Stéphane Gaffet

The 1997 Umbria‐Marche, Italy, Earthquake Sequence: A first look at the main shocks and aftershocks

A long sequence of earthquakes, six with magnitudes between 5 and 6, struck Central Italy starting on September 26, 1997, causing severe damages and loss of human lives. The seismogenic structure consists of a NW-SE elongated fault zone extending for about 40 km. The focal mechanisms of the largest shocks reveal normal faulting with NE-SW extension perpendicular to the trend of the Apennines, consistently with the Quaternary tectonic setting of the internal sector of the belt and with previous earthquakes in adjacent regions. Preliminary data on the main shocks and aftershocks show that extension in this region of the Apennines is accomplished by normal faults dipping at low angle (∼40°) to the southwest, and confined in the upper ∼8 km of the crust. These normal faults might have reactivated thrust planes of the Pliocene compressional tectonics. The aftershock distribution and the damage patterns also suggest that the three main shocks ruptured distinct 5 to 15 km-long fault segments, adjacent and slightly offset from one another.

A boundary integral equation-discrete wavenumber representation method to study wave propagation in multilayered media having irregular interfaces

We present a method which combines boundary‐integral equation techniques with the discrete wavenumber Green’s function representation to study wave propagation in multilayered media having irregular interfaces. The approach is based on the representation of the interfaces by distributions of body forces, the radiation from which is equivalent to the scattered wave field produced by the diffracting boundaries. The Green’s functions are evaluated by the discrete wavenumber method. Propagator matrices are introduced to relate force distributions on neighboring interfaces. The solution then requires the inversion of a matrix at each interface. The dimensions of the linear system are independent of the number of layers considered, and the computation time varies linearly with the number of interfaces. We apply the method to calculate surface and vertical seismic profiles in the presence of synclinal or anticlinal structures.

Spatio-temporal distribution of seismic activity during the Umbria-Marche crisis, 1997

We present the spatio-temporal distribution of more than 2000 earthquakesthat occurred during the Umbria-Marche seismic crisis, between September 26and November 3, 1997. This distribution was obtained from recordings of atemporary network that was installed after the occurrence of the first two largest shocks (Mw =, 5.7, Mw = 6.0) of September 26. This network wascomposed of 27 digital 3-components stations densely distributed in theepicentral area. The aftershock distribution covers a region of about 40 km long and about2 km wide along the NW-SE central Apennines chain. The activity is shallow,mostly located at less than 9 km depth. We distinguished three main zonesof different seismic activity from NW to SE. The central zone, that containsthe hypocenter of four earthquakes of magnitude larger than 5, was the moreactive and the more complex one. Sections at depth identify 40–50°dipping structures that agree well with the moment tensor focalmechanisms results. The clustering and the migration of seismicity from NW to SE and the generalfeatures are imaged by aftershock distribution both horizontally and at depth.

Effects of two‐dimensional topographies using the discrete wavenumber‐boundary integral equation method in P–SV cases

The effect of topography on surface motion is studied in the case of incident P and SV waves and in presence of an explosive source. The discrete wavenumber‐boundary integral equation method is formulated for this case and is applied to the problem of diffraction of an elastic wave field by a ridge‐shaped topography. The amplitude of the scattered field, which mostly consists of surface P waves and Rayleigh waves, is strongly dependent on the steepness of the topography. The generation of surface waves by an explosion located in the vicinity of the ridge topography is studied and an increase in Rayleigh wave amplitude and a broadening of the Rayleigh pulse when the explosion occurs within the ridge is found.

Frequency-Scaled Curvature as a Proxy for Topographic Site-Effect Amplification and Ground-Motion Variability

We introduce a new methodology to predict the topographic site‐effect amplification. Ground motions obtained from a large database of 3D earthquake simulations show that the curvature of the Earth’s surface, defined as the second spatial derivative of the elevation map, is correlated with the topographic site amplification. The highest correlation between the frequency‐dependent topographic amplification and the topographic curvature is reached when the curvature is smoothed over a characteristic length equal to the S wavelength divided by two (i.e., frequency‐scaled curvature [FSC]). This implies the amplification is caused by topographic features for which horizontal dimensions are similar to half of the S wavelength. The largest ground‐motion variabilities are found at sites located on slopes and on the largest summits, whereas intermediate variabilities occur over narrow ridges and a stable behavior in the bottom valleys. The FSC proxy allows the identification of topographic features with similar characteristic dimensions and probabilistic estimates of amplification values accounting on the variability of ground motions due to source–site interactions. Amplification estimates using the FSC proxy are robust and easily computed from digital elevation maps provided that reasonable values of S‐wave velocities are available in the area of interest.

