São Paulo had a major earthquake 2.5 million years ago, says study

Geological features found underground in the center of the capital of São Paulo indicate that the region recorded an earthquake of great magnitude at least 2.5 million years ago – a very short time from a geological point of view. A recent work, published by researchers from USP and the Federal University of ABC, is the first to document records of earthquakes of such high magnitude in the region. The tremors may have been caused by the fall of a meteorite or by tectonic activity.

Published in Sedimentary Geology, the study reveals that at least one major earthquake occurred in the region, reaching a magnitude of at least 6 degrees on the Richter scale – enough to destroy much of the city center, if it occurred today. Researchers do not know exactly what could have caused such an intense earthquake, but one of the main suspicions is that it was the fall of a meteorite.

In fact, there is a crater in the south of the city, about 40 km from the center, 3.6 km in diameter and 450 m deep, the Colônia Crater, which went unnoticed for a long time. It is covered by sediments that form a plain. The neighborhoods of Colônia and Vargem Grande were built in the region. It wasn't until the 1960s that aerial and satellite images revealed the crater. By analyzing the edges and sediments at its bottom, it is possible to say that it was caused by the fall of a meteorite.

Tectonic activity

The other hypothesis raised by the researchers is that the earthquake was caused by tectonic activity. Brazil is located right in the center of the South American Plate and, for this reason, earthquakes are rare here. Earthquakes are much more frequent in regions where different plates meet, as there is a risk of collision between them. However, scientists warn, tremors can occur in other areas due to the simple movement of plates.

If it was this movement that caused the earthquakes, there is a risk, albeit remote, of this happening again in the region. “If we can prove that there is no relationship between the tremor and Colônia Crater, the only option we would have left would be an earthquake of tectonic origin,” said geologist Maurício Guerreiro dos Santos, from the Federal University of ABC, one of the authors of the study. “It would not be unprecedented, it occurs with some frequency, although it is more common at the edges of tectonic plates. If an earthquake like that occurred today it would be a complete catastrophe, the buildings in São Paulo are not made for this type of occurrence,” he said.

“Large earthquakes, in general, occur on the continental shelf, 2,000 km away, and when they arrive here it is with very weak tremors”, said geologist Renato Henrique Pinto, from the USP Institute of Geosciences, who also signed the study . “The fall of the meteorite makes more sense from both a geological and stratigraphic point of view.”

Isolation in the pandemic

The researchers decided to research the city's subsoil when they found themselves in isolation forced by the Covid-19 pandemic in 2020. Studying the São Paulo subsoil is not a simple task, because of the paving and construction. Geologists looked for rock outcrops in the city itself. USP campus and also in squares in the center. The type of rock found is typical of regions where there have been earthquakes of great magnitude. The researchers also used underground surveys carried out by Metro.

“We need to preserve these outcrops,” said Renato Henrique Pinto. “If they are cemented, no one will ever be able to see them again. So I wrote a project, so we could put together a visitation itinerary and, if we can get funding, set up a date.”

The information is from the newspaper The State of S. Paulo.