More Earthquakes for Chile? Seismic gap has not been closed

GPS measurements of the displacement vectors. Credit: GFZ

After the strong earthquake that struck Chile on April 2 (CEST), numerous aftershocks, some of them of a considerable magnitude, have struck the region around Iquique. Seismologists from the GFZ German Research Centre for Geosciences doubt that the strong earthquake closed the local seismic gap and decreased the risk of a large earthquake. On the contrary, initial studies of the rupture process and the aftershocks show that only about a third of the vulnerable zone broke.

This vulnerable area is referred to as the seismic gap of Iquique and a strong earthquake is expected to strike here. The Pacific Nazca plate meets the South American plate at South America’s west coast. “In a subsea trench along the coast, the Pacific Ocean floor submerges beneath the continent building up tension that is released in earthquakes,” explains Professor Onno Oncken of the GFZ. “In the course of about 150 years the entire plate boundary from Patagonia in the South to Panama in the North breaks completely with a segmented series of strong earthquakes.” This cycle has been completed except for a last segment west of Iquique in northern Chile. As expected, the strong earthquake of April 2 took place exactly at this seismic gap.

No All-Clear

Initial analyses conducted by GFZ seismologists have shown that there is no sign that tension in the earth’ crust has significantly decreased: “So far tension has been released only in the central section of this vulnerable zone,” Oncken further explains. The series of earthquakes began on March 16 with a 6.7-magnitude earthquake. Although the main earthquake with a magnitude of 8.1 broke the central section of the seismic gap of a length of some 100 kilometres, two large segments further north and south remain intact, and these segments are able to cause strong earthquakes with a high risk of ground shaking and tsunamis.Oncken: “This means that the risk of one or even several earthquakes with a magnitude clearly above 8 still exists.” Furthermore, the location and magnitude of the aftershocks suggest such a scenario.

Since the main quake struck, hundreds of aftershocks have been registered, the strongest that of April 2 (CEST) of a magnitude of 7.6. This earthquake struck about 100 kilometres south of the main earthquake’s epicentre. Together with the its associated aftershocks, it forms a second rupture zone.

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Scientists getting ready for a field trip

For such extreme events, the GFZ has a task force called HART (Hazard and Risk Team) that will travel to the area affected to conduct further studies. The assignment aims at gaining a better and more detailed understanding of the rupture process based on the aftershocks, and defining the rupture surface more precisely based on the distribution of the aftershocks. Currently 25 seismometers are being prepared for air transport. Early next week a team of eight GFZ scientists will fly to Chile. The 25 portable seismometers will be used to expand the existing observatory network IPOC (Integrated Plate Boundary Observatory Chile) in order to be able to determine the earthquake epicentres more precisely. In addition highly precise surface displacements will be measured at 50 GPS measuring points. Two new additional continuous GPS stations will be installed to determine how the earthquake has deformed the earth’ crust.The Helmholtz Centre for Ocean Research Geomar in Kiel intends to support the measuring campaign. Ocean floor seismometers will supplement land-based seismic data by providing measurements of the aftershocks on the seafloor.

The Plate Boundary Observatory IPOC in Chile

The GFZ initiated the setup of an observatory directly within the seismic gap in northern Chile in order to be able to precisely measure and capture tectonic processes before, during and after the expected strong earthquake. The observatory called Integrated Plate Boundary Observatory Chile (IPOC) is a European-American network of institutions and scientists. Together with several Chilean and German universities, German, French, Chilean and American non-university research institutions operate a decentralized instrumentation system located at Chile’s convergent plate boundary to gather data on earthquakes, deformations, magmatism, and surface processes.The mission succeeded in the case of the April 2 earthquake: “All our instruments survived the quake and aftershocks unscathed. We now have a set of data that is unique in the world,” says GFZ seismologist Günter Asch with a smile, who was responsible for checking the instruments on site right after the earthquake and who is once again on his way to the region. “We believe that these data will help us understand the entire earthquake process — from the phase that tension builds up to the actual rupture, and also during the post-seismic phase.” This understanding will provide insights into earthquake risks in this part of the world as well as elsewhere.

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The IPOC will further expand. To this day more than 20 multi-parameter stations have been set up. They comprise broadband seismographs, accelerometers, continuous GPS receivers, magneto-telluric probes, expansion measuring devices and climate sensors. Their data is transferred to Potsdam in real time. The European Southern Observatory on Cerro Paranal is now also part of the observatory network.

Note : The above story is based on materials provided by Helmholtz Centre Potsdam – GFZ German Research Centre for Geosciences.