Geoffrey A. Abers

Subduction factory 2. Are intermediate‐depth earthquakes in subducting slabs linked to metamorphic dehydration reactions?

[1] New thermal-petrologic models of subduction zones are used to test the hypothesis that intermediate-depth intraslab earthquakes are linked to metamorphic dehydration reactions in the subducting oceanic crust and mantle. We show that there is a correlation between the patterns of intermediate-depth seismicity and the locations of predicted hydrous minerals: Earthquakes occur in subducting slabs where dehydration is expected, and they are absent from parts of slabs predicted to be anhydrous. We propose that a subductingoceanicplatecanconsistoffourpetrologicallyandseismicallydistinctlayers:(1) hydrated, fine-grained basaltic upper crust dehydrating under equilibrium conditions and producing earthquakes facilitated by dehydration embrittlement; (2) coarse-grained, locally hydrated gabbroic lower crust that produces some earthquakes during dehydration but transformschieflyaseismicallytoeclogiteatdepthsbeyondequilibrium;(3)locallyhydrated uppermost mantle dehydrating under equilibrium conditions and producing earthquakes; and (4) anhydrous mantle lithosphere transforming sluggishly and aseismically to denser minerals. Fluid generated through dehydration reactions can move via at least three distinct flowpaths:percolationthroughlocal,transient,reaction-generatedhigh-permeabilityzones; flow through mode I cracks produced by the local stress state; and postseismic flow through fault zones. INDEX TERMS: 7218 Seismology: Lithosphere and upper mantle; 7230 Seismology: Seismicity and seismotectonics; 8123 Tectonophysics: Dynamics, seismotectonics; 8135 Tectonophysics: Evolution of the Earth: Hydrothermalsystems (8424); 3660 Mineralogyand Petrology: Metamorphicpetrology;

Subduction factory 1. Theoretical mineralogy, densities, seismic wave speeds, and H 2 O contents

[1] We present a new compilation of physical properties of minerals relevant to subduction zones and new phase diagrams for mid-ocean ridge basalt, lherzolite, depleted lherzolite, harzburgite, and serpentinite. We use these data to calculate H2O content, density and seismic wave speeds of subduction zone rocks. These calculations provide a new basis for evaluating the subduction factory, including (1) the presence of hydrous phases and the distribution of H2O within a subduction zone; (2) the densification of the subducting slab and resultant effects on measured gravity and slab shape; and (3) the variations in seismic wave speeds resulting from thermal and metamorphic processes at depth. In considering specific examples, we find that for ocean basins worldwide the lower oceanic crust is partially hydrated (<1.3 wt % H2O), and the uppermost mantle ranges from unhydrated to � 20% serpentinized (� 2.4 wt % H2O). Anhydrous eclogite cannot be distinguished from harzburgite on the basis of wave speeds, but its � 6% greater density may render it detectable through gravity measurements. Subducted hydrous crust in cold slabs can persist to several gigapascals at seismic velocities that are several percent slower than the surrounding mantle. Seismic velocities and VP/VS ratios indicate that mantle wedges locally reach 60–80% hydration. INDEX TERMS: 3040 Marine Geology and Geophysics: Plate tectonics (8150, 8155, 8157, 8158); 3660 Mineralogy and Petrology: Metamorphic petrology; 3919 Mineral Physics: Equations of state; 5199 Physical Properties of Rocks: General or miscellaneous; 8123 Tectonophysics: Dynamics, seismotectonics; KEYWORDS: subduction, seismic velocities, mineral physics, H2O

Potentially induced earthquakes in Oklahoma, USA: Links between wastewater injection and the 2011 Mw 5.7 earthquake sequence

Significant earthquakes are increasingly occurring within the continental interior of the United States, including five of moment magnitude (Mw) ≥ 5.0 in 2011 alone. Concurrently, the volume of fluid injected into the subsurface related to the production of unconventional resources continues to rise. Here we identify the largest earthquake potentially related to injection, an Mw 5.7 earthquake in November 2011 in Oklahoma. The earthquake was felt in at least 17 states and caused damage in the epicentral region. It occurred in a sequence, with 2 earthquakes of Mw 5.0 and a prolific sequence of aftershocks. We use the aftershocks to illuminate the faults that ruptured in the sequence, and show that the tip of the initial rupture plane is within ∼200 m of active injection wells and within ∼1 km of the surface; 30% of early aftershocks occur within the sedimentary section. Subsurface data indicate that fluid was injected into effectively sealed compartments, and we interpret that a net fluid volume increase after 18 yr of injection lowered effective stress on reservoir-bounding faults. Significantly, this case indicates that decades-long lags between the commencement of fluid injection and the onset of induced earthquakes are possible, and modifies our common criteria for fluid-induced events. The progressive rupture of three fault planes in this sequence suggests that stress changes from the initial rupture triggered the successive earthquakes, including one larger than the first.

