Remote Seismicity following Landers Earthquake Steve Kidder

Outline Observations Analyses Conclusions

Remote seismicity all to N (cf. Hector mine, mostly to S) Gomberg et al 2001

Details on post-Landers activity in Long Valley, Linde et al, 1994 E-W tilt Dilatation # of Earthquakes -Pre- and post- Landers seismicity similar -Seismicity associated with dilatation and tilt Pre-Landers Post-Landers

Gao et al, 2000 Red areas all active geothermal/volcanic areas

Outline Observations Analyses & possible explanations - Static stress framework - Dynamic stress framework - General effects of heat and fluids - Local fluid effects - Enhancement of fault connectivity of static strain - Bubbles & liquified magma - Combination of above

Static stress approach Coulomb stress changes in central and northern California are on the order of bars (below tidal). King et al (1994) found little correlation with Lander’s aftershocks and Coulomb stresses <1 bar. (though Ziv & Rubin, 2000 find statistical significance at.1 or even.01 bar given a large sample size)....little reason to expect the observed dramatic increase in seismicity given these low stresses 1 bar 2 bar 5 bar Static stress change decreases with distance as r -3 whereas dynamic stresses decrease as r -2 or r -1.5 a

coulomb regime rate & state regime EQ dynamic stresses have frequencies >> earth tides, so we are certainly within the rate & state regime of Beeler & Lockner (2003). Lander’s EQ dynamic stresses were ~1-3 bar in central California (~2 orders of magnitude > earth tides), so we might expect some triggering if dynamic stresses are large enough and occur and near failure threshold. A lull is predicted following passage of dynamic stress. Dynamic stress approach Tides ~ Hz Annual pressure change ~ Hz Dynamic Stress Change (Gomberg, 2001) Beeler & Lockner, bar 1 bar.1 bar staticdynamic Seismic Waves ~1 Hz

Other factors: effects of heat and fluids in volcanic areas Lots of fluids (higher pore pressures) & heat (elevated seismogenic zone) Earth tides correlate with earthquake swarms in volcanic regions (Kasahara, 2002; Sholz, 2003) frequency Oscillating stress 1/relaxation time = 1/t n = τ“dot”/ aσ frequency Oscillating stress Possibly a higher stress rate or reduced effective normal stress related to high fluid pressures in volcanic regions lowers t n and allows lower amplitude stresses to trigger seismicity seismicity no seismicity

Local fluid effects (Hill et al, 93; Brodsky et al, 2003) - water levels in wells are observed to fluctuate greatly as seismic surface waves pass by, these changes may be sustained for long time periods. - compaction of saturated fault gauge or sediments may occur as surface waves pass by (Gomberg et al, 2000) - to the extent that these effects increase pore pressure, σ is affected resulting in shorter nucleation times and reduced Coulomb stress change required for to induce seismicity

Bubbles Liquifying Magma (Hill et al, 1993) If barometric pressure caused annual variation in seismicity, these are unlikely causes Rising bubble(s) shaken loose by seismic waves might significantly increase pressures along a fault (Linde et al, 1994) ?

Conclusions Seismic waves and/or smaller-than-expected Coulomb stress changes can trigger earthquakes at remote distances Influence of fluids is probably necessary in order to reconcile remote seismicity with Coulomb or rate-and-state theories and observations