- Why is a large-magnitude submarine earthquake along a strike-slip fault significantly less likely to generate a destructive, ocean-wide tsunami than a slightly lower-magnitude earthquake along a subduction zone thrust fault?
Answer: Strike-slip faulting involves predominantly horizontal motion of the seafloor, which does not efficiently displace large volumes of water vertically.
Tsunami generation is overwhelmingly dependent on the vertical displacement of a large volume of water. Subduction zone thrust faults produce this via dip-slip motion (one plate moving up and over another). Strike-slip faults, like the San Andreas Fault, involve two plates sliding horizontally past each other. This horizontal motion is very inefficient at lifting or lowering the overlying water column, and thus even very large strike-slip earthquakes typically generate only small, localized tsunamis, if any.
- A coastal region is protected by extensive offshore coral reefs. While these reefs can be damaged by a tsunami, what effect do they typically have on the tsunami wave itself as it approaches the shore?
Answer: They can dissipate a significant amount of the wave's energy through friction and breaking, reducing its impact on the coast.
Healthy, extensive coastal ecosystems like coral reefs and mangrove forests can act as natural barriers that mitigate tsunami impact. The complex, rough structure of a reef increases friction, forcing the wave to break and dissipating a substantial portion of its energy before it reaches the coastline. This can lead to lower wave heights, reduced inundation distances, and slower floodwater velocities, thus reducing the overall coastal risk. While a powerful tsunami will still overtop and damage the reef, its protective effect can be significant.
- A coastal community receives a tsunami warning and evacuates to higher ground. The first wave arrives and causes moderate flooding, but it is not as large as predicted. Twenty minutes later, a much larger wave arrives, causing catastrophic damage. This sequence of events best illustrates which key principle of tsunami hazards?
Answer: A tsunami consists of a series of waves, known as a wave train, and the first wave is not necessarily the largest.
A common and dangerous misconception is that a tsunami is a single event. In reality, it is a series of waves, or a 'wave train'. The first wave to arrive may be relatively small, leading to a false sense of security. Subsequent waves, arriving minutes to hours later, can be significantly larger and more destructive. Choice A is incorrect as tsunamis are unrelated to tides. Choice B is incorrect because the tsunami is generated by the main shock, not aftershocks. Choice D describes a possible precursor but doesn't explain why a later wave would be larger than the first.
- Residents of a low-lying coastal area observe the sea rapidly and unexpectedly receding far beyond the normal low-tide mark, exposing the sea floor for hundreds of meters. What is the most critical and immediate implication of this observation?
Answer: This indicates the arrival of the tsunami's trough, and a destructive crest is likely to follow within minutes, requiring immediate evacuation.
The rapid withdrawal of the sea is a classic and extremely dangerous precursor to a tsunami. It signifies that the trough (the lowest point) of the tsunami wave has arrived first. The corresponding crest (the highest point), which carries the destructive energy, is typically only minutes behind. This is one of the most important natural warning signs, and the correct response is to evacuate to high ground immediately.
- A tsunami warning system relies on a network of seismometers and deep-ocean DART buoys. What is the primary reason that data from DART buoys is essential for confirming a tsunami threat, rather than relying solely on seismic data?
Answer: Not all large submarine earthquakes with vertical displacement efficiently transfer energy to the water to create a significant tsunami.
Seismometers can quickly detect an earthquake, determine its location, magnitude, and focal mechanism (e.g., vertical motion). This allows scientists to identify an earthquake as potentially tsunamigenic. However, the actual generation of a tsunami depends on the complex details of the seafloor rupture and how efficiently its energy is coupled to the water column. A DART buoy provides direct confirmation by measuring the passage of the actual tsunami wave in the deep ocean. This confirmation is crucial to avoid false alarms and to verify the threat level.
- Following a major tsunami, emergency managers report that the danger to public health is not over even after the floodwaters recede. Which of the following represents a critical secondary hazard directly resulting from the coastal inundation?
Answer: The contamination of drinking water sources and agricultural land by saltwater, sewage, and hazardous materials.
When analyzing tsunami hazards, you need to distinguish between primary effects (the immediate physical impact of the waves) and secondary effects (the ongoing dangers that persist after the water recedes). This question tests your understanding of how natural disasters create cascading problems that extend far beyond the initial event.
The correct answer is A because contamination represents a classic secondary hazard. When tsunami waters surge inland, they pick up sewage from damaged treatment facilities, saltwater that kills crops and contaminates freshwater supplies, industrial chemicals, fuel from vehicles and storage tanks, and debris containing toxic materials. This contaminated mixture then soaks into soil and groundwater systems. Even after floodwaters drain away, this contamination remains, creating serious public health risks including waterborne diseases, poisoned food supplies, and long-term environmental damage that can persist for months or years.
Option B describes another wave in the same tsunami event, which is still part of the primary hazard, not a secondary effect. Option C refers to seismic aftershocks, which are related to the original earthquake that caused the tsunami, not a result of the coastal flooding itself. Option D mentions liquefaction, but this typically occurs during the earthquake phase due to seismic shaking, not as a consequence of the tsunami inundation.
