The Utah Mine Collapse

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The Utah Mine Collapse

• Seismology solves the case

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Utah Mine Collapse The Problem Earthquake or
Collapse?
AP PHOTO/Rick Bowmer
3.9M Event
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Utah Mine Collapse The Data
16 seismometers used in the analysis
Seismometers in the region operating on August 6,
2007
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Utah Mine Collapse Where to find answers
Seismogram analysis in two parts
P wave starts downwards
1. Analyze the first motions of the P waves to
identify the forces (fault slip or collapse) that
generated the recorded seismic waves
P wave starts upwards
2. Model the entire P waveform for a 1)
theoretical collapse and 2) theoretical slip
along a fault (earthquake), and compare with the
observed P waves.
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Utah Mine Collapse Analysis 1 P wave first motion
P waves are compressional waves, just like sound
waves. Along the paths traveled by P or sound
waves the ground compresses and dilates. When the
waves reach the surface, the compressional forces
push the ground up while the dilatations pull it
down. Explosions start off P waves as
compressions implosions start them off as
dilatations.
dilatation
compression
P wave that starts as a dilatation
P wave that starts as a compression
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Utah Mine Collapse Analysis 1 P wave first motion
Do the P waves start as compressions (upward) or
dilatations (downward) at these seismic stations?
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Utah Mine Collapse Analysis 1 P wave first motion
First-motion diagram
Right-lateral strike-slip fault
During an earthquake, which is an abrupt motion
along a geological fault, some parts of the
ground first get pulled into the hypocenter while
other quadrants first get pushed out.
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Utah Mine Collapse Analysis 1 P wave first motion
First motion diagram for the mine collapse
What should the first-motion diagram of the Utah
mine collapse look like, considering that all the
P waves began downwards?
What type of seismic event is represented by this
diagram?
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Utah Mine Collapse Analysis 2 Modeling the data
Finding the best match to the P waveforms
What kind of movements at the source produced the
recorded P waveforms?
Model (dotted line)
The model seismogram should align as closely as
possible with the data
Data (solid line)
Although we can predict, or model, P waveforms
from given source parameters (fault orientation,
slip, first-motion diagram), we cannot measure
such source parameters directly off a seismogram.
So seismologists have a computer try a wide
variety of source parameters and quantify how
well each source predicts waveforms like the
recorded waveforms. The source parameters for the
best-fitting waveforms are good estimates of the
actual fault orientation and direction and amount
of slip that occurred at the earthquake
source. To predict waveforms as accurately as
possible, the predictions are based on previously
recorded waveforms from small earthquakes in the
same area. This ensures that the way the waves
propagate from the source to the seismic station
are properly taken into account.
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Utah Mine Collapse Analysis 2 Modeling the data
The fit from a model requiring a double-couple
The model that fits best
Forces characteristic of slip on a fault
Modeling the event like a tectonic earthquake
produces a poor fit to the waveforms.
The parameters in this model indicate that the
seismic event was an implosion (collapse).
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Utah Mine Collapse Conclusion
The 3.9 magnitude seismic event at the time of
the mine collapse was caused by the collapse of
the mine itself, not by an earthquake.
AP PHOTO/Rick Bowmer
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Utah Mine Collapse EarthScope and USArray
Why are there so many seismometers in Utah,
anyway?

• The seismometers were installed primarily to
  facilitate research into the formation,
  development and evolution of the North American
  continent.
• The seismometers are part of the national
  EarthScope project - 400 portable seismometers
  started in the ground in the West and will slowly
  leapfrog eastward, eventually covering the entire
  lower 48 states.
• EarthScope is like the Hubble Telescope of the
  earth science community. The data are free to
  anyone.

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What other instruments does EarthScope have?
Utah Mine Collapse EarthScope and USArray
Drilling into the San Andreas Fault Portable
Seismometers Permanent Seismometers GPS
Stations Borehole Strainmeters Long-baseline
Laser Strainmeters
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USArray
Utah Mine Collapse EarthScope and USArray

• USArray is the seismic component of EarthScope.
• A dense grid of seismometers is leapfrogging
  across the country.
• Each will stay in its site for 1-2 years.
• About 20 seismometers are installed each month
  throughout the year. Engineers are likely
  installing a new one right now.