T. Monfret

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Flat versus normal subduction zones: a comparison based on 3-D regional traveltime tomography and petrological modelling of central Chile and western Argentina (29°–35°S)

Geophysical Journal International (2014)

Marianne Marot T. Monfret Muriel Gerbault Guust Nolet G. Ranalli

Our study compares the seismic properties between the flat and normal subduction regions in central Chile, to better understand the links between the slab geometry, surface deformation and the deeper structures. In comparison with previous studies, we show the most complete 3-D regional seismic tomography images for this region, in which we use (1) a larger seismic data set compiled from several short-term seismic catalogues, (2) a denser seismic array allowing a better resolution of the subduction zone from the trench to the backarc and into the upper ∼30 km of the slab and (3) a starting 1-D background velocity model specifically calculated for this region and refined over the years. We assess and discuss our tomography results using regional seismic attenuation models and estimating rock types on the basis of pressure and temperature conditions computed from thermomechanical models. Our results show significant seismic differences between the flat and normal subduction zones. As expected, the faster seismic velocities and increased seismicity within the flat slab and overriding lithosphere are generally consistent with a cooler thermal state. Our results are also consistent with dehydration of the mantle above the subducted Juan Fernandez Ridge at the eastern tip of the flat slab segment, indicating that the latter retains some fluids during subduction. However, fluids in the upper portion of the flat slab segment are not seismically detected, since we report instead fast slab seismic velocities which contradict the argument of its buoyancy being the cause of horizontal subduction. The forearc region, above the flat slab, exhibits high Vs and very low Vp/Vs ratios, uncorrelated with typical rock compositions, increased density or reduced temperature; this feature is possibly linked with the aftershock effects of the Mw7.1 1997 Punitaqui earthquake, the flat slab geometry and/or seismic anisotropy. At the surface, the seismic variations correlate with the geological terranes. The Andean crust is strongly reduced in seismic velocities along the La Ramada–Aconcagua deformation belt, suggesting structural damage. Slow seismic velocities along the Andean Moho match non-eclogitized hydrated rocks, consistent with a previous delamination event or a felsic composition, which in turn supports the extent of the Chilenia terrane at these depths. We confirm previous studies that suggest that the Cuyania terrane in the backarc region is mafic and contains an eclogitized lower crust below 50-km depth. We also hypothesize major Andean basement detachment faults (or shear zones) to extend towards the plate interface and canalize slab-derived fluids into the continental crust.

Seismic Tomography

Slab

Forearc

Slab window

10.1093/gji/ggu355

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Citations (72)

TheMw= 8.0 Antofagasta (northern Chile) earthquake of 30 July 1995: A precursor to the end of the large 1877 gap

Bulletin of the Seismological Society of America (1997)

