Earth Science Quiz: Seismic Hazards

What this quiz covers

This quiz focuses on Seismic Hazards, giving you a quick way to practice the rules, question types, and explanations that matter most for Earth Science.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

Question 1

1. They are evidence of past seismic ground shaking strong enough to cause liquefaction. (correct answer)
2. They represent the locations of ancient tree roots that have been filled in and replaced by sand deposits.
3. They are remnants of drilling activity or human-made boreholes that were backfilled with sand.
4. They were formed by slow, continuous groundwater seepage that preferentially deposited sand in vertical cracks.

Explanation: Sand dikes and sand blows are classic paleoseismic indicators of liquefaction. During intense earthquake shaking, saturated sandy layers can lose their strength and behave as a fluid. The high pore water pressure can force this sand-water slurry upward through cracks in overlying, less permeable layers (like silt and clay), where it solidifies after the shaking stops. This process creates the observed intrusive features. The other explanations are inconsistent with the forceful, intrusive nature of sand dikes.

Question 2

1. Liquefaction of the basin sediments caused the buildings to lose their foundation support, leading to their collapse.
2. The S-waves from the earthquake slowed down as they entered the soft sediments, causing their amplitude to increase to conserve energy. (correct answer)
3. The seismic waves were reflected off the bedrock-sediment boundary, focusing all their energy directly underneath the downtown area.
4. The natural frequency of the tall downtown buildings matched the high frequency of the seismic waves traveling through the bedrock.

Explanation: This phenomenon is known as site amplification. As seismic waves travel from a denser material (bedrock) to a less dense, softer material (sediments), their velocity decreases. To conserve the total energy of the wave, the amplitude of the shaking must increase. This leads to much stronger ground motion on soft soils compared to solid rock. While liquefaction (A) can be a result of this strong shaking, it is the amplification itself that causes the shaking to be stronger in the first place. Reflection (C) occurs, but 'focusing all their energy' is an oversimplification and not the primary mechanism. Resonance (D) involves matching frequencies, but the key error is that waves in soft sediments have lower frequencies (longer periods), not higher frequencies.

Question 3

1. The mean recurrence interval of 210 years is not a useful metric for hazard assessment because the process is too random.
2. The variability indicates that the fault's slip rate is not constant and is currently slowing down.
3. The hazard cannot be considered low simply because the time since the last event is less than the mean interval of 210 years. (correct answer)
4. The next earthquake will most likely occur after an interval of 350 years to balance out the previous shorter intervals.

Explanation: This variability, or aperiodicity, is a critical feature of real fault behavior. The fact that recurrence intervals can be significantly shorter than the average (e.g., 120 years) means that a large earthquake can occur well before the mean of 210 years has passed. Therefore, one cannot assume the hazard is low just because the 'average' time has not elapsed. The probability of an earthquake remains significant throughout the cycle. The mean (A) is still a crucial measure of the long-term activity rate. The variability doesn't necessarily mean the slip rate is changing (B). Choice D represents the 'gambler's fallacy' and is not how physical fault systems behave.

Question 4

1. A single-story, wood-frame house with a natural period of 0.2 seconds.
2. A 5-story commercial building with a natural period of 0.5 seconds.
3. A 20-story high-rise steel-frame building with a natural period of 2.0 seconds. (correct answer)
4. A reinforced concrete parking garage with a very stiff structure and a natural period of 0.1 seconds.

Explanation: Resonance occurs when the natural period of vibration of a structure matches the period of the ground shaking. The sediment basin will preferentially amplify ground motions with a 2-second period. Therefore, a building with a natural period close to 2 seconds will be shaken most violently. As a rule of thumb, a building's natural period in seconds is approximately its number of stories divided by ten. A 20-story building has a natural period of about 20/10 = 2.0 seconds, matching the site period and creating the conditions for severe resonant damage. The other buildings have periods that do not match the site's dominant period.

Question 5

1. Area 1 experienced intense liquefaction, and Area 2 experienced site amplification.
2. Area 1 experienced resonant amplification that matched the buildings' frequencies, while Area 2 experienced severe liquefaction and ground failure. (correct answer)
3. Both areas experienced identical ground shaking, but the buildings in Area 1 were of poorer construction quality.
4. The earthquake's epicenter was directly beneath Area 1, while Area 2 was affected by a secondary tsunami.

