Physical Geology: Crustal Deformation and Earthquakes

Comprehensive practice flashcards covering structural geology, crustal deformation (folds, joints, faults, stress, and strain), stratigraphy, relative and absolute dating, seismology, earthquake mechanics, seismic waves, tsunamis, and hazard prediction.

What is crustal deformation, and where does most of it occur?

Deformation is a general term referring to all changes in the original form and/or size of a rock body; most crustal deformation occurs along plate margins.

How does stress differ from force and differential stress?

Force puts stationary objects in motion or changes the motions of moving objects; stress is the force applied to a given area ($\text{Stress} = \frac{F}{A}$); differential stress is stress applied unequally in different directions.

What are the three types of differential stress, and how do they affect a rock body?

1. Compressional stress: squeezes and shortens a body.
2. Tensional stress: stretches and pulls a body apart.
3. Shearing stress: pushes different parts of a body past each other in opposite directions.

How does brittle deformation differ from ductile deformation, and in what crustal regimes does each prevail?

Brittle deformation occurs when increasing stress causes rocks to fracture, typical of the upper crust where lower temperatures and confining pressures prevail. Ductile deformation occurs when stress causes rocks to undergo smooth, continuous flow and contortion without fracturing, typical of the deep crust characterized by high temperatures and high confining pressures.

What four factors determine how a rock deforms?

1. Confining pressure
2. Temperature
3. Rock type (e.g., rock salt deforms ductiley; granite and crystalline rocks fracture; igneous and metamorphic rocks resist deformation more than sedimentary rocks)
4. Time (longer stress application increases the likelihood and magnitude of deformation)

Based on this diagram, how are strike and dip defined and measured on dipping strata?

Strike is the compass direction of a line formed by the intersection of a dipping rock layer with a horizontal plane. Dip is the angle at which the bed tilts relative to that horizontal plane, measured at right angles ($90^\circ$) to the strike line in the direction water would trickle down the slope.

What are the structural components of a fold: limbs, hinge line, and axial plane?

The limbs are the two dipping sides of a fold. The hinge line is the imaginary line along which opposite limbs meet and defines the strike of the fold. The axial plane is an imaginary surface that divides the fold into two symmetrical halves.

How do anticlines and synclines differ regarding limb dip directions and the relative ages of strata exposed along their hinge lines?

In an anticline (an arch), limbs dip away from the hinge line, and the oldest strata are exposed along the hinge line. In a syncline (a trough), limbs dip toward the hinge line, and the youngest strata are exposed along the hinge line.

What type of fold is illustrated in this diagram?

An overturned fold, in which the axial plane is steeply inclined and both limbs dip in the same direction, but at different angles.

How do symmetrical, asymmetrical, overturned, and recumbent folds differ in their axial planes and limb dips?

Symmetrical fold: axial plane is vertical; limbs dip in opposite directions at equal angles. Asymmetrical fold: axial plane is inclined; limbs dip in opposite directions at different angles. Overturned fold: axial plane is steeply inclined; both limbs dip in the same direction at different angles. Recumbent fold: axial plane is horizontal.

What outcrop map pattern is produced when plunging folds are eroded, and how does this pattern distinguish plunging anticlines from plunging synclines?

Plunging folds produce curved horseshoe- or hairpin-shaped outcrop patterns. In a plunging anticline, the horseshoe pattern closes in the direction of plunge. In a plunging syncline, the horseshoe pattern opens in the direction of plunge.

What geologic feature is shown in this block diagram, and what is its age relationship from center to edge?

A dome; it is an anticlinal circular upwarping where flanking strata dip away from the central point in all directions, exposing the oldest strata at the center.

What is a structural basin, and what are its dip characteristics and relative rock ages?

A basin is a synclinal, bowl-shaped circular depression produced by downwarping where rock layers dip toward a central point from all directions, with the youngest strata situated in the center (e.g., the Michigan Basin).

How does a joint differ from a fault, and what type of deformation do both represent?

Both represent brittle deformation. A joint is a fracture or crack in rock along which no appreciable movement or displacement has occurred. A fault is a plane of dislocation along which rocks on one side have moved (offset) relative to rocks on the other side.

What geologic fracture feature is exhibited at Devils Tower, Wyoming, and how did it form?

Columnar joints; they formed as igneous rock cooled and shrank, developing columnar shrinkage fractures.

How does the release of confining pressure produce expansion or sheeting joints in an exhumed pluton?

As overlying rock is stripped away by uplift and erosion, the reduction in confining pressure causes the exhumed pluton to expand and fracture into two prominent sets of joints that intersect at approximately $90^\circ$.

In an inclined fault, how are the hanging wall and the footwall distinguished?

The hanging wall is the block of rock positioned directly above the inclined fault plane; the footwall is the block of rock situated directly below the inclined fault plane.

What stress produces normal faults, what is the relative block motion, and what topographic structures do they form?

Normal faults are produced by tensional stress (crustal extension). The hanging wall moves downward relative to the footwall. They characterize continental rifts and mid-ocean ridges, commonly forming uplifted mountain blocks called horsts and down-dropped valleys called grabens (e.g., the Basin and Range Province).

