Subcontinental-scale crustal velocity changes along the Pacific–North America plate boundary

Abstract

Transient tectonic deformation has long been noted within approximately 100 km of plate boundary fault zones and within active volcanic regions, but it is unknown whether transient motions also occur at larger scales within plates. Relatively localized transients are known to occur as both seismic and episodic aseismic events, and are generally ascribed to motions of magma bodies, aseismic creep on faults, or elastic or viscoelastic effects associated with earthquakes. However, triggering phenomena and systematic patterns of seismic strain release at subcontinental (approximately 1,000 km) scale along diffuse plate boundaries have long suggested that energy transfer occurs at larger scale. Such transfer appears to occur by the interaction of stresses induced by surface wave propagation and magma or groundwater in the crust, or from large-scale stress diffusion within the oceanic mantle in the decades following clusters of great earthquakes. Here we report geodetic evidence for a coherent, subcontinental-scale change in tectonic velocity along a diffuse approximately 1,000-km-wide deformation zone. Our observations are derived from continuous GPS (Global Positioning System) data collected over the past decade across the Basin and Range province, which absorbs approximately 25 per cent of Pacific-North America relative plate motion. The observed changes in site velocity define a sharp boundary near the centre of the province oriented roughly parallel to the north-northwest relative plate motion vector. We show that sites to the west of this boundary slowed relative to sites east of it by approximately 1 mm yr(-1) starting in late 1999.

Detecting Large-scale Intracontinental Slow-slip Events (SSEs) Using Geodograms

“…Average velocities remain relatively constant across eastern Nevada, progressively rotating northwestward and increasing in magnitude up to 12 mm/ yr across western Nevada, accommodating both extension and right-lateral shear associated with the Pacific-North America transform plate boundary to the west (e.g., Bennett et al 2003;Hammond and Thatcher 2004). Position time series over the entire decade of data indicate significant secular changes in velocity (Davis et al 2006), such as in the east component of motion for site EGAN (Figure 2A). …”

“…We used the decade-long continuous time series for the first 18 sites installed in BARGEN in 1996-97 (Wernicke et al 2000), supplemented with shorter time series for five additional sites (GABB, BAMO, TOIY, MONI, and SPIC) installed more recently (Davis et al 2006;Wernicke et al 2008) (Figure 1). Previous analyses of this network (Bennett et al 2002;Davis et al 2006;Wernicke et al 2008) show that average horizontal velocities (relative to a North American reference frame) rise from near zero on the eastern side of the network to about 3 mm/yr oriented due west near the Nevada-Utah border, indicating crustal extension in the eastern Basin and Range (Figure 1).…”

“…We used the decade-long continuous time series for the first 18 sites installed in BARGEN in 1996-97 (Wernicke et al 2000), supplemented with shorter time series for five additional sites (GABB, BAMO, TOIY, MONI, and SPIC) installed more recently (Davis et al 2006;Wernicke et al 2008) (Figure 1). Previous analyses of this network (Bennett et al 2002;Davis et al 2006;Wernicke et al 2008) show that average horizontal velocities (relative to a North American reference frame) rise from near zero on the eastern side of the network to about 3 mm/yr oriented due west near the Nevada-Utah border, indicating crustal extension in the eastern Basin and Range (Figure 1). Average velocities remain relatively constant across eastern Nevada, progressively rotating northwestward and increasing in magnitude up to 12 mm/ yr across western Nevada, accommodating both extension and right-lateral shear associated with the Pacific-North America transform plate boundary to the west (e.g., Bennett et al 2003;Hammond and Thatcher 2004).…”

“…These plots have been effective in demonstrating the existence of slow-slip events (SSEs) on subduction megathrust interfaces around the globe (e.g., Miller et al 2002;Melbourne et al 2005;Schwartz and Rokosky 2007). A largescale intraplate SSE in the northern Basin and Range Province that occurred between 1999 and 2005 was initially identified by plotting continuous time series with a vertical time axis, arranged according to a spatial position coordinate for each site (Davis et al 2006, their Figure 3A). …”

Characterization of site‐specific GPS errors using a short‐baseline network of braced monuments at Yucca Mountain, southern Nevada

“…[2] The Global Positioning System (GPS) is now being used to measure not only ground deformations at the level of 1 mm, but also transient and/or high-rate signals that occur over relatively short periods of time [e.g., Dragert et al, 2001;Lowry et al, 2001;Ozawa et al, 2003;Larson et al, 2003;Davis et al, 2006;Freed et al, 2007]. It is therefore important that we understand the degree to which systematic errors and unmodeled local effects can contribute to both time-and space-dependent signals in the GPS time series.…”