Arizona Earthquakes

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Three earthquakes struck late last night, centered near Black Canyon City north of Phoenix. Tom Sharp of Arizona State University’s School of Earth & Space Exploration will tell us more.

TED SIMONS: Good evening and welcome to "Arizona Horizon," I'm Ted Simons. Arizona become earthquake country for a few hours last night as three quakes registering from 3.2 to 4.1 on the Richter Scale made for an interesting evening. Here with more on last night's seismic activity is Tom Sharp from ASU's School of Earth and Space Exploration. Thanks so much for joining us, we appreciate it.

TOM SHARP: Oh, thanks for asking me.

TED SIMONS: So let's start with where these quakes were located.

TOM SHARP: So they were located just north-northeast of Black Canyon City. A little bit east of I-17, making kind of a north-south line from Black Canyon City.

TED SIMONS: And how deep in the ground were these?

TOM SHARP: They were fairly shallow. The pre-quake was quite shallow, three kilometers and the others were about five kilometers.

TED SIMONS: So the 3.2, we're looking at the map right here, the 3.2 was relatively shallow?

TOM SHARP: Yes, that was the shallowest of them.

TED SIMONS: Does it matter how deep these things are in terms of how we feel them?

TOM SHARP: It does some but it doesn't matter that much. In this case it really wouldn't matter for us in the valley. They are all fairly shallow.

TED SIMONS: These are fairly shallow as earthquakes go.


TED SIMONS: What parts of the valley and the state that you understand, what parts shook the most?

TOM SHARP: It would have shaken most in that area around Black Canyon City in the north part of the valley. People felt it all the way from north of Flagstaff to south of Casa Grande. It was a large area.

TED SIMONS: More or less shaking if you live in a low-lying area?

TOM SHARP: If you live in a sedimentary basin type of area you tend to have more shaking than if you live on bedrock. Most of us live in a great basin in the valley and very few people actually live on bedrock so it wouldn't have made much difference around the valley.

TED SIMONS: The lower elevations, I guess near rivers and things like that, you're going to feel it right?

TOM SHARP: Not necessarily more than the other areas in town.

TED SIMONS: But like places like Fountain Hills and hilly areas? More so, less so?

TOM SHARP: Probably a bit less.

TED SIMONS: Okay. As far as this area that we looked at getting hit by an earthquake, how uncommon? What are you looking at?

TOM SHARP: Earthquakes of this magnitude are pretty uncommon in Arizona. We get a few. It turns out we have a lot of very small earthquakes that we don't normally notice. So a few years ago, a program called earth scope came through with lots of seismometers and we were measuring all kinds of small-scale seismic activity in the state. We know there are lots of little earthquakes on these faults but not many large ones.

TED SIMONS: Was this a recognizable fault? It looked like they were lined up in a row.

TOM SHARP: There was no mapped active fault there so it really isn't a fault that we know of. The nearest fault to there is the Horseshoe Fault along Horseshoe Reservoir and the Verde Valley. And a very similar pattern, north south sort of fault about 30 miles east.

TED SIMONS: Is this a kind of situation where now we know there is a fault there or could this be a very, very old fault line that just recently decided to wake up?

TOM SHARP: It's probably or it could be an older fault that we didn't recognize as active that has been reactivated. There are lots of old faults throughout the area.

TED SIMONS: And as far as -- I asked how uncommon it would be for this part of the state to get this kind of activity. Magnitude 4.0, 4.1, how common is that?

TOM SHARP: I don't really have the data on it but we don't see much that's larger than that in Arizona. There have been a few of that magnitude.

TED SIMONS: So I think you gave this away earlier but I always wonder about these things. Were these earthquakes, were these possibly foreshocks to something to come? Were these aftershocks of something happening a while ago that we weren't aware of? What do you think?

TOM SHARP: The interpretation is that the first one was a foreshock, the largest one was the main shock and the last one was an aftershock and we've been getting aftershocks since then but they've been too small to notice and too small to locate with the seismometers that are out.

TED SIMONS: Do these kinds of things come in bunches like this, three in a couple of hours?

TOM SHARP: They do. When the fault ruptures, you have a main event and smaller event around that rupture zone.

TED SIMONS: And so with that in mind, you say we're getting more all the time in terms of aftershocks but should we expect anything in the threes and fours related to this?

TOM SHARP: I doubt it I think it will be trailing off.

TED SIMONS: Having gone through the 1989 quake personally, there was a lot of these kinds of questions. It happened October 17th, it was a warm day in San Francisco. This one happens in early November. Is there such a thing as earthquake season? Is there such a thing as earthquake weather? Do high-pressure systems, low-pressure systems make any difference at all?

TOM SHARP: Not that we know of. If there was a difference, it would be very insignificant.

TED SIMONS: So basically, if you're trying to predict earthquakes, good luck fella?

TOM SHARP: No, the prediction comes from understanding the tectonic stresses in the crust and recognizing whether there is recent motion on faults. So what people will do is study these faults and look at their history and you can excavate the faults and see what the earthquake history is and from that, predict what its likelihood of activity is.

TED SIMONS: But when the biggest is 4.1 and the aftershock is 4.0 and you don't really know where the fault is, there's not a heck of lot more information to get there, is there?

TOM SHARP: No, and this one didn't rupture the surface so it's not obvious that there's a fault to be mapped from this one.

TED SIMONS: Did you feel the earthquake?

TOM SHARP: I did not, I slept through it.

TED SIMONS: You slept through it! Thank you so much for joining us, we appreciate it.

TOM SHARP: You're welcome.

Tom Sharp : Arizona State University's School of Earth & Space Exploration

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