Characterization of microseismicity at the Húsmúli reinjection area, Hengill Geothermal Field, Southwest Iceland​

Induced seismicity remains a major challenge for geothermal projects, with implications for public acceptance and operational risk management. Understanding how fluid injection interacts with fault structures to generate seismicity is therefore essential. The Húsmúli reinjection area in the Hengill geothermal field (SW Iceland) provides an ideal setting to investigate these processes due to its sustained induced seismicity and long operational history. Here, we present an improved seismicity catalog (2018–2021; COSEISMIQ project) and a waveform-based detection workflow that substantially increases catalog completeness and enhances spatiotemporal resolution.

We first improve the initial automatic catalog by re-picking events with unrealistic Vp/Vs ratios (Wadati analysis), high RMS location misfits, or unrealistic depths (e.g., airquakes). Phase picks are then refined using a cross-correlation (CC)-based repicking approach: events are clustered into waveform-similar families, traces are aligned and stacked to increase signal-to-noise, and consistent arrival times are obtained from a single-family reference pick. Missing picks are recovered by inspecting waveforms around the expected arrival time window and estimating phase onsets, accepting only traces with  CC ≥ 0.65 with respect to other family members.

3D spatial clustering of the refined catalog reveals NE–SW oriented seismic lineaments consistent with mapped faults and inferred fluid migration pathways. In contrast, nearby E-W structures show little to no seismicity, suggesting permeability barriers and reservoir compartmentalization. Repeating earthquakes occur along narrow fault segments, indicating repeated rupture of localized slip patches. To further enhance detection, we use QuakeMatch, a single-station template matching workflow using high-SNR events as templates at the station with the best waveform quality and data completeness. This expands the catalog from 3,647 to 12,899 events, lowering the magnitude of completeness and revealing numerous low-magnitude earthquakes previously missed by the automatic STA/LTA processing due to low signal-to-noise or waveform overlap. The resulting catalog shows swarm-like activity typical of fluid-driven seismicity and episodic bursts. A prominent sequence on 15 November 2020 (M_LX  = 4.08) is preceded by foreshocks and followed by multiple M_LX ≥ 3.0 aftershocks. Gutenberg–Richter analysis indicates a decrease in b-values prior to the mainshock, consistent with stress build-up and suggesting potential precursory behaviour relevant for operational monitoring.