Abstract
Earthquakes are frequently accompanied by public reports of audible low-frequency noises. In 2018, public reports of booms or thunder-like noises were linked to induced earthquakes during an Engineered Geothermal System project in the Helsinki Metropolitan area. In response, two microphone arrays were deployed to record and study these acoustic signals while stimulation at the drill site continued. During the 11 day deployment, we find 39 earthquakes accompanied by possible atmospheric acoustic signals. Moment magnitudes of these events ranged from -0.07 to 1.87 with located depths of 4.8–6.5 km. Analysis of the largest event revealed a broadband frequency content, including in the audible range, and high apparent velocities across the arrays. We conclude that the audible noises were generated by local ground reverberation during the arrival of seismic body waves. The inclusion of acoustic monitoring at future geothermal development projects will be beneficial for studying seismic-to-acoustic coupling during sequences of induced earthquakes.
Highlights
Earthquakes are frequently accompanied by public reports of audible low-frequency noises
We have presented evidence for infrasonic and audible atmospheric acoustic signals generated by at least one low magnitude fluid-induced earthquake. These observations are notable for two reasons: (1) these are the first recorded earthquake-generated acoustic signals from induced earthquakes, and (2) they represent the lowest magnitude events to be recorded by acoustic microphones. (There are reports of audible noises from earthquakes with magnitudes as low as −2 but these events were not recorded with microphones5.) Manual inspection of data identified at least 39 events where possible acoustic waves were recorded propagating across at least one array of sensors (Fig. 2)
We have presented acoustic events recorded within the Helsinki Metropolitan area in July 2018 during hydraulic stimulation at a pilot Engineered Geothermal System project
Summary
Earthquakes are frequently accompanied by public reports of audible low-frequency noises. During the 11 day deployment, we find 39 earthquakes accompanied by possible atmospheric acoustic signals Moment magnitudes of these events ranged from −0.07 to 1.87 with located depths of 4.8–6.5 km. We describe a case study of local acoustic waves generated by earthquakes during a hydraulic stimulation project in Finland, one of the first documented recordings of acoustic signals from an induced earthquake sequence and are amongst the lowest magnitude events to be recorded. From 4 June to 1 August 2018, a total of 8412 earthquakes were automatically recorded by the network out of which 1977 were suitable for relocations and magnitude c alculations[27] These events were located across three distinct clusters ranging in depths of 4.8–6.6 km and moment magnitudes ( Mw ) of −0.76 to 1.87 (Fig. S1 in Supporting Information). Spatial distributions of the reports were strongly correlated with the SH radiation pattern of the reverse faulting mechanism in the event[29]
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