Natalia A. Ruppert

Trans-Alaska Crustal Transect and continental evolution involving subduction underplating and synchronous foreland thrusting

We investigate the crustal structure and tectonic evolution of the North American continent in Alaska, where the continent has grown through magmatism, accretion, and tectonic under-plating. In the 1980s and early 1990s, we conducted a geological and geophysical investigation, known as the Trans-Alaska Crustal Transect (TACT), along a 1350-km-long corridor from the Aleutian Trench to the Arctic coast. The most distinctive crustal structures and the deepest Moho along the transect are located near the Pacific and Arctic margins. Near the Pacific margin, we infer a stack of tectonically underplated oceanic layers interpreted as remnants of the extinct Kula (or Resurrection) plate. Continental Moho just north of this underplated stack is more than 55 km deep. Near the Arctic margin, the Brooks Range is underlain by large-scale duplex structures that overlie a tectonic wedge of North Slope crust and mantle. There, the Moho has been depressed to nearly 50 km depth. In contrast, the Moho of central Alaska is on average 32 km deep. In the Paleogene, tectonic underplating of Kula (or Resurrection) plate fragments overlapped in time with duplexing in the Brooks Range. Possible tectonic models linking these two regions include flat-slab subduction and an orogenic-float model. In the Neogene, the tectonics of the accreting Yakutat terrane have differed across a newly interpreted tear in the subducting Pacific oceanic lithosphere. East of the tear, Pacific oceanic lithosphere subducts steeply and alone beneath the Wrangell volcanoes, because the overlying Yakutat terrane has been left behind as underplated rocks beneath the rising St. Elias Range, in the coastal region. West of the tear, the Yakutat terrane and Pacific oceanic lithosphere subduct together at a gentle angle, and this thickened package inhibits volcanism.

Supershear rupture of the 5 January 2013 Craig, Alaska (Mw 7.5) earthquake

Supershear rupture, in which a fractures crack tip expansion velocity exceeds the elastic shear wave velocity, has been extensively investigated theoretically and experimentally and previously inferred from seismic wave observations for six continental strike-slip earthquakes. We find extensive evidence of supershear rupture expansion of an oceanic interplate earthquake, the 5 January 2013 Mw = 7.5 Craig, Alaska earthquake. This asymmetric bilateral strike-slip rupture occurred on the Queen Charlotte Fault, offshore of southeastern Alaska. Observations of first-arriving Sn and Sg shear waves originating from positions on the fault closer than the hypocenter for several regional seismic stations, with path calibrations provided by an empirical Greens function approach, indicate a supershear rupture process. Several waveform inversion and modeling techniques were further applied to determine the rupture velocity and space-time distribution of slip using regional seismic and geodetic observations. Both theoretical and empirical Greens functions were used in the analyses, with all results being consistent with a rupture velocity of 5.5 to 6 km/s, exceeding the crustal and upper mantle S wave velocity and approaching the crustal P wave velocity. Supershear rupture occurred along ~100 km of the northern portion of the rupture zone but not along the shorter southern rupture extension. The direction in which supershear rupture developed may be related to the strong material contrast across the continental-oceanic plate boundary, as predicted theoretically and experimentally. The shear and surface wave Mach waves involve strongly enhanced ground motions at azimuths oblique to the rupture direction, emphasizing the enhanced hazard posed by supershear rupture of large strike-slip earthquakes.

Cooperation among tectonic and surface processes in the St. Elias Range, Earth's highest coastal mountains

Investigations of tectonic and surface processes have shown a clear relationship between climate-influenced erosion and long-term exhumation of rocks. Numerical models suggest that most orogens are in a transient state, but observational evidence of a spatial shift in mountain building processes due to tectonic-climate interaction is missing. New thermochronology data synthesized with geophysical and surface process data elucidate the evolving interplay of erosion and tectonics of the colliding Yakutat microplate with North America. Focused deformation and rock exhumation occurred in the apex of the colliding plate corner from > 4 to 2 Ma and shifted southward after the 2.6 Ma climate change. The present exhumation maximum coincides with the largest modern shortening rates, highest concentration of seismicity, and the greatest erosive potential. We infer that the high sedimentation caused rheological modification and the emergence of the southern St. Elias, intercepting orographic precipitation and shifting focused erosion and exhumation to the south.

