Smaller, more frequent earthquakes help reduce tectonic tension

The San Andreas fault in California is famous for its large and infrequent earthquakes. However, some segments of the San Andreas Fault (SAF) are instead characterized by frequent low to moderate magnitude earthquakes and high rates of continuous or episodic aseismic creep. With tectonic stress released in near-frequent motion, this reduces the potential for large earthquakes along these segments.

Now researchers assert that ubiquitous evidence of continued geologic sequestration of carbon in mantle rocks in creeping sections of the SAF is one of the underlying causes of aseismic creep the extent of a SAF section of approximately 150 very long kilometers between San Juan Bautista and Parkfield, California, and the extensive of several others. fault segments.

“Although there is no consensus regarding the underlying cause of aseismic creep, aqueous fluids and mechanically weak minerals appear to play a central role,” say the researchers in a new post, “Carbonation of serpentinite in creeping faults of California,” published in Geophysical Research. Letters.

The new study incorporates field observations and thermodynamic modeling “to examine possible relationships between the presence of serpentinite, silica-carbonate rock and aqueous fluids Rich in CO2 in the rampant flaws of California ”, indicates the doc. “Our models predict that the carbonation of serpentinite leads to the formation of talc and magnesite, followed by silica-carbonate rock. While copious exposures of silica-carbonate rock indicate complete carbonation, CO2-rich spring fluids hosted in serpentinite are highly supersaturated with talc at elevated temperatures. Consequently, the carbonation of serpentinitis is probably in progress in certain parts of the San Andres fault system and operates in conjunction with other Talc formation modes which can further increase the potential for Asismic Flimper, thus limiting the potential of large earthquakes.

The article states that because wet talc is a mechanically weak mineral, “its development by carbonation favors tectonic movements without large earthquakes”.

The researchers recognized several possible underlying mechanisms causing aseismic creep in FAS, and they also noted that because aseismic creep rates are significantly higher in some functions of the SAF system, an additional or different mechanism – serpentinite carbonation – is required to account for the full extent of creep.

With fluids almost everywhere the extent of the FAS, but with only certain parts of the fault lubricated, the researchers considered that a rock could be responsible for the lubrication. Some previous studies had suggested that the lubricant could be talc, a soft and slippery component commonly used in baby powder. A well-established mechanism for forming talc is by adding silica to mantle rocks. However, the researchers here focused on another mechanism of talc formation: the addition of CO2 to mantle rocks to form soapstone.

“The addition of CO2 to mantle rocks – which is the process of mineral carbonation or carbon sequestration – had not previously been studied in the context of formation of earthquakes or the natural prevention of earthquakes. Using baseline geological constraints, our study showed where these carbonate-weathered mantle rocks are and where there are resources along the fault line in California that are enriched in CO2. It turns out that when you plot the occurrence and distribution of these kinds of rocks and the occurrence of CO2-rich resources in California, they all line up along the San Andreas Fault in creeping sections. of the fault where you don’t have major earthquakes,” said Frieder Klein, lead author of the journal article.

Klein, associate researcher at the department of Marine Chemistry and Geochemistry from the Woods Hole Oceanographic Establishment, explained that carbonation is essentially the absorption of CO2 by a rock. Klein noted that he used existing databases from the US Geological Study and Google Earth to plot the locations of carbonate-altered rocks and CO2-rich resources.

“Geological evidence suggests that this mineral carbonation process is underway and that the talc is an intermediate reaction product of this process,” said Klein. Although the researchers did not find soapstone on rock outcrops in the mantle, the results of the theoretical models “strongly suggest that carbonation is a continuous process and that soapstone could indeed form in the SAF at depth”, notes the document.

These theoretical models “suggest that carbon sequestration with SAF is taking place today and that the process is actively helping to lubricate the fault and minimize strong earthquakes. Land in the rampant events of SAF, ”said Klein.

The Take document also notes that this mechanism can also be present in other failure systems. “Because CO2-rich aqueous fluids and ultramafic rocks are particularly common in young orogenic belts and subduction zones, the formation of talc by means of mineral carbonation may play a critical role in controlling the seismic behavior of major tectonic faults in the world.”

“Our study allows us to better understand the fundamental processes that take place in fault zones where these ingredients are present, and allows us to better understand the seismic behavior of these faults, some of which are in densely populated areas and some of which are in sparsely populated or oceanic settings. , said Klein.

This work was supported by grants from the Countrywide Science Basis.

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