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

Multiple Scattering and Coda Localization at Merapi Volcano

AB: Due to their eruptive history the cones of strato volcanoes consist of different materials such as hardened lava, tephra, and volcanic ash. Additionally, due to their rough topography, e.g. caused by erosion, deposition is irregular and the volcanic structure cannot be described by a simple 1D layered model. The 3D small scale heterogeneities with large impedance contrast cause multiple scattering of seismic waves and are important features for the modelling of seismic wave propagation in strato volcanoes. Active seismic experiments at Merapi and Vesuvius volcanoes have shown that the transport mean free path of strato volcanoes is as small as some hundreds of meters and, therefore, is about three orders of magnitude smaller than the transport mean free path of usual Earth's crust. Moreover, the transport mean free path is at least one order of magnitude smaller than the characteristic scale length of intrinsic attenuation. Finally, the transport mean free path is in the same order as the inverse of the wave number. This indicates, that in strato volcanoes heterogeneity is so strong that we approach the regime of strong scattering where the classical theories such as radiative transfer and diffusion become invalid. All this makes strato volcanoes a natural laboratory for the application of multiple scattering theories. One important recent observation at Merapi volcano is an abnormal spatial concentration of coda energy in the summit region. This observed coda localization can be interpreted as an indication of Anderson localization, which is a theoretically predicted effect of strong scattering beyond the validity of diffusion theory. We show that the Anderson localization model better fits the data observed at Merapi than a standard half space diffusion model. However, we also show that, alternatively, the observation can also be explained within the classical diffusion approach by assuming leakage of energy from the strongly scattering volcanic edifice into the much more homogeneous underlying earth crust. Similar to Anderson localization the leakage results in an inhomogeneous distribution of energy in space, where the energy is low near the volcano-crust boundary and large inside the strongly scattering volcano far from that boundary. Additionally to the Anderson localization model, we use two classical models to explain coda localization: The first one is based on an analytical solution of the diffusion equation for a scattering cylinder (representing the volcano) embedded in a homogeneous half-space (representing the surrounding crust). The second model is based on a Monte-Carlo simulation of the acoustic equation of radiative transfer. In this simulation we take into account multiple scattering inside the volcanic edifice as well as leakage at the bottom of the volcano into the less heterogeneous crust. Additionally, in this model we also consider the true topography of the volcano by simulating reflections at the free surface, where we use a digital elevation model of the volcano and the Kirchhoff tangent plane method

Spatial concentration of coda energy in the summit region of volcanoes

In general the energy of the seismic coda becomes uniformlydistributed in space at some late lapse time, a fact thatis used for example in the coda normalization method. Somerecent observations at strato volcanoes, on the contrary,indicate an abnormal spatial concentration of coda energyin the summit regions of these volcanoes. This observedcoda localization can be explained by the leakage of energyfrom the strongly scattering volcanic edifice into the muchmore homogeneous underlying earth crust. The leakage resultsin an inhomogeneous distribution of energy in space, wherethe energy is low near the volcano-crust boundary and largeinside the strongly scattering volcano far from that boundary.We present two models for this observation: The first one isbased on an analytical solution of the diffusion equationfor a scattering cylinder (representing the volcano) embeddedin a homogeneous half-space (representing the surroundingcrust). The second model is based on a Monte-Carlo simulationof the acoustic equation of radiative transfer. In thissimulation we take into account multiple scattering insidethe volcanic edifice as well as leakage at the bottom of thevolcano into the less heterogeneous crust. Additionally, inthis model we also consider the true topography of the volcanoby simulating reflections at the free surface, where we use adigital elevation model of the volcano and the Kirchhofftangent plane method. Both models can explain the observedcoda localization. We compare theoretical seismogram envelopesof both models to data of shallow volcano-tectonic earthquakespreceding the 1998 eruption of Merapi volcano (Indonesia)

A repeatable seismic source for tomography at volcanoes

One major problem associated with the interpretation of seismic signals on active volcanoes is the lack of knowledge about the internal structure of the volcano. Assuming a 1D or a homogeneous instead of a 3D velocity structure leads to an erroneous localization of seismic events. In order to derive a high resolution 3D velocity model ofMt. Merapi (Java) a seismic tomography experiment using active sources is planned as a part of the MERAPI (Mechanism Evaluation, Risk Assessment and Prediction Improvement) project. During a pre-site survey in August 1996 we tested a seismic source consisting of a 2.5 l airgun shot in water basins that were constructed in different flanks of the volcano. This special source, which in our case can be fired every two minutes, produces a repeatable, identical source signal. Using this source the number of receiver locations is not limited by the number of seismometers. The seismometers can be moved to various receiver locations while the source reproduces the same source signal. Additionally, at each receiver location we are able to record the identical source signal several times so that the disadvantage of the lower energy compared to an explosion source can be reduced by skipping disturbed signals and stacking several recordings

A repeatable seismic source for tomography at volcanoes

One major problem associated with the interpretation of seismic signals on active volcanoes is the lack of knowledge about the internal structure of the volcano. Assuming a 1D or a homogeneous instead of a 3D velocity structure leads to an erroneous localization of seismic events. In order to derive a high resolution 3D velocity model of<r>Mt. Merapi (Java) a seismic tomography experiment using active sources is planned as a part of the MERAPI (Mechanism Evaluation, Risk Assessment and Prediction Improvement) project. During a pre-site survey in August 1996 we tested a seismic source consisting of a 2.5 l airgun shot in water basins that were constructed in different flanks of the volcano. This special source, which in our case can be fired every two minutes, produces a repeatable, identical source signal. Using this source the number of receiver locations is not limited by the number of seismometers. The seismometers can be moved to various receiver locations while the source reproduces the same source signal. Additionally, at each receiver location we are able to record the identical source signal several times so that the disadvantage of the lower energy compared to an explosion source can be reduced by skipping disturbed signals and stacking several recordings

Unified Green's function retrieval by cross-correlation: Connection with energy principles

Civil Engineering and Geoscience