Aftershock Hazards in Haiti

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David Hebert: Hello and welcome to this USGS podcast, I'm David Hebert. On Wednesday of January 20th, a magnitude 5.9 aftershock struck Haiti which was alarming and notable considering the magnitude seven earthquake that devastated that country last week. So I'm here with Mike Blanpied, USGS Associate Earthquake Hazards Program Coordinator and he's going to fill us in on sort of the earthquake picture in the near future for Haiti and for the Caribbean at large.

Mike, I know you're very busy, thank you for joining us once again.

Mike Blanpied: I'm very happy to be here.

David Hebert: Let's start with the aftershocks. Again there was a significant aftershock yesterday. There had been other notable aftershocks since the main earthquake last week. Can you just talk a little bit about the aftershock picture, you know, what sort of probabilities are we looking at, how big, that kind of thing?

Mike Blanpied: Yes. Aftershocks have continued since immediately after the main magnitude seven earthquake.

01:03

As is always the case for large earthquakes, there are a number of aftershocks. It turns out that this sequence turns out to be particularly vigorous. We're seeing more aftershocks and we would expect on average say in California. There have been at least 15 aftershocks of magnitude five or larger over the nine days since the main shock.

Looking forward for the next few days and weeks, we expect that the aftershock sequence will continue. Of course we can't say exactly where or when any particular aftershock will occur. However, we can look at the sequence and using simple established models for the rate and statistics of aftershocks, we can make a projection of how likely aftershocks are at various sizes over the coming weeks.

For example, if we just look for the month ahead, we give in a probability that there will be more aftershocks that magnitude five level or above. That's about 90%.

02:01

In other words, it's almost assured that we'll see more aftershocks of that size and we may see even two or three of those. At a higher level of magnitude, magnitude six, we've not seen one of those in this sequence. We've seen five nine but not quite up to the six level and there's probably a one in four chance that we will see a magnitude six earthquake sometime in the next month.

And of course, the trouble there is that an earthquake of that magnitude can cause considerable damage especially to the weakened structures already existing in the region due to the previous earthquake.

David Hebert: OK, now with that in mind, what kinds of precaution should people in Haiti be taking right now?

Mike Blanpied: Right now people need to be extremely cautious when working around structures that are still standing or those that have been damaged in the main shock. If you're on the ground in Haiti, you must maintain some awareness of the situation with regard to your personal safety. You need to be aware that the ground could shake at any time so if you're working in or around structures, be ready to take quick action to take your self to safety if the ground begins to shake.

03:09

People should be aware that really it takes an authoritative engineer who's qualified in structural design to determine if a damaged building is safe to re-occupy. In some cases, they maybe but if you're not sure, the best guess is the structure that appears to be damaged is probably unsafe.

David Hebert: So what are some of the short-term concerns for Haiti as far as seismic activity goes?

Mike Blanpied: Right now scientists from USGS and elsewhere are working very hard to gain a better understanding of exactly how the earth behaved in the earthquake of nine days ago. The earthquake occurred on a very long fault; the Enriquillo fault zone that stretches through Dominican Republic, Haiti and then out to sea toward Jamaica, only a portion of that fault ruptured in the earthquake of magnitude seven.

04:04

This means that other sections of the fault which have not slipped recently remained hazardous and we're looking at a variety of types of evidence from a space photography, LiDAR and other types of investigations in order to determine what that fault looks like, how long ago it was that it slipped, how hazardous it is, so we can make an improved assessment of how likely it is that earthquakes will occur in the future.

David Hebert: Most transition were long-term picture both in terms of the infrastructure there in Haiti in trying to rebuild and geologic hazards on that fault system that you mentioned, what are some of the long-term concerns?

Mike Blanpied: Well, looking at the seismic situation in Haiti, it is part of the seismic zone that cuts through the Caribbean and Haiti itself has two major plate boundary fault zones; this one and one farther to the north.

05:06

And over the past centuries, three to four centuries, there had been several earthquakes of the size of the one last Tuesday that if they occurred today would cause considerable damage. And it's really been a century or in some cases, two or three centuries since portions of these faults have ruptured, meaning that they remain highly stressed today and pose a hazard.

As Haiti works hard to recover from this earthquake and begin the process of rebuilding, it's very important that there be an improved understanding of the seismic hazard that exists due to these large faults and there needs to be a sufficient understanding about the likely shaking in the urban area and elsewhere in Haiti so that building codes can be developed that will safeguard these buildings such that they don't fall down when the earth shakes some time in the future.

05:58

In the coming years and coming decades there will certainly be other earthquakes in the area. We can't say when, we can't say where but it is very likely that during the lifetime of buildings that are built now and the recovery from this earthquake, they will be shaking. And we need to make sure that those buildings don't fall down and hurt people when that shaking occurs.

David Hebert: So let's expand the picture a little bit here and look at the Caribbean as a whole. What sort of hazards are presented in the Caribbean as region?

Mike Blanpied: Well, looking at the Caribbean as a region, as you know it's an arch of violence, the Lesser Antilles and the Greater Antilles and what those islands represent is a boundary between two large tectonic plates; those of the Caribbean and North America. So this entire region is seismic reactive because these two plates are sheering past each other.

We talk frequently about a ring of fire around the Pacific due to the plate boundaries there, well, this is a little ring of fire located just to the southeast of the United States.

06:58

So every one of these countries has a certain degree of seismic hazard and looking forward now that this earthquake has highlighted so dramatically for us that there are these large risks. It's really incumbent upon everyone to make sure that earthquake safety policy is founded on a really a good understanding that there is a seismic hazard and a good assessment about what that hazard is and that goes both for the hazard of shaking and also that for tsunamis which can also occur.

David Hebert: Well, thank you again for your time, Mike. Is there anything else that you like to add or mention that you haven't had a chance to yet?

Mike Blanpied: Oh, I just like to conclude by saying that we in the United States are working hard to understand this earthquake and help in the recovery process. Our hearts go out to those who are living in the affected area.

David Hebert: Absolutely. Thank you very much for your time, Mike. Thank you to all of you out there for listening. If you'd like more information on USGS earthquake research and the earthquakes that are happening around the world, go to our earthquake.usgs.gov. If you like this podcast or any other USGS podcast, go to usgs.gov/socialmedia.

08:03

This podcast is a product of the U.S. Geological Survey, Department of the Interior. Thank you for listening.

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Related resources:

• USGS Issues Assessment of Aftershock Hazards in Haiti
• earthquake.usgs.gov