ASBOG Geologic Hazards 2

Question 1

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

Accepted 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

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Hazard assessment measures lava viscosity; risk assessment measures eruption frequency

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Hazard assessment applies only to active volcanoes; risk assessment applies to all geologic hazards

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Hazard assessment is conducted by volcanologists; risk assessment is conducted only by insurance companies

Question 2

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

Accepted 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

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A lahar is a volcanic bomb ejected from a cinder cone; it remains dangerous because it stays hot for weeks after eruption

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A lahar is a submarine volcanic landslide; it remains dangerous because it generates repeated tsunamis for months

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A lahar is a volcanic gas cloud; it remains dangerous because of long-term carbon dioxide seepage from the edifice

Question 3

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

Accepted 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

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Site amplification refers to strong ground motion near active faults where fault slip amplifies P-waves

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Site amplification is the increase in earthquake magnitude reported at seismic stations close to the epicenter due to proximity

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Site amplification refers to the concentration of soil failure (liquefaction) in thick alluvial deposits, not to shaking intensity

Question 4

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

Accepted 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

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A tsunami is a wind-driven ocean wave generated by offshore hurricanes; its speed is controlled by wave period and is independent of water depth

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A tsunami is a harbor resonance wave caused by distant earthquakes; its speed is controlled by tidal forcing and varies with the position of the moon

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A tsunami is a standing wave trapped in a bay or harbor; its speed increases in shallow water as the seabed friction decreases

Question 5

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

Accepted 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

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Debris flows are dry granular flows; rockfalls are water-saturated mass movements on gentle slopes

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Debris flows are slow-moving (mm/year) slope failures; rockfalls are seismically triggered events only

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Debris flows occur only in volcanic terrain; rockfalls occur only in glacially eroded alpine terrain

Question 6

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

Accepted 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

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Induced seismicity is vibration caused by heavy industrial machinery near active faults; mining operations for coal are the primary cause in the US

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Induced seismicity is seismic noise from wind farms interfering with seismograph recordings; it is a monitoring problem, not an actual earthquake hazard

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Induced seismicity is vibration from traffic and construction; urban infrastructure development is the primary cause globally