Earthquake Source Physics
An earthquake is the sudden frictional slip of a fault that releases stored elastic strain, radiating seismic waves whose pattern and size encode the geometry, energy, and dynamics of the rupture.
Definition
Earthquake source physics is the study of the mechanical processes by which a fault ruptures and radiates seismic energy, characterizing the source through its focal mechanism, seismic moment, stress drop, and rupture dynamics.
Scope
This topic covers the physics of the earthquake source: the elastic-rebound and stick-slip framework, fault friction and rupture nucleation and propagation, the double-couple force system and its radiation pattern, focal mechanisms, the seismic moment tensor, and moment magnitude. It treats source parameters such as stress drop, rupture velocity, and slip distribution, and the scaling relations that connect small and great earthquakes. The emphasis is on how the mechanics of faulting determines the radiated seismic field.
Core questions
- What governs the nucleation, propagation, and arrest of a fault rupture?
- How is the seismic moment defined, and why is moment magnitude preferred for large events?
- What does the radiation pattern reveal about fault geometry and slip direction?
- How do stress drop and rupture velocity scale across the range of earthquake sizes?
Key concepts
- Stick-slip faulting and the earthquake cycle
- Double-couple force system and radiation pattern
- Seismic moment and the moment tensor
- Moment magnitude and earthquake scaling
- Stress drop, rupture velocity, and slip distribution
Key theories
- Elastic rebound and stick-slip faulting
- Reid's elastic-rebound model holds that tectonic loading stores elastic strain across a locked fault until frictional failure permits sudden slip; laboratory stick-slip experiments later supplied the frictional mechanism underlying this earthquake cycle.
- Seismic moment and moment magnitude
- The seismic moment, the product of rigidity, fault area, and average slip, provides a physically grounded measure of earthquake size; Kanamori used it to define a moment magnitude that does not saturate for great earthquakes.
Mechanisms
Tectonic stress loads a fault held by friction; when shear stress exceeds frictional strength, slip nucleates and propagates as a rupture front, the dislocation across the fault being equivalent to a double-couple force system that radiates P and S waves with a four-lobed pattern, and the stress drop accompanying slip controls the radiated energy and ground-motion amplitudes.
Clinical relevance
Source physics determines the size, location, and rupture characteristics reported after an earthquake, informs ground-motion prediction and seismic hazard models, and distinguishes natural earthquakes from explosions in nuclear-test monitoring.
History
Reid formulated elastic rebound from the 1906 San Francisco earthquake; the double-couple representation was resolved in mid-century debates, and Kanamori's 1977 moment magnitude, together with routine moment-tensor inversion, made source characterization quantitative and globally standardized.
Key figures
- Harry Fielding Reid
- Hiroo Kanamori
- Christopher Scholz
Related topics
Seminal works
- reid1910
- kanamori1977
- scholz2019
Frequently asked questions
- Why has moment magnitude replaced the Richter scale for large earthquakes?
- The original Richter and related magnitudes saturate, underestimating the size of the greatest earthquakes because they are based on fixed-period wave amplitudes; moment magnitude is derived from the seismic moment, a direct physical measure of the slip and fault area, so it remains accurate for the largest events.
- What is stress drop and why does it matter?
- Stress drop is the difference between the shear stress on a fault before and after it slips; it influences how strongly an earthquake radiates high-frequency energy and therefore how intense the shaking is for a given magnitude.