Analysis of a high quality seismic catalog reveals that the average of seismic b‐values in the crust beneath most part of northeastern Japan island arc decreased from 0.86 between 1984 and 1990, to 0.73 between 1991 and 1995. The two areas with the largest decrease are found to be in the same areas where the coupling between the North American and the Pacific plates is the highest, as suggested by a recent geodetic study. In the same time period, the annual seismic moment release increased by 10 times. In addition, there seems to be a corresponding increase in volcanic activities in the same area. One of the most likely interpretations for the observations is an increase in the subduction rate starting from 1991. The timing of this possible increase in subduction rate is consistent with an apparent increase in global seismic activity.

No abstract

[1] The current data set from USArray provides an unprecedented opportunity to investigate mantle transition zone structures beneath the western United States. We have made transition zone images with the Common Converted Point (CCP) stacking method. More than 9600 high quality receiver functions were stacked with reference to two different three-dimensional tomography models and a one-dimensional velocity model. Where the Gorda plate passes through the transition zone, the 410 discontinuity has been elevated ∼25 km and the 660 discontinuity has been depressed ∼35 km. We interpret the transition zone topography in terms of mineral physics results in several different ways, noting in particular that recent measurements on the Clapeyron slope for the ringwoodite-toperovskite phase transition under dry conditions give a phase boundary slope of ∼−1.3 to −0.4 MPa/K. The ∼35 km deflection of the 660 discontinuity observed in the receiver functions seems to be the evidence that the subducted slab can carry abundant water from the surface to the transition zone, and in the transition zone the water in the slab may be fully saturated (e.g. the water content is ∼2.0 wt%). Analyses of the velocity perturbations in the tomography models and the transition zone thickness indicate that the deep water is likely well confined within the subducted slab. We infer that the presence of water in the subducted Gorda slab might have contributed ∼15 km and the thermal anomaly in the slab might have contributed ∼20 km to the depression of the 660 discontinuity.Citation: Cao, A., and A. Levander (2010), High-resolution transition zone structures of the Gorda Slab beneath the western United States: Implication for deep water subduction,

Constraining the topography of the inner-core boundary is important for studies of core-mantle coupling and the generation of the geodynamo. We present evidence for significant temporal variability in the amplitude of the inner core reflected phase PKiKP for an exceptionally high-quality earthquake doublet, observed postcritically at the short-period Yellowknife seismic array (YK), which occurred in the South Sandwich Islands within a 10-year interval (1993/2003). This observation, complemented by data from several other doublets, indicates the presence of topography at the innercore boundary, with a horizontal wavelength on the order of 10 km. Such topography could be sustained by small-scale convection at the top of the inner core and is compatible with a rate of super rotation of the inner core of Ϸ0.1-0.15°per year. In the absence of inner-core rotation, decadal scale temporal changes in the innercore boundary topography would provide an upper bound on the viscosity at the top of the inner core.PKiKP ͉ super rotation ͉ doublet

The seismic phase PKJKP, which traverses the inner core as a shear wave and would provide direct evidence for its solidity, has been difficult to detect. Using stacked broadband records from the Grafenberg array in Germany, we documented a high signal-to-noise phase, the arrival time and slowness of which agree with theoretical predictions for PKJKP. The back azimuth of this arrival is also consistent with predictions for PKJKP, as is the comparison with a pseudoliquid inner core model. Envelope modeling of the PKJKP waveform implies a shear velocity gradient with depth in the inner core that is slightly larger than that in the preliminary reference Earth model.

Using an updated data set of ballistic PKIKP travel time data at antipodal distances, we test different models of anisotropy in the Earth's innermost inner core (IMIC) and obtain significantly better fits for a fast axis aligned with Earth's rotation axis, rather than a quasi‐equatorial direction, as proposed recently. Reviewing recent results on the single crystal structure and elasticity of iron at core conditions, we find that an hcp structure with the fast c axis parallel to Earth's rotation is more likely but a body‐centered cubic structure with the [111] axis aligned in that direction results in very similar predictions for seismic anisotropy. These models are therefore not distinguishable based on current seismological data. In addition, to match the seismological observations, the inferred strength of anisotropy in the IMIC (6–7%) implies almost perfect alignment of iron crystals, an intriguing, albeit unlikely situation, especially in the presence of heterogeneity, which calls for further studies.

S U M M A R YThe density jump ( ρ ICB ) at the inner core boundary (ICB) is an important constraint on the dynamics and history of the Earth's core. Two types of seismological data sensitive to ρ ICB have been studied since the 1970s: free oscillation eigenfrequencies and the amplitudes of core reflected phases (PKiKP/PcP). The preliminary reference earth model (PREM) of Dziewonski & Anderson, based largely on normal mode data, has a relatively low value of ρ ICB = 0.60 g cm −3 , whereas most studies based on PKiKP/PcP amplitude ratios find significantly larger values, sometimes in excess of 1.0 g cm −3 . It has been argued that, because PKiKP is rarely observed in the distance range considered (10-70 • ), the latter type of measurement provides only upper bounds on ρ ICB . We have analysed 10 yr of high-quality global broad-band data accumulated since the work of Shearer & Masters. We systematically analysed over 4500 seismograms from intermediate/deep events (depth >70 km) and nuclear explosions in the distance range 10-70 • . The data were filtered in the bandpass 0.7-3 Hz. We performed rigorous data selection and identified five pairs of very clear (quality A), and 15 possible (quality A − ) PKiKP and PcP arrivals. In addition, 58 records showed no PKiKP but a clear PcP. Together, we obtain a much less dispersed data set than previously available, with the quality A data at the lower end of the ensemble of amplitude ratios versus distance. We combine our high-quality measurements with two measurements from the literature that fall within our rigorous selection criteria and obtain estimates of ρ ICB in the range 0.6-0.9 g cm −3 and β ICB in the range 2-3 km s −1 . Our estimate of ρ ICB is in agreement with a recent re-evaluation of normal mode data, thus reconciling results from body wave and mode studies and providing a tighter constraint on ρ ICB for geodynamicists. Our study also provides evidence for a shear velocity gradient at the top of the inner core.

A new breed of in-plane bi-directional MEMS actuators based on controlled electrothermal buckling and electromagnetic Lorentz force has been demonstrated under both dc and ac operations. Experimentally, bi-directional actuators made by the standard surface-micromachining process have a lateral actuation range of several microns and can exert forces over 100 µN, while those made by SOI and MetalMUMPs processes have an operation range up to several tens of microns and can exert more than 20 mN of force. Reliability tests show that SOI/MetalMUMPs and surface-micromachined actuators can operate for more than 1 and 100 million cycles, respectively, with no signs of degradation. As such, these micro-actuators could be used for MEMS devices that require a bi-directional movement with a large force output such as bi-directional micro-relays.