Shallow seismicity between 0 and 3-km depth has persisted at Mount St. Helens, Washington (MSH) during both eruptive and non-eruptive periods for at least the past thirty years. In this study we investigate the source mechanisms of shallow volcano-tectonic (VT) earthquakes at MSH by calculating high-quality hypocenter locations and fault plane solutions (FPS) for all VT events recorded during two eruptive periods (1981-1986 and 2004-2008) and two non-eruptive periods (1987-2004 and 2008-2011). FPS show a mixture of normal, reverse, and strike-slip faulting during all periods, with a sharp increase in strike-slip faulting observed in 1987-1997 and an increase in normal faulting in 1998-2004. FPS P-axis orientations show a similar to 90 degrees rotation with respect to regional sigma(1) (N23 degrees E) during 1981-1986 and 2004-2008, bimodal orientations (similar to N-S and similar to E-W) during 1987-2004, and bimodal orientations at similar to N-E and similar to S-W from 2008-2011. We interpret these orientations to likely be due to pressurization accompanying the shallow intrusion and subsequent eruption of magma as domes during 1981-1986 and 2004-2008 and the buildup of pore pressure beneath a seismogenic volume (located at 0-1 km) with a smaller component due to the buildup of tectonic forces during 1987-2004 and 2008-2011. (C) 2013 Elsevier B.V. All rights reserved.

Volcanic tremor is an important precursor to explosive eruptions and is ubiquitous across most silicic volcanic systems. Oscillations can persist for days and occur in a remarkably narrow frequency band (i.e. 0.5-7 Hz). The recently proposed magma-wagging model of Jellinek & Bercovici provides a basic explanation for the emergence and frequency evolution of tremor that is consistent with observations of many active silicic and andesitic volcanic systems. This model builds on work suggesting that the magma column rising in the volcanic conduit is surrounded by a permeable vesicular annulus of sheared bubbles. The magma-wagging model stipulates that the magma column rattles within the spring like foam of the annulus, and predicts oscillations at the range of observed tremor frequencies for a wide variety of volcanic environments. However, the viscous resistance of the magma column attenuates the oscillations and thus a forcing mechanism is required. Here we provide further development of the magma-wagging model and demonstrate that it implicitly has the requisite forcing to excite wagging behaviour. In particular, the extended model allows for gas flux through the annulus, which interacts with the wagging displacements and induces a Bernoulli effect that amplifies the oscillations. This effect leads to an instability involving growing oscillations at the lower end of the tremor frequency spectrum, and that drives the system against viscous damping of the wagging magma column. The fully non-linear model displays tremor oscillations associated with pulses in gas flux, analogous to observations of audible 'chugging'. These oscillations also occur in clusters or envelopes that are consistent with observations of sporadic tremor envelopes. The wagging model further accurately predicts that seismic signals on opposite sides of a volcano are out of phase by approximately half a wagging or tremor period. Finally, peaks in gas flux occur at the end of the growing instability several tens of seconds after the largest tremors, which is consistent with observations of a 30- to 50-s lag between major tremor activity and maximum gas release. The extended magma-wagging model, thus, predicts tremor frequency and its evolution before and during an eruption, as well as a driving mechanism to keep the tremor excited for long periods.

Telica Volcano, Nicaragua, is a 'persistently restless' basaltic-andesite stratovolcano located in the Central American volcanic front. A high rate of low-frequency seismic events (LFs) has been recorded at Telica since the installation of a single, vertical-component 1 Hz seismic sensor (TELN) near its summit in 1993. Due to the high rate of LFs at Telica, traditional methods of forecasting volcanic activity based on increases in the overall rate of seismicity are not applicable; therefore an understanding of the nature of precursory changes in Telica's seismicity is necessary to forecast future volcanic activity. In May 1999 a nine-month eruptive episode started at Telica, consisting of phreatic to phreatomagmatic explosions. Here we analyse over 29,000 seismic events recorded during a fifteen-month period of seismicity bracketing this eruptive episode, in an attempt to retrospectively identify precursory changes in seismicity. Seismic event rates between January 1999 and March 2000 show a reduction in the LF event rate three months before the onset of eruptive activity, closely followed by a short-lived swarm of high-frequency (HF) (>5 Hz) events. After a three month data gap a second reduction in the LF event rate started in August 1999, directly following eruptive activity in August and approximately two months before a series of explosions in October 1999. This reduction in the LF event rate was closely followed by a short-lived swarm of HF events that was coincident with the onset of numerous (22) short-lived, but populous, LF multiplets. A further reduction in event rate for both LFs and HFs is evident in the months between the October 1999 explosions and explosions on the 29th of December 1999. We suggest that these changes in seismicity reflect a transition from open-system degassing to closed-system degassing at Telica and could signify a change in the volcanic system preceding future episodes of phreatic to phreatomagmatic activity at Telica and similar persistently restless volcanic systems worldwide. We note that these signals are for phreatic to phreatomagmatic activity and thus may not pertain to magmatic volcanism or to other persistently restless volcanoes prior to their magmatic activity. (C) 2013 Elsevier B.V. All rights reserved.

