Presented here are some important topics that do not easily fit into the structure of the first ten chapters.

Instruments

Throughout this book, we have often discussed Earth motion in terms of the displacement field, u(x, t), but have not mentioned how these movements are actually measured. A device that detects seismic wave motion is termed a seismometer; the entire instrument package, including the recording apparatus, is called a seismograph. The most common type of seismometer is based on the inertia of a suspended mass, which will tend to remain stationary in response to external vibrations.

As an example, Figure 11.1 shows a simple seismometer design that will detect vertical ground motion. A mass is suspended from a spring and connected to a lever such that it can move only in the vertical direction. Motions of the lever are damped using a “dashpot” to prevent excessive oscillations near the resonant frequency of the system. The differential motion between the mass and the seismometer case (which is rigidly connected to Earth) is measured using the voltage induced in a coil by the motion of a magnet. The induced voltage is proportional to the velocity of the mass for the instrument shown in Figure 11.1. In alternative seismometer designs, the displacement or acceleration of the mass may be recorded. As we will see later, the frequency response of the seismometer is a strong function of whether the displacement, velocity, or acceleration of the mass is measured.