Catherine Dorbath

A Teleseismic Study of the Altiplano and the Eastern Cordillera in Northern Bolivia: New Constraints on a Lithospheric Model

The Altiplano, one of the worlds great plateaus, is the most significant geomorphological unit of the Central Andes. Knowledge of its lithospheric structure is essential to the understanding of the mountain building in this region. In order to study this, we performed a teleseismic field experiment in northern Bolivia. During a 4-month period, 34 short-period seismic stations were installed along a 320-km-long profile, crossing the Altiplano and the Eastern Cordillera, from the volcanic arc zone to the sub-Andean zone, perpendicular to the main structural direction. A careful study of relative travel time residuals led us to propose a qualitative model related to the structural patterns; the maximum amplitude of these residuals along the profile reaches 3 s for P phases and have a strong azimuthal dependence. The residuals show a noticeable sudden increase, clearly associated with the fault system bordering the Eastern Cordillera to the west, the Cordillera Real fault system. The high quality of the data allows us to perform a velocity inversion and to calculate at what depth lateral variations occur. The origin of the perturbations in the upper crust is reasonably accounted for by the depth variations of the sedimentary fill, with the maximum thickness found under the Altiplano basin. The velocity perturbations in the lower crust are interpreted as variations of the Moho depth which decreases from about 60 km below the Altiplano to 50 km below the Eastern Cordillera. In the upper mantle, a high-velocity zone is observed below the Eastern Cordillera. The major feature of this model is a subvertical boundary, dipping slightly to the southwest, which separates two strongly contrasting velocity units throughout the model, from the surface down to 140 km. This boundary coincides with the Cordillera Real fault system and is interpreted as an old suture. As the Altiplano is characterized by low velocities in the crust, the high velocities beneath the Eastern Cordillera down to 120 km may correspond to the Brazilian craton; in the region studied, the western limit of the underthrusting of the craton corresponds to the western border of the Eastern Cordillera.

Crustal and upper mantle structure across the Dead Sea rift and Israel from teleseismic P-wave tomography and gravity data

Abstract New findings of the structure of the crust and the upper mantle across the Dead Sea rift and Israel were obtained by applying P and PKP wave relative travel time residuals inversion of 612 teleseisms, recorded by the seismic networks in Israel and Jordan. Independently of the relative teleseismic travel time residuals, the Bouguer gravity anomalies were used as an additional aid to examine the structure of the crust and upper mantle. An examination of the teleseismic travel time residuals in Israel suggests the existence of two prominent provinces of distinct crustal structure. The northward increase of the teleseismic travel time residuals is in good agreement with a gradual thickening of the sedimentary sequence and a simultaneous thinning of the crust. Examination of the inverted velocity anomalies indicates that the upper and lower crusts are sub-divided into a number of structures. We identify several velocity anomalies that are associated with prominent geological structures. The southern Dead Sea basin is characterized with the largest decrease of velocity, in both upper and lower crustal layers, due to the infilling of light material relative to the surrounding material. The velocity anomaly in the Carmel structure is probably associated with variations in the upper part of the upper mantle. Examination of a cross-section that traverses the Dead Sea transform supports the existence of lighter mass density material on the eastern side relative to that on the western side.

Crustal structure and magmato‐tectonic processes in an active rift (Asal‐Ghoubbet, Afar, East Africa): 1. Insights from a 5‐month seismological experiment

We seek to characterize how magmatic and tectonic activities combine and interact during the continental rifting process. We address this question in two companion papers. In both, we analyze the seismicity that occurs in an active magmato-tectonic rift, Asal-Ghoubbet (East Africa), to identify the features and/or processes responsible for its activity. Here, we report results from a 5-month experiment that we conducted in the rift. Eleven seismometers were deployed to complement the eight-station permanent network. This allowed recording ∼400 earthquakes in the rift; 200 events could be well located (precision 5–6 km) magma reservoir. Most events concentrate at the roof of the pipe (at 3–4 km) and result from up and down slip ruptures on both the volcanic (ring) and tectonic faults that enclose the pipe at depth. The up and down motions are likely driven by pressure changes in the magma reservoir. Hence, although a few rift faults were associated with seismicity, most remained seismically silent during the experiment. In the companion paper, we analyze the seismic activity in the rift over the 23 years that followed its last rifting episode. This confirms the importance of the Fieale-Shark Bay plumbing system in the overall rift behavior.

