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Origin of earthquake swarms in West Bohemia inferred from observations of non- doble -couple components in seismic moment tensors. Václav Vavryčuk Institute of Geophysics, Prague. West-Bohemia n earthquake swarm in 2008. Springs of mineral water and emanations of CO2. - PowerPoint PPT Presentation
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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