Principles of Tsunami Generation, Mechanics, and Global Hazards

Differentiation from Wind Waves:
- Tsunami are not generated by wind; wind only affects the surface of the ocean.
- Tsunami are typically generated at depth on the ocean floor.
- For a devastating tsunami, the ocean floor must be displaced vertically.
Energy and Water Column:
- Unlike wind-driven surface waves, tsunami energy involves the entire water column.
- The displacement affects water from the ocean floor up to the surface, potentially involving tens of thousands of feet of water.
- This vertical displacement causes the water to "bounce up" and attempt to return to equilibrium, creating surface wave energy proportional to the floor displacement.

Initiation:
- The life of a tsunami begins when the ocean floor is displaced vertically, either pushing the water up or pulling it down (if rocks are down-dropped).
The Split:
- The wave energy travels in multiple directions: toward the nearest coastline and toward distant coastlines across the ocean basin.
- Energy spreads out in all directions from the point of initiation.
Amplification:
- As the energy reaches shallow waters near a coastline, the waves begin to "steepen up."
- In a large tsunami, water is often sucked back from the shoreline toward the incoming energy source. This abnormal, quick recession is a primary warning sign for an impending arrival.
Run-up:
- When the wave becomes too tall and unstable, it rushes forward and "runs up" onto the beach and inland.
- Depending on the tsunami's strength and the terrain (flat vs. mountainous), the water can travel significant distances inland.

Visual Appearance:
- Eyewitness footage (notably from the 2011 Japan event) shows that a tsunami often resembles a fast-moving "rushing tide" or a "splash forward" rather than a single, vertical wall of water.
The Wave Series:
- A tsunami is always a series of waves, not a single individual wave.
- The first wave in the series is usually not the most devastating.
- The water arrives with significant momentum, following one wave after another into the shoreline.

Geologic Context:
- The earthquake occurred along a subduction zone where two plates meet.
- The epicenter was located on the subducting slab at depth.
- The rupture happened along the plate boundary (indicated by the pink line on maps provided in class).
Impact of Depth and Proximity:
- Depth: The earthquake was recorded at a depth of $15.2\,\text{miles}$ . This is considered "extremely shallow."
- Why Depth Matters: At shallow depths, there is less overlying rock to absorb or radiate the displacement energy through cracking. Consequently, the displacement occurs almost entirely at the surface/ocean floor, pushing the water up more effectively.
- Proximity: The nearby coastline of Japan received the brunt of the energy and the most damage because the energy had not yet dissipated.
Distant Impact:
- Tsunami energy was measured across the Pacific Ocean, including Hawaii and the Bay Area.
- In the Bay Area, effects were seen approximately $10\,\text{hours}$ after the initial earthquake. The region sustained damage and one death attributed to the tsunami.
- While energy dissipates as it spreads across the basin (evidenced by color-coded maps where red indicates higher waves and blue/green indicates lower waves), danger remains even at distant points.

Magnitude and Tsunami Triggers:
- Magnitude < $6.5$ : Very unlikely to trigger a tsunami, even if the rupture is at the surface, because the seafloor offset is insufficient.
- Magnitude $6.5$ to $7.5$ : Small changes in sea level may be noticed, but dangerous tsunami are generally not expected unless an underwater landslide is triggered.
- Magnitude $7.6$ to $7.8$ : High enough energy to produce disruptive tsunami near the epicenter; distant areas may see no damage.
- Magnitude \ge $7.9$ : Capable of producing significant sea level changes and damage both near the epicenter and at distant coastlines.
Comparison of the 2011 Event ( $M = 9.0$ ):
- The 2011 Japan earthquake was approximately $30 \times$ stronger than the $1906$ San Francisco earthquake ( $M = 7.8$ ).
- It was $900 \times$ stronger than the $1989$ Loma Prieta earthquake and the $2010$ Haiti earthquake (both roughly $M = 6.8$ to $M = 6.9$ ).

The Pacific Northwest:
- This region contains a subduction zone boundary in Northernmost California, Oregon, Washington, and Canada.
- The area of this boundary is comparable in size to the subduction zone that ruptured in Sumatra in $2004$ .
- It has the capability of producing a $M = 9.0$ earthquake or larger. Since no large event has happened in "historic time," scientists are concerned the region is overdue.
California and the San Andreas Fault:
- The $1906$ San Andreas earthquake was offshore but did not produce a significant tsunami.
- This is because the San Andreas is a lateral (strike-slip) fault. Lateral motion does not push the water column up.
- California’s primary tsunami threats come from:
  1. Large underwater landslides triggered by earthquakes.
  2. Tsunami traveling from distant subduction zones (e.g., Japan or the Pacific Northwest).
Hawaii:
- Described as "Subduction Zone Mecca" because it is centered in the Pacific Rim, which is almost entirely surrounded by subduction zones.
- Hawaii houses the Tsunami Warning Center and is considered one of the most prepared states due to its history of devastating tsunami hits.
The East Coast (Atlantic):
- The danger is significantly lower than the Pacific Rim due to fewer subduction zones.
- Primary risks are underwater landslides or small, distant subduction zones.
- The East Coast faces higher risks from hurricanes than tsunami.

The Role of NOAA:
- The National Oceanic Atmospheric Administration (NOAA) issues automatic warnings based on earthquake magnitude and depth.
- They utilize a system of buoys across the Pacific Ocean to detect energy traveling through the water.
Scientific Advancement:
- Buoys allow for the prediction of travel times and arrival at different shorelines.
- The $2004$ Indian Ocean tsunami resulted in a high death toll partly because a warning system was lacking.
- Increased cell phone technology and cameras have provided scientists with more footage (especially from Japan) to study the physical behavior of tsunami.
Public Safety:
- Evacuation route signage has increased significantly along the California coastline since the $2004$ event.