Earthquake swarms

Ge 277, 2012Thomas 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

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)

Examples

(Cappa et al., JGR, 2009)

(Cappa et al., JGR, 2009)

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

• 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

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

b-valueDecrease 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.

cum

ulat

ed s

lip

fault length

Slowly developing rupture

new earthquake triggering

• 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/• 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