Abstract
Hurst's analysis of natural background microtremors at 10 stations in Yun-Li and Chia-Yi Counties, Taiwan, yields high values of H, ranging from 0.733 to 0.934. These values are found to be site-specific and stationary. Microtremors recorded in the foothills, on a hillside and at rock sites have higher H values than those on the alluvial plain, near the coast or at soil sites although they may very well be affected by the local environment and geology. Since low-frequency microtremors tend to be more persistent in time, we suggest that the high values of the Hurst exponent H may be causally correlated with the presence of low frequencies. This seems likely because Hurst exponents H＞0.5 mean a higher statistical persistence of a random signal.
Highlights
Microtremors are the sum of different types of ground motion and cannot be identified as being directly associated with any particular event of finite duration, such as earthquakes or explosions
Stations C008, C027, C031, C044, C055, C104 and C106 (Fig. 1) are located on the Western alluvial plain of Taiwan, but among these, stations C027 and C044 are close to the sea, and C104 and C106 are on a hillside
The H values of the microtremors on the alluvial plain, near the coast or at soil sites are lower than those in the foothills, near the mountain or at rock sites they could be very well affected by the local environment and geology. These results imply that the strength of the long-term statistical dependence of the microtremors recorded in the foothills near the mountain and at the rock sites is greater than that on the alluvial plain, near the coast and at the soil sites
Summary
Microtremors are the sum (or mixture) of different types of ground motion and cannot be identified as being directly associated with any particular event of finite duration, such as earthquakes or explosions Most commonly, they are classified as noise. More than half a century ago, Hurst (1951) first proposed an empirical relationship to describe the long-term characteristics of the discharge from the River Nile Since his methodology has come to be known as ‘the rescaled range technique’. To estimate the H values, the same dataset may exhibit both persistent and antipersistent properties whether they are suitably differentiated or integrated (Karner 2001) It has been 50 years since Hurst first proposed the rescaled range technique, the technique still remains an open problem in stochastic geophysical signals. We applied Hurst’s rescale range technique to analyze microtremors in the Yun-Chia area, Taiwan, first to examine whether or not they are uncorrelated independent random noise and, secondly, to determine the implications of the distribution of the H values in this area
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