IN THIS SECTION
Spring 2017 Colloquia
January 9: Jessica Hawthorne, University of Leeds, UK. Title: Perspectives on slow and fast slip from repeating earthquakes and tremor
Abstract: The last few decades have revealed several new types of slow and fast earthquakes, including slow slip events, low frequency earthquakes, and tremor. By investigating these novel slip behaviors, we can probe the physics that controls how faults work. Here I present three investigations of repeating and low frequency earthquakes on the San Andreas Fault. First, I look for aseismic slip that occurs just after repeating earthquakes. Repeating earthquakes are earthquakes that repeatedly rupture a specific patch on the fault. But surprisingly, summing the estimated earthquake slips gives a total slip much smaller than the long-term slip on the slip on the fault, so large aseismic slip on the earthquake patches has been proposed. Models including this slip in the postseismic interval further suggest that repeater patches may be close in size to the smallest possible earthquake, which would place repeaters in a very useful part of parameter space for testing models of earthquake nucleation. However, my observations imply that the postseismic moment following repeaters is just 50 to 150% times the coseismic moment, consistent with models of "normal-size" earthquakes. So in the second investigation, I look for any peculiar properties of seismic slip in repeating earthquakes. Using a method I have recently developed, I am able to directly probe the spatial extent of the earthquake ruptures, rather than just the durations as in many previous studies. I find that the rupture extents of repeating earthquakes are consistent with their being "normal" earthquakes, following standard scaling laws. However, the similarity of each repeating earthquake with others in its group suggest that within these standard dimensions, rupture patterns are nearly identical from event to event, perhaps as a result of variations in the fault properties within the patch. These events may turn out to be relatively typical though geometry-controlled earthquakes. However, the earthquakes in the last part of my presentation – low frequency earthquakes that comprise tremor – show a more unusual scaling relationship. Non-volcanic seismic tremor is often inferred to consist of tens of thousands of small earthquakes, but it is dominated by frequencies lower than would be expected for such small earthquakes, suggesting that the earthquakes last 10 or 100 times longer than other events with similar magnitudes. Standard earthquake models would interpret such long durations as the result of larger-than-typical spatial extents. However, my results imply that the spatial extent of low frequency earthquakes near Parkfield are a factor of 4 smaller than would be predicted with standard rupture models. These small dimensions suggest a difference in the rupture dynamics of low frequency earthquakes. Tremor may be composed of earthquakes that rupture more slowly or have a more complex rupture pattern than typical earthquakes. Determining which frictional models can match these rupture dynamics may help reveal how small and large earthquakes grow and propagate.
January 16: Dr. Martin Luther King Day, no colloquium.
January 23: Sarah Penniston-Dorland, University of Maryland Title: Strength and heat in the subduction channel: Evidence from metamorphic rocks and geodynamic models
Abstract: The thermal structure and flow of material within subduction zones are closely linked and are important for our understanding of seismicity within subduction zones and for the generation of arc magmas. This is a talk in two parts investigating evidence from metamorphic rocks for the thermal structure and degree of material flow within subduction zones. Evidence from natural rocks is compared to that generated from computational geodynamic models.
Part 1. Thermal structure: The maximum-pressure P-T conditions (Pmax-T) and prograde P-T paths of exhumed subduction-related metamorphic rocks are compared to predictions of P-T conditions from computational thermal models of subduction systems. While the range of proposed models encompasses most estimated Pmax-T conditions, models predict temperatures that are on average colder than those recorded by exhumed rocks. In general, discrepancies are greatest for Pmax > 2 GPa where only a few of the highest-T modeled paths overlap typical petrologic observations and model averages are 100-300 °C colder than average conditions recorded by rocks. Prograde P-T paths similarly indicate warmer subduction than typical models. Our compilation and comparison suggest that exhumed high-P rocks may closely represent the subduction geotherm. While exhumation processes in subduction zones require closer petrologic scrutiny, the next generation of models should more comprehensively incorporate all sources of heat. Subduction-zone thermal structures from currently available models do not match the rock record, and this mismatch has wide-reaching implications for our understanding of global geochemical cycles, the petrologic structure of subduction zones, and fluid-rock interactions and seismicity within subduction zones.
Part 2. The Catalina Schist contains a spectacular, km-scale amphibolite facies mélange zone, thought to be part of a Cretaceous convergent margin plate interface. In this setting, mafic and ultramafic blocks ranging from cms up to 100s of m in diameter are surrounded by finer-grained matrix. All blocks throughout the mélange contain assemblages consistent with upper amphibolite-facies conditions, suggesting a relatively restricted range of depths and temperatures over which the mélange formed. This apparent uniformity contrasts with other mélanges, such as the Franciscan Complex, where rocks with highly variable peak metamorphic grade suggest extensive mixing of materials along the subduction interface. This mixing has been ascribed to flow of material within relatively low viscosity matrix. The Zr content of rutile in samples from the amphibolite facies of the Catalina Schist was measured to determine peak metamorphic temperatures, identify whether these temperatures were different among blocks (within measurement error), and whether the spatial distribution of temperatures throughout the mélange was systematic or random. Resolvably different Zr contents are found among the blocks, corresponding to different peak metamorphic temperatures of 650 to 730°C at an assumed pressure of 1 GPa. No systematic distribution of temperatures was found, however. Therefore material flow within the Catalina Schist mélange was likely chaotic, but appears to have occurred on a relatively restricted scale.
January 30: Craig Lundstrom, University of Illinois. Title: Do granites reflect crystallization from rhyolitic melt?