Stress transfer among en echelon and opposing thrusts and tear faults: Triggering caused by the 2003 Mw = 6.9 Zemmouri, Algeria, earthquake

Stress transfer among en echelon and opposing thrusts and tear faults: Triggering caused by the 2003 M w = 6.9 Zemmouri, Algeria, earthquake [1] The essential features of stress interaction among earthquakes on en echelon thrusts and tear faults were investigated, first through idealized examples and then by study of thrust faulting in Algeria. We calculated coseismic stress changes caused by the 2003 M w = 6.9 Zemmouri earthquake, finding that a large majority of the Zemmouri afterslip sites were brought several bars closer to Coulomb failure by the coseismic stresses, while the majority of aftershock nodal planes were brought closer to failure by an average of ∼2 bars. Further, we calculated that the shallow portions of the adjacent Thenia tear fault, which sustained ∼0.25 m slip, were brought >2 bars closer to failure. We calculated that the Coulomb stress increased by 1.5 bars on the deeper portions of the adjacent Boumerdes thrust, which lies just 10–20 km from the city of Algiers; both the Boumerdes and Thenia faults were illuminated by aftershocks. Over the next 6 years, the entire south dipping thrust system extending 80 km to the southwest experienced an increased rate of seismicity. The stress also increased by 0.4 bar on the east Sahel thrust fault west of the Zemmouri rupture. Algiers suffered large damaging earthquakes in A.D. 1365 and 1716 and is today home to 3 million people. If these shocks occurred on the east Sahel fault and if it has a ∼2 mm/yr tectonic loading rate, then enough loading has accumulated to produce a M w = 6.6–6.9 shock today. Thus, these potentially lethal faults need better understanding of their slip rate and earthquake history. among en echelon and opposing thrusts and tear faults: Triggering caused by the 2003 M w = 6.9 Zemmouri, Algeria, earthquake,

Deriving microstructure and fluid state within the Icelandic crust from the inversion of tomography data

The inversion of seismic data to infer rock microstructural properties and fluid flow patterns in the crust is a challenging issue. In this paper, we develop an effective medium model for estimating velocities in porous media including both pores and cracks and use it to derive the distribution of crack density beneath the Reykjanes Peninsula from accurate tomography data. Outside the active hydrothermal areas, crack density is shown to decrease with depth. There are two main reasons for this: the closure of cracks because of the increasing overburden and the secondary filling of cracks because of hydrothermal flows. However, crack density may locally increase with depth beneath the southwestern part of the Kleifarvatn lake. This is consistent with the presence of a deep reservoir with supercritical fluids under pressure, which may activate hydrofracturing processes. We recognize that capturing the link between seismic data and the physical properties of crust is very difficult. This study shows that a combination of mechanical concepts and effective medium theory contributes to improve our understanding of the phenomena occurring within the Icelandic crust

Local Earthquake Tomography in the Southern Tyrrhenian Region of Italy: Geophysical and Petrological Inferences on the Subducting Lithosphere

We obtained a high-resolution seismic tomography of the Ionian lithosphere subduction using a new approach based on: (a) the Double-Difference technique for inversions and (b) the statistical post-processing of a great number of preliminary models (Weighted Average Model, WAM method); the latter was used to increase reliability and resolution. In the tomographic model, the high-velocity portion of the steeply dipping Ionian slab is well imaged, as is an underlying low-Vp (≈7.0 km/s) aseismic region. We propose that the low-velocity region can be assigned to a partially hydrated (serpentinized) mantle of the subducting Ionian slab, which progressively dehydrates with depth in dense high-pressure phases. In the mantle wedge overlying the slab, large areas, characterized by low-Vp (≤7.0 km/s) and high Vp/Vs (≥1.85), have been imaged beneath the Stromboli and Marsili volcanoes down to a maximum depth of 180 km. We have interpreted these areas as being regions where mantle partial melts form and accumulate and which then feed the present-day Aeolian Archipelago magmatism

Deep downward fluid percolation driven by localized crust dilatation in Iceland

From a detailed seismic survey on a transform-like plate setting in Iceland, we propose a spatial relationship between highly deformed crustal columns and areas of low Vp/Vs ratio, which are best explained by high fluid pressures at the base of the seismogenic crust. Combined with geological observations, our seismic data suggest transient and local increases in crust permeability at specific highly dilating areas triggered by dynamical stresses from high-magnitude regional earthquakes. Fluids are then trapped at supercritical conditions in the deep brittle crust which, in turn, increases the capacity of the rocks to deform through cracking and faulting.

Small Scale Earthquake Mechanisms Induced by Fluid Injection at the Enhanced Geothermal System Reservoir Soultz (Alsace) in 2003 using Alternative Source Models

