Rychert Catherine A.

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Catherine A.

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  • Article
    Seismicity properties of the chain transform fault inferred using data from the PI‐LAB experiment
    (American Geophysical Union, 2023-02-23) Leptokaropoulos, Konstantinos ; Rychert, Catherine A. ; Harmon, Nicholas ; Kendall, John Michael
    Oceanic transform faults are intriguing in that they do not produce earthquakes as large as might be expected given their dimensions. We use 1‐year of local seismicity (370 events above MC = 2.3) recorded on an array of ocean bottom seismometers (OBSs) and geophysical data to study the seismotectonic properties of the Chain transform, located in the equatorial Mid‐Atlantic. We extend our analysis back in time by considering stronger earthquakes (MW ≥ 5.0) from global catalogs. We divide Chain into three areas (east, central, and west) based on historical event distribution, morphology, and multidimensional OBS seismicity cluster analysis. Seismic activity recorded by the OBS is the highest at the eastern area of Chain where there is a lozenge‐shaped topographic high, a negative rMBA gravity anomaly, and only a few historical MW ≥ 5.5 events. OBS seismicity rates are lower in the western and central areas. However, these areas accommodate the majority of seismic moment release, as inferred from both OBS and historical data. Higher b‐values are significantly correlated with lower rMBA and with shallower bathymetry, potentially related to thickened crust. Our results suggest high lateral heterogeneity along Chain. Patches with moderate to low OBS seismicity rates that occasionally host MW ≥ 6.0 earthquakes are interrupted by segments with abundant OBS activity but few historical events with 5.5 ≤ MW < 6.0. This segmentation is possibly due to variable fluid circulation and alteration, which may also change in time.
  • Article
    Slab to back-arc to arc: fluid and melt pathways through the mantle wedge beneath the Lesser Antilles
    (American Association for the Advancement of Science, 2023-02-01) Hicks, Stephen P. ; Bie, Lidong ; Rychert, Catherine A. ; Harmon, Nicholas ; Goes, Saskia ; Rietbrock, Andreas ; Wei, Songqiao Shawn ; Collier, Jenny S. ; Henstock, Timothy J. ; Lynch, Lloyd ; Prytulak, Julie ; Macpherson, Colin G. ; Schlaphorst, David ; Wilkinson, Jamie J. ; Blundy, Jonathan D. ; Cooper, George F. ; Davy, Richard G. ; Kendall, John-Michael
    Volatiles expelled from subducted plates promote melting of the overlying warm mantle, feeding arc volcanism. However, debates continue over the factors controlling melt generation and transport, and how these determine the placement of volcanoes. To broaden our synoptic view of these fundamental mantle wedge processes, we image seismic attenuation beneath the Lesser Antilles arc, an end-member system that slowly subducts old, tectonized lithosphere. Punctuated anomalies with high ratios of bulk-to-shear attenuation (Qκ−1/Qμ−1 > 0.6) and VP/VS (>1.83) lie 40 km above the slab, representing expelled fluids that are retained in a cold boundary layer, transporting fluids toward the back-arc. The strongest attenuation (1000/QS ~ 20), characterizing melt in warm mantle, lies beneath the back-arc, revealing how back-arc mantle feeds arc volcanoes. Melt ponds under the upper plate and percolates toward the arc along structures from earlier back-arc spreading, demonstrating how slab dehydration, upper-plate properties, past tectonics, and resulting melt pathways collectively condition volcanism.
  • Article
    Elastic and anelastic adjoint tomography with and full Hessian kernels
    (Oxford University Press, 2023-03-17) Xie, Yujiang ; Rychert, Catherine A. ; Harmon, Nicholas
    The elastic and anelastic structures of the Earth offer fundamental constraints for understanding its physical and chemical properties. Deciphering small variations in the velocity and amplitude of seismic waves can be challenging. Advanced approaches such as full-waveform inversion (FWI) can be useful. We rewrite the anelastic Fréchet kernel expression of Fichtner & van Driel using the displacement–stress formulation. We then derive the full Hessian kernel expression for viscoelastic properties. In these formulations, the anelastic Fréchet kernels are computed by the forward strain and a shift of the adjoint strain. This is complementary to the quality factor Q (i.e., inverse attenuation) Fréchet kernel expressions of Fichtner & van Driel that are explicit for the velocity–stress formulation. To reduce disk space and I/O requirements for computing the full Hessian kernels, the elastic full Hessian kernels are computed on the fly, while the full Hessian kernels for Q are computed by a combination of the on-the-fly approach with the parsimonious storage method. Applications of the Fréchet and full Hessian kernels for adjoint tomography are presented for two synthetic 2-D models, including an idealized model with rectangular anomalies and a model that approximates a subduction zone, and one synthetic 3-D model with an idealized geometry. The calculation of the full Hessian kernel approximately doubles the computationally cost per iteration of the inversion; however, the reduced number of iterations and fewer frequency stages required to achieve the same level of convergence make it overall computationally less expensive than the classical Limited-memory Broyden–Fletcher–Goldfarb–Shanno (L-BFGS) FWI for the 2-D elastic tested models. We find that the use of full Hessian kernels provides comparable results to the L-BFGS inversion using the improved anelastic Fréchet kernels for the 2-D anelastic models tested for the frequency stage up to 0.5 Hz. Given the computational expense of the Q full Hessian kernel calculation, it is not advantageous to use it in Q inversions at this time until further improvements are made. For the 3-D elastic inversion of the tested model, the full Hessian kernel provides similar image quality to the L-BFGS inversion for the frequency stage up to 0.1 Hz. We observe an improved convergence rate for the full Hessian kernel inversion in comparison to L-BFGS at a higher frequency stage, 0.1–0.2 Hz, and we speculate that at higher frequency stages the use of full Hessian kernels may be more computationally advantageous than the classical L-BFGS for the tested models. Finally, we perform 3-D elastic and Q L-BFGS inversions simultaneously using the rederived Q kernels, which can reduce the computational cost of the inversion by about 1/3 when compared to the classical anelastic adjoint tomography using the additionally defined adjoint source. The recovered Q model is smeared when compared to the recovered elastic model at the investigation frequencies up to 0.5 Hz. Q inversion remains challenging and requires further work. The 2-D and 3-D full Hessian kernels may be used for other purposes for instance resolution analysis in addition to the inversions.