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Strain Accumulation Along the Cascadia Subduction Zone

Mark Murray

Introduction

Geodetic measurements of present-day deformation show that the Cascadia subduction zone (CSZ) (Figure 24.1) is accumulating strain, and geologic evidence of abrupt episodic coastal subsidence in the past suggests that this strain is periodically released in large earthquakes [Clague, 1997]. The most recent event may have been a great M$\sim $9 earthquake that ruptured much of the megathrust in January 1700 [Satake et al., 1996], although other evidence suggests that the overlying plate is weak and rapidly deforming, making it more likely that the entire megathrust ruptures in a sequence of smaller M$\sim $8 events [McCaffrey and Goldfinger, 1995].

A conventional model for strain accumulation at a subduction zone assumes that the plate interface slips at the long-term convergence rate at great depths but is fully or partially locked at shallower depths [Savage, 1983]. A model with 10$^\circ $ dipping megathrust, a 100-km wide locked zone and a 75-km wide transition zone, where aseismic slip increases linearly with depth until it equals the plate convergence rate, adequately explains crustal shortening and vertical uplift observations in western Washington [Savage et al., 1991]. Similar models are consistent with deformation observations in Vancouver Island, vertical uplift along the CSZ margin, and thermal modeling [Flück et al., 1997].

Horizontal deformation within the southern CSZ is more poorly known. Between 1987 and 1999, the U. S. Geological Survey (USGS) used GPS to recover the positions of survey benchmarks in this region previously measured by triangulation, trilateration, or GPS techniques. We use these observations to estimate horizontal strain rates, which indicate that strain is accumulating everywhere along the CSZ from Cape Mendocino to the Strait of Juan de Fuca and do not rule out the possibility that the entire CSZ could rupture in a major M$\sim $9 earthquake [Murray and Lisowski, 2000].

Data Analysis

Geodetic measurements have been collected in 16 regions (networks) along the CSZ (Figure 24.1). First- and second-order triangulation surveys were conducted by the National Geodetic Survey (NGS) in 6 of the networks during the last 120 years (Crescent City, Portland, Bellingham, Columbia, Eureka, Juan de Fuca). Between 1972 and 1991, the USGS used a Geodolite, an electro-optical distance-measuring instrument, to survey 5 networks (Hanford, Olympic, Seattle, Cape Mendocino, and Helens). Since 1987, the USGS has collected GPS measurements in 10 of the networks. We use these measurements to determine new strain estimates within all the networks except Olympic, Crescent City, Portland, and Bellingham.

We assume horizontal strain rates are constant in time and uniform in space within each network. For networks with Geodolite and GPS observations, we estimate the tensor-strain rate (given by the direction of maximum contraction $\beta$ and the maximum $\dot{\varepsilon}_1$ and minimum $\dot{\varepsilon}_2$ principal strain rates) We estimate engineering shear-strain rates $\dot{\gamma_1}$ and $\dot{\gamma_2}$ in networks with triangulation data or NGS GPS solutions. For comparison, we also determine the maximum shear-strain rate $\dot{\gamma}~=
\dot{\varepsilon}_1~-~\dot{\varepsilon}_2~=
(\dot{\gamma_1}^2~+~\dot{\gamma_2}^2)^{1/2}$ in all networks.

Results

The estimated strain rates for the 16 networks are shown in Figure 24.1. For networks within the forearc, $\dot{\gamma}$ are 26 to 167 nanoradian/yr and significantly greater than zero (95% confidence), and $\beta$ is generally east-northeast, about N55$^\circ $-80$^\circ $E. For most of the networks with well resolved tensor-strain rates, the strain is predominantly uniaxial contraction. In Figure 24.1, we show $\dot{\gamma}$ in the $\beta$ direction for networks with poorly resolved dilatation rates, which is equivalent to assuming the shear-strain rates are due to uniaxial contraction.

The geodetic networks from Cape Mendocino to the Strait of Juan de Fuca for which horizontal strain rates are measurable span almost the entire forearc region of the CSZ (Figure 24.1). In general, the directions and magnitudes of the strain rates are consistent with strain accumulation predicted by a megathrust that is mostly slipping at the plate convergence rate, but locked at shallow depths. We assume a convergence rate v that ranges from 32 mm/yr near Cape Mendocino to 42 mm/yr near the Strait of Juan de Fuca [Wilson, 1993].

The directions of maximum contraction $\beta$ in most networks in the forearc are nearly parallel (within 2-sigma) to plate convergence (Figure 24.2). For networks in or near the Cascade arc region (Nisqually, Portland, Sisters, and Saint Helens), $\beta$ significantly differs from plate convergence or strain is not consistent with uniaxial contraction. Additional deformation may be responsible for these anomalies, such as post-eruption volcanic deformation near Mt. Saint Helens, extension of the Eastern California Shear Zone into the arc and backarc regions [Pezzopane and Weldon, 1993], and clockwise rotation of the Oregon forearc [Wells et al., 1998; Savage et al., 2000].

In the Cape Blanco, Newport, and Portland networks, $\beta$ is 15-20$^\circ $ more eastward than plate convergence and is more consistent with the direction normal to the deformation front (Figure 24.2). Deformation in Oregon may be partitioned into arc-normal convergence (the observed strain) and arc-parallel shear accommodated on strike-slip faults in the forearc [McCaffrey and Goldfinger, 1995].

