1. A system for measuring seismic activity to detect a threat to a structure, comprising: an optical accelerometer configured to be mounted to the structure, the optical accelerometer outputting an electrical signal that represents modulation of light, the modulation of the light being caused by seismic activity affecting the structure; and a computer operatively connected to the accelerometer to compare the electrical signal to a plurality of seismic activity signals indicative of seismic activity associated with structures stored in a database for determining whether the electrical signal is indicative of a threat, wherein the optical accelerometer comprises: a light source to direct light signals through a first light path; a modulator formed of a supple, resilient material that deforms in response to seismic activity, the modulator being positioned to modulate a light signal propagating through the first light path in response to the deformation; and a photoreceiver positioned in a second light path to receive the modulated light signal; wherein the modulator is configured to change a diameter of the first and second light paths located at their junction.