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!