Diana Comte

Subduction and collision processes in the Central Andes constrained by converted seismic phases

The Central Andes are the Earths highest mountain belt formed by ocean–continent collision. Most of this uplift is thought to have occurred in the past 20 Myr, owing mainly to thickening of the continental crust, dominated by tectonic shortening. Here we use P-to-S (compressional-to-shear) converted teleseismic waves observed on several temporary networks in the Central Andes to image the deep structure associated with these tectonic processes. We find that the Moho (the Mohorovičić discontinuity—generally thought to separate crust from mantle) ranges from a depth of 75 km under the Altiplano plateau to 50 km beneath the 4-km-high Puna plateau. This relatively thin crust below such a high-elevation region indicates that thinning of the lithospheric mantle may have contributed to the uplift of the Puna plateau. We have also imaged the subducted crust of the Nazca oceanic plate down to 120 km depth, where it becomes invisible to converted teleseismic waves, probably owing to completion of the gabbro–eclogite transformation; this is direct evidence for the presence of kinetically delayed metamorphic reactions in subducting plates. Most of the intermediate-depth seismicity in the subducting plate stops at 120 km depth as well, suggesting a relation with this transformation. We see an intracrustal low-velocity zone, 10–20 km thick, below the entire Altiplano and Puna plateaux, which we interpret as a zone of continuing metamorphism and partial melting that decouples upper-crustal imbrication from lower-crustal thickening.

Reappraisal of great historical earthquakes in the northern Chile and southern Peru seismic gaps

A critical reappraisal of great historical interplate earthquakes in the occidental margin of South America, including southern Peru and northern Chile, is carried out.A spacetime distribution of the earthquakes associated to the seismotectonics regions defined by the rupture zones of the greatest events (1868, Mw = 8.8 and 1877, Mw = 8.8) is obtained. Both regions are seismic gaps that are in the maturity state of their respective earthquake cycles. The region associated to the 1868 earthquake presents a notable seismic quiescence in the present century.

Geodetic, teleseismic, and strong motion constraints on slip from recent southern Peru subduction zone earthquakes

We use seismic and geodetic data both jointly and separately to constrain coseismic slip from the 12 November 1996 M_w 7.7 and 23 June 2001 M_w 8.5 southern Peru subduction zone earthquakes, as well as two large aftershocks following the 2001 earthquake on 26 June and 7 July 2001. We use all available data in our inversions: GPS, interferometric synthetic aperture radar (InSAR) from the ERS-1, ERS-2, JERS, and RADARSAT-1 satellites, and seismic data from teleseismic and strong motion stations. Our two-dimensional slip models derived from only teleseismic body waves from South American subduction zone earthquakes with M_w > 7.5 do not reliably predict available geodetic data. In particular, we find significant differences in the distribution of slip for the 2001 earthquake from models that use only seismic (teleseismic and two strong motion stations) or geodetic (InSAR and GPS) data. The differences might be related to postseismic deformation or, more likely, the different sensitivities of the teleseismic and geodetic data to coseismic rupture properties. The earthquakes studied here follow the pattern of earthquake directivity along the coast of western South America, north of 5°S, earthquakes rupture to the north; south of about 12°S, directivity is southerly; and in between, earthquakes are bilateral. The predicted deformation at the Arequipa GPS station from the seismic-only slip model for the 7 July 2001 aftershock is not consistent with significant preseismic motion.

Subduction of the Chile Ridge: Upper mantle structure and flow

We deployed 39 broadband seismometers in southern Chile from Dec. 2004 to Feb. 2007 to determine lithosphere and upper mantle structure in the vicinity of the subducting Chile Ridge. Body-wave travel-time tomography clearly shows the existence of a long-hypothesized slab window, a gap between the subducted Nazca and Antarctic lithospheres. P-wave velocities in the slab gap are distinctly slow relative to surrounding asthenospheric mantle. Thus, the gap between slabs visible in the imaging appears to be filled by unusually warm asthenosphere, consistent with subduction of the Chile Ridge. Shear wave splitting in the Chile Ridge subduction region is very strong (mean delay time ~3 s) and highly variable. North of the slab windows, splitting fast directions are mostly trench parallel, but, in the region of the slab gap, splitting fast trends appear to fan from NW-SE trends in the north, through ENE-WSW trends toward the middle of the slab window, to NE-SW trends south of the slab window. We interpret these results as indicating flow of asthenospheric upper mantle into the slab window.

A DOUBLE-LAYERED SEISMIC ZONE IN ARICA, NORTHERN CHILE

A double layered seismic zone is determined in Arica, northern Chile using locally recorded events. At depths >100 km two planes of seismicity can be observed: one dipping at ∼30°E with ∼10 km of thickness and a second parallel plane 20–25 km deeper, with the same average thickness. Fault plane solutions for both layers show a wide variability, even between nearby events. The genesis of the Arica double seismic zone seems to be independent of the age, the relative convergence rate and direction of the Nazca plate, because all of these parameters are almost the same along the whole northern Chile and a clear separation of the seismicity into two layers is only observed around the Arica elbow. Moreover, it cannot be responsible of the double layered seismic zone because no similar significant changes are observed in the other well studied double seismic zones.

