Bertrand Guillier

Compatibility constraints on folded and faulted strata and calculation of total displacement using computational restoration (UNFOLD program)

Abstract A balanced-surface method is proposed that allows one to test the reliability of the interpretation of the structural geometry of folded and faulted strata. It also estimates both the finite total displacement field linked to the folding and faulting processes and the finite displacement field linked to the folding. The method describes a thin competent folded and faulted sedimentary layer using rigid (triangular) elements, their sizes depending on the curvature of the surface. The elements are laid flat (and are automatically fit) to form a horizontal surface, which represents the initial state of the layer. The degree of compatibility (given by different indicators) tests the reliability of the geometric fitting of the layer. If folding and faulting occur without bed stretching (or if this change is known and introduced as a parameter in the code) a plausible interpretation can be perfectly retrodeformed, just as a folded and torn sheet of paper may be smoothed with an iron. The method has been applied to two natural examples in oil-field regions using three- or two-dimensional depth-migrated seismic data. The main results reveal in general that the petroleum companys interpretations of the data were nonoptimal. A careful reinterpretation of the seismic data was necessary to obtain balanced folded and faulted surfaces. The estimation of the finite displacement fields revealed the compatibility between fold and fault deformation, and also the strike-slip movement or rotation associated with the deformation.

Restoration and balance of a folded and faulted surface by best-fitting of finite elements: principle and applications

Abstract A computer program is presented which allows us to test the restoration of a folded and faulted thin competent layer and then to balance this surface. The balance of such a surface is useful both to constrain the three-dimensional shape of the folds and the geometry of the limits of the faults. If a part of the surface is fixed the restoration can also give the finite displacement field linked to the deformation of the layer. The principle of the method is given and its accuracy is tested for the restoration of an experimentally folded sheet of paper. Finally the applicability to the restoration of natural structures is discussed.

An indication of the soil topmost layer response in Quito (Ecuador) using noise H/V spectral ratio

Abstract H/V noise spectral ratio (HVSR), standard spectral ratio (SSR), and receiver functions (RF) techniques have been used in the Quito (Ecuador) urban area to estimate the frequency dependence of soil response. Two amplified frequencies obtained by the HVSR method appear on about 60 sites. Taking into account the most amplified frequency rather than considering only the first amplified frequency, generally associated with the fundamental frequency, we find that iso-frequency curves tightly fit the surface geology. The second amplified frequency is interpreted as the fundamental frequency of the soft thin topmost layer, which in some cases amplifies the surface ground motion more than the rest of the soil column. This hypothesis is further supported by the results provided from the SSR and RF studies at a station located on top of a solid waste landfill, and by a study of known thickness of a waste landfill, using the HVSR method.

Use of Ambient Noise: From Spectral Amplitude Variability to H/V Stability

The study of the variation over time of both spectral amplitudes and H/V curves, has been performed on three different sites, two close to cities and one in the countryside, during periods varying from week to over a month. It demonstrates the robustness of the H/V technique to give consistent peak frequency values. In particular, H/V peak frequencies, either fundamental (f0) or natural (fx, x•1), are not affected by weather nor the level of human activity. However, while fundamental H/V peak amplitudes are stable, they proved rather unstable for natural (secondary) peak. Spectral amplitude curves are very variable but follow human activity cycles from week-week end and day-night variations down to a very small scale, such as lunch breaks. Finally, the frequency limit between anthropic noise and natural noise, commonly taken at 1 Hz, is not straightforward and is varying from site to site from 0.7–0.8 Hz up to 2–3 Hz.

Coseismic and postseismic motion of a landslide: Observations, modeling, and analogy with tectonic faults

We document the first time series of a landslide reactivation by an earthquake using continuous GPS measurements over the Maca landslide (Peru). Our survey shows a coseismic response of the landslide of about 2 cm, followed by a relaxation period of 5 weeks during which postseismic slip is 3 times greater than the coseismic displacement itself. Our results confirm the coseismic activation of landslides and provide the first observation of a postseismic displacement. These observations are consistent with a mechanical model where slip on the landslide basal interface is governed by rate and state friction, analogous to the mechanics of creeping tectonic faults, opening new perspectives to study the mechanics of landslides and active faults.

Basement Mapping with Single‐Station and Array Ambient Vibration Data: Delineating Faults under Boumerdes City, Algeria

Investigations of geological structures, such as layer thickness, depth to bedrock, and tectonic features, are important tasks for geologists and engineers. Direct investigational methods, such as boreholes and trenches, can provide accurate data. However, these direct methods are usually expensive and time consuming. More often, geophysical methods that are less expensive and faster to implement (e.g., seismic and electric surveys and ground penetrating radar) are used. However, these geophysical methods may be difficult, or even impossible, to implement in some cases, such as regions with steep slopes or those highly urbanized. In contrast, the single‐station horizontal‐to‐vertical spectral‐ratio (HVSR) method (Nogoshi and Igarashi, 1970, 1971; Nakamura, 1989) and/or dense‐array techniques (Aki, 1957; Lacoss et al. , 1969) based on ambient vibration recordings have fewer constraints. The HVSR method has been used extensively to evaluate site effects, seismic microzonation, and basin structure (e.g., Fah et al. , 1997; Gueguen et al. , 2000; Alfaro et al. , 2001; Duval et al. , 2001; Navarro et al. , 2001; Panou et al. , 2005; Chatelain, Guillier, Parvez, 2008; Bensalem et al. , 2010). In such studies, one assumes a 1D soil column. Variations in the fundamental frequency are used to estimate the first‐order geometry of the interface between a sediment layer and the underlying bedrock (e.g., Yamanaka et al. , 1994; Ibs‐von Seth and Wohlenberg, 1999; Delgado et al. , 2000; Parolai et al. , 2002; Oliveto et al. , 2004). By imaging the soil–bedrock interface, the HVSR method has the ability to highlight blind faults. For a simple 1D soil column, the shear‐wave velocity V S, the fundamental …

Earthquake risk management pilot project in Quito, Ecuador

An earthquake risk management project was conducted in Quito (Ecuador), consisting of evaluating the consequences of destructive earthquakes on the city. After choosing seismogenic sources that can affect the city, on historical and seismotectonics bases, intensities produced by these events were calculated, in order to estimate damages to the buildings and city networks. The scientific and technical studies were completed by interviews of the directors of the main city services in order to produce a vivid nontechnical description of the events during and at various time scales after the occurrence of one of the chosen earthquakes. Finally, recommendations were proposed to minimize the consequences of the next major earthquake on the city.