Magma Migration at Laguna Del Maule, Chile, Using Well Constrained Seismicity and Receiver Function Imaging

Continental volcanic arcs are driven by melting in the mantle wedge between a subducting oceanic plate and overriding continental plate. These melts are emplaced within the continental crust where they then fractionate and evolve, producing silicic volcanic rocks. From observations of these systems globally, we understood them to have magmatic plumbing networks organized into transcrustal systems, and while the geometries of these systems are somewhat constrained, the links between magma storage regions, melt migration and surface unrest remains poorly understood. At Laguna Del Maule (LdM) in central Chile, where volcanic unrest is being currently monitored, we use two densely deployed seismic datasets of nodal and broadband seismometers to study these connections in detail. Here, we present interpretations using well constrained earthquake locations and high resolution crustal-scale seismic imaging using Receiver Functions (RFs), to infer the magma plumbing network and interconnectivity within this modern arc volcano setting.

Over 4300 events were detected within the periods between 2015-2018 and 2022-2024 and are divided into shallow and deep groups. Shallow seismicity is separated into clusters consistent with prior observations that link fault activity to shallow magma intrusion. Those events occurring within the deep crust (~12-30 km) are a new observation, containing a mixture of high and low frequency earthquakes. Through Frequency Index Analysis, we classify those deep events with low frequencies as Deep Long Period earthquakes (DLP). These have been observed in other volcanic arcs, but this data contains the first evidence of DLP seismicity within the Andes. The deep higher frequency events are provided in a pronounced one-day swarm of activity in 2018, all with similar magnitude and frequency index. The swarm has a vertical extent between ~21-26 km depth, and we interpret this activity to be a Volcano-Tectonic swarm (VT) related to magma migration within the middle-lower crust. In the RF images, the VT swarm is located between the top of a low velocity zone (LVZ) in the lower crust, and the base of an upper crust LVZ. The lower crust LVZ likely represents an area of deep magma storage that intermittently incubates the upper crust system with batches of basic magmas. RF images of the upper crust LVZ are consistent with prior geophysical estimates of the geometry and approximate spatial extent of LdMs shallow magma chamber.

Three months following the deep VT swarm, vertical surface uplift in the local GPS record accelerates. We therefore infer that the VT swarm was driven by the delivery of a new batch of magma from lower to upper crustal magma reservoirs. This applied additional pressure to the base of the upper crustal reservoir, leading to a surficial response in a lag-time consistent with the systems hydraulic diffusivity (~20 m²/s). Since this inflation rate has been maintained at least until 2020, the VT swarm may represent the establishment of a new preferred magma ascent path. These results indicate that volcanic unrest is preceded months in advance by seismic activity occurring within the middle-lower crust, applying bottom-up reservoir pressurization in arc volcanoes.