Explanation: The damage patterns are classic indicators of two different seismic hazards. In Area 1, the collapse of buildings without significant ground failure points to intense shaking amplified at frequencies that matched the buildings' natural frequencies (resonance). In Area 2, the warped ground, sand boils (ejected sand and water), and floating of buoyant buried objects (pipes) are hallmark signs of liquefaction, where the soil lost its strength and behaved like a fluid. Choice B correctly identifies both phenomena. Choice A reverses the correct causes. Choice C ignores the clear evidence of ground failure in Area 2. A tsunami (D) would cause flooding and scour, not liquefaction features.

Question 6

1. The recurrence interval is 500 years, meaning the fault is overdue for another major earthquake and the risk is extremely high.
2. The recurrence interval is 600 years; since the last event was 150 years ago, the immediate seismic risk is currently low.
3. The recurrence interval of 600 years represents an average, implying a roughly constant probability for a major earthquake occurring in any given year. (correct answer)
4. The recurrence interval is 500 years, which indicates that the probability of a major earthquake is highest in exactly 350 years from now.

Explanation: The recurrence interval is the total time period divided by the number of intervals between events. With 6 events, there are 5 intervals. Thus, the interval is 3000 years / 5 intervals = 600 years. This interval is an average, not a precise timer. It implies a long-term probability of an event occurring, which can be modeled as being roughly constant from year to year. Therefore, the risk is not necessarily 'low' just because it has not been 600 years. Choices A and D incorrectly calculate the interval as 3000/6 = 500. Choice B correctly calculates the interval but incorrectly concludes that the risk is low based on the time since the last event, which misinterprets the probabilistic nature of recurrence.

Question 7

1. Metropolis will experience the most financial damage in the region during the next major earthquake.
2. The ground in Metropolis is expected to shake more intensely during a future earthquake than ground in lower-hazard zones. (correct answer)
3. The building codes in Metropolis are inadequate to withstand the expected level of shaking.
4. A large earthquake is more likely to originate directly beneath Metropolis than in other areas.

Explanation: A seismic hazard map depicts the potential for a natural phenomenon—in this case, strong ground shaking (PGA). A high-hazard zone is an area where stronger shaking is expected in the future. Therefore, B is a direct interpretation of the map. In contrast, 'risk' combines hazard with exposure (population, infrastructure) and vulnerability (construction quality). Financial damage (A) and building code adequacy (C) relate to risk and vulnerability, which are not on the hazard map. The source of the shaking (D) could be a nearby small fault or a distant large fault; the map integrates all sources and only shows the resulting shaking potential at a location.

Question 8

1. Site A will experience greater amplification of high-frequency (short-period) seismic waves. (correct answer)
2. Site B will experience greater amplification of high-frequency (short-period) seismic waves.
3. Both sites will experience identical amplification because they are the same distance from the fault.
4. Site A will experience liquefaction, while Site B will not, due to the thinner sediment layer.

Explanation: The natural period of vibration for a soil layer is proportional to its thickness. A thin sediment layer (Site A) has a short natural period, causing it to resonate with and amplify high-frequency (short-period) seismic waves. A thick sediment layer (Site B) has a long natural period and will amplify low-frequency (long-period) waves. Therefore, Site A will preferentially amplify high-frequency waves. Choice C is incorrect because it ignores local site effects. Choice D makes an unsupported claim about liquefaction, which depends on more factors than just thickness.

Question 9

1. The steady accumulation and periodic release of elastic strain along a locked fault segment. (correct answer)
2. The process of liquefaction, which reoccurs in saturated sediments with each major shaking event.
3. The constant rate of radioactive decay within the Earth's mantle, which drives large-scale plate tectonics.
4. The amplification of seismic waves in soft sediment basins, which causes shaking to repeat in predictable patterns.

Explanation: The physical basis for earthquake recurrence is the elastic rebound theory. Tectonic plates move at a relatively steady rate, causing elastic strain to accumulate along a fault that is locked by friction. When the accumulated strain exceeds the frictional strength of the fault, the fault ruptures in an earthquake, releasing the strain. The process then repeats, leading to a cycle of strain accumulation and release. This cycle is what produces quasi-periodic earthquakes and allows for the calculation of an average recurrence interval. The other options describe effects (B, D) or drivers at the wrong scale (C) for the stick-slip behavior of faults.