Sharp increase in central Oklahoma seismicity since 2008 induced by massive wastewater injection

Wastewater disposal linked to earthquakes The number of earthquakes is increasing in regions with active unconventional oil and gas wells, where water pumped at high pressure breaks open rock containing natural gas, leaving behind wastewater in need of disposing. Keranen et al. show that the steep rise in earthquakes in Oklahoma, USA, is likely caused by fluid migration from wastewater disposal wells. Twenty percent of the earthquakes in the central United States could be attributed to just four of the wells. Injected fluids in high-volume wells triggered earthquakes over 30 km away. Science, this issue p. 448 The recent surge in central U.S. seismicity is likely attributable to injection of wastewater at a small number of wells. Unconventional oil and gas production provides a rapidly growing energy source; however, high-production states in the United States, such as Oklahoma, face sharply rising numbers of earthquakes. Subsurface pressure data required to unequivocally link earthquakes to wastewater injection are rarely accessible. Here we use seismicity and hydrogeological models to show that fluid migration from high-rate disposal wells in Oklahoma is potentially responsible for the largest swarm. Earthquake hypocenters occur within disposal formations and upper basement, between 2- and 5-kilometer depth. The modeled fluid pressure perturbation propagates throughout the same depth range and tracks earthquakes to distances of 35 kilometers, with a triggering threshold of ~0.07 megapascals. Although thousands of disposal wells operate aseismically, four of the highest-rate wells are capable of inducing 20% of 2008 to 2013 central U.S. seismicity.

High resolution image of the subducted Pacific (?) plate beneath central Alaska, 50–150 km depth

Abstract A receiver function transect across the Alaska Range images the subducting Pacific plate at 50–150 km depth. Across a 200 km long array of 30 receivers, the largest observed P-to-S conversions come from the top of the subducting slab. This signal is coherent across the array and is strongly asymmetric, requiring a complicated interface at the top of the slab. Waveform inversion shows that the conversion is generated by a 11–22 km thick low velocity zone at the top of the slab, as much as 20% slower than the surrounding mantle. The velocity of this zone increases with increasing depth of the slab, approaching velocities of the mantle near 150 km depth. All intermediate depth earthquakes occur within the zone, along a plane dipping 5° steeper. The layer is too thick to represent metamorphosed oceanic crust, as proposed for other subduction zones. It may represent a thick serpentinized zone or, more likely, a thick exotic terrane subducting along with the Pacific plate.

Arc-parallel flow in the mantle wedge beneath Costa Rica and Nicaragua

Resolving flow geometry in the mantle wedge is central to understanding the thermal and chemical structure of subduction zones, subducting plate dehydration, and melting that leads to arc volcanism, which can threaten large populations and alter climate through gas and particle emission. Here we show that isotope geochemistry and seismic velocity anisotropy provide strong evidence for trench-parallel flow in the mantle wedge beneath Costa Rica and Nicaragua. This finding contradicts classical models, which predict trench-normal flow owing to the overlying wedge mantle being dragged downwards by the subducting plate. The isotopic signature of central Costa Rican volcanic rocks is not consistent with its derivation from the mantle wedge or eroded fore-arc complexes but instead from seamounts of the Galapagos hotspot track on the subducting Cocos plate. This isotopic signature decreases continuously from central Costa Rica to northwestern Nicaragua. As the age of the isotopic signature beneath Costa Rica can be constrained and its transport distance is known, minimum northwestward flow rates can be estimated (63–190 mm yr-1) and are comparable to the magnitude of subducting Cocos plate motion (∼85 mm yr-1). Trench-parallel flow needs to be taken into account in models evaluating thermal and chemical structure and melt generation in subduction zones.

