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The University of Texas Institute for Geophysics (UTIG) has actively researched earthquake seismology since it was founded under another title in 1972 (see Institutional History). Initially, the founding Institute's research program in lunar seismology outpaced the terrestrial program. Its scientists included one of the pioneers in research concerning terrestrial surface waves, however, and two well-known papers on booming dunes were co-authored by another Institute scientist. Between 1977 and 1989, UTIG's ocean-bottom seismograph (OBS) program ramped up and remained particularly productive, deploying several hundred earthquake-recording units. Generally, the scientific questions that have motivated OBS earthquake research concern determining where active tectonic movement is taking place today. This involves obtaining accurate locations for small earthquakes and relating the pattern of these locations to the regional geology. The same techniques are useful for evaluating earthquake patterns and tectonics when using conventional non-OBS earthquake data. In 1997, for example, UTIG seismologists analyzed earthquakes along the Macquarie Ridge south of New Zealand. This study complemented a UTIG marine geophysics program investigating the origin of the central Macquarie Ridge; an important question concerned whether this central region was a product of both transform motion and incipient subduction. Both the earthquake investigations and the cruise data suggested that, contrary to previously published accounts, the area was not undergoing incipient subduction. Because earthquake monitoring methods are also useful for monitoring massive explosions, seismic research may play an important role in preventing nuclear escalation and warfare. The Comprehensive Test Ban Treaty, drafted in 1996 and signed by many countries including the United States and Russia, implies that signatory nations even as they await its ratification will not undertake nuclear testing with devices corresponding to seismic magnitudes larger than about 3.0. Many nations are unwilling to sign the treaty unless there are methods to monitor small seismic events, at least in politically sensitive regions. Most current methods locate events by comparing the travel times of seismic phases arriving at several monitoring stations. Unfortunately, explosions with magnitudes as small as 3.0 are often recorded by only one or two stations and go unreported. To address this problem, UTIG scientists are currently developing methods that use inverse theory to more accurately locate earthquakes or explosions with data from a single broadband digital station. Whereas conventional seismic analysis focuses on phasal peaks only, this approach exploits the informational wealth of the entire waveform, against which the scientists attempt to match synthetic waveforms generated by sophisticated models. |
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Location of the events in the Tonga-Fiji region
(white stars at lower center) that give rise to anomalous shear-wave splitting,
recorded at Hockley, Texas.
Of course, not all earthquake research at UTIG is of such societal relevance. A significant fraction of the UTIG research papers in earthquake seismology concern 'deep' earthquakes, the focal depths of which exceed 70 kilometers. Roughly one of every four of the Earth's quakes is a deep one, but these seldom present a hazard to humankind. That such seismic activity can occur at all under the pressures and temperatures within the Earth's mantle is a puzzle, however.
Because deep earthquakes tend to have particularly sharp and impulsive onsets, they are useful sources for probing the Earth's structure. A recent paper by UTIG staff scientists used recordings of deep earthquakes to investigate the anisotropic structure at the base of the mantle, the region just above the liquid core.
Within Texas, UTIG scientists have always led efforts to monitor earthquakes and advise State officials about appropriate safety measures. Over the past half century, Texas has grown from a sparsely populated state with an agrarian economy to the nation's second most populous state with several of its largest urban centers. Since 1997, when such rapid demographic changes were seen to warrant a State-supported program, UTIG scientists have been engaged in a major reassessment of Texas seismicity and resulting hazards.
An important part of this program involves increasing the number of
high-quality seismic stations in Texas. In 1973, UTIG scientists installed
seismometers deep within a salt mine in
Hockley,
Texas. With the cooperation of the mine's owner, United Salt, Inc., they have
operated equipment there ever since. Because the station is 470 meters
beneath the surface, the Hockley station (station code HKT) is among the most
sensitive stations on the entire Gulf coast, especially since July 1995 when
UTIG installed modern, broadband, digital seismometers whose recorded signals
are continuously relayed to the global network by satellite. Recently, scientists
at UTIG and Texas Tech University collaborated to similarly upgrade the
seismic station in Junction, Texas (station code JCT). As funding becomes
available, UTIG and Texas Tech will continue to install other stations at
various sites in Texas as part of the
TexSeis network.
