Characteristics of vertical and horizontal ground motions recorded during the Niigata-ken Chuetsu, Japan Earthquake of 23 October 2004

The Niigata-ken Chuetsu, Japan Earthquake of 23 October 2004 (Mw = 6.6) was the most significant earthquake that had affected Japan since the Kobe earthquake of 1995. The dense network of seismographs deployed in this region generated a quality dataset and as such, offers a good opportunity to understand events similar to those that can be expected in other parts of the world. In this paper, characteristics of the free-field ground motion recorded during the earthquake are examined. The database includes 156 records at 52 sites ranging from rock to stiff soil. Special attention of this study is placed on the behavior of the recorded vertical ground motion and its relation to the horizontal ground motion. The analysis shows that the vertical response spectra generally display low frequency contents at distant sites and high frequency contents at hard sites. Depending on site distance and site condition, the period for peak response in the spectra may vary from 0.05 to 0.15 s. The analysis also shows that the vertical to horizontal spectral ratio V/H is strongly dependent on spectral frequency, site-to-source distance, and site condition. The V/H ratio can be significantly larger than the rule-of-thumb value of 2/3 at short periods and tends to exceed 2/3 again at very long periods (greater than 5 s).