The Utah Mine Collapse Seismology solves the case

Utah Mine Collapse The Problem: Earthquake or Collapse? AP PHOTO/Rick Bowmer Very early in the morning on August 8th, 2007, a mine collapsed in central Utah and six miners were trapped. Soon after, the news media flocked to Crandall Canyon mine to report on the mine disaster. The mine owner emphatically maintained that an earthquake caused the collapse. A magnitude 3.9 magnitude seismic event had been recorded at the time of the collapse. It could have been an earthquake, that is, big chunks of rock moving along a geological fault, but an event of that magnitude could also represent the collapse of the mine itself. In a couple of days, seismologists from the University of Utah and UC-Berkeley would come out with reports that the event was a collapse, but the mine owner still asserted that an earthquake caused the collapse. How did seismologists read the seismograms to conclude the event was not an earthquake? 3.9M Event

Utah Mine Collapse The Data 16 seismometers used in the analysis On August 6th, hundreds of USArray seismometers were recording ground motion in the western US. These seismometers are placed in a grid about 70-km from each other. For their analysis seismologists choose 16 seismograms from the closest stations that had signal levels well above the typical noise levels. Seismometers in the region operating on August 6, 2007

Seismogram analysis in two parts: 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. The direction and shape of the recorded seismic P waves provide important clues towards the origin of the seismic waves: The shape and size of the P waves can be modeled on a computer for different origins, for example an underground collapse or an actual earthquake, and then compared to the P waves in the recorded seismograms. A quicker look at this comes from determining the initial direction of the P waves (up or down) in each of the recorded seismograms. A P wave going up means that the ground is going up; a P wave going down means the ground is going down. The ground goes up when the first motion of the ground at the origin (hypocenter) is outwards of the hypocenter.

Analysis 1: P wave first motion 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

Utah Mine Collapse Analysis 1: P wave first motion Here are four of the clearest seismograms from the event. They recorded P waves on very different sides of the epicenter (yellow star). In which direction does each P wave start? Are those compressions or dilations? The next step is to map these direction back to the epicenter, to determine where the ground was initally pushed out and where it was initially pulled in. Do the P waves start as compressions (upward) or dilatations (downward) at these seismic stations?

Utah Mine Collapse Analysis 1: P wave first motion First-motion diagram The simplest example is to imagine plate movement along the San Andreas Fault. The black and white circle diagram is what seismologists create to describe the initial ground motion, more specifically the orientation of the fault and the direction of slip after an earthquake. Black quadrants on the circle are pushed out from the focus and white quadrants are pulled in towards the focus. Seismometers located in the paths of P waves that left through the black quadrants record the P wave first going upwards, away from the origin, and seismometers located in the paths of P waves that left through the white quadrants record the P wave going down, towards the source, first. We have seen that P waves from the seismograms of the Utah mine collapse all began downwards, corresponding to inwards motion at the source. If all P waves from this event move down first, what should the first-motion diagram (the circle diagram) look like? 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.

First motion diagram for the mine collapse Utah Mine Collapse Analysis 1: P wave first motion First motion diagram for the mine collapse On the left is the diagram lifted from the seismic moment tensor report from the Berkeley Seismological Laboratory. On the right is a cartoon version of the diagram. How is it different from the strike-slip fault diagram? If the ground is initially pulling inward from all directions, what kind of seismic event occurred? The diagram is all white, indicating that all sixteen seismometers used in the analysis recorded a P wave that began downwards. For the P wave to begin downwards at every seismometer, the ground must have been pulled inwards from all directions towards the focus of the seismic event. Therefore, the event was an implosion. 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?

Finding the best match to the P waveforms 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.

Utah Mine Collapse Analysis 2: Modeling the data The model that fits best The fit from a model requiring a double-couple Forces characteristic of slip on a fault The parameters in this model indicate that the seismic event was an implosion (collapse). Modeling the event like a tectonic earthquake produces a poor fit to the waveforms. These are two of the models used in the Berkeley report. For the model that fits best, the parameters indicated that the seismic event was an implosion. They also tried to fit the data into a model with double-couple forcing. You can see that the model fit is much worse.

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 The conclusion to this case is that the seismic event was caused by the collapse of the mine itself, not by the earthquake. The question now is, what did cause the collapse? Were poor mining practices involved? Scientists cannot speculate about this, but a private government investigation is currently underway (Dec. 2007).

Why are there so many seismometers in Utah, anyway? 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. Looking at the map on Slide 3, you might wonder why there are so many seismometers in Utah. The seismometers aren’t in the ground to record earthquake magnitudes, per se, but they are really there to help earth scientists understand the formation, structure and evolution of the North American continent. Seismology is a tool to image the ground deep deep beneath our feet.

What other instruments does EarthScope have? Utah Mine Collapse EarthScope and USArray What other instruments does EarthScope have? Drilling into the San Andreas Fault Portable Seismometers Permanent Seismometers GPS Stations Borehole Strainmeters Long-baseline Laser Strainmeters EarthScope consists of a vast amount of instruments - GPS receivers, strainmeters, seismometers, magnetotelluric sensors and a hole drilled into the San Andreas Fault at Parkfield. 400 portable seismometers (16 of which were used in the Utah Mine study) and 100 reference seismometers 20 transportable and 7 permanent magnetotelluric sensors A pool of over 2000 seismic stations available for researchers 116 new and 20 existing GPS receivers along plate boundaries 875 permanent GPS receivers in dense clusters along faults and magmatic centers. A pool of ~100 GPS receivers available for researchers. 103 borehole strainmeters A hole drilled into the San Andreas Fault

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.