Repeating caldera collapse events constrain fault friction at the kilometer scale

Fault friction is central to understanding earthquakes, yet laboratory rock mechanics experiments are restricted to, at most, meter scale. Questions thus remain as to the applicability of measured frictional properties to faulting in situ. In particular, the slip-weakening distance dc strongly influences precursory slip during earthquake nucleation, but scales with fault roughness and is challenging to extrapolate to nature. The 2018 eruption of Kīlauea volcano, Hawaii, caused 62 repeatable collapse events in which the summit caldera dropped several meters, accompanied by MW 4.7 to 5.4 very long period (VLP) earthquakes. Collapses were exceptionally well recorded by global positioning system (GPS) and tilt instruments and represent unique natural kilometer-scale friction experiments. We model a piston collapsing into a magma reservoir. Pressure at the piston base and shear stress on its margin, governed by rate and state friction, balance its weight. Downward motion of the piston compresses the underlying magma, driving flow to the eruption. Monte Carlo estimation of unknowns validates laboratory friction parameters at the kilometer scale, including the magnitude of steady-state velocity weakening. The absence of accelerating precollapse deformation constrains dc to be ≤10 mm, potentially much less. These results support the use of laboratory friction laws and parameters for modeling earthquakes. We identify initial conditions and material and magma-system parameters that lead to episodic caldera collapse, revealing that small differences in eruptive vent elevation can lead to major differences in eruption volume and duration. Most historical basaltic caldera collapses were, at least partly, episodic, implying that the conditions for stick–slip derived here are commonly met in nature.

Our knowledge of rock friction comes from laboratory experiments on samples from centimeters to at most meter scale (1, 2). These experiments have led to rate- and state-dependent friction laws (3, 4), which together with continuum fault models explain many features of natural earthquakes (5, 6). Extrapolation of laboratory-derived constitutive parameters to faults in situ, however, has been challenging, particularly for the characteristic slip weakening distance, dc, the displacement scale over which friction degrades from nominally static to dynamic values. In the laboratory dc ranges from several to tens of micrometers, but scales with fault roughness (7). Some seismological estimates are up to five orders of magnitude larger (8), but are sensitive to the decrease in shear strength at earthquake rupture fronts, leading to weakening lengths that scale with dc, but can be much larger (9, 10). Understanding the magnitude of dc in situ is crucial because the amount of potentially observable precursory slip scales with dc (11). Significant insights have been gained from in situ fluid injection experiments into faults that induce aseismic slip and seismicity (12–14), yet constraints on the parts of faults that actually generate earthquakes are rare.Collapse at basaltic shield volcanoes typically occurs in repeated discrete events, generating characteristic deformation transients and very long period (VLP) earthquakes (15–17). Rapid outflow of magma causes the pressure in subcaldera magma reservoirs to decrease, leading to an increase in stress in the overlying crust. Collapse initiates if this stress reaches the crustal strength, forming ring faults bounding down-dropped block(s) (18). Once initiated, collapse transfers the weight of the overlying crust onto the magma reservoir, maintaining pressure necessary for the eruption to continue (19). Thus, caldera collapse is not simply a response to the rapid withdrawal of magma, but is also an essential process in sustaining these eruptions.The 2018 Kīlauea collapses were quasi-periodic and exceptionally well monitored by nearby global positioning system (GPS) and tilt stations, including GPS stations on the down-dropped block(s). These data can be used to infer stress changes on the caldera-bounding ring faults, making them effectively kilometer-scale stick–slip experiments. The highly repeatable nature of the collapses, as well as constraints on the changes in magma pressure prior to the onset of collapse (20), minimizes uncertainty due to otherwise difficult to constrain initial conditions.