D. Legrand | UNAM Universidad Nacional Autónoma de México (original) (raw)

Papers by D. Legrand

Bulletin of the Seismological Society of America, 2005

Using nearby strong-motion recordings and a finite-source model, we determine the focal mechanism... more Using nearby strong-motion recordings and a finite-source model, we determine the focal mechanisms and identify the activated fault planes of two earthquakes of seismotectonic interest in Taiwan: the 15 December 1993 Tapu ( M w 5.5) and the 17 July 1998 Rueyli ( M w 5.7). These events occurred about 35 km apart, in the same geological province, which is characterized mainly by north-northeast trending thrust faults as well as some strike-slip faults. They are moderate-sized events that did not produce surface ruptures. Therefore, the identification of fault planes appears a significant issue for this region accommodated in a complex tectonic environment and constitutes the main objective of this article. Both events were located in the vicinity of the Chukou-Tachienshan fault system, which is known to be dominated by east-dipping thrust faults, and both earthquakes resulted in almost pure reverse source mechanisms. However, we find that the two ruptures dip in opposite directions, the northwest-dipping fault of the Tapu earthquake being in disagreement with the nearby fault system. For the Rueyli event, we show that fault-plane discrimination becomes effective when more than three stations are used, and that certain stations contribute more than others. We infer that this latter event occurred on one of the secondary east-dipping thrust faults parallel to the Tachienshan fault.

Journal of South American Earth Sciences, 2007

... This network represents a joint effort of the Swiss Seismological Service (SED), of the Feder... more ... This network represents a joint effort of the Swiss Seismological Service (SED), of the Federal Swiss Institute of Technology, the Swiss Agency for Development and Cooperation (DEZA), and the Departments of Geophysics and Civil Engineering of the University of Chile ...

Earth and Planetary Science Letters, 2007

ASAR-ENVISAT Interferometric Synthetic Aperture Radar (InSAR) data collected over the Lastarria-C... more ASAR-ENVISAT Interferometric Synthetic Aperture Radar (InSAR) data collected over the Lastarria-Cordon del Azufre complex (Chile-Argentina) between March 2003 and May 2005 show the persistence of the large wavelength ground inflation revealed by Pritchard and Simons in 2002 from the analysis of ERS InSAR data [Nature 418 (2002) 167-170]. After reducing the tropospheric contribution in the interferograms using a combination of data network adjustment and analysis of MODIS images, we produced an accurate interferometric time series showing a 2 yr long temporal evolution of the ground displacements patterns. Two distinct inflating signals are detected. The main signal covers an elliptical area with a 45 km NNE-SSW major axis and a 37 km minor axis. It is correlated with a regional topographic dome. We estimated its maximum inflation rate to ∼ 2.5 cm yr − 1. We inverted the InSAR data for a range of source geometries (spherical, prolate ellipsoids, penny-shaped cracks). The inferred source parameters for 2003-2005 period are consistent with an over-pressured reservoir at shallow to intermediate crustal depths (7-15 km), with an average volumetric rate of inflation of about 14 × 10 6 m 3 yr − 1. In addition to this main signal a new feature highlighted by the ASAR data is short wavelength inflation (6 km wide) at the location of Lastarria volcano on the northern margin of the large wavelength signal. We explain this short wavelength signal by a spherical over-pressured source lying 1000 m below the summit of Lastarria volcano. We estimate the average volumetric rate of inflation during the observation period to be ∼ 35 × 10 3 m 3 yr − 1. It is remarkable that both volumetric variations for the large and small inflations exhibit the same evolution during the 2003-2005 period, suggesting that both processes could be related. On the basis of the inversion results and of arguments provided by field evidences and a morpho-structural analysis of the Digital Elevation Model of the area, we propose that the deep source have a magmatic origin while the shallow source is most likely related to hydrothermal fluids. In our interpretation, the ongoing deformation processes observed at Lastarria-Cordon del Azufre volcanic complex could represent an evolving pre

Journal of Volcanology and Geothermal Research, 2009

An algorithm of automatic detection and discrimination of volcanic tremors, defined as events con... more An algorithm of automatic detection and discrimination of volcanic tremors, defined as events containing only P-type wave, and tectonic earthquakes, containing both P-and S

