Space geodetic data recorded rates and directions of motion across the convergent boundary zone between the oceanic Nazca and continental South American plates in Peru and Bolivia. Roughly half of the overall convergence, about 30 to 40 millimeters per year, accumulated on the locked plate interface and can be released in future earthquakes. About 10 to 15 millimeters per year of crustal shortening occurred inland at the sub-Andean foreland fold and thrust belt, indicating that the Andes are continuing to build. Little (5 to 10 millimeters per year) along-trench motion of coastal forearc slivers was observed, despite the oblique convergence.

Abstract. We present results from two-dimensional (2-D) numerical experiments on the thermal and dynamical evolution of the subducting slab and of the overlying mantle wedge for a range in subduction parameters. These include subduction rate and the age and rheology of both subducting and overriding plates. Experiments also consider the influence of slab forcing conditions (from purely kinematic to purely dynamic) on the evolution of both the slab and mantle wedge. One goal is to determine how different parameters control thermal evolution of the slab-wedge interface, from just after subduction initiation up through roughly 500-600 km of subduction, where temperatures are approaching steady state. An additional goal is to define optimal conditions for the melting of slab sediments and crust. Results show slab surface temperatures (SSTs) depend strongly on subduction velocity, plate thermal structure, and upper mantle (or wedge) viscosity structure. Fast subduction beneath a thick (>70 km) overriding plate results in the coolest SSTs. Maximum SSTs are recorded as an early transient event for cases of slow subduction (<3 cm/yr) beneath young, thin lithosphere (<45 km). The latter result supports a model for melting of slab sediments, and possibly crust, early on in cases where young plates subduct beneath thin lithosphere, such as in the Cascades. Maximum wedge temperatures are recorded at higher subduction rates and are found to be strongly dependent on factors influencing return flow into the wedge, such as age of the overriding plate and the ratio of retrograde to longitudinal slab motion. Assuming a model for arc magma genesis driven by fluids migrating into the wedge, these results predict higher-temperature, Mg-rich melts coming up beneath subduction zones with fast, steep slabs and young overriding plates, such as in Japan. The influence of variable viscosity is most pronounced in the slab-wedge corner, which tends to stagnate, or freeze out, with time. Moreover, a region of highly viscous mantle develops above the slab at intermediate depths (> 100 km) which deflects the zone of maximum shear away from slab-wedge interface.

No abstract

No abstract

No abstract

Peridotite is widely considered to be the dominant component of the upper most mantle. Seismic velocities of a dry peridotite determined in the laboratory at high pressure and hypersolidus temperature show a rapid decrease with increasing temperature. The strong temperature dependence of velocities can be used to estimate temperature and melt fraction in the low‐velocity zone of the Earth. The laboratory results show that the pressure dependence of both velocity and melt fraction appears to be well accounted for by the pressure dependence of the solidus temperature of the peridotite, i.e., homologous temperature dependence. This observation allows us to extrapolate laboratory results to higher pressures (greater depths) with some confidence, requiring only a knowledge of the solidus as a function of pressure. Using the ratios of lithospheric to asthenospheric velocities, temperature and melt fraction in the asthenosphere (the low‐velocity zone) can be determined from laboratory velocity data. Examples of thermal structures of the upper mantle beneath oceanic plates are presented. We choose three locations of geophysical interest: Iceland Plateau, Pacific Ocean, and Philippine Sea, where reliable seismic velocities have been determined in the upper mantle from surface wave studies. A large amount of partial melt (≥5 vol %) and a higher temperature than the dry peridotite solidus are inferred in 0–5 m.y. asthenosphere under the Iceland Plateau and the active marginal basin of the east Philippine Sea. The partial melt zone appears to extend deeper (>100 km) and to greater ages (>20 m.y.) in the Pacific Ocean region than under the slowly spreading Iceland Plateau. Partial melting is not expected in the asthenosphere older than 5 m.y. under the Iceland Plateau. Even though the volume fraction of partial melt is not so large (≤3 vol %) beneath the Pacific plate, the melting zone may extend to a distance of above 2000 km from the ridge. Many seamounts observed in the Pacific Ocean may result from this vast region of melting under the plate. We suggest a linear relation between the width of melting zone and the plate velocity. The laboratory results are also applied to other low‐velocity zones. It is suggested that a velocity drop up to about 6% in the asthenosphere does not necessarily require the existence of partial melt but can be explained by high subsolidus temperature. This implies that partial melt may not exist generally but only in limited areas in the low‐velocity zone. Comparison of seismic results with theoretical models may overestimate the temperature and melt fraction, if the velocity drop at subsolidus temperature is neglected. Temperature and melt fraction obtained in this study are discussed together with results from heat flow and electrical studies. Almost the same temperatures as inferred from seismic velocity and a dry peridotite solidus are calculated from heat flow data, indicating that the upper mantle under mid‐ocean ridges may be fairly dry. Even a water‐ or CO2‐undersaturated solidus (0.1 wt ...

