5:15 PM - 6:45 PM
[SCG47-P03] Changes in groundwater level due to bubble growth caused by earthquakes: Observations at Yachigashira hot spring in Hokkaido from January 2004 to September 2010
Keywords:coseismic groundwater level change, carbon dioxide, bubble growth, predominant frequency of seismic waves, surface wave
Following earthquakes, coseismic groundwater level changes have been observed in many locations, which would be related to the earthquakes. While some coseismic changes are understood to be due to static volumetric strain changes associated with fault slip during earthquakes, there are many cases that cannot be explained by the volumetric strain changes, and various mechanisms have been proposed. One of the proposed mechanisms is coseismic changes associated with gas bubble nucleation and growth. However, few studies have compared observed groundwater level changes with coseismic changes related to bubble growth. Therefore, we focused on bubble nucleation and growth among the proposed coseismic mechanisms, and analyzed groundwater level data recorded at the Yachigashira hot spring No. 5 well in Hakodate, Hokkaido from January 2004 to September 2010. The well has been found to contain carbon dioxide.
We found two different coseismic changes: one in which the water level continued to rise for several days after earthquakes, five times during the period, and one in which the water level changes ended within a few minutes to an hour after earthquakes, three times. First of all, we confirmed that these coseismic changes were not due to permeability changes of the aquifer related to earthquakes. Next, we examined the seismic velocity waveforms recorded by the radial component of the broadband seismometer, and found that seismic waves between 0.10 Hz and 1.0 Hz were dominant for the coseismic changes in several days after earthquakes. Similarly, for the coseismic changes in a few minutes to an hour after earthquakes, seismic waves between 0.010 Hz and 0.10 Hz were dominant.
Because of the dependence of the coseismic changes on specific frequencies of seismic waves and the presence of carbon dioxide associated with aquifers, the relatively long and continuous increase in water level changes over the days following the earthquake is likely to be due to the nucleation and growth of bubbles. We therefore estimated the groundwater pressure from the radial component of the seismic velocity waveform of the earthquakes that caused the changes in groundwater level from the bubble growth model. The amount of groundwater level changes in Yachigashira Onsen Well No.5 was estimated using the amount of bubble growth calculated from the seismic wave velocity of the 2004 Kushiro-oki earthquake, and found that the water level increased by 0.29 m, which was consistent with the observed change. The short duration of the coseismic changes, which varies from a few minutes to an hour after the earthquakes, were difficult to explain by the bubble growth model, and may be due to the seismic wave amplification in the borehole.
We found two different coseismic changes: one in which the water level continued to rise for several days after earthquakes, five times during the period, and one in which the water level changes ended within a few minutes to an hour after earthquakes, three times. First of all, we confirmed that these coseismic changes were not due to permeability changes of the aquifer related to earthquakes. Next, we examined the seismic velocity waveforms recorded by the radial component of the broadband seismometer, and found that seismic waves between 0.10 Hz and 1.0 Hz were dominant for the coseismic changes in several days after earthquakes. Similarly, for the coseismic changes in a few minutes to an hour after earthquakes, seismic waves between 0.010 Hz and 0.10 Hz were dominant.
Because of the dependence of the coseismic changes on specific frequencies of seismic waves and the presence of carbon dioxide associated with aquifers, the relatively long and continuous increase in water level changes over the days following the earthquake is likely to be due to the nucleation and growth of bubbles. We therefore estimated the groundwater pressure from the radial component of the seismic velocity waveform of the earthquakes that caused the changes in groundwater level from the bubble growth model. The amount of groundwater level changes in Yachigashira Onsen Well No.5 was estimated using the amount of bubble growth calculated from the seismic wave velocity of the 2004 Kushiro-oki earthquake, and found that the water level increased by 0.29 m, which was consistent with the observed change. The short duration of the coseismic changes, which varies from a few minutes to an hour after the earthquakes, were difficult to explain by the bubble growth model, and may be due to the seismic wave amplification in the borehole.