???? The great Californian faults ready to generate powerful earthquakes

2023-07-13 04:00:04

The precise relocation of earthquakes shows faults with smooth, flat or arcuate surfaces, on scales ranging from a few hundred meters to a few tens of kilometers, at the seismogenic depth. This regularity can play a crucial role in the genesis of great earthquakes, and can transform our understanding of the physics (Physics (from the Greek φυσις, nature) is etymologically the…) of rupture (Ruptures is the second album of the sci-fi series Orbital consisting of…) and seismic risks.
Seismicity along the San Andreas fault around Parkfield environment through reception and…) in perspective. The M>1 hypocenters from 01/01/1984 to 02/22/2022 were relocated with NNL-SSST-coherence.

The physical behavior of faults, and the resulting seismic risks, strongly depend on whether they are rough or smooth at the depth at which energy is released during earthquakes (Earth is the third planet in the Solar System in order of distance …). At this depth of about 4-15 km in California, the localization of earthquakes suggested that the faults are irregular on scales greater than one kilometer (The meter (symbol m, from the Greek metron, measure) is the basic unit of length of the system…). Moreover, the layout of faults mapped on the surface (A surface generally designates the superficial layer of an object. The term a…) is also generally complex and presents shifts at all scales. This leads to suppose a strong roughness of the major faults in depth, the rupture of a large earthquake would thus amount to trying to slide two egg boxes along their bumpy sides.

The authors, including a CNRS-INSU researcher, apply a new earthquake location procedure to large earthquake sequences and microseismicity along strike-slip faults in California. This multi-scale method makes it possible to correct certain distortion effects and the relocation of earthquakes reveals that fault surfaces are smooth at depth, flat or arcuate on scales ranging from a few hundred meters to a few tens of kilometers.

Scientists therefore demonstrate that seismic rupture is more like egg boxes sliding on their smooth sides, and this has obvious consequences. The presence at depth of multi-scale smooth surfaces in major strike-slip fault zones can influence the initiation, failure, direction and arrest of seismic ruptures, and these smooth faults may even be necessary for large earthquakes occur.

Seismicity along the Calaveras fault seen on the map. The hypocenters M>1.5 from 01/01/1984 to 26/1°/2022 were relocated with NNL-SSST-coherence. The hypocenters of the main earthquakes (1984 Mw 6.2, 1979 M5.8 and 2007 M 5.4) are indicated. The 1984 earthquake and the aftershocks recorded in the month that followed are in yellow (There are (at least) five definitions of yellow which designate roughly the same one…). The lower panel shows the alignment of the seismicity on an arc of a circle (A circle is a closed plane curve made up of points located at equal…).

These results can help map the seismic hazard and reinforce recent work on surface failures. This work shows that surface failures largely reflect shallow and often complex secondary deformations, not the seismically active slip surfaces at depth.