Bertrand Delouis T. Monfret L. Dorbath M. Pardo Luis Rivera

Abstract An Mw = 8.0 earthquake occurred on 30 July 1995 in the Antofagasta region (northern Chile). The main rupture, corresponding to thrust faulting, developed from 10 to 50 km in depth along the subduction interface between the Nazca and the South American plates. The 1995 earthquake took place just south of the large seismic gap where a great earthquake (M = 9) had occurred in 1877. Most of the 1995 rupture was located within a local network consisting of nine short-period stations that had been previously installed at the southern end of the 1877 gap, and the aftershock sequence could be accurately monitored. Little destruction resulted from the 1995 earthquake in spite of its large size. Ground acceleration in Antofagasta reached 29% of gravity. A tsunami wave, 2 to 2.5 m high, was observed along the coast from Mejillones to Taltal. One strong foreshock (Mw = 6.2) occurred in the 1995 hypocentral region 6 months before the main event. Body-wave modeling of broadband seismograms from the global network, along with the analysis of the aftershock distribution, allows us to propose a well-constrained model for the whole rupture process. Some additional details of the rupture were obtained from an accelerometer record at Antofagasta. The main rupture started as a double even with thrust mechanism below the southern part of the Mejillones peninsula, and it propagated southward in a N200°E direction with an average velocity of 2.8 km/sec. It ended near the trench in normal faulting. The total rupture area and seismic moment are 185 × 90 km2 and 1.2 × 1028 dyne-cm, respectively. The aftershock distribution delineates a well-defined rupture surface along the subduction interface. The distribution of epicenters during the first 20 h of aftershock activity shows a sharp northern boundary beneath the Mejillones peninsula. Hence, the 1995 main rupture did not propagate north of the Mejillones peninsula into the 1877 gap. Aftershocks during the following 2 weeks indicate a growth of the initial rupture zone toward the north. The mechanisms of the strongest aftershocks are similar to that of the mainshock. The down-dip termination of the main rupture corresponds to the maximum depth (50 km) of the region that had been identified as the locked part of the subduction interface from the analysis of the microseismicity recorded by the local network prior to the 1995 event. A well-constrained dislocation model is proposed for the 1995 main rupture, which produces surface displacements in good agreement with available observations of coastal uplift and GPS measurements. The dislocation model, as well as Global Positioning System (GPS) measurements, indicate that the 1995 earthquake generated E-W extension in the coastal region of Antofagasta. The Atacama fault, located 40 to 50 km above the 1995 main rupture, showed small fresh surface ruptures near Sierra Remiendos (70 km to the SSE of Antofagasta) with a maximum vertical offset of 20 cm. This offset corresponds to normal faulting, which is in agreement with the E-W co-seismic extension. The Mejillones peninsula appears to be the surface expression of a barrier that interrupted the propagation of the 1995 rupture to the north into the region of the 1877 gap. Modeling of static stress changes induced by the Antofagasta earthquake indicates an increase in compressive stresses along a direction transverse to the trench immediately to the north of the 1995 rupture surface. Thus, the chances for the reactivation of the 1877 gap after this event are greater now.

Seismogram

Thrust fault

Peninsula

10.1785/bssa0870020427

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Citations (118)

Seismicity and average velocities beneath the Argentine Puna Plateau

Geophysical Research Letters (1999)

Bernd Schurr G. Asch Andreas Rietbrock R. Kind M. Pardo

A network of 60 seismographs was deployed across the Andes at ∼23.5°S. The array was centered in the backarc, atop the Puna high plateau in NW Argentina. P and S arrival times of 426 intermediate depth earthquakes were inverted for 1‐D velocity structure and hypocentral coordinates. Average velocities and υ p /υ s in the crust are low. Average mantle velocities are high but difficult to interpret because of the presence of a fast velocity slab at depth. Although the hypocenters sharply define a 35° dipping Benioff zone, seismicity in the slab is not continuous. The spatial clustering of earthquakes is thought to reflect inherited heterogeneties of the subducted oceanic lithosphere. Additionally, 57 crustal earthquakes were located. Seismicity concentrates in the fold and thrust belt of the foreland and Eastern Cordillera, and along and south of the El Toro‐Olacapato‐Calama Lineament (TOCL). Focal mechanisms of two earthquakes at this structure exhibit left lateral strike‐slip mechanisms similar to the suggested kinematics of the TOCL. We believe that the Puna north of the TOCL behaves like a rigid block with little internal deformation, whereas the area south of the TOCL is weaker and currently deforming.

