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Earthquake swarms
Ge 277, 2012Thomas Ader
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Outline
• Presentation of swarms• Analysis of the 2000 swarm in Vogtland/NW
Bohemia:Indications for a successively triggered rupture growth
underlying the 2000 earthquake swarm in Vogtland/NW Bohemia, S. Hainzl & T. Fischer, JGR 2002
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Definition of swarms ?
• Large number of earthquakes clustered strongly in space and time
• Not characterized by a dominant earthquake
No law comparable to the Omori law: no exact definition of swarms can be formulated
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Examples
Long valley Caldera:
dome-like uplift of the caldera:- 1ft in summer 1979- 2.5ft since then
swarm of earthquakes (3 M6 earthquakes the same day)
(USGS)
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Examples
(Cappa et al., JGR, 2009)
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(Cappa et al., JGR, 2009)
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- surface rupture- local uplift of about 75cm- outflows of deep origin brine water (i.e., NaCl) saturated with CO2
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• Mostly associated with volcanic activity• Sometimes to geothermal activity• Occasionally observed at the boundary of
tectonic plates [Holtkamp & Brudzinski, 2011]
Swarms ?
Possible mechanisms:- fluids trigger swarms, which trace the migration of fluids- self-organization of earthquakes in regions which prevent the occurrence of mainshocks
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2000 earthquake swarm in Bohemia
swarm area
more than 8000 earthquakes
quaternary volcanoes in the region
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b-value
Usually:- for swarms 1<b<1.5- for tectonic plate boudaries 0.7<b<1.1
Decrease of b-value: earthquakes tend to become larger
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b-valueDecrease of b-value: earthquakes tend to
become larger.
Increase of mean seismic moment release
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Increase of the Coulomb failure stress:- shear stress increase- pore pressure rise
2 possible scenarios:- successive stress accumulation due to propagating rupture front- gradual inflow of fluids in the seismogenic zone.
Why do the b-value increase ?
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Interevent time distribution
First phase: exp distribution
random occurrence in time
Other phases seem time correlated
different triggering mechanisms
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Spatial migration in fault plane
No specific organization of the migration
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Spatiotemporal analysis
Propagation not controlled by fluids diffusion.
Rupture starts at the edge of the previous ruptured area
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Spatiotemporal analysis
Moment-radius relationship:
Rupture starts at the edge of the previous ruptured area
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Comparison with tectonic earthquakes
The swarms appears to behave like a single large earthquake that would develop slowly.
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cum
ulat
ed s
lip
fault length
Slowly developing rupture
new earthquake triggering
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• Intrusion of fluids probably initiated the swarm seismicity
• Swarm evolution then influenced by earthquakes and stress transfer (locally induced fluid flows ?).
• Cumulative behavior of the swarm activity single large earthquake that ruptures the fault segment at once.
Conclusions of the study
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• Dynamic pore creation: fluid flows out of a localized high-pressured fluid compartment with onset of earthquake rupture [Yamashita, 1999]
• Structural inhomogeneities + visco-elastic coupling (magma filled dikes) [Hill, 1977]
• Behavior reproduced by block model with local stress transfer and viscous coupling [Hainzl et al., 1999]
Discussion: which mechanism(s) ?
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• USGS website (Long Valley Caldera): http://pubs.usgs.gov/fs/fs108-96/• Modeling crustal deformation and rupture processes related to upwelling of
deep CO2-rich fluids during the 1965 – 1967 Matsushiro earthquake swarm in Japan, F. Cappa, J. Rutqvist, and K. Yamamoto, JGR, 114, 2009
• Indications for a successively triggered rupture growth underlying the 2000 earthquake swarm in Vogtland/NW Bohemia, S. Hainzl & T. Fischer, JGR, 107, 2002
• Pore creation due to fault slip in a fluid-permeated fault zone and its effect on seismicity: Generation mechanism of earthquake swarm, T. Yamashita, Pure Appl. Geophys., 155, 625 – 647, 1999.
• Similar power laws for foreshock and aftershock sequences in a spring-block model for earthquakes, S. Hainzl, G. Zoller, and J. Kurths, JGR, 104, 7243 – 7254, 1999.
• Earthquake swarms in circum-Pacific subduction zones, S.G. Holtkamp, M.R. Brudzinski, Earth and Planetary Science Letters, 305, 215-225, 2011
References