fieldwork in the jungle
The reason for the first visit of Argentina is the field-school organized within our StRATEGy program. This field trip is about 10 days only and we thought it would be a good idea to prolong the stay a few more days and get an impression of the area and search for possible sites for our seismographs. Hence, my second PI Gela and me left Germany 4 days earlier and teemed up with the Argentinian Professor Antonio and his student Ahmad to explore the Candelaria range 150km north of Tucumán.
Unfortunately, March is the end of the monsoon season; we got already surprised when we arrived in Tucumán by a typical convective rainstorm which lasted only 10-15 minutes, but enough time to flood all streets and sidewalks. Also the landscape responded to these weather conditions and I want to explain in this post how everything was affected.
I may should add that in this region you have 500-700 mm of precipitation during the year, but it will mainly happen in the summer monsoon. But due to El Niño this year, it is a lot more rainfall this summer.
First of all you will see in the most of the pictures in this post green as the dominant color. Everything is growing everywhere. On the first day we were mainly driving through cow fields but you couldn’t see any cows 1 meter away from the path, because they would’ve been completely covered by grass and other plants. Also we were searching for paths which are not used regularly and therefore densely vegetated. Often we couldn’t reach our final destination because the paths were completely overgrown.
The rivers in this region have no constant water flow during the whole year, due to the seasonality of the rainfall. But with stronger rainfall you will have more water in a short time in the river channels. This results sometimes in more erosion, sediment transport, incision of old river beds and/or new flood plains. These results affecting our work quite a lot as we only drive on dirt roads to go into the field. Somewhere every road crosses a river channel and the roads are completely destroyed then as you can see in the pictures above. The local people will fix these dirt roads as long as they use them but not before the end of the monsoon. Sometimes you could see that other parts of some roads are former river channels, where rivers incised several meters into the ground and created a natural path for cars.
At some point there was no flooding or vegetation blocking the path at all. The bigger problem was the slippery and muddy soil which covered the path, make it impossible to go further down the road safe, even with a 4WD Toyota Hilux.
On the picture above you can see what we were searching for the last few days. In the left third of the image there is a straight line with a sharp color change from dark green to lighter green. This line is interpreted as a fault scarp created by one or multiple earthquakes and can be seen as a steep offset in the landscape. But as described before it was not possible to access this area. Only once we had the chance to come close and actually see the scarp.
Actually this site was already prospected by our Argentinean colleagues and we could get access easily and doing already some GPS-profiles and geoelectrical measurements. To do so we used a differential GPS system which has a much higher precision than the ones you have in your smartphone or car navigation system. The profile is just a line of point measurements of the exact GPS position of the surface and we can calculate out of a set of profiles a high resolution DEM (Digital Elevation Model) of the fault scarp for further analyses. The geoelectrical method is a little bit more tricky to explain, but I’ll give it a try. For this we inject electricity into the ground with two electrodes and a car battery. With two other electrodes the resistivity of the ground is measured. Depending on the distance between the electrodes you can measure the resistivity at different depths (up to several hundreds of meters) and get an idea if there are changes in the composition of the earth below us.
Enough technical stuff; in the end we were happy that we found at least one place to get to the fault scarp. From the pictures you get an idea what strong forces are working down in the earth: along several kilometers the ground was moved upwards up to 10 meter. And these are not the big earthquakes. But the impact to the people living in this area is quite striking; in the pictures there is a paddock right in front of the scarp an the house of the gauchos who owns the land have there house directly on top of the fault. I don’t want to be in this one when the next earthquake would hit this area and moving up the whole area.
The first three days in the field were mainly influenced by throwbacks due to the road and natural conditions in this area, besides my personal throwback due to the asado on the first dinner. But anyway, now we are grounded and prepared for the field school the next ten days with the StRATEGy group and the future fieldwork after the trip.
For the end some pictures I’ve made and which could not be included into the text before.