On 31 August a new eruption began from the same fissure and is still ongoing at the time of writing. After 4 September the movement associated with the dyke was minor, suggesting an approximate equilibrium between inflow of magma into the dyke and magma flowing out of it feeding the eruption. Minor eruptions may have occurred under Vatnajškull; shallow ice depressions marked by circular crevasses (ice cauldrons) were discovered in the period 27/08-07/09, indicating leakage of magma or magmatic heat to the glacier causing basal melting ( Fig. 1 and 2b). On 5 September, aircraft radar profiling showed that the ice surface in the centre of the B ‡r!arbunga caldera had subsided 16 m relative to the surroundings, resulting in a 0.32±0.08 km 3 subsidence bowl ( can be compared to a 1 day interferogram over the ice surface spanning 27 -28 August (Fig. 1), that has maximum line-of-sight (LOS) increase of 57 cm, indicating 55-70 cm of subsidence, during 24 hours. From 24 August to 6 September 16 M≥5 earthquakes occurred on the caldera boundary.Over 22000 earthquakes were automatically detected 16/08-06/09 2014, 5000 of which have been manually checked. Four thousand of these have been relatively relocated, defining the dyke segments. Ground deformation in areas outside the Vatnajškull ice cap, and on nunataks within the ice cap, is well mapped by a combination of InSAR, continuously recording GPS sites, and campaign GPS measurements. The GPS observations and analysis give the temporal evolution of the three-dimensional displacements used in the modelling (Fig. 1). Interferometric analysis of synthetic aperture radar images from the COSMO-SkyMed, RADARSAT-2 and TerraSAR-X satellites was used to form 11 interferograms showing LOS change spanning different time intervals (Supplementary Fig. 2). The analysis of seismic and geodetic data is described in Methods.Initial modelling of the dyke, with no a priori constraints on position, strike or dip, show the deformation data require the dyke to be approximately vertical and line up with the seismicity (Extended Data item 4). We therefore fixed the dip to be vertical and the lateral position of the dyke to coincide with the earthquake locations.We modelled the dyke as a series of rectangular patches and estimated the opening and slip on each patch ( Fig. 3a; see Supplementary Figures 3-4 for slip and standard deviations of opening). We used a Markov-chain Monte Carlo approach to estimate 7 the multivariate probability distribution for all model parameters (Methods) on each day 16/08-06/09 2014 (Fig. 2d). The results suggest that most of the magma injected into the dyke is shallower than the seismicity, which mostly spans the depth range from 5 to 8 km below sea level (see Fig. 2c and Methods). While magma may extend to depths greater than 9 km near the centre of the ice cap, towards the edge of the ice cap where constraints from InSAR and GPS are much better, significant opening is all shallower than 5 km (Fig. 3a). The total volume intruded into the dyke by 28 August was 0.48-0...

[1] We investigate the distribution of active deformation in the northern Basin and Range province using data from continuous GPS (CGPS) networks, supplemented by additional campaign data from the Death Valley, northern Basin and Range, and Sierra Nevada-Great Valley regions. To understand the contemporary strain rate field in the context of the greater Pacific (P)-North America (NA) plate boundary zone, we use GPS velocities to estimate the average relative motions of the Colorado Plateau (CP), the Sierra Nevada-Great Valley (SNGV) microplate, and a narrow north-south elongate region in the central Great Basin (CGB) occupying the longitude band 114-117°W. We find that the SNGV microplate translates with respect to the CP at a rate of 11.4 ± 0.3 mm yr À1 oriented N47 ± 1°W and with respect to NA at a rate of $12.4 mm yr À1 also oriented N47°W, slower than most previous geodetic estimates of SNGV-NA relative motion, and nearly 7°counterclockwise from the direction of P-NA relative plate motion. We estimate CGB-CP relative motion of 2.8 ± 0.2 mm yr À1 oriented N84 ± 5°W, consistent with roughly east-west extension within the eastern Great Basin (EGB). Velocity estimates from the EGB reveal diffuse extension across this region, with more rapid extension of 20 ± 1 nstr yr À1 concentrated in the eastern half of the region, which includes the Wasatch fault zone. We estimate SNGV-CGB relative motion of 9.3 ± 0.2 mm yr À1 oriented N37 ± 2°W, essentially parallel to P-NA relative plate motion. This rate is significantly slower than most previous geodetic estimates of deformation across the western Great Basin (WGB) but is generally consistent with paleoseismological inferences. The WGB region accommodates N37°W directed right lateral shear at rates of (1) 57 ± 9 nstr yr À1 across a zone of width $125 km in the south (latitude $36°N), (2) 25 ± 5 nstr yr À1 in the central region (latitude $38°N), and (3) 36 ± 1 nstr yr À1 across a zone of width $300 km in the north (latitude $40°N). By construction there is no net extension or shortening perpendicular to SNGV-CGB relative motion. However, we observe about 8.6 ± 0.5 nstr yr À1 extension on average in the direction of shear from southeast to northwest within the Walker Lane belt, comparable to the average east-west extension rate of 10 ± 1 nstr yr À1 across the northern Basin and Range but implying a distinctly different mechanism of deformation from extension on north trending, rangebounding normal faults. An alternative model for this shear parallel deformation, in which extension is accommodated across a narrow, more rapidly extending zone that coincides with the central Nevada seismic belt, fits the WGB data slightly better. Local anomalies with respect to this simple kinematic model may reveal second-order deformation signals related to more local crustal dynamic phenomena, but significant improvements in velocity field resolution will be necessary to reveal this second-order pattern.

