Earthquake scaling laws for the 2000 dike Earthquake scaling laws for the 2000 dike at Miyakejima (Japan) andat Miyakejima (Japan) and

relation to stress changes and focal relation to stress changes and focal mechanisms.mechanisms.

Luigi Passarelli (1)

Eleonora Rivalta(1)

Simone Cesca(1) Yosuke Aoki(2)

(1) GFZ Potsdam(2) ERI, University of Tokyo

Motivation: Expected focal mechanisms during volcanic swarms

Belachew et al., 2012

Manda Hararo dikes

Belachew et al., 2012

Manda Hararo dikes

Izu peninsula dike

Ukawa and Tsukahara 1996

Motivation: Expected focal mechanisms during volcanic swarms

Hill, 1977

Passarelli et al 2015 submitted JGR

Motivation: Expected b-values during volcanic swarms

Jacobs and McNutt, USGS pp 1769

Pre-eruptive Earthquake Swarm at Augustine Volcano 2005–2006

✔ Marked deviation from GR for largest magnitudes

✔ 2 different GR distributions

✔ Is GR a good model for volcanic seismicity?

LP events at Mt Etna and Turrialba volcanoes

Bean et al 2014

Motivation: Expected b-values during volcanic swarms

Seismic b-values in volcanic areas have large variability.

Influenced by:

State of stress:● Stress (Scholz, 1968, Schorlemmer, 2005), ● Thermal stresses ● Pore pressure (Wyss, 1973)

Material heterogeneities:● Fracture density (Mogi, 1962)● Material rheology (Amitrano, 2003, Bean et al., 2013)● Limited thickness of competent rock layers (Becker et al., 2012)

The 2000 dike intrusion at Miyakejima, Japan

Seismicity and Focal mechanisms (FMs)

Hypocenters catalog (JMA) Ml>1.5, FM catalog (NIED) Mw>3

Passarelli et al 2015 JGR in press

Seismicity: Cross-section

Seismic data courtesy of JMA

Ml>6

Passarelli et al 2015 JGR in press

Migration of the seismicityPropagation phase M

l>6

Seismic data courtesy of JMA

Thickening phase

Focal mechanisms: Clustering results

Passarelli et al 2015 JGR in press

Focal mechanisms: Clustering results

Passarelli et al 2015 JGR in pressDike strike

Spatio-Temporal distribution of FMs

● Strike-slip sets (red and cyan) activate at the intrusion start● Normal faulting sets (dark blue and orange) occur at the center of the cloud,

activate 2 days later and peak after dike arrest

Passarelli et al 2015 JGR in press

Depth of Focal Mechanisms

Passarelli et al 2015 JGR in press

Coulomb stress changes around the dike edges

Free surface

Dike upper tip

Dikepopagatingtip

Passarelli et al 2015 JGR in press

Focal Mechanisms induced by dike stress

Frequency-magnitude distribution

Passarelli et al 2015 JGR in press

Conclusions

✔ Frequency-Magnitude distribution deviates from GR only for NF (lack of large

events):

➔ Finiteness of the seismogenic layer on top of the dike (HET)

➔ Weakness of the shallow layer and presence of magmatic fluids limit accumulation of stress (HET+ STRESS) ➢ Fractures cannot grow large

✔ T-axes all consistent with local stress field and dike-induced tension.

✔ All possible P-axes (given T-axis)

→ FMs spatially consistent with dike-induced stresses: strike-slip off dike tips, normal faulting on top of dike (with all intermediate possible FMs)

Conclusions

Thank you

✔ Frequency-Magnitude distribution deviates from GR only for NF (lack of large

events):

➔ Finiteness of the seismogenic layer on top of the dike (HET)

➔ Weakness of the shallow layer and presence of magmatic fluids limit accumulation of stress (HET+ STRESS) ➢ Fractures cannot grow large

✔ T-axes all consistent with local stress field and dike-induced tension.

✔ All possible P-axes (given T-axis)

→ FMs spatially consistent with dike-induced stresses: strike-slip off dike tips, normal faulting on top of dike (with all intermediate possible FMs)

Passarelli et al 2015 JGR in press

Passarelli et al 2015 JGR in press

Focal Mechanisms induced by dike stress