Václav Vavryčuk Institute of Geophysics, Prague

Václav VavryčukInstitute of Geophysics, Prague

Origin of earthquake swarms in West Bohemia

inferred from observations of non-doble-couple components

in seismic moment tensors

Origin of earthquake swarms in West Bohemia

inferred from observations of non-doble-couple components

in seismic moment tensors

West-Bohemian earthquake swarm

in 2008

West-Bohemian earthquake swarm

in 2008

Springs of mineral water and emanations of CO2

Seismicity in West Bohemia, Czech republic

Data and methods

Data• 249 selected micro-earthquakes from the 2008 swarm • Magnitudes between 0.5 – 3.7, depth between 7 and 11 km• 18-22 local short-period seismic stations • Sampling rate 250 Hz

Method• Double-difference location method

– P and S wave arrivals obtained using cross-correlation• Frequency-domain waveform inversion for moment tensors

– P waves, 1-D smooth model, ray-theoretical Green’s functions

Double-difference locations: map view

main active fault

2 km

-2 -1.5 -1 -0.5 0 0.5 1 1.5 2-2

-1.5

-1

-0.5

0

0.5

1

1.5

2main active fault

4 km

Bouchaala, Vavryčuk, Fischer, J. Seismology, 2013

Vavryčuk Bouchaala, Fischer, Tectonophysics, 2013

Examples of focal mechanisms

Waveform inversion of P waves

good focal sphere coverage, slightly non-DC mechanisms

Variety of focal mechanisms

249 most accurate focal mechanisms

three basic types of focal mechanisms

Nodal lines P/T axes

o P axis, + T axis

Locations & focal mechanisms: map view

main active fault

2 km

-2 -1.5 -1 -0.5 0 0.5 1 1.5 2-2

-1.5

-1

-0.5

0

0.5

1

1.5

2main active fault

4 km

most frequent focal mechanism

Mohr’s diagramfirst

principalfault

secondprincipal

fault

.

.P

T

PT

20 km

EGR

MLF

Tectonic sketch and principal faults

maximum compression coincides with that for western and central Europe

Focal mechanisms and non-DC components

-30 -20 -10 0 10 20 30-15

-10

-5

0

5

10

15CLVD-ISO: all events

CLVD [%]

ISO

[%]

Nodal linesNon-DC components

ISO

[%

]

CLVD [%]

Focal mechanisms and non-DC components

Shear-tensile source model

Positive ISO and CLVD : positive slope angle, fault is openingNegative ISO and CLVD: negative slope angle, fault is closing

][u

Moment tensors

2.0 1.7

1.5

fault is opening

Moment tensors components: ISO – isotropic component

CLVD – compensated linear vector dipoleDecomposition into ISO, DC and CLVD is after Vavryčuk (2001)

Diamond source-type plot

ISO

CLVD

Diamond plot

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

ISO3

23

1.51.4

1.31.2

CLVD

Diamond plot

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

DC

Diamond plot

0.2 0.4 0.6 0.8 1

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Moment tensors Source tensors

opening of a fault

Non-DC components

Non-DC components and shear-tensile faulting: 1997

Slip deviation from the fault

crackclosing crack opening

Slope angle [ º ]CLVD [% ]

-30 -20 -10 0 10 20 30-15

-10

-5

0

5

10

15CLVD-ISO: all events

CLVD [%]

ISO

[%]

-6 -4 -2 0 2 4 60

10

20

30

40

50

60All mechanisms

slope angleN

eve

nts

Slope angle [ � ]

N e

ven

ts

closing of a fault, rock compaction

Non-DC components Slip deviation from the fault

crack closing crack opening

Non-DC components and shear-tensile faulting: 2008

Origins of rock compaction: fluid-rock interaction

• Permanent fluid flow in the Earth’s crust

• Hydrothermal alteration of rocks

• Dissolution of minerals

• Transport of dissolved material to the surface

• Fault erosion by fluids

Borehole picture of open fluid-filled fracture at depth of 111 mThe fracture width is 1-2 cmAfter Heinicke et al. (2009)

Seismic cycles: alternative scenarios

Stress accumulation

Fault weakening

stre

ss

time

fault strength

stress

time

faul

tstr

engt

h

(seismicity at margins of continental plates)

(intraplate seismicity)

• intraplate seismicity in a geothermal area with numerous springs • repeated occurrence of earthquake swarms at the same focal zone (1985/86, 1997, 1997, 2000, 2008, 20011)

• prevailing focal mechanisms are slightly compressive (negative ISO and CLVD)

• swarms are triggered by fault weakening rather than by stress accumulation

Swarm triggering

-2 -1.5 -1 -0.5 0 0.5 1 1.5 2-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

main active fault

4 km

left-lateral

right-lateral

σ1

Swarm 2008

Thank you for your attention

Karlovy Vary spa