Geosphere, 2018

The Central Andes is a key global location to study the enigmatic relation between volcanism and ... more The Central Andes is a key global location to study the enigmatic relation between volcanism and plutonism because it has been the site of large ignim briteforming eruptions during the past several million years and currently hosts the world's largest zone of silicic partial melt in the form of the Alti plano Puna Magma (or Mush) Body (APMB) and the Southern Puna Magma Body (SPMB). In this themed issue, results from the recently completed PLUTONS project are synthesized. This project focused an interdisciplinary study on two regions of largescale surface uplift that have been found to represent ongoing movement of magmatic fluids in the middle to upper crust. The loca tions are Uturuncu in Bolivia near the center of the APMB and Lazufre on the Chile Argentina border, on the edge of the SPMB. These studies use a suite of geological, geochemical, geophysical (seismology, gravity, surface defor ma tion, and electromagnetic methods), petrological, and geomorphological techniques with numerical modeling to infer the subsurface distribution, quantity, and movements of magmatic fluids, as well as the past history of eruptions. Both Uturuncu and Lazufre show separate geophysical anomalies in the upper, middle, and lower crust (e.g., low seismic velocity, low resistiv ity, etc.) indicating multiple distinct reservoirs of magma and/or hydrothermal fluids with different physical properties. The characteristics of the geophysical anomalies differ somewhat depending on the technique used-reflecting the different sensitivity of each method to subsurface melt (or fluid) of different compositions, connectivity, and volatile content and highlight the need for integrated, multidisciplinary studies. While the PLUTONS project has led to significant progress, many unresolved issues remain and new questions have been raised.

Geophysical Journal International, 1997

The Racha-Dzhava earthquake (Ms=7.0) that occurred on 1991 April 29 at 09: 12:48.1 GMT in the sou... more The Racha-Dzhava earthquake (Ms=7.0) that occurred on 1991 April 29 at 09: 12:48.1 GMT in the southern border of the Great Caucasus is the biggest event ever recorded in the region, stronger than the Spitak earthquake (Ms=6.9) of 1988. A field expedition to the epicentral area was organised and a temporary seismic network of 37 stations was deployed to record the aftershock activity. A very precise image of the aftershock distribution is obtained, showing an elongated cloud oriented N105", with one branch trending N310" in the western part. The southernmost part extends over 80 km, with the depth ranging from 0 to 15 km, and dips north. The northern branch, which is about 30 km long, shows activity that ranges in depth from 5 to 15 km. The complex thrust dips northwards. A stress-tensor inversion from P-wave first-motion polarities shows a state of triaxial compression, with the major principal axis oriented roughly N-S, the minor principal axis being vertical. Body-waveform inversion of teleseismic seismograms was performed for the main shock, which can be divided into four subevents with a total rupture-time duration of 22s. The most important part of the seismic moment was released by a gentle northerly dipping thrust. The model is consistent with the compressive tectonics of the region and is in agreement with the aftershock distribution and the stress tensor deduced from the aftershocks. The focal mechanisms of the three largest aftershocks were also inverted from body-wave records. The April 29th (Ms= 6.1) and May 5th (Ms= 5.4) aftershocks have thrust mechanisms on roughly E-W-oriented planes, similar to the main shock. Surprisingly, the June 15th (Ms = 6.2) aftershock shows a thrust fault striking N-S. This mechanism is explained by the structural control of the rupture along the eastdipping geometry of the Dzirula Massif close to the Borzhomi-Kazbeg strike-slip fault. In fact, the orientation and shape of the stress tensor produce a thrust on a N-S oriented plane. Nappe tectonics has been identified as an important feature in the Caucasus, and the source mechanism is consistent with this observation. A hidden fault is present below the nappe, and no large surface breaks were observed due to the main shock. The epicentral region is characterized by sediments that are trapped between two crystalline basements: the Dzirula Massif, which crops out south of Chiatoura, and the Caucasus Main Range north of Oni. Most, if not all, of the rupture is controlled by the thrusting of overlapping, deformed and folded sediments over the Dzirula Massif. This event is another example of blind active faults, with the distinctive feature that the fault plane dips at a gentle angle. The Racha Range is one of the surface expressions of this blind thrust, and its growth is the consequence and evidence of similar earthquakes in the past.

