Seismicity Near Western Greenland’s Draining Lakes Author: Joshua D Carmichael | U. of Washington | Seismic Contact: [email protected]
Ian Joughin, (U.W.) | Mark Behn (WHOI) | Sarah Das (WHOI) | Dan Lizarralde (WHOI)
Contributing Authors
NASA Grants: 0230338 and 0233823
Keywords: multiplets, seismicity, Greenland Ices Sheet, correlation detection, clustering, surface energy balance, seismic network, statistical signal processing, modeling, supraglacial lakes, icequakes
Correlation Detectors Multiplets are identified using a adaptive, network-based correlation detector. The detection capability is maximized at a fixed false alarm rate by appropriately weighting each correlogram to maximize the signal degrees of freedom (DOF) in the stack. The area under the PDF in the acceptance region for the correlation coefficient under the signal absent case (versus the signal case) give the false alarm rate (versus detection rate). Waveforms and their envelopes are correlated and clustered in distinct applications.
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The Western Greenland Ice Sheet hosts site to lakes that drain from transient hydrofracture events in early summer. These establish surface-to-bed pathways (moulins) for melt water that may facilitate sliding and alter the morphology of the drainage system. Mid-summer melt can supply water to the bed through moulins to trigger speedup. Subglacial models suggests melt variance has a greater influence on ice speed compared to volume. We therefore address the transient response of the ice sheet to melt using seismic and GPS observables near these lakes.
The Lake Network is composed of GPS and seismic receivers. Sites that include a 3-C L-28 geophone are indicated with a triangle. The current network configuration is designed to capture drainage.
Pre-Drainage Seismicity includes 74 repeating seismic events (multiplets) and a total of ! 1000 events, detected network-wide 3 days prior to lake tapping. The 3 operational stations are indicated by bold instrument outlines. The most populous multiplet (21 events) is composed of apparent surface waves that are likely due to fracture propagation. The signal envelopes (green) were used to identify clusters and are shown with raw waveforms.
Drainage Seismicity includes 32 multiplets and a total of ! 1000 network-wide detected events. Most of the wavefield includes ambiguous events that may be due to discrete rupture events, or cavitating bubbles coincident with air + water moulin drainage. Nearly all events originate within the array and include surface waves and coda, and locate near NL07
Post-Drainage Seismicity observations include initial quiescence and additional station coverage ( ! 7 geophones), and reduction in local multiplet generation. Additional clusters were sparse and contained ! 5 events or less. Additional coverage increased confidence in locations. Epicenters for additional events were downstream of the drainage site, and had less scattering. We suggest a basal source, to be evaluated with coincident GPS.
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Start Date 18!Jun!2011 00:47:24
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Installation of a geophone network is difficult due to melt rates that tend to expose the geophones and cause tilt-out or submergence in melt water. Counter-measures may also have a limited effect. We installed geophones on pole-platforms buried 2m beneath the ice to increase coupling. Upon melt out, the geophone poles acted as axially-loaded columns with resonance frequencies matching those of the seismic wavefield. This caused signal interference, and reduced detection rates during the late summer. Eventually, the geophones titled out. However, data integrity was improved overall, prior to resonance.
Waveform Envelopes provide temporal localization of seismogram energy. They have two applications here. Right: Envelopes are temporally shifted according to theoretical travel times over a source grid and then summed to compute a spatially dependent map of signal coherency. Below: Envelopes are cross correlated and the differential arrival time between receivers is compared to cross correlation times. The grid point that matches the double difference provides a epicentral location. Envelopes are products of Ricean random variables and therefore, correlation detection performance can be quantified.
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Start Date 06!Jul!2011 00:15:15
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High Meltwater Variance in spring/early summer is coincident with high-variance seismicity. However, sustained melt production " 1 day is coincident with decreases in seismicity. Further, net seismicity seems to decrease after peak melt. Our interpretation of the role of melt is ongoing, and will supplemented with new (as of Nov. 2012) GPS data.
Detectability of seismic signals is quantified using an adaptive energy detector. This detector estimates the effective degrees of freedom (DOF) of the noise, any present signal, and the relevant SNR/hour. We correct the null and alternative hypothesis distributions using this DOF. Temporally correlated noise lowers detection probabilities and is accounted for in our monitoring program. We find detection probabilities and SNR are seasonally dependent, with higher variance and a higher floor in spring compared to summer. Summer detection rates are likely reduced relative to true rates.
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Spatially- Resolved Waveform Coherency Post-Drainage
References & More
Geophone NLBS installed in ice, no pole or platform
SNR of detections
Geophone NLBS installed on pole/platform
Geophone NLBS installed on pole/platform, and South-Lake receiver with same configuration
North Lake Network installed along with reinstall of NLBS
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Carmichael, J. D., E. C. Pettit, M. Hoffman, A. Fountain, and B. Hallet (2012), Seismic multiplet response triggered by melt at Blood Falls, Taylor Glacier, Antarctica, J. Geophys. Res., 117, F03004, doi:10.1029/2011JF002221 Research Material: http://earthweb.ess.washington.edu/~joshuadc/ Steven M. Kay. Fundamentals of Statistical Signal Processing: Detection Theory. Prentice-Hall Inc., Upper Saddle River, New Jersey, USA, 1st edition, 1998. Sarah B. Das, Ian Joughin, Mark D. Behn, Ian M. Howat, Matt A. King, Dan Lizarralde, and Maya P. Bhatia. Fracture Propagation to the Base of the Greenland Ice Sheet During Supraglacial Lake Drainage. Science, 320(5877):778–781, May 2008.
The Lake Tapping Seismogram?
This website includes presentations, a paper, and the movie of a detection methodology if you are interested.
