Real-time inversion of VLP source functions at Stromboli Volcano, Italy.

We present a comprehensive processing tool for the real-time analysis of the source mechanism of Very-Long-Period (VLP) seismic data based on waveform inversions performed in the frequency domain for a point source. A search for the source providing the best-fitting solution is conducted over a three-dimensional grid of assumed source locations, in which the Green’s functions associated with each point source are calculated by finite differences using the reciprocal relation between source and receiver. Tests performed on 62 nodes of a Linux cluster indicate that the waveform inversion and search for the best-fitting signal over 100,000 point sources require roughly 30 s of processing time for a 2-min-long record. The procedure is applied to post-processing of a data archive and to continuous automatic inversion of real-time data at Stromboli, providing insights into different modes of degassing at this volcano

Shallow subsurface structures and geotechnical characteristics of Tal El-Amarna area, middle Egypt

Abstract The shallow seismic refraction profiling was carried out at 18 sites in Tal El-Amarna, which is a flat area on the eastern bank of the Nile River, 50 km south of El Minia Governorate, middle Egypt. The collected data are used to estimate the P-wave velocity and to delineate the near-surface ground model beneath the study area. This study is supported by the National Research Institute of Astronomy and Geophysics due to the historical interest of the Tal El-Amarna area as a famous tourist place where there exist many Pharaoh temples and tombs. This area is low seismically active, but it is probably of high vulnerability due to the influence of the local geological conditions on earthquake ground motion, as well as the presence of poor constructions in the absence of various issues such as building designs, quality of building materials, etc. Another dataset at the study area is obtained by multi-channel passive source (microtremor) measurements, which have been recorded at four arrays. The frequency–wavenumber (f–k) method was used to derive the dispersion curves from the raw signals at each array. The resulted dispersion curves were inverted using the neighborhood algorithm to obtain the shear and P-wave velocity models. The concluded Vs and Vp values provide a preliminary estimation of the geotechnical parameters and site classification for the shallow soil as they are of great interest in civil engineering applications

Toward Forecasting Volcanic Eruptions using Seismic Noise

During inter-eruption periods, magma pressurization yields subtle changes of the elastic properties of volcanic edifices. We use the reproducibility properties of the ambient seismic noise recorded on the Piton de la Fournaise volcano to measure relative seismic velocity variations of less than 0.1 % with a temporal resolution of one day. Our results show that five studied volcanic eruptions were preceded by clearly detectable seismic velocity decreases within the zone of magma injection. These precursors reflect the edifice dilatation induced by magma pressurization and can be useful indicators to improve the forecasting of volcanic eruptions.Comment: Supplementary information: http://www-lgit.obs.ujf-grenoble.fr/~fbrengui/brenguier_SI.pdf Supplementary video: http://www-lgit.obs.ujf-grenoble.fr/~fbrengui/brenguierMovieVolcano.av

Source Mechanism of Small Long-Period Events at Mount St. Helens in July 2005 Using Template Matching, Phase-Weighted Stacking, and Full-Waveform Inversion

Long-period (LP, 0.5-5 Hz) seismicity, observed at volcanoes worldwide, is a recognized signature of unrest and eruption. Cyclic LP “drumbeating” was the characteristic seismicity accompanying the sustained dome-building phase of the 2004–2008 eruption of Mount St. Helens (MSH), WA. However, together with the LP drumbeating was a near-continuous, randomly occurring series of tiny LP seismic events (LP “subevents”), which may hold important additional information on the mechanism of seismogenesis at restless volcanoes. We employ template matching, phase-weighted stacking, and full-waveform inversion to image the source mechanism of one multiplet of these LP subevents at MSH in July 2005. The signal-to-noise ratios of the individual events are too low to produce reliable waveform inversion results, but the events are repetitive and can be stacked. We apply network-based template matching to 8 days of continuous velocity waveform data from 29 June to 7 July 2005 using a master event to detect 822 network triggers. We stack waveforms for 359 high-quality triggers at each station and component, using a combination of linear and phase-weighted stacking to produce clean stacks for use in waveform inversion. The derived source mechanism points to the volumetric oscillation (∼10 m3) of a subhorizontal crack located at shallow depth (∼30 m) in an area to the south of Crater Glacier in the southern portion of the breached MSH crater. A possible excitation mechanism is the sudden condensation of metastable steam from a shallow pressurized hydrothermal system as it encounters cool meteoric water in the outer parts of the edifice, perhaps supplied from snow melt

The propagation and seismicity of dyke injection, new experimental evidence

To reach the surface, dykes must overcome the inherent tensile strength of the country rock. As they do, they generate swarms of seismic signals, frequently used for forecasting. In this study we pressurize and inject molten acrylic into an encapsulating host rocks of (1) Etna basalt and (2) Comiso limestone, at 30 MPa of confining pressure. Fracture was achieved at 12 MPa for Etna basalt and 7.2 MPa for Comiso limestone. The generation of radial fractures was accompanied by acoustic emissions (AE) at a dominant frequency of 600 kHz. During “magma” movement in the dykes, AE events of approximately 150 kHz dominant frequency were recorded. We interpret our data using AE location and dominant frequency analysis, concluding that the seismicity associated with magma transport in dykes peaks during initial dyke creation but remains significant as long as magma movement continues. These results have important implications for seismic monitoring of active volcanoes

Dynamic Evolution of Microscopic Wet Cracking Noises

Characterizing the interaction between water and microscopic defects is one of the long-standing challenges in understanding a broad range of cracking processes. Different physical aspects of microscopic events, driven or influenced by water, have been extensively discussed in atomistic calculations but have not been accessible in microscale experiments. Through the analysis of the emitted noises during the evolution of individual, dynamic microcracking events, we show that the onset of a secondary instability known as hybrid events occurs during the fast healing phase of microcracking, which leads to (local) sudden increase of pore water pressure in the process zone, inducing a secondary instability, which is followed by a fast-locking phase on the microscopic faults (pulse-like rupture)