Spots 
Early earthquake study relied on unverified accounts and local terror. Modern observational seismology transforms ground shocks into measurable physical events by recording wave arrival times, mapping intensity iso-seismal lines, and locating deep subterranean foci.
Robert Mallet established modern earthquake investigation in the Kingdom of Naples by measuring fractures in stone walls across damaged villages. By tracing lines perpendicular to wall cracks back to their intersection beneath the surface, Mallet estimated the depth of the focal shock at approximately five to nine miles.
Clarence Dutton gathered thousands of reports following the shock in South Carolina to plot concentric intensity lines across North America. Precise time records along railway networks allowed investigators to compute the velocity of seismic waves radiating through the Earth's crust at over thirteen thousand feet per second.
Bunjiro Koto's field survey across central Japan revealed a continuous subterranean rift breaking the surface along the Neo Valley. The vertical slip of up to twenty feet along this fault line demonstrated that sudden tectonic displacement causes the ground vibration, reversing the older belief that faults were mere secondary effects.
Richard Dixon Oldham documented massive land slips and vertical displacement along the Chedrang fault in northeast India. Analysis of distant seismograph records from this event revealed three distinct wave phases, establishing the existence of longitudinal primary waves and transverse secondary waves traveling through the Earth's deep interior.
An examination of major seismic events that shaped modern observational seismology and subterranean fault theory.