Ph.D., California Institute of Technology, 1978
seth AT earth.northwestern.edu
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Plate tectonics, space-based geodesy, and seismology.
Carol Stein (University of Illinois at Chicago) and I are analyzing the variations in seafloor depth and heat flow with age that provide the primary constraints on the thermal structure and evolution of the oceanic lithosphere. Moreover, regions of midplate volcanism and swells are identified by shallow seafloor depths. In turn, investigation of the processes giving rise to these regions rely on assessments of how the depths, heat flow, and flexural properties differ from those for unperturbed lithosphere. Such comparisons have been inhibited because reference thermal models assumed to describe unperturbed lithosphere predict deeper depths and lower heat flow than typically observed for lithosphere older than 70 Ma. As a result, depth and heat flow anomalies can be significantly overestimated. To address this difficulty, we derive a new model, GDH1, by joint fitting of heat flow and bathymetry, which implies that the lithosphere is hotter at depth and thinner than previously assumed. GDH1 fits the data, including that data from older (>70 Ma) lithosphere previously treated as anomalous, significantly better than previous models. GDH1 thus facilitates analysis of processes including midplate volcanism and swells, differences in regional subsidence, and hydrothermal circulation near spreading centers.
The new model reduces inferred anomalies at swells like Hawaii, implying little or no reheating of the lithosphere. Hence it is important for the ongoing debate about whether hotspot swells are due to plumes rising from deep the the mantle, or are instead due to shallow process. This leads naturally to consideration of "Superswells", such as the Darwin Rise region shown, where multiple hotspot tracks may indicate that in the Cretaceous (pre-70 Myr) an unusual outpouring of mantle heat produced a broad upwelling. Depth anomalies relative to different reference models yield quite different maps, and hence tectonic inferences. The entire Rise is shallow relative to a halfspace. Relative to PSM, much of the area is also shallow, suggesting a remnant regional thermal signature of the volcanism that formed the swells. However, because almost all lithosphere of this age is shallower than these models predict, the anomalies need not indicate that the Rise presently differs from lithosphere of this age elsewhere. In contrast, relative to GDH1, swells associated with volcanic chains are shallow, whereas depths between them are within a standard deviation of that predicted. Because of the three models GDH1 best describes average old lithosphere, it indicates that much of the Darwin Rise is not significantly deeper than lithosphere of the same age elsewhere, implying that the region between the swells retains no significant large-scale thermal signature of the Cretaceous events.
These studies have been extended, in joint research with recent graduates Phil Richardson and John Delaughter, using the earth's geoid measured from satellite altimetry. The geoid is very valuable for these studies, because it gives a constraint on the geotherm complementary to those given by bathymetry and heat flow, and is in principle the best of these three data types for resolution of deep thermal structure.
One major effort is understanding how global plate motions over geologic time (millions of years) compare with those over a few years measured from space geodesy, and exploring these data`s implications for the evolution of the continents. This is being addressed by programs using using the Global Positioning System (GPS) satellites to study the central U.S. seismic zone and South America. Coworkers and I are conducting a multi-institutional GPS program to quantify the rate and distribution of strain accumulation in the New Madrid, Missouri, seismic zone. Global Positioning System measurements were made in 1991, 1993 and again in 1997 at two dozen sites within 300 km of the seismic zone. The results show minimal differential motion. These data are consistent with platewide continuous GPS data away from the NMSZ, showing no motion within uncertainties. Both these data and the frequency-magnitude relation for seismicity imply that had the 1811-1812 earthquakes been magnitude 8, their recurrence interval should well exceed 2500 yr, longer than typically assumed. Alternatively, both the 1811-1812 earthquakes and those in the paleoseismic record may have been much smaller than typically assumed. Hence the hazard posed by great earthquakes in the NMSZ appears to be significantly overestimated.
Data from the Andes GPS program , are being analyzed. These data provide the first direct measurement of crustal shortening rates across the Andes. The rate is less than inferred geologically, and closer to that inferred from seismic moments. These data also provide estimates of the fraction of the total plate motion that may be released by large trench earthquakes. The data are being combined with geological and seismological data to develop models of the mechanics of the plate boundary.
Giovanni Sella, Emile Okal, and I have been looking into the tectonic setting of the January, 26, 2001, M = 7.7 Bhuj earthquake in India. Although it has been suggested that this earthquake was a continental intraplate earthquake with analogies to the New Madrid seismic zone in the central U.S., it seems more plausible to us that the earthquake occurred within the Indian plate's diffuse western boundary.
E. Okal, S. Kirby from U.S.G.S., D. Rubie from Bayreuth (Germany) and I are exploring the possibility that deep-focus earthquakes result from phase changes in metastable minerals within the subducting lithosphere. Thermo-kinetic modeling shows that younger and slower subducting slabs (e.g., Aleutian) are hot enough that transformation of olivine to spinel keeps pace with the descent rate and is completed near the equilibrium phase boundaries. At most a small metastable region forms and deep earthquakes do not occur. Older and faster subducting slabs (e.g., Tonga) are colder and kinetic hindrance prevents transformation from keeping pace with the descent rate. These predictions are consistent with the variation in earthquake depths between and along subduction zones, and have interesting consequences for the subduction process. Recent work with grad student Fred Marton, and C. Bina is looking into the effect of metastability on subduction rates.
For references to any of these, see my Publications list