Alexandre Nercessian

Towards forecasting volcanic eruptions using seismic noise

Volcanic eruptions are preceded by increased magma pressures, leading to the inflation of volcanic edifices1. Ground deformation resulting from volcano inflation can be revealed by various techniques such as spaceborne radar interferometry2, or by strain- and tiltmeters3. Monitoring this process in real time can provide us with useful information to forecast volcanic eruptions. In some cases, however, volcano inflation can be localized at depth with no measurable effects at the surface, and despite considerable effort4, 5 monitoring changes in volcanic interiors has proven to be difficult. Here we use the properties of ambient seismic noise recorded over an 18-month interval to show that changes in the interior of the Piton de la Fournaise volcano can be monitored continuously by measuring very small relative seismic-velocity perturbations, of the order of 0.05%. Decreases in seismic velocity a few weeks before eruptions suggest pre-eruptive inflation of the volcanic edifice, probably due to increased magma pressure. The ability to record the inflation of volcanic edifices in this fashion should improve our ability to forecast eruptions and their intensity and potential environmental impact.

3‐D surface wave tomography of the Piton de la Fournaise volcano using seismic noise correlations

[1] We invert Rayleigh waves reconstructed from cross-correlations of 18 months of ambient seismic noise recorded by permanent seismological stations run by the Piton de la Fournaise Volcanological Observatory. By correlating noise records between 21 receivers, we reconstruct Rayleigh waves with sufficient signal-to-noise ratio for 210 inter-station paths. We use the reconstructed waveforms to measure group velocity dispersion curves at periods between 1.5 and 4.5 s. The obtained measurements are inverted for two-dimensional group velocity maps and finally for a 3-D S-wave velocity model of the edifice from +2 to A1 km above sea level. Our results clearly show a high velocity body spatially delimited by the borders of the active 10 km wide caldera. The preferential N30°-N130° orientations of this anomaly at A0.5 km below sea-level is an evidence of the preferential paths of magma injections associated to the NE-SE Rift Zones. This structure is surrounded by a low-velocity ring interpreted as effusive products associated to the construction of the Piton de la Fournaise volcano on the flank of the older Piton des Neiges volcano. Citation: Brenguier, F., N. M. Shapiro, M. Campillo, A. Nercessian, and V. Ferrazzini (2007), 3-D surface wave tomography of the Piton de la Fournaise volcano using seismic noise correlations, Geophys.

Seismicity during lateral dike propagation: Insights from new data in the recent Manda Hararo–Dabbahu rifting episode (Afar, Ethiopia)

Seismicity released during lateral dike intrusions in the Manda Hararo-Dabbahu Rift (Afar, Ethiopia) provides indirect insight into the distribution and evolution of tensile stress along this magma‐assisted divergent plate boundary. In this paper, 5 dike intrusions among the 14 that form the 2005-present rifting episode are analyzed with local and regional seismic data. During dike intrusions, seismicity migrates over distances of 10-15 km at velocities of 0.5-3.0 km/h away from a single reservoir in the center of the rift segment, confirming the analogy with a slow spreading mid‐ocean ridge segment. Comparison with geodetic data shows that the reservoir is located 7 km down rift from the topographic summit of the axial depression. Dikes emplaced toward the north are observed to migrate faster and to be more voluminous than those migrating southward, suggesting an asymmetry of tension in the brittle‐elastic lithosphere. Seismicity during dike injections is concentrated near the propagating crack front. In contrast, faults and fissures in the subsurface appear to slip or open aseismically coeval with the intrusions. The seismic energy released during dike intrusions in the Manda Hararo Rift appears to be primarily modulated by the local magnitude of differential tensile stress and marginally by the rate of stress change induced by the intrusion. The low level of seismic energy accompanying dike intrusions, despite their significant volumes,is likely an indicator of an overall low level of tension in the lithosphere of this nascent plate boundary.