Question 10

1. The probability of an M>6.5 earthquake in the next 10 years is significantly higher than it was 100 years ago.
2. According to a simple stationary (Poisson) model, the probability of an M>6.5 earthquake occurring this year is approximately 1/150. (correct answer)
3. No M>6.5 earthquake is likely to occur for at least another 10 years, until the full 150-year interval has passed.
4. The fault has accumulated 93% (140/150) of the strain needed for the next rupture, making an earthquake imminent.

Explanation: The simplest model for earthquake probability, often used for introductory hazard concepts, is a Poisson process. In this model, the event has no 'memory' of past events. The recurrence interval of 150 years defines the average rate, meaning the probability of an event in any given year is constant at 1/150, regardless of when the last one occurred. Choice B correctly states this. Choice A implies a time-dependent model (the 'overdue' concept), which is a common misconception in simpler models. Choice C incorrectly treats the average as a fixed, minimum time. Choice D implies a level of deterministic knowledge about strain accumulation that is not possible to infer from the recurrence interval alone.

Question 11

1. Area 1 experienced intense liquefaction, and Area 2 experienced site amplification.
2. Area 1 experienced resonant amplification that matched the buildings' frequencies, while Area 2 experienced severe liquefaction and ground failure. (correct answer)
3. Both areas experienced identical ground shaking, but the buildings in Area 1 were of poorer construction quality.
4. The earthquake's epicenter was directly beneath Area 1, while Area 2 was affected by a secondary tsunami.

Explanation: The damage patterns are classic indicators of two different seismic hazards. In Area 1, the collapse of buildings without significant ground failure points to intense shaking amplified at frequencies that matched the buildings' natural frequencies (resonance). In Area 2, the warped ground, sand boils (ejected sand and water), and floating of buoyant buried objects (pipes) are hallmark signs of liquefaction, where the soil lost its strength and behaved like a fluid. Choice B correctly identifies both phenomena. Choice A reverses the correct causes. Choice C ignores the clear evidence of ground failure in Area 2. A tsunami (D) would cause flooding and scour, not liquefaction features.

Question 12

1. Valley A, because the low seismic wave velocity in its loose, saturated sediments will cause an increase in wave amplitude. (correct answer)
2. Valley B, because the higher density of its clay deposits will transmit seismic energy more efficiently to the surface.
3. Valley A, because the narrow canyon shape will focus seismic waves through constructive interference.
4. Both valleys will have equal amplification because the sediment thickness is the same.

Explanation: Seismic amplification is primarily controlled by the contrast in properties between bedrock and the overlying soil/sediment, especially the shear wave velocity (Vs). Softer, looser, more saturated materials have a lower Vs. As seismic waves travel from high-velocity bedrock into low-velocity sediment, they slow down and their amplitude increases to conserve energy. The loose, saturated gravel in Valley A will have a significantly lower Vs than the dense, dry clay in Valley B, leading to greater amplification. While topography (C) can have an effect, the material property difference is the dominant factor described. Thickness (D) is important, but not the only factor.

Question 13

1. The recurrence interval on the San Andreas Fault is likely shorter than on the North Anatolian Fault. (correct answer)
2. The recurrence interval on the North Anatolian Fault is likely shorter than on the San Andreas Fault.
3. Both faults will have identical recurrence intervals because they are both major continental strike-slip faults.
4. Recurrence interval cannot be estimated from slip rate; it can only be determined from the historical earthquake catalog.

Explanation: A fault's slip rate represents the rate at which strain accumulates. A higher slip rate means strain builds up faster. To release this strain, earthquakes must occur more frequently. Therefore, the fault with the higher slip rate will have a shorter average recurrence interval, assuming the size of earthquakes is comparable. Since the San Andreas Fault has a higher slip rate (35 mm/yr) than the North Anatolian Fault (25 mm/yr), it will have a shorter recurrence interval. Slip rate is a fundamental input for calculating long-term recurrence, especially when historical records are short, making D incorrect.

