Sierra Negra’s eight-hour gap between earthquake and magma intrusion

At 09:15 UTC on 26 June 2018, a magnitude-5.4 earthquake struck Sierra Negra in the Galápagos. Magma intrusion began around 17:00; the eruption followed around 19:40. A January 2026 reanalysis finds small tremors within that long interval, suggesting a physical step between the quake and moving magma.[1]

Detecting those tremors requires recognising a different kind of seismic signal. Ordinary rock fractures often produce abrupt arrivals with substantial high-frequency energy. Long-period events build more gradually and contain less of that fast vibration. In 2024, Yiyuan Zhong and Yen Joe Tan showed that widely used AI phase pickers systematically lost performance as earthquake frequency content fell.[2]

They retrained models using volcanic recordings that deliberately included long-period events. The task was to mark when the seismic waves arrived, enabling subsequent earthquake location; classification came later. Tests on volcanic regions excluded from training showed improved detection. The distinction matters physically: a catalogue dominated by easily recognised fractures can leave fluid-related activity poorly represented.[2]

The Sierra Negra team used that Volpick model, then waveform matching, to expand the record. Five families of repeating long-period events appeared about two hours after the quake and continued for roughly six hours, near the later intrusion’s starting point. Some paired signals reversed polarity, supporting rapidly changing local stresses.[1]

Fluids can alter when stressed rock gives way. USGS seismologist David Shelly explains the mechanism using Yellowstone: water-rich fluids enter a fault and reduce the force clamping its sides together. The energy released can still come from accumulated tectonic stress. The fluid supplies the trigger by changing the fault’s resistance to slipping.[3]

That also explains why a fluid signal does not automatically identify rising magma. At Yellowstone, water-rich fluids can emerge as deeper magma cools, then migrate through existing faults. Fault motion and volcanic fluids can therefore act together. The useful distinction is between what supplies the stored stress and what changes the conditions for its release.[3]

Sierra Negra’s authors infer fluid weakening around the reservoir allowed magma to escape. Ground-position measurements showed no significant intervening deformation. The fluid’s identity and causal role remain uncertain, but its seismic signature supplies a plausible missing step in the eruption’s initiation.[1]

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