Unit 4 Tsunami Generation

Overview
1 Introduction to
   Tsunamis
2 Tsunamis of
   the Past
3 Plate Tectonics
4 Tsunami Generation
5 Tsunami Propagation
6 Tsunami Inundation
7 Tsunami Aftermath
   and Response

4.1 Causes of Tsunamis


In this unit, learn about how tsunamis are generated.

Essential Question: How are tsunamis generated?
Enduring Understanding: Tsunamis are generated by the massive displacement of water caused by earthquakes, landslides, and volcanic eruptions.



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Overview

Earth is a dynamic system; earthquakes, slope failure and volcanoes are some of the obvious and visible results of unseen forces and processes continually interacting within the planet. One of the dramatic and potentially dangerous outcomes from these activities is the production of tsunamis. If earthquakes, landslides or volcanic eruptions occur with enough force and displace a large volume of water, a tsunami will be produced.


Every year on average Earth experiences more than 30,000 earthquakes strong enough to be felt by people. While earthquakes are a primary cause of tsunamis, only a fraction of the earthquakes produced annually will result in tsunamis. Earthquakes that produce tsunamis occur underwater or near the coast and usually measure above magnitude 6. Not all tsunamis result in damage or fatalities. From 2000-2010, 113 tsunamis were recorded, 12 of which resulted in fatalities.

Each tsunami event leads to greater understanding of the physical processes of Earth and water, reveals an urgent need for better warning systems, and underscores the importance of continuous education and disaster preparedness in coastal communities.






4.2 Earthquakes


What is an Earthquake?

An earthquake is a vibration of Earth produced by the rapid release of energy in the form of waves, which radiate in all directions from the focus, or source, of the earthquake. The energy dissipates as it moves farther from the source. Seismometers, sensitive instruments, are able to record even small earthquakes from a great distance.


A Modern Concept of Earthquakes

Prior to 1906, the cause of earthquakes was still debated. After the Great San Francisco Earthquake of 1906, scientist H.F. Reid deduced tectonic forces slowly deform the crust on both sides of the San Andreas fault.

Reid proposed moving plates become locked together and build up strain in rocks, storing potential energy. Eventually the friction resistance holding the plates together is overwhelmed, and the plates slip past each other starting at the weakest point, called the focus, producing an earthquake. The rocks rebound and plates may shift, releasing energy in the form of waves. This moves the stress further along the fault, where sooner or later another earthquake will occur. 

Most earthquakes occur below the surface of Earth. The below ground location of the earthquake is called the focus. The epicenter is directly above the focus on the surface.

 

 


4.3 Earthquake Waves


The shaking ground associated with an earthquake is the result of P (primary), S (secondary), and surface waves. The waves travel at different speeds and through different substances. Generally, the intensity of shaking increases with magnitude and decreases with distance from the epicenter.


Locating the Epicenter

The epicenter is the location on the surface directly above the underground source, or focus, of the earthquake.

Comparing the time it takes for P-and S-waves to arrive at a monitoring station gives researchers the ability to identify the distance to the epicenter. To visualize this,imagine a circle is drawn around the location of the monitoring station.

Scientists can pinpoint the location of the epicenter by finding the distance between the epicenter and at least three different seismometers. The intersection of three rings around each monitoring location indicates the epicenter.

Rapid identification of earthquake epicenters helps inform people responsible for responding to hazards. Locating epicenters leads to a greater understanding of Earth’s plate activity and potential tsunami sources.


Earthquakes and Tsunamis

If an earthquake of magnitude 6 or larger occurs near the coast or underwater, it may produce a tsunami. Earthquakes can generate tsunamis when they cause a sudden uplift in the seafloor or change the shape of the seafloor by triggering landslides. In the Pacific Ocean, earthquakes generate 83% of tsunamis. Locating the epicenter of an undersea earthquake gives an indication of where a tsunami may have originated.

Scientists consider three important factors when determining the likelihood of an earthquake to generate a tsunami:
1. They determine whether the earthquake had a magnitude greater
    than 6.0;
2. They consider the total area of seafloor disturbance; and
3. determine the amount of vertical displacement caused by the earthquake.



