The first reports(1,2) 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(3-8). It has been suggested(9) that the slow events may trigger ordinary earthquakes (in a context supported by numerical modelling(10)), 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(11,12), and earthquakes may be triggered during the passage of teleseismic waves(13), a phenomenon now recognized as being common(14-17). 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.

The CALIPSO collaborative volcano monitoring system on the Caribbean island of Montserrat includes observations of strain at depths similar to 200 m using Sacks-Evertson strainmeters. Strain data for the March 2004 explosion of the Soufriere Hills Volcano are characterized by large, roughly equal but opposite polarity changes at the two near sites and much smaller changes at a more distant site. The strain amplitudes eliminate a spherical pressure (Mogi-type) source as the sole contributor. The initial changes are followed by smaller recoveries, but with differing relative recovery magnitudes. This dissimilarity requires a minimum of two pressure sources, which we model as a deep spherical pressure source and a shallow dike. The spherical source is fixed at the location derived from data for the massive dome collapse in July 2003. We solve for the best fitting dike plus sphere source combination. The dike geometry is consistent with earlier interpretations of dikes based on GPS data and other lines of evidence. Citation: Linde, A. T., et al. (2010), Vulcanian explosion at Soufriere Hills Volcano, Montserrat on March 2004 as revealed by strain data, Geophys. Res. Lett., 37, L00E07, doi: 10.1029/2009GL041988.

Five Vulcanian explosions were triggered by collapse of the Soufriere Hills Volcano lava dome in 2003. We report strainmeter data for three explosions, characterized by four stages: a short transition between the onset of disturbance and a pronounced change in strain; a quasi-linear ramp accounting for the majority of strain change; a more gradual continued decline of strain to a minimum value; and a strain recovery phase lasting hours. Remarkable similar to 800 s barometric gravity waves propagated at similar to 30 m s(-1). Eruption volumes estimated from plume height and strain data are 0.32-0.42 x 10(6), 0.26-0.49 x 10(6), and 0.81-0.84 x 10(6) m(3), for Explosions 3-5 respectively, consistent with quasi-cylindrical conduit drawdown <2 km. The duration of vigorous explosion is given by the strain signature, indicating mass fluxes of order 10(7) kg s(-1). Conduit pressures released reflect static weight of porous gas-charged magma, and exsolution-generated overpressures of order 10 MPa. Citation: Voight, B., et al. (2010), Unique strainmeter observations of Vulcanian explosions, Soufriere Hills Volcano, Montserrat, July 2003, Geophys. Res. Lett., 37, L00E18, doi: 10.1029/2010GL042551.

Vulcanian explosions with plumes to 12 km occurred at Soufriere Hills volcano (SHV) between July 2008 and January 2009. We report strainmeter and barometric data, featuring quasi-linear strain changes that correlate with explosive evacuation of the conduit at rates of similar to 0.9-2 x 10(7) kg s(-1). July and January explosion-generated strains were similar, similar to 20 nanostrain at similar to 5 km, and interpreted as contractions of a quasi-cylindrical conduit, with release of magmastatic pressure, and exsolution-generated overpressure of order 10 MPa. The 3 December 2008 event was distinctive with larger signals (similar to 140-200 nanostrain at 5-6 km) indicating that a rapid pressurization preceded and triggered the explosion. Modeling suggests a dike with ENE trend, implying that feeder dikes at SHV had diverse attitudes at different times during the eruption. All explosions were associated with acoustic pulses and remarkable atmospheric gravity waves. Citation: Chardot, L., et al. (2010), Explosion dynamics from strainmeter and microbarometer observations, Soufriere Hills Volcano, Montserrat: 2008-2009, Geophys. Res. Lett., 37, L00E24, doi: 10.1029/2010GL044661.

Extrusion of viscous magma and the subsequent formation of a lava dome is often interspersed by short-lived vigorous (Vulcanian) explosions. The causes for and the timing of the transition from effusive to explosive activity during dome formation are poorly understood and forecasting this transition remains a challenge. Here, we describe and interpret a robust and unique multi-parameter data set documenting the subsurface processes associated with Vulcanian explosions at Soufriere Hills Volcano, Montserrat (W.I.) in July and December 2008. We quantify explosion priming by processes in either the shallow (< 2 km depth) or the deep magmatic system and quantify syn-eruptive processes. The July 29 explosion has a signature related exclusively to shallow dynamics including conduit destabilisation, syn-eruptive decompression and magma fragmentation, conduit emptying and expulsion of juvenile pumice. In contrast, the December 3 explosion was triggered by unprecedented sudden pressurisation of the entire plumbing system from depths of about 10 km (including the magma chambers) resulting in surficial dome carapace failure, a violent cannon-like explosion, propagation of pressure waves and pronounced ballistic ejection of dome fragments. With timescales for explosion priming on the order of a few minutes, the precursory geophysical signatures are indicative of the nature of ensuing Vulcanian explosions. The short precursory phases characterise Vulcanian explosions as freak events triggered by abrupt rather than gradual changes in subsurface dynamics. Our findings provide important constraints for theoretical and experimental investigations of the effusive to explosive transition, forecasting of Vulcanian explosions and volcanic risk mitigation. (C) 2011 Elsevier B.V. All rights reserved.

