Earthquakes: Faults, Seismic Waves, Magnitude, and Hazards

Comprehensive practice flashcards covering fault mechanics, rock deformation, seismic wave propagation, seismometry, earthquake magnitude and intensity, and secondary earthquake hazards based on Geology Lecture 16 and Lecture 17.

What physical process generates an earthquake and produces seismic waves?

An earthquake occurs when the Earth's crust "breaks" along a fault, releasing stored elastic energy that travels through the Earth as seismic waves.

What are the five main structures or fabrics studied in structural geology that result from rock deformation?

Joints, fractures, faults, folds, and deformation fabrics (such as cleavage).

What are the three components of deformation, and which component specifically defines "strain"?

Deformation consists of changing location, changing orientation, and changing shape. The change in shape is specifically defined as strain.

What is elastic deformation?

A temporary change in shape or size that is fully recovered when the deforming force is removed.

How do brittle deformation and ductile deformation differ?

Brittle deformation is discontinuous and produces fractures, whereas ductile deformation is continuous, more evenly distributed, and accommodates deformation through flow.

What is the mathematical definition of mechanical stress?

$$\text{Stress} = \frac{\text{Force}}{\text{Area}}$$

How are the footwall and hanging wall defined along a sloping fault?

The footwall is the block of rock located below the fault plane, while the hanging wall is the block located above the fault plane.

What relative block motion characterizes a normal fault?

The hanging wall moves downward relative to the footwall, typically accommodating crustal extension or stretching.

How do reverse faults and thrust faults differ in their dip angle?

Both exhibit hanging wall motion upward relative to the footwall, but reverse faults have a steep dip ($\approx 60^\circ$), whereas thrust faults have a gentle dip ($\approx 30^\circ$ or less).

What type of fault is depicted in this cross-section diagram, characterized by a hanging wall moving up along a gentle dip angle of $30^\circ$?

A thrust fault.

What orientation and slip motion characterize strike-slip faults?

Strike-slip faults tend to have vertical fault planes where crustal blocks slide laterally past one another horizontally.

What causes stick-slip behavior along active faults?

Friction generated by asperities (bumps) along the fault plane arrests motion ("stick"), causing strain to build up over time until the applied stress overcomes friction and results in sudden rupture and movement ("slip").

What is the distinction between an earthquake's hypocenter (focus) and its epicenter?

The hypocenter (focus) is the point beneath the surface where fault rupture initiates, while the epicenter is the geographic point on Earth's surface directly above the hypocenter.

What is a Wadati-Benioff zone and what mineral phase transitions occur along it?

A dipping planar zone of shallow, intermediate, and deep earthquakes along a subducting slab at a convergent plate boundary, where mantle minerals transition from olivine to spinel at $410\,km$ and from spinel to perovskite at $660\,km$.

What underlying tectonic structure causes the high intraplate seismicity observed in New Madrid, Missouri?

A buried, reactivated Precambrian rift zone within the North American Plate.

What post-glacial process accounts for deformation and seismicity in eastern Canada and the St. Lawrence River valley?

Post-glacial isostatic rebound (crustal rebound following the melting and retreat of continental ice sheets).

What are the particle motion and relative velocity characteristics of primary (P) waves?

P-waves are compressional waves where particle motion is parallel to the direction of wave propagation, and they travel roughly twice as fast as S-waves.

What particle motion characterizes secondary (S) waves, and how does their speed compare to P-waves?

S-waves are shear waves where particle motion is perpendicular to the direction of wave propagation; they travel slower than P-waves.

What historical seismic device is illustrated in this drawing, designed with dragon mouths and toads to indicate the direction of ground shaking?

An ancient Chinese seismoscope (invented by Zhang Heng).

What physical principle allows a seismometer to record earthquake vibrations?

Inertia: an inert suspended mass (pendulum or spring-mounted weight) remains relatively stationary while the supporting frame and the ground vibrate around it.

How do seismologists determine the distance from a seismic station to an earthquake epicenter?

By measuring the difference in arrival time ($S - P$ time interval) between the faster P-wave and the slower S-wave on a seismogram.

What is the minimum number of seismographic stations required to locate an earthquake epicenter through triangulation?

A minimum of three stations, by plotting circles with radii equal to the computed distances from each station and finding their single common point of intersection.

What does the Modified Mercalli Intensity (MMI) Scale measure?

Earthquake destructiveness based on human perceptions of ground shaking and the observed severity of structural and environmental damage (ranging from I to XII).

Why has seismic moment magnitude replaced the Richter scale in modern seismology?

The Richter scale is based solely on the peak wave amplitude of the largest wave and is no longer used, whereas the seismic moment measures the total energy released by all ground motion and fault slip.

How do shaking amplitude and energy release increase with each whole-number increase in earthquake magnitude?

Each 1-unit increase corresponds to a 10-fold increase in ground shaking amplitude and approximately a 30-fold increase in released energy.

What magnitude and maximum epicenter intensity define a "Great" earthquake according to standard descriptive tables?

A magnitude of $> 8.0$ and an approximate maximum epicenter intensity of X to XII on the Modified Mercalli scale.

How is an earthquake hazard distinguished from an earthquake risk?

An earthquake hazard is the physical natural phenomenon (e.g., ground shaking, liquefaction), while earthquake risk is the likelihood of loss of human life, injury, or property damage.

What surface ground motions occur when vertical P-waves and vertical S-waves arrive at the surface?

Vertical P-waves cause the ground to displace up and down, whereas vertical S-waves cause the ground to shake back and forth horizontally.

What type of seismic surface wave is illustrated here, producing side-to-side, lateral undulations of the ground surface?

Love wave.

What type of seismic surface wave is illustrated here, causing the ground surface to roll in an elliptical, wave-like motion?

Rayleigh wave.

What three environmental conditions are required for soil liquefaction to occur during an earthquake?

1) Loose, granular sediment; 2) Water-saturated sediment; 3) Strong seismic shaking.

What geological surface and subsurface features indicate that liquefaction took place?

Sand volcanoes (sand boils) erupting at the surface and disrupted bedding in the subsurface.

What faulting mechanism is responsible for generating large ocean tsunamis?

Sudden vertical displacement of the seafloor caused by submarine normal faulting or thrust faulting, which abruptly displaces the overlying water column.

In paleoseismology, what four geological and biological indicators visible in trench excavations provide evidence of prehistoric earthquakes?

Disrupted bedding layers, offset ancient soil horizons (paleosols), layers of sand volcanoes, and bent or tilted trees displaying asymmetric growth rings.