GEOL101 - Faults and Earthquakes

Flashcards reviewing forces, stresses, strains, fault classifications, earthquake mechanisms, seismology, magnitude scales, and intensity measurements from GEOL101.

How many detectable earthquakes occur globally each year, and what primary force causes them?

There are more than 1 million detectable earthquakes per year. Earth shaking is caused by a rapid release of energy, most of which is due to tectonic forces.

What depth and magnitude characterized the January 2010 Haiti earthquake, and why was it so destructive?

The January 2010 Haiti earthquake had a magnitude of $7.0$ and occurred only $13\,km$ ($6\,miles$) below the earth. Its shallow epicenter caused extensive surface damage and 316,000 deaths.

What economic impact did the 2011 Honshu earthquake and Fukushima disaster in Japan have on global energy markets?

In the aftermath of the Fukushima disaster caused by the Honshu earthquake, uranium prices dropped $60\%$ in the next 3 years.

According to Newton's Second Law of Motion, how is force mathematically defined?

Force is defined as mass multiplied by acceleration ($F = ma$), where acceleration is the rate of increase in velocity.

How is stress defined mathematically, and how does surface area affect the stress exerted by a given force?

Stress is defined as force divided by area ($\text{Stress} = \frac{\text{Force}}{\text{Area}}$). A force distributed over a smaller area results in greater stress.

What is the physical distinction between pressure and directional stress?

Pressure occurs when an object experiences equal stress on all sides from every direction (such as a scuba diver underwater), whereas directional stress acts unevenly across different directions.

What is strain in geological terms?

Strain is the change in shape that an object or rock body experiences in response to an applied stress.

How do tension, compression, and shear stresses deform rocks?

Tension pulls ends apart (stretching and thinning material), compression squeezes material (shortening and thickening material), and shear slides surfaces past one another (neither thickening nor thinning the crust).

How are fault planes defined, and what is the difference between a footwall block and a hanging-wall block?

Faults are planar breaks in the crust (mostly sloping dip-slip faults). The footwall block is the body of rock below the fault plane, while the hanging-wall block is the body of rock above the fault plane.

Based on the provided diagram of earthquake origins, what is the difference between the focus (hypocenter) and the epicenter?

The hypocenter (or focus) is the exact underground location where fault slip occurs on the fault surface, while the epicenter is the location on the land surface directly above the hypocenter.

What stress type produces a normal fault, and how do the crustal blocks move relative to each other?

Tensional stresses in the Earth's crust lead to normal faults, in which the hanging wall slips downward relative to the footwall. They are most common at divergent plate boundaries.

What stress type produces a reverse fault, and how do the crustal blocks move relative to each other?

Compressive stresses in the Earth's crust lead to reverse faults, in which the hanging wall is pushed upward relative to the footwall. They are most common at convergent plate boundaries.

How do you distinguish between a right-lateral and a left-lateral strike-slip fault?

For a right-lateral strike-slip fault, a person standing on one side of the fault observes the rock on the opposite side move to their right. For a left-lateral strike-slip fault, the rock on the opposite side moves to their left.

What type of fault is the San Andreas Fault, and how much movement has occurred along it over the past 20 million years?

The San Andreas Fault is a right-lateral strike-slip fault, evidenced by offset streams, that has undergone $500\,km$ of fault movement in $20\text{ million years}$.

What is 'stick-slip behavior' in fault dynamics?

Stick-slip behavior describes cyclical fault movement where friction prevents motion ('stick') until friction is briefly overwhelmed by movement ('slip'), releasing accumulated elastic strain before locking again.

What are asperities along a fault plane, and how do they relate to foreshocks, main shocks, and aftershocks?

Asperities are bumps along a fault plane that slow movement. As stress builds, asperities break, causing a sequence of small earthquakes (foreshocks) that build to a larger earthquake (main shock), followed by smaller earthquakes (aftershocks).

What is InSAR, and how is it used to measure strain in rocks?

InSAR (interferometric synthetic aperture radar) is a satellite technique that compares ground elevation changes over time to create maps displaying ground distortion and elastic strain buildup as color bands.

According to this seismogram record, what is the order of arrival for seismic waves generated by an earthquake?

P-waves arrive first, followed by S-waves, and finally surface waves, which produce the largest amplitude ground vibrations.

What fundamental relationship between P-waves and S-waves allows seismologists to determine distance to an epicenter?

P-waves move faster than S-waves. Therefore, the time lag between the first arrival of the P-wave and the first arrival of the S-wave ($S - P$ time) increases systematically with distance from the earthquake.

In the triangulation method shown in the diagram, how is the epicenter of an earthquake determined using three seismic stations?

Circles with radii corresponding to calculated distances from three seismic stations (e.g., $2,000\,km$, $4,000\,km$, and $6,000\,km$) are drawn, and the point where all three circles intersect is the epicenter.

How does the Richter magnitude scale quantify earthquake magnitude?

The Richter scale is a logarithmic scale based on the maximum ground displacement in micrometers ($10^{-6}\,m$) measured by a standard seismometer at a distance of $100\,km$ from the epicenter.

How much more energy is released when earthquake magnitude increases by $1.0$ on the moment magnitude scale?

An increase of $1.0$ in moment magnitude ($M_w$) represents a $32\times$ increase in energy release.

What three main factors control the intensity of ground shaking experienced in a given area during an earthquake?

Shaking intensity depends on: 1) the energy released during the earthquake, 2) the distance from the site to the hypocenter, and 3) the type of bedrock (ground or substrate) where shaking is felt.

What does the Modified Mercalli Intensity scale measure, and how is its data gathered?

The Modified Mercalli Intensity scale is a subjective determination that assigns Roman numerals (I to XII) based on observed damage severity and eyewitness reports of shaking intensity.

Why was the 1886 Charleston earthquake ($M_w = 7.3$) felt with a Modified Mercalli Intensity of V as far away as Chicago and Indianapolis?

It was recorded as intensity V due to the energy released, the regional bedrock/substrate characteristics, and the amplification of shaking in city streets and taller buildings.