Question 14

1. The tectonic plates permanently shifted, causing the entire region to drop in elevation.
2. The intense seismic P-waves compressed the soil particles, permanently reducing their size and causing the ground to sink.
3. The shaking caused the soft soil layers to be ejected from the ground, leaving empty voids that then collapsed.
4. As shaking stopped, the liquefied soil particles settled into a denser, more compact arrangement, reducing the total volume. (correct answer)

Explanation: When you encounter earthquake damage scenarios involving saturated sandy soils and widespread building settlement, you're dealing with soil liquefaction—one of the most destructive secondary effects of seismic activity. During an earthquake, saturated loose sandy soils can temporarily lose their strength and behave like a liquid when shaken. The soil particles become suspended in groundwater, creating a fluid-like mixture. Once the shaking stops, gravity takes over and the suspended particles settle back down into a new, denser configuration. This settling process reduces the total soil volume, causing the ground surface to subside permanently. This explains both the building tilting (differential settlement) and the meter-deep subsidence described in the question, making D correct. Option A misunderstands the scale of tectonic movement—while plates do shift during earthquakes, the localized subsidence described here is far too rapid and concentrated to result from regional tectonic displacement. Option B incorrectly suggests that P-waves physically compress soil particles themselves; seismic waves don't permanently alter particle size, and P-waves alone wouldn't cause the liquefaction necessary for this type of settlement. Option C describes soil ejection, which can occur during liquefaction but represents only a small fraction of the affected soil—the widespread subsidence indicates the bulk of the soil remained in place but compacted. Remember: when you see saturated sandy soils plus earthquake damage involving settlement and tilting, immediately think liquefaction followed by consolidation. The key indicator is that loose, water-saturated sediments rearrange into denser configurations after shaking stops.

Question 15

1. Liquefaction was triggered on the ridge but not in the valley due to differences in groundwater levels.
2. The P-waves were converted to S-waves at the base of the ridge, doubling their amplitude.
3. Seismic waves became trapped within the ridge, reflecting off the steep sides and interfering constructively at the crest. (correct answer)
4. The ridge, being higher in elevation, was physically closer to the earthquake's hypocenter in the crust below.

Explanation: This is an example of topographic amplification. The geometry of landforms like ridges and hilltops can focus seismic wave energy. As waves enter the ridge, they can reflect off the free surfaces (the steep sides) and interfere constructively, leading to significantly stronger shaking at the crest compared to the base or adjacent flat areas. Liquefaction (A) is highly unlikely on a ridge where the water table is deep. Wave conversion (B) is a real process but doesn't fully explain this effect. The difference in distance to the hypocenter due to elevation (D) is negligible and would not cause a substantial difference in shaking intensity.

Question 16

1. Fault Y poses a greater hazard because its earthquakes are far more frequent.
2. Hazard cannot be compared without knowing the exact time since the last earthquake on each fault.
3. The hazards are roughly equivalent because the product of magnitude and recurrence interval is similar for both.
4. Fault X poses a greater hazard because its earthquakes are significantly more powerful, and this larger magnitude often dominates hazard calculations. (correct answer)

Explanation: Seismic hazard assessment requires balancing earthquake magnitude against frequency of occurrence. When evaluating hazard, you need to consider both how often earthquakes occur and how much energy they release when they do occur. The key insight is that earthquake magnitude follows a logarithmic scale. A magnitude 7.0 earthquake releases approximately 32 times more energy than a magnitude 6.0 earthquake. This massive difference in energy translates to significantly stronger ground shaking, greater potential for structural damage, and higher casualties. Fault X produces M7.0 earthquakes every 500 years, while Fault Y produces M6.0 earthquakes every 50 years. Despite Fault Y's 10-times higher frequency, the enormous energy difference means Fault X poses the greater hazard. The devastating potential of the much larger earthquake typically outweighs the advantage of lower frequency in hazard calculations. Option A incorrectly assumes frequency alone determines hazard, ignoring the crucial magnitude difference. Option B suggests timing uncertainty prevents comparison, but hazard assessment uses statistical averages rather than precise timing predictions. Option C proposes that multiplying magnitude by recurrence interval creates equivalent hazards (7×500 vs 6×50), but this mathematical approach doesn't reflect how seismic energy actually scales with magnitude. For earth science exams, remember that earthquake magnitude differences are exponential, not linear. A single unit increase in magnitude represents roughly 32 times more energy release. This exponential scaling means that when comparing earthquake hazards, magnitude differences often dominate over frequency differences, especially for settlements and infrastructure designed to withstand smaller, more frequent events.

