Kazuaki Masaki

Source model of the 2007 Noto-Hanto earthquake (Mw 6.7) for estimating broad-band strong ground motion

A source model for estimating broad-band ground motions from the 2007 Noto-Hanto earthquake (Mw 6.7) is estimated from a comparison of the observed records of the mainshock and synthesized motions based on the characterized asperity model using the empirical Green’s function method. The observed records of aftershocks used as the empirical Green’s functions are carefully selected to have almost the same radiation characteristics and source distance as the asperities of the mainshock. The best-fit source model consists of two asperities of different size. A large one is located just above the hypocenter, with an area of 6.3×6.3 km2 and stress drop of about 26 MPa. A smaller one is located north-east of the large one, with an area of 3.6×3.6 km2 and stress drop of about 10 MPa. The stress drop of the large one is about twofold higher than the average values of inland crustal earthquakes so far estimated, while that of smaller one is almost average. We found that the remarkable directivity pulses from the source model struck the northern part of the Noto peninsula, causing heavy damage in some towns there.

Application of a simplified calculation for full-wave microtremor H / V spectral ratio based on the diffuse field approximation to identify underground velocity structures

Under the diffuse field approximation, the full-wave (FW) microtremor H/V spectral ratio (H/V) is modeled as the square root of the ratio of the sum of imaginary parts of the Green’s function of the horizontal components to that of the vertical one. For a given layered medium, the FW H/V can be well approximated with only surface waves (SW) H/V of the “cap-layered” medium which consists of the given layered medium and a new larger velocity half-space (cap layer) at large depth. Because the contribution of surface waves can be simply obtained by the residue theorem, the computation of SW H/V of cap-layered medium is faster than that of FW H/V evaluated by discrete wavenumber method and contour integration method. The simplified computation of SW H/V was then applied to identify the underground velocity structures at six KiK-net strong-motion stations. The inverted underground velocity structures were used to evaluate FW H/Vs which were consistent with the SW H/Vs of corresponding cap-layered media. The previous study on surface waves H/Vs proposed with the distributed surface sources assumption and a fixed Rayleigh-to-Love waves amplitude ratio for horizontal motions showed a good agreement with the SW H/Vs of our study. The consistency between observed and theoretical spectral ratios, such as the earthquake motions of H/V spectral ratio and spectral ratio of horizontal motions between surface and bottom of borehole, indicated that the underground velocity structures identified from SW H/V of cap-layered medium were well resolved by the new method.

Seismic Ground Motion and Damage Caused by Large Earthquakes in Nagoya, Japan

By the response analysis of soil layers with regard to a fault rupture model, seismic ground motions in about 300 mesh areas in Nagoya city were estimated for three earthquakes: the Nobi earthquake of 1891, the Tonankai earthquake of 1944 and the Mikawa earthquake of 1945. On the other hand, damage ratios to wooden houses in each mesh area due to these earthquakes were obtained by rearranging original data. From the comparison of seismic ground motions to damage ratios in each mesh area, it was found that maximum acceleration at the ground surface was the major factor of earthquake damage. Another factor was liquefaction which occurred in loose sandy deposits and contributed significantly to damage even at rather low acceleration levels.