NWS Monterey: What is a Tsunami and How is it Generated?

This presentation covers the basics of Tsunamis
here in California. We will learn more about what a tsunami is, and how it is generated,
as well as how earthquakes are related to tsunamis and what types of hazards these can
bring to California. What does a tsunami look like? Each tsunami
is unique. A tsunami moves with incredible speed potentially moving more than 500 miles
per hour over the deep ocean. In the ocean the wave can be only a few feet high but as
it gets closer to land, the water begins to rise. It looks more like a river in flood
stage than a wave that can be surfed. Tsunamis look different from the normal waves
that we see at the beach. The tsunami moves very fast across the open ocean and then slows
down as it gets close to shore. A tsunami is better described as a surge of water instead
of a wave that breaks. The water may flow onto land in a surge for 10 minutes or more,
then the water will rush back out. When the water rushes out the current can be just as
strong as when it rushed in. Let’s learn first about earthquakes and
the types of earthquakes that can occur in California. There are two different kinds
of faults. The first is called a “strike slip” fault where the two different plates
will move side by side in opposite directions. The famous San Andreas Fault is this type
of fault. The second type of fault is called a “Subduction Fault” this is where one
plate will slide underneath of another plate. There is a fault called the “Cascadia Subduction
Zone” which runs under the Pacific Ocean from Northern California to British Columbia.
Both kinds can produce large and destructive earthquakes. The Subduction fault is more
likely to produce stronger earthquakes and larger tsunamis.
Some tsunamis are created by earthquakes that are far away. These tsunamis are known as
“distant source tsunamis”. The area many many far away earthquakes occur is the Pacific
“Ring of Fire”. These orange areas show the faults in the earth’s crust where earthquakes
are typically generated. The orange area near the Pacific Northwest is the “Cascadia Subduction
Zone”. This is a subduction fault where the plate beneath the ocean slides under the
plate beneath the continent, this action discplaces a large amount of water and is more likely
to produce a destructive tsunami. Here in California we can also have earthquakes
generated locally, in addition to the far away earthquakes. In the Monterey Bay region,
there are numerous faults that are present. The San Andreas Fault is the most commonly
known. It has produced several large and destructive earthquakes throughout history. This includes
the 1906 San Francisco Earthquake and the 1989 Loma Prieta Earthquake. Fortunately for
the area, these faults are not “subduction faults” which means they are not likely
to produce large and destructive tsunamis. One particular hazard for this area would
be from a local earthquake that produces an underwater landslide. There is an underwater
feature in Monterey Bay, known as the Monterey Canyon. A large earthquake in this area could
produce a landslide into this canyon. This could produce a large tsunami in Monterey
Bay. The numbers here show possibly how high a tsunami could be in the Monterey Bay area
from one of these underwater landslides. This type of tsunami is dangerous because it would
take only 10 to 15 minutes to reach the coast. If the type of landslide shown here occurred,
some of the highest tsunami inundation would be observed in Monterey and could do damage
to the water front areas of the city. Scientists have found evidence that this type of landslide
has happened in the past and it is possible that it could happen again in the future.
If you are in this area and you feel an earthquake, you should move away from the water’s edge
to safer areas. Here is a summary of what we’ve covered
today. Tsunamis are a strong surge of water. They are not and do not look like a wave that
can be surfed. The tsunami is often filled with dangerous debris. A Tsunami is often
generated by an earthquake. It can be generated from an earthquake that is far away, but can
also be created from a local earthquake here in California. Not every earthquake will generate
a tsunami. Strong earthquakes are more likely to generate a tsunami. The type of fault that
is most likely to generate a tsunami is called a “subduction fault”. This type of fault
will produce a large displacement of water generating a tsunami.
Here are some sources on the web for Tsunami Information. The National Weather Service
is a good place to learn more about the weather and types of weather hazards that can impact
you on a daily basis. The West Coast – Alaska Tsunami Warning Center is responsible for
monitoring all earthquakes in the area and they will send out information on tsunami
alerts, watches, or warnings, if a tsunami is expected to occur. And finally, the marine
debris program can provide you more information on debris in the ocean, including debris that
may be present in the Pacific from the large tsunami occurring in Japan in 2011.
We want to give special thanks to the California Emergency Management Agency and the California
Geological Survey for their assistance in creating this presentation. These agencies
help our local communities and citizens to better understand and prepare for tsunami
threats here along the coast of California. Visit the National Weather Service Websites
for the latest and most up to date forecasts and watches and warnings. The National Weather
Service covers all of the United States, and our office in Monterey provides information
for both the San Francisco and Monterey Bay areas. Information is available through various
social media outlets as well.

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