The principle by which the seismometers work can be thought of as a heavy mass freely supported by a spring from a frame fixed to the Earth. When an earthquake or explosion occurs, sesimc waves traveling through the Earth reach the seismometer. The frame is shaken in response to the motion of the wave. Although the frame is displaced by the ground motion, the heavy mass tends to remain stationary because of its inertia. The displacement of the grounded frame is therefore a measure of the ground motion.
Stress drop reduction in stress across a fault, e.g. caused by an earthquake.
Intraplate earthquakes have in general higher stress drop than interplate earthquakes. S-wave a seismic body wave with particle motion perpendicular to the direction of propagation, also called shear wave. The passage of an S-wave involves a pure shear of the medium. Vulnerability degree of damage caused by various levels of loading. The vulnerability may be calculated in a probabilistic or deterministic way for a single structure or groups of structures.