Study brings unexpected data about the southern Andes

Monday, 26 de September

High seismicity in uneforeseen areas and a fault system moving in directions never seen before, are some of the data produced by a geological research that measured for 16 months in depth activity of the north Liquiñe-Ofqui Fault System.

Its a study never carried out before in Chile: the installation of a seismic of 35 stations between Alto Biobío and Liquiñe, was a unique investigation for Chilean geology because, for the first time, so much data about the seismotectonic nature of the area was gathered in real time.

To obtain this information, a team led by Jose Cembrano (Andean Geothermal Center of Excellence, CEGA), Dietrich Lange (Helmholtz Centre for Ocean Research Kiel, GEOMAR) and Gerd Sielfeld (PhD CEGA student in Engineering Sciences Program at Catholic University of Chile) obtained 27 short period seismometers and 8 broadband, provided in loan by the GIPP (Geophysical Instrument Pool Potsdam), a German institution dependent from the German Research Centre for Geosciences (GFZ).

Aboiut this negotiations José Cembrano says: “Originally we had the equipment for a year, but they accepted our request to extend the period, something complicated because the GIPP receives urgent requirements from other countries. Then, it is not normal to have these instruments for 16 months. It was a luxury to have this temporary network for the first time on the continent.”

After getting the instruments, a team of 20 young geologists participated during 16 days in the installation campaign. Gerd Sielfeld, who was in charge of coordinating the field work, says that this implied a special logistic, because it was necessary to get transportation, storage and to coordinate the maintenance for more than 1 year for each one of the seismometers: “You can check the instruments while you are in place, but then you can not know what happens with them, so you need to go there periodically to check them. Also, we needed a power source . In this case, the simplest way to do it was connecting them to a house or installing solar panels”.

On average between 3 and 5 stations were installed daily. For this task, the collaboration and technical expirience of the German researcher Dietrich Lange was fundamental. Meanwhile, it was also important to find the best geographical setting to distribute the instruments, as the challenge was to create a consistent and homogeneous network, with a good balance of deopth and coverage. About this process, Gerd emphasizes that the team looked the most optimal formula to generate this balance:”We left between 20 and 25 kilometers between each station, because if you want to have more coverage near the surface, you have to leave a smaller distance among the seismometers, on the other hand, if you want to get an idea of what happens at a larger scale, you have to put more spaced stations, so if there is a superficial earthquake at 1 km, it will not be registered”

From fieldwork to data processing

After fieldwork the data analysis began, an extensive stage which involved looking carefully the records of each one of the stations. In this process, the team have already detected interesting events such as a seismic cluster of 48 hours in the northern of Caburgua Lake. This has called the attention of researchers, because there is a register for more than 170 events of homogeneous magnitudes, without a triggering seism, and concentrated in two days.

On this data, Gerd says: “This was totally unexpected. We were lucky enough to register this seismic cluster in northern Caburgua , it was completely unexpected. Its origin still being uncertain, as it is a new data, it needs a detailed and accurate review”. He adds that although it might have been ccaused by the activation of a failure by some fluid, it can also correspond to a reactivation of the main fault Liquiñe-Ofqui, with null or very low participation of fluids.

Even though until now they know little about the origin of this high seismicity, the researchers point out that it is possible that many of these events have a volcano-tectonic origin, where the fluid breaks the rock causing this concentrated seismicity. This seismic cluster, the say, could be the typical case where this happens.

In addition to this, there is another interesting fact: the cross presence of activity on the Liquiñe-Ofqui fault system. Even though they were expecting to identify these type of events, they never speculated about the place where this would exactly happen. On this subject, Gerd explains: “there are geological and geomorphological evidence that suggests the existence of oblique deformation systems, but the activity of these had not been identified and pinned until now”.

He also says that thanks to the data they have identified 2 types of orientations of these cross-cutting systems: one, parallel to the alignment of Villarrica-Quetrupillán-Lanín volcanoes; and another near the town of Liquiñe, close to Llaima, Sierra nevada and North of the Lonquimay volcanoes. About this discovery, Gerd says that “the interesting thing about these cross-cutting domains is that they may have an origin dating from periods where the Andes mountain rage did not exist neither did exist the current subduction in the area. This could be an Earth’s scar and could be reactivated according to the deformation and orientation rate of the main tectonic efforts, so much in convergent margin as in the dynamics of the same fault system parallel to the margin”.

From Germany, Gerd Sielfield and Dietrich Lange continue to work on the extracted data and seek to develop alternative studies, even to further and set focus in the deep earthquakes that were reported: “in other words, the data – which cost a fair amount of time, money and effort- are already in pur hands, we only need more human resourses to start processing the information”, concludes Gerd.