Source geometry from exceptionally high resolution long period event observations at Mt Etna during the 2008 eruption

During the second half of June, 2008, 50 broadband seismic stations were deployed on Mt Etna volcano in close proximity to the summit, allowing us to observe seismic activity with exceptionally high resolution. 129 long period events (LP) with dominant frequencies ranging between 0.3 and 1.2 Hz, were extracted from this dataset. These events form two families of similar waveforms with different temporal distributions. Event locations are performed by cross-correlating signals for all pairs of stations in a two-step scheme. In the first step, the absolute location of the centre of the clusters was found. In the second step, all events are located using this position. The hypocentres are found at shallow depths (20 to 700 m deep) below the summit craters. The very high location resolution allows us to detect the temporal migration of the events along a dike-like structure and 2 pipe shaped bodies, yielding an unprecedented view of some elements of the shallow plumbing system at Mount Etna. These events do not seem to be a direct indicator of the ongoing lava flow or magma upwelling

Seismic imaging and petrology explain highly explosive eruptions of Merapi Volcano, Indonesia

Our seismic tomographic images characterize, for the first time, spatial and volumetric details of the subvertical magma plumbing system of Merapi Volcano. We present P-and S-wave arrival time data, which were collected in a dense seismic network, known as DOMERAPI, installed around the volcano for 18 months. The P-and S-wave arrival time data with similar path coverage reveal a high Vp/Vs structure extending from a depth of >= 20 km below mean sea level (MSL) up to the summit of the volcano. Combined with results of petrological studies, our seismic tomography data allow us to propose: (1) the existence of a shallow zone of intense fluid percolation, directly below the summit of the volcano; (2) a main, pre-eruptive magma reservoir at >= 10 to 20 km below MSL that is orders of magnitude larger than erupted magma volumes; (3) a deep magma reservoir at MOHO depth which supplies the main reservoir; and (4) an extensive, subvertical fluid-magma-transfer zone from the mantle to the surface. Such high-resolution spatial constraints on the volcano plumbing system as shown are an important advance in our ability to forecast and to mitigate the hazard potential of Merapi's future eruptions.We gratefully acknowledge the French Agence Nationale pour la Recherche for funding the DOMERAPI ANR project (ANR- 12-BS06-0012) and BMKG for providing data used in this stud

Transient deformation associated with explosive eruption measured at Masaya volcano (Nicaragua) using Interferometric Synthetic Aperture Radar

Deformation caused by processes within a volcanic conduit are localised, transient, and therefore challenging to measure. However, observations of such deformation are important because they provide insight into conditions preceding explosive activity, and are important for hazard assessment. Here, we present measurements of low magnitude, transient deformation covering an area of ∼4 km2 at Masaya volcano spanning a period of explosive eruptions (30th April - 17th May 2012). Radial uplift of duration 24 days and peak displacements of a few millimetres occurred in the month before the eruption, but switched to subsidence ∼27 days before the onset of the explosive eruption on 30th of April. Uplift resumed during, and continued for ∼16 days after the end of the explosive eruption period. We use a finite element modelling approach to investigate a range of possible source geometries for this deformation, and find that the changes in pressurisation of a conduit 450 m below the surface vent (radius 160 m and length 700 m), surrounded by a halo of brecciated material with a Young’s modulus of 15 GPa, gave a good fit to the InSAR displacements. We propose that the pre-eruptive deformation sequence at Masaya is likely to have been caused by the movement of magma through a constriction within the shallow conduit system. Although measuring displacements associated with conduit processes remains challenging, new high resolution InSAR datasets will increasingly allow the measurement of transient and lower magnitude deformation signals, improving the method’s applicability for observing transitions between volcanic activity characterised by an open and a closed conduit system

Earthquake-volcano interaction imaged by coda wave interferometry

Large earthquakes are often assumed to influence the eruptive activity of volcanoes. A major challenge to better understand the causal relationship between these phenomena is to detect and image, in detail, all induced changes, including subtle, non-eruptive responses. We show that coda wave interferometry can be used to image such earthquake-induced responses, as recorded at Yasur volcano (Vanuatu) following a magnitude 7.3 earthquake which occurred 80 km from its summit. We use repeating Long-Period events to show that the earthquake caused a sudden seismic velocity drop, followed by a slow partial recovery process. The spatial distribution of the response amplitude indicates an effect centered on the volcano. Our result demonstrates that, even if no major change in eruptive activity is observed, volcanoes will be affected by the propagation of large amplitude seismic waves through their structure, suggesting that Earthquake-volcano interaction is likely a more common phenomenon than previously believed. Citation: Battaglia, J., J.-P. Metaxian, and E. Garaebiti (2012), Earthquake-volcano interaction imaged by coda wave interferometry, Geophys. Res. Lett., 39, L11309, doi:10.1029/2012GL052003

