscience, 37, 4, 295-302.
Wessel, P., & Smith, W.H.F. (1991). Free software
helps map and display data, EOS Trans., 72, 441.

Conclusions

Seismic waveforms and pore pressure records
were examined to elucidate the characteristics of theshaking and the ground water response in the filledslope. The analysis revealed that updown component
was dominant in the S-part, equivalent to thehorizontal ones, probably due to the existence of
alluvium and saturated layers in the filled slope. Thisis crucially important to estimate the slope stability
during shaking, though only horizontal accelerations
are considered in the stability analysis currently used.
Moreover, it was found that shear waves with theacceleration larger than 10 gal made the soil plastic.
In the next step, we should figure out that thealluvium and the ground water layers actually
produce large amplification at 5-8 Hz and around 20
Hz through simulation.

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