Filed study

Volcanoes are heterogeneous and unstable structures prone to collapse and, as such, they present a physical and economic risk to the population living in their proximity. Hydrothermal alteration, common to many volcanoes, changes rock strength and is thought to be involved in the process of stability decrease. To understand the stability of volcanoes, large-scale numerical models incorporate petrophysical and mechanical parameters that have been measured in the laboratory. Point load testing, on the other hand, is a field technique that can quickly measure a large sample suite, which removes the challenges associated with rock transportation or selecting appropriate samples that accurately reflect the heterogeneity of the volcanic structure. Therefore, we tested almost 550 rocks from seven different locations on La Soufrière de Guadeloupe, an active andesitic stratovolcano in the Eastern Caribbean. We chose to collect irregular-shaped rocks, from 35 to 85 millimetres in height. We sampled four landslides on the flanks of the dome that exhibited different alteration intensities, two fumaroles at the summit of the dome, and a large fault on the side of the volcano. The rocks were assigned an alteration grade value depending on the visible intensity of the alteration, using parameters such as colour and rock texture. After measuring the bulk rock density of each rock using the Archimedes density method, they were broken in a point load tester. We observed that strength increases with the increase in bulk rock density. We also observed a large range in strength for a given density, highlighting the heterogeneity of the sample suite. For example, at a bulk density of around 2000 kg/m3, we noticed that the breaking force can vary between 0,43 and 9,66 kN. Also, rocks that were assigned with higher alteration grades were found to be less dense and weaker. Point load testing can provide a deeper understanding of the heterogeneity of the volcano and spatial strength distributions to improve large-scale volcano stability modelling, hence helping monitor volcanic unrest and mitigate future risks. 

High strain rates generated by moderate to large earthquake rupture cycles cause wall rock fragmentation, brecciation, and in-situ shattering, and enhance fluid flow in damage zones of continental thrust faults. This process reduces the rigidity of the hanging wall and raises the question of how hanging wall blocks slide on low angle faults without significant internal structural thickening. Our mapping shows that the Muddy Mountain Thrust slid for 10s-100s km on a low angle basal plane, which suggests the fault was weaker than the hanging wall wedge, an inference which requires significant hanging wall healing. To test this hypothesis, we conducted a uniaxial dynamic compressive loading test using an 8.5 m-long Split Hopkinson Pressure Bar apparatus to observe fracturing behavior at extreme stressing rates similar to conditions in wall rocks next to a seismic fault. We tested samples of healed breccias from the Sevier Muddy Mountain Thrust (MMT) in southern Nevada, USA, and their intact dolostone equivalent. By comparing the natural vs. laboratory deformation features, we constrained the strength recovery associated with breccia healing mechanisms in nature. The uniaxial compressive strength of the pristine dolostone ranges from 0.27 to 0.56 GPa at strain rates of 100 - 110 s-1. In contrast, the healed breccia is significantly weaker, with a uniaxial compressive strength of approximately 0.116 to 0.118 GPa, and deformed at strain rates up to 225 - 240 s -1. The pristine samples developed thicker (> 2.5 mm) throughgoing fractures during lab experiments, than in healed breccia (< 1.5 mm), and micro-cracks mainly concentrated around grain boundaries are more prominently seen in the pristine samples than in the healed breccia. The strength of the brecciated samples constrains an upper limit to the strength of the fault and our new data provide valuable insights into the strength recovery associated with breccia healing mechanisms in nature.

Geometrical heterogeneity and variation of mechanical properties along and across active fault zones may affect the seismicity of such faults. Additionally, the nature of host-rocks juxtaposed by the fault representing lithological units of distinct mechanical property plays a key role in controlling the deformation associated with faulting. In this study, we utilized excellent outcrops in Baza sub-basin of Southern Spain along a 30km active intrabasinal fault; namely, Galera Fault (GF), which is geographically exposed at different structural positions. GF acts as a transfer fault that is kinematically coupled with the normal Baza fault to the west. As part of our investigation, we conducted scanline surveys that included structural analysis, in-situ measurements such as Young's Modulus (E) using Schmidt hammer, permeability as well as P-wave velocity (Vp) and XRD analysis from the collected samples. Structural analysis reveals that the orientation of the segments of GF strike slightly changes from north to south, mostly along NE-SW to ENE-WSW and show strike-slip or normal fault kinematics. In the northern sector of GF, XRD analysis suggests that the fault rocks and the corresponding host rocks are calcite rich (~94-99%). On the other hand, the host-rock is very heterogeneous in the central and southern segments with either dolomite or gypsum as the most dominant mineral. E estimates generally show lower values along the fault planes relative to the adjacent host rocks due to the presence of higher proportion of clay minerals in the fault rocks (Figure 1). However, E and Vp in fault rocks exhibit higher values as compared to their adjacent host-rock only when the proportion of calcite in the fault rocks is high coupled with no detectable clay minerals.  Our study reveals that compartmentalization of geomechanical properties at different structural positions of active fault zone in Tertiary rocks is significantly controlled by the mineral composition of the host rocks through which the faults cut across. Therefore, we conclude that in addition to the regional tectonic forces, the distribution of the different types of host-rock representing the mechanical stratigraphy of the basin is expected to play a vital role in influencing the style of faulting and seismicity.



Figure 1. Plots showing the distribution of Young's Modulus and location of subsidiary faults along two scanlines oriented along and across the strike of the major fault (GF) in its central segment.