[1] The Wasatch fault and adjacent fault zones provide an opportunity to compare present-day deformation rate estimates obtained from space geodesy with geologic displacement rates over at least four temporal windows, ranging from the last millennium up to 10 Myr. The three easternmost GPS sites of the Basin and Range Geodetic Network (BARGEN) at this latitude define a $130-km-wide region spanning three major normal faults extending east-west at a total rate of 2.7 ± 0.4 mm/yr, with an average regional strain rate estimated to be 21 ± 4 nstrain/yr, about twice the Basin and Range average. On the Wasatch fault, the vertical component of the geologic displacement rate is 1.7 ± 0.5 mm/yr since 6 ka, <0.6 mm/yr since 130 ka, and 0.5-0.7 mm/yr since 10 Ma. However, it appears likely that at the longest timescale, rates slowed over time, from 1.0 to 1.4 mm/yr between 10 and 6 Ma to 0.2 to 0.3 mm/yr since 6 Ma. The cumulative vertical displacement record across all three faults also shows time-variable strain release ranging from 2 to 4 mm/yr since 10 ka to <1 mm/yr averaged over the past 130 kyr. Conventional earthquake recurrence models (''Reid-type'' behavior) would require an accordingly large variation in strain accumulation or loading rate on a 10-kyr timescale, for which there appears to be no obvious geophysical explanation. Alternatively, seismic strain release, given a wide range of plausible constitutive behaviors for frictional sliding, may be clustered on the 10-kyr timescale, resulting in the high Holocene rates, with comparatively low, uniform strain accumulation rates on the 100-kyr timescale (''Wallace-type'' behavior). The latter alternative, combined with observations at the million-year timescale and the likelihood of a significant contribution of postseismic transients, implies maxima of spectral amplitude in the velocity field at periods of $10 Myr (variations in tectonic loading), $10 kyr (clustered strain release), and of 100 years (postseismic transients). If so, measurements of strain accumulation and strain release may be strongly timescaledependent for any given fault system.

We use Global Positioning System (GPS) estimates of horizontal site velocity to constrain slip rates on faults comprising the Pacific-North America plate boundary in southern California and northern Mexico. We enlist a simple elastic block model to parameterize the distribution and sum of deformation within and across the plate boundary. We estimate a Pacific-North America relative plate motion rate of 49 _+ 3 mm/yr (one standard deviation), consistent with NUVEL-1A estimates. We are able to resolve robust slip rate estimates for the southernmost San Andreas, San Jacinto, and Elsinore faults (26 _+ 2, 9 +_ 2, and 6 _+ 2 mm/yr, respectively) and for the Imperial and Cerro Prieto faults (35 _+ 2 and 42 _+ 1 mm/yr, respectively), accounting for about 86% of the total plate motion. The remaining 14% appears to be accommodated to the west of these fault systems, probably via slip along the San Clemente fault and/or the San Miguel, Vallecitos, Rose Canyon, and Newport-Inglewood fault systems. These results are highly consistent with paleoseismic estimates for slip rates implying that off-fault strain accumulation within the deforming zone of the plate boundary is largely elastic. We estimate that the seismically quiescent, southernmost San Andreas fault has incurred about 8.2 m of slip deficit over the last few hundred years, presumably to be recovered during a future large earthquake. Introduction Sometime around 30 Ma, the ancestral East Pacific Rise collided with the trench that then separated the North America and Farallon plates [Atwater, 1970] bringing the Pacific and North America plates into contact. Although the details of the early evolution of this boundary are hazy, by about 20 Ma the existence of a right-lateral proto-San Andreas transform near the present-day continental borderland is probable [e.g., Sedlock and Hamilton, 1991]. Eastward migration of the proto-San Andreas transform to its present location and the associated rifting of the Gulf of California appear to be as recent as 5 Ma. The NUVEL-1A global plate motion model [DeMets et al., 1990, 1994] predicts Pacific-North America relative motion across southern California of 46 _+ 1 mm/yr. This estimate represents plate motions averaged over the last 3 Myr. Recently, however, DeMets [1995] compared seafloor spreading rates in the Gulf of California (used in the calculation of the NUVEL-1A rate) with the NUVEL-iA closure fitting rate (CFR) for Pacific-North America relative motion (estimated in the absence of these spreading rate data) and found that the NUVEL-1A model may underestimate the true rate by as much as 8-9%. That is, the true rate of Pacific-North America relative motion may be closer to 50 mm/yr. It is the ongoing motion of these plates that drives the contemporary accumulation of elastic strain within the plate boundary and is therefore ultimately responsible for earthquakes throughout the region. •Now at the Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts. While the San Andreas fault accommodates a significant frac...

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