Most of us have some concept of what an earthquake is because of our constant exposure to mass media coverage of large-scale seismic events as they take place in the western U.S. and other areas along the Pacific Ocean's "Ring of Fire." We who live in Texas, however, are not generally concerned about an earthquake happening here because we are not usually informed of local seismic activity. Is this complacency justified? Do earthquakes happen in Texas and are they a threat to the safety of the state's residents? This paper will address these questions and explain causes and potential threats of future earthquakes in Texas.
"Earthquake." The word alone can bring to mind scenes from end-of-the-world movies in which the ground shakes violently, the earth cracks open, and cars, people, and buildings are swallowed whole in a storm of flame and smoke. The aftermath in this terrible vision is a city in ruins, devastation and debris littering the landscape. While this scenario is probably not inconceivable in a major quake, most earthquakes cause little or no damage and are seldom even felt. In fact, areas like southern California and the Mississippi and Tennessee River valleys have numerous quakes, sometimes several a day, that go unnoticed. If, then, a cataclysmic event is not necessarily representative of an earthquake, what actually constitutes one?
An earthquake is a seismic event in which there is a sudden upheaval or shifting of an area of the Earth's surface due to movement or collapse of subterranean supportive masses; "...these disturbances, whether weak or strong, are caused by sudden slippage of one huge part of the earth's solid exterior on, or against, another part. The motion itself may be very small, only a fraction of an inch." Because of the shear amount of mass involved, however, if only several cubic miles of rock drops an eighth of an inch (as in a very small quake), it would create a substantially jarring experience (Udden 5).
Numerous complex mechanisms contribute to create an earthquake. The concept of large, continent-sized tectonic plates sliding into and across each other is a relatively new theory originated in the 1960's, but it is generally accepted as the mechanical principle underlying most catastrophic seismic events around the world today. It is believed that when these plates collide, carrying on their backs the 22 miles of upper layers and exposed lands of the earth, great mountainous regions such as the Rocky Mountains are formed. This obviously does not happen quickly, but is an ongoing process. As these plates move, they snag and bind at their common edges, creating areas of enormous tension known as faults. Faults not only develop at adjacent tectonic plate surfaces, but any place where shifting or bending of the earth's geological layers occurs- which is just about everywhere, to some degree. The tension at these faults continues until a weak point buckles under the stress. Again, the actual slip at the stress point may be minute, but this movement, when translated to the surface, may be felt as an immense lurch. Surface movement may be anywhere from less than an inch to many feet. Between 18,000 and 22,000 shallow earthquakes (those with an initial fracture point less than 37 miles deep and considered to be the most dangerous) occur every year worldwide (refer to the appendix for a map illustrating worldwide distribution of earthquakes). Tectonic-based seismic events generally account for the largest earthquakes, which occur at least once a year on average (Finkl).
A second earthquake-causing mechanism, with particularly far reaching implications for Texas and other oil-bearing locations, is that of fluid injection and withdrawal. This a common retrieval technique used in areas where oil that is under insufficient pressure to be pushed to the surface (typically when a well has been pumped for an extended amount of time) is re-pressurized by injecting water or steam into the oil reservoir, thus exceeding the previous limitations on withdrawal. As the fluid injection/retrieval process is continued, the earthen masses around the oil reservoir weaken and may eventually collapse. When this happens, the collapse is felt as an earthquake on the surface.
Less frequently, small earthquakes may be caused by explosions (dynamite, atomic, etc.) and the pressure exerted downward by large lakes behind dams (Finkl). In a discussion with an engineer associated with NASA's Kennedy Space Center, this author learned that the last of the Apollo rocket launches caused a minor earthquake, as well.
Earthquakes have always been a subject of great interest with some of the earliest records going back over 3000 years to the Shang Dynasty in China (Finkl). Generally, records (when kept) have amounted mainly to subjective observations made by those who experienced the earthquakes, or in collections of these descriptions made by authors who recognized that there may exist some future use for the information. Advances in the scientific measurement of these events, however, have aided in objectively defining and recording earthquake magnitudes, locations, and depths for statistical analysis.
Currently, there are two measurement scales that are used to describe an earthquake's intensity. That preferred by geologists is the Modified Mercalli Intensity Scale. It consists of 12 increasing levels of severity based on ground acceleration. The other system is the familiar Richter Scale, which is based on the energy released through surface seismic waves accompanying an earthquake. Each whole number increase in the scale (1 through 10) represents a magnitude of released energy that is 10 times stronger than the previous level; this means that an earthquake with a magnitude of 8 is 10,000 times stronger than one with a magnitude of 4. The Richter Scale, which is itself a modification of an older standard called the Rossi-Forel Intensity Scale, is usually estimated from the Mercalli Scale and reported through the mass media. (Refer to the appendix for a comparison of the scales). The smallest earthquakes that can be felt have a magnitude of about 2. Even though the Richter Scale has an upper limit of 10, the highest theoretical magnitude that could be generated by a tectonic-based earthquake is 9 (Finkl).
