Wednesday, March 16, 2011
600 ft Crack Follows Michigan Quake
The Pop-Up Model
In the case of most pop-ups observed, the limestone has been lying under additional layers of limestone (or ice), and quarrying (or the retreat of a glacier) results in the sudden release of overburden weight holding the rock in place. The layer of limestone has been subjected, for a geologically long time, to the overburden, and this weight has been supported by stresses in the rock: because rock cannot deform by spreading out laterally, horizontal stresses exist in the rock as each piece pushes against its neighbor, supporting the load. When that load is removed, many of these lateral stresses may still exist, having been “locked in” the rock over geologic time; in some cases of quarries, the neighboring rocks still have the overburden weight on top of them, and the resulting lateral stresses can be large. Because the newly exposed layer no longer is confined from above, any small instability may initiate an upward buckling of the layer, which rapidly accelerates, resulting in a sudden noise and quite violent upheaval of rock (figure 11).
Pop-ups are known from many quarries worldwide, where the rock is exposed. They are also recognized in the geologic record, where they are buried under more-recent sediments, and can generally be explained by unloading due to the retreat of continental glaciers about 10,000 years ago. (Another usage of the term “pop-up” can refer to larger, tectonic-scale structures; this is not what is meant here.)
We are currently unaware of other examples of pop-ups that have been observed by humans that are not the result of quarrying operations; this may be the first known case of a contemporary pop-up that resulted from natural causes.
Despite this lack of other modern examples, we postulate that the Menonimee Crack is a result of a limestone pop-up immediately beneath the ridge. The limestone violently buckled upward, heaving the uppermost rock layers and the overlying clay layer, resulting in the ridge (figure 12). The crack is simply a superficial feature, resulting from the stretching of the surface clay as it bent upward, much as a crack forms in the top of a loaf of bread as the dough rises.
There are two remarkable aspects to this feature and the pop-up explanation: (1) there is no evidence for recent removal of the overburden that could have unloaded the limestone and triggering the buckling that initiates the pop-up; and (2) this is the first (to our knowledge) example of a modern, natural (not due to quarrying) pop-up that has been clearly documented by human observation. It is quite possible that other previously unexplained earthquake-like sounds and sensations have been due to similar events, but that the associated ridge (and crack) were never observed or reported.
Was it an Earthquake?
We have chosen to identify the sound and shaking that local residents experienced as evidence of an earthquake because the event fits the technical definition – a sudden movement of rock mass that resulted in the release of seismic energy through the earth. The event was too small to be recorded on seismographs, none of which are operating in the immediate area, but the observations of the residents clearly identify the seismic energy that was released, and formation of the ridge confirms that the sounds originated from rock movement. Therefore, this event can be labeled an earthquake, albeit a very small one, with magnitude of about 1 or 2. This classification means that it can be considered to be the first clearly documented earthquake in the Upper Peninsula that was not associated with an underground mine collapse (as are known from the Houghton/Hancock area in the early 1900’s, for example). This earthquake, however, has almost nothing in common with the usual concept of a natural tectonic earthquake, which would occur on a fault and result from large stresses rooted deeply within the earth’s crust.
What Does this Mean for the Future?
The limestone underneath the ridge has already had its stresses relieved, and is extremely unlikely to experience another, similar event. The areas beyond the current ridge and crack, to the SE and NW, may possibly be under stresses that are similar to those which caused the Menominee Crack, and could, conceivably, be at risk for a similar event. But because we do not (currently) understand what triggered this event, we do not know if another trigger could occur in those areas.
The region is not experiencing increased stress that would result in future larger earthquakes. The stresses that caused this event were located in the uppermost few tens of feet, regardless of the (unknown) trigger, whereas large earthquakes result from stresses that are miles deep in the earth.
More information and images can be found here: http://www.geo.mtu.edu/MenomineeCrack/index.html