4.4 Measuring Earthquake Energy

Measuring the size of earthquakes allows for a common method or standardized language to compare different events, prepare for the future, and develop structures that can withstand the forces exerted by the earthquake.

Seismic Recording

Seismometers are instruments that record earthquake activity. Chang Heng, a Chinese astronomer and mathematician, invented an early seismic recorder in 132 CE. This beautiful and utilitarian cast bronze vessel depicts eight dragons holding balls in their mouths poised to drop into the open mouth of frogs waiting below. This device could detect an earthquake event, but not the strength of the earthquake, the direction from which it came, or the location of the epicenter.

Modern seismographs record seismic waves such as P, S and surface waves. Early seismographs were functionally simple devices consisting of a weight suspended within a stable enclosure. When seismic waves shake the earth the enclosure moves, but the weight remains stable. Recording devices as simple as ink and paper can record these movements. Modern digital seismometers are much more sensitive than previous versions and can be widely distributed to increase earthquake data recording.

One example of distributed seismographs is the Quake Catcher Network, currently comprised of 1450 citizen scientists who record seismographic data collected by laptop motion or USB sensors. This data provides seismic scientists with a worldwide network of seismographic data.



  Intensity and Magnitude Scale

Intensity Scales

Intensity scales measure earthquakes based on observations of how strong the shaking is felt and surface damage. The Modified Mercalli Intensity Scale has 12 steps differentiating how clearly the earthquake was felt by “few,” “nearly all” or “all” persons, and whether or not damage was slight or considerable. These scales are site-specific measurements that do not allow for quantitative comparisons between regions with different building types and population densities.


Magnitude Scales

Magnitude is a measure of the energy of waves, similar to how a thermometer measures temperature. The first widely accepted magnitude scale was developed in 1935 by Richter, Beno, and Guttenburg and called the Richter scale. The Richter scale uses seismograms and not survivor’s observations to measure earthquakes.

The Richter scale is logarithmic, so that a magnitude 6.0 earthquake has 10 times the shaking effect and releases 31.6 times the energy of a magnitude 5.0 earthquake. An 8.0 earthquake has 1,000 times the shaking effect and releases 31,600 times more energy as a 5.0 earthquake, and so on.

The Moment Magnitude scale was developed in 1979 by Caltech scientists to address the inability of the Richter scale to accurately measure earthquakes greater than 7. The Moment Magnitude scale is calculated by both seismometer data and field measurements of the rigidity of the crust and the amount of slippage the fault experienced.  For medium-sized earthquakes the Moment and Richter scales are equivalent, but the Moment Magnitude scale is now the accepted standard for measuring large earthquakes worldwide.



4.5 1960 Chile


The largest recorded earthquake

Location         Chile, 39.5W, 74.5W
Date and time         May 22, 1960, 7:11 PM
Magnitude         9.5
Tsunami generated         Yes
Dead or Missing         1,886
Injured         > 3,000
Homeless         2 million
Damage         more than $675,500,000

The 9.5 magnitude 1960 Chile earthquake is the largest ever recorded. It was three times bigger than the Japan 2011 earthquake and produced 5.6 times more energy. Four earlier earthquakes, or foreshocks, bigger than magnitude 7.0 preceded the main quake. The resulting tsunami waves, killed people in Chile, Hawai‘i, Japan, and the Philippines, and caused over a half billion dollars worth of damage in many coastal communities along the Pacific Rim.

How did it happen?

The Nazca plate slid 15 meters beneath the South American continent, causing a rupture over a 1000 kilometer section of the fault that reshaped the lower slopes of the continent and ocean floor. In southern Chile, closest to the epicenter, most of the casualties were due to the resulting tsunami.

Impacts on Hilo, Hawai‘i

In Hawai‘i, the most damage and deaths occurred in Hilo on the Big Island, which previously had been devastated by the 1946 April Fool’s Day tsunami from Alaska’s Aleutian Islands. Two small tsunamis in the 1950s, which caused little to no damage, may have made the populace complacent.

In Hilo, curious residents flocked to the waterfront to witness the tsunami, forecasted to arrive around midnight. The tsunami arrived later than predicted with the third and largest wave catching many onlookers in the rising water. The Hilo main business district along Kamehameha Avenue and the residential areas of Waiākea and Shinmachi were completely destroyed. Property damage was estimated as high as $50 million, and 61 people lost their lives. Survivors tell harrowing stories of narrow escapes.

