Past and ongoing work under this initiative focuses on gaining insight into the physical processes that generate earthquakes and the faults that control how they evolve. This knowledge is fundamental in assessing earthquake hazards and risk, and improve disaster preparation and prevention policies.

A New Model for the Repeating M6 Parkfield Earthquakes
This study investigates the fine-scale geometry and structure of the San Andreas Fault near Parkfield, CA, and their role in the development of the 1966 and 2004 ~M6 earthquakes. Long-term surface fault traces indicate that structural heterogeneities associated with secondary reverse and normal fault structures are present at both rupture tips, near Middle Mountain and Gold Hill. Detailed analysis of almost 50 years of high-resolution seismicity reveals a fault plane that has been twisted into a helicoid between Middle Mountain and Gold Hill. Numerical models support our conclusion that this shape is the result of long-term torqueing of a strong stuck patch surrounded by a weak creeping region. The changes in fault friction behavior and related geometric discontinuities act as barriers to rupture propagation of moderate size earthquakes at Parkfield, and as areas of stress concentrations where rupture initiates. Our study demonstrates also that smooth strike-slip faults with large cumulative offset can form new fault segments at a late stage in their evolution. For more details see Perrin et al. (2019).

Shear Deformation Zone and Fault Maturity
Active fault zones worldwide are 3D features made of a parent fault and secondary faults and fractures that damaged the surrounding medium. During and soon after a large earthquake, these structures are reactivated, highlighted by numerous smaller events, also called aftershocks. Their distribution allows us to characterize the zone of shear deformation around the fault plane. In this study, we show that the width of the shear deformation zone is narrower around mature faults than around immature faults. It decreases as a power law with cumulative fault displacement as the result of the smoothing of the fault with wear through geological times. Our study provides some relations to better understand and anticipate the size of off-fault deformation reactivated during and after an earthquake, based on geological fault parameters. For more details see Perrin et al. (2021).

Strengthening Tidal Signal Observed Before the 2019 M7.1 Ridgecrest, CA, Earthquake
The solid Earth experiences tides, like the ocean, as it deforms under the gravitational attraction of the Sun and Moon. This deformation induces an oscillatory stress change in the Earth's crust with small peak-to-peak amplitudes on the order of 1 kPa (about 1% of the atmospheric pressure). Tidal stresses can influence the rate of earthquake occurrence and the characteristics of such modulations carry information about the physics of the earthquakes and the properties of the crust. It has been proposed, based on experimental and field observational studies, that the modulation of seismicity by the tides increases before a large earthquake. We tested this hypothesis by developing a comprehensive catalog of 10 years of seismicity before the Mw7.1 2019 Ridgecrest, CA earthquake and use a novel method to extract the signal of tidal modulation throughout the study period. We find that seismicity is increasingly modulated by the tides starting about 2 years before the mainshock, thus corroborating the hypothesis. For more details see Beauce et al. (2023).

RELATED PUBLICATIONS

Perrin, C., F. Waldhauser, E. Choi, C. Scholz (2019). Persistent fine-scale fault structure and rupture development: A new twist in the Parkfield, California, story, Earth Planet. Sci. Lett., 521, doi:10.1016/j.epsl.2019.06.010. [PDF]

Perrin, C., F. Waldhauser, C. Scholz (2021). The Shear Damage Zone and the Smoothing of Faults with Displacement, J. Geophys. Res., 126, e2020JB020447. https://doi.org/10.1029/2020JB020447. [PDF]

Beaucé, E., P. Poli, F. Waldhauser, B. Holtzman, and C. Scholz (2023). Enhanced tidal sensitivity of seismicity before the 2019 magnitude 7.1 Ridgecrest, California earthquake, Geophys. Res. Lett., 50, e2023GL104375. https://doi.org/10.1029/2023GL104375. [PDF]