Governor William L. Guy’s secretary thought she felt a sonic boom on Monday, July 8, 1968. The State Capitol Building shook a bit, but most people did not feel the shaking or, if they did, they did not recognize it for what it was, a 4.4-magnitude earthquake. The earthquake was centered just southwest of Huff and it was felt over a 3,000-square-mile area. In Huff, and on nearby farms, the quake rattled dishes, window and wood-frame houses creaked, but no damage was reported anywhere in the state.
At least 12 additional earthquakes have been felt in North Dakota. The most widely felt earthquake in North Dakota occurred at about 9 p.m on May 15, 1909. It was a shock that rocked the northern Great Plains. The epicenter of this tremor was near Avonlea, Saskatchewan, near the North Dakota-Montana- Saskatchewan border. The Avonelea earthquake was felt throughout North Dakota and western Montana as well as in the adjacent Canadian Provinces. It broke windows and dishes and cracked plaster and masonry.
Some of the largest earthquakes in U.S. history, in the early part of the 19th century, were likely felt in North Dakota. They are known as the New Madrid quakes, after the town of New Madrid in southeastern-most Missouri. A series of four strong quakes (and hundreds of smaller ones) occurred – two on December 16, 1811, and one each on January 23, 1812 and February 7, 1812. The largest of the quakes was felt from the Gulf of Mexico to Canada and from the Rocky Mountains to the Atlantic coast. The potential remains for more devastating earthquakes in the New Madrid area, and if that happens, we will likely feel it in North Dakota.
Other earthquakes that have been centered and felt in North Dakota include one in the southeastern part of the state in 1872; one near Pembina in 1900; three in the Williston area in 1915, 1946, and 1982; and one each in the Hebron area in 1927; near Havana in 1934; and the Selfridge area in 1947. Earthquakes centered near Morris, Minnesota were felt in southeastern North Dakota in 1975 and 1993.
Almost all earthquakes are caused by sudden slippage along faults in the upper few hundred miles of the Earth’s outermost shell (the crust). Most of them occur at the boundaries between the several large plates, which fit together to form the crust. These plates move, in some places pulling away from one another, as along the spreading parts of the mid-Atlantic Ridge, sliding past one another, such as along at the San Andreas Fault in California, or colliding into one another, such as in the Pacific Northwest (the Cascadia Fault in British Columbia, Oregon, and Washington) where the Pacific Plate is pushing under the North American Plate. In all of these, and other comparable areas, the potential is higher for a severe earthquake.
Movement along these plates is slow but steady, most of the time approximating one to ten millimeters per year (about the rate at which your fingernails grow), but the continual slow movement causes stress to build. When the stress finally exceeds the strength of the rocks, they break and snap violently into a new position (the last time that happened in the Pacific Northwest was in 1700).The point of rupture, which may be many miles beneath the Earth’s surface, is known as the focus or hypocenter of an earthquake; the epicenter is the point directly above the hypocenter. The process of breaking (known as faulting) creates vibrations called seismic waves. We feel these waves as earthquakes. Earthquakes can occur anywhere enough elastic strain builds up to drive fracture propagation along a fault plain.
The sides of a fault may move past each other smoothly, without causing an earthquake if there are no irregularities along the fault surfaces. However, nearly all faults do have irregularities, which tend to cause frictional resistance. If a fault becomes locked–“stuck” in place, continued relative motion between the plates leads to increasing stress, and therefore, stored strain along the fault surface. This continues until the stress has risen sufficiently to break through the friction, allowing sudden sliding over the locked portion of the fault, releasing the stored energy.
Large earthquakes are among the most devastating natural events that can occur. If an earthquake occurs near the edge of a continent, it may generate a tsunami, which can result in a massive flood when it comes ashore. Tsunamis are long-wavelength, long-period sea waves produced by the sudden or abrupt movement of large volumes of water. In the open ocean, the distance between the wave crests can be more than 60 miles, and the wave periods can vary from five minutes to an hour. A tsunami wave can travel from 375 to 500 miles per hour, faster in deep water, slower if the water is shallow. Tsunamis can travel thousands of miles across open ocean, inundating far shores several hours after the actual earthquakes that generated them. One of the most devastating tsunamis in recent years occurred on December 26, 2004 as a result of a massive earthquake (known as the Sumatra-Andaman Earthquake), with a Richter Scale rating of 9.3 off the west coast of Sumatra, Indonesia. The earthquake that triggered the tsunami was the third largest earthquake ever recorded and it also had an unusually long duration of shaking, about ten minutes. The resulting 100-foot-high tsunami wave killed more than 230,000 people when it came ashore in Sumatra and Thailand.
