Chapter 11: Earthquakes and Earthquake Hazards

Earthquakes occur when accumulated stress along faults exceeds frictional resistance, causing sudden rupture and energy release as seismic waves. The elastic rebound theory describes how rocks progressively deform under stress and then abruptly return to their original shape during slip, generating the ground motion that characterizes seismic events. The hypocenter identifies the subsurface location where rupture initiates, while the epicenter marks the surface point directly above it. Earthquakes frequently produce aftershocks as the crust readjusts to stress redistribution, and occasionally foreshocks precede major ruptures. Different fault geometries dominate specific plate boundary configurations: normal faults characterize divergent spreading centers, reverse and thrust faults dominate compressional convergent zones, and strike-slip faults typify transform plate margins. The most powerful earthquakes occur along megathrust faults within subduction zones, where oceanic plates descend beneath continental lithosphere, generating magnitude 9 events capable of displacing the seafloor by tens of meters. Seismology uses seismographs to record and analyze body waves, which include primary compressional waves that travel fastest through all materials and secondary shear waves that propagate only through solids, along with slower surface waves that produce the greatest structural damage. Earthquake magnitude is quantified through multiple scales, including the amplitude-based Richter scale and the moment magnitude scale, which better represents total energy release in large events. The intensity of ground shaking depends on wave amplitude, duration, local soil composition, and structural engineering, with soft sedimentary deposits amplifying seismic motion and potentially triggering liquefaction, the transformation of water-saturated soils into flowing masses that destabilize infrastructure. Beyond ground shaking, earthquakes trigger cascading hazards including landslides, subsidence, dam failures through seiche formation, and fires from ruptured utilities. Megathrust earthquakes displace ocean floors, generating tsunamis that propagate across ocean basins at extreme velocities, posing transoceanic hazards. Seismic energy concentrates along narrow belts, particularly the circum-Pacific Ring of Fire and the Alpine-Himalayan collision zone, though intraplate earthquakes demonstrate that seismic hazards exist even far from active plate boundaries.