Geofísica Internacional, 2010

Dos tipos de sismos intraplaca en la placa de Cocos que subduce debajo de la placa Norte America ... more Dos tipos de sismos intraplaca en la placa de Cocos que subduce debajo de la placa Norte America ocurren en Guerrero, México, y áreas adyacentes: (A) inversos de gran echado y (B) de fallamiento normal. Los de tipo A se localizan a ~10-35 km de la costa, a una profundidad de ~35 km, y revelan compresión en la dirección del echado de la placa, probablemente causada por su desdoblamiento. Los de tipo B son ligeramente más profundos que los del tipo A cuando ocurren cerca de la costa, pero si ocurren más adentro del continente, donde la placa se vuelve horizontal, alcanzan profundidades de 40-50 km. Estos eventos revelan extensión en la placa subducida orientada en la dirección de su echado. El análisis de los sismos del 27 de abril y del 22 de mayo de 2009 revela que se trata de eventos intraplaca en la placa de Cocos subducida del tipo A y B, respectivamente. Los espectros de fuente obtenidos a partir de datos locales y regionales dan una caída de esfuerzos de Brune, Î”Ïƒ, de ~49 y 3...

ABSTRACT Colima is one of the most active volcanoes in Mexico. Since the beginning of the most re... more ABSTRACT Colima is one of the most active volcanoes in Mexico. Since the beginning of the most recent eruptive period in 1998, its activity has been characterised by repeated episodes of dome growth and collapse, leading to pyroclastic flows. The current dome at Colima began growing in 2007, but the steep slopes and explosive nature of the volcano limit the possibilities for monitoring it directly. However, measuring deformation in the region of the crater is important to determine the rate of the ongoing eruption and the stability of the dome. Since June 2011, the seismic and magmatic activity at Colima has decreased significantly and remained low ever since. The activity in the summit region has been recorded by a video monitoring system installed by the University of Colima volcano observatory. We have analysed the optical camera data obtained between February and June 2011 using spatial digital image correlation techniques. We show that the velocity of dome extrusion varies strongly on a daily basis, reaching up to 3m/day, and then systematically decreased over the following months. Deformation was barely above the detection threshold of 30cm/day in the weeks prior to June 21st, when a significant explosion occurred, removing part of the dome. Camera data recorded after this event does not show any displacements, possibly due to the low spatial resolution of the camera data. In order to analyse slower deformation processes, we have acquired TerraSAR-X data in spotlight mode for ascending and descending tracks over Colima, obtaining a high spatial resolution of up to 2 m, and a temporal resolution of up to 11 days. In combination with a high resolution digital elevation model, the InSAR data allow the detection of modifications of the dome at a resolution that is two orders of magnitude below the detection threshold of the cameras. The different temporal and spatial scales of deformation detectable by camera and radar monitoring (metre to centimetre, respectively), highlight the benefit of combining these methods to observe the full range of activites at Colima. The results reveal that explosions may occur suddenly after a period of declined dome growth.

Journal of Geophysical Research: Solid Earth, 2013

Estimating the magnitude of dome eruptions is one of the main challenges in volcano monitoring. A... more Estimating the magnitude of dome eruptions is one of the main challenges in volcano monitoring. Although modern monitoring networks are in place at many dome-building volcanoes, the type and occurrence of explosive activity and the scale of the eruptions are commonly estimated by visual inspection. Quantifying the deformation of dome-building volcanoes and the occurrence of explosions is highly valuable, not only for enabling the provision of early warnings but also for facilitating an understanding of the physics of explosive volcanoes, as demonstrated by this study of one of the most active volcanoes in Mexico. The Volcán de Colima is currently experiencing a phase of viscous dome growth, which is associated with episodic "Vulcanian" eruptions and rock falls. Little is known about the dynamics of this dome, its growth rates, or the scale of the associated explosions. We present the results from an analysis of nighttime time-lapse infrared images and compare these data with local seismic amplitude recordings. By digital image correlation, we track temperature features in infrared images. Images taken before and after the explosions reveal the location of the hot dome to be subject to significant and systematic lateral pixel offsets. Dome deformation is shown to occur intermittently every 3-4 h, with lateral displacements exceeding 0.3 m within periods of less than 120 s. Only the thermally elevated regions of the western dome, which may represent a coulée-like flow, are displaced. This movement is often, but not always, associated with seismic amplitude peaks. Therefore, our analysis of the infrared image correlation suggests the occurrence of aseismic dome-deformation episodes, thereby challenging the current understanding of dome growth and/or the appropriateness of commonly used volcano surveillance techniques.