Other recent, large earthquakes include one in 1960 in Chile and one in 1964 in Alaska (the Good Friday Earthquake). The most devastating (in terms of casualties) earthquakes in recorded history were the 1556 Shaanxi earthquake in China, which killed 830,000 people, and the 1976 Tangshan earthquake, also in China, which killed 655,000 people. The fatalities in the Chinese earthquakes were due to direct earthquake damage, not tsunamis, which more often account for most of the fatalities.
Seismic waves (from the Greek seismos, meaning “caused by an earthquake”), actually consist of two kinds of waves: surface waves and body waves. Body waves travel deep into the Earth’s mantle, and even through its core, before reaching the surface, whereas surface waves travel along the Earth’s surface.
The seismic waves of large earthquakes can induce natural oscillations in the Earth and cause the entire planet to ring like a bell for hours, or even days. The reverberating tone is much too low for us to hear, but seismographs can record the low-frequency oscillations. The recorded sound can be played back at, say, 10,000 times faster than the original. We can then, as it were, listen to the Earth. It is a strange experience that sounds like being in a forest on a windy day, with occasional brief falling tones and longer, rather melodic tones, similar to an orchestra tuning up. Every now and again we hear sharp noises that sound like a branch breaking. Sometimes we hear sounds like a herd of animals stampeding through a forest, smashing off branches and breaking them underfoot. I have listened to several such recordings. You can access samples of earthquake sounds on a variety of internet sites.
Most earthquakes that originate in North Dakota are probably related to deeply buried structures in the Precambrian basement. These structures contain numerous faults, but because they are so deeply buried, their extent and locations are poorly known. Movement on any of the faults could produce small to moderate earthquakes. Small earthquakes can also occur when layers of sedimentary rock collapse into voids left by the dissolution of underlying salt beds. Northwestern North Dakota is underlain by thick layers of salt at depths ranging from 4,000 to 12,000 feet. Salt is a geologically unstable mineral, readily dissolved in water and, when burdened under the tremendous mass of overlying sediments, it can flow and deform. As the salt moves, the support for overlying layers may be removed. The overlying layers can settle downward gradually, or they may collapse suddenly, creating a comparatively shallow, small earthquake.
Seismographs around the world record earthquakes with a magnitude of about 4.5 or greater; seismic waves of smaller tremors dissipate before being recorded by distant instruments. For an earthquake with an epicenter in North Dakota to be recorded, it would have to have a magnitude of 3.3 or greater. The 1968 Huff earthquake, which I mentioned earlier, is one of only about a half-dozen that have been instrumentally verified to have epicenters in North Dakota, although it is likely that other small reported earthquakes have had epicenters within the state. Tremors of Richter magnitude 3.0 or less are often felt by persons favorably situated, so more small tremors could have occurred in the state than instrumentally-verified records suggest. The only permanent seismic monitoring station in North Dakota is located near Maddock, southwest of Devils Lake.
In 1978, along with Alan Kehew, Erling Brostuen, and Ken Harris, I investigated reports by farmers of loud, “banging” sounds in Emmons and Dickey counties in south-central North Dakota. It was a drought year and cracks formed in fields of alfalfa. We determined that the alfalfa, which has a particularly deep root system, was de-watering the soil, causing it to shrink. The cracks – and the noises – were due to shrinkage of the soil, which caused deep cracks to form. As the cracks formed, material at their edges fell into them, many of which were 10 to 15 feet deep. The banging noises may have been due either to material falling into the cracks or, perhaps, the soil may have pulled apart with force as it shrank, causing the sounds. The occurrences I just described are not earthquakes, as they do not involve tectonic forces. They might be characterized as examples of “pseudo-earthquakes,” or perhaps “soilquakes.” Similar events – landslides, etc. – might also be mistaken for earthquakes. We published the results of our “alfalfa study” in a scientific journal: (Bluemle, J. P., Kehew, A. E., Brostuen, E. A.,. and Harris, K. L., 1978, Alfalfa and the occurrence of fissures on the North Dakota prairies, The Prairie Naturalist, Vol. 10, pages 53 – 59).
As a sort of afterthought, because I am so often asked about it, I’ll add a note on current concerns about possible earthquakes being triggered by hydraulic fracturing (“fracking”) activity in our oil-producing areas. At the depths at which they are performed in North Dakota, hydraulic fracturing procedures are unlikely to cause earthquakes. It is possible, though, that injection of waste fluids into certain geologic formations could trigger small earthquakes, as has been reported in some places (Texas, Oklahoma, Ohio, etc.). The likelihood of a damaging earthquake due to this activity in North Dakota is remote, although pollution of groundwater is possible.
The closest I ever came to directly experiencing a significant earthquake was on September 26, 1997, when my wife, Mary, and I were in Assisi, Italy. We spent several hours sightseeing in Assisi, some of it in the Basilica of St. Francis of Assisi. That evening, while we were staying in Chianciano Terme, a nearby village, an earthquake caused extensive damage in Assisi, killing several people in the church we had been in a few hours earlier.
