Robert Mallet - 1851 Seismoscope

Explosion seismology was born in 1851, when Robert Mallet used dynamite explosions to measure the speed of elastic waves in surface rocks. Mallet wished to obtain approximate values for the velocities with which earthquake waves were likely to travel. To detect the waves from the explosions, Mallet looked through an eleven-power magnifier at the image of a cross-hairs reflected in the surface of mercury in a container. A slight shaking caused the image to blur or disappear. Transit velocities were measured over distances of the order of a thousand feet. For granite, Mallet obtained velocities of about 1600 feet per second. He had expected to find velocities of 8000 feet per second. The unexpectedly low elastic-wave velocity was attributed to the heterogeneity of the rock through which the wave traveled.

Robert Mallet advocated the use of fallen objects and cracks in buildings as aids in the study of earthquakes. He made a detailed investigation of the Neapolitan earthquake of 1857, in which he paid particular attention to the way buildings were cracked, walls overthrown, and soft ground fissured. Mallet believed that an earthquake consisted primarily of a compression followed by a dilatation. For such a shaking, he suggested, the resulting cracks in structures would be transverse to the direction of wave propagation. Overturned objects would fall along the horizontal projection of the direction of wave propagation. By observing the directions of arrival from a number of different points, he plotted an origin from which the wave seemed to spread. Mallet also published a set of formulae for calculating the velocities necessary to overturn structures of various simple shapes. From these, and observations of overturned objects, he estimated the velocity of particle motion at different sites.

Robert Mallet's assumption that earthquakes consisted mainly of longitudinal motion was proven invalid as soon as seismometers were built which recorded the large transverse component of ground motion.