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4.2. Strong motion observation and processing

In strong motion seismology, accelerometers are commonly used as seismic sensors because in contrast to the instruments used in classical seismology they are able to record strong ground shaking (e. g. peak accelerations between 1-3 g) without  being driven off scale. In the old days, accelerometers were realized as mechanical, displacement proportional seismometers with high natural frequency which were operated in the frequency range below the natural eigenfrequency of the instrument (Fig. 4.2.1). In the frequency range where the displacement frequency response function is constant (above the natural eigenfrequency of 25 Hz), an input signal A sin(ω t) would result in a phase shifted output signal A sin(ω with being the phase delay time at frequency ω (cf. Scherbaum (2001)). On the other hand, in the frequency range below the natural eigenfrequency where the displacement frequency response function is proportional to , an input signal A sin(ω t) would result in a phase shifted and attenuated output signal  sin(ω . The multiplication with in the frequency domain corresponds to a double differentation in the time domain which means that below the natural eigenfrequency, a mechanical displacement seismometer outputs a signal proportional to acceleration.

Figure 4.2.1       Modulus of the displacement frequency response function  of a mechanical  seismometer. Below the natural eigenfrequency, the output is proportional to acceleration

Most modern accelerometers operate on different principles (without a moving mass), which has considerable technical advantages in comparison to mechanical acclerometers which become less and less sensitive for smaller frequencies. Independent of the physical principle on which the recording is based, however, any sensor acts as a filter. As a consequence, in order to obtain true ground motion , which is the prerequisite for the calculation of a response spectrum by whatever algorithm, e. g. through the numerical evaluation of the Duhamel integral equation (4.5),  the effects of the sensor and of the recording process need to be corrected for as good as possible.

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Frank Scherbaum (2015), Fundamental concepts of Probabilistic Seismic Hazard Analysis, Hazard Classroom Contribution No. 001