S-velocity model of the Cascadia Subduction Zone using Ambient Seismic Noise
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PNW10-S combines the state of the art ambient noise tomography method with time tested analyst selection to ensure the highest quality data is used as input to the model inversion. Incorporating spatially and temporally long paths with this manual selection step allows recoverable structure from the surface to ~120km depth. This model focuses on the US Pacific Northwest to address a series of questions relating to variations in arc volcanism, seismicity, tremor activity, and the relation to subduction complex structure. In this model we image lateral discontinuities of the subducting slabs in line with offshore transform faults in the Juan de Fuca plate. This suggests zones of weakness in the subducting plate creating pathways for upwelling material through the plate. This observation along the continuation of the Blanco transform fault explains the high heat production observed in the southern Cascades as arising from an upper mantle source. Variations within the crustal structure align with long term tremor segmentation boundaries suggesting the structure of the overriding plate has an effect on tremor frequency.
N-S cross-section throught the Cascadia forearc. See Fig 7
Porritt et al 2011
PNW10-S is a Vsv model derived from fundamental mode Rayleigh
waves on ambient seismic noise cross correlations
Cross correlations span the entire US, with a handful of
stations in Canada and Mexico. The inversion solves for relative structure
from 33N to 49.5N latitude and 126W to 110W longitude, but we extract the well
resolved section from 38.2N to 49N and 126W to 119W due to its data density.
- Data source:
While our dataset focuses on two Flexible Array Experiment
(FACES and Mendocino) more than 1200 stations where used from the USArray transportable array, regional seismic networks, and temporary seismic deployments.
- Data type:
Phase velocities measured from inter-station paths with a
Frequency-Time Analysis procedure to prepare period dependent maps.
Phase velocities are converted to perturbations to the 1D WUS
model of Pollitz, 2009 with a crustal thickness determined by prior
receiver function analysis (Audet et al., 2010, Levander et al., 2007). The
relative phase velocities are then inverted with damped LSQR.
Horizontal resolution varies with spatial coverage and can
perhpas be best estimated around the minimum station spacing of ~50km. Vertical
resolution varies with depth as the shallower regions are more densely sampled
and can thus be considered better resolved, but we estimate an overall
vertical resolution of ~10km.
See the published manuscript for more detailed resolution analysis.
Porritt, R. W., Allen, R. M, Boyarko, D. C., and Brudzinski, M. R.,
Investigation of Cascadia Segmentation with Ambient Noise Tomography. Earth
and Planet. Sci. Lett., 309, 67-76. doi: 10.1016/j.epsl.2011.06.026, 2011.
Along strike variation in the characteristics of subduction zone processes has
been observed throughout the Cascadia Subduction Zone through analysis of arc
magmas and the distribution of seismicity. We investigate links between these
observations and subduction zone structure by imaging three-dimensional
lithospheric scale shear velocity with ambient noise tomography (ANT). The
crustal portion of the model is well resolved through typical ANT processing
techniques. We expand the methodology to use longer period phase velocities in
order to recover structure to ~120km depth. The resulting model, PNW10-S,
represents structural information in terms of relative shear velocity in the
crust and uppermost mantle. Crustal structure mirrors surface geology to ~10
km depth and then transitions to a structure that is dominated by the
subducting slab. The subducting slab and overriding crust appear segmented
into three parts with boundaries near 43°N and 46°N. This three-way structural
segmentation is aligned with the variation in recurrence of episodic tremor
and slip along the subduction zone (Brudzinski and Allen, 2007). Upper to
middle crustal boundaries between the Klamath Mountains and Siletzia Terrane
(43N) and between the Crescent Formation and Olympic Peninsula (47N) are also
coincident with locations of increased occurrence of tremors raising the
question of whether there is a link between the intensity of tremor activity
and shallow (less than 10km) crustal structure. The slab-segment boundary at
43N is a stronger feature than the northern segment boundary at 46N and
appears to be the continuation of the Blanco Fracture Zone separating the
Gorda segment of the plate from the rest of the Juan de Fuca plate. The
southern half of the arc system, south of 45N, shows lower velocities from the
surface to ~80 km depth relative to the northern portion of the arc. We
propose this is due to clockwise plate rotation, which causes extension in the
south, and results in increased melting. Along the arc, four broad
low-velocity features are also imaged just below the Moho and centered at 42N,
44N, 47N, and 49N. We interpret these as ponding of melt just below the crust
where differentiation can occur before further ascent through the crust.