8.4 Earthquake rupture processes and dynamics

Earthquake rupture processes are like dominoes falling. Once stress overcomes rock strength, the rupture spreads along the fault at high speeds. Factors like fault geometry and rock properties influence how the rupture behaves and grows.

During an earthquake, stress changes dramatically. Energy gets split between seismic waves, creating new fault surfaces, heat, and shifting rocks. Understanding these processes helps predict ground shaking and aftershock patterns.

Rupture Characteristics

Initiation and Propagation of Ruptures

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• Rupture initiation occurs when stress exceeds rock strength at a specific point on a fault
• Stress concentration at crack tips drives rupture propagation along the fault plane
• Rupture velocity typically ranges from 2-3 km/s for shallow crustal earthquakes
• Supershear ruptures can exceed local shear wave velocity in some large events
• Slip distribution varies along fault, with areas of high and low slip
• Asperities represent regions of high slip or stress drop on fault plane
• Barriers act as obstacles to rupture propagation, influencing earthquake size and duration

Factors Influencing Rupture Behavior

• Fault geometry affects rupture propagation path and complexity
• Pre-existing stress state on fault determines ease of rupture growth
• Rock properties like strength and fracture toughness impact rupture dynamics
• Pore fluid pressure changes can facilitate or inhibit rupture progression
• Rupture branching may occur at fault intersections or geometrical complexities
• Directivity effects cause asymmetric ground motion patterns around fault
• Near-field versus far-field rupture behavior differs due to wave propagation

Stress and Energy

Stress Changes During Earthquakes

• Dynamic stress drop measures stress change during rupture propagation
• Static stress drop represents final stress change after rupture completion
• Stress drop typically ranges from 1-10 MPa for most earthquakes
• Directivity enhances ground motion in rupture propagation direction
• Stress changes on nearby faults can trigger or inhibit subsequent earthquakes
• Coulomb stress analysis predicts likely locations of aftershocks and triggered events

Earthquake Energy Partitioning

• Seismic energy radiates as elastic waves during fault slip
• Fracture energy creates new fault surface area and damage zone
• Heat energy dissipates through friction on fault plane
• Gravitational potential energy changes due to vertical fault displacement
• Energy partitioning varies with earthquake size and tectonic setting
• Seismic efficiency (radiated energy / total energy) typically 5-20% for earthquakes
• Larger earthquakes tend to have higher seismic efficiency than smaller events

Rupture Phases

Stopping and Healing Processes

• Stopping phases generate when rupture terminates at fault edges or barriers
• Healing phases occur as slip ceases and fault locks up behind rupture front
• Stopping phases produce high-frequency seismic radiation at rupture edges
• Healing phases control final slip distribution and stress state on fault
• Rupture arrest occurs when driving stress falls below frictional resistance
• Fault healing rate influences earthquake recurrence intervals and clustering
• Afterslip and postseismic deformation follow main rupture as fault re-equilibrates

Seismic Wave Generation and Propagation

• P-waves (compressional) and S-waves (shear) radiate from earthquake source
• Surface waves (Rayleigh and Love) develop along Earth's surface
• Wave amplitudes decay with distance due to geometric spreading and attenuation
• Seismic waves reflect, refract, and diffract at velocity contrasts in Earth's interior
• Body wave and surface wave trains separate at large distances from source
• High-frequency content attenuates more rapidly than low frequencies
• Local site effects (soil amplification, basin resonance) modify incoming waves