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Most of the world's explosive volcanoes are located in volcanic arcs, formed by fluid-fluxed melting of upper mantle rocks. The fluids that facilitate melting are released from subducted tectonic plates as they sink into the mantle. Yet, we have sparse knowledge of the migration pathways of melts through the upper mantle (i.e., between the surface of the subducted plate and arc volcanoes). We are also uncertain of the time required for this migration to occur. Here, we show evidence of two earthquake swarms that occur in the upper mantle beneath the Mariana and Izu-Bonin arc systems. The best-resolved swarm occurs beneath the Mariana arc, where the earthquakes define a sub-vertical pipe-like structure with a diameter of ∼50 km and occurs between depths of ∼10–250 km. To test the robustness of depth locations, we used a fully non-linear grid search algorithm, double-difference relocation, as well as an analysis of pP-P arrival times and depth sensitive phases. In addition, we calculated centroid moment tensor solutions using a 3D Earth model to understand the mechanism of failure within the swarms. These data demonstrate that the sub-vertical earthquake swarm occurs within the upper mantle between the subducted slab and the overriding volcanic arc, with seismicity concentrated within discrete day- to month-long swarms of activity over a single two-year period. The Izu-Bonin example shares a similar sub-vertical pipe-like geometry with seismic activity bracketed within a two-year period. We infer that these rare earthquake swarms record the ascent of hydrous melt and/or fluid, from dehydration of the subducting plate. This implies that hydrous minerals within subducted slabs continue to dehydrate to depths of at least 200–250 km. Also, the short duration of earthquake swarms implies that fluids/melts can be rapidly transported through the sub-arc mantle at rates in the order of km/hr. This is consistent with rapid melt ascent rates inferred from geospeedometry and exp

Classically, wide-area synchronized power system monitoring has been an expensive task requiring significant investment in utility communications infrastructures for the service of relatively few costly sensors. The purpose of this research is to demonstrate the viability of power system monitoring from very low voltage levels (120 V). Challenging the accepted norms in power system monitoring, the document will present the use of inexpensive GPS time synchronized sensors in mass numbers at the distribution level. In the past, such low level monitoring has been overlooked due to a perceived imbalance between the required investment and the usefulness of the resulting deluge of information. However, distribution level monitoring offers several advantages over bulk transmission system monitoring. First, practically everyone with access to electricity also has a measurement port into the electric power system. Second, internet access and GPS availability have become pedestrian commodities providing a communications and synchronization infrastructure for the transmission of low-voltage measurements. Third, these ubiquitous measurement points exist in an interconnected fashion irrespective of utility boundaries. This work offers insight into which parameters are meaningful to monitor at the distribution level and provides applications that add unprecedented value to the data extracted from this level. System models comprising the entire Eastern Interconnection are exploited in conjunction with a bounty of distribution level measurement data for the development of wide-area disturbance detection, classification, analysis, and location routines. The main contributions of this work are fivefold: the introduction of a novel power system disturbance detection algorithm; the development of a power system oscillation damping analysis methodology; the development of several parametric and non-parametric power system disturbance location methods, new methods of power system phenom

Using data from the frequency disturbance recorders (FDRs) that comprise the nation-wide frequency monitoring network known as FNET, disturbances in the eastern interconnected system (EI) have been monitored and recorded over the past several years. Analysis of this and other data by a wide variety of research scientists and engineers has rendered the idea that frequency disturbances from generator trips, transmission line trips, load trips, and other events, travel with finite speed as electromechanical waves throughout any power system (in this case the EI). Using FNET data as a tool, it is possible to measure and output the arrival times of these disturbance waves with a time resolution of 100 ms. To observe with certainty the arrival time of the frequency disturbance waves, field data collected by the FDRs must first be conditioned in a robust manner. The current method that uses the moving mean of raw FDR data is analyzed and two computationally efficient robust methods are suggested in this report. These new methods that rely on robust statistics are more resistant to the effect of outliers contained within the raw FDR data. Furthermore, like the moving mean, these methods smooth the raw data without removing the general trend. Having recorded and conditioned the FDR data, three conventional triangulation techniques taken from the field of seismology are proposed and analyzed. This study reconfirms the fact that the EI is not a medium of continuous elasticity though which the frequency perturbations travel but rather a discontinuous patchwork of varying elasticities. Within this report, nine generator trip events are analyzed and the aforementioned triangulation methods are applied. The advantages and disadvantages of each method are discussed. To conclude, axioms of future research are proposed and delineated.

The Costarican isthmus is located at the western limit of the Caribbean oceanic plateau, where the Cocos plate subducts along the Middle American Trench. This plate shows strong lateral variations in its morphology and structure. The main objective of this study is to investigate the effects of subduction-related magmatism as a function of morphology and structure of the subducting plate, by performing a simultaneous inversion of the 3-D crustal velocity field and hypocenter locations from local earthquakes. For that, we used the traveltimes of P-waves first arrivals from more than 5000 events, recorded at the Costarican seismic networks between 1991 and 1998. In order to prevent data and modeling inaccuracies, we followed an inversion scheme consisting of four steps. (1) Selection of an adequate data subset for tomographic inversion, (2) estimation of the best reference 1-D model, (3) determination of the finest parameterization and (4) evaluation of resolution. The results show that the upper-most levels of the crust (0-6 km) are consistent with geology, since the velocity anomalies reflect the most meaningful geological features observed in the surface. Below these levels (6-20 km deep), we found two different zones separated by a SW-NE seismic alignment. The northern part is characterized by a highly heterogeneous velocity field and is seismically active, while the southern part is much more homogeneous and practically inactive. Moreover, the northern part shows a certain accumulation of low velocity material within the upper mantle. We suggest and illustrate that the structural differences between the northern and southern zones can be a consequence of the differences on the geometry and structure of the subducting slab. © 2001 Elsevier Science B.V. All rights reserved