The Birth of Modern Seismology in the Nineteenth and Twentieth Centuries

The earliest seismic instruments were seismoscopes, which could only indicate that a ground shaking had occurred. Modern seismology started in the late 19th century when, mostly in Italy, Japan, Russia, and Germany, seismic instruments were developed, which were able to record ground movements as function of time and orientation. During the decade before World War I, the fundamental development of seismic instruments was completed with seismograph systems of high resolution in time and enough sensitivity to record the most important seismic phases. Since seismic waves traverse the whole Earth and do not stop at political borders, the seismological discoveries were only possible after developing new structures for internationally organized data exchange and cooperation. In parallel to instrumental and organizational developments, seismologists had to learn the principles of seismic wave propagation in a solid body based on elasticity and ray theory. A stepwise deciphering of seismic wave speeds inside the Earth for the different seismic phase types led to the discovery of the basic structure of our planet consisting of crust, mantle and a core divided in two parts.