Seismic Ground Motion - Dynamic Behavior of Concrete and Seismic Engineering - page 96

Only those aspects linked with seismic motion itself will be dealt with here:
readers interested in seismology can read [BER 03], [LAM 96], [LAM 97] and
[MAD 91].
This chapter briefly outlines three areas:
i) techniques for the measurement of seismic motions for engineering purposes;
ii) qualitative characterizations, with a view to para-seismic dimensioning; and
iii) physical descriptions of the phenomena affecting the characteristics of such
motions, including their origin, propagation and site effects.
3.2. Measuring seismic motions
3.2.1. Differences between seismological and accelerometer networks
The first seismological instruments were built almost 2,000 years ago by the
Chinese, who developed a device for locating the epicenter of an earthquake. The
first “modern” instruments, which date back to the end of the 19th century, aimed at
detecting and recording motions that were imperceptible to man. Since then, a vast
range of seismological instrumentation has come online, with increased sensitivities
and frequency responses, designed to locate quake event maxima (whether local,
regional or global) and to provide information on the inner structure of the terrestrial
globe.
For technological reasons, these very sensitive instruments were initially
intended to monitor violent events in their immediate vicinity. This is why
Californian engineers since the 1930s have tended to design low gain instruments.
The first accelerometers (called “strong motion” instruments) were low gain devices
because these make it easier to record acceleration directly. Even now, when the
velocimeters available have wide recording dynamics, the “strong motion” ground
acceleration has endured as a testing parameter. It has also been kept alive by the
fact that most of the instrument networks set up globally to monitor violent motions
still use accelerometers. The logic behind the siting of stations remains quite
different; “strong motion” instruments are typically installed in high economic stake
areas (city areas, important works). The ways in which seismological networks use
data on strong motions also remain quite different. Besides “traditional” seismology,
a more practical “violent motion seismology” has developed, which does not
necessarily leave aside relatively sophisticated treatment or modeling: this forms the
subject matter of this chapter.