Earthquakes and Society: Chapter 5 - Earthquake Faults

Earthquakes: Result from the long-term application of forces on rocks, which can last from hundreds to millions of years.
- Types of Rocks:
- Brittle Rocks: Examples include granite and basalt, which are solid but can break under stress.
- Less Solid Rocks: Examples include mudstone, chalk, and sandstone that can crumble or deform under force.
Stress on Rocks: Unlike the transient forces applied by humans, tectonic forces are enduring and cause significant geological changes.

Plastic vs. Brittle Behavior:
- Weaker rocks can deform plastically under sustained pressure, akin to silly putty, especially in sedimentary and metamorphic layers.
Folding of Rocks:
- Fundamental Geology Principle: Sediments are initially deposited horizontally.
- Under long-lasting tectonic stresses, horizontal layers can bend into folded rocks, changing from parallel to curved or zigzag shapes.
- Example: Folded rock layers seen in California (e.g., at the San Andreas Fault Zone).

Brittle vs. Ductile Responses:
- In most cases, tectonic forces lead to fault ruptures, generating earthquakes when the stress surpasses the internal cohesion of the rocks.
- Creeping Faults: Some faults can move without breaking. These faults slowly crawl past each other without generating significant earthquakes, characterized by gradual movement (typically a few centimeters per year).
- Example: The Calaveras Fault in Hollister, California—experiences microearthquakes but mainly releases stress through aseismic creep.
- Observation: This gradual movement can visibly shift curbs and walls over time.

Definition of Locked Faults: This term describes faults that do not slip under stress and remain interlocked, similar to two people locking arms. They require considerable force to unlock (i.e., to break the internal cohesion).
- Elastic Rebound Theory: Explained by Harry Fielding Reid, identifying how locked faults accumulate stress until they release it in an earthquake.
Mechanics of Earthquakes via Elastic Rebound:
- During the buildup of tectonic forces, a fault may deform elastically; once the threshold is surpassed, it slips causing an earthquake.
- Coseismic Slip: Sudden movement during an earthquake can result in several meters of offset.
- Following this, the fault undergoes postseismic slip, allowing further minor movements without additional earthquakes.

Understanding Earthquake Cycles: The interval between earthquakes on a fault is cyclic, where stress accumulates after each quake, leading to renewed potential for future earthquakes.
- Cycle Example: The San Andreas Fault shows an average recurrence time of about 23.4 years for quakes averaging magnitude 6.0 since 1881.
- Migration of Earthquakes: Some faults exhibit “migration” where earthquake hypocenters move along the fault line, as observed in the North Anatolian Fault in Turkey.
- Historical Context: Major earthquakes from this fault have caused widespread devastation, with clear westward migration patterns noted.
- Implications: Concerns arise over future quakes that may approach populated areas, exemplified by worries for Istanbul.

Hybrid Faults: Most faults are not purely locked or creeping; instead, they often exhibit mixed behavior. They can show some aseismic creep while being capable of producing significant earthquakes.
- Example: The Hayward Fault in California, which has shown creeping behavior at the surface but remains locked at greater depths.
- Impact on Infrastructure: The Hayward Fault has interacted with urban settings, highlighting risks to structures including those of the University of California, Berkeley.

Surface Expression of Faults: Sometimes fault lines are obvious (like the 2010 Baja California quake). Other times, they may be hidden due to vegetation or erosive processes.
Technology in Fault Detection: Lidar technology assists geologists in identifying faults obscured by plant growth by using pulsed laser light to measure ground distance, distinguishing between vegetative and bare ground features.

Deformation Categories: Rocks can either break (rupture) or deform (plastic behavior) under long-lasting forces.
Fault Types:
- Creeping Faults: Move continuously without generating significant earthquakes.
- Locked Faults: Accumulate stress until they release it in sudden earthquakes.
- Hybrid Faults: Exhibit both creeping and the potential for large seismic events.
Overall Process: Earth's fault behavior encapsulates a constant cycle of stress accumulation and release through earthquakes, underpinned by geological principles such as elastic rebound theory.

An illustration of the Vuache Fault in the French Alps where the rock surface shows remarkable polishing (slickensides) due to friction during fault movements. It provides direct evidence for tectonic activity.

Describes the SAFOD project that involved drilling to obtain rock samples from the San Andreas Fault, revealing that the fault generates material (talc) that reduces friction, aiding in the gliding motion of fault surfaces during earthquakes.