Anomalous Vp/Vs ratios in the focal zone of West/Bohemia earthquake swarms

T. Fischer 1,2 , T. Dahm 3

(1) Institute of Geophysics, Czech Academy of Sciences (2) Faculty of Science, Charles University in Prague

(3) GFZ Potsdam AIM meeting 2012, Smolenice 1

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. v P /v S AIM meeting 2012, Smolenice 2

• Standard method for v P /v S : Wadati diagram • events

i

, stations

j

• constant v P /v S • simple and robust method • average v P /v S in the area AIM meeting 2012, Smolenice 3

Modified Wadati method – remove t

0 • • •

t ij S

a b

t ij P

= d

i

d

i

= æ ææ 1 a b æ ææ

t i

0 • Remove t 0 events i, stations j constant v P /v S simple and robust method average v P /v S in the area AIM meeting 2012, Smolenice 4

Double-difference Wadati method

Δ

t P 12

, Δ

t S 12 …

arrival time differences D

t ij S

a b D d

i

D

t ij P

= D d

i

= æ ææ 1 a b æ ææ D

t i

0 • • 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 5

Anomaleous source region: Single-difference Wadati method Synthetic data: α 0 /β 0 = 1.89

α 1 /β 1 = 1.50

The normal velocity ratio of the crust is retrieved AIM meeting 2012, Smolenice 6

Anomaleous source region: Double-difference Wadati method Δ

t P 12

, Δ

t S 12 …

arrival time differences Synthetic data: α 0 /β 0 = 1.89

α 1 /β 1 = 1.50

The anomalous velocity ratio of the focal zone is retrieved AIM meeting 2012, Smolenice 7

Inversion technique

• Single-differences minimize A = • Double-differences minimize A = g = a b , d

i

= ( 1 g )

t i

0 • Two-step inversion 1. Grid-search in γ 2. For each γ we calculate δ by minimizing median [sum(A

i

) 2 ] (Least Median of Squares) AIM meeting 2012, Smolenice 8

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 9

v

P

/v

S

ratios of the West-Bohemia swarms

AIM meeting 2012, Smolenice 10

v

P

/v

S

of 1997 swarm

AIM meeting 2012, Smolenice 11

v

P

/v

S

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 12

v

P

/v

S

of 2000 swarm

• dd AIM meeting 2012, Smolenice 13

v

P

/v

S

of 2000 swarm

AIM meeting 2012, Smolenice • 7 periods Constant regional ratio • • Focal zone Normal ratio during the swarm onset Low ratio of 1.3 during the onset of the 2 nd swarm period (15 Oct) 14

v

P

/v

S

of 2008 swarm

• AIM meeting 2012, Smolenice 15

v

P

/v

S

of 2008 swarm

AIM meeting 2012, Smolenice • 8 periods Constant regional ratio • • Focal zone Low ratio of 1.46 during the swarm onset Normal ratio afterwards 16

Velocity ratio during swarms

• 1997: ratio of 0.9 related to small volume, one week period • • small Δt

P

range -0.15 to +0.15 s • 2000: ratio of 1.3 related to larger volume, ca. 5 days period • • larger Δt

P

range -0.3 to +0.5 s 2008: ratio of 1.48 related to small volume, ca. 1 day period small Δt

P

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 17

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 ∆t P ≈ ± 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 18

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 19

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 20

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 21

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 22