Estimation of the near-surface velocity structure of the Yasur-Yenkahe volcanic complex, Vanuatu

Small-aperture array measurements of seismic noise at seven sites around Yasur volcano (Vanuatu) are performed to estimate the V-P and V-S velocities of the shallow structure. The spatial autocorrelation (SPAC) and the frequency-wavenumber (f-k) methods are used to determine Rayleigh phase velocity dispersion curves. Phase velocities computed with the SPAC method vary between 580 m/s and 960 m/s at 1 Hz and between 270 m/s and 420 m/s at 15 Hz. F-k dispersion curves show velocities of 300-340 m/s and 800-940 m/s at 1 Hz and 200-230 m/s at 15 Hz. An inversion technique based on the use of the neighbourhood algorithm has been used to calculate the one-dimensional velocity model at each site. Velocity models reach 200 m deep and mainly contain two layers and a half-space. For sites close to the Siwi caldera rims, comparisons with geology and hydrothermal system studies suggest that the two layers highlighted in models may correspond to two large pyroclastic sequences related to caldera collapses based on the flank of an old volcano. Results obtained for the other three sites, located inside the caldera, show the influence of the hydrothermal system on P- and S-wave velocities. For these sites, fluid circulation inside the volcanic deposits causes lower velocities at depth. To obtain a near-surface velocity model of the volcanic structure, each 1D velocity model is spatially extrapolated according to the surface geology. Results highlight four distinct areas, the Siwi caldera edges with high velocities and the resurgent block, the ash plain and the Yasur edifice with lower velocities at depth

Preliminary study results of crustal structure beneath Mount Merapi, Central Java, Indonesia

International audienceIn this study, we put an effort to estimate crustal depth and image crustal structure beneath Merapi volcano by employing multicomponent analysis popularly known as Receiver Function technique. We collected a series of waveforms from teleseismic events recorded from October 2013 to mid-April 2015 at 53 stations as a part of DOMERAPI project. We processed selected seismograms by simple deconvolution process between radial and vertical components to estimate the depth of Moho discontinuity beneath the volcano. Current results show complex structure beneath the volcano and a relatively potential Moho depth at about 30 km, which becomes shallower to the North at about 23 km. Stations located at Southern and Northern area show potential low velocity zone though a velocity modelling is necessary to confirm its depth and how low the velocity is

Broad-band ambient noise characterization by joint use of cross-correlation and MUSIC algorithm

International audienceSeveral days of passive seismic broad-band recordings (vertical component) from a dense 3 × 6 km array installed near Chémery (France), with about 100 seismometers, are analysed for wavefield characterization between 0.1 and 3 Hz. Backazimuth is determined by using the Multiple Signal Characterization (MUSIC) algorithm at frequencies below 1 Hz, and non-coherent cross-correlation beamforming above 1 Hz, since the latter is less sensitive to aliasing issues. A novel method of determining the wavefield velocity is introduced, consisting of processing a cross-correlation common-offset gather by the MUSIC algorithm. The fundamental and three higher modes of Rayleigh waves (R0, R1, R2 and R3) are identified under 1 Hz. Above 1.5 Hz, the Lg phase is detected, while R0 and R1 are also present. Roughly between 1 and 1.5 Hz, a quicker phase, probably Pg, is detected. Both Pg and Lg are dominant during night time, suggesting they have a natural origin, which is also consistent with their backazimuth pointing towards the Atlantic. Large scale 2-D spectral-element simulations using deep- and shallow-water ocean sources confirm the possibility of the Lg phase excitation. Thus, even above 1 Hz, natural sources can explain the major part of the ambient noise energy during quiet time periods

Locating volcano-seismic signals in the presence of rough topography : wave simulations on Arenal volcano, Costa Rica

Quantifying the scattering effects of pronounced volcano topography on the seismic wavefield is an important component in locating and interpreting volcano seismic sources. In this study, we perform seismic wave simulations to quantify the scattering generated by a 3-D digital elevation map and 1-D velocity model of Arenal volcano, Costa Rica. Full waveform synthetic seismograms were generated using a 3-D elastic lattice method including complex topography. Several different simulations were performed where the source location, source type and topographic models were varied. Synthetic seismograms were calculated for 35 seismic arrays each one comprising nine stations. At each array, the slowness vector of wave propagation is estimated from the time delays between the sensors obtained using the cross-spectral method. Results show that the backazimuth estimated for some arrays, in particular those close to the source, deviate from the true source position suggesting strong topographic effects in these regions. The maximum of the probability density function, obtained by crossing the backazimuths of the remaining arrays, coincides exactly with the true source location. We also compare our synthetic seismograms with array results from a physical field study. The true and calculated location misfit depends largely on the topography, but also on the number of antennas, the distance from the source and the spatial resolution of the antennas. The results show that this kind of study could be undertaken prior to the installation of seismic arrays to select the sites that minimize the topographic effects leading to improved source locations