Presentation on theme: "Earthquake swarms Ge 277, 2012 Thomas Ader. Outline Presentation of swarms Analysis of the 2000 swarm in Vogtland/NW Bohemia: Indications for a successively."— Presentation transcript:
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
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
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)
- surface rupture - local uplift of about 75cm - outflows of deep origin brine water (i.e., NaCl) saturated with CO 2
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
2000 earthquake swarm in Bohemia swarm area more than 8000 earthquakes quaternary volcanoes in the region
b-value Decrease of b-value: earthquakes tend to become larger. Increase of mean seismic moment release
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 ?
Interevent time distribution First phase: exp distribution random occurrence in time Other phases seem time correlated different triggering mechanisms
Spatial migration in fault plane No specific organization of the migration
Spatiotemporal analysis Propagation not controlled by fluids diffusion. Rupture starts at the edge of the previous ruptured area
Spatiotemporal analysis Moment-radius relationship: Rupture starts at the edge of the previous ruptured area
Comparison with tectonic earthquakes The swarms appears to behave like a single large earthquake that would develop slowly.
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
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) ?
USGS website (Long Valley Caldera): http://pubs.usgs.gov/fs/fs108-96/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