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When is an Earthquake not an Earthquake?

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Detailed Description

We sit down with USGS Senior Science Advisor for Earthquakes and Geologic Hazards David Applegate to talk about some of the subtle nuances and uses of seismic networks.




Public Domain.


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Music credit:

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Dave Hebert

Hello there, and welcome to the USGS CoreCast for September 13, 2007. I'm Dave Hebert.

Before we get started, I want to give a quick shout out and thank you to those of you listening across the U.S. and around the world in places like France, Japan, Iran, Sri Lanka, the U.K., Argentina, Canada, Taiwan, and everywhere else. Thanks for listening, and tell your friends about CoreCast.

Without further adieu, I'd like to welcome today's guest and geologic hazards supergenius Davis Applegate, Senior Science Advisor for Earthquakes and Geologic Hazards here at the USGS. Dave, thanks so much for joining us.

David Applegate

Well, thank you.

Dave Hebert

Now, we have brought Dave here today to address the question of when is an earthquake not an earthquake and to talk about some of the subtle nuances of earthquake detection. Should be fun and enlightening.

Dave, as you well know, the past month or so has been a busy one, seismically, with a major earthquake off the coast of Peru, the tragic events in Utah, volcanic activity in Alaska, and the list goes on and on.

The USGS has been involved in some capacity in all of these events, and many others, but the Utah event in particular seems to touch on an angle of seismic science that I haven't seen discussed too often, and that is what are some of the odd or out-of-the-ordinary things the USGS seismic network has been involved in detecting?

David Applegate

Well, as you know, our primary purpose in maintaining seismic networks, both nationally and globally, is for reporting on earthquakes, but indeed these very sensitive networks of seismic stations that are distributed around the globe pick up any manner of earth shaking that takes place.

One of the more common types that is recorded just as an earthquake would be recorder is mine blasts, particularly in quarries and open-pit mining. So those typically take place as many as 50 a day, the large concentration in the Powder River Basin of Wyoming...the big open-pit mines there. And those are recorded...some of the blasts can be as large as a magnitude-4 event.

Dave Hebert

Oh, wow. So fairly significant in terms of how it registers on the network.

David Applegate

Absolutely. Our networks also pick up activity within mines, so as was the case with the Utah mine collapse, these types of rock bursts, or as there referred to in coal mining as "coal bumps," are picked up, both the large ones by our national network but also regional networks, such as the one that's operated by the University of Utah for that Utah region of the country . . . pick up much larger number of those.

There, instead of there being many a day, we're looking instead at maybe 8 to10 of these events per year that are picked up by our national network.

Dave Hebert

OK. I've also, in doing some digging and some reading, I've noticed some other interesting events around the world that have involved our seismic networks, including nuclear testing. Is that correct?

David Applegate

Absolutely. It's important to realize that the global seismographic network that the USGS operates in partnership with the National Science Foundation and the IRIS Consortium has multiple purposes.

USGS's involvement in it is because of the ability to detect earthquakes worldwide and report on them from our National Earthquake Information Center. The National Science Foundation's involvement reflects the tremendous research tool for seismology that having a global network presents.

And then finally, this network is a secondary network for nuclear test ban monitoring. And so there is a long history going back to the precursor network to the global seismic network, the World-Wide Standardized Seismographic Network, or the WWSSN, was established back in the 1960s specifically for monitoring nuclear explosions.

Dave Hebert

Oh, OK. I was not aware of that. Very interesting. I guess . . . certainly a compelling topic in the 1960s.

David Applegate

Absolutely, and most recently the network reported the event in North Korea, which was then subsequently confirmed by the Department of Energy as having been a nuclear blast.

Dave Hebert

OK. Just quickly; a couple of the other things I've heard about or read about . . . the World Trade Center collapse registered on the networks, and I saw on ESPN that they were actually . . . they brought seismographs to drag races to measure how . . . what the dragsters put off. Just a couple of the other compelling items that were picked up by our networks.

David Applegate

Absolutely. One of the interesting things that networks pick up is not in the kind of individual events such as you describe, but just in the background noise that exists on these seismic stations. They can pick up things like, for example, storms. [Dave: Wow]

So one of the things that the scientists are looking at is not the event themselves but what happens in between to see if there are changes and trends. So seismic networks can actually help us to look climate change. These networks have been in place for decades, and we can look at storm intensity shifts over that timeframe. So there's some very interesting research that can be done with these continuously operating, fully open and available data streams.

Dave Hebert

Oh, wow. I had no idea-that's very interesting. Very cool stuff. How often do these sorts of things happen compared to actual earthquakes in terms of what we register. Do we have a number on the total events that our network detects in a day or a month or a year? And I know you talked about mine blasts and whatnot . . . but what are the percentages in terms of how these show up versus earthquakes and how many hits we get?