Crustal thickness variations across the Colorado Rocky Mountains from teleseismic receiver functions

Variations in crustal thickness from the Great Plains of Kansas, across the Colorado Rocky Mountains, and into the eastern Colorado Plateau are determined by receiver function analysis of broadband teleseismic P waveforms recorded during the 1992 Rocky Mountain Front Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL) experiment. The receiver functions are calculated using a time domain deconvolution approach and are interpreted in terms of a single crustal layer, with thickness determined by a grid-search comparison of observed receiver functions with synthetics. The average crustal thicknesses determined by these methods are Kansas Great Plains, 43.8±0.4 km; Colorado Great Plains, 49.9±1.2 km; Colorado Rocky Mountains, 50.1±1.3 km; and northeast Colorado Plateau, 43.1±0.9 at latitudes of 38°–40°N. The main variations in crustal thickness that we observe are between the Kansas Great Plains and the Colorado Great Plains and between the Rocky Mountains and the Colorado Plateau. There is not a significant crustal thickness difference between the Colorado Great Plains and the Colorado Rocky Mountains. Together with gravity data and mass balance calculations, these results are incompatible with the hypothesis that the compensation of the Rocky Mountains relative to the Great Plains is accommodated purely by an Airy-type crustal root or any other mechanism that restricts compensation solely to the crust and requires significant support for the excess topography of the Rocky Mountains to come from the mantle. Models with a rigid elastic plate may match receiver function estimates of crustal thickness but underpredict the amplitude of the gravity low over the Rockies. Our favored model includes lateral variations in crustal velocities obtained from refraction studies and crustal thickness variations constrained by the receiver functions. These models indicate that there is a profound transition in mantle density structure near the eastern range front.

Seismic imaging of subduction zone metamorphism

Combined analysis of high-resolution seismic images of the Alaska and Cascadia subduction zones reveals where metamorphic fl uids are released. Both images show the subducted oceanic crust as a dipping low-velocity layer with a clear termination depth. However, in Alaska the crust is thicker (15-20 km compared to 8 km) and terminates at greater depth (120 km com- pared to 40 km) than in Cascadia. Based on metamorphic reaction estimates and geodynamic models, we demonstrate that the termination depth corresponds to eclogitization of the crust triggered by dehydration of water-bearing minerals, and that the location of this reaction is dependent on the thermal structure of the subducted slab.

Enhanced remote earthquake triggering at fluid-injection sites in the midwestern United States

Movers and Shakers We tend to view earthquakes as unpredictable phenomena caused by naturally shifting stresses in Earths crust. In reality, however, a range of human activity can also induce earthquakes. Ellsworth (p. 10.1126/science.1225942) reviews the current understanding of the causes and mechanics of earthquakes caused by human activity and the means to decrease their associated risk. Notable examples include injection of wastewater into deep formations and emerging technologies related to oil and gas recovery, including hydraulic fracturing. In addition to directly causing increased local seismic activity, activities such as deep fluid injection may have other ramifications related to earthquake occurrence. Van der Elst et al. (p. 164; see the news story by Kerr) demonstrate that in the midwestern United States, some areas with increased human-induced seismicity are also more prone to further earthquakes triggered by the seismic waves from large, remote earthquakes. Improved seismic monitoring and injection data near deep disposal sites will help to identify regions prone to remote triggering and, more broadly, suggest times when activities should, at least temporarily, be put on hold. Wastewater injected deep underground can make some faults more susceptible to triggering by large remote earthquakes. A recent dramatic increase in seismicity in the midwestern United States may be related to increases in deep wastewater injection. Here, we demonstrate that areas with suspected anthropogenic earthquakes are also more susceptible to earthquake-triggering from natural transient stresses generated by the seismic waves of large remote earthquakes. Enhanced triggering susceptibility suggests the presence of critically loaded faults and potentially high fluid pressures. Sensitivity to remote triggering is most clearly seen in sites with a long delay between the start of injection and the onset of seismicity and in regions that went on to host moderate magnitude earthquakes within 6 to 20 months. Triggering in induced seismic zones could therefore be an indicator that fluid injection has brought the fault system to a critical state.

Hydrated subducted crust at 100–250 km depth

Abstract Seismic waves that travel along the surface of subducted slabs provide a means to infer petrology to considerable depth. At high frequencies (0.5–10 Hz) they are particularly sensitive to the presence and state of subducted oceanic crust. New observations reveal systematic distortion of body waves in all north Pacific subduction zones, when signals traverse slabs at 100–250 km depths, suggesting that crust remains distinct to these depths. The signals show waveguide behavior at the scale of a few kilometers: short-wavelength, high-frequency energy (≥3 Hz) is delayed 5–7% relative to that of low frequencies (≥1 Hz), systematically at all subduction zones. To explain these observations, velocities in a low-velocity layer 1–7 km thick, likely subducted crust, must remain seismically slow relative to surrounding mantle at these depths. Hence, it seems unlikely that subducted crust has completely converted to eclogite, as often assumed. Inferred velocities within subducted crust are similar to those estimated for blueschists, suggesting that hydrous assemblages persist past the volcanic front.