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University of Fribourg – Department of Geosciences

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1 University of Fribourg – Department of Geosciences
Optimizing event detection and location in low-seismicity zones: Case study from Western Switzerland N. Vouillamoz1, M. Abednego2, G.H. Wust-Bloch3, J. Mosar2 1 Universitat Stuttgart, Institut fur Geophysik, Stuttgart, DE 2 University of Fribourg, Department of Geosciences, Fribourg, CH 3 University of Tel-Aviv, Department of Geophysics and Planetary Sciences, Tel-Aviv, IL

2 General context Geophys. J. Int. 2007 “… the inferred fault carries the potential of a M 6 earthquake…” M 6 ‘Ben et Racalbuto’ Le Matin Dimanche

3 Station network: SED permanent stations + UNIFR mini-arrays
SED permanent network upgraded by 2 SNS* ( ): Seismic Navigating System suited for beam processing *Wust-Bloch & Joswig, 2006; Joswig, 2008

4 Earthquake detection by visual sonogram* screening
Sonoview module (NanoseismicSuite package by Sick et al. 2012) Up to several hours in one screen Straightforward visual pattern recognition of earthquake Sonograms* = Spectrograms with dynamic frequency-dependent noise adaptation Enhance the display of weak signal energy down to noise threshold * Joswig, 1990, 1995, 2008

5 Analyst guided earthquake location by HypoLine*
1) Real-time updated display of all location constraints: tS – tP circles tP – tP hyperboloids array beam 2) Real-time update of simulated onsets at the selection of a location 3) Erroneous parameters discarded by jackknifing (no match with global solution) Testing/sliding depth solution through 1D velocity profile * Wust-Bloch & Joswig, 2006 ; Joswig, 2008

6 Analyst guided location vs. residual based solutions
Benchmark of HypoLine solutions with standard approach to location (NonLinLoc)

7 Interactive solutions vs. residual based solutions
HypoLine is better at processing low SNR data and provides robust location with minimal data Erroneous parameters which are unambiguously recognized as not matching the global solution are discarded routinely This is not an option for standard approaches (black box)

8 Earthquake Catalog: FRICAT (2009-2013, 304 events)
1) Detection ( ) 34 ECOS events (0.3 < ML < 2.5) 270 new sonogram detection (-1.7 < ML < 1.5) 2) Location quality (by HypoLine) A-class (50%): 162 events (-1.0 < ML < 2.5) 5 high quality constraints (beams, hyperboloids, circles) all consistent to the global solution B-class (20%): 46 events (not displayed) Unlocated (30%): 96 events

9 Magnitude (ML) estimation

10 Distance attenuation function residuals
And the winner is… HypoLine (three calibration shots at the dead sea valley… ) Uncalibrated below km Calibrated below km

11 Thank you for your attention!!!
Conclusion Thank you for your attention!!! Sonogram analysis results in a 90% increase of earthquake detection relative to ECOS ( ) The magnitude of completeness MC is reduced by about one magnitude unit Sonogram SCOU & STAF Detection rate is not increased by SNS (higher noise level!) but array processing is a powerful tool to constrain location of weak events When waveform quantity and quality drops, the analyst-guided graphical approach of HypoLine is superior to residual-based approach to location as it screens out effects of potential outliers

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