Ground Coupling with Attenuation

The energy of the earthquake is ground-coupled and attenuated according to equation 5.1 - T where a is the maximum ground acceleration (cm/sec ), at the location of interest, M is the magnitude of the earthquake (Richter or local magnitude), R is the distance (km) to the ene ter or the causative fault, and b, and bj are 

CO ients that are determined from strong-motion data. For example, Donovan and Borstein, 1977, reported b, = 2.154E6 R h, = 0.046 + 0.445 login(R), and bj = 

United States, where most recorded earthquake data are in MM intensity units, there are two approaches for determining the attenuation of ground motion. Method 1 is an intensity-atte ition 

United States (equation 5.1-4), where /, is the site intensity in MM units and / is the ep ntral intensity in MM units. The site intensity is converted to the instmmental peak ground acceleration using an equation like equation 5.1-5. 

1 )ther method uses computer codes that predict ground acceleration  

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A physically separate and remote reference electrode is desirable becau.se the reference electrode is not electrically isolated from the inputs to the differential amplifier. The reference electrode is in contact with the person and consequently is electrically coupled to both inputs of the amplifier by the electrical impedance of the body. This impedance is a function of distance, i.e., the further away the reference electrode is, the greater is the attenuation. The reference electrode forms a floating ground signal rather than a true ground. Furthermore, the noise signal that can be introduced by the reference electrode is not necessarily a conunon-mode signal that will be removed by the differential amplification process. 

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