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  • This is Dr. Elizabeth Vanacore from the University of Puerto Rico Mayagüez

  • Department of Geology and I work as a seismologist at the Puerto Rico Seismic Network

  • Today I'm going to talk a little bit about the different types of

  • waves you find in an earthquake. An earthquake isn't just a simple explosive

  • source event. So what happens when we have an earthquake you have multiple types of

  • waves produced. There are three major types of waves: one is the P wave or the

  • primary wave, the second one is the S wave or the secondary wave and the third

  • one are the surface waves. Now, these waves have different motions and arrive at

  • different times and so the seismologists like myself can use these arrivals to

  • help locate earthquakes, specifically using the P and the S wave arrival times.

  • Now I'm going to show you the slinky. Some of the basic movement of both

  • p-waves and s-waves with a little assistance, now spread out on this table we have a slinky.

  • The first one I'm gonna show is the P wave. And the P wave is like a compressional wave, same line as

  • propagation or easy way to show it is the slinky you pull back and at the release the rings is like moving back and forwards across the table in line with the slinky.

  • Now the second wave is the S wave. Now the S waves there are something called a Axe vertical and the Axe

  • horizontal so essentially these are the S waves will move perpendicular to the

  • propagation. So in other words they'll move a bit more like a snake along the ground so if they both are

  • moving in and out along the plane where my hand is moving now, that would be

  • horizontal. If we're moving up or down that would be the Axe vertical. To show you what the Axe

  • horizontal then we pull the slinky sideways and It'll bounce back and forth

  • across the table. So those are two of the waves. The third one is are the surface

  • waves and these waves move... There's two different ones. They are Rayleigh waves

  • that moves like ocean waves and Love waves which kind of like the Shear waves

  • moving back and forth and the reason we care about surface waves is because

  • usually surface waves have the largest amplitudes and therefore they are the ones that

  • are most felt and most destructive. So as far as when an earthquake occurs these really

  • waves all move at different rates. The one that is the rabbit in the race is the P

  • wave. It's the fastest and the first to arrive. However the P wave also tend to have a

  • smaller amplitude which means that that's the wave that people don't

  • necessarily feel. Now, The second one in the race are the S waves. It's a little bit

  • slower than the P waves but not as slow as the surface waves. So the S waves

  • usually kind of get a bit higher amplitude than the P waves and then the

  • third one in the race, last one are the surface waves which tend to have the

  • largest amplitudes. To show you an example what this looks like in a

  • seismogram I have a video. This is of our 6.4 and southwest and from the

  • southwest Puerto Rico sequence that we had in January this year and on this

  • video is you'll be able to hear and see an event coming in. Now what does that

  • sound like?

  • So I chose this one because of the difference in time the further

  • away you get from that location the more spread out the waves get because the P

  • wave is faster than the S wave which is faster than the surface wave. So, we have

  • the P wave arriving first, then you have our S wave arriving and then you see

  • this big packet of waves coming through. These are the surface waves. So I've

  • already shown you one of a more distant or regional event where the P wave, the S

  • wave and the surface wave are separated. However what does that look

  • like for a local event like we look measure here at the Puerto Rico Seismic Network

  • Well the reality is if you're looking at a local event, the race hasn't

  • really started really much yet. So they haven't had much spreading out

  • of the different waves due to different velocities. So essentially you

  • have the P wave and S wave and the surface waves are very close to each other so

  • what happens over multiple seconds, um sorry over tens of seconds or 20 or

  • hundreds of seconds in a distant earthquakes occurs within just a few seconds.

  • So I'm showing you here is the 6.4 event recorded in Aguadilla. Now the event

  • itself only goes up to about 1,500 1,600 seconds on this slide and then you

  • can see some of the replicas are the aftershocks coming in afterwards

  • You can hear in this video

  • essentially that 6.4 we heard before a little bit you hear a hit for the

  • P wave and hit for the S wave and then a rumble for the surface waves. All you

  • really hear is that first big boom prevent for that 6.4 and that's because

  • the waves are all very close to each other in time. And it'd be very difficult to

  • spread that out and make it audible for a human to hear cause in order to make seismic

  • signals audible you actually have to speed it up.

  • This video is 500 times faster. The other one was 200 times faster. So with that I just

  • want to do a quick recap. Remember what I told you is there are

  • three major types of waves: P waves, S waves and Surface waves.

  • The P waves are faster than the S waves which are faster than the Surface waves and the

  • largest amplitude waves tend to be the Surface waves therefore and also the

  • most destructive waves, they're the ones you feel the

  • most during the earthquake. So what I want to remind you because you'll be

  • feeling these earthquakes is one: drop cover and hold on and then wait till

  • the shaking has stopped and then once the shaking stops

  • take another deep breath, look around your surroundings and decide what you

  • would do next. Thank you, have a good day!

This is Dr. Elizabeth Vanacore from the University of Puerto Rico Mayagüez

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Earthquake Waves with Dr. Vanacore

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    林宜悉 posted on 2020/03/07
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