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 ~50-100 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 BD) 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)

2002-2003 field season deployed• 6 GPS• 8 Seismometers

Seismicity concentrated in three “swarms”10 days 24 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. 2005

•Glaciological stresses seem to be primary (only?) ingredient

•Several modeling/remote sensing studies have come to similar conclusion?

•Might be modulated by environmental stresses

Str

ess

Time

f

•Glaciological stress accumulates

•At critical stress, failure occurs

•Cycle repeats

•Regularly spaced intervals of propagation

A Very Basic Model

But not quite right

Str

ess

Time

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

Str

ess

Time

f

+

Stress or Strength Perturbations

Str

ess

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-DuctileOne (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

SpectragramsSwarm

Sumatra

Swarm

Hypothesis Testing

•We detected over 8,000 during 48 days

•The Sumatra Earthquake was much larger than everything else

x