Anomalous Vp/Vs ratios Anomalous Vp/Vs ratios in the focal zone ofin the focal zone of
West/Bohemia earthquake swarmsWest/Bohemia earthquake swarms
T. Fischer1,2, T. Dahm3
(1) Institute of Geophysics, Czech Academy of Sciences(2) Faculty of Science, Charles University in Prague
(3) GFZ Potsdam
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What drives mid-crustal earthquake swarms What drives mid-crustal earthquake swarms in West Bohemia? in West Bohemia?
• fluid injection (e.g. Hainzl et al. 2012)
• magmatic intrusions, e.g. Dahm, Fischer & Hainzl (2008)?
• degassing events, e.g. Hainzl & Fischer (2002)?
• aseismic creep and structure, e.g. Bankwitz et al. (2003), Neunhöfer & Hemman (2005)?
=> Search for anomalous processes in the focal zone, e.g. vP/vS
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Standard method for Standard method for vvPP/v/vSS: Wadati diagram: Wadati diagram
•
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• events i, stations j
• constant vP/vS
• simple and robust method
• average vP/vS in the area
Modified Wadati method – remove Modified Wadati method – remove tt00
Remove t0
•events i, stations j
•constant vP/vS
•simple and robust method
•average vP/vS in the area
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Double-difference Wadati methodDouble-difference Wadati method
• Pairs of events• For each event pair
arrival time differences• S-P: double difference=> Sensitive to the vP/vS in
the focal zone
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ΔtP12, ΔtS
12
… arrival time differences
Anomaleous source region:Anomaleous source region:Single-difference Wadati method Single-difference Wadati method
Synthetic data:α0/β0 = 1.89
α1/β1 = 1.50
The normal velocity ratio of the crust is retrieved
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Anomaleous source region:Anomaleous source region:Double-difference Wadati methodDouble-difference Wadati method
Synthetic data:α0/β0 = 1.89
α1/β1 = 1.50
The anomalous velocity ratio of the focal zone is retrieved
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ΔtP12, ΔtS
12
… arrival time differences
Inversion techniqueInversion technique• Single-differences
minimize A =
• Double-differencesminimize A =
• Two-step inversion1. Grid-search in γ2. For each γ we calculate δ by minimizing
median [sum(Ai)2] (Least Median of Squares)
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Synthetic testSynthetic testRobustness test using Data = Synthetic travel times +
+ 20 outliers with σ = 1s ++ normal noise with increasing σ
=> Able to retrieve both the normal and anomaleous ratios- L1 and LMS inversion performes similar
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vvPP/v/vSS ratios of the West-Bohemia swarms ratios of the West-Bohemia swarms
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vvPP/v/vS S of 1997 swarmof 1997 swarm
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vvPP/v/vS S of 1997 swarmof 1997 swarmTwo periods during swarm: 1.-17.1., 18.1.-29.1.Post-swarm period 30.1.-31.12.-Constant regional ratio-Focal zone vP/vS <1.3 (down to 0.94) during swarm
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vvPP/v/vS S of 2000 swarmof 2000 swarm• dd
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vvPP/v/vS S of 2000 swarmof 2000 swarm
7 periods•Constant regional ratioFocal zone•Normal ratio during the swarm onset•Low ratio of 1.3 during the onset of the 2nd swarm period (15 Oct)
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vvPP/v/vS S of 2008 swarmof 2008 swarm•
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vvPP/v/vS S of 2008 swarmof 2008 swarm
8 periods•Constant regional ratioFocal zone•Low ratio of 1.46 during the swarm onset•Normal ratio afterwards
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Velocity ratio during swarmsVelocity ratio during swarms1997: ratio of 0.9 related to •small volume, one week period •small ΔtP range -0.15 to +0.15 s
2000: ratio of 1.3 related to •larger volume, ca. 5 days period•larger ΔtP range -0.3 to +0.5 s
2008: ratio of 1.48 related to •small volume, ca. 1 day period•small ΔtP range -0.2 to +0.25 s
Anomalous ratios correspond to different areas of the fault plane=> rather process-related that structural feature
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Effect of arrival time errorsEffect of arrival time errors- WadatiDDrequires high precision – short rays through the
focal zome compared to the rays to stations+ Large number of event – station pairs• Test of noisy input data for swarm and station geometries
similar to the studied ones:– typical spread of ∆tP ≈ ±0.4 s,
=> standard deviations of arrival times up to 0.15 s acceptable
– picking errors estimated as 1 sample (4 ms) for P- and 2 samples (8 ms) for S-waves
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Physical interpretation of low vP/vSPhysical interpretation of low vP/vSPorous sediments saturated by brine and gas: Gassmann equations (Mavko, 2003)
N, K – saturated shear and bulk moduliK1, K2 – bulk moduli of the rock and fluidα – Biot-Willies parameterΦ – porosity
=> Low velocity ratios explainable by spontaneous transition of fluid to gaseous phases
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Other results of low vP/vS for swarmsOther results of low vP/vS for swarmsVavryčuk (JGR 2011)•vP/vS derived from the slope of linear relation between ISO and CLVD of MT •small vP/vS in the focal volume; 1.45 for the 1997 swarm 1.35 for the 2008 swarm
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Estimates of fluid pressure in the focal zoneEstimates of fluid pressure in the focal zone
• Vavryčuk (2002): 5 MPa based on the MT and stress analysis
• Hainzl et al. (2012): 20-30 MPa based on – Stress deficits during the 2000 and 2008 swarms– Fracture model applied to spreading of the 2008 swarm
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ConclusionsConclusions• WadatiDD method designed to analyze the vP/vS in the
focal zones • Synthetic tests show rubustness against arrival time
errors• Application to the 1997, 2000 and 2008 swarms gives
anomaleous low vP/vS of 0.94, 1.3, 1.46• Such unphysical values possible in the presence of
gaseous phase• Consistent with vP/vS results of Vavrycuk for 2008
swarm
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