I don’t believe that I have ever felt an earthquake, although I am told that there are many earthquakes underfoot. Truly there are parts of North America that have a staggering number of naturally occurring earthquakes (Alaska, Yukon, California, Ontario, Yellowstone,) and man-made earthquakes (Kansas, Oklahoma, Texas, Dakotas, Colorado, Alberta, Saskatchewan) but I have yet to feel the earth move beneath me. This has not dampened my curiosity in all things geologic and as such, much of the web surfing I do could be called ‘earthquake tourism’. Keeping track of seismic events and viewing the locations by commonly available satellite imaging teaches me a great deal about how our natural and man-affected world functions.
Naturally Occurring Earthquakes
When sifting through archives of seismic and geological history recorded around the world it becomes apparent that our Earths’ rocky crust is split up into sections like a broken glass ball that has been hastily glued back together. Each section of our crust (plate) has its own characteristics which differ in crustal thickness, number of volcanoes, height of mountains, depth of rifts, and amount of water that is present but the most interesting difference is the speed and direction with which the sections move. Our earths sections don’t just sit there collecting dust; they float and move about much like ice breaking up on a frozen river, motivated by the water flowing beneath. Movement of our Earths crust is motivated by a liquid mantle which wells up from hotter, denser depths and supports the individual crust sections like boats in a gently boiling kettle. Crust sections, like boats, have weight and volume which press down on the mantle creating pressure which displaces some of the fluid mantle either outwards between other crust sections or upwards as volcanic magma.
As you can imagine with all the flowing and bobbing about our plates undergo a lot of stress, most of it at the edges where they bang and rub against each other with great shuddering and quaking, some even riding up over top of neighbouring sections like bullies in a pool. Generally the bullies are bigger than their submerging victim and indeed those crustal plates with the greatest depth and mass tend to ride over thinner crust sections, pushing them back down into the hot mantle where they slowly melt, only to emerge as new, thin crust at different plate edges. However the bully does not come out of this conflict unscathed as his victim is not willingly submerged but resists with all its might, the stress rippling and breaking off sections of the bully and pushing up great hills and mountains, often causing parts of the bully behind those mountains to be pulled down towards the mantle by the victim in its death throes.
Stress is also induced at the centers of crustal plates during the continuous motions of over-riding or subducting because only one edge of the plate is moving vertically. After a number of these motions the crust weakens and ‘faults’, just like a metal coat-hanger will weaken and eventually break after only a few flexes of one end. Our North American crustal plate is a bully as it overrides the thin crusts of the Pacific and Atlantic Ocean floors but in doing so has created large ‘faults’ within the continent, most notably along the St. Lawrence River valley and down the Mississippi River valley. Although the action at both of these faults is continuous and results in ground movement (earthquakes) it is the Mississippi River valley between Indianapolis, Indiana and Memphis, Tennessee that appears to have suffered the worst and is nearing failure at the New Madrid seismic zone. Failure of a fault means just what it means for the coat-hanger; it breaks apart, often with a sudden snap.
Thin crust sections, like those found beneath the Atlantic and Pacific oceans have a depth of between 3,000 and 10,000 meters from surface to mantle while larger, ‘continental’ crust sections average 30,000 meters; the 60,000 meter depth beneath the Rocky Mountains or the 100,000 meter depth below the Alps or Sierra Nevada is very rare. Where I am standing atop the Alberta Basin and east of (behind) the Rocky Mountains, the crust has been measured to a depth of between 37,000 and 40,000 meters, however exact measurements are impeded by the 6,000 meters of sediment (Craton) which lies atop the crustal rock. In fact the crust which begins 6,000 meters below and extends another 38,000 meters has been pulled downwards by the Pacific plate as it is forced into the mantle far beneath and this crustal depression is known as a basin.
Over the millennia, limestone, shale, gravel, coal, water, oil, gas, sand and igneous intrusions (lava flows) have been deposited along the length of North America in a valley formed between the Rocky Mountains and the Appalachian Mountains. Although varying in depth with distance from each mountain range, much of this deposited material sits in distinct basin formations with consistent depths of 4,000 to 7,000 meters. It is from these shallow basins that we draw our ground water, oil and natural gas and it is back into these same stones that we inject at high pressure our drilling waste, captured CO2, toxic sludge and those chemicals used in hydrocarbon recovery.
Increasingly and against all that science knows about crustal faulting and the cause of earthquakes, it is within these shallow basins that the earth is moving and being recorded as earthquakes with levels of energy approaching those occurring at the plate edges. It is possible that there exists an active and moving fault in the crust below theses basins but in the vast majority of recorded North American quakes the movement is occurring either within the basin sediment or at the convergence of sediment and crustal rock. If there are no known faults in the crust to cause the ground movement and if the movement originates ‘above’ the crustal rock then what is it that excerpts enough force on the sediments to cause a sudden shift and resulting earthquake?
If you answered with ‘ground water removal’ and ‘deep well injection of chemical waste and fracturing fluids’ then you are on the right track but don’t expect to meet up with everyone from corporate science or main-stream media along the way. It’s not that these phenomenons remain unproven or that government is unaware, it’s just that admitting to altering the Earth in ways which we cannot fully understand raises the ire of most people and threatens the businesses responsible. We don’t really know what happens when we inject fluids into the transition between sediment and bedrock but we are in possession of years of seismic data that tells us the effects are broadly felt and manifest in fractures between sediment carrying water and sediment carrying natural gas and oil. Similarly we don’t really know what the final effects of ground water removal will be but we have years of data revealing massive surface uplifting, salt intrusion, chemical contamination and local subsidence in the form of sinkholes.
Not every earthquake occurring within a continent is man made, just like not every earthquake occurring near a known crustal fault is natural; it all depends on the depth of the quake and the material from which the quake originated. Shallow quakes centered less than 10,000 meters below the surface are generally occurring at a transition between crust and sediment or within layers of sediment and are a result of external pressures like the upwelling of magma/steam through a vent or the addition or removal of pressure from within those layers. Earthquakes centered at depths greater than 10,000 meters are generally the result of crustal plate movement and the stresses imposed.
Two sources of real-time, world wide seismic information:
Google Maps (if you don’t have Google Earth)
I would strongly urge you to visit the locations of shallow earthquakes in Oklahoma, Alberta and other areas within continental North America using Google satellite view. As you zoom in on coordinates given by either seismic source please pay attention to ground structures, roads and the presence of drilling rigs, well sites and injection wells. Although wells and gas fields are not named they become highly visible as a patchwork of bare dirt rectangles in the middle of nowhere and connected via a spider web of roads.