anomalous vp/vs ratios in the focal zone of west/bohemia earthquake swarms

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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