 



4.6 Slope Failure and Volcanoes


Slope failure is a potential tsunami trigger and is the second most common cause of tsunamis. Landslides, rockfalls, avalanches and slumps are all examples of slope failure and occur above and below water. Slope failure tsunami triggers include earthquakes, volcanic and human activity.

So-called surprise tsunamis are those generated by slope failure caused by small earthquakes that may not alert scientists to tsunami danger. A massive slope failure may produce a larger tsunami than an earthquake due to the amount of water that is displaced.

Submarine Slope Failure

Above Water Slope Failure

Lituya Bay, Alaska

Lituya Bay, Alaska is the site of many past tsunamis. The Fairweather fault, a strike-slip fault, crosses the head of the bay and is surrounded by steep slopes, magnifying the effect of tsunamis. On July 10, 1958, a magnitude 7.9 earthquake triggered an enormous landslide at the head of the bay. This landslide triggered a tsunami that reached 525 meters in height, killing two people.

Lituya Bay has a history of earthquakes, landslides, tsunamis, and violent tidal action. The Tlingit Indians who lived around this area for thousands of years have stories that describe the danger of this locale.

 


Loihi

Thirty-five kilometers off the southern coast of the Big Island of Hawai‘i under 975 meters of water lies Loihi, the youngest volcano of the island chain. Loihi means “long” in Hawaiian, which describes the shape of the volcano and seamount, topped by a 4-kilometer-wide caldera.

In 1996, Loihi experienced intense seismic activity producing over 4,000 earthquakes, more than any other volcano on the Hawaiian Islands. While the earthquakes were mostly in the 3.0-4.0 magnitude range and not considered severe, there was the possibility the caldera could rapidly collapse and produce a tsunami. Fortunately, the caldera underwent a gradual settling and formed crater 1 kilometer long and 300 meters deep.


Human-induced Tsunamis
On April 11, 1994, construction in the Skagway harbor in Alaska triggered a landslide, which in turn triggered a tsunami wave 11 meters tall. The wave caused $21 million in property damage and killed one person.

Volcano and Tsunami Triggers

Volcano tsunami triggers
Volcanoes are vents in Earth’s crust through which lava, gas and ash are forced out. Most volcanoes form over subduction or divergent zones. Some volcanoes such as the Hawaiian Islands form above hotspots where narrow plumes of magma rise to Earth’s surface.

Volcanic activity known to produce tsunamis, includes earthquakes accompanying eruptions, pyroclastic flows, submarine explosions, calderas collapse or volcanic landslides

Kilauea, Hawai‘i
In 1975, Kilauea, on the Hawaiian Islands, produced a volcano-generated earthquake with a magnitude of 7.2, triggering a tsunami. Two people died from the tsunami in Halapē, Hawaii where the maximum water height reached 14.6 meters.

Krakatau
Krakatau, a volcanic island in Java, violently erupted in 1883, reducing the island to one third its previous size. The eruption produced pyroclastic flows that crossed several miles of open water and destroyed communities on neighboring islands. The collapse of the volcano, pyroclastic flows, and displacement from the eruption produced tsunami waves that claimed at least 36,000 lives throughout the region.


4.7 Review


Take the following practice quiz to review content covered in Unit 4.

  1. What geophysical process is responsible for more than 80% of tsunamis produced in Earth's oceans?





  1. Where is an earthquake's epicenter located?
    Between the core and the mantle




  1. How do scientists determine the distance from a seismic recording station to an earthquake's epicenter?





  1. What is the minimum number of seismic recording stations required to locate an epicenter?





  1. What are intensity earthquake scales used to measure?





  1. What was one of the limitations of the Richter scale that the Moment magnitude scale corrected?





  1. Why are logarithmic scales used in earthquake measurements?





  1. How far away did damage occur from the tsunami produced by the largest earthquake ever recorded (Chile, 1960)?





  1. How is it possible for small earthquakes to produce surprisingly large tsunamis?





  1. When may undersea volcanoes produce tsunamis?