The United States Geological Survey lists North Dakota among ten states that are least likely to suffer earthquake damage. Some other nearby “earthquake-poor” states include Iowa, Minnesota and Wisconsin. Infrequent, small earthquakes may occur near to and within North Dakota, but unless one occurs in a remarkably unfortunate location, it is unlikely that any serious damage will occur. It ought to be safe to visit churches here.
Every summer, even during the coldest part of the Ice Age, some melting took place on a glacier’s surface and along its margin. Melting occurred during each summer season – even more when the climate warmed for periods of several years at a time – 20 or 30 years – periods of time comparable to the kinds of swings we see in North Dakota’s climate today. The farther south the glacier advanced, into more temperate zones, the more the amount of melting challenged the health of the glacier in those areas until a balance was finally struck between 1) the rate at which the glacial was ice advancing, 2) warmer climates to the south, and 3) overall climate warming due to the approaching end of the Ice Age. Gradually, the balance among these three factors shifted farther north (and east) and ice began to disappear in those parts of North Dakota that were glaciated.
The position of the edge of an ice sheet at any given time was determined by the balance between melting and the rate at which the glacier was flowing. While the climate remained cold (at average annual temperatures below freezing), a continental ice mass became thicker and the edge of a glacier advanced. When it warmed a little, perhaps with average temperatures a bit cooler than those we have now, the glacier margin melted back about as fast as new ice could be supplied. Even though the glacier was moving, its edge neither advanced nor receded (but melting was taking place on the glacier surface). Given still warmer conditions, the surface of a glacier melted more rapidly; the ice thinned, and the glacier’s edge melted back faster than new ice was being supplied. Areas the ice had covered gradually became deglaciated.
As the margin of a glacier melted, debris that had been frozen into the ice many miles to the north was freed and deposited on the ground. This “glacial sediment” consisted of a blended sampling of the various kinds of rock and sediment over which the glacier had flowed. Glaciers advancing into North Dakota from the northeast deposited mainly sandy, granite-rich materials they had picked up as they flowed over the Precambrian rocks of northwest Ontario. Ice coming from the northwest brought a mixture of sandy and clayey materials it had accumulated as it flowed over broad expanses of Cretaceous shale and Tertiary sands of southern Alberta and Saskatchewan. Ice advancing straight southward from Manitoba, up the Red River Valley, deposited carbonate-rich sediment it had picked up north of Winnipeg where Paleozoic limestone and dolostone are exposed today. If you travel north of Winnipeg to Stony Mountain or Stonewall, Manitoba, be sure to notice the quarries, now producing some of the same materials that glaciers brought to North Dakota, perhaps 17,000 years ago. Studying the composition of the glacial sediment is one way that geologists can determine the direction from which the ice came, and the kind of land it flowed over.
Eventually, as each glacier melted (North Dakota was probably glaciated between 10 and 20 times during the past three million years), the land gradually became free of ice. No reversal of ice flow is involved when the glacier recedes; I emphasize that “retreat” of a glacier refers to the melting of the ice. Three different kinds of ice wasting occurred, at different times and places in North Dakota. The first occurred when the glacier margin may have been far to the south, in Iowa and South Dakota. The result of melting, perhaps over a period of relative warmth of hundreds or thousands of years, resulted in the loss of much of the ice mass off the glacier’s surface. The “view from above” in North Dakota would not have changed much – everything was still all ice – but the thickness of glacial ice covering the land was diminished in thickness by hundreds or thousands of feet, even before the glacier margin had receded into North Dakota.
The second way a glacier wasted (at least from our North Dakota perspective) was when the ice margin was nearby. When that happened, wasting involved frequent change in the position of the edge of the active glacier. As a glacier melted, and after it had become thinner, its active margin gradually receded because the volume of ice arriving was insufficient to replace the ice lost at the edge due to melting. Shrinkage of this kind caused the ice margin to melt back, sometimes in a step-like fashion, the flow of ice pausing long enough at times for the forward movement of the glacier to deliver piles of sediment (moraines) to the receding ice margin. A year of glacial activity might involve the margin moving forward a short distance during a winter; then, during the following summer, the margin receding a slightly greater distance. During this phase, one of the most important things taking place, at least during summer seasons, was the deposition of large amounts of gravel and sand being deposited in front of the glacier by water flowing from the melting, sediment-laden ice. The net effect of this second phase of glacial melting was deglaciation; land that had been covered by ice saw the light of day again, after about 20,000 years.
The third way a glacier wasted involved large-scale stoppage of ice movement, leaving large parts of the glacier stranded, sometimes over broad, mainly upland areas, detached from the main body of still-actively-flowing ice on surrounding lowlands (the plains surrounding Turtle Mountain, for example). In North Dakota, this was important over upland places like the Missouri Coteau and Turtle Mountain. Areas of “stagnant,” or “dead” ice on the uplands then continued to melt slowly. Landforms resulting from the melting of such stagnant ice are distinctive and much different from those that were constructed during the step-wise retreat of active glacial ice I described earlier.