The geothermal reservoir at Soultz-sous-Forêts is a valuable natural laboratory for understanding the mechanisms of microearthquakes generated during stimulations and circulation tests. An ongoing effort currently exists regarding the retrieval of mechanisms aimed to indicate the type of fracturing of the rock massif. As a default, a moment tensor description has been applied. Nevertheless, the retrieval of the mode of fracturing still remains ambiguous. Recent studies indicate a prevailing shear slip but, rarely, a non-shear pattern has also been observed. The moment tensor, used today as a universal tool for descriptions of the mechanism, captures general balanced dipole sources. However, in the case of small-scale earthquakes, the moment tensor need not always be reliably determined. In an effort to fit the data, there may be notable non-shear components caused by the low quality of input data. Constraining the source model to directly determine a simpler one is convenient for describing the physical phenomena expected for a particular focus. An opening of new fractures can be described, to a first approximation, by a tensile crack, optionally combined with a shear slip. Such an alternative model is called a shear-tensile crack (STC) source model. The combination is practical, and can be used to both identify events that reflect purely mode-I (tensile) failure and to determine the dilation angle of the fracture undergoing shear. The latter is particularly important in enhanced geothermal system reservoirs such as Soultz, where shear-related dilation is believed to be the primary mechanism underpinning permeability creation during stimulation injections. We performed a synthetic case study by simulating seismic data as recorded by the actual seismic array installed at Soultz-sous-Forêts. Synthetic P and S amplitudes for several shear-tensile source models were inverted for several types of station coverage. The analysis explored how results were influenced by mislocation, mismodeling, and noise contamination of data. In most cases, the orientation of the mechanism was well resolved. Determination of shear vs. non-shear content within the mechanism was more difficult. From all of the factors influencing resolution that we explored, the quality of the monitoring system (the number of stations and their distribution with respect to the focus) and noise contamination were of the highest impact. The STC source model yielded considerably less spurious non-shear fracture components than the moment tensor. From the bulk of the seismicity recorded during the stimulation in 2003, we concentrated on the first phase of the injection when only a single borehole at the site was stimulated. We processed thirteen small earthquakes with magnitudes larger than 1.4 that were not treated in previous studies. We determined that their source mechanisms were dominantly pure shear slips on pre-existing faults, just as the earthquakes investigated earlier were. The results were also in agreement with the stress pattern from in situ measurements.

Crustal seismicity and subduction morphology around Antofagasta, Chile: preliminary results from a microearthquake survey

During September-October 1988, 13 analog and 16 digital seismographs were installed in northern Chile within 100 km around the city of Antofagasta (22.5–24.5°S; 68.5–70.5°W). The purposes of this study were to observe the microseismicity, to describe the morphology of the subducting slab near the southern edge of the rupture of the last great 1877 earthquake (Mw= 8.8) in the northern Chile seismic gap, and to monitor the seismic activity probably associated with the Atacama fault system that is roughly parallel to the coast. The analysis of the analog records provides a total of 552 reliable events (2.0 < M < 5.0), whose hypocentres delineate the morphology of the subducting plate in the region. The Nazca plate subducts to the east with a dip of 10° along the trench from 22°S to 25°S down to 30 km depth. At 30–60 km depth a slight variation in the dip angle is observed from 17° (22–23.5°S) to 14° (24–25°S). Downplate, from 60 to 100 km in depth, the dip angle increases more rapidly to the north of 23.5°S than to the south of this latitude, where an almost constant dip (14–16°) is observed and the subducting plate becomes more subhorizontal. For greater depths (100–150 km), the dip of the subducting Nazca plate gradually varies from 36° to 18° between 22°S and 24.5°S. South of 24°S and below 100 km depth, an absence of seismicity is observed. However, a cluster of intermediate depth activity is located near the hypocentre of the December 9, 1950 (Mw= 8.2) intraplate normal fault earthquake, around 500 km inland from the trench. Shallow seismicity (depth ⩽ 30 km) is located near the Atacama fault system. Focal mechanisms show normal faulting with slight left-lateral motion along an average strike in the north-northeast-south-southwest direction, which is in agreement with the observed superficial orientation of the fault. Shallow seismicity is also observed on the Mejillones Peninsula, the main irregularity along the coastline. Focal mechanisms of microearthquakes located near the Cerro Moreno fault, which is in the Mejillones Peninsula show left-lateral motion along a north-south fault plane, similar to the fault orientation observed in the field. South of Antofagasta, the Coastal Scarp presents shallow seismicity. Focal mechanisms were possible to obtain only for events with depths between 20 and 30 km which are characterized by thrust faults and are probably associated with the interplate subducting zone.

On the geometry of the Nazca plate subducted under central Chile (32-34.5°S) as inferred from microseismic data

Abstract Results obtained from the distribution of local seismicity recorded with a temporarily expanded network in Central Chile during two months in 1986 are presented. Data from the Bulletin of Regional Seismicity for South America (SISRA), between 1965 and 1981 and for depths over 50 km, are added to extend the spatial covering. All of this information evidences the geometry of the subducted plate in ten E-W-oriented cross-sections, 33 km wide. The passage from subhorizontal subduction north of 33°S to normal subduction is well established as a continuous transition. The geometry of the subducted lithosphere beneath the Chilean territory remains unchanged throughout with a dip of 25°. A difference between segments may be seen to the east of the high Andes: an almost horizontal seismic zone, 300 km wide in the northern part, narrows gradually as we move south and disappears completely near 33°S. This correlates very well with the beginning of active volcanism. It is shown that the vanishing of the horizontal part of the subducted lithosphere represents the transition from subhorizontal to normal subduction. No clear activity is observed deeper than 150 km.