Earthquake detection probabilities in Japan

We present a complete history of earthquake detection probabilities and detection completeness levels for the Japanese seismic network from 1923 to, and including, 2014 in high resolution over space and time. In our study, we employ the probability-based magnitude of completeness method developed by Schorlemmer and Woessner (2008). Using this method, for each station we derive detection capabilities over time from empirical data only, namely earthquake information, phase data, station information, and network-specific attenuation relations. From the entire phase-data history, we estimate the operational times of each station and then synthesize detection-probability maps for specific magnitudes or completeness maps. We compute these maps for every month in the period 1923-1987 and for approximately every 10 days in the period 1988-2014. We document the improvement of the Japanese seismic network from its early stage in 1923, when the network reaches completeness levels for shallow seismicity of magnitude 4 only in the wider Tokyo area, to the recent years exhibiting completeness levels of approximately magnitude 1 in all of Japan. We provide detailed completeness estimates for the eight major periods of the Japanese seismic network, defined by the types of seismometers used and the location technique employed, and show the development of completeness levels for selected regions. Finally, we discuss the effects of the 2011 Tohoku-Oki earthquake on completeness levels in Japan.