The 2013 Ruisui earthquake represents the first unequivocal evidence of the activity of the Central Range fault in central Longitudinal Valley, Taiwan. Using a joint Bayesian finite-fault source inversion of Global Navigation Satellite System and strain time series, we infer that coseismic rupture occurred between 4 to 19 km depth with maximum slip of 0.5 m located near the hypocenter. We then apply a variational Bayesian Independent Component Analysis approach to displacement signals to infer a 3-month long afterslip located in the near-source region. This observation represents the first evidence of aseismic slip on the Central Range fault. Combining geodetic and seismological analysis with simulations based on rate-and-state friction mechanics, we analyze the interplay between seismic and aseismic deformation during the earthquake sequence. We observe that afterslip represents the dominant postseismic deformation mechanism, with > 95% of the moment being released aseismically in the postseismic phase. Besides, afterslip likely represents the driving force controlling aftershock productivity and the spatiotemporal migration of seismicity. Finally, we infer the presence of a shallow velocity strengthening zone (∼ 0-4 km depth) associated with spatially heterogeneous slip during the postseismic phase with maximum slip of 0.15 m located above the zone of maximum coseismic deformation.

The continuous redistribution of water mass involved in the hydrologic cycle leads to deformation of The continuous redistribution of water involved in the hydrologic cycle leads to deformation of the solid Earth. On a global scale, this deformation is well explained by the loading imposed by hydrological mass variations and can be quantified to first order with space-based gravimetric and geodetic measurements. At the regional scale, however, aquifer systems also undergo poroelastic deformation in response to groundwater fluctuations. Disentangling these related but distinct 3D deformation fields from geodetic time series is essential to accurately invert for changes in continental water mass, to understand the mechanical response of aquifers to internal pressure changes as well as to correct time series for these known effects. Here, we demonstrate a methodology to accomplish this task by considering the example of the well-instrumented Ozark Plateaus Aquifer System (OPAS) in central United States. We begin by characterizing the most important sources of groundwater level variations in the spatially heterogeneous piezometer dataset using an Independent Component Analysis. Then, to estimate the associated poroelastic displacements, we project geodetic time series corrected for hydrological loading effects onto the dominant groundwater temporal functions. We interpret the extracted displacements in light of analytical solutions and a 2D model relating groundwater level variations to surface displacements. In particular, the relatively low estimates of elastic moduli inferred from the poroelastic displacements and groundwater fluctuations may be indicative of aquifer layers with a high fracture density. Our findings suggest that OPAS undergoes significant poroelastic deformation, including highly heterogeneous horizontal poroelastic displacements.

The 2016-2017 Central Italy earthquake sequence struck the central Apennines between August 2016 and October 2016 with Mw ∈ [5.9; 6.5], plus four earthquakes occurring in January 2017 with Mw ∈ [5.0; 5.5]. Here we study Global Positioning System (GPS) stations active during the post-seismic phase including near and far-field domains. We separate the post-seismic deformation from other, mainly seasonal, hydrological deformation signals present in ground displacement time-series via a variational Bayesian Independent Component Analysis technique. For each component, realistic uncertainties are provided to the related ICA-reconstructed displacement field. We study the distribution of afterslip on the main structures surrounding the mainshock, and we highlight the role played by structures that were not activated during the co-seismic phase in accommodating the post-seismic deformation. In particular, we report aseismic deformation occurring on the Paganica fault, which hosted the Mw 6.1 2009 L’Aquila earthquake, and is located further south of the 2016-2017 epicenters; and on a 〜2-3 km thick subhorizontal shear-zone, clearly illuminated by seismicity, which bounds at depth the west-dipping normal faults where the mainshocks nucleated. Since afterslip alone underestimates the displacement in the far-field domain, we consider the possibility that the shear zone marks the brittle-ductile transition, assuming the viscoelastic relaxation of the lower crust as a mechanism contributing to the post-seismic displacement. Our results suggest that multiple deformation processes are active in the first two years after the mainshocks.

A comprehensive surface displacement monitoring system installed in the recently deglaciated bedrock slopes of the Aletsch Valley shows systematic reversible motions at the annual scale. We explore potential drivers for this deformation signal and demonstrate that the main driver is pore pressure changes of phreatic groundwater in fractured granitic mountain slopes. The spatial pattern of these reversible annual deformations shows similar magnitudes and orientations for adjacent monitoring points, leading to the hypothesis that the annually reversible deformation is caused by slope-scale groundwater elevation changes and rock mass properties. Conversely, we show that the ground reaction to infiltration from snowmelt and summer rainstorms can be highly heterogeneous at local scale, and that brittle-ductile fault zones are key features for the groundwater pressure-related rock mass deformations. We also observe irreversible long-term trends (over the 6.5 yr dataset) of deformation in the Aletsch valley composed of a larger uplift than observed at our reference GNSS station in the Rhone valley, and horizontal displacements of the slopes towards the valley. These observations can be attributed respectively to the elastic bedrock rebound in response to current glacier mass downwasting of the Great Aletsch Glacier and gravitational slope deformations enabled by cyclic groundwater pressure-related rock mass fatigue in the fractured rock slopes.