CLEAR IDENTIFICATION OF FUNDAMENTAL IDEA OF NAKAMURA ' S TECHNIQUE AND ITS APPLICATIONS

A method which employs microtremor has been introduced for estimating dynamic characteristics of surface layers, in early 1950. Then usage of this method has received lots of criticism considering uncertainty about source of microtremor. After an introduction of the Nakamura’s technique (H/V or QTS technique; Nakamura, 1989), many people have paid a renewed great attention for estimating dynamic characteristics of ground and structures using microtremor, since clear and reliable information was provided by very simple and inexpensive noise measurements. In recent years, although several researchers claimed that theoretical ground of this technique is not clear and consensus based on experiment couldn’t be reached, there have been many successful experimental studies based on these technique. Many theoretical studies have been performed, for explaining the amount of types of waves included in microtremor and checking the applicability of the QTS technique. And some of them are suggested that the peak on H/V ratio can be explained with the fundamental peak of Rayleigh waves. From the output of these researches, explanation of microtremor with Rayleigh waves caused some confusion between users and the author decided to clear out this problem. The basic idea and the main goal of QTS technique are tried to be re-explained in present paper. The author’s explanation about the effects of contents of Rayleigh waves in microtremor is also given. Other possible usage of products from QTS technique (predominant frequency and amplification factor) for hazard estimation is also given. As it is well known, occurrence of earthquake damage depends upon strength, period and duration of seismic motions. And these parameters are strongly influenced by seismic response characteristics of surface ground and structures. This reality makes investigation of vulnerability of ground and structures an important issue, before the earthquake occurs. For this purpose, vulnerability indices called K values were proposed by Nakamura (1996). K values are simply derived from strains of ground and structures. Formulation of K values for ground (Kg) and some application examples are also given in present paper. These new values give a chance to estimate vulnerabilities of all types of structures and ground, before the real damage occurs.