Multiscale Mapping of Completeness Magnitude of Earthquake Catalogs

We propose a multiscale method to map the spatial variations of the com- pleteness magnitude Mc of earthquake catalogs. The Gutenberg-Richter law describ- ing the earthquake frequency-magnitude distribution (FMD) might not hold over the entire magnitude range, and small areas may exhibit a specific type of seismicity, especially in volcanotectonic contexts. For these reasons, any scaling relation should be obtained by adapting the dimension of the studied zone to the range of the event magnitude. Here, we associate ranges of larger magnitudes with increasing areas for data selection based on empirical relations in seismotectonics. Then, for each point in space, we document the earthquake FMD at all length scales within the corresponding earthquake magnitude ranges. High resolution of the Mc-value is achieved through the determination of the smallest space-magnitude scale in which the Gutenberg-Richter law is verified. The multiscale procedure isolates the magnitude range that meets the best local seismicity and local record capacity. Using artificial catalogs and earthquake catalogs of the Lesser Antilles arc, this Mc-mapping method is shown to be efficient in regions with mixed types of seismicity, a variable density of epicenters, and various levels of registration.

Volcanic plume above Mount St. Helens detected with GPS

Eruptions can produce not only flows of incandescent material along the slopes of a volcano but also ash plumes in the troposphere [Sparks et al., 1997] that can threaten aircraft flying in the vicinity [Fisher et al., 1997]. To protect aircraft, passengers, and crews, the International Civil Aviation Organization and the World Meteorological Organization created eight Volcanic Ash Advisory Centers (VAAC, http://www.ssd.noaa.gov/VAAC/vaac.html) around the globe with the goal of tracking volcanic plumes and releasing eruption alerts to airports, pilots, and companies. Currently, the VAAC monitoring system is based mostly on the monitoring systems of any local volcano observatories and on real-time monitoring of data acquired by meteorological satellites.

Deep reflection seismic images of the crustal thinning in the eastern Pyrenees and western Gulf of Lion

Abstract The results of the MCS LISA seismic survey in the Mediterranean Sea and the wide angle data recorded by the onland stations during this experiment are merged and compared with the ray tracing modeling and the deep structure known from previous refraction studies. The results are very good, arriving at a new deep seismic image of the crustal structure. The LISA seismic profile 26, shown is this publication, crosses in strike line a deep basin close to the eastern Pyrenees. The shallow part of the seismic profile is poorly imaged but, thanks to the very good ELF seismic profiles crossing the LISA survey, the acoustic basement and a decollement level can be drawn. The deep part of the LISA profile data has a much better quality than the shallow part and reveals a high velocity (7.1 km/s) lower crust, overlying a shallow Moho interpreted at 8 s TWTT. The wide angle reflections enable the comparison between the ray tracing measured by distance and the seismic image measured by time thereby making the relationship between the continent and the continental shelf clearly delineated for the first time. The thinning of the thickened crust of the eastern Pyrenees does not occur at the shore but inland at the western boundary of the Roussillon basin. However, the central part of the Gulf of Lion continental shelf forms a deep basin that overlies a thinned crust. The age of the thinning is discussed in the general context of the convergence and divergence of the European and Iberian plates including the Sardinia-Corsica block. The geometry of the transfer faults is studied and shown as not steep and to have normal and strike-slip components.

Une coupe de la province volcanique Caraı̈be : premiers résultats de la campagne sismique Casis 2

The eastwards motion of the Caribbean plate is supposed to be related to a Cretaceous flip of subduction from eastwards to westwards vergence. However, we do not observe on the seismic profiles recorded during the CASIS 2 cruise any evidence of subduction beneath the Aves Ridge and Nicaragua Rise. Aves volcanic arc has been probably formed after the collision of the Caribbean volcanic plateau as shown by a wedge of volcanic-clastic sediments imaged by the seismic cruise CASIS 2. A recent left-lateral transtensional tectonics is observed in the lower Nicaragua Rise; the Colombia basin might have a motion towards the northeast relative to the rise.