Successful eruption forecasts are heavily dependent on the recognition of well-established patterns in volcano monitoring data. Therefore, it is critical to develop, in retrospect, an understanding of the physical basis for cases of abnormal precursory behavior, as the basis for (a) a complete understanding of the range of precursory signals that may be expected at a particular volcano and (b) development of new monitoring approaches to detect more subtle signals of the underlying processes responsible for common patterns of seismic unrest. Here, using a hybrid analysis of shear-wave splitting (SWS) and double-couple fault-plane solutions (FPS), we document the timing and nature of local stress field changes in the months to days preceding the 2009 eruption of Redoubt Volcano, Alaska, which was characterized by an abnormally long period of precursory low-frequency seismicity reflected in multiple escalations of alert levels prior to the eruption. We find that an approximately similar to 90 degrees change in the polarization of fast S-wavelets (Phi) accompanied the earliest signs of seismic unrest in 2008 and continued through the eruption before diminishing in 2009. A similar change in the orientation of VT FPS occurred 18-48 h prior to the eruption onset on March 23, 2009, but almost two months after a strong increase in the rate of shallow VT earthquakes. Combined, our SWS and FPS results show the earliest-, and latest-known changes in seismic monitoring data, respectively, and are suggestive of a protracted period of slow magma ascent followed by a short period of rapidly increasing magma pressurization beneath the volcano. These results demonstrate the power of a combined stress-field analysis for clarifying the processes driving ambiguous seismic unrest at active volcanoes. (C) 2013 Elsevier B.V. All rights reserved.

The metal-silicate partition coefficients of Ni and Co in a model C1 chondrite were determined at pressures ranging from 1.2 to 12.0 GPa and temperatures between 2123 and 2750 K. At 5.0 GPa and 2500 K, the effect of variable oxygen contents on the partitioning of Ni and Co was also investigated. Graphite was chosen as the sample container. Carbon is an integral part of the system because about 5 wt% C dissolved in the metal liquid.

We present multidisciplinary observations of the March-June 2011 VEI 2 eruptive episode of the basalticandesite Telica volcano, Nicaragua, which allow for a comprehensive study of the eruption mechanics of low-explosivity eruptions at persistently active volcanoes. The observations are from a dense network of seismic and GPS instrumentation augmented by visual observations of the eruptive episode, geochemical and petrologic analysis of eruptive products, plume SO2 measurements, and temperature measurements of fumaroles inside and outside the active vent. The 2011 eruptive episode was Telica's most explosive since 1999 and consisted of numerous vulcanian explosions, with maximum column heights of 1.5-2 km above the crater rim, depositing a low volume of dominantly hydrothermally altered ash. Based on observed variations in seismicity, temperature, and SO2 flux, the lack of deformation of the edifice, the non-juvenile origin of and predominance of accretionary lapilli in the ash, we propose that temporary sealing of the hydrothermal system between similar to 0.5 and 2 km depth, allowed pressure to build up prior to vulcanian explosions, making this a phreatic eruptive episode. (C) 2013 Elsevier B.V. All rights reserved.

We present a method for calculating quantitative melting reactions in systems with multiple solid solutions that accounts for changes in the mass proportions of phases between two points at different temperatures along a melting curve. This method can be applied to any data set that defines the phase proportions along a melting curve. The method yields the net change in mass proportion of all phases for the chosen melting interval, and gives an average reaction for the melting path. Instantaneous melting reactions can be approximated closely by choosing sufficiently small melting intervals. As an application of the method, reactions for melting of model upper mantle peridotite are calculated using data from the system CaO-MgO-Al2O3-SiO2-Na2O (CMASN) over the pressure interval 0.7-3.5 GPa. Throughout almost this entire pressure range, melting of model Iherzolite involves the crystallization of one or more solid phases, and is analogous to melting at a peritectic invariant point, In addition, we show that melting reactions for small melting intervals(< 5%) along the solidus of mantle peridotite are significantly different from those calculated for large melting intervals. For large melting intervals (> 10%), reaction stoichiometries calculated in CMASN are usually in good agreement with those available for melting of natural peridotite, The coefficients of melting reactions calculated from this method can be used in equations that describe the behavior of trace elements during melting. We compare results from near-fractional melting models using (1) melting reactions and rock modes from CMASN, and (2) constant reactions representative of those used in the literature. In modeling trace element abundances in melt, significant differences arise for some elements at low degrees of melting(< 10%). In modeling element abundances in the residue, differences increase with increase in degree of melting. Reactions calculated along the model Iherzolite solidus in CMASN are the only ones available at present for small degrees of melting so we recommend them for accurate trace element modeling of natural lherzolite.