Much of North Dakota’s modern landscape reflects its latest encounter with glaciers during the Ice Age. While glaciers flowed into and over the state, carrying the pulverized rock and soil debris they had picked up along their routes, they sheared off old bedrock landforms, smeared on new layers of sediment, and built new landforms. They filled old river valleys with sediment at the same time rivers of meltwater were flowing from the glaciers carving new valleys. In some places, the glacial ice forced existing rivers to follow different routes; in other places it completely obliterated and concealed what had been rivers and valleys. Cold winds blowing over sand that had earlier been deposited on floodplains and in lakes built dunes and spread a veneer of silt (loess) over much of the state.
Most of the sediment associated with the action of glaciers of the most-recent glaciation is soft. It is “unconsolidated,” and does not hold together well (you can dig it with a shovel). An exception: earlier glaciers also deposited sediment. Nearly all of this earlier sediment has eroded away, but in those places where we have found it exposed, or drilled into it, it may be cemented. A jackhammer may be more appropriate than a shovel for digging in such cemented deposits. However, the softer, looser materials that form most North Dakota glacial deposits are much more common. Sediments related to glaciation in North Dakota can be grouped into three main types: till, lake sediment, and outwash.
1. Till was deposited directly from the ice, mostly in the form of mud flows, which slumped or flowed into their current position as the ice melted. Till consists of silty, sandy, pebbly clay, as well as cobbles, or even large boulders.
2. Lake sediment is layered material that was deposited in lakes, which formed on and near the glacier. Such sediment consists mainly of layers of fine-grained silt and clay, deposited on lake floors, along with some sand and gravel, which collected as beaches along the shores of lakes , many of which were dammed by glaciers.
3. Outwash consists of material deposited by running water. Some outwash may be cemented into a kind of stony concrete, but most of it is loose sand and gravel that was washed out of the melting glacier (hence the name “outwash”). Outwash was deposited by streams and rivers flowing through meltwater valleys or as broad, often nearly level sheets of sand ahead of a melting glacier.
Where they are present, sediments deposited directly by glaciers, and by wind and water associated with glaciation, form a thick covering on top of much of the preglacial (bedrock) surface. In central North Dakota, in Sheridan County, the glacial sediment is over 700 feet thick in places. Ten miles northwest of Tolley in northwestern Renville County, it is at least 800 feet thick, the thickest I can document in the state. The amount and thickness of glacial sediment can vary considerably over short distances so it is likely that even thicker deposits than I mentioned exist in places. Over much of the glaciated part of the state, the glacial materials average 150 feet thick.
At any given location, the glacial deposits may consist of two or more layers of till, interbedded with lake beds, alluvial sediments or other materials. In some places, soils, which had developed on the surface of an earlier glacial, river, lake, or wind-blown deposit, were buried when a new layer of glacial material was deposited. These old, buried soils (paleosols) were formed during long intervals of weathering and exposure, like the one we are enjoying now. Paleosols are among the best indicators in the geological record for multiple episodes of glaciation during the Ice Age. The characteristics of a paleosol also help us understand the climatic conditions (forest or grassland, wet or dry, cool or warm, etc.) at the time it formed.
The best places to see several multiple layers of glacial deposits in North Dakota are near Riverdale, at the Wolf Creek inlet to Lake Sakakawea in McLean County and in Beulah Bay, about 15 miles north of the town of Beulah in Mercer County. In both locations, two and, in some places, three discrete till units, separated by cemented gravel layers or paleosols, are being eroded by waves along the lake.
Drill-hole data in eastern and southeastern North Dakota provide evidence that at least a half dozen glacial advances have occurred there since the Ice Age began. Parts of southwestern North Dakota were glaciated during some of the earlier glaciations, but (apart from some rare exceptions) glacial landforms are not found there today because they were eroded away long ago. Glacial lake sediments and river gravels containing glacially derived materials can be found as far southwest as Dickinson and near the Killdeer Mountains, and I have tentatively identified patches of hard, cemented till and glacial river gravels near Bowman and Rhame, places usually considered never to have been glaciated.
Modern soils are an important link to our geologic past. Fresh glacial deposits consist of a mixture of materials, and, because their sources are so varied, they provide the combination of nutrients necessary for fertile soil. In the glaciated part of the state, North Dakota’s soils consist of the weathered exterior of materials left by glacial action. In the thousands of years that have elapsed since the ice sheets disappeared, constantly changing climate, physical and chemical weathering, accumulation of prairie and woodland plant litter, development of root systems, and burrowing activity by organisms have all contributed to the transformation of glacial deposits into the rich soils that form the basis for much of our agricultural wealth.