Germany's Lake Laach volcano, long considered dormant, has recently exhibited signs of activity, with over 1,000 tiny earthquakes recorded in a year. These quakes have revealed a deeper, slanted reservoir beneath the lake, a stark contrast to the vertical structure initially expected. This discovery, made by Torsten Dahm at the GFZ Helmholtz Centre for Geosciences, significantly alters our understanding of the volcano's internal dynamics.
The earthquakes are concentrated in a narrow zone, suggesting a complex fault system beneath the lake. This system is tilted towards the Neuwied Basin, a valley on the Rhine, where many small quakes have also been detected. While the geometry of these faults does not definitively prove an imminent eruption, it does indicate a highly active underground system that warrants close monitoring.
To gather data, Dahm's team deployed over 500 sensors across the Eifel hills and utilized a 40-mile fiber-optic cable. This dense network of sensors, with spacing as close as one mile, provided an unprecedented view of the underground, revealing tiny structures that were previously obscured.
The quakes, mostly occurring 6 to 10 miles deep, often in short bursts, are indicative of a system influenced by moving fluids. Pressure from these fluids can weaken rock and cause faults to slip, a process that could be ongoing beneath Lake Laach. However, near one southern step in the fault zone, the sequence resembles standard aftershocks rather than a fluid swarm, adding complexity to the interpretation.
The study also uncovered unusual seismic reflections, suggesting the presence of magmatic fluids or hot gases and liquids released by melt, pooling between rock layers. The strength of these reflections indicates significant fluid accumulation, but the nature of these fluids (magma or magmatic fluids) remains uncertain and is a subject of ongoing investigation.
Lake Laach last erupted 13,006 years ago, and since 2013, deeper rumblings have suggested the presence of fresh material rising from below the earthquake zone. Gas studies at nearby mineral springs have also detected repeated pulses from deep sources, indicating active fluid pathways.
The new earthquake map, combined with earlier imaging, points to a cylinder of unusual rock beneath the volcano, extending approximately 7 miles deep. The active part of this reservoir is estimated to be located 5 to 6 miles down and tilted towards the basin, which may explain the clustering of quakes along one side of the lake.
The regional stress field near the volcano has also been found to rotate, indicating overpressure, where swelling material pushes sideways and alters fault behavior. Most quakes along the main fault slipped sideways, while a nearby cluster dropped downward, suggesting a hidden basin edge. A few quakes even pushed upward, indicating a localized pressure pattern distinct from the broader region.
The Eifel region, where Lake Laach is located, is not a single-vent volcano but a spread-out volcanic field with multiple old vents. This means that future eruptions might not necessarily occur at the last vent or follow the same route, making precise mapping crucial for hazard assessment, gas monitoring, and land-use planning.
Despite the intriguing findings, there are still uncertainties. The model does not confirm overpressure in the current reservoir, and pre-existing weak zones in the crust could also influence the quakes. Further detailed quakes and stress tests are needed to distinguish between these explanations.
In conclusion, the new data significantly enhances our understanding of the Eifel region's volcanic activity, providing a sharper baseline for assessing future unrest. The study, published in the Geophysical Journal International, underscores the importance of continued monitoring and research to better understand and manage volcanic hazards.
