Join us for this week's Smith Lecture as three of our own grad students will be giving mini talks. See below for info on each lecture:

Speaker 1: Sydney Gable
Title: Temporal and Spatial Variations in b-Value for Foreshock Sequences in Southern California
Abstract: Our current understanding of seismic hazard relies heavily on a parameter known as the b-value, which describes the relative proportion of small to large earthquakes in a catalog, and is routinely used in probabilistic seismic hazard assessments to estimate magnitude exceedance probabilities over a long time-span. Recent studies have suggested that fine-scale spatial and temporal variations in the b-value may be used to describe short-term behavior of an earthquake sequence (Gulia and Weimer 2019) particularly during the foreshock period. While the b-value variations of several large earthquake sequences have already undergone thorough investigation, smaller earthquake sequences have not been analyzed systematically.

In this work, we re-estimate earthquake magnitude using a relative amplitudes method which standardizes magnitude measurement for earthquake foreshock sequences. Additionally, we employ the b+ method for b-value estimation to reduce bias due to short-term incompleteness (van der Elst 2021). We apply this methodology to several earthquake foreshock sequences with moderately sized manshocks (~M 4.0 - 5.5) in Southern California and compare the spatial and temporal variations b-value. This allows us to assess the utility of using fine-scale variations in b-value as a proxy for short term hazard analysis and identify spatio-temporal trends that may shed light on the processes of foreshock evolution.

Speaker 2: Cameron Tripp
Title: Evaluating Ba/Ca as an Upwelling Tracer in Galápagos Porites lobata Corals
Abstract: In the Galápagos Islands, strong seasonal upwelling transports cold, nutrient-rich deep waters to the surface ocean. Local sea surface temperatures (SST) fluctuate as a function of upwelling intensity, while interannual shifts in this dynamical pattern influence the El Niño Southern Oscillation. The Galápagos phytoplankton stock, supporting a uniquely diverse marine ecosystem, depends upon upwelled nutrient supply. With no long-term, direct measure of Galápagos upwelling, we rely on proxy data to track its behavior over time. Prior analyses established empirical links between lattice-bound barium (Ba/Ca) in scleractinian coral samples and the abundance of Ba 2+ in the coral environment. Ba 2+ accumulates in the deep ocean through passive inclusion in organic matter and the biological pump mechanism. Barium substitution in
coral aragonite suggests Ba 2+ enrichment in the surface ocean, facilitated by upwelling. This relationship has informed several upwelling reconstructions via Ba/Ca.
Documented impacts of temperature, calcification rate, and coral physiology on barium substitution complicate these reconstructions. Further, terrestrial sources of Ba 2+ can override the upwelling signal. To isolate the influence of these conflating factors in the Galápagos upwelling region, we generated 10 novel Ba/Ca records from modern and fossil coral skeletons – the largest assemblage from this area to date. Paired Sr/Ca records isolate the thermal upwelling signal, enabling direct comparison. Considering temperature and extraneous factors such as location, age, and coral extension rate, we evaluate Ba/Ca fidelity to the local upwelling pattern. The data reveal consistently weak relationships between Ba/Ca and the thermal upwelling signature.

Additionally, mean barium concentrations vary significantly between cores, increasing in records with lower extension rates and closer proximity to basal aquifers. The inverse relationship to extension matches a kinetic model of Rayleigh fractionation identified in prior studies. Elevated mean values are not intrinsically linked to Ba/Ca disagreement with the thermal upwelling signal, but low extension rates are associated with both features. These findings challenge present understanding of coral Ba/Ca as a paleo-upwelling tracer.

Speaker 3: Yiruo Xu
Title: "Did Archean metamorphic terranes cool slower? Insights from garnet diffusion studies on the Superior Province"
Abstract: Archean metamorphic terranes are traditionally suggested to have cooled significantly slower than their Phanerozoic counterparts. Many have argued that this contrast in metamorphic timescale reflects changes in Earth’s tectonic regime. However, diffusion chronometry-based cooling rate data on Precambrian rocks are very limited. We present case studies of metamorphic timescales on the Superior Province, one of Earth's largest Archean craton. We combine conventional thermobarometry and phase-equilibrium modeling to constrain the peak temperatures and pressures and estimate metamorphic cooling rates from major element diffusion in garnet. The resulting cooling rates exhibit large variability, with the fastest estimates comparable to those from the Phanerozoic eon and the lowest rates slower by two orders of magnitude. We then discuss the uncertainties and potential biases in determining diffusion timescales. The results complicate the conventional idea of slow-cooling Precambrian orogens. Careful evaluation of the geologic context of individual cooling rate data and more targeted diffusion studies are necessary to address the question of long-term change in tectonics.