How do reverse faults and thrust faults differ?

Both result from compressional stress where the hanging wall moves upward relative to the footwall. Reverse faults have fault dip angles greater than $45^\circ$, are generally small, and accommodate local displacements. Thrust faults have fault dip angles less than $45^\circ$ and accommodate large horizontal displacements (tens to hundreds of kilometers).

What structural feature is depicted at Chief Mountain and the Lewis Overthrust Fault?

A low-angle thrust (overthrust) fault where compressional stress shoved older Precambrian rocks horizontally on top of younger Cretaceous rocks, violating the principle of superposition; Chief Mountain is an isolated erosional remnant (klippe) of the hanging wall.

What are strike-slip faults, what stress forms them, and what is a transform fault?

Strike-slip faults are fractures with near-vertical fault planes where movement is primarily horizontal and parallel to the strike of the fault, resulting from shear stress. Plate-boundary-scale strike-slip faults that accommodate motion between tectonic plates are called transform faults (e.g., the San Andreas Fault and the Alpine Fault).

What are the three types of unconformities, and how are they identified?

1. Angular unconformity: an erosional surface separating tilted or folded sedimentary rocks below from flat-lying sedimentary rocks above.
2. Nonconformity: an erosional surface separating underlying crystalline igneous or metamorphic rocks from overlying sedimentary rocks.
3. Disconformity: an erosional surface separating parallel sedimentary rock layers above and below.

What are the four principles of relative dating?

1. Law of Superposition: in undeformed sedimentary rocks, younger rocks overlie older rocks.
2. Principle of Original Horizontality: sedimentary layers are originally deposited horizontally.
3. Principle of Cross-Cutting Relationships: a fault or igneous intrusion is always younger than the rock it cuts.
4. Principle of Inclusions: rock fragments enclosed within another rock are older than the rock containing them.

How do alpha emission, beta emission, and electron capture alter the atomic number and atomic mass of a daughter isotope?

Alpha emission: releases $2$ protons and $2$ neutrons; atomic number decreases by $2$, atomic mass decreases by $4$. Beta emission: a neutron emits an electron and converts into a proton; atomic number increases by $1$, atomic mass remains unchanged. Electron capture: a proton captures an electron and converts into a neutron; atomic number decreases by $1$, atomic mass remains unchanged.

If a rock sample has a parent-to-daughter isotope ratio of $25\%$ parent and $75\%$ daughter, and the half-life is $1{,}000{,}000\,\text{years}$, how old is the sample?

The sample is $2{,}000{,}000\,\text{years}$ old (two half-lives have elapsed: $100\% \rightarrow 50\% \rightarrow 25\%$; $2 \times 1{,}000{,}000\,\text{years} = 2{,}000{,}000\,\text{years}$).

How does a subsequent metamorphic reheating event affect radiometric dating of individual minerals versus whole-rock samples?

Heating causes loosely fitting daughter isotopes to escape from the mineral lattice into the surrounding rock matrix, resetting the mineral's isotopic clock to date the time of the metamorphic event. Whole-rock analysis retains both the retained and escaped daughter isotopes, yielding the age of initial igneous crystallization.

How are the ages of non-fossiliferous sedimentary rocks determined through radiometric dating?

Because sedimentary rocks contain minerals weathered from diverse older sources, their ages cannot be determined directly; instead, their ages are bracketed between the radiometric dates of cross-cutting igneous intrusions, underlying crystalline rocks, or interbedded volcanic ash layers using the principles of relative dating.

What are the major subdivisions of the Geologic Time Scale from largest to smallest, and what are the eras of the Phanerozoic Eon?

Subdivisions in decreasing order: Eon $\rightarrow$ Era $\rightarrow$ Period $\rightarrow$ Epoch. The two eons are the Precambrian (origin of Earth at $4.5\,\text{billion years}$ to $542\,\text{million years ago}$) and the Phanerozoic (last $542\,\text{million years}$). The Phanerozoic is divided into the Paleozoic, Mesozoic, and Cenozoic eras.

What is an earthquake, and what is the sequence of events described by the Elastic Rebound Theory?

An earthquake is a vibration of Earth produced by the rapid release of energy. Elastic Rebound Theory stages:

1. A locked fault resists movement as tectonic forces are applied.
2. Continued stress causes elastic strain to build up in surrounding rocks.
3. The frictional strength is exceeded, and the fault abruptly slips, releasing energy as seismic waves.
4. Rocks elastically snap back to an unstrained state, but the fault blocks remain permanently displaced.

How does an earthquake's hypocenter (focus) differ from its epicenter?

The hypocenter (or focus) is the exact underground location along the fault plane where initial slippage and energy release begin. The epicenter is the point on Earth's surface situated directly above the hypocenter.

What type of seismic wave and particle motion is illustrated by this slinky model?

Primary (P) waves; particles compress and expand parallel to the direction of wave travel.

What are the properties, velocities, and transmission media of Secondary (S) waves?

Secondary (S) waves are shear waves that move particles at right angles ($90^\circ$) to the path of travel. They travel through solid rock at roughly half the speed of P-waves ($\sim 2.5\,\text{km/s}$ vs. $\sim 5\,\text{km/s}$) and cannot travel through liquids or gases.

