Tsunami Hazard Zone Study Notes
Tsunami Hazard Zone
What is a Tsunami?
Definition: A tsunami is a large wave (or series of waves) formed by a sudden and widespread movement of the Earth's crust, displacing the water above it.
Causes of Tsunamis:
Commonly Generated by Earthquakes: Most tsunamis are generated during earthquakes.
Other Causes: Landslides and submarine volcanic eruptions can also form tsunamis.
Notable Tsunami Events:
Tohoku, Japan (2011)
Indonesia (2004)
Tonga (2022)
Russia (2025)
Characteristics of Tsunamis
Travel Efficiency: Tsunamis can efficiently travel across entire ocean basins, affecting communities far from their sources.
Historical Example:
1946 Aleutian Island Earthquake:
Magnitude: M8.6 earthquake struck the Alaskan Aleutian Islands at 4 AM.
Travel Time: Approximately 5 hours later, the tsunami reached Hawaii, causing widespread flooding and resulting in the death of 159 people.
Consequences: This event led to the formation of the first US Tsunami Warning Center, which is now one of two within the US and initiated a push to better understand tsunamis and improve forecasting.
Formation of a Tsunami
Requirements for Generation:
A sudden and widespread deformation of the seafloor is essential for generating a tsunami.
Tsunamis often occur along plate boundaries, particularly at subduction zones.
Factors Influencing Deformation:
The degree of deformation depends on:
Fault geometry
Depth
Amount of slip
Visual Representation:
Left: Map view of a fault with low dip angle (depth = 25 km) and uniform slip.
Right: Expected deformation on the seafloor (red = uplift, blue = subsidence).
Impact of Earthquake Depth on Tsunami Generation
Parameters:
Strike = 0°, Depth = 14°
Slip = 1 m
Depth (center point): 25 km, 50 km, 75 km
Observation:
For the same fault with identical geometry/orientation and slip, surface deformation decreases as depth increases.
Implication: Depth of the earthquake significantly impacts the tsunami generation.
Slip Patterns and Complexity in Tsunami Generation
Source Description:
Increasing complexity in the earthquake source leads to increased complexity in the tsunami generated.
Example:
M8.2 Chignik, Alaska earthquake (July 2021) demonstrates how large earthquakes rupture across a fault, deforming the crust and perturbing the water surface, thus generating a tsunami.
Properties of Tsunamis
Anatomy of a Wave
Crest: Maximum point above mean sea level.
Trough: Maximum point below mean sea level.
Wavelength: Distance from two crests (or troughs).
Wave Height: Elevation of wave from crest to trough.
Amplitude: Elevation of wave from mean sea level to crest.
Difference from Typical Waves
Deep Water Waves:
Occur when the ocean depth is greater than the wavelength (e.g., Wind-driven beach waves).
Ocean water depth: ~4,000 - 6,000 m; Wavelengths: ~meters.
Shallow Water Waves:
Occur when the ocean depth is significantly less than the wavelength (e.g., Tsunamis).
Ocean water depth: ~4,000 - 6,000 m; Wavelengths: 50-100 km.
Wave Speed Calculation
Formula:
Where:
= tsunami wave speed
= acceleration due to gravity
= water column height.
Observation: Tsunamis move faster in open ocean and slow down as they approach land, traveling nearly at the speed of a large airplane and slowing down to the speed of a car in traffic near shorelines.
Shoaling Effect
Definition: Shoaling refers to the increase in wave amplitude and decrease in wavelength that occurs as a tsunami approaches a coastline.
Transition in Size: A tsunami that is less than 1 m in the open ocean can shoal to over 1 m along coastlines.
Characteristics of Tsunami Wash: Unlike typical beach waves, tsunamis typically do not "break" and instead flood coastlines with a surge.
Edge Waves
Description: Once a tsunami reaches the coastline, it can become 'trapped', reflecting off the coast and continental shelf, forming edge waves.