Tectonics, 2012

A 6 day duration earthquakes swarm occurred in the Andaman Sea, 31 days after the giant 2004 Suma... more A 6 day duration earthquakes swarm occurred in the Andaman Sea, 31 days after the giant 2004 Sumatra-Andaman earthquake (Mw 9.2). The swarm occurred less than 100 km from the eastern edge of the 2004 earthquake rupture and is the most energetic ever recorded in the world. The earthquakes swarm appear to have occurred on en echelon fault system bounded by the two main right-lateral strike-slip faults, namely, the Seulimeum Strand of Sumatra Fault system (SEU) and the West Andaman Fault (WAF). At the beginning of the swarm, earthquakes with predominantly strike-slip focal mechanisms occurred which were followed by earthquakes with predominantly normal faulting focal mechanisms having significant deviatoric component. High b value, presence of double slope in the Gutenberg-Richter relation, presence of monogenetic submarine volcanoes in the region of the swarm and the earthquake focal mechanisms suggest that the swarm was of volcano-tectonic origin and fluid flow played an important role in its occurrence. Indeed, our modeling suggests that it was triggered by the 2004 Sumatra-Andaman earthquake through poroelastic relaxation of the coseismic stresses.

Journal of Volcanology and Geothermal Research, 2000

Locations and focal mechanisms of long-period volcanic events observed at Aso volcano, Japan, are... more Locations and focal mechanisms of long-period volcanic events observed at Aso volcano, Japan, are determined by waveform inversion. Near-field broadband three-component seismograms of four to seven stations are simultaneously inverted in the time domain in order to find the six-component seismic moment tensor. A linear inversion is performed at each point of a 3D grid located under the volcano in

Journal of South American Earth Sciences, 2011

Geophysical Research Letters, 2007

HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific re... more HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Geochemistry, Geophysics, Geosystems, 2012

Epicentral locations of non-volcanic tremors (NVT) in the Mexican subduction zone are determined ... more Epicentral locations of non-volcanic tremors (NVT) in the Mexican subduction zone are determined from the peak of the energy spatial distribution and examined over time. NVT is found to occur persistently at a distance of ~215 km from the trench, which we term the "Sweet Spot" because this region probably has the proper conditions (i.e. temperature, pressure, and fluid content) for the NVT to occur with minimum shear slip. High-energy NVT episodes are also observed every few months, extending ~190 km to ~220 km from the trench with durations of a few weeks. During the 2006 slow slip event (SSE) the duration and the recurrence rate of the NVT episodes increased. Low-energy episodes were also observed, independent from the high-energy episodes, ~150 km to ~190 km from the trench during the 2006 SSE. Both the high and low energy episodes were made up of many individual NVT's that had a range of energy-release-rates. However, the highest energyrelease-rates of the high-energy episodes were consistently double those of the low-energy episodes and the persistent activity at the Sweet Spot. We suggest that all of the high-energy episodes are evidence of small, short repeat interval SSE. Given this model, the increased recurrence rate of the high-energy NVT episodes during the 2006 long term SSE implies that short term SSE's also increase during the SSE and are therefore triggered by the SSE.

Bulletin of the Seismological Society of America, 2006

There is an important need for seismologists and engineers to determine seismic displacement incl... more There is an important need for seismologists and engineers to determine seismic displacement including its quasi-static components. The existence of commercial low-cost and low-noise instruments has prompted researchers to try to derive displacement-time histories from acceleration records by using different integration techniques. In this article we show that an important reason for the failure of the integration process is the presence, in translational acceleration records, of tilt effects or rotational components of motions. The importance of these effects in seismic records has been considered in analytical studies before. In this work we show, using controlled experimental tests on a six-component shake table, that small tilts generate a distortion of the translational acceleration records that limits their validity, especially for low-frequency components. We show that knowledge of the complete history of tilt is necessary to obtain an accurate displacement after a double-time integration and that the knowledge of the initial and final tilt magnitudes is not sufficient to obtain the true displacements. Hence, it is crucial, as a minimum, to record at the same place and at the same time the complete time history of the three rotational and the three translational components or to have a non-rotation-sensitive instrument to calculate properly the translational ground displacement, including its permanent components.