David Applegate

Well, we record about 30,000 earthquakes a year on our our National Earthquake Information Center. So that's using the national network, that's using the global network, and also using data that comes in subsequently from many, many partner networks operated around the world.

Our job is, we want to get a report out very quick using all the available real-time data. But then there's additional data that comes in that's very important from a research standpoint. We want to capture the whole event. And that's about 30,000 a year at a magnitude-2-and-a-half or greater. Our regional then networks pick up even more events—smaller events—in their area of responsibility. So some pretty big numbers in terms of events. It's an active planet that we live on.

These other kinds of events, typically-sort of human-induced events...I mentioned the number of, the really multiple per day of these mine blasts. With the events such as mine explosions, we actually maintain a separate catalog from our earthquake catalog. And there's about one event every 2 days that's large enough that we put...we can actually get an accurate location of the event, and then we put that into the mine explosion catalog.

Dave Hebert

OK. So fairly regular, but clearly not as frequent as the number of earthquakes we register.

David Applegate

That's right.

Dave Hebert

When a non-earthquake seismic event happens, can we tell that it isn't an earthquake, and if so, do we have any way of determining what it is?

David Applegate

Well, usually, these kind of events have a very different signal. For a typical earthquake, it's being generated by a fault, and so you've got two sides of a fault moving against one another. So you get sort of a shearing motion.

So there are a number of different kinds of waves that are generated from an earthquake. There are p-waves, or primary or pressure waves, as well as s-waves, which are secondary or shear waves. P-waves travel much faster-those are like sound waves, they move straight at you [Dave Hebert: OK.]—whereas the s-waves are moving in the opposite direction, perpendicular to the direction of transport.

Those shear waves are very pronounced with a typical earthquake, but with, for example, an explosion like a blast, that's all coming out from one single point and emanating out. So it's got a very big p-signal and a much smaller s-signal.

So there are a number of ways looking at the seismograms themselves...and again, all of this data is made freely and openly available as soon as it comes out. It's one of the real strengths of our seismic data collection is that a scientist anywhere in the world can look at this data and can make judgments about it.

Dave Hebert

Sure. Is that what's posted at

David Applegate

Well, the information on the events themselves is at, but we actually rely on a very important partner, the IRIS Consortium, which is a university consortium funded by the National Science Foundation to maintain all the data archives. So that's a very key partnership for us and a wonderful resource for the academic seismology community.

Dave Hebert

Oh, sure. Sure. In these non-earthquake situations, do we have an official role? Is there some capacity in which we automatically come into play when there is a seismically registered or network-registered event that isn't an earthquake, or does it just depend on the situation?

David Applegate

Well, it's somewhat situation dependant. Our specific statutory responsibility is for earthquakes is for earthquakes themselves. Obviously we're also a resource for other kinds of events—specifically when it comes to things like blasts that could have a potentially nuclear origin, there are other entities that have responsibility for that.

And so all we're doing is making the data available for a seismic event, and then it falls to others to identify the specifics associated with that event. But that said, we really are a key resource—it's a unique role that the USGS plays with this global reporting of earthquakes and taking advantage of the wonderful networks that are operated not only by the USGS, but again, by so many partners.

Dave Hebert

Great—thanks very much. You've touched on this a little bit, but in addition to our statutory mandate for earthquake monitoring, do we have any other roles in seismic monitoring in the United States and around the world?

David Applegate

In addition to having this responsibility for reporting on earthquakes, we also have the similar statutory responsibility for other geologic hazards, including volcanoes and landslides. And another very important aspect of our seismic monitoring is seismic monitoring that's done through our Volcano Hazards Program at active volcanoes. And there are a number of key ways that we can monitor the activity in these active centers, and seismic is one of them.

We also rely on other types of data, both for earthquakes and for volcanoes, and that's 

. . . a good example would be Geodyssey, or using the global positioning system of satellites to measure the actual deformation in the Earth's crust that takes place when, say, a volcano is inflating or when an earthquake moves a piece of crust. So we rely on a lot more than just more than seismic, but seismic has proved to just be a tremendous tool.

Dave Hebert

Well, that about does it for this episode of CoreCast. I'd like to thank Dave Applegate again for joining us, and I'd like to thank all of you out there for listening.

If you'd like to know more about some of the topics we've discussed today, please check out or We also have a great Natural Hazards Gateway that covers information on a variety of natural hazards, and you can find that

Also, make sure you check out some of the other episodes of CoreCast by going to and clicking the podcasts tab in the upper-right corner, or you can check us out on iTunes.

CoreCast is a product of the U.S. Geological Survey, Department of the Interior. Until next time, I'm Dave Hebert—thanks for listening, and take it easy.

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Music credit:

"When the Levee Breaks" by Kansas Joe McCoy and Memphis Minnie

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