NIED Hi-net Automatic Hypocenter Determination

We, NIED, observe ground motion at each NIED Hi-net station and send the data to the NIED DMC continuously 24 hours a day, 365 days a year. We automatically analyze these ground motion data and detect occurrences of earthquakes in and around the Japanese Islands. When the Hi-net system detects an earthquake, it tries to estimate the hypocenter location and earthquake magnitude. The seismometers installed at each Hi-net station are highly sensitive, so they can record weak signals caused by construction work and traffic. Of course, these signals are "noise" for micro-earthquake observation. Since computers sometime mistake this noise for signals from earthquakes in our automatic analysis, estimated hypocenter information may contain large errors. Moreover, ground shaking caused by manmade explosions is also observed and is analyzed as a natural earthquake. In order to avoid showing hypocenter information with large errors, we automatically check the accuracy of the hypocenter estimation procedure using various criteria. On our website, we release hypocenter information that has passed all of these criteria. Because our preliminary hypocenter information is estimated by a completely automatic procedure, you should also check information provided by the Japan Meteorological Agency (JMA). Because of system maintenance and/or other reasons, we sometimes stop updating our data without any prior announcement.