Earthquakes happen almost everywhere in the world, even in Texas.
Although earthquakes represent less of a hazard in Texas than in other states...damaging earthquakes do occasionally occur in Texas.... A comprehensive review of Texas earthquakes from 1847 to 1986 [reveals] 106 earthquakes of magnitude 3 or greater [86 of which were felt]; of these, 24 are earthquakes for which reports of damage are available, and 1 was responsible for a human fatality (Davis et al. 1,2).
The largest known Texas earthquake occurred near Valentine, in West Texas, in 1931 with a magnitude of 6. It was felt over an area of 400,000 square miles. Damage was widespread throughout the region. This author's grandparents were living and teaching in Valentine at that time. By their account, a fissure one-foot wide in the floor broke apart the walls of the school. A description of the earthquake's local effects is found in United States Earthquakes (Neumann 1932):
The outstanding feature was the damage at Valentine.... Adobe buildings suffered most, although cement and brick walls were in some cases badly cracked.... All but frame houses were badly damaged and all brick chimneys toppled over or were badly damaged. The schoolhouse, consisting of one section built of cement blocks and another of brick, was reported to require practically complete rebuilding.... Tombstones in the cemetery were rotated in both directions... only a few [people] were injured by falling adobe. This is accounted for by the fact that nearly everyone was sleeping outdoors. (Qtd. in Davis et al. appendix).
The earthquake happened at 5:40 AM, local time, on August 16, 1931. In El Paso, 140 miles northwest of the epicenter, a mirror was thrown from a wall in a 6-foot arc (Davis et al. appendix). The August 17, 1931 issue of the San Antonio Express (2) reported that this earthquake caused the injuries of "many" people as far away as Oaxaca, Mexico, 200 miles northwest of Mexico City. The paper also quoted the head of the seismograph station at St. Louis University in St. Louis, where the quake was recorded, as describing the event as an earthquake of "great intensity."
By comparison, the largest quakes known to have happened in the continental U.S. were the three Mississippi River Valley Earthquakes of the winter of 1811-12 with magnitudes estimated at 8.6. There were few deaths because the area was mostly unpopulated (Finkl). Some windows in New York City were broken, however.
North and West Texas have for many years been seismically active. The cause of this activity is due mainly to fault-related tectonic stresses, though some of the quakes in the Panhandle region have also been connected with oil field activity (Davis et al. 11-15).
East Texas, especially the region between Austin and Houston, has had a history of minor earthquakes associated with oil field production, sediment loading in the Gulf of Mexico, large deposits of salts (salt domes), and fault stresses since the 1800's. Oil field production is likely responsible for most of this activity. "The East Texas oil field was, at the time of discovery in 1930, the largest field in the Western Hemisphere (...192 million square miles). By the time of the earthquakes [4 in 1957 near Gladewater with the first having a magnitude of 4.7 and felt over an area of 20,000 square miles], more than 3.5 [billion barrels] of oil had been extracted from the field...." Fluid withdrawal is also associated with tremors and land subsidence [sinking] near Beaumont. At one oil field, a 5 acre plot of land sank 160 feet in October 1929. A 7 foot drop in the Houston area between 1943 and 1974 has also been measured. Both of these subsidence actions are thought to be the result of massive withdrawals of oil and water from the underlying strata. Fluid withdrawal, though a major contributor to East Texas seismic activity, does not account for all earthquakes in the region. During a 5 month period in 1964, more than 70 earthquakes with magnitudes up to 4.4 were recorded near Hemphill. Though these tremors are thought to have been caused by sediment loading, "the reason for a burst of activity ...as opposed to random seismicity, is unexplained" (Davis et al. 1, 17-18, 21). South Texas has begun to experience an increase in frequency of earthquakes and related seismic activity in recent years. Areas in and around Bexar and Atascosa counties, particularly near the cities of Seguin, Jourdanton, Pleasanton, and Fashing have been sites for relatively mild earthquakes for several decades. The August 10, 1992 issue of the San Antonio Express (10A) reported that there were at least 4 earthquakes with magnitudes between 2.3 and 3.8 around Jourdanton and Pleasanton from 1991 to 1992. One tremor moved a small house several inches off its foundation in Pleasanton. These tremors have generally been attributed to oil field production, but some are thought to be the result of an increase in fault line activity (Davis et al. 21).
Today, the risks of an earthquake due to fluid injection and withdrawal for oil field operations appears to be increasing. The business section of each Sunday paper contains a continually active listing of operations in South Texas employing fluid injection and horizontal drilling techniques to withdraw as much oil as possible from deposits that were, at one time, thought to have been played out.
