Microseismicity Linked to Gas Migration and Leakage on the Western Svalbard Shelf

Abstract

The continental margin off Prins Karls Forland, western Svalbard, is characterized by widespread natural gas seepage into the water column at and upslope of the gas hydrate stability zone. We deployed an ocean bottom seismometer integrated into the MASOX (Monitoring Arctic Seafloor‐Ocean Exchange) automated seabed observatory at the pinch‐out of this zone at 389 m water depth to investigate passive seismicity over a continuous 297 day period from 13 October 2010. An automated triggering algorithm was applied to detect over 220,000 short duration events (SDEs) defined as having a duration of less than 1 s. The analysis reveals two different types of SDEs, each with a distinctive characteristic seismic signature. We infer that the first type consists of vocal signals generated by moving mammals, likely finback whales. The second type corresponds to signals with a source within a few hundred meters of the seismometer, either due east or west, that vary on short (∼tens of days) and seasonal time scales. Based on evidence of prevalent seafloor seepage and subseafloor gas accumulations, we hypothesize that the second type of SDEs is related to subseafloor fluid migration and gas seepage. Furthermore, we postulate that the observed temporal variations in microseismicity are driven by transient fluid release and due to the dynamics of thermally forced, seasonal gas hydrate decomposition. Our analysis presents a novel technique for monitoring the duration, intensity, and periodicity of fluid migration and seepage at the seabed and can help elucidate the environmental controls on gas hydrate decomposition and release.

Spatial and temporal dynamics of gas-related processes in the Sea of Marmara monitored with ocean bottom seismometers

“…In the Galicia Margin, Diaz et al (2007) interpreted long-lasting tremors and SDEs as the resonance of fluid-filled cracks related to hydrothermal circulation, following the model proposed by Chouet (1988) for volcanic tremors and long-period events (LPE). The same mechanism was proposed to explain similar tremors and SDEs in Taiwan (Hsu et al 2013), the Svalbard Margin (Franek et al 2017) or the SoM (Tary et al 2012;Bayracki et al 2014;Embriaco et al 2014). In all these cases, the fluid-filled crack model was well supported by evidence of cold seeps, increased methane concentration or temperature variations at the bottom, indicating gas seepage out of the seafloor.…”

“…3). As already noticed by other studies (Diaz et al 2007;Franek et al 2017), SDEs can be classified according to their frequency content or waveform. Even though we observed some common and possibly discriminating parameters in the spectrum and waveform of all SDEs, we did not perform such an analysis because of the size of our data set (more than 40 000 events at 2011-OBS1, for instance) but also because such categorization does not necessarily resolve the nature of the SDE source.…”

“…In this framework, we propose that tremors might indicate that gas is migrating out of the seafloor. SDEs and tremors recorded by OBSs in a gas-rich setting could thus be a useful tool to monitor gas activity as already proposed by Franek et al (2017).…”

“…Strong earthquakes (M > 7) might create pathways for free gas to migrate by fracturing the sediments, like in the Arabian Sea (Fischer et al 2013) or in the Gulf of Izmit (Kuscu et al 2005). However, no such phenomena was observed in long-term data in the Svalbard Margin (Franek et al 2017) nor offshore Vancouver Island (Römer et al 2016), where ground motion was weaker. This lack of consistency between gas emission and earthquake can be explained by magnitudes too small to induce gas mobilization (Lapham et al 2013) or site effects induced by the seismic energy radiation pattern (Römer et al 2016).…”

Correlation between tectonic stress regimes and methane seepage on the western Svalbard margin

“…Some of the mechanisms commonly invoked to explain seepage activity across passive margins include climate related gas hydrate dissociation, tidal or seasonal sea-level changes, and pressure in-creases in shallow reservoirs due to fast sedimentation (e.g. Bünz et al, 2003;Hustoft et al, 2010;Karstens et al, 2018;Riboulot et al, 2014;Skarke et al, 2014;Berndt et al, 2014;Wallmann et al, 2018;Westbrook et al, 2009;Franek et al, 2017). While all of these mechanisms may influence seepage systems as deep as the Vestnesa Ridge (> 1000 m deep; as discussed further in Sect.…”