Chapter 10: A Violent Pulse: Earthquakes

Earthquakes occur when accumulated stress along faults exceeds frictional resistance, causing sudden rupture and elastic energy release—a process called elastic rebound. The chapter establishes foundational vocabulary including hypocenter (rupture initiation point), epicenter (surface location directly above the hypocenter), foreshocks (smaller events preceding major ruptures), and aftershocks (continued adjustments following the main event). Seismic wave behavior is central to earthquake detection and analysis: body waves including primary waves and secondary waves travel through Earth's interior at different velocities, while surface waves cause the most destructive ground motion at the surface. Seismometers detect and record these waves on seismograms, providing essential data for earthquake characterization. The chapter explains critical distinctions between intensity measurements based on observed damage and magnitude scales that quantify energy release, including the Richter scale and the moment magnitude scale which accounts for rigidity and fault area. Geographically, earthquakes concentrate along plate boundaries—convergent zones where subduction generates mega-thrust earthquakes, divergent boundaries where rift spreading occurs, and transform boundaries with lateral slip. Intraplate earthquakes also occur where continental stress builds far from plate edges. Earthquake hazards extend beyond ground shaking to include liquefaction (loss of soil strength from increased pore pressure), landslide triggering, tsunami generation from submarine rupture displacement, and secondary fires from ruptured gas lines. The chapter concludes by addressing earthquake mitigation through building code implementation, seismic retrofitting of older structures, hazard zoning based on recurrence intervals and seismic gaps, and early warning systems that alert populations seconds to minutes after rupture initiation.