San Francisco: Increased risk of quake

Newly discovered connection between two faults makes future quake more violent

Two large faults (red) in the San Francisco Bay Area are connected (orange) - this increases the strength of the already threatening earthquake © USGS / scinexx
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Danger for San Francisco: The megacity threatens a stronger earthquake than previously expected. Because two hitherto considered separate dislocations in the San Francisco Bay are connected to each other - this can tearing the underground on a longer route. Within the next 30 years, the city could threaten a quake of magnitude 7.4, warn researchers in the journal "Science Advances".

The city of San Francisco and the surrounding cities sit on a tectonic ejection seat: in the area around the San Francisco Bay run in addition to the San Andreas Graben equal to six other active dislocations. Already in 2014, seismologists warned that four of them are so tense that a massive earthquake threatens in the near future.

Especially dangerous are the south of San Pablo Bay lying Hayward and the north lying Rogers Creek fault. "They are the zone where an earthquake with a magnitude greater than 6.7 is most likely in the next 30 years, " said Janet Watt and her colleagues from the US Geological Survey.

The longer, the more dangerous

But just with these two dislocations so far a crucial question remained open: Are they separated from each other and run parallel or is there a connection under the San Pablo Bay? "Answering this question is important because both variants have very different effects on seismic risk, " say the researchers.

The Hayward and Rodgers Creek Fault is one of the disruptions in the Bay Area where quake risk is currently highest. © USGS

If Hayward and Rodgers Creek were connected, they would form a 190-kilometer potential quake zone. If the rock breaks under the pressure of the moving earth plates along this area, an earthquake can continue over the entire length - and is correspondingly stronger than the crack of only one of the two sections, as the scientists explain. display

Merger under the Bay

So far, however, the exact course of the two faults under the San Pablo Bay could not be determined, because the rock there contains very many gas inclusions. Watts and her colleagues have now used a so-called Chirp Subbottom Profiler. This sonar device pulled behind a boat emits particularly high-frequency sound waves between 2 and 12 kilohertz. In addition, they conducted targeted magnetic field measurements from the ship.

The result: Although both upheavals on land are offset by up to six kilometers, they converge under water. "The Hayward Dam curls under the bay ten degrees to the right toward the Rodgers Creek Fault, " the researchers report. At the northern edge of the bay they both meet and merge.

The new measurements show that there is a connection between Hayward and Rodgers Creek Fault. Watts et al / Science Advances / CC-by-nc 4.0

"Huge damage and worldwide consequences"

For San Francisco and the surrounding area, this is not good news: "The discovery of a link between the Hayward and Rodgers Creek spans facilitates a simultaneous sweeping of both faults, " warn Watts and her colleagues. "That would have the potential to cause an earthquake of up to 7.4 magnitudes."

The consequences of such an event would be fatal: "Estimates of the intensity of the shocks for a quake of magnitude 7.2 have shown that very strong ground movements occur throughout the bay area would be, "explain the researchers. "Especially in areas with soft ground such as the cities of Santa Rosa, Livermore and San Jos these vibrations would be strengthened."

A scenario with a quake of magnitude 7.4 could be even more serious: "It would cause enormous damage and cause fatalities and even worldwide economic consequences, " warn Watts and her colleagues. Let's hope that San Francisco is spared from this worst-case scenario. (Science Advances, 2016; doi: 10.1126 / sciadv.1601441)

(AAAS, 20.10.2016 - NPO)