Imaging Shallow Cascadia Structure with Ambient Noise Tomography

Robert W. Porritt and Richard M. Allen


Along strike variation has been observed throughout the Cascadia Subduction Zone in multiple studies with a variety of data sets. Body-wave tomography shows a broad zone in the center of the slab beneath Oregon with a weak high velocity signal in an atypically quiescent seismic zone (Obrebski and Allen, 2009). Characteristics of primitive basalts found in the arc volcanoes change along strike, defining four distinct magma sources or plumbing systems (Schmidt et al, 2007). However, the most striking variation is in the recurrence rate of episodic tremor and slip throughout the region (Brudzinski and Allen, 2007). These separate observations may reflect lithospheric variations on a regional scale. This study seeks to connect these previous observations by developing a short period surface wave model of structure in the region using ambient seismic noise as the source.

Data Processing

Data for this study comes from the Berkeley Digital Seismic Network, the Southern California Earthquake Center, the Canadian National Seismic Network, and USArray with focus on two Flexible Array Experiments. The Flexible Array deployments, FlexArray Along Cascadia Experiment for Segmentation (FACES) and Mendocino Broadband, were deployed in 2007 and have completed their deployments as of summer 2010. This is one of the first studies utilizing the approximately one hundred stations in these broadband experiments.

Detailed processing flow for computing group and phase velocity maps can be found in Benson et al (2007). While the typical measurements from 7-40 seconds were well constrained from signal-to-noise ratio (SNR) and wavelength criterion, in order to obtain reliable longer period measurements (40-92) seconds, the empirical Green's functions and measured dispersion curves were checked manually to ensure that the measurements were reasonable and consistent with realistic Earth models. The inter-station dispersion curves were inverted with a ray theoretical approach onto a 0.1$^{\circ}$ by 0.1$^{\circ}$ grid with a smoothing radius of one wavelength for each period from 7-92 seconds. Initial models exhibited strongly biased maps with clear artifacts from the heterogeneous distribution of noise sources. This bias was removed by binning the paths into 15$^{\circ}$ bins, normalizing the number of paths in each bin, and applying post-inversion smoothing regularization. To estimate the uncertainties in the phase and group velocity maps, a bootstrapping procedure was applied by randomly choosing 70% of the paths to invert and repeating the process 30 times.

Dispersion curves over the model space and their corresponding uncertainties from the bootstrapping and a static value proportional to period are utilized in a Monte Carlo inversion scheme (Shapiro and Ritzwoller, 2002). The inversion used PREM (Dziewonski and Anderson, 1982) as a starting model in the mantle and GIL7 (Dreger and Romanowicz, 1994) in the crust. Crustal thickness was imposed from the receiver function model of Levander et al. (2008). Slab depth estimates from Audet et al (2009) were also used for better constaints in the subduction zone.


It is not possible to discuss al the features of the model in this short summary; instead we discuss just two key areas of interest. Figure 2.17 details the crustal structure of the Klamath Mountain range. These mountains primarily overlay the Gorda plate portion of the Gorda-Juan de Fuca system, and the region has the shortest tremor recurrence interval of the Cascadia Subduction Zone at 10 months. The fastest velocities in the crust are around 3.6km/s and occur around 20km deep in the core of the mountains. The variability of the top of the Gorda slab with respect to the crustal variations implies some dynamic system of crust-mantle interaction in this zone.

Figure 2.18 details the three dimensional structure of the Siletzia terrain. This piece of accreted ocean terrain is often seen in high-resolution 2D active source studies (Trehu et. al, 1994), and anomalies in geochemical studies are often attributed to it. This study clearly illustrates the structure in a 3D capacity. From this it is clear the Siletzia terrain consists of high velocity material above the plate interface which may be driving the very long recurrence interval of episodic tremor and slip.

Comparing the structure of the two regions can provide insight into the observed variations. The Klamath range is overall slower than the Siletzia. While mapping velocity to rheology is a non-unique process, in general the low shear velocity could be due to a higher silicic or fluid content, or lower shear modulus, both of which imply a lower density and thus less compressive stress on the plate interface. Both of these factors are probable causes for an area to be more likely to have non-volcanic tremors. Future studies, such as receiver function analysis for $V_p$/$V_s$ ratio and tomographic $V_p$ and $V_s$ studies will further improve our constraints on the region.

Figure 2.17: Slices to illustrate the structure of the Klamath terrain. Left: Map slice at sea level. Right: Sections of constant latitude showing high velocity mountain core and the top toe of the subducting slab.
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Figure 2.18: Slices illustrating the 3D structure of the Siletzia terrain. Left: Map slice at 15km. Right bottom: Cross section at 123$^{\circ}$W longitude. Right top and middle: Cross sections at 46$^{\circ}$N and 45$^{\circ}$N respectively.
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We would like to acknowledge our co-PIs and collaborators on the flexible array experiments. The Mendocino Broadband experiment was made possible through NSF grants EAR0643392 and EAR0745934, with help from Gene Humphreys, Leland O'Driscoll, Alan Levander, and Yongbo Zhao for fieldwork and discussions. The FlexArray Along Cascadia was funded through NSF grant EAR0643007 with co-PI Mike Brudzinksi and his students Devin Boyarko and Stefany Sit.

Data from this study came from the Earthscope USArray/Transportable Array, the Canadian National Seismic Network through the AutoDRM system, the Berkeley Digital Seismic Network, and the Southern California Earthquake Center.

This work has been made possible with the resources available through the PASSCAL instrument center at New Mexico Tech.


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