Hazards: The periodic reflection can mean hazardous waves may persist for many hours after the tsunami's initial arrival. Positive interference can result in larger tsunami waves arriving later, rather than as the initial wave.
Example of Simulation: Simulation of a tsunami arriving at the California coast shows increased amplitude as the tsunami approaches the coast compared to open ocean.
Measuring Tsunamis
Open Ocean Measurements
DART (Deep-Ocean Assessment and Reporting of Tsunamis) Gauges:
Positioned on the seafloor to record water column pressure.
Pressure increases when a tsunami passes, allowing for conversion to water column height calculations.
Typically deployed in depths greater than 4,000 m to avoid damage and signal contamination from seismic activity.
Near Shore Measurements
Tide Gauges:
Used along coastlines for accurate tsunami observations as the wave shoals and inundates land.
These measurements are subject to noise from the environment, making them more challenging for earthquake source studies.
Real-time monitoring by Tsunami Warning Centers for forecasting.
Example Data from Tide Gauges: Tide gauge recordings of the 2022 Manzanillo tsunami.
On Land Measurements
Post-event Measurements Include:
Wave Height: Difference between tsunami amplitude and mean sea level.
Flow Depth: Height of tsunami amplitude above ground level.
Inundation Distance: Furthest horizontal distance the tsunami traveled inland.
Run-up Elevation: Highest point the tsunami reached on land.
Paleotsunami Studies
Analysis:
Studies track ancient tsunami deposits in environments with low energy.
Includes examining subsidence along coast, paleo-tsunami, and liquefaction deposits.
Sources of Tsunamis
Earthquake-triggered Tsunamis
Historical Record:
The 1700 Cascadia Earthquake had records of tsunami damage with no determined source initially.
Studies analyzing tsunami deposits around Oregon and Washington indicate the last major event was approximately 300 years ago, with an average recurrence interval of about 500 years.
Landslide-triggered Tsunamis
Types of Landslide Tsunamis:
Generated when a landslide displaces water (either subaerially or submarine).
The tsunami's formation depends on the landslide's volume and velocity.
1968 Lituya Bay Example: A landslide triggered by a M7.8 earthquake generated a tsunami with a run-up of 524 m, the largest recorded.
Ongoing Concerns: Recent observations indicate potential hazards from retreating glaciers destabilizing conditions, like in the Barry Arm.
Volcanic Eruptions as Tsunami Sources
2018 Sunda Strait Tsunami:
Resulted from the collapse of Anak Krakatau volcano flank, reaching neighboring islands quickly and resulting in significant damage.
2022 Hunga Tonga Tsunami: Featured explosive volcanic activity generating a global tsunami, marking one of the largest volcanically generated tsunamis since the 1883 Krakatoa eruption.
Tsunami Preparedness
Hazard Analyses
Conducted Studies: Probabilistic tsunami hazard analyses examine the likelihood of a tsunami affecting a specific coastal area over a duration of time, facilitating the creation of evacuation maps and shelter placement.
Community Safety Guidelines
Actions During an Earthquake:
If shaking is felt, evacuate inland immediately.
If water recedes, do not wait for an official alert, evacuate.
Avoidance Advice: Do not approach the coast during expected tsunami activity.
Understanding Tsunami Alerts
**Types of Alerts:
Warning: A tsunami is occurring or imminent; dangerous flooding expected.
Advisory: Strong currents may be hazardous to those in the water; caution advised.
Watch: Possible tsunami occurrence; public and emergency officials alerted.
Information Statement: No immediate threat detected. No action needed.
Response During a Tsunami Emergency
Steps to Take:
In an earthquake, drop, take cover, and hold on.
Move to higher ground if near the coastline and shaking persists.
Stay inland; do not return to coastal areas for several hours or until cleared by emergency management.
Potential Persistence: Tsunami waves can last several hours, and strong currents can pose risks even from smaller waves.