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Volcanic Earthquakes

Volcanic Earthquakes. Seismic Swarms: A Case Study EPSC 330 Presented by Fabien Rasselet April 4 th , 2007. Volcanic vs. Tectonic Seismicity. Seismicity that precedes, accompanies and follows volcanic eruptions and magmatic intrusions.

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Volcanic Earthquakes

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  1. Volcanic Earthquakes Seismic Swarms: A Case Study EPSC 330 Presented by Fabien Rasselet April 4th, 2007

  2. Volcanic vs. Tectonic Seismicity • Seismicity that precedes, accompanies and follows volcanic eruptions and magmatic intrusions. • # of earthquakes are higher than in an average strength (shear) crust. • Hot fluids (magma) and low rock strengths lead to numerous, simultaneous small events (M=I; not felt) • Events occur at depths of 1-9 km. • Events occur as “swarms” as opposed the tectonic mainshock-aftershock sequence.

  3. Variety of Volcanic Earthquakes • A - High frequency events (volcano-tectonic) • B - Hybrid events • C,F - Long Period (Low Frequency) Events • E - Explosion Quake • D,G,H - Volcanic Tremors (McNutt et al., 1995)

  4. Defined as numerous earthquakes of the same size occurring in a small volume. Figure shows # of events/ unit time (N) vs. time. The mainshock (MS) indicates the sharp increase in rate for the top two distributions. What is a seismic swarm? (McNutt et al., 1995)

  5. Possible Mechanisms • Opening of pathways associated with fluid transport. • Traction on conduit surfaces due to viscous fluid flow. • Sustained crustal deformation and increase in stressing rate (specific to the 2000 Izu Islands’ earthquake swarm).

  6. The 2000 Izu Islands’ Seismic Swarm. • Located 150 km south of Tokyo • Time: June-August 2000. • Recorded more than 7000 M>3 and 5 M>6 events. • Evidence magmatic intrusion (hypothesis).

  7. The Model (Todo et al., 2002) • Seismic activities produced by stress transfers due to crustal deformations initiated by magma intrusions/extrusions. • Very high stress generated for a relatively short period rapidly elevates the earthquake rate episodes. • Model: a vertical dike spread 15km long x 5km wide , continuously propagated for ~2months until the dike opened up by 20m. Real-time monitoring of displacement of ground surface in relation to seismicity. Figure constructed from seismic and GPS (land survey) data. (Todo et al., 2002)

  8. Stress Step vs. Stress Rate Increase • Shear stress rate calculated as a strike-slip fault. • Estimated stress rate near dike = 10 MPa/yr. • Normal stress rate before event = 0.01 MPa/yr. • More than a 1000-fold increase in stressing rate. • Observations consistent with fault friction theory experiments: • Area of seismic activities expanded over time. • The duration of aftershocks of earthquakes of M≥6 were very short. 0.01MPa/yr Figs. c, d - Typical Mainshock-aftershock earthquake. Figs. a, b – Earthquake Swarm. (Toda et al., 2002)

  9. So far we know of two earthquake groups: • Mainshock-aftershock types where the sudden increase in stressing rate triggers temporal burts of seismic activities where aftershocks decay over time. • Earthquake swarm types where the seismic activities increase in proportion to the gradual and sustained increase in stressing rate.

  10. Why are we interested in low-frequency seismic swarms? • Have preceded major volcanic eruptions in the past. Possible predictive tool? • Can provide direct link between surface observations and internal igneous (magmatic) processes. • Link seismicity to deformation; contrast pressure (volumetric) and shear sources.

  11. References Cited • http://kiska.giseis.alaska.edu/dbases/swarmcat/ofr/intro.html • http://sicarius.wr.usgs.gov/animations.html • http://www.vulkaner.no/v/vulkinfo/ordbok/vulktrem-e.html • http://kiska.giseis.alaska.edu/dbases/swarmcat/GVESD.HTML • Benoit, J. P. and S. R. McNutt, Global volcanic earthquake swarm database and preliminary analysis of volcanic earthquake swarm duration, Annali deGeofisca, 39, 221-229, 1996. • Karpin T.L. The relationship between earthquake swarms and magma transport – Kilauea Volcano, Hawaii. Pure and Applied Geophysics 125:971 (1987) • McNutt, S. R. and J. P. Benoit. Generic Earthquake Swarm Model. (extended abs.). Periodico di Mineralogia, 64, 229-230 (1995) • Nishimura, T. et al. Crustal deformation caused by magma migration in the northern Izu Islands, Japan. Geophys. Res. Lett. 28, 3745-3748 (2001) • Toda et al. Evidence from AD 2000 Izu islands earthquake swarm that stressing rate governs seismicity.Nature, 419, p58-61 (2002)

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