Researchers told the hearing on Tuesday 6 October that 4,730 alerts recorded over four nights in May at Woldrandsveen reserve were not signs of earthquakes. Sensors beside a restored peat bog had picked up a loud chorus of European tree frogs, mixed with small bursts of methane from the wet ground. No earthquake warning was issued, but the volume of alarms sent staff scrambling to explain the readings.
The reserve, in Drenthe province, is part of a long-running effort to restore drained peatland and track how the land settles as it becomes wetter. Its monitoring network cost €2.1 million (about $2.4 million) and was designed to detect tiny shifts in the ground. The inquiry began after an automated system forwarded the extraordinary cluster of readings to a provincial emergency desk.
How the bog fooled sensors
The Lowland Seismology Centre’s field team compared recordings from 11 sensor pods with sound captured by microphones and measurements from pressure plates. Pods fixed to a wooden boardwalk registered a repeated vibration pattern during the frogs’ nightly chorus; units anchored in firmer ground recorded far less. The team estimated that about 5,900 male frogs were calling across the wetland during the busiest night.
“The bog gave us a concert, a percussion section and a false earthquake report, all before midnight,” said Anna D. Wendt, an ecologist who led the field study. The hearing was told that bursts of gas rising through peat added isolated thumps, while the boardwalk carried the frogs’ low-frequency calls into the sensors. Together, the signals confused software trained to sort ground movement from background noise.
The bog gave us a concert, a percussion section and a false earthquake report, all before midnight.
Anna D. Wendt, ecologist
The network did not mistake the calls for a damaging quake, said Kutay Karaer, the centre’s monitoring director. Instead, its screening system labelled the readings “unusual seismic activity” and passed them to a human review queue. Investigators found the first cluster within 38 minutes, but the alert system continued counting similar pulses until dawn because it treated each one as a fresh event.
A new tool for peat research
The episode has prompted a simple fix: three pods will be moved from the boardwalk to solid ground, and the software will be taught to compare vibration records with nearby sound levels. Researchers also plan to keep one boardwalk sensor in place as a frog-activity monitor. Wendt said the chorus may help track wetland recovery because frogs return when shallow pools and reed margins improve.
Karaer said the team’s revised system would be tested through the next spring breeding season, with monthly summaries shared at public meetings. The reserve has no plans to quiet the frogs or drain the pools; staff say the nightly calls are a welcome sign of habitat returning. A sensor meant to listen for a restless Earth, the hearing concluded, had instead found one of the bog’s liveliest residents.
