Earthquake initiation within the deep crust requires very high stresses, which are not expected elsewhere in the Earth’s crust. Several mechanisms have been proposed to generate these unusually high stresses, but direct measurements are lacking. This is a fundamental gap of knowledge, because models of earthquake hazard must be constrained by a thorough understanding of the mechanisms of earthquake faulting and of the associated stress levels. Earthquakes in the deep crust are common in the continents, where they represent approximately 20% of seismicity of magnitude 5 or more. Intracontinental earthquakes are often devastating and, over the past century, have killed significantly more people than earthquakes that occurred at subduction zones. An in-depth understanding of intracontinental earthquakes requires knowledge of the origin and significance of deep-crustal seismicity.
The project CONTINENT – “CONdiTIons for earthquake NuclEatioN in the lower crusT” - aims to determine the state of stress of earthquake-generating faults in the deep continental crust. The project proposes to achieve this goal using cutting-edge scanning electron microscopy techniques. These techniques will be applied to rock samples that contain “fossil earthquakes” and that were brought to the surface from the deep crust, in order to measure the residual stress retained in mineral grains subjected to earthquakes.
The samples come from Western Norway and Lofoten, two localities that have inspired the research on deep crustal earthquakes and that are known worldwide for the preservation of “fossil earthquakes” in their rocks.
Measurements will be coupled to numerical models of stress distribution in deep crustal fault systems. The project will provide for the first time a comprehensive view of the state of stress in faults that represent the most common earthquake source in the lower crust of the continental interiors and will thus inform models of earthquake hazard in areas at risk.
Lower crustal earthquakes are an important component of the earthquake-cycle deformation in the continents. Intracontinental earthquakes are often devastating and, over the past century, have killed significantly more people than earthquakes that occurred at subduction zones. An in-depth understanding of them requires knowledge of the origin of deep crustal seismicity. However, the origin of lower crustal earthquakes is controversial. Deep crustal earthquakes in dry rocks require mechanisms capable of generating very high differential stresses. This requirement contrasts with the current models of continental lithospheric deformation, which typically favor a distributed flow of weak viscous lower crust. Such flow would limit the capability of rocks to build up the high stresses necessary for brittle failure and earthquake generation. Thus, there is a fundamental disconnect between the main rheological models of crustal-scale faults and the inference that high stresses are required to generate earthquakes in the dry lower crust. Determining the locations and magnitude of such transient high stresses from the rock record of exhumed deep crustal earthquake sources is a fundamental challenge, and indeed the main goal of the project. Recent tremendous technological improvements of micro-analytical techniques (e.g., high-angular resolution electron backscatter diffraction: HR-EBSD) have created unprecedented opportunities for a quantitative determination of the residual stress retained in geological materials. We propose to determine the state of stress of lower crustal faults using the HR-EBSD technique on samples of seismogenic faults exhumed from the lower crust, coupled to numerical models of stress distribution and of cyclical aseismic-seismic deformation in lower crustal fault systems. Thus, the project will also fill the major gap of knowledge related to the mechanisms controlling cyclical switches from aseismic to seismic behavior in lower-crustal faults.