Geological Fieldwork in Post-Earthquake Haiti

In the wake of the devastating earthquake that struck Haiti, a group of U.S. scientists - including University of Texas at Austin Cockrell School of Engineering Professor Ellen Rathje - flew to Port-au-Prince to complete the first technical survey of the city's geology.

Transcript

00:00:10 so this is the most heavily damaged area yeah yeah this is bad when the earthquake hit even though a magnitude 7 earthquake is not the maximum earthquake that this place could experience i knew it was large enough to cause a lot of damage here we are at the presidential palace

00:00:37 but it was quickly apparent within a few days that some areas had been affected much more so than others so now what we're trying to do is to learn what is a relationship between the type of geology and the amount of destruction when an earthquake happens it's a very unfortunate event but it's something

00:01:14 that we need to take the opportunity and learn from there were some complicated things that happened with this earthquake in terms of understanding the damage patterns why were these buildings damaged in this area when they weren't damaged over here and the construction appeared to be very

00:01:33 similar well there was something different about the way the ground shook in certain areas of the city during the earthquake it's really bad over there so an earthquake is caused by a slip in the earth and that slip creates these seismic waves that travel from deep in the earth

00:01:55 out and as the waves come up under your building site they're propagating out of the rock and through the soil that's why we try and characterize the stiffness of the soil because it really has a profound effect on the level of ground shaking this is where we tested before and i just want to do some shallow testing

00:02:17 here and see if the results are similar to what we got before because this was a bit of a strange location so just walk down there and i'll stop you the stiffness of the soil is quantified by something we call a sheer wave velocity okay i'm going to hook up some cables

00:02:34 and that's simply the speed that shear waves which are the waves that are generated by the earthquake is how fast those waves travel through the soil so brady is setting up these geophones and they're going to measure vertical motion up and down and we're going to use the sledgehammer

00:02:55 okay to generate surface waves so waves that travel along the surface and as they travel underneath the geophones the geophones will vibrate so we'll be able to see the speed of the waves as they travel from sensor to sensor now as long as the analyzer works it'll be good

00:03:16 okay go ahead okay again that's good got 1500 feet per second 10 feet deep is what it's looking like right now it's pretty good stuff yeah it is it's stiff it's strong it's not going to increase the level of shaking significantly so the shaking level here

00:03:41 might have been moderate all right one more okay that's it so what we'll do now is check out different roads make a stop collect a data point and a gps measurement okay go ahead again we've observed and we've measured the shear wave velocities around the rest of port-au-prince looks good again

00:04:20 and the majority of port-au-prince actually has stiff what we would call pretty good soil the place where it's not as well off is right downtown within about one or two kilometers to the coast and this is where you start to see softer sediments and that's where

00:04:39 a significant amount of damage was we've begun to really piece together the puzzle where we can say those really soft soils have a tendency to amplify waves that are coming from the earthquake so you will have to design your structure to resist higher forces if it's built on soft soil usually it gets stiffer as you go deeper and we're trying to help the structural

00:05:03 engineers predict the level of ground shaking that the structure needs to be designed for be a weathered soil horizon on top or it could be some when i am designing something and i'm i am calculating the the structures elements like the beams for the house i must know about the

00:05:24 seismic effect and i should include it in the building the reconstruction process is starting so that's why it's important to make smart decisions based on the data that is being acquired and delivered this week plus general considerations about what

00:05:44 we know about how to build from experiences elsewhere in the world there are a lot of geologic correlations between port-au-prince in the san francisco bay area first off they both are in very active fault zones both of them have similar soil conditions and fairly hard bedrock materials so the

00:06:10 level of ground shaking would be pretty comparable in the bay area we're probably looking at very similar levels of shaking as what was experienced in downtown port-au-prince certainly san francisco the bay area has been really at the forefront of seismic design and has a particular concentration of of experts research

00:06:31 and actual buildings that are constructed so it really is a working test laboratory in earthquakes the ground's shaking and the building starts to shake back and forth and the biggest risk they carry is the separation of the walls from the floors what we're trying to do is

00:06:53 utilize continuous cables to stabilize the massive wall and bring that structure back so these extend all the way down into the foundation mat that's right we essentially have hollow ducts that we've been casting into the concrete and those hollow ducts provide a pathway for cables once the cables are inserted we

00:07:13 take those cables and stretch them and compress the concrete walls and during an earthquake it rocks back and forth it forms cracks and those continuous cables help bring the wall back close the cracks and bring the building back to its original position the cables are positioned essentially to

00:07:34 be centered in the wall so for each length of wall there's a cable that's really running right down there while these cables are incorporated in a structure in a slightly different way technologically there's supplies and materials that are available throughout the world it's a technology that's

00:07:48 available to almost all societies basically doing concrete structures for a lot of poor countries it's difficult for them to accommodate seismic design but then on the other hand the cost of a disaster is extreme we know that january 12th earthquake was not the

00:08:13 large earthquake that may hit port-au-prince so we have two solutions we can say well earthquakes are a tough problem to deal with and we have other priorities or we are proactive about it and say let's use a reconstruction as an opportunity to build better as an opportunity to move forward as an opportunity to

00:08:34 develop a seismic risk reduction program because if we don't do it now we will never do it you