Plate Margin Deformation and Active Tectonics Along the Northern Edge of the Yakutat Terrane in the Saint Elias Orogen, Alaska and Yukon, Canada

Structural syntaxes, tectonic aneurysms, and fault-bounded fore-arc slivers are important tectonic elements of orogenic belts worldwide. In this study we used high-resolution topography, geodetic imaging, seismic, and geologic data to advance understanding of how these features evolved during accretion of the Yakutat Terrane to North America. Because glaciers extend over much of the orogen, the topography and dynamics of the glaciers were analyzed to infer the location and nature of faults and shear zones that lie buried beneath the ice. The Fairweather transform fault system terminates by oblique-extensional splay faulting within a structural syntaxis, where thrust faulting and contractional strain drive rapid tectonic uplift and rock exhumation beneath the upper Seward Glacier. West of the syntaxis, oblique plate convergence created a dextral shear zone beneath the Bagley Ice Valley that may have been reactivated by reverse faulting when the subduction megathrust stepped eastward during the last 5–6 Ma. The Bagley fault zone dips steeply through the upper plate to intersect the subduction megathrust at depth, forming a fault-bounded crustal sliver capable of partitioning oblique convergence into strike-slip and thrust motion. Since ca. 20 Ma the Bagley fault accommodated more than 50 km of dextral shearing and several kilometers of reverse motion along its southern flank during terrane accretion. The fault is considered capable of generating earthquakes because it is suitably oriented for reactivation in the contemporary stress field, links to faults that generated large historic earthquakes, and is locally marked by seismicity.

Stress Map for Alaska From Earthquake Focal Mechanisms

This paper introduces a stress map for Alaska based on inversion of earthquake focal mechanisms. A composite catalog of earthquake focal mechanisms was compiled, for both crustal and slab events. This catalog includes mechanisms obtained with the regional and global seismic data and data available in the published literature. The resulting stress field varies greatly across the region. The subducting slab is characterized by normal and strike-slip faulting in the shallower depth range (<100 km) and by reverse and strike-slip faulting at greater depths. The maximum compressive axes are generally aligned in the strike direction, while the minimum compressive stress is aligned in the down-dip direction. The largest deviation from this pattern is within the Kenai block, which is restricted by slab bends on both sides. The crustal earthquakes exhibit a variety of faulting types and stress axis orientations in the region. The general pattern is consistent with the maximum compression radiating from the plate interface in southern Alaska. The highest complexity is observed within the Wrangell crustal block indicative of its internal deformation.

Seismicity, Earthquakes and Structure Along the Alaska‐Aleutian and Kamchatka‐Kurile Subduction Zones: A Review

We present a review of great earthquakes and seismicity patterns along the Alaska-Aleutian and Kamchatka-Kurile arcs as an overview of one of the longest subduction zone complexes on the planet. Seismicity patterns, double seismic zones and focal mechanism solutions are described and used to illustrate the distribution of stress in the Pacific plate as it collides with North America and Eurasia. Seismicity along the Alaska-Aleutian arc is relatively shallow as compared to the Kamchatka-Kurile arc where the plate is considerably older and thicker prior to entering the subduction zone. Tomographic inversions of the slab generally show high velocity anomalies where seismicity is high, presumably tracking the cold subducting lithosphere.

Rapid Ice Mass Loss: Does It Have an Influence on Earthquake Occurrence in Southern Alaska?

The glaciers of southern Alaska are extensive, and many of them have undergone gigatons of ice wastage on time scales on the order of the seismic cycle. Since the ice loss occurs directly above a shallow main thrust zone associated with subduction of the Pacific-Yakutat plate beneath continental Alaska, the region between the Malaspina and Bering Glaciers is an excellent test site for evaluating the importance of recent ice wastage on earthquake faulting potential. We demonstrate the influence of cumulative glacial mass loss following the 1899 Yakataga earthquake (M=8.1) by using a two dimensional finite element model with a simple representation of ice fluctuations to calculate the incremental stresses and change in the fault stability margin (FSM) along the main thrust zone (MTZ) and on the surface. Along the MTZ, our results indicate a decrease in FSM between 1899 and the 1979 St. Elias earthquake (M=7.4) of 0.2 - 1.2 MPa over an 80 km region between the coast and the 1979 aftershock zone; at the surface, the estimated FSM was larger but more localized to the lower reaches of glacial ablation zones. The ice-induced stresses were large enough, in theory, to promote the occurrence of shallow thrust earthquakes. To empirically test the influence of short-term ice fluctuations on fault stability, we compared the seismic rate from a reference background time period (1988-1992) against other time periods (1993-2006) with variable ice or tectonic change characteristics. We found that the frequency of small tectonic events in the Icy Bay region increased in 2002-2006 relative to the background seismic rate. We hypothesize that this was due to a significant increase in the rate of ice wastage in 2002-2006 instead of the M=7.9, 2002 Denali earthquake, located more than 100km away.

