Kahori Iiyama

Shallow subsurface structure estimated from dense aftershock records and microtremor observations in Furukawa district, Miyagi, Japan

We conducted single-site and array observations of microtremors in order to revise the shallow subsurface structure of the Furukawa district, Miyagi, Japan, where severe residential damage was reported during the Great Eastern Japan Earthquake of 2011, off the Pacific coast of Tohoku. The phase velocities of Rayleigh waves are estimated from array observations at three sites, and S-wave velocity models are established. The spatial distribution of predominant periods is estimated for the surface layer, on the basis of the spectral ratio of horizontal and vertical components (H/V) of microtremors obtained from single-site observations. We then compared ground motion records from a dense seismometer network with results of microtremor observations, and revised a model of the shallow (~100 m) subsurface structure in the Furukawa district. The model implies that slower near-surface S-wave velocity and deeper basement are to be found in the southern and eastern areas. It was found that the damage in residential structures was concentrated in an area where the average value for the transfer functions in the frequency range of 2 to 4 Hz was large. Severe residential damage in the Furukawa district, Miyagi, Japan, occurred during the Great Eastern Japan Earthquake of 2011. We model the shallow subsurface structure based on aftershock records from a dense seismometer network and results of microtremor observations. Site amplifications estimated from the model explain the damage distribution well.

Practical Method for Damage Evaluation Based on Point Estimate Considering Uncertainty of Structural Properties

This study introduces a practical method for evaluating structural damage based on a large-scale simulation targeting expansive areas, like whole cities. In such a seismic simulation that deals with numerous building structures, it is desirable to estimate the damage based on a stochastic evaluation considering the uncertainty of structural properties. This is because an accurate modeling of numerous building structures, according to each designed value, would require a great deal of time. However, a damage evaluation considering the model uncertainty generally involves numerous calculations and is inadequate for such a large-scale simulation. Therefore, we propose a method using the point estimate technique which can estimate the probability of damage under model uncertainty from a small number of calculations. The applicability and usefulness of the proposed method is evaluated by comparing it to the method based on a Monte Carlo simulation.

Identification of Dynamic Characteristics of a Historical Church in Armenia Using Microtremor Survey Technique

Historical Churches in Armenia may have many experiences of earthquake ground motion. Thus, some of them have been damaged partially or severely. We had carried out the microtremor survey at a historical churches, which is Cathedral at Etchimiadzin around 5th century, to identify the dynamic characteristic. Four sensors are used simultaneously for the observations and modes of vibration are identified from some methods such as the Fourier spectrum and coherence function, and frequency domain decomposition (FDD) technique.

A Study on Stochastic Properties of Auto-Correlation Coefficients for Microtremor Data Simultaneously Observed at Two Sites

We discussed stochastic properties of the spatial auto-correlation (SPAC) coefficients for microtremor data which are observed simultaneously at two sites. Usually, the SPAC method can provide the phase velocities using the data observed simultaneously on circular array with four or seven sites. The method, however, suggests analytically the possibility that we can estimate the phase velocities using only the data observed simul- taneously at two sites. To clarify the limitation of this idea, some mathematical analyses are performed and stochastic properties of the SPAC coefficients are derived. Furthermore, some real data of microtremors are applied to the above analytical results and the validity is discussed.