Dynamic Full‐Field Imaging of Rupture Radiation: Material Contrast Governs Source Mechanism

Abstract

In seismology, the rupture mechanisms of an earthquake, a glacier stick‐slip and a landslide are not directly observed, but inferred from surface measurements. In contrast, laboratory experiments can illuminate near field effects. The near field reflects the rupture mechanism but is highly attenuated in the case of real‐world surface data. We directly image the elastic wave‐field of a nucleating rupture non‐invasively in its near‐field with ultrasound speckle correlation. Our imaging yields the particle velocity of the full shear wave field at the source location and inside the 3D frictional body. We experimentally show that a strong bimaterial contrast, as encountered in environmental seismology, yields a unidirectional or linear force mechanism for pre‐rupture microslips and decelerating supershear ruptures. A weak contrast, characteristic for earthquakes, generates a double‐couple source mechanism for sub‐Rayleigh ruptures, sometimes preceded by slow deformation at the interface. This deformation is reproduced by the NF of a unidirectional force.

Internal Structure of the Central Garlock Fault Zone From Ridgecrest Aftershocks Recorded by Dense Linear Seismic Arrays

“…The depths of events that generate FZHWs imply that the bimaterial interface extends to at least 8 km (Figure 1a). The sense of velocity contrast and left‐lateral motion across the central Garlock fault suggest a preferred propagation direction of subshear earthquake ruptures to the east‐north‐east, and an opposite preferred direction for supershear ruptures (Aichele et al., 2022; Andrews & Ben‐Zion, 1997; Shi & Ben‐Zion, 2006; Weertman, 1980, 2002). The velocity contrast interface likely does not extend continuously beyond array A5 in the southwest, as FZHW detections were not found for events southwest of array A5.…”

Seismological Stress Drops for Confined Ruptures Are Invariant to Normal Stress

“…Like previous scaled experiments (Aichele et al, 2023;Baumberger et al, 2003;Corbi et al, 2011Corbi et al, , 2017Latour et al, 2013), one of the most important differences between this experiment and traditional laboratory faults is the compliance of the system. The elastic modulus of the PDMS utilized in these experiments is ∼50,000X smaller than that of rock, indicating that our 8.5 × 8.5 cm fault is the equivalent of ∼4 × 4 km of fault area in the earth.…”

“…Softer materials have been shown to be excellent analog materials for studying geophysical systems (Aichele et al., 2023; van Otterloo & Cruden, 2016) and using PDMS offers many advantages for an experimental fault. First, it allows us to embed ∼75 μm aluminum oxide tracer particles 1 mm below the frictional interface, whose displacement can be imaged providing a direct measurement of deformation, and thus calculation of slip velocities.…”

“…However, by using both a softer, transparent material for our sample slabs, we gain the unique capabilities of directly measuring strains at the frictional interface, actively controlling normal stress heterogeneity, and having events that are predominantly contained within the fault, each of which are difficult or impossible in traditional rock friction experiments. Using softer materials as analogs (Aichele et al, 2023;Baumberger et al, 2003;Brune, 1993;Corbi et al, 2011Corbi et al, , 2017Latour et al, 2013;Schallamach, 1971) and directly imaging material deformations (Rubino et al, 2017(Rubino et al, , 2022 have proven to be valuable approaches for understanding the behavior of faults.…”