Active Tectonics of Interior Alaska: Seismicity, Gps Geodesy, and Local Geomorphology

Interior Alaska is an actively deforming landscape that is part of a broad zone of deformation extending hundreds of kilometers inboard of the convergent plate boundary. This chapter provides an overview of recent studies on the seismicity, tectonic geomorphology, and crustal motions from GPS measurements in interior Alaska and the surrounding regions. Although the combined data sets do not point to one single mechanism of deformation for interior Alaska, we suggest that the interactions between North America and two crustal blocks, the Bering and Wrangell, are responsible for the broad diffuse belt of deformation that characterizes this region. We propose that the deformation zone extending along the NNE-striking, left-lateral seismic zones north of the Denali fault and along the Susitna seismic zone south of the fault represents the eastern extent of the distributed boundary between the Bering block and the North American plate. Double-difference earthquake relocations document the fine details of seismic structures and help define the seismogenic part of the crust in interior Alaska. Frequency―magnitude and stress analysis provide insights into faulting parameters and earthquake behavior in the region. Stream channel morphologies and stream profile perturbations help define active surface deformation in interior Alaska. GPS measurements show that the relative motions across the region are different than what would be expected for a stable plate interior, and include the effects of multiple deformation sources. Results from the study show that three dominant types of structures characterize interior Alaska: right-lateral strike-slip faults, left-lateral strike-slip seismic zones, and thrust faults.

Earthquake nucleation and fault slip complexity in the lower crust of central Alaska

Earthquakes start under conditions that are largely unknown. In laboratory analogue experiments and continuum models, earthquakes transition from slow-slipping, growing nucleation to fast-slipping rupture. In nature, earthquakes generally start abruptly, with no evidence for a nucleation process. Here we report evidence from a strike-slip fault zone in central Alaska of extended earthquake nucleation and of very-low-frequency earthquakes (VLFEs), a phenomenon previously reported only in subduction zone environments. In 2016, a VLFE transitioned into an earthquake of magnitude 3.7 and was preceded by a 12-hour-long accelerating foreshock sequence. Benefiting from 12 seismic stations deployed within 30 km of the epicentre, we identify coincident radiation of distinct high-frequency and low-frequency waves during 22 s of nucleation. The power-law temporal growth of the nucleation signal is quantitatively predicted by a model in which high-frequency waves are radiated from the vicinity of an expanding slow slip front. The observations reveal the continuity and complexity of slip processes near the bottom of the seismogenic zone of a strike-slip fault system in central Alaska.A strike-slip fault zone in central Alaska exhibits a range of earthquake slip processes, including very-low-frequency earthquakes, some of which transition into regular, fast earthquakes.

Evidence of Wadati-Benioff zone triggering following the Mw 7.9 Little Sitkin, Alaska intermediate depth earthquake of 23 June 2014

The 23 June 2014 Mw=7.9 earthquake that struck the Rat Islands region of Alaska was a rare intermediate-depth event followed by a vigorous aftershock sequence. The earthquakes location within a regional network has allowed us to study it in rich detail. Double-difference relocations of aftershocks and static stress modeling reveal some intriguing features; most aftershocks are not located on the main shock rupture plane, but are concentrated in the adjacent Wadati-Benioff zone in areas of increased stress. Further, a secondary plane of seismicity aligns well with a moderately dipping nodal plane reported by the Global Centroid Moment Tensor Project, and the separation of these locations and the main shock hypocenter from the slab indicates that the main shock ruptured into the oceanic mantle.