Presenter: Ryota HINO (Tohoku University)

Description:
Long-term continuous observation of seafloor pressure is effective for detecting seafloor vertical deformations associated with transient tectonic phenomena such as slow slip events. A series of seafloor motion before and after the 2011 Tohoku earthquake were captured by seafloor pressure observations and played significant roles in constraining the distributions of coseismic slip and aseismic slip preceded the mainshock. In this paper, we inspect the quality of seafloor pressure data obtained by Seafloor observation network for earthquakes and tsunamis along the Japan Trench (S-net) in view of seafloor geodesy. The characteristics of continuous seafloor pressure records at all the 150 stations were examined after the tidal components were removed. It turned out that many records contain remarkable slow fluctuations with a time constant of several months or more. These include irregular variations of unknown cause that cannot be interpreted as those due to pressure sensor drift or transient instrument response during the settlement on the seabed after the deployment. In a broad frequency range, evident correlations to temperature variation are identified in most of the pressure records and amplitudes of pressure data are effectively reduced by subtracting a temperature dependent component. Based on the duration of the tremor episodes observed by the S-net, noise level in a period range shorter than a month is important to assess the ability of SSE detection. As a measure of the noise level, we inspected RMS amplitudes of highpass filtered pressure records, after applying the temperature correction. At ~ 50 stations, the RMSs are less than 5 hPa, a guideline for detecting transient tectonic signals. Most of the spatial correlating pressure variations seem to be of oceanographical noise and no possible tectonic signals have been detected so far, although nine of the low noise stations are located in the area of tectonic tremor activities.