EARTHQUAKE FAQ

A: Yes (Figure 1). In historical times no earthquake with magnitude larger than about 6 has occurred in Texas. However, seismographs near El Paso record small earthquakes with magnitude of 2 or smaller every few days. Nearly every year earthquakes large enough to be felt by ordinary citizens occur somewhere in Texas.

Q: What is the largest earthquake in Texas in recorded history?

A: The largest earthquake in Texas history had a magnitude of about 6.0 and occurred on 16 August 1931 near the town of Valentine, 220 km southeast of El Paso. It caused severe damage to adobe and brick structures in Valentine, and was felt by Texans as far away as Dallas. Texas' second largest earthquake occurred on 14 April 1995, also in west Texas. It had a magnitude of 5.8, and was felt in Austin.

Q: What is the highest death toll from a historical Texas earthquake?

A: Only one Texas earthquake ever took a human life, and this wasn't in Texas. On 6 March 1923 an earthquake with magnitude of 4.7 occurred near El Paso and Juarez. Juan Ortiz, a Mexican tenant farmer who lived in Mexico six miles from Juarez, suffocated when his adobe house caved in during the quake.

Q: What was the most damaging earthquake in Texas history?

A: No Texas earthquake has caused much damage as compared to some earthquakes in other states or throughout the world. The most damaging was the 16 August 1931 earthquake in Valentine, Texas, where "all but frame houses were badly damaged, and all brick chimneys toppled over....The schoolhouse, consisting of one section built of cement blocks and another of brick....required practically complete rebuilding."

Texas Earthquakes Occurring Since 1990

Q: Has an earthquake occurred in Austin in historic times?

A: On 9 October 1902 an earthquake with magnitude about 3.9 occurred about 15 km south of Austin near the town of Creedmoor; this event was felt in Austin. However, there were no felt reports in Austin from an earthquake with magnitude of 3.1 that occurred in Manor (15 km east of Austin) on 1 May 1873. Every few years Austin residents in tall buildings may feel the effects of large, distant earthquakes, such as the 19 September 1985 Michoacan, Mexico earthquake, and the 14 April 1995 earthquake in west Texas.

Q: What sort of earthquakes should we expect from the Balcones Fault?

A: Probably none. The Balcones Fault is the boundary between the central Texas hill country to the west of Austin and the flatter plains regions to the east. About 10 million years ago this was an active fault, but it is not active any longer. A "fault" simply represents a boundary between two sections of the earth's crust that move or slip relative to one another. When slip is ongoing, the fault is active. After the slip stops, the fault remains. Thus, in Texas and elsewhere the land is riddled with faults which no longer present any significant earthquake risk.

Q: Where can I get a map showing the geologic faults of Texas?

A: The University of Texas at Austin Walter Geology Library (http://www.lib.utexas.edu/Libs/GEO/geology.html) has a geologic map of Texas which has the major faults on it (but not all the faults). You might also check out the Bureau of Economic Geology: http://www.utexas.edu/research/beg/pub.html

Q: Have earthquakes occurred in Houston in historic times?

A: No. The closest earthquake occurred on 8 May 1910 near Hempstead, Texas, about 70 km northwest of Houston. This earthquake had a magnitude of about 3.8, and was felt by individuals within about 30 km of Hempstead, but not in Houston. Of course, Houston residents in tall buildings may occasionally feel large very distant earthquakes that occur in Mexico or western Texas.

    Near Houston there may be examples of faults which are active, but which don't have any earthquakes. Land in some communities southeast of Houston such as Clear Lake and Dickenson is sinking because water has been pumped out from the ground for many years; this sinking may be associated with slip along faults. However, when such slip is a slow or continuous "creep," it isn't an earthquake. To cause an earthquake the faults need to "stick" then slip suddenly enough to radiate seismic waves.

Q: Have earthquakes occurred in Dallas-Fort Worth in historic times?

A: No. The closest earthquakes were both very small; on 18 September 1985 one with magnitude about 3.4 occurred in Valley View, Texas, about 80 km north of Fort Worth; one with magnitude about 3.3 occurred on 20 March 1950 in Chico, Texas, about 70 km northwest of Fort Worth. Recently, an earthquake with magnitude about 3.4 occurred on 31 May 1997 near Commerce, about 100 km northwest of central Dallas. None of these quakes were felt in Dallas or Fort Worth. Of course, Dallas-Fort Worth residents in tall buildings may occasionally feel effects of very distant earthquakes in Mexico or elsewhere.

Q: Have earthquakes occurred in San Antonio in historic times?

A: No. However, since 1973 a number of earthquakes have occurred near the cities of Fashing and Pleasanton about 50 km southeast of San Antonio. The largest of these had a magnitude of 4.3 and occurred on 9 April 1993 in Fashing. Of course, San Antonio residents in tall building may occasionally feel effects of very distant earthquakes in Mexico or west Texas.
Figure 2, an isoseismal map of the cities of Fashing and Pleasanton.

Q: Have earthquakes occurred in Corpus Christi in historic times?

A: On 24 March an earthquake with magnitude 3.8 shook Alice, Texas, a community situated about 60 km west of Corpus Christi. This is the only known earthquake in this local area in historic times. Although the Corpus Christi Caller Times reported that this quake was felt in Corpus Christi, interviews by a team of UT students determined that this was erroneous; apparently reporters were confused by reports of people "near" Corpus (but actually in Alice) feeling the quake. A felt report study determined that the entire felt area of the earthquake was about 900 km²; the highest Mercalli intensities of about VI were from a sparsely populated region about 10-15 km ESE of Alice, Texas. This region lies along the Vicksburg fault zone that runs nearly parallel to the Fulf coast and is within a few kilometers of the currently producing Stratton oil field. Although this location suggests that the earthquake may have been triggered by hydrocarbon withdrawal, another likely possibility is that the quake has a tectonic origin and falls within a band of weak, natural seismic activity occurring inland from the Gulf coast eastward to the Texas-Louisiana border.

Q: Does drilling for oil and gas cause or trigger earthquakes?

A: Research carried out by UTIG scientists suggests that earthquakes in some parts of Texas may be induced by the pumping of fluids at oil and gas fields, or by the injection of fluids to dispose of chemical wastes. The earthquakes in the Fashing-Pleasanton area southeast of San Antonio are almost certainly caused by or triggered by pumping; such earthquakes also seem to occur in the Texas Panhandle near Snyder, Texas.

Q: If pumping oil and gas cause earthquakes, is it safe to continue pumping?

A: Yes, it is almost always safe. Earthquakes induced by the injection or pumping of fluids from wells are generally very small; most have magnitudes of 3 or less. Moreover, while tens of thousands of oil and gas wells exist in Texas, only a few fields show any evidence that oil and gas pumping induces earthquakes.

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