For each network, we determined the rate of extension $\bar{e}$ in the direction of plate convergence $\theta$, normalized by the convergence rate v: $\bar{e}~= [\dot{\varepsilon}_2~\cos^2(\theta~-~\beta)~+
\dot{\varepsilon}_1~\sin^2(\theta~-~\beta)]/v$. For networks with only shear-strain rates, we assumed the dilatational component that minimizes extension perpendicular to plate convergence, which gives: $\bar{e}~=
[\dot{\gamma_2}~\sin2\theta~- \dot{\gamma_1}~\cos2\theta]/v$. Except in the Saint Helens network, all the rates show contraction.

The contraction rates are highest in networks close to the CSZ deformation front, such as in northern California, and decrease with distance from it, becoming indistinguishable from zero at the most distant network near Hanford (Figure 24.3). The contraction rates in most of the forearc networks compare well (within 2-sigma) with the Savage et al. [1991] dislocation model (denoted by 100+75, Figure 24.3). Contraction rates in the Cape Blanco and Newport networks are less than predicted by the dislocation model, but are consistent with models assuming more narrow locked and transition zones (30+30 for Cape Blanco and 40+40 for Newport, Figure 24.3). This generally agrees with previous studies [Flück et al., 1997; Savage et al., 2000], although the locked zone near Cape Blanco is more narrow than previously suggested.

The observed horizontal strain rates indicate that strain is accumulating everywhere along the CSZ from Cape Mendocino to the Strait of Juan de Fuca. These observations do not indicate whether the strain will ultimately be released by a single great M$\sim $9 earthquake, by a sequence of smaller M$\sim $8 earthquakes, or by some other scenario. However, we find no evidence that the megathrust is segmented by shallow regions where strain is not accumulating, so the hypothesis that the strain could be released by a single great earthquake cannot be rejected. Measurements by recent GPS surveys and permanent GPS stations installed since 1996 [e.g.,Khazaradze et al., 1999], should soon provide greater spatial resolution of deformation and strain accumulation along the CSZ.

References

Clague, J. J., Evidence for large earthquakes at the Cascadia subduction zone, Rev. Geophys., 35, 439-460, 1997.

Flück, P., R. D. Hyndman, and K. Wang, Three-dimensional dislocation model for great earthquakes of the Cascadia subduction zone, J. Geophys. Res., 102, 20,539-20,550, 1997.

Khazaradze, G., A. Qamar, and H. Dragert, Tectonic deformation in western Washington from continuous GPS measurements, Geophys. Res. Lett., 26, 3153-3156, 1999.

McCaffrey, R., and C. Goldfinger, Forearc deformation and great subduction earthquakes: Implications for Cascadia offshore earthquake potential, Science, 267, 856-859, 1995.

Murray, M. H., and M. Lisowski, Strain accumulation along the Cascadia subduction zone, Geophys. Res. Lett., (submitted), 2000.

Pezzopane, S. K., and R. J. Weldon II, Tectonic role of active faulting in central Oregon, Tectonics, 12, 1140-1169, 1993.

Satake, K., K. Shimazaki, Y. Tsuji, and K. Ueda, Time and size of a giant earthquake in Cascadia inferred from Japanese tsunami records of January 1700, Nature, 379, 246-249, 1996.

Savage, J. C., A dislocation model of strain accumulation and release at a subduction zone, J. Geophys. Res., 88, 4984-4996, 1983.

Savage, J. C., M. Lisowski, and W. H. Prescott, Strain accumulation in western Washington, J. Geophys. Res., 96, 14,493-14,507, 1991.

Savage, J. C., J. L. Svarc, W. H. Prescott, and M. H. Murray, Deformation across the forearc of the Cascadia subduction zone at Cape Blanco, Oregon, J. Geophys. Res., 105, 3095-3102, 2000.

Wells, R. E., C. S. Weaver, and R. J. Blakely, Fore-arc migration in Cascadia and its neotectonic significance, Geology, 26, 759-762, 1998.

Wilson, D. S., Confidence intervals for motion and deformation of the Juan de Fuca plate, J. Geophys. Res., 98, 16,053-16,071, 1993.


  
Figure 24.1: Map of strain rates along the CSZ. Shaded areas show approximate extent of geodetic networks. Black arrows, principal-strain rates. White arrows, maximum shear-strain rates and direction of maximum contraction. Gray arrow, convergence rate of Juan de Fuca plate (JDF) with respect to North America [ Wilson, 1993]. Open triangles, Quaternary volcanoes in the Cascade arc.
\begin{figure}\epsfig{file=murray00_1_1.ps}\end{figure}


  
Figure 24.2: Direction of maximum contraction as a function of distance along the CSZ from the Mendocino triple junction. Solid line, plate convergence direction [ Wilson, 1993]. Dashed line, approximate direction of normal to the Cascadia deformation front.
\begin{figure}\epsfig{file=murray00_1_2.ps, width=8cm}\end{figure}


  
Figure: Observed and predicted normalized extension rates as a function of distance from the CSZ deformation front. Models with different locked and transition zone widths are shown by the solid and dashed lines (e.g., 100+75 denotes 100-km width locked and 75-km width transition zones). The 100+75 model geometry assuming a 10$^\circ $-dipping plate interface, with shallow locked, linear transition, and deep aseismically slipping zones, is shown to scale at top, where the extension of the aseismic zone to great depth is omitted for clarity.
\begin{figure}\epsfig{file=murray00_1_3.ps, width=8cm}\end{figure}


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