coseismic

Coseismic describes processes and effects that occur during the rupture of an earthquake and the immediate ground shaking. It derives from the prefix co- meaning "together" and seismic, referring to seismic waves. The coseismic phase includes the initiation and propagation of rupture along a fault and the associated radiated energy that produces ground motion.

Coseismic slip is the displacement that occurs on the fault during the quake. The rupture propagates as shear failure, with slip distributed along the fault plane; the amount of slip and the area determine the seismic moment. Ground shaking results from dynamic rupture and wave radiation, and surface displacements may appear as fault rupture at the surface or distributed cracks and landforms.

Measurement and analysis: coseismic deformation is observed with seismographs, Global Positioning System (GPS) networks, and Interferometric Synthetic Aperture Radar (InSAR). Seismology often distinguishes coseismic slip from postseismic slip; tectonic models use the concept of coseismic moment to quantify the earthquake size. The seismic moment M0 equals the shear modulus mu times average slip D multiplied by rupture area A.

Relation to other terms: interseismic refers to the period between earthquakes when stresses accumulate; postseismic denotes processes that occur after rupture, such as afterslip, poroelastic rebound, and viscoelastic relaxation. Coseismic research helps understand fault behavior, rupture dynamics, and seismic hazard.