Geologic Hazards Flashcards

What is the difference between volcanic hazard assessment and volcanic risk assessment?

Answer: Hazard assessment evaluates the probability and potential impact of volcanic phenomena (lava flows, pyroclastic density currents, lahars, tephra); risk assessment combines hazard with vulnerability and exposure of human and built environment to estimate potential losses

Volcanic hazard assessment maps the probability and spatial extent of volcanic processes; risk equals hazard multiplied by vulnerability and exposure — how many people and assets are in harm's way and how susceptible they are to damage.

What is a lahar, and under what conditions do lahars remain dangerous long after a volcanic eruption has ended?

Answer: A lahar is a volcanic mudflow or debris flow of water and volcanic material; it can remain hazardous for years because loose volcanic debris on slopes is repeatedly remobilized by rainfall into rivers and valleys downstream of the volcano

Lahars are volcanic mudflows composed of water mixed with pyroclastic debris; they can persist for years to decades as rainfall mobilizes unstabilized tephra deposits on volcano flanks into rivers, causing repeated floods and channel aggradation far from the volcano.

In the context of earthquake ground shaking, what is site amplification and which type of site is most susceptible?

Answer: Site amplification refers to the magnification of ground motion by local soil or rock conditions relative to a reference bedrock site; soft sediments (loose sand, saturated clay, bay mud) amplify shaking most due to their low shear-wave velocity and impedance contrast with underlying rock

Site amplification is the local increase in ground shaking intensity due to trapping and resonance of seismic waves in soft sediment overlying bedrock; sites with soft, thick, low-velocity sediments (San Francisco Bay mud, Mexico City lake sediments) amplify ground motion most severely.

What is a tsunami, and how does the wave speed of a tsunami in the open ocean relate to water depth?

Answer: A tsunami is a large ocean wave generated by rapid displacement of the seafloor (earthquake, submarine landslide, volcanic flank collapse); its speed in the open ocean follows v = √(gd), where g is gravitational acceleration and d is water depth — reaching ~800 km/h in the deep ocean

Tsunamis are long-wavelength ocean waves generated by seafloor displacement; their speed is given by the shallow-water wave approximation v = √(gd), reaching ~800 km/h in deep ocean (4,000 m depth) and slowing to 30–50 km/h in shallow coastal water while growing in height.

What is the difference between a debris flow and a rockfall in terms of geologic hazard characteristics?

Answer: Debris flows are fast-moving, water-saturated mixtures of sediment and organic material that travel in channels; rockfalls involve free-falling, bouncing, or rolling of individual blocks detached from a cliff or steep rock face — both are rapid and difficult to warn against

Debris flows are rapid, channelized flows of water-saturated sediment and organic debris; rockfalls involve the free fall or rolling of detached rock blocks from steep cliffs. Both are rapid mass movements but differ in mechanism, volume, and travel path.

What is 'induced seismicity,' and which industrial activity has been most widely linked to significant induced earthquake sequences in the United States?

Answer: Induced seismicity is earthquakes triggered by human activities; wastewater disposal (injection of produced water from oil and gas operations into deep disposal wells) has caused the most significant induced earthquake sequences in the central US since 2008

Induced seismicity refers to earthquakes triggered by human activities; deep-well injection of wastewater (primarily produced water from oil and gas operations) into crystalline basement aquifers in the central US has been responsible for a dramatic increase in earthquake rates, including M5+ events in Oklahoma, Kansas, and Ohio.