Question 17

1. A uniformly sorted, loose deposit of fine sand.
2. A deposit of silty sand with a low plasticity index.
3. A well-graded mixture of sand, gravel, and cobbles with high relative density. (correct answer)
4. A layer of non-plastic silt with a uniform grain size.

Explanation: Liquefaction susceptibility is highest in loose, uniformly sorted (i.e., particles are all the same size), cohesionless materials like fine sands and silts (choices A, B, and D). Conversely, factors that inhibit liquefaction include high density (particles are tightly packed), being well-graded (a wide mix of particle sizes, which promotes interlocking), and the presence of cohesive clay. Choice C describes a soil that is both well-graded (sand, gravel, cobbles) and has high density, making it the most resistant to liquefaction among the options.

Question 18

1. It assumes that sites with lower Vs30 values (like Class E) will experience less amplification due to energy dissipation in the soft soils.
2. It assumes that all site classes experience the same amplification, but that structures on softer soils are inherently weaker.
3. It is primarily used to predict liquefaction potential, with Site Class E being the most likely to liquefy, independent of amplification.
4. It uses Vs30 as a proxy for the potential for ground shaking amplification, with lower Vs30 values requiring structures to be designed for higher seismic forces. (correct answer)

Explanation: When you encounter questions about seismic site classification, think about how different soil and rock types affect earthquake wave propagation. The key relationship is between material stiffness (measured by shear wave velocity) and ground motion amplification. Seismic amplification occurs when earthquake waves slow down and increase in amplitude as they travel from stiff bedrock into softer surface materials. This happens because wave energy is conserved - as velocity decreases, amplitude must increase to maintain the same energy flux. The Vs30 classification system directly captures this relationship: lower shear wave velocities indicate softer materials that will amplify ground motion more significantly. Answer D correctly identifies that Vs30 serves as a proxy for amplification potential, with softer soils (lower Vs30) requiring stronger structural designs to handle the increased seismic forces. This is exactly why building codes use these classifications. Answer A gets the physics backwards - softer soils (lower Vs30) actually amplify motion more, not less. While some energy dissipation occurs, the amplification effect dominates. Answer B incorrectly suggests equal amplification across all sites, missing the fundamental point that different soil types produce vastly different ground motions. Answer C conflates liquefaction with amplification; while soft soils can liquefy, the Vs30 system primarily addresses amplification, not liquefaction potential. Remember this pattern: in seismic engineering, softer always means more amplification. When you see site classification questions, focus on the inverse relationship between soil stiffness and ground motion amplification.

Question 19

1. Valley A, because the low seismic wave velocity in its loose, saturated sediments will cause an increase in wave amplitude. (correct answer)
2. Valley B, because the higher density of its clay deposits will transmit seismic energy more efficiently to the surface.
3. Valley A, because the narrow canyon shape will focus seismic waves through constructive interference.
4. Both valleys will have equal amplification because the sediment thickness is the same.

Explanation: Seismic amplification is primarily controlled by the contrast in properties between bedrock and the overlying soil/sediment, especially the shear wave velocity (Vs). Softer, looser, more saturated materials have a lower Vs. As seismic waves travel from high-velocity bedrock into low-velocity sediment, they slow down and their amplitude increases to conserve energy. The loose, saturated gravel in Valley A will have a significantly lower Vs than the dense, dry clay in Valley B, leading to greater amplification. While topography (C) can have an effect, the material property difference is the dominant factor described. Thickness (D) is important, but not the only factor.

Question 20

1. The slip-predictable model, which posits that the amount of slip in the next event can be predicted from the time elapsed since the last one.
2. The time-predictable model, which posits that the time to the next event can be predicted from the amount of slip in the previous one. (correct answer)
3. The characteristic earthquake model, which posits that the fault always ruptures with the same characteristic slip and recurrence interval.
4. A random (Poisson) model, which posits that the timing and size of earthquakes are independent of the previous event's history.

Explanation: The observation describes a direct relationship between the size (slip) of an earthquake and the length of the following quiet period. This is the basis of the time-predictable model. This model assumes that a fault ruptures when a certain stress level is reached. A larger slip event releases more stress, so it takes a longer time for the constant tectonic loading to build the stress back up to the failure point. The slip-predictable model (A) is the reverse idea. The characteristic (C) and random (D) models do not account for such a systematic relationship between the size of one event and the timing of the next.