68 巻 (2015-2016) 2 号 p. 31-44
The dense seismic array “MeSO-net” (Metropolitan Seismic Observation network), in which 296 accelerometers, at this moment, are installed with several kilometer intervals, was established in 2007 for the purpose of the disaster mitigation for forthcoming large earthquakes. Whether the actual azimuths of MeSO-net seismometers newly installed after 2009 were really in the magnetic north or not has not been verified yet, while the azimuths of three seismometers installed before 2008 were already confirmed to be in the opposite direction. Since such obvious errors in the azimuths badly affect subsequent data processing, we evaluate the azimuths of all seismometers based on cross-correlations with seismograms recorded at nearby Hi-net tiltmeters and F-net broadband seismometers. Our result suggests that the northward components at more than 80% of stations are determined to be within 10 degrees from the magnetic north, while those at the three stations are reconfirmed to rotate more than 90 degrees as the previous study pointed out. Correcting azimuths of all seismograms based on the result, the estimated coherence is clearly improved in the frequency band less than 0.10Hz.