The anelastic properties of compressional waves in a peridotite have been determined in the laboratory at sufficiently high temperatures (to 1280°C) and pressures (to 0.73 GPa) to warrant comparison with seismic measurements of the Earth. A substantial decrease of Qp is observed at temperatures well below the onset of partial melting. Qp systematically increases with increasing pressure over the entire temperature range. Of major significance is the finding that Qp is dependent on the ratio of the temperatures to the melting (solidus) temperature; i.e., Qp depends on the homologous temperature. The pressure dependence of Qp appears through the pressure dependence of the solidus of the peridotite. Within the uncertainties of measurement of both Qp and the phase diagram, it appears that melting and high‐temperature anelastic properties have a common origin in peridotite. The homologous temperature dependence of Qp suggests that we may estimate the temperature and pressure dependence of Qp for peridotites of different compositions and possibly even for hydrous peridotites, if solidus temperatures are known as a function of pressure (a far easier measurement than elastic and anelastic properties). The activation volume of Qp is greatly reduced at high pressure, since the slope of solidus versus pressure rapidly decreases with increasing pressure. Pressure dependence of seismic velocity and melt fraction in peridotite also appears to be related to the homologous temperature. The Qp‐homologous temperature relation suggests a connection between Qp and the properties of the grain boundaries; that is, the major loss of seismic energy occurs at the grain boundaries. Grain boundary relaxation or high‐temperature background attenuation is a possible mechanism for the grain boundary damping. No frequency dependence of Qp is resolved (0<α<0.2 in Qp ∝ fα) over the pressure, temperature and frequency ranges of the measurement. The present results and the model of grain boundary relaxation suggest that an appropriate choice of grain size may give an ultrasonic Q that is applicable to the Earth. Experimentally determined anelastic properties of a peridotite are critical for modeling mechanical properties of the upper mantle. Implications of the results are as follows: (1) Seismic data commonly interpreted as indicating a partially molten asthenosphere may instead reflect a hot solid asthenosphere at 90–100% of the solidus temperature. (2) Partial melting may not produce any abrupt change of seismic velocity and Q; rather, elastic and anelastic properties of the upper mantle will change gradually at the boundary where the geotherm crosses the solidus. (3) There may be no sharp mechanical boundary between the lithosphere and the asthenosphere.

The first reports on a slow earthquake were for an event in the Izu peninsula, Japan, on an intraplate, seismically active fault. Since then, many slow earthquakes have been detected. It has been suggested that the slow events may trigger ordinary earthquakes (in a context supported by numerical modelling), but their broader significance in terms of earthquake occurrence remains unclear. Triggering of earthquakes has received much attention: strain diffusion from large regional earthquakes has been shown to influence large earthquake activity, and earthquakes may be triggered during the passage of teleseismic waves, a phenomenon now recognized as being common. Here we show that, in eastern Taiwan, slow earthquakes can be triggered by typhoons. We model the largest of these earthquakes as repeated episodes of slow slip on a reverse fault just under land and dipping to the west; the characteristics of all events are sufficiently similar that they can be modelled with minor variations of the model parameters. Lower pressure results in a very small unclamping of the fault that must be close to the failure condition for the typhoon to act as a trigger. This area experiences very high compressional deformation but has a paucity of large earthquakes; repeating slow events may be segmenting the stressed area and thus inhibiting large earthquakes, which require a long, continuous seismic rupture.