Remember that secondary hazards are the "disaster after the disaster." On earth science exams, look for effects that result from the primary event but create new, different types of problems that outlast the original phenomenon.
- A DART buoy in a 4,000-meter-deep ocean basin detects the passage of a tsunami wave, measuring its speed at approximately 720 km/h and its amplitude at 0.8 meters. As this same wave train approaches a coastline and enters water 40 meters deep, which observation would be most expected at a near-shore sensor?
Answer: A wave speed significantly less than 720 km/h and an amplitude significantly greater than 0.8 meters due to shoaling.
As a tsunami enters shallower water, its propagation speed, which is proportional to the square root of water depth, decreases significantly. To conserve energy, the wave's kinetic energy (from its speed) is converted into potential energy, which results in a dramatic increase in its amplitude (height). This process is known as shoaling. Therefore, the wave's speed will decrease and its amplitude will increase.
- The 1883 eruption of Krakatoa generated a devastating tsunami primarily through the collapse of the volcanic caldera. How would the resulting hazard likely differ from a tsunami generated by a typical M9 subduction zone earthquake?
Answer: The earthquake-generated tsunami would propagate from a long, linear source, while the volcano collapse would act as a point source.
A large subduction zone earthquake generates a tsunami along the entire length of the fault rupture, which can be hundreds of kilometers long. This creates a linear source that directs energy efficiently across an ocean basin. A volcanic collapse, while displacing a massive volume of water, does so from a much more localized or 'point' source. This causes the wave energy to spread out radially and decrease in intensity with distance more rapidly than from a line source. While both can be devastating, this difference in source geometry is fundamental.
- A magnitude 9.0 megathrust earthquake occurs at a subduction zone. The earthquake's epicenter is located 100 km offshore. However, the resulting tsunami appears to originate from a much broader area—a long, linear zone extending for hundreds of kilometers along the trench. What is the best explanation for this observation?
Answer: The epicenter only marks the starting point of the rupture, which then propagates for a great distance along the fault plane, displacing the entire seafloor above it.
This question tests the distinction between an earthquake's epicenter and its rupture area. The epicenter is the point on the Earth's surface directly above where the earthquake rupture begins (the hypocenter). In a large megathrust event, this rupture then 'un-zips' along the fault plane for hundreds of kilometers. The entire overlying block of seafloor is what uplifts or subsides, displacing the water column. Thus, the tsunami's source is not a point (the epicenter) but a large rectangular area corresponding to the fault rupture.
- The 1960 Valdivia, Chile earthquake (M9.5) generated a tsunami that caused fatalities in Japan, over 17,000 km away. What property of a tsunami in the deep ocean is most responsible for its ability to travel across entire ocean basins with minimal energy loss?
Answer: Its extremely long wavelength, which means its energy extends through the entire water column and experiences little frictional loss at the deep seafloor.
The key to a tsunami's ability to travel vast distances is its incredibly long wavelength (often hundreds of kilometers) in the deep ocean. A wave's energy is only significantly affected by friction with the seabed when the water depth is less than about half its wavelength. Because a tsunami's wavelength is so much greater than the ocean depth, it effectively does not 'feel' the bottom and thus loses very little energy to friction as it propagates, allowing it to traverse entire oceans.
- An M8.2 earthquake occurs offshore. Seismologists quickly issue a tsunami warning for nearby coastlines. Which of the following seismological observations would provide the strongest justification for their rapid decision?
Answer: The focal mechanism solution indicated predominantly vertical motion on a shallow-angle thrust fault.
The primary mechanism for tsunami generation by an earthquake is the large-scale vertical displacement of the seafloor. A shallow-angle thrust fault, characteristic of subduction zones, produces exactly this kind of upward motion over a large area, efficiently lifting the water column above it. A deep focus (A) is less likely to cause significant seafloor deformation. Horizontal motion (B) displaces water sideways but not vertically, making it inefficient at generating tsunamis. A long rupture (D) is a factor in the tsunami's size, but the type of motion (vertical) is the essential prerequisite for generation.
- Consider the tsunami risk to the west coast of the United States from two major fault systems: the San Andreas Fault and the Cascadia Subduction Zone. How do the primary tsunami threats from these two sources compare for a coastal city?
Answer: The Cascadia Subduction Zone poses a major tsunami risk due to its potential for a large megathrust earthquake, while the San Andreas Fault poses a minimal direct tsunami risk.
This question requires applying knowledge of fault types to a real-world setting. The Cascadia Subduction Zone is a long thrust fault capable of producing a M9+ megathrust earthquake, which would cause massive vertical seafloor displacement and a devastating tsunami along the entire west coast. The San Andreas Fault is a classic strike-slip fault. While a large earthquake on it would be devastating due to ground shaking, its horizontal motion would not generate a significant tsunami directly. It might cause submarine landslides that could create smaller, localized tsunamis, but the primary threat is from Cascadia.
- Which of the following is a necessary condition for an earthquake to generate a powerful tsunami, but is not, on its own, a sufficient condition?
