Papers by Sigrún Hreinsdóttir
Geophysical Monograph Series, 2000
Tectonophysics, 2010
We provide the first synthesis of seismic reflection data and active present-day crustal deformat... more We provide the first synthesis of seismic reflection data and active present-day crustal deformation for the greater Wasatch fault zone. We analyzed a number of previously unpublished seismic reflection lines, horizontal and vertical crustal velocities from continuous GPS, and surface geology to investigate the relationships between interseismic strain accumulation, subsurface fault geometry, and geologic slip rates on seismogenic faults across the eastern third of the northern Basin and Range Province. The seismic reflection data show recent activity along high-angle normal faults that become listric with depth and appear to sole into preexisting décollements, possibly reactivating them. We interpret these listric normal faults as reactivated Sevier-age structures that are connected at depth with a regionally extensive detachment horizon. These observations of subsurface structure are consistent with the mapped geology in areas that have experienced significant extension. We modeled the crustal deformation data using a buried dislocation source in a homogeneous elastic half space. The estimated model results include a low-angle dislocation (~8-20°) at a locking depth of~7-10 km and slipping at 3.2 ± 0.2 mm/yr. Despite the model's relative simplicity, we find that the predicted location of the dislocation is consistent with the interpreted seismic reflection data, and suggests an active regionally extensive sub-horizontal surface in the eastern Basin and Range. This result may imply that this surface represents aseismic creep across a reactivated low-angle fault plane or the onset of ductile flow in the lower crust at or beneath the brittle-ductile transition zone under the presentday Basin and Range extensional regime. This result may also have implications for crustal rheology, and suggests that geodesy might, under some circumstances, serve as an appropriate tool for inferring deeper crustal structure.
Journal of Geophysical Research, 2006
1] During the first 2 years following the 2002 M w = 7.9 Denali, Alaska, strike-slip earthquake, ... more 1] During the first 2 years following the 2002 M w = 7.9 Denali, Alaska, strike-slip earthquake, a large array of Global Positioning System (GPS) receivers recorded rapid postseismic surface motions extending at least 300 km from the rupture and at rates of more than 100 mm/yr in the near field. Here we use three-dimensional (3-D) viscoelastic finite element models to infer the mechanisms responsible for these postseismic observations. We consider afterslip both from an inversion of GPS displacements and from stress-driven forward models, poroelastic rebound, and viscoelastic flow in the lower crust and upper mantle. Several conclusions can be drawn: (1) No single mechanism can explain the postseismic observations. (2) Significant postseismic flow below a depth of 60 km is required to explain observed far-field motions, best explained by a weak upper mantle with a depth-dependent effective viscosity that ranges from >10 19 Pa s at the Moho (50 km depth) to 3-4 Â 10 18 Pa s at 100 km depth. (3) Shallow afterslip within the upper crust occurs adjacent to and beneath the regions of largest coseismic slip. (4) There is a contribution from deformation in the middle and lower crust from either lower crustal flow or stress-driven slip. Afterslip is preferred over broad viscoelastic flow owing to the existence of seismic velocity discontinuities across the fault at depth, though our modeling does not favor either mechanism. If the process is viscoelastic relaxation, the viscosity is a factor of 3 greater than the inferred mantle viscosity. (5) Poroelastic rebound probably contributed to the observed postseismic deformation in the immediate vicinity of the Denali/Totschunda junction. These conclusions lead us to infer an Alaskan mechanical lithosphere that is about 60 km thick, overlying a weak asthenosphere, and a Denali fault that cuts through the entire lithosphere with shear accommodated by faulting in the top $20 km and time-dependent aseismic shear below.
Journal of Geophysical Research, 2006
1] We estimate coseismic displacements from the 2002 M w 7.9 Denali Fault earthquake at 232 GPS s... more 1] We estimate coseismic displacements from the 2002 M w 7.9 Denali Fault earthquake at 232 GPS sites in Alaska and Canada. Displacements along a N-S profile crossing the fault indicate right-lateral slip on a near-vertical fault with a significant component of vertical motion, north-side up. We invert both GPS displacements and geologic surface offsets for slip on a three-dimensional (3-D) fault model in an elastic half-space. We restrict the motion to right-lateral slip and north-side-up dip slip. Allowing for oblique slip along the Denali and Totschunda faults improves the model fit to the GPS data by about 30%. We see mostly right-lateral strike-slip motion on the Denali and Totschunda faults, but in a few areas we see a significant component of dip slip. The slip model shows increasing slip from west to east along the Denali Fault, with four localized higherslip patches, three near the Trans-Alaska pipeline crossing and a large slip patch corresponding to a M w 7.5 subevent about 40 km west of the Denali-Totschunda junction. Slip of 1-3 m was estimated along the Totschunda Fault with the majority of slip being at shallower than 9 km depth. We have limited resolution on the Susitna Glacier Fault, but the estimated slip along the fault is consistent with a M w 7.2 thrust subevent. Total estimated moment in the Denali Fault earthquake is equivalent to M w 7.89. The estimated slip distribution along the surface is in very good agreement with geological surface offsets, but we find that surface offsets measured on glaciers are biased toward lower values.
