Proceedings of International Forum for Green Asia 2019
5th International Exchange and Innovation Conference on Engineering & Sciences (IEICES 2019)

GA17 / P29
The impact of seasonal rainfall toward crustal pore pressure: Insight from seismic
velocity change monitoring
1

Rezkia Dewi Andajani1, Takeshi Tsuji1,2, Fernando Lawrens Hutapea1
Department of Earth Resources Engineering, Faculty of Engineering, Kyushu University
2
International Institute for Carbon-Neutral Energy Research, Kyushu University
Email: rezkiadewiandajani90@ mine.kyushu-u.ac.jp

Abstract: Pore pressure can be seasonally time-variant depending on surface perturbations. Pore pressure conditions
can be evaluated from the changes in seismic velocity. This study focuses on the response of crustal pressure from seismic
velocity monitoring during a rainy season in the Chugoku-Shikoku region, Japan. We computed the cross-correlation
between rainfall and seismic velocity change. We discovered that the correlation between rainfall and seismic velocitychanges is mostly negative. The inverse relationship infers high pore pressure (velocity decrease) condition in the deep
formation, due to groundwater level fluctuation. Such fluctuations are caused by aquifer recharges from rainfall.
Moreover, time-lag information further implies the near-surface layer’s permeability that controls the water percolation
reaching the water table. We concluded that the lithology above and below groundwater contributes to the crustal porepressure variations related to the seasonal rainfall.
Keywords: pore pressure, seismic velocity changes, rainfall precipitation, lithology.
1. INTRODUCTION
Pore pressure is one of the most critical parameterscharacterizing crustal dynamics. Under high porepressure conditions, any fluctuations in pore pressure
generate cracks [1, 2]. Since pore pressure changes affectseismic wave propagation in the earth's crust, several
studies have been applied to estimate seismic velocitychanges [2, 3, 4]. Several studies reported that seismic
velocity changes are seasonally influenced by rainfall
precipitation [5,6]. Such variations could reflect seasonalchanges in pore pressure. Since the fluid flow in porousrock is the primary influence of pore pressure change, we
aim to investigate the relationship between seismic
velocity changes and rainfall infiltration. Here, weinvestigate the relationship between rainfall and seismic
velocity data monitoring changes in the ChugokuShikoku regions. In order to assess the impact of rainfall
on seismic velocity changes more accurately, we applieda band-pass filter and computed the cross-correlation
between the time series data.
For stations with high correlation coefficients, weobserved mostly a negative relationship between seismic
velocity and rainfall changes. At the same time, rainfall
is known to directly recharge an unconfined aquifer. The
negative correlation indicates the high pore pressure in
the formation below groundwater due to overburdenpressure. The result of the time-lag further implies thepermeability of the unsaturated zone in the aquifer. We
conclude that the seasonal variations of pore pressure
related to seasonal rainfall are also influenced by thelithology of near-surface and in the deep formation,below the groundwater.

station and its pairs. To obtain seismic velocity change ateach station, we applied a spatial averaging of the pairstations within a radius <40 km from the main station.
We also averaged rainfall data from all meteorologicalstations within a radius of 40 km of the main seismicstation.
In the Chugoku-Shikoku region, the intensity of snow
cover is minimal; thus, it can be negligible. Surface watermass is known to influence crustal deformation [7]; atsome stations, seismic velocity change exhibit variations
reflecting the seasonal sea-level rise [6]. To minimize theeffects of ocean perturbations, we applied a band-passfilter to all data-set. We used periodic bands when thecorrelation coefficient between rainfall and sea-levelchanges can be suppressed. Using the band-pass filteredtime-series, we computed the cross-correlation between
rainfall and seismic velocity changes. In our case, wedefined seismic velocity change as the reference trace.
We considered only the positive time as we focused on
the indication where seismic velocity is influenced byrainfall precipitation. Finally, we show the correlation
coefficient and the time lag map in the result section (Fig.
1).
3. RESULTS AND DISCUSSION
Fig. 1 shows the correlation coefficient and time lag
results of the cross-correlation analysis. Some stations
display positive and negative correlation coefficients.
The relatively high correlation (> 0.2) between rainfall
and velocity changes was mostly negative (Fig. 1a). In
this study, we only consider stations with negative
correlations where it represents high pore pressureconditions. The time lag information is significant only ifthere is a relatively high correlation coefficient. We
included only stations with an absolute negative
correlation greater than 0.2 (Fig. 1b and 1c).
It has been discussed that the ground surface variations
could be reflected in a deep confined aquifer [8]. We
compared the seismic velocity change and rainfall
precipitation with the closest groundwater level data. We
observed that seismic velocity change decreases not too

2. DATA AND METHODS
We analyzed the seismic velocity change, rainfall, andsea-level changes obtained in the Chugoku-Shikoku
region from 2015 to 2017. We used the seismic velocitychange calculated from the stretching interpolation
method [2]. We used a frequency range of 0.1-09 Hz,
reflecting the depth resolution within the surface - 8 km.
Seismic velocity change was measured between the main
- 45 -A., Takeda, T., Aoki, Y., Seasonal Crustal Seismic
Velocity Changes Throughout Japan. J. Geophys. Res.
Solid Earth 122, 2017, 7987–8002.
[7] van Dam, T., Blewitt, G., Larson, K.M., Lavalee, D.,Milly, P.C.D., Shmakin, A.B., Wahr, J.M., Crustal
displacements due to continental water loading.
Geophys. Res. Lett. 28, 2001, 651–654.
[8] Kamp, G. & Maathuis, H., Annual fluctuations ofgroundwater levels as a result of loading by surface
moisture. J. Hydrol. 127, 1991,137–152.

long after rainfall increases the groundwater level. We
interpret decreasing seismic velocity changes due tooverburden pressure induced by groundwater level
fluctuation. As the overburden pressure increases, thepore pressure in a deep formation will be subjected tovertical stress. If the fluid is trapped inside a low
permeability rock, a negative correlation will occur. The
fluid that is filling the pore space cannot escape(undrained conditions), resulting in high pore pressure
conditions. Conversely, a positive correlation occurs if
the rock has sufficient permeability to allow fluid todiffuse as the rock is subjected to stress.
The time-lag result in Fig. 1c further implies the nearsurface lithology. High permeability rocks will allow
rainfall to reach water-table in a relatively short period.
Time-lag result can also be an indicator of the fractured
consolidated rocks (e.g., igneous and crystalline rock)which also produces secondary permeability that could
allow rainfall percolation from the surface.
4. SUMMARY
Based on the observed changes in the seismic velocity,
the status of pore pressure associated with rainfall can bepredicted. The negative correlation indicates high porepressure conditions in deep formation due to overburdenpressure. The time lag results show that rainfall
penetrates the water surface due to the permeability of thecrust. It can be concluded that the lithology above andbelow groundwater contribute to changes in crustal porepressure during seasonal rainfall.
ACKNOWLEDGMENT
The seismic data (Hi-net) were obtained from theNational Research Institute for Earth Science andDisaster Resilience. The rainfall and sea-level changes
data were obtained from the Japan MeteorologicalAgency. We are fully grateful for the support provided
by the Kyushu University, Advanced Program in Global
Strategy for Green Asia.
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negative correlation coefficient > 0.2. (c) Time-lag result
obtained from panel (b).

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