Northern Chile is an ideal natural laboratory for studying the origin of earthquakes. Here, the Pacific Nazca Plate slides under the South American continental plate at a speed of about 65 millimeters per year. This process, known as subduction, creates tension between the two plates, so scientists expected a mega-earthquake here sooner or later, like the last one in 1877. But although northern Chile was one of the focal points of global earthquake research, until now there was no complete data set on the structure of the seabed – until nature itself even intervene to help.
On April 1, 2014, a segment of the subduction zone finally ruptured northwest of the city of Iquique. The 8.1 moment magnitude earthquake released at least some of the accumulated stresses. Subsequent seismic measurements off the coast of Chile along with seabed mapping and terrestrial data have provided a unique insight so far into the architecture of the plate boundary. “Among other things, this allows us to explain why a relatively severe earthquake like the one in 2014 only triggered a relatively small tsunami,” explains Florian Petersen of the GEOMAR Helmholtz Center for Ocean Research Kiel. He is the lead author of the study, which has now been published in the journal Geophysical research letters.
As early as December 2014, just eight months after the main earthquake, the Kiel team deployed 15 specially developed seismic measuring devices for the deep sea off the Chilean coast. “The logistical and administrative challenges associated with the deployment of these ocean floor seismometers are difficult and eight months of preparation are very short. However, since investigations are crucial to better understand the potential for danger of the plate margins off northern Chile, even the Chilean navy has finally supported us by providing its patrol vessel COMANDANTE TORO ”, reports the head of project and co-author Dr. Dietrich Lange from GEOMAR.
At the end of 2015, these ocean floor seismometers (OBS) were recovered by the German research vessel SONNE. The crew on board repaired the devices, read the data and replaced the OBS on the seabed. It was not until November 2016 that the American research vessel MARCUS G. LANGSETH finally recovered them. “With terrestrial data, we got a 24-month earthquake region seismic dataset, in which we can find the signals for many aftershocks. It’s unique to this day, ”explains Florian Petersen, for whom the study is part. of his doctoral thesis.
The evaluation of long-term measurements, in which colleagues from the Universidad de Chile and Oregon State University (USA) also participated, showed that a surprisingly high number of aftershocks were located between the earthquake rupture zone and the high seas trench. “But what surprised us even more was that many aftershocks were quite shallow. They occurred in the overlying South American continental plate and not along the edge of the Nazca Plunge Plate.” , says Petersen.
During many earthquake cycles, these aftershocks can severely disrupt and disrupt the sea edge of the continental plate. The resulting vacancies fill with pore fluids. As a result, the authors conclude, the energy of earthquakes can only propagate downward, but not to the deep-water trench off the coast of Chile. “As a result, there were no significant and sudden changes to the seabed during the 2014 earthquake and fortunately the tsunami was relatively small,” says Florian Petersen.
It remains to be seen whether the 2014 Iquique earthquake was already the major earthquake expected in the region or whether it only relieved some of the stress that had accumulated since 1877. “The region remains very exciting. for us. The current results were only possible, due to the close cooperation of several countries and the use of research vessels from Germany, Chile and the United States. This shows the immense effort needed to study natural hazards at sea. However, this is essential for a detailed risk assessment for the coastal cities of northern Chile, so everyone was dedicated to this task, ”says co-author Prof Heidrun Kopp from GEOMAR.
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