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I live in a seismic zone (the Pacific NW), so I "think about" Earthquake Events occasionally.

Knowing how to react, when the building you're in, when itself "starts to react" -- could save your life.

First the Physics ...

Everyone go find your old Slinky's  ...

Remember how Slinky's can "Walk", and how they can act like an "Accordion" too?

Well the "Physics" of the Earth's surface, can do very much the same kind of things. And often does ...

P-waves and S-waves - which are faster?
Berkeley Seismological Laboratory

Compressional waves are also called P-Waves, (P stands for "primary") because they are always the first to arrive. They gave us the first jolt last Friday.

Shear waves propagate more slowly through the Earth than compressional waves and arrive second, hence their name S- or secondary waves. They were responsible for the second rumble.

The difference in arrival time between these two types of seismic waves can be used as a rough estimate of the distance to the earthquake focus.

As a rule of thumb: Multiply the time between the two jolts by 5 and you get the distance to the focus in miles.

1st the Accordion action.

2nd the Walking action.  Check.

and there's a 5-second Delay in between those wave types, for every mile distant you are form the Quake's epicenter.

Hmmm? kind of like Lightning and Thunder, that way.  Except with Lightning, the most dangerous event, strikes first.  With Quakes, the dangerous thing strikes second.

The "Up and Down" one is the most violent event with Quakes.   (vs. the "Side to Side" one, that hits first.)

FAQs -- Measuring Earthquakes
United States Geological Survey’s (USGS)

Earthquake Hazards Program

What is a P wave? An S wave?

The different types of energy waves shake the ground in different ways and also travel through the earth at different velocities. The fastest wave, and therefore the first to arrive at a given location, is called the P wave.

The P wave, or compressional wave, alternately compresses and expands material in the same direction it is traveling.

The S wave is slower than the P wave and arrives next, shaking the ground up and down and back and forth perpendicular to the direction it is traveling.

Surface waves follow the P and S waves.

Primary waves "roll in" first.  Check.

Secondary waves "jolt" you life, next.   They have the potential to "rock your world".

Once that Slinky starts down those steps, there's no stopping it.

But then thirdly, Surface Waves?  they follow up the pack, with a burst of "surface movement" at the end of the event.  They are the final "ripples on the pond" ...

I was unfamiliar with "Surface Waves" til today, here is one description of them.
I suspect if you survive the Shearing "Up and Down" Secondary waves, that the Surface Waves will be "no big shakes".  

Assuming you're clear of "falling hazards" by then, of course.

The Southern California Integrated GPS Network Education Module

The third type of wave, and the slowest, is the surface wave. These waves move close to or on the outside surface of the ground.

[... here's S. Cal's explanation of the P and S waves -- which is a bit more Techie, than previous descriptions above ...]

There are three types of waves that are created when stress is released as energy in earthquakes: P, S, and surface waves. The P wave, or primary wave, is the fastest of the three waves and the first detected by seismographs. They are able to move through both liquid and solid rock. P waves, like sound waves, are compressional waves, which means that they compress and expand matter as they move through it. S waves, or secondary waves, are the waves directly following the P waves. As they move, S waves shear, or cut the rock they travel through sideways at right angles to the direction of motion. S waves cannot travel through liquid because, while liquid can be compressed, it can't shear. S waves are the more dangerous type of waves because they are larger than P waves and produce vertical and horizontal motion in the ground surface.

Now about those "falling hazards" -- it's not the shaking that hurts you (usually) -- it's the stuff that the "shaking" turns into "flying objects" -- Ouch!

Ever have a Bookshelf fall on your head?  (I hope not.)  THAT could ruin your whole day!

Video: Earthquake Preparedness Week - Eliminating Falling Hazards
Tennessee Emergency Management Agency

Yes indeed, I'm glad we still have some Govt Agencies in tact -- those dedicated to informing and protecting the Public.  Having some "common goals" for Society -- it's a wonderful thing.  Despite whatever Tea Party advocates say about "their burden" on you and me, Government DOES do some very useful things, most of the time.

Earthquakes -- Emergency Preparedness and Response
U.S. Department of Labor -- Occupational Safety & Health Administration (OSHA

What hazards are associated with earthquakes?

When an earthquake occurs in a populated area, it may cause deaths and injuries and extensive property damage. Ground movement during an earthquake is seldom the direct cause of death or injury. Most earthquake-related injuries result from collapsing walls, flying glass, and falling objects as a result of the ground shaking, or people trying to move more than a few feet during the shaking. Much of the damage in earthquakes is predictable and preventable.

What can I do to prepare before an earthquake occurs?

 -- Pick "safe places". A safe place could be under a sturdy table or desk or against an interior wall away from windows and bookcases, or tall furniture that could fall on you. The shorter the distance to move to safety, the less likely you will be injured. Injury statistics show that people moving as little as 10 feet during an earthquake's shaking are most likely to be injured.

 -- Practice drop, cover, and hold-on in each safe place. Drop under a sturdy desk or table and hold on to one leg of the table or desk. Protect your eyes by keeping your head down. Practice these actions so that they become an automatic response.

 -- Wait in your safe place until the shaking stops, then check to see if you are hurt. You will be better able to help others if you take care of yourself first, then check the people around you. Move carefully and watch out for things that have fallen or broken, creating hazards. Be ready for aftershocks.

 -- If you must leave a building after the shaking stops, use the stairs, not the elevator. Earthquakes can cause fire alarms and fire sprinklers to go off. You will not be certain whether there is a real threat of fire. As a precaution, use the stairs.

 -- If you're outside in an earthquake, stay outside. Move away from buildings, trees, streetlights, and power lines. Crouch down and cover your head. Many injuries occur within 10 feet of the entrance to buildings. Bricks, roofing, and other materials can fall from buildings, injuring persons nearby. Trees, streetlights, and power lines may also fall, causing damage or injury.

And ... as you probably know IF you live next to Seismic Coastline,

SEEK HIGHER GROUND, whenever Tsunami waves are likely to occur!

Usually there is enough "lead time" after the Jolt, before the Tsunami waves start to roll in, to get up and find somewhere safe -- somewhere that 20 foot+ waves cannot reach.   Tsunami waves move at the speed of a Jet plane -- 500 to 600 miles an hour. (or so I just heard a Reporter on the Oregon Coast say.)

Taking Action, when Nature's warning signs are prompting you to do so, could very well save your life.  

"One Mississippi, Two Mississippi, Three ... Get somewhere Safe!"  LOOK OUT for falling objects, especially around old buildings.

I always knew  --  all that Sciency-book-learning would be good for something, someday!


Where can I find information about faults in my area?
United States Geological Survey’s (USGS) -- Dept of the Interior

 ... brought to you by another one of those Government Agencies that belong to Us, the Public -- Agencies that some in Congress would like to outsource, drown, or dismantle and sell for pieces.  

I suspect those Austere-minded downsizing folks, opted out of most of their Science courses.  Who needs Public Safety anyways, when Corporations are perfectly willing to take care of everything?

Originally posted to Digging up those Facts ... for over 8 years. on Fri Mar 11, 2011 at 10:19 AM PST.

Also republished by SciTech.

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