Popular myth busted – new findings challenge outdated earthquake models

Recent research suggests that earthquakes in subduction zones occur across a complex network of fault planes, rather than across a single plane. This finding, derived from detailed seismic data collected in Ecuador and analyzed using artificial intelligence, challenges traditional views of how seismic stress is released. The study, which also provides new insights into the behavior of aftershocks, could significantly impact future earthquake models and hazard assessments in subduction zones.

The first high-resolution image of a seismic fault zone changes our knowledge of earthquakes.

The idea that earthquakes release stress through a single strong earthquake along a single fault plane may need to be corrected. Recent research by the Karlsruhe Institute of Technology (KIT), in cooperation with the German Research Center for Geosciences of the GFZ and other international institutions, suggests that earthquakes occur within a zone containing multiple fault levels, some of which are parallel.

According to the authors, the study results could help create more realistic models of earthquakes and seismic hazards in subduction zones. The study was published in the journal Nature.

The international team led by first author Caroline Chalomieux from KIT investigated a series of earthquakes in Ecuador on the west coast of South America. There, the Pacific plate is subducted beneath the continental South American plate. Subduction frequently causes very intense earthquakes. The latest series of earthquakes in Taiwan, the main one of which killed nine people and caused extensive damage on Taiwan's east coast at the beginning of April, can also be attributed to subduction.

The diagram shows the 100 to 600 m thick seismic zone in which fault planes (5 to 20 m thick) and thus cracks occur. Credit: Dr. Carolyn Chalomo, Dr. Hans Argorto-Ditzel, Prof. Andreas Rietbrück, Dr. Michael Fritsch. Professor Onu Onken, Dr. Monica Segovia, Dr. Audrey Galve: Seismic evidence for a multi-fault network at the subduction interface. Nature, 2024. DOI: 10.1038/s41586-024-07245-y

The series of earthquakes in Ecuador that the team analyzed began on March 12, 2022 and ended on May 26, 2022. The most severe earthquake (magnitude 5.8) occurred on March 27 and caused many smaller aftershocks over a short period of time. There was a dense network of 100 seismometers in the area at that time. It has been prepared for the offshore experiment “High-Resolution Imaging of a Subduction Fault in the Pedernales Earthquake Rupture Zone” (HIPER for short).

Detailed seismic data analysis

Thanks to the extraordinarily detailed data from the HIPER experiment and using artificial intelligence, researchers were able to map more than 1,500 earthquakes and their fault levels at a depth of 15 to 20 kilometers with very high resolution. “We observed that the seismicity of the earthquakes occurred in a primary zone – the main earthquake, so to speak – and in a secondary zone, i.e. the aftershocks,” says first author Dr. Caroline Chalomieux from the Geophysical Institute (GPI) at KIT. “Within the primary zone, we observed that seismicity occurred on several different fault levels, often on top of each other. In some places, parallel seismically active levels occurred, and in other places only single levels occurred.”

The parallelism of the earthquakes was not related to a specific depth. “We have found evidence that the previous idea that stress releases from a single strong earthquake along a single fault plane could be a thing of the past,” says Professor Andreas Rietbroek from GPI. “Instead, we should be talking about a fault network in which a series of cracks are triggered during a single earthquake.”

Analysis of the Ecuadorian earthquake series also provides new insights into aftershocks. Chalomo says these first occurred near the main epicenter and then gradually spread in other directions. It is concluded from this that the propagation of aftershocks in the region is mainly controlled by subsequent collapses. “With this work, Caroline Chalomieux's team has provided the first sharp seismic image of seismic plate boundaries,” says Professor Onno Onken from GFZ. “On the one hand, it confirms existing geological observations, and on the other hand, it successfully explains the propagation of aftershocks with a new approach. The assumptions have thus been refuted The precedent that says, for example, that the spread of fluids causes aftershocks.

The results are also important for assessing seismic hazards in subduction zones. “The study will impact future earthquake modeling, but also on seismic slip, that is, plate movements without earthquakes occurring,” says Andreas Rietbroek.

Reference: “Seismic evidence for a multi-fault network at the subduction interface” by Caroline Chalomo, Hans Agorto-Dietzl, Andreas Rietbrück, Michael Fritsch, Ono Onken, Monica Segovia, and Audrey Galvi, 17 April 2024, Nature.DOI: 10.1038/s41586- 024- 07245-Y

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