Seismic diffracted waves from topography using 3-D discrete wavenumber-boundary integral equation simulation

Compressional (P) and shear (S) wave diffraction by free‐surface topography plays a prominent part in the prediction of site responses for seismic risk estimation. Wave propagation modeling in 3-D media is required for an accurate estimation of these diffractions. We have extended the discrete wavenumber‐indirect boundary integral equation method for a 3-D geometry in the case of irregular topography. The Green’s functions are expressed as finite sums of analytical density functions over the horizontal wavenumbers using the spatial periodicity of the topography and a discretization of the surface. We show that the evaluation over vertical wavenumber kz of the analytical integral is possible because a new factor in 1/k2 exists. When the force point and the receiver point are at the same vertical position, we develop a numerical strategy to choose the sign of the exponential factor, which is not given by the analytical formulation. The free‐stress boundary conditions at the topography lead to a large linear...

A Robust Method for Assessing 3-D Topographic Site Effects: A Case Study at the LSBB Underground Laboratory, France

By means of three-dimensional (3-D) numerical simulations, including the Laboratoire Souterrain à Bas-Bruit (LSBB) topography, we carefully analyze site effects assessments yielded by two approaches: the classical site to reference spectral-ratio method (SRM) and the statistical median reference method (MRM). We show for both isotropic and double-couple point sources that a 94% reduction in the number of stations of a regularly spaced array yields MRM site-effect estimates within 5% of those obtained from the absolute regional median, and within 20% using a 98% station reduction with irregularly located sites. In contrast, the SRM yielded site-effect overestimates greater than 50% in some areas and up to 100% in specific sites, which makes the MRM much more robust than the SRM. We determined a 33% probability to exceed an amplification factor of 2, and an 8% probability to exceed a factor of 3 due to topography in the surroundings of the sharpest summit of the LSBB area.

Energetic Charged Particles Above Thunderclouds

The French government has committed to launch the satellite TARANIS to study transient coupling processes between the Earth’s atmosphere and near-Earth space. The prime objective of TARANIS is to detect energetic charged particles and hard radiation emanating from thunderclouds. The British Nobel prize winner C.T.R. Wilson predicted lightning discharges from the top of thunderclouds into space almost a century ago. However, new experiments have only recently confirmed energetic discharge processes which transfer energy from the top of thunderclouds into the upper atmosphere and near-Earth space; they are now denoted as transient luminous events, terrestrial gamma-ray flashes and relativistic electron beams. This meeting report builds on the current state of scientific knowledge on the physics of plasmas in the laboratory and naturally occurring plasmas in the Earth’s atmosphere to propose areas of future research. The report specifically reflects presentations delivered by the members of a novel Franco-British collaboration during a meeting at the French Embassy in London held in November 2011. The scientific subjects of the report tackle ionization processes leading to electrical discharge processes, observations of transient luminous events, electromagnetic emissions, energetic charged particles and their impact on the Earth’s atmosphere. The importance of future research in this area for science and society, and towards spacecraft protection, is emphasized.

Map of low-frequency electromagnetic noise in the sky

The Earths natural electromagnetic environment is disturbed by anthropogenic electromagnetic noise. Here we report the first results from an electromagnetic noise survey of the sky. The locations of electromagnetic noise sources are mapped on the hemisphere above a distributed array of wideband receivers that operate in a small aperture configuration. It is found that the noise sources can be localized at elevation angles up to ∼60° in the sky, well above the horizon. The sky also exhibits zones with little or no noise that are found toward the local zenith and the southwest of the array. These results are obtained by a rigorous analysis of the residuals from the classic dispersion relation for electromagnetic waves using an array analysis of electric field measurements in the frequency range from ∼20 to 250 kHz. The observed locations of the noise sources enable detailed observations of ionospheric modification, for example, caused by particle precipitation and lightning discharges, while the observed exclusion zones enable the detection of weak natural electromagnetic emissions, for example, from streamers in transient luminous events above thunderclouds.

Array analysis of electromagnetic radiation from radio transmitters for submarine communication

The array analyses used for seismic and infrasound research are adapted and applied here to the electromagnetic radiation from radio transmitters for submarine communication. It is found that the array analysis enables a determination of the slowness and the arrival azimuth of the wave number vectors associated with the electromagnetic radiation. The array analysis is applied to measurements of ∼20–24 kHz radio waves from transmitters for submarine communication with an array of 10 radio receivers distributed over an area of ∼1 km ×1 km. The observed slowness of the observed wave number vectors range from ∼2.7 ns/m to ∼4.1 ns/m, and the deviations between the expected arrival azimuths and the observed arrival azimuths range from ∼−9.7° to ∼14.5°. The experimental results suggest that it is possible to determine the locations of radio sources from transient luminous events above thunderclouds with an array of radio receivers toward detailed investigations of the electromagnetic radiation from sprites.