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[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 subducting oceanic plate can consist of four petrologically and seismically distinct layers: (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 transforms chiefly aseismically to eclogite at depths beyond equilibrium; (3) locally hydrated 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 flow paths: percolation through local, transient, reaction-generated high-permeability zones; flow through mode I cracks produced by the local stress state; and postseismic flow through fault zones.

[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 H 2 O 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 H 2 O 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 % H 2 O), and the uppermost mantle ranges from unhydrated to $20% serpentinized ($2.4 wt % H 2 O). 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 V P /V S ratios indicate that mantle wedges locally reach 60-80% hydration.

[1] The location and motion of subducting plates relative to volcanic arcs provide a first-order constraint on theories of arc magmagenesis. We compile volcano-specific subduction parameters for 33,000 km of the global arc system at 839 volcanic centers, measuring the depth to the top of the slab (H) beneath each volcano. The compilation also includes estimates of slab strike and dip, incoming plate velocity, and age, all available in accompanying auxiliary material. The slab geometry is contoured from the top surface of Wadati-Benioff zones (WBZs) for a variety of teleseismic and local seismicity catalogs, which provides a reference surface for evaluating the distribution of seismicity within subducting plates. The WBZ thickness exceeds that expected from hypocentral errors in a manner correlating with plate age, indicating that old plates have thicker regions in which earthquakes can occur. When averaged over 500-km-long arc segments, H ranges from 72 to 173 km with a global average of 105 km, increasing by 20 km when hypocentral error effects are taken into account. These depths correlate poorly with most subduction parameters, but significant correlations exist between H and slab dip (correlation coefficient is 0.54 for 45 arc segments). The dip correlation can be explained if the melting region is displaced from the WadatiBenioff zone by a constant-thickness boundary layer. For the north Pacific, H varies inversely with descent rate; this trend may reflect the manner in which wedge thermal structure affects arc location. Over short distances some arc segments exhibit abrupt variations in arc location but not slab geometry, indicating that upper-plate tectonic processes also exert control on H. These along-strike trends in H also correlate with geochemical proxies for the degree of melting, at least in one test case. Thus slab geometry and kinematics provide an important control on the melting that produces arc volcanoes.

Signifi cant earthquakes are increasingly occurring within the continental interior of the United States, including fi ve of moment magnitude (M w) ≥ 5.0 in 2011 alone. Concurrently, the volume of fl uid 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 M w 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 M w 5.0 and a prolifi c 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 fl uid was injected into effectively sealed compartments, and we interpret that a net fl uid volume increase after 18 yr of injection lowered effective stress on reservoir-bounding faults. Signifi cantly, this case indicates that decades-long lags between the commencement of fl uid injection and the onset of induced earthquakes are possible, and modifi es our common criteria for fl uid-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 fi rst.