Seismometer

Lineament

Slab

10.1029/1999gl005385

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Citations (49)

Forcasting the end of a gap : the large Antofagasta (northern Chile) earthquake of july 30, 1995

B. Delouis T. Monfret L. Dorbath Martin Velez‐Pardo L. Rivera

Source

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Citations (0)

Seismotectonic and stress distribution in the central Chile subduction zone

Journal of South American Earth Sciences (2002)

M. Pardo D. Comte T. Monfret

Slab

Eclogitization

Slab window

Convergent boundary

10.1016/s0895-9811(02)00003-2

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Citations (145)

The October 15, 1997 Punitaqui earthquake (Mw=7.1): a destructive event within the subducting Nazca plate in central Chile

Tectonophysics (2002)

M. Pardo D. Comte T. Monfret Rubén Boroschek Maximiliano Astroza

Hypocenter

Focal mechanism

Thrust fault

10.1016/s0040-1951(01)00213-x

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Citations (39)

Algunas características espectrales de la sismicidad asociada a la actividad del Volcán de Colima

Geofísica Internacional (1993)

J. Lermo Julio Cuenca Tinitana T. Monfret F. J. Hernández E. Nava

Entre el 5 y 15 de marzo de 1991 se instaló una red de sismógrafos digitales de tres componentes para registrar la actividad sísmica asociada al Volcán de Colima. Estos eventos fueron clasificados por su forma de onda y/o por observaciones directas en campo. Con el objeto de caracterizarlos por su frecuencia dominante se estimaron sus espectros de Fourier, tomando en cuenta el efecto de sitio de las estaciones. Cuatro tipos diferentes de eventos fueron observados: Eventos tipo B de alta y baja frecuencia, tremores armónicos y tremores de avalancha. El tipo B-Alta frecuencia presenta un máximo en 5 Hz. El pico dominante para el tipo B-Baja frecuencia es aproximadamente de 2 Hz que tiene una forma espectral característica y bien definida. La frecuencia dominante de los tremores armónicos varia de 3 a 5 Hz, presentando una forma espectral muy característica compuesta de dos máximos estrechos. El segundo pico de menor amplitud guarda una relación del doble de la frecuencia dominante del primero. Finalmente los tremores de avalancha muestran también una forma espectral característica compuesta de un solo máximo en 3 Hz.

10.22201/igeof.00167169p.1993.32.4.612

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Citations (3)

The Mw 7.7 Tocopilla Earthquake of 14 November 2007 at the Southern Edge of the Northern Chile Seismic Gap: Rupture in the Deep Part of the Coupled Plate Interface

Bulletin of the Seismological Society of America (2009)

Bertrand Delouis M. Pardo D. Legrand T. Monfret

The slip distribution of the Mw 7.7 Tocopilla earthquake was obtained from the joint inversion of teleseismic and strong-motion data. Rupture occurred as underthrusting at the base of the seismically coupled plate interface, mainly between 35 and 50 km depth. From the hypocenter, located below the coast 25 km south of the town of Tocopilla, the rupture propagated 50 km northward and 100 km southward. Overall, the slip distribution was dominated by two slip patches, one near the hypo- center and the other 70 km to the south where slip reached its maximum value (3 m). An additional branch of moderate slip propagated at shallower depth toward the west near the northern tip of the Mejillones peninsula. Rupture velocity remained close to 2:8 km=sec, with a total rupture duration of 45 sec. The first 2 weeks of aftershocks located with a local seismic network display a strong correlation with the slip distri- bution. The 2007 rupture ended below the Mejillones peninsula, where the 1995 An- tofagasta rupture also ended (Ruegg et al., 1996; Delouis et al., 1997; Pritchard et al., 2006). This corroborates the role of barrier played by this structure. The downdip end of the seismically coupled zone at 50 km depth, evidenced by previous studies for the 1995 event, is also confirmed. The 2007 Tocopilla earthquake contributed only mod- erately to the rupturing of the great northern Chile seismic gap, which still has the capacity for generating a much larger underthrusting event.

Hypocenter

Peninsula

Seismic moment

10.1785/0120080192

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Citations (75)

A New Approach for In Situ Characterization of Rock Slope Discontinuities: The "High-Pulse Poroelasticity Protocol" (HPPP)

HAL (Le Centre pour la Communication Scientifique Directe) (2008)

Yves Guglielmi Frédéric Cappa Stéphane Gaffet T. Monfret J. Virieux

Classification of discontinuities

Poromechanics

Characterization