Lecture 16

Stages of Earthquake-Generated Tsunamis

Tsunamis triggered by undersea earthquakes progress through four distinct primary stages:

• Stage 1: Initiation

  • An earthquake rupture occurring in the seafloor causes vertical displacement of the water column.

  • The tectonic movement pushes water upwards, creating an uplifted dome of water on the ocean surface and initializing the tsunami wave train.

• Stage 2: Split

  • The uplifted water dome collapses under gravity and splits into two separate waves traveling in opposite directions: one heading toward the open ocean and the other heading toward the nearest coastline.

• Stage 3: Amplification

  • In the deep ocean, the tsunami wave moves extremely rapidly at speeds up to $900\,m/h$ ($900\,\text{km/hr}$) while maintaining a low wave height of less than $1\,\text{m}$.

  • As the tsunami approaches shallower coastal waters, frictional drag with the seabed slows the wave speed down to approximately $45\,\text{km/hr}$.

  • The continuous energy behind the wave compresses and squeezes the water column upward, dramatically increasing the overall wave height.

• Stage 4: Runup

  • The elevated tsunami wave heads inland, sweeping over the shoreline and destroying structures, infrastructure, and vegetation in its path.

  • In many instances, the trough of the wave reaches the shore first, resulting in a dramatic drawdown of water that temporarily exposes the seafloor before the main wave surge arrives.

Comparison Between Wind-Caused Waves and Tsunami Waves

Wind-generated surface ocean waves and earthquake-generated tsunami waves differ fundamentally in their fluid dynamics, energy profile, and coastal impact:

• Wind-Caused Waves

  • Motion Dynamics: As wind-driven waves approach shallow water, they rise up and break by rolling forward in closed circular orbital paths ($\text{water rotates in circles}$).

  • Behavior & Frequency: Consist of isolated, discrete waves that come and go quickly across brief time intervals.

  • Surfing Feasibility: Surfing is possible on standard wind-driven ocean waves because they break predictably along the crest. Surfing a tsunami is impossible due to the continuous mass water surge, extreme volume, and turbulent destruction.

• Tsunami Waves

  • Motion Dynamics: Tsunamis do not form standard rolling breakers; instead, the water column flows straight forward ($\text{water flows straight}$) like a rapidly rising, turbulent wall of tide.

  • Energy & Power: Characterized by a quick, massive increase in sea level carrying enormous energy and momentum.

  • Duration: Rather than a single quick wave, a tsunami event consists of multiple surges that can continuously inundate coastal areas for hours.

Tsunami Wave Propagation: Distant vs. Local Tsunamis

When an offshore earthquake lifts the seafloor and creates an initial dome of water, the collapse of that dome generates two separate wave fronts propagating in opposite directions:

• Initial Wave Height Division

  • Both the distant and local tsunamis inherit an initial wave height equal to approximately one-half ($\frac{1}{2}$) of the height of the original uplifted water dome.

• Distant Tsunami

  • Travels outward across deep open ocean basins at high velocities (up to $900\,\text{km/hr}$).

  • Can cross entire ocean basins over thousands of kilometers with very little loss of energy, posing hazards to distant coastlines hours after the seismic event.

• Local Tsunami

  • Travels directly toward the adjacent landmass.

  • Arrives at nearby shores very quickly following the earthquake, giving coastal populations minimal advance warning time.

Landslide-Generated Tsunamis

While the majority of large, destructive tsunamis are produced in subduction zones at tectonic plate boundaries, mass wasting events (landslides) generate localized mega-tsunamis and contribute to seismic tsunami events:

• Submarine Landslides

  • Occur entirely underwater along continental slopes or oceanic trenches, displacing large volumes of seawater vertically as sediment and rock move downward.

• Terrestrial Rock Avalanches and Landslides

  • High-velocity rock avalanches from mountains and cliffs plunge directly into bays, lakes, or oceans.

  • The rapid impact violently displaces the water column vertically.

  • Provides extremely little warning, causing coastal villages or shorelines to be swept away in less than an hour.

Case Study: The 1958 Lituya Bay, Alaska Tsunami

The most extreme recorded landslide-generated tsunami occurred at Lituya Bay, Alaska, in 1958, generating the highest wave splash runup in recorded history:

• Seismic Trigger

  • A magnitude $7.7$ ($\text{M} 7.7$) earthquake triggered a massive rock avalanche from mountains bordering Lituya Bay.

• Displacement Volume and Dimensions

  • Approximately $30.5\text{ million }\text{m}^3$ ($30.5 \times 10^6\,\text{m}^3$) of rock fell from a cliff directly into the ocean bay.

  • The rock slide detaching zone spanned widths between $730\,\text{m}$ and $915\,\text{m}$, sliding from an elevation of up to $915\,\text{m}$ down to water level ($0\,\text{m}$ elevation).

  • The bay inlet spanned approximately $\sim 1350\,\text{m}$ across.

• Impact and Splash Runup

  • The plunge of the massive rock volume instantly displaced a huge volume of bay water.

  • The displaced water splashed upward against the opposite mountain ridge to a peak runup height of $524\,\text{m}$ ($1720\,\text{ft}$) above normal water level.

  • Compared to normal waves (which reach up to $20\,\text{ft}$ high), the maximum wave height at Lituya Bay ($1720\,\text{ft}$) was nearly $1000\times$ larger, completely stripping trees and soil along the bay's perimeter trimline.