What Causes a Rift to Propagate? (and then why does it stop?) Project funded by the AAD, NSF, NASA J. N. Bassis, H. A. Fricker, J.B. Minster, R. Coleman Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography

Why Study Iceberg Calving? Icebergs account for 2/3 of mass loss Large tabular bergs detach sporadically (recurrence intervals ~ years) Little is known about how rifts initiate and then propagate Absence of calving physics in numerical models results in large uncertainties in predictions Crack! Need better understanding of rift mechanics in order to improve models

Enables hypothesis testing How sensitive is rift propagation to environmental perturbations? Can large swell/storms trigger rift propagation? Use the earthquake and tsunami as control variables Like “shake” test on buildings What the heck does this have to do with the Sumatra earthquakes?

Questions? What forces drive propagation? A) Glaciological stress B) Tides, wind stress, storms, swell, etc. C) A and B D) None of the above What conditions are necessary and sufficient for rift propagation? Crack!

What do we already know? Blobs of seismicity around rift tip (Bassis et. al. 2005) field season deployed 6 GPS 8 Seismometers Seismicity concentrated in three “swarms” 10 days24 days Amery Ice Shelf

What does this mean? Rapid widening (transverse to the rift) during seismic swarms over 4 hours Rift widening very small (~1 cm) Not (instantaneously) triggered by winds, or tides Two of the three swarms were preceded by elevated winds insufficient data to determine if it is statistically significant.... but interesting (Bassis et. al., 2005)

Implications? Bassis et. al Glaciological stresses seem to be primary (only?) ingredient Several modeling/remote sensing studies have come to similar conclusion? Might be modulated by environmental stresses

Stress Time   ff Glaciological stress accumulates At critical stress, failure occurs Cycle repeats Regularly spaced intervals of propagation A Very Basic Model But not quite right

Stress Time   ff Stress accumulates At critical stress, failure occurs Cycle repeats Regularly spaced crack propagation Random perturbations to the stress or strength can cause randomized recurrence intervals Long term stress + tidal bending + winds + current + A Very Basic Model

Stress Time  ff + Stress or Strength Perturbations Stress  ff Random perturbations to the stress or strength can cause randomized recurrence intervals Long term stress + tidal bending + winds + current + A Very Basic Model

What does this have to do with Earthquakes? Good news: Rift propagation appears to be insensitive to non- glaciological stresses Earthquake doesn’t directly trigger propagation Tsunami might - but not until much later Sumatra EQ Tide gauge data courtesy of AAD Days Since Dec 16 Ice-ocean non-interaction

What does this have to do with Earthquakes? Good news: Rift propagation appears to be insensitive to non- glaciological stresses Holdsworth (1970) hypothesis: Large icebergs produces by swell induced vibrations in the ice shelf Unlikely for swell to cause anything remotely as large as vibrations for the earthquake Days Since Dec 16 Ice-ocean non-interaction

Spatial Patterns of Seismicity? Days Since Dec 16 Ice-ocean non-interaction

Spatial Patterns of Seismicity? Days Since Dec 16 Ice-ocean non-interaction Joint hypocenters show trend in propagation Each burst ~ 200 m

Spatial Patterns of Seismicity Events tend to occur at the surface

Models Necking in Ductile layer Strong layer Brittle-Ductile One (speculative) hypothesis: Rifting controlled by ductile deformation at depth Thin brittle layer at surface responds to strain at depth

Typical spectrum of growth rate factor Wavelength varies from about 1.2 x thickness – 2 x thickness For 400 m thick ice shelf : characteristic wavelength is 600 m – 800 m Peak wavelength ~ 600 m

Gravitational stresses drive rift propagation Even large external stresses do not trigger propagation Long term effects like fatigue are difficult to measure but may be important Most of the seismicity occurs at the surface suggesting a surface brittle layer Conclusions

Spectragrams Swarm Sumatra Swarm

Hypothesis Testing We detected over 8,000 during 48 days The Sumatra Earthquake was much larger than everything else x