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

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

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

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

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

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

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

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

Synthetic test Robustness 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 AIM meeting 2012, Smolenice

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

vP/vS of 1997 swarm AIM meeting 2012, Smolenice

vP/vS of 1997 swarm Two 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 AIM meeting 2012, Smolenice

vP/vS of 2000 swarm dd AIM meeting 2012, Smolenice

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

vP/vS of 2008 swarm AIM meeting 2012, Smolenice

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

Velocity ratio during swarms 1997: 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 AIM meeting 2012, Smolenice

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

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

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; 1.45 for the 1997 swarm 1.35 for the 2008 swarm AIM meeting 2012, Smolenice

Estimates 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 AIM meeting 2012, Smolenice

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