Kaoru Sawazaki

Time-lapse changes of seismic velocity in the shallow ground caused by strong ground motion shock of the 2000 Western-Tottori Earthquake, Japan, as revealed from coda deconvolution analysis

A color image forming apparatus provided with plural charging devices, plural image exposure devices, and plural developing devices which are arranged around the outer circumference of a photoreceptor so that a color image is formed during a single rotation of the photoreceptor. The apparatus comprises a supporting cylinder for enclosing the photoreceptor and for supporting the plural image exposure devices thereon.

Stress rotations due to the M6.5 foreshock and M7.3 main shock in the 2016 Kumamoto, SW Japan, earthquake sequence

A shallow M7.3 event with a M6.5 foreshock occurred along the Futagawa-Hinagu fault zone in Kyushu, SW Japan. We investigated the spatiotemporal variation of the stress orientations in and around the source area of this 2016 Kumamoto earthquake sequence by inverting 1,218 focal mechanisms. The results show that the σ3-axis in the vicinity of the fault plane significantly rotated counterclockwise after the M6.5 foreshock and rotated clockwise after the M7.3 mainshock in the Hinagu-fault segment. This observation indicates that a significant portion of the shear stress was released both by the M6.5 foreshock and M7.3 mainshock. It is estimated that the stress release by the M6.5 foreshock occurred in the shallower part of the Hinagu-fault segment, which brought the stress concentration in its deeper part. This might have caused the M7.3 mainshock rupture mainly along the deeper part of the Hinagu-fault segment after 28 hours.

Imaging the high‐frequency energy radiation process of a main shock and its early aftershock sequence: The case of the 2008 Iwate‐Miyagi Nairiku earthquake, Japan

To understand the energy release process that operates at the end of the main shock rupture and start of the aftershock activity, we propose an inversion method that uses continuous high-frequency seismogram envelopes of the main shock and early aftershocks (i.e., events that occur at short times after the main shock). In our approach, the aftershock sequence is regarded as a continuous energy release process, rather than a discrete time series of events. To correct for the contribution of coda wave energy excited by multiple scattering, we use the theoretical envelope synthesized on the basis of the radiative transfer theory as a Greens function. The site amplification factors are corrected considering the conservation of energy flux and using the coda normalization method. The inverted temporal energy release rate for the 2008 MW 6.9 Iwate-Miyagi Nairiku earthquake, Japan, decays following t−1.1, at the lapse time t of 40–900 s after the main shock origin time. This exponent of the decay rate is similar to the p value of the modified Omori law. The amount of estimated energy release is consistent with that calculated from the magnitude listed in the aftershock catalog. Although the uncertainty is large, the location of large energy release at the lapse times of 40–900 s approximately overlaps to that of the aftershocks, which surrounds the large energy release area during the main shock faulting. The maxima of the energy release rate normalized by the average decay rate distributes following a power law, similar to the Gutenberg-Richter law.

Automatic Aftershock Forecasting: A Test Using Real‐Time Seismicity Data in Japan

Real‐time aftershock forecasting is important to reduce seismic risks after a damaging earthquake. The main challenge is to prepare forecasts based on the data available in real time, in which many events, including large ones, are missing and large hypocenter determination errors are present due to the automatic detection process of earthquakes before operator inspection and manual compilation. Despite its practical importance, the forecast skill of aftershocks based on such real‐time data is still in a developmental stage. Here, we conduct a forecast test of large inland aftershock sequences in Japan using real‐time data from the High Sensitivity Seismograph Network (Hi‐net) automatic hypocenter catalog (Hi‐net catalog), in which earthquakes are detected and determined automatically in real time. Employing the Omori–Utsu and Gutenberg–Richter models, we find that the proposed probability forecast estimated from the Hi‐net catalog outperforms the generic model with fixed parameter values for the standard aftershock activity in Japan. Therefore, the real‐time aftershock data from the Hi‐net catalog can be effectively used to tailor forecast models to a target aftershock sequence. We also find that the probability forecast based on the Hi‐net catalog is comparable in performance to the one based on the latest version of the manually compiled hypocenter catalog of the Japan Meteorological Agency when forecasting large aftershocks with M >3.95, despite the apparent inferiority of the automatically determined Hi‐net catalog. These results demonstrate the practical usefulness of our forecasting procedure and the Hi‐net automatic catalog for real‐time aftershock forecasting in Japan. Online Material: Figures and tables showing detailed forecast results for all considered aftershock sequences and all forecast time frames.

Strong ground motions recorded by a near-source seismographic array during the 16 August 2005 Miyagi-Ken-Oki, JAPAN, earthquake (Mw 7.2)

An earthquake of Mw 7.2 took place on August 16, 2005 at a plate boundary between the Pacific plate and the North American plate off the coast of Miyagi Prefecture, Northeast Japan. During the Miyagi-Ken-Oki event, we succeeded in recording strong ground motions at six stations in a seismograhic array with an epicentral distance of about 70 km, where we have been operating seven strong-motion seismometers in an aperture of about 500 m since April 2004. The predominant period of the ground motion was shorter than 0.3 s. The peak ground acceleration exceeded 1.7 g at a station where non-linear site response may have occurred during the mainshock. The short-period strong ground motions show a large spatial variation, with up to a ten-fold difference in amplitude even within the array. However, there is a similarity between waveforms registered at different stations for periods longer than 0.4 s. Therefore, the difference in the ground motions may be mainly attributed to the difference in the shallow structure just beneath the stations.