Journal of Geophysical Research, 2012
1] Most magmatic systems on Earth are located at actively deforming plate boundaries. In these sy... more 1] Most magmatic systems on Earth are located at actively deforming plate boundaries. In these systems, the magmatic and plate boundary deformation signals are intertwined and must be deconvolved to properly estimate magma flux and source characteristics of the magma plumbing system. We investigate the inter-rifting and inter-seismic deformation signals at the Eastern Volcanic Zone (EVZ) -South Iceland Seismic Zone (SISZ) ridge -transform intersection and estimate the location, depth, and volume rate for magmatic sources at Hekla and Torfajökull volcanoes, which are located at the intersection. We solve simultaneously for the source parameters of the tectonic and volcanic deformation signals using a new ten-year velocity field derived from a dense network of episodic and continuous GPS stations in south Iceland. We find the intersection of the axes of the EVZ and the SISZ is located within the Torfajökull caldera, which itself is subsiding. Deformation at Hekla is statistically best described in terms of a horizontal ellipsoidal magma chamber at 24 À2 +4 km depth aligned with the volcanic system and increasing in volume by 0.017 À0.002 +0.007 km 3 per year. A spherical magma chamber centered at 24 À2 +5 km depth with a volume rate of 0.019 À0.002 +0.011 km 3 per year, or a vertical pipe-shaped magma chamber between 10 À1 +3 km and 21 À4 +7 km with a volume rate of 0.008 À0.001 +0.003 km 3 per year are also plausible models explaining the deformation at Hekla. All three models indicate magma accumulation in the lower crust or near the Moho under Hekla.
Geophysical Research Letters, 2003
Grímsvötn is a subglacial volcano, under the Vatnajökull ice cap at the center of the Icelandic h... more Grímsvötn is a subglacial volcano, under the Vatnajökull ice cap at the center of the Icelandic hotspot. This highly active volcano erupted in December 1998. GPS measurements at a single station on a nunatak at the caldera rim were made 7 times during 1992-2001. The measurements prior to the 1998 eruption reveal pre-eruption inflation, but subsidence of more than 15 cm was measured during the eruption. Following the eruption, re-inflation occurred initially, at a rate of 20 mm/month, then declined to 5 mm/month. Measurements were fitted to a Mogi model, assuming that the source was located under the center of the Grímsvötn caldera complex. Results indicate a source depth of at least 1.6-km. The calculated amount of magma outflow during the eruption is comparable to field estimates of the erupted volume. Grímsvötn continues to inflate, but has not reached its 1998 pre-eruption level.
Journal of Geophysical Research, 2007
Abstract[1] Glaciers in Iceland began retreating around 1890, and since then the Vatnajökull ice ... more Abstract[1] Glaciers in Iceland began retreating around 1890, and since then the Vatnajökull ice cap has lost over 400 km3 of ice. The associated unloading of the crust induces a glacio-isostatic response. From 1996 to 2004 a GPS network was measured around the southern edge of Vatnajökull. These measurements, together with more extended time series at several other GPS sites, indicate vertical velocities around the ice cap ranging from 9 to 25 mm/yr, and horizontal velocities in the range 3 to 4 mm/yr. The vertical velocities have been modeled using the finite element method (FEM) in order to constrain the viscosity structure beneath Vatnajökull. We use an axisymmetric Earth model with an elastic plate over a uniform viscoelastic half-space. The observations are consistent with predictions based on an Earth model made up of an elastic plate with a thickness of 10–20 km and an underlying viscosity in the range 4–10 × 1018 Pa s. Knowledge of the Earth structure allows us to predict uplift around Vatnajökull in the next decades. According to our estimates of the rheological parameters, and assuming that ice thinning will continue at a similar rate during this century (about 4 km3/year), a minimum uplift of 2.5 meters between 2000 to 2100 is expected near the current ice cap edge. If the thinning rates were to double in response to global warming (about 8 km3/year), then the minimum uplift between 2000 to 2100 near the current ice cap edge is expected to be 3.7 meters.
Journal of Geophysical Research, 2010
Abstract[1] We present the first results from a dense network of 36 campaign and 46 continuous GP... more Abstract[1] We present the first results from a dense network of 36 campaign and 46 continuous GPS stations located in the Eastern Transverse Ranges Province (ETR), a transition zone between the southernmost San Andreas fault (SSAF) and eastern California shear zone (ECSZ). We analyzed the campaign data together with available data from continuous GPS stations for the period 1994–2009. We used the GPS velocity estimates to constrain elastic block models to investigate fault-loading rates representing four hypotheses characterized by different fault-block geometries. Fault-block scenarios include blocks bounded by the east-striking left-lateral Pinto Mountain, Blue Cut, and Chiriaco faults of the ETR; blocks bounded by a right-lateral north-northwest striking structure (the “Landers-Mojave earthquake line”) that cuts obliquely across the ETR and mapped Mojave Desert faults; and combinations of these end-member hypotheses. Each model implies significantly different active fault geometries, block rotation rates, and slip rates for ETR and ECSZ structures. All models suggest that SSAF slip rate varies appreciably along strike, generally consistent with rates derived from tectonic geomorphology and paleoseismology, with a maximum of ∼23 mm/yr right-lateral along the southernmost Coachella Valley strand, decreasing systematically to <10 mm/yr right-lateral through the San Gorgonio Pass region. Slip rate estimates for the San Jacinto fault are ∼12 mm/yr for all models tested. All four models fit the data equally well in a statistical sense. Qualitative comparison among models and consideration of geologic slip rates and other independent data reveals strengths and weaknesses of each model.
… 2010, held 2-7 May …, May 1, 2010
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Papers by Sigrún Hreinsdóttir