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Earthquake swarms

Earthquake swarms. Ge 277, 2012 Thomas Ader. 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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Earthquake swarms

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  1. Earthquake swarms Ge 277, 2012 Thomas Ader

  2. 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

  3. 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

  4. 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)

  5. Examples (Cappa et al., JGR, 2009)

  6. (Cappa et al., JGR, 2009)

  7. - surface rupture - local uplift of about 75cm - outflows of deep origin brine water (i.e., NaCl) saturated with CO2

  8. Swarms ? • Mostly associated with volcanic activity • Sometimes to geothermal activity • Occasionally observed at the boundary of tectonic plates [Holtkamp & Brudzinski, 2011] • Possible mechanisms: • - fluids trigger swarms, which trace the migration of fluids • - self-organization of earthquakes in regions which prevent the occurrence of mainshocks

  9. 2000 earthquake swarm in Bohemia • more than 8000 earthquakes • quaternary volcanoes in the region • swarm area

  10. b-value • Usually: • - for swarms 1<b<1.5 • - for tectonic plate boudaries0.7<b<1.1 • Decrease of b-value: earthquakes tend to become larger

  11. b-value • Decrease of b-value: earthquakes tend to become larger. • Increase of mean seismic moment release

  12. Why do the b-value increase ? 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.

  13. Interevent time distribution First phase: exp distribution • random occurrence in time Other phases seem time correlated • different triggering mechanisms

  14. Spatial migration in fault plane No specific organization of the migration

  15. Spatiotemporal analysis Propagation not controlled by fluids diffusion. Rupture starts at the edge of the previous ruptured area

  16. Spatiotemporal analysis Moment-radius relationship: Rupture starts at the edge of the previous ruptured area

  17. Comparison with tectonic earthquakes The swarms appears to behave like a single large earthquake that would develop slowly.

  18. Slowly developing rupture new earthquake triggering cumulated slip fault length

  19. Conclusions of the study • 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.

  20. Discussion: which mechanism(s) ? • 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]

  21. References • 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

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