Akio Katsumata

Rapid magnitude determination from peak amplitudes at local stations

The rapid determination of its magnitude soon after a great earthquake is necessary for the issuing of effective tsunami warnings, as demonstrated in the great earthquake off Tohoku district in Japan on March 11, 2011. The earthquake magnitude for the first tsunami warning was underestimated due to magnitude saturation. This paper proposes a method to determine magnitude rapidly from peak velocity and displacement of long-period seismic waves up to 100 seconds at local stations. When waveform data at local stations are available, the magnitude from S-wave peaks is expected to be determined faster than that from only P-wave peaks. It takes about 140 seconds to estimate a magnitude of about 9 for the March 11, 2011, earthquake, which would enable us to issue the first tsunami warning within three minutes after the same type of earthquake.

Fast Hypocenter Determination in an Inhomogeneous Velocity Structure Using a 3D Travel-Time Table

Abstract The computer processing time required for raytracing and hypocenter determination in a 3D inhomogeneous velocity structure is too great for the method to be used to locate a large number of hypocenters or for interactive processing. In this study, hypocenter determinations were conducted using 3D travel-time tables (3D-TTs) to reflect travel times of a 3D inhomogeneous velocity structure in event locations, and the results were compared with those achieved using the 3D raytracing method. The use of 3D-TTs reduced the calculation time by a factor of about 1800 compared with the raytracing method. Whereas minor differences between interpolated travel times and the raytracing method caused some corresponding location differences, the travel-time table was effective for correction of hypocenter locations according to an inhomogeneous velocity-structure model.

Secular and coseismic changes in S-wave velocity detected using ACROSS in the Tokai region

We discovered a secular change in the travel time of direct S-waves over a 10-year observation period by means of continuous operation of an artificial and stable seismic source, called Accurately Controlled Routinely Operated Signal System (ACROSS), which is deployed in the central part of Japan along the Nankai Trough. We used 13 High Sensitivity Seismograph Network Japan (Hi-net) stations around the ACROSS source to monitor the temporal variation in travel time. Green’s functions were calculated for each station daily from March 29, 2007, through October 30, 2017. Secular advance in the temporal variation in travel time was seen for the whole operation period, in addition to a steplike delay associated with the 2011 Tohoku earthquake. We estimated the rate of secular change and the amount of coseismic step by modeling the transfer function of S-waves with a linear trend and the coseismic step of the 2011 Tohoku earthquake. Distance dependences of the travel time changes can be explained as a combination of common bias and dispersion for each station, for both the secular and coseismic changes. This can be interpreted as a randomly distributed change in seismic velocity over the range of the observation region. An azimuthal dependence exists for both changes and shows larger changes in the NE–SW direction than in the NW–SE direction from the ACROSS source.