Detection of Coseismic Changes of Underground Water Level

Abstract

An underground water level data set, regularly observed over a 10-year period in the Tokai area, one of the most intensively observed areas in Japan, is analyzed. First, the 500,000+ point data set was smoothed using a non-Gaussian state-space model to interpolate missing observations and to correct outlying observations. Then the effects of barometric pressure, earth tide, and precipitation on the water level were estimated using state-space modeling. After subtracting these effects, the coseismic effects on 16 examined earthquakes are clear. The amount of the coseismic effect is explained as a function of magnitude and hypocentral distance of the earthquake. It is also shown that the underground water level had a tendency to increase approximately 6 cm/year. Presumably, this corresponds to the increase of strain in this area due to the subduction of the Philippine Sea tectonic plate under the Eurasian plate.

Key Words:

• Coseismic effect
• Earthquake
• Missing observations
• Outliers
• State-space model
• Underground water level