1. Anomalous Vp/Vs ratios in the focal zone of West/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
AIM meeting 2012, Smolenice

2. What drives mid-crustal earthquake swarms 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
AIM meeting 2012, Smolenice

3. Standard method for vP/vS: Wadati diagram
events i, stations j
constant vP/vS
simple and robust method
average vP/vS in the area
AIM meeting 2012, Smolenice

4. Modified Wadati method – remove t0
events i, stations j
constant vP/vS
simple and robust method
average vP/vS in the area
AIM meeting 2012, Smolenice

5. Double-difference Wadati method
ΔtP12, ΔtS12 … arrival time differences
Pairs of events
For each event pair arrival time differences
S-P: double difference
=> Sensitive to the vP/vS in the focal zone
AIM meeting 2012, Smolenice

6. Anomaleous source region: Single-difference Wadati method
Synthetic data: α0/β0 = 1.89 α1/β1 = 1.50 The normal velocity ratio of the crust is retrieved
AIM meeting 2012, Smolenice

7. Anomaleous source region: Double-difference Wadati method
Synthetic data: α0/β0 = 1.89 α1/β1 = 1.50 The anomalous velocity ratio of the focal zone is retrieved
ΔtP12, ΔtS12 … arrival time differences
AIM meeting 2012, Smolenice

8. Inversion technique Single-differences minimize A =
Double-differences minimize A =
Two-step inversion
Grid-search in γ
For each γ we calculate δ by minimizing median [sum(Ai)2] (Least Median of Squares)
AIM meeting 2012, Smolenice

9. Synthetic test
Robustness test using Data = Synthetic travel times outliers with σ = 1s + + normal noise with increasing σ => Able to retrieve both the normal and anomaleous ratios - L1 and LMS inversion performes similar
AIM meeting 2012, Smolenice

10. vP/vS ratios of the West-Bohemia swarms
AIM meeting 2012, Smolenice

11. vP/vS of 1997 swarm
AIM meeting 2012, Smolenice

12. vP/vS of 1997 swarm Two periods during swarm: 1.-17.1., 18.1.-29.1.
Post-swarm period
Constant regional ratio
Focal zone vP/vS <1.3 (down to 0.94) during swarm
AIM meeting 2012, Smolenice

13. vP/vS of 2000 swarm dd
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14. vP/vS of 2000 swarm 7 periods Constant regional ratio Focal zone
Normal ratio during the swarm onset
Low ratio of 1.3 during the onset of the 2nd swarm period (15 Oct)
AIM meeting 2012, Smolenice

15. vP/vS of 2008 swarm
AIM meeting 2012, Smolenice

16. vP/vS of 2008 swarm 8 periods Constant regional ratio Focal zone
Low ratio of 1.46 during the swarm onset
Normal ratio afterwards
AIM meeting 2012, Smolenice

17. Velocity ratio during swarms
1997: ratio of 0.9 related to
small volume, one week period
small ΔtP range to 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 s
Anomalous ratios correspond to different areas of the fault plane => rather process-related that structural feature
AIM meeting 2012, Smolenice

18. Effect 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
AIM meeting 2012, Smolenice

19. Physical interpretation of low vP/vS
Porous sediments saturated by brine and gas: Gassmann equations (Mavko, 2003)
N, K – saturated shear and bulk moduli
K1, K2 – bulk moduli of the rock and fluid
α – Biot-Willies parameter
Φ – porosity
=> Low velocity ratios explainable by spontaneous transition of fluid to gaseous phases
AIM meeting 2012, Smolenice

20. Other results of low vP/vS for swarms
Vavryčuk (JGR 2011)
vP/vS derived from the slope of linear relation between ISO and CLVD of MT
small vP/vS in the focal volume; for the 1997 swarm for the 2008 swarm
AIM meeting 2012, Smolenice

21. Estimates of fluid pressure in the focal zone
Vavryčuk (2002): 5 MPa based on the MT and stress analysis
Hainzl et al. (2012): MPa based on
Stress deficits during the 2000 and 2008 swarms
Fracture model applied to spreading of the 2008 swarm
AIM meeting 2012, Smolenice

22. Conclusions
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
AIM meeting 2012, Smolenice