The largest fluid injection-induced earthquake reported in Texas was of a magnitude of 4.5; the largest outside the state had a magnitude of 5.5. These numbers sound relatively small when compared to some of today's larger events, but when such an earthquake occurs in a highly populated area, many deaths are often recorded. Three earthquakes between 1960 and 1986 with magnitudes ranging from 5.4 to 6.2 claimed 23,000 lives (Finkl). A geologic circular issued by the Bureau of Economic Geology of the University of Texas at Austin warns
These activities affect the
analysis of seismic risk in Texas because fluid injection and withdrawal
for [oil] recovery is so widespread in Texas. Siting sensitive structures
such as power plants, dams, and hazardous waste repositories requires careful
consideration of the local seismic history (Davis et al. 22).
An examination of the earthquake damage risk maps, provided in the appendix, will attest to the belief that earthquakes pose little threat to the people of Texas when compared to places like the Western United States and Japan. These maps are generated as guidelines for engineers who must produce shockproof systems and structures. They offer statistical evidence of the chances of damaging earthquakes occurring in a given region. The maps do not, however, necessarily reflect the chances of an earthquake happening in that region. Historical evidence clearly illustrates that earthquakes can happen anywhere. Currently, these maps, and a variety of seismic immunity tests, are all that design engineers and communities have to second guess what nature may throw at them. The future, however, looks more promising.
Serious studies are underway to develop early warning systems for the occurrence of seismic events. "Routine prediction of earthquakes of magnitudes 6 or less may be possible in well instrumented areas [in the near future]" (National Research Council 33). Prediction of earthquakes would allow people to prepare for the event in the manner they now prepare for hurricanes; they could secure their property and evacuate the area with enough of an advance notice.
At this point in time, areas of California, Japan and China are issued earthquake advisories during periods of anticipated activity. Accuracy of these warnings has been fair for small earthquakes but mediocre for others because an appearance of known earthquake precursors is not always followed by a seismic event (National Research Council 31-33). Earthquake notification after the event is rapid, however. Fast notification ensures seismic monitoring facilities worldwide will be anticipating the possibilities of aftershocks and watching for related events. Up-to-the-minute information on earthquakes can be obtained from the Internet by those have access via a personal computer and phone line connection (modem). One may also be put on the U.S. Geological Survey's e-mail list for instant earthquake notification and information. Other postings and seismic information may be obtained off the Internet using GOPHER or the World Wide Web. ( Refer to http://quake.wr.usgs.gov/QUAKES/CURRENT/current.html for a recent list).
Then there is the possibility of earthquake control.
Earthquake control is likely to be farther in the future than earthquake prediction. Nevertheless…. earthquake control by fluid injection or by other means that may be developed. Earthquake control experiments have been tried successfully on a small scale in an oil field at Rangely, Colorado (National Research Council 33).
Indeed, the future holds some exciting possibilities for the prevention of damage to property and loss of life due to earthquakes.
Initially, the perceived lack of seismic activity in Texas was brought into question. We then proceeded to examine historical evidence, which indicates that earthquakes are a growing potential threat for Texas in general and the eastern and southern parts of the state in particular.
Certainly, Texas does not have the kind of seismic activity that the more well known earthquake-prone regions possess. There may never again be a magnitude 6 earthquake in this state. The purpose of this paper has not been to alarm, but rather, to inform those who might be interested that the possibility of earthquakes occurring in Texas not only exists, but that the chances of one happening locally appears to be increasing.
Davis, Scott D., et al. A Compendium of Earthquake Activity in Texas. Austin, Tx: Bureau of Economic Geology, University of Texas at Austin, 1989.
"Earthquake Felt In West Texas; Damage Slight." San Antonio Express 17 Aug. 1931: 1-2.
Finkl, Charles W. "Earthquakes." Compton's Interactive Encyclopedia. CD-Interactive. Compton's New Media, Inc.,1992. Philips CD-I, Compact Disc.
National Research Council, Panel on Earthquake Prediction. Predicting Earthquakes: A Scientific and Technical Evaluation- with Implications for Society/ Panel on Earthquake Prediction of the Committee on Seismology, Assembly of Mathematical and Physical Sciences. Washington: National Academy of Sciences, 1976.
"Portions of Jourdanton area shook up by minor tremors, officials say." San Antonio Express-News 10 Aug. 1992: A10.
Udden, Johan August. The Southwest Earthquake of July 30, 1925. Austin, Tx: The University, 1926.
Vogel, Andreas, ed. Terrestrial and Space Techniques in Earthquake Prediction Research. Weisbaden, Ger.: Viewveg, 1979.
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