Crustal Seismogenic Thickness and Thermal Structure of NW South America

Abstract. The crustal seismogenic thickness (CST) has direct implications on the magnitude and occurrence of crustal earthquakes, and therefore, on the seismic hazard of any region. Amongst other factors, the seismogenesis of rocks is affected by in-situ conditions (temperature and state of stress) and by the rocks’ heterogeneous composition. Diverse laboratory experiments have explored the frictional behavior of the most common materials forming the crust and uppermost mantle. However, it remains a matter of debate how to "up-scale" to the scale of the crust the conclusions derived from these studies. In this study, we propose a workflow to up-scale and validate these experiments to natural geological conditions of crustal and upper mantle rocks. We used NW South America as a case study to explore the three-dimensional spatial variation of the CST and the potential temperatures at which crustal earthquakes occur. The 3D steady-state thermal field was computed with a finite element scheme using the software GOLEM, taking into account the uppermost 75 km of a previously published 3D data-integrated lithospheric configuration and lithology-constrained thermal parameters. We found that the majority of events nucleate at temperatures of less than 350 °C, in general agreement with frictional experiments of crustal and mantle materials. A few outliers in the hypocentral temperatures showcase nucleation conditions consistent with the seismogenic window of olivine-rich rocks, and can be linked to uncertainties in the Moho depths and/or in the earthquake hypocenters, or to the presence of ultramafic rocks within the allochthonous crustal terranes accreted to this complex margin. Our results suggest that the two largest earthquakes recorded in the region (Ms=6.8 and Ms=7.3, Murindó sequence, in 1992) nucleated at the calculated lower boundary of the seismogenic crust, highlighting the importance of considering this transition when characterizing the seismogenic source in hazard assessment studies. The approach presented in this study can be applied to other tectonic settings worldwide, and it could be further refined as new, high-quality heat flow and temperature observations became eventually available for testing and validating the thermal models.