What are surface waves, and why do they cause the most structural damage during an earthquake?

Surface waves are seismic waves confined to Earth's exterior and outer crust that travel slightly slower than S-waves. They produce ocean-wave-like rolling motion and severe side-to-side horizontal motion, which severely damages and shears building foundations.

How do seismologists locate the epicenter of an earthquake using triangulation?

Seismologists calculate the time interval between the first arrival of the P-wave and the first arrival of the S-wave on a seismogram, which determines the distance from that station to the epicenter. By drawing circles corresponding to those distances around at least three separate seismograph stations, the unique point where all three circles intersect marks the epicenter.

What is the Modified Mercalli Intensity Scale, and what is its primary limitation?

The Modified Mercalli Intensity Scale is a qualitative ranking system designated by Roman numerals from I (imperceptible) to XII (catastrophic total destruction) based on observed structural damage and human perception. Its limitation is that it lacks an objective mathematical basis and varies with local building quality, distance, and ground geology.

How does Richter magnitude scale with seismic wave amplitude?

Richter magnitude measures maximum wave amplitude recorded on a seismogram (adjusted for epicentral distance). A difference of $1$ whole unit on the Richter scale corresponds to a factor of $10$ increase in the measured amplitude of ground motion.

What physical parameters determine an earthquake's moment magnitude ($M_w$), and by what factor does total energy increase per unit magnitude?

Moment magnitude depends on the amount of fault slip, the surface area of the fault rupture, and the rigidity of the rock. The total energy released increases by a factor of approximately $32$ (or $32\text{ to }33$) for each whole unit increase on the magnitude scale.

Why did the Marina District of San Francisco and the Cypress Structure in Oakland experience severe damage during the 1989 Loma Prieta earthquake?

Earthquake destructiveness strongly depends on the nature of substrate material. The Marina District and the Cypress Structure were built on unconsolidated water-saturated mud, sand, gravel, or landfill, which amplified seismic wave vibrations far more than solid bedrock.

What is liquefaction, and what surface feature can it produce?

Liquefaction occurs when intense seismic shaking draws groundwater upward into unconsolidated, water-saturated soils, transforming stable ground into a mobile fluid slurry incapable of supporting buildings; it frequently vents slurry to the surface, creating sand volcanoes.

Along which plate boundaries do intermediate- and deep-focus earthquakes occur?

Intermediate- ($70\text{--}300\,\text{km}$) and deep-focus ($301\text{--}700\,\text{km}$) earthquakes occur exclusively along convergent plate boundaries within subducting oceanic lithospheric slabs (Wadati-Benioff zones); shallow earthquakes ($0\text{--}69\,\text{km}$) occur along all plate boundaries.

What fault mechanism is indicated by this focal mechanism ('beachball' plot)?

A normal fault, indicated by a light/white dilatational (tensional pull) quadrant in the center bounded by dark compressional (push) quadrants.

How do tsunami waves change in speed, height, and spacing as they travel from the deep open ocean into shallow coastal waters?

In the deep open ocean, tsunami waves travel at high velocities ($\sim 835\,\text{km/h}$) with very low wave heights (often under a meter, nearly imperceptible) and wide wavelengths. Upon approaching shallow coastal waters, friction with the seafloor slows them down ($340\,\text{km/h}$ to $50\,\text{km/h}$), causing wave heights to increase dramatically into destructive walls of water that bunch closer together.

What were the magnitudes and impacts of the 2004 Sumatra and 2011 Sendai earthquakes?

The December 26, 2004 Sumatra earthquake ($M\,9.0$) triggered an Indian Ocean tsunami that surged several kilometers inland, claiming over $230{,}000$ lives. The March 11, 2011 Sendai, Japan earthquake ($M\,8.9$) generated $30\text{-foot}$ walls of water that swept $6\,\text{miles}$ inland.

What physical precursors have been studied for short-range earthquake predictions, and what is their current reliability?

Precursors include unusual animal behavior, foreshock frequency increases, ground tilting/deformation, micro-fracture opening (which alters rock electrical conductivity and P-wave velocities), and changes in well water levels or radon gas concentrations. Currently, no reliable method exists for making short-range predictions.

What are seismic gaps, and what role do they play in paleoseismology and long-range forecasting?

Seismic gaps are locked segments along active faults that have not experienced major ruptures or earthquakes for decades or centuries. Because strain continues to accumulate without release, these gaps are interpreted as the most probable locations for future major earthquakes.

How do different segments of the San Andreas Fault behave regarding earthquake hazards?

1. Creeping segments: exhibit slow, continuous displacement (fault creep) that constantly bleeds off strain, reducing the likelihood of a major earthquake.
2. Regularly slipping segments: slip periodically to produce moderate earthquakes.
3. Locked stick-slip segments: remain locked and accumulate elastic strain for $50\text{ to }200\,\text{years}$ or more before rupturing in great earthquakes (e.g., the southernmost segment, locked for over $300\,\text{years}$, is the most likely source of the next major rupture).