A new network of geodetic field stations has greatly improved monitoring of relative motion across a seismic zone in the central United States. It seems that rapid deformation is occurring across this fault system.
The New Madrid seismic zone lies 50–200 km from Memphis, Tennessee, and was the site of devastating earthquakes in 1811 and 1812. These earthquakes included three mainshocks and many aftershocks, with the largest earthquake having an estimated1,2 magnitude of 7.4–8.1. Historically, New Madrid has been the most seismically active region in central and eastern North America — what hazard might it pose today?
This question has been the subject of vigorous debate in the Earth science and earthquake engineering communities3,4. The report by Smalley et al. (page 1088 of this issue)5 will enlighten that debate. From high-precision Global Positioning System (GPS) measurements, made with a newly installed network of field stations, they conclude that the New Madrid seismic zone is rapidly deforming at rates of the same order of magnitude as those at the boundaries of tectonic plates. This result contradicts earlier estimates of low rates of deformation or strain accumulation6, but is consistent with geological evidence for the occurrence of repeated 1811–1812-type (New Madrid) events in the past 2,000 years7,8.
During the past 12 years, geologists found a record of New Madrid events in the form of earthquake-related features, known as sand blows (Fig. 1). The sand blows formed as a result of liquefaction, a process by which water-saturated sandy sediment below the surface is liquefied and vented on the ground in response to strong earthquake shaking. Detailed study of hundreds of sand blows, some of which are associated with Native American archaeological sites, led to the interpretation that they formed during three, possibly four, New Madrid events of magnitude 7.6 or greater in the past 2,000 years8.
In the 1990s, geophysicists undertook GPS measurements using a network of field stations spanning the New Madrid region to ascertain the rate at which the seismic zone is deforming in response to tectonic forces6. Measurements were collected for several days in 1991, 1993 and 1997, the upshot being estimated relative motion across the seismic zone of 1.4 mm yr−1 with uncertainties of ±3 mm yr−1. These motions were interpreted to be indistinguishable from zero, and therefore indicative of low rates of strain accumulation. Given that earthquake frequency is related to the build-up and release of strain energy, it was concluded that the New Madrid seismic zone produces either magnitude 8 earthquakes every 5,000–10,000 years or magnitude 7 earthquakes every 1,000 years6. This finding differed from that of the geological studies.
In the late 1990s, a network of permanent GPS stations was installed in the New Madrid region. The new network included many improvements; for example, stations were located close to and on both sides of major New Madrid faults, and strong H-beams were used that are less susceptible to non-tectonic movements than the 1-inch-diameter steel rods used in the previous network5. Because the new stations are permanent and collect data continuously, the repeated setting up of field stations, which introduced measurement errors in the previous studies, could be avoided.
Smalley et al.5 have analysed four years of continuous measurements from the new network. They calculate relative motions across the seismic zone that are similar (1–2.7 mm yr−1) to those measured during the 1990s but with much smaller uncertainties — at most 25% of those of the previous studies. Smalley et al. point out that in the earlier GPS data the tectonic signal was lost in the noise, and interpret their results to indicate high rates of strain in the New Madrid seismic zone.
They also find relative motions across the seismic zone that are consistent with expected fault movements as inferred from present-day seismicity9 and recent fault studies7. For example, relative motion indicates that bedrock slips over itself along a major northwest-oriented fault, known as the Reelfoot thrust fault, that is inclined towards the southwest (see Fig. 2 on page 1089). The new findings are persuasive because they help to explain the geological observations of frequent New Madrid earthquakes, and they make sense in terms of the active faulting in the region.
One of the most interesting results is that motions in the surrounding region are low compared with motion in the seismic zone itself. This unusual behaviour differs from that at plate boundaries, raising questions about the driving forces and earthquake processes within plates. Post-seismic afterslip — a process by which fault displacements at depths of several kilometres are expressed at the surface for a period of time following an earthquake10 — seems a reasonable explanation for the regional pattern of motions. However, there is currently insufficient information about the physical properties of the Earth in the New Madrid region to test this and competing models.
Smalley and colleagues' results are consistent with the findings of geological studies that the seismic zone produced earthquakes about every 500 years of magnitude 7.6 or greater. As such, they provide scientific justification for the adoption of stricter earthquake provisions in the building codes for Memphis and other cities in the central United States4. Looking ahead, installation of additional field stations close to known faults would help to define their extent and further quantify their strain rates. One of the most daunting challenges will be to develop and test models that can explain how such large and frequent earthquakes are produced in the New Madrid region, and to see if the models also apply to other intraplate regions.
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Frontiers in Earth Science (2019)
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Geophysical Journal International (2008)
Journal of Earthquake Engineering (2007)