The 15 March, 2007 Vulcanian paroxysm at Stromboli volcano was recorded by several instruments that allowed description of the eruptive sequence and unraveling the processes in the upper feeding system. Among the devices installed on the island, two borehole strainmeters recorded unique signals not fully explored before. Here we present an analysis of these signals together with the time-lapse images from a monitoring system comprising both infrared and visual cameras. The two strainmeter signals display an initial phase of pressure growth in the feeding system lasting similar to 2 min. This is followed by 25 s of low-amplitude oscillations of the two signals, that we interpret as a strong step-like overpressure building up in the uppermost conduit by the gas-rich magma accumulating below a thick pile of rock produced by crater rim collapses. This overpressure caused shaking of the ground, and triggered a number of small landslides of the inner crater rim recorded by the monitoring cameras. When the plug obstructing the crater was removed by the initial Vulcanian blast, the two strainmeter signals showed opposite sign, compatible with a depressurizing source at similar to 1.5 km depth, at the junction between the intermediate and shallow feeding system inferred by previous studies. The sudden depressurization accompanying the Vulcanian blast caused an oscillation of the source composed by three cycles of about 20 s each with a decreasing amplitude, as well recorded by the strainmeters. The visible effect of this behavior was the initial Vulcanian blast and a 2-3 km high eruptive column followed by two lava fountainings displaying decreasing intensity and height. To our knowledge, this is the first time that such a behavior was observed on an open conduit volcano. (C) 2012 Elsevier B.V. All rights reserved.

In December 2008/January 2009 a 4-week episode of lava extrusion characterised by rapid dome growth and ground deflation occurred at Soufriere Hills Volcano (SHV), Montserrat W.I. Recorded strain dilatometer data show strain changes up to -3500 ns during this event. Modelling the collected data assuming existing structural models of the magmatic system, best-fit solutions are obtained for a simultaneous decompression of the shallow and the mid-crustal magma chambers together with a dilation of the shallow dyke-conduit. We interpret our model results as magma ascending from the two chambers into the eruption feeding dyke-conduit and partly extruding at the surface. Dome growth volume estimations from visual observations at SHV fit the extrusion volume inferred from our best-fit model and support our results. The reported data is the first set of geodetic data that documents the dynamic coupling within the entire crustal magmatic system of SHV. (C) 2012 Elsevier B.V. All rights reserved.

We investigated the eruptive episodes that occurred at Etna volcano on 15 November 2011 and 18 March 2012 using different types of data. We present novel data from two recently installed strainmeters that recorded unique signals during the lava fountain phases of these events. The strainmeter data, integrated with those recorded by the magnetic network, and with satellite and ground thermal data, allowed us to follow the path of a gas-rich magma batch from the source inside the volcano to the surface and atmosphere. The amplitude ratio of the volumetric strain changes constrained the storage depth of the magma feeding the lava fountains above 1.5 km below sea level. Magnetic data revealed an attempted shallow lateral intrusion, whereas ground and satellite thermal data furnished a quantification of the total erupted volumes of similar to 2.2x10(6)m(3) for the 15 November event and similar to 3.0x10(6)m(3) for the 18 March event. Despite different durations of the explosive and effusive phases of the two lava fountain events, the total erupted volume was quite similar, suggesting the emptying of a shallow storage system displaying a steady behaviour.

In January 2011, eruptive activity resumed at Etna producing a new phase with frequent lava fountain episodes until April 2012. In November 2011, the first two borehole strainmeters were installed, which detected negative strain changes (similar to 0.15-0.8 strain) during the paroxysmal events. A Finite Element Model was set up to estimate accurately the tilt and volumetric strain, taking into account the real profile of the volcano and the elastic medium heterogeneity. The numerical computations indicated an elongated depressurizing source located at 0 km b.s.l., which underwent a volume change of similar to 2 x 10(6) m(3) which is the most of the magma volume erupted, while a smaller remaining part is accommodated by the magma compressibility. This shallow source cannot accumulate large magma volumes and, thus, favors short-term periodic eruptive events with a fairly constant balance between the refilling and the erupted magma.

A Sacks-Evertson borehole volumetric strainmeter (SE strainmeter) at a site located 105 km from the epicenter of the mainshock recorded a clear slow strain event following the 2003 M-w 8.0 Tokachi-oki earthquake (September 25, 2003, 19:50:06 UTC). This consisted of an episode of contraction for 4 days followed by expansion for 23 days. GPS sites in southeastern Hokkaido also recorded displacement changes during the same time interval. We use quasi-static calculations to generate synthetic waveforms for the measured quantities. All the data are satisfied by a propagating line source 2-stage model of slow reverse slip, uniform amplitude of 50 cm, with rupture propagation velocities of constant 9 cm/s (first stage) and exponentially decreasing from 3 to 0.7 cm/s (second stage). This post-seismic slip event is taken to be coplanar with the main shock rupture on the upper plane of the double Wadati-Benioff seismic zone (DSZ), and largely overlaps the seismic rupture. Regular earthquakes release only about 30% of the plate motion in this section of the subduction zone; post-seismic slip appears to account for at least some of the deficit.