Abstract: Earth has a bimodal crust with continents considerably more silicic than ocean crust. This composition is generally attributed to making silicic igneous rocks; however the process of making silicic igneous rocks remains somewhat nebulous. Tuttle and Bowen (1958) produced hydrous melt coexisting with quartz and feldspar at a minimum melt point (˜65°C at 1kbar) which we take as the granite solidus. However TB58 as well as other literature and new experiments show quartz and two feldspars coexist with a hydrous melt at temperatures as low as 33°C. I will present an alternative view of granite formation by a near equilibrium process involving crystal-melt reactions in a thermal gradient. Two case studies, the making of plagiogranites within the Troodos ophiolite and the making of granites in Torres del Paine, will be used as evidence to support this proposal. The idea that silicic igneous rocks form by this process and not by crystallization from rhyolitic melt has wide reaching implications.
February 6: Nicole Gasparini, Tulane University. Title: Rainfall gradients and bedrock river incision: A tale of two landscapes
February 13: Abhijit Basu, Department of Geological Sciences, IU. Title: Sedimentary Provenance at IU Yesterday, Today, Tomorrow
Abstract: Modern sedimentary provenance studies at IU started with Lee J Suttner’s 1969 paper in the Bulletin of AAPG. There has been no stopping since then. The Dickinsonian quartz-feldspar-lithics (QFL) approach to tectonic provenance of sands and sandstones (1979) – a paradigm to be – was effectively moderated with quantitative documentation of climatic controls on QFL. IU’s major contribution, however, has been to promote the principle that properties of single minerals or mineral groups more effectively track sedimentary provenance. IU studies have unraveled genetically significant crystallographic, chemical, and luminescence properties of quartz and feldspar, which indicate if one had crystallized rapidly or slowly from a magma (volcanic or plutonic), or, re-crystallized under stress (metamorphic). Uranium-lead and uranium-thorium/helium isotopic dating of single zircon and apatite grains in sandstones are helping to reconstruct ancient (>1000 million years) plate positions and to identify sediment transport rates in Patagonia in the last ˜100 million years. With futuristic hopes, we are tracking lutetium-hafnium isotopic evolution of individual zircon grains in sandstones to find the zircons’ ultimate source deep inside the crust (up to ˜80 km) and in deep time (usually >1500 million years).
February 20: Paul Segall, Stanford University. TUDOR LECTURE Title: Beyond Rangely: Understanding the Mechanics of Induced Seismicity
Abstract: Since the pioneering study at Rangely, Colorado, induced seismicity due to fluid injection has been understood to result from a decrease in effective normal stress acting on faults due to increase in pore-fluid pressure. Much attention has thus been given to the spatiotemporal distribution of pore-pressure resulting from injection. Yet, there are well documented cases in which oil and gas production, with dramatic decreases in reservoir pressure have triggered earthquakes. This is opposite to expectation based on the effective stress concept, but can be well understood with Biot’s theory of poroelasticity.
Normal fault scarp on the margin of the Goose Creek oil field in south Texas in the 1920’s associated with a series of felt earthquakes. Oil production there caused the field to subside by as much as 1 meter between 1917 and 1925.
I will explore whether poroelastic effects are important in injection induced seismicity. Several surprising results follow from simple models, including that abrupt shut-in (cessation of injection) can lead to locally sharp increases in seismicity rate. In addition, poroelastic stressing can destabilize faults that are hydraulically isolated from injection horizons. Finally, the maximum magnitude of induced events has been observed to occur post-injection, which presents a clear problem for so-called ‘stop light’ mitigation systems. I suggest that under low ambient shear stresses rupture extents are limited by the time varying volume of perturbed crust. This leads to time dependence in the frequency magnitude distribution of earthquake sizes, as has been observed in Basel, Switzerland. In this limit, larger events post shut-in are not unexpected.
Stress changes during finite duration injection at constant rate. Dotted vertical line indicates the shut-in time.
About Paul Segall Paul Segall has been a Professor of Geophysics at Stanford University since 1989. He received a Ph.D. from Stanford in 1981 and an M.S. from Case Western Reserve University in 1976. He worked as a Geologist for the USGS from 1981-1989. He studies active earthquake and volcanic process through data collection, inversion, and theoretical modeling. Using techniques such as the Global Positioning System (GPS) and Interferometric Synthetic Aperture Radar (InSAR) he and his students measure deformation in space and time and invert these data for the geometry of faults and magma chambers, and spatiotemporal variations in fault slip-rate and magma chamber dilation. He develops and tests models of active plate boundaries such as the San Andreas, and the Cascade subduction zone, the nucleation of earthquakes, slow slip events, and the physics of magma migration leading to volcanic eruptions. He is an AGU and GSA Fellow and has received the prestigious AGU Macelwane (1990) and Whitten (2014) medals. He was elected a member of the National Academy of Sciences in 2016.
February 27: Patrick McLaughlin, Indiana Geological Survey Title: Frontiers in chemostratigraphy: transformative framework for Earth systems science
March 6: Greg Retallack, University of Oregon. Title: TBA
March 13: Spring Break - no colloquium.
March 29: Tiffany Shaw, University of Chicago. Title: TBA
March 27: Darren Ficklin, IU Department of Geography. Title: TBA.
April 3: David Mohrig, University of Texas. Title: TBA
April 10: Allison Wing, LDEO. Title: TBA
April 17: Jose Constantine, Williams College. Title: TBA
April 24: Karen Johannesson, Tulane University. Title: TBA