Answer: The earthquake must involve significant vertical displacement of the seafloor.
Vertical displacement (dip-slip faulting) is absolutely necessary to generate a tsunami; horizontal (strike-slip) motion is ineffective. However, this condition is not sufficient by itself. A small M6.0 earthquake might occur on a thrust fault, causing some vertical displacement, but it would not displace enough water to create a powerful tsunami. To be sufficient, the vertical displacement must be large in both offset and area, which requires a high-magnitude earthquake (typically M7.5+). Therefore, vertical displacement is necessary but not sufficient.
- A post-tsunami survey of a coastline reveals that a V-shaped, narrow bay experienced wave run-up of 15 meters, while a broad, open beach just 5 km away experienced run-up of only 5 meters from the same tsunami. Which phenomenon best accounts for this significant difference?
Answer: Topographic amplification, where the funnel-like shape of the bay constricts the wave's energy into a smaller area, increasing its height.
Coastal geography plays a critical role in tsunami risk. Bays, inlets, and harbors, particularly those that are V-shaped or funnel-like, can dramatically amplify the height of an incoming tsunami. As the wave enters the narrowing and shallowing bay, its energy is squeezed into a progressively smaller volume of water, forcing the wave amplitude (height) to increase significantly. This is a form of topographic or bathymetric amplification.
- Seismologists determine that the seafloor rupture that generated a tsunami was approximately 400 km long and 100 km wide. This information about the source dimensions is most directly useful for tsunami modelers in determining which initial parameter of the tsunami wave?
Answer: The initial wavelength, which is fundamentally controlled by the dimensions of the displaced block of seafloor.
The physical size of the source of displacement on the seafloor is the primary factor that determines the initial wavelength of the tsunami. A larger block of uplifted seafloor will create a wave with a longer wavelength. While the amount of vertical slip over this area determines the initial amplitude (A), and the speed (C) is determined by water depth, the length and width of the rupture area directly constrain the initial wavelengths of the waves that propagate away from the source.
- A geological survey of the seafloor near a densely populated coastal region reveals a previously unknown fault. Seismic data show it is a large, shallow-angle thrust fault. Core samples indicate major slip events have occurred on this fault roughly every 500 years, with the last one estimated to be 480 years ago. Which statement best synthesizes the tsunami risk for this region?
Answer: The risk is high; the fault type is tsunamigenic and the recurrence interval suggests significant elastic strain has accumulated.
This scenario combines several key indicators of high tsunami risk. First, the fault is a shallow-angle thrust fault, the primary type that generates large tsunamis. Second, there is evidence of regular, large slip events (a recurrence interval). Third, the time since the last event is very close to the average recurrence interval, implying that the fault may be late in its seismic cycle and has accumulated significant strain, making a future large earthquake plausible. This combination points to a high-risk situation.
- The diagram below shows two conceptual models of coastal bathymetry. A tsunami is approaching both coastlines from the left. Which statement correctly predicts the relative tsunami impact?
Answer: Coastline X will be more impacted because the submarine canyon will focus the tsunami's energy toward the city.
Submarine topography (bathymetry) significantly influences tsunami impact. Submarine canyons can act as channels that focus wave energy, leading to much higher wave heights and greater inundation at the head of the canyon. Conversely, submarine ridges (underwater mountains) tend to block and reflect wave energy, providing a degree of protection to the coastline directly behind them. Therefore, Coastline X is at much greater risk due to the focusing effect of the canyon.
- The Lituya Bay, Alaska, tsunami in 1958 was triggered by a massive rockslide into a fjord, causing a wave with a record run-up height of 524 meters. How does the primary generation mechanism of this event explain the extreme but localized nature of the hazard?
Answer: The sudden impact of a large mass of rock into a confined body of water generated a wave with immense initial amplitude that could not propagate far.
When you encounter tsunami questions, focus on the relationship between the generation mechanism, the local environment, and how wave energy behaves in different settings.
The 1958 Lituya Bay tsunami demonstrates how landslide-generated tsunamis differ from earthquake-generated ones. When the massive rockslide hit the water, it instantly displaced an enormous volume, creating a wave with exceptional initial amplitude. However, this type of tsunami has limited propagation ability because the energy is concentrated in the initial impact zone rather than being transmitted efficiently across ocean basins like earthquake tsunamis. The confined fjord setting meant this immense energy had nowhere to spread laterally, resulting in the record-breaking 524-meter run-up height but remaining geographically constrained.
Option B incorrectly suggests tidal amplification was the primary factor, but the rockslide's massive displacement dwarfed any tidal influence. Option C gets the mechanism backwards—the rockslide actually displaced a huge volume of water, not a small one, and while the narrow fjord did contribute to focusing energy, the primary factor was the enormous initial displacement. Option D misidentifies the generation mechanism entirely; while an earthquake may have triggered the rockslide, the tsunami's extreme height resulted from the direct water displacement by falling rock, not ground shaking.
Remember that landslide tsunamis typically create localized but extremely dangerous waves because they generate maximum energy at the source but don't efficiently transmit that energy over long distances like seismic sea waves do.