Source-side shear wave splitting and upper mantle flow in the Chile Ridge subduction region

The actively spreading Chile Ridge has been subducting beneath Patagonian Chile since the Middle Miocene. After subduction, continued separation of the faster Nazca plate from the slow Antarctic plate has opened up a gap—a slab window—between the subducted oceanic lithospheres beneath South America. We examined the form of the asthenospheric mantle flow in the vicinity of this slab window using S waves from six isolated, unusual 2007 earthquakes that occurred in the generally low-seismicity region just north of the ridge subduction region. The S waves from these earthquakes were recorded at distant seismic stations, but were split into fast and slow orthogonally polarized waves at upper mantle depths during their passage through the slab window and environs. We isolated the directions of fast split shear waves near the slab window by correcting for upper mantle seismic anisotropy at the distant stations. The results show that the generally trench-parallel upper mantle flow beneath the Nazca plate rotates to an ENE trend in the neighborhood of the slab gap, consistent with upper mantle flow from west to east through the slab window.

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

Abstract The 1943 Illapel seismic gap, central Chile (30–32°S), was partially reactivated in 1997–1998 by two distinct seismic clusters. On July 1997, a swarm of offshore earthquakes occurred on the northern part of the gap, along the coupled zone between Nazca and South American plates. Most of the focal mechanisms computed for these earthquakes show thrust faulting solutions. The July 1997 swarm was followed on October 15, 1997 by the Punitaqui main event (Mw=7.1), which destroyed the majority of adobe constructions in Punitaqui village and its environs. The main event focal mechanism indicates normal faulting with the more vertical plane considered as the active fault. This event is located inland at 68-km depth and it is assumed to be within the oceanic subducted plate, as are most of the more destructive Chilean seismic events. Aftershocks occurred mainly to the north of the Punitaqui mainshock location, in the central-eastern part of the Illapel seismic gap, but at shallower depths, with the two largest showing thrust focal mechanisms. The seismicity since 1964 has been relocated with a master event technique and a Joint Hypocenter Determination (JHD) algorithm, using teleseismic and regional data, along with aftershock data recorded by a temporary local seismic network and strong motion stations. These data show that the 1997 seismic clusters occurred at zones within the Illapel gap where low seismicity was observed during the considered time period. The analysis of P and T axis directions along the subduction zone, using the Harvard Centroid Moment Tensor solutions since 1977, shows that the oceanic slab is in a downdip extensional regime. In contrast, the Punitaqui mainshock is related to compression resulting from the flexure of the oceanic plate, which becomes subhorizontal at depths of about 100 km. Analog strong motion data of the Punitaqui main event show that the greatest accelerations are on the horizontal components. The highest amplitude spectra of the acceleration is in the frequency band 2.5–10 Hz, in agreement with the energy band responsible for the collapsed adobe constructions. The isoseismal map derived from the distribution of observed damage show that a high percentage of destruction is due to the proximity of the mainshock, the poor quality of adobe houses and probably local site amplification effects.

Seismological Studies of the Central and Southern Andes

The central Andes have formed by the complex interaction of subduction-related and tectonic processes on a lithospheric scale. The deep structure of the entire mountain range and underlying subduction zone has been investigated by passive and active seismological experiments. Detailed tomographic features are interpreted to represent the ascent paths of fluid and melts in the subduction zone and provide new insights about the mechanisms of lithospheric deformation. Receiver functions from teleseismic events have been used to observe the upper-plate continental Moho and subducted oceanic Moho, as well as the interaction of subducted oceanic lithosphere and mantle discontinuities. A second working area was established in the southern Andes to compare two different types of Andean subduction and to identify the principal controlling parameters. Besides the first accurate definition of the Wadati-Benioff zone in south-central Chile, a three-dimensional, tomographic velocity model based on local earthquakes in the southern Andes is presented.

Seismicity near the slip maximum of the 1960 Mw 9.5 Valdivia earthquake (Chile): Plate interface lock and reactivation of the subducted Valdivia Fracture Zone

Understanding the processes behind subduction-related hazards is an important responsibility and major challenge for the Earth sciences. Few areas demonstrate this as clearly as south-central Chile, where some of the largest earthquakes in human history have occurred. We present the first observation of local seismicity in the Villarrica region (39°–40°S), based on a temporary local network of 55 stations installed from the Chilean coast into the Argentinian back-arc for one year. While consistent with the Chilean national catalog (SSN), our results allow us to observe smaller magnitudes with a completeness of about 2.0 and image the geometry of the Wadati-Benioff Zone from the Chile Trench down to 200 km. Offshore, a gap in interplate seismicity is observed in the region of the 1960 Valdivia earthquake slip. Above the interface, two offshore seismicity clusters possibly indicate ongoing stress relaxation. In the subducting Nazca Plate, we find a prominent seismicity cluster along the extrapolated trace of the oceanic Valdivia Fracture Zone (VFZ). The seismicity cluster is observed between 70 and 130 km depth and comprises mainly strike-slip events. It indicates weakening and reactivation of the major VFZ by dehydration of oceanic crust and mantle. Interpreting the subducted VFZ section as a localized reservoir of potential fluid release offers an explanation for the Villarrica volcanic complex that is located above the reactivated VFZ and shows the highest volcanic activity in South America. Crustal seismicity is observed near Puyehue volcano, which recently started to erupt (June 2011). Key Points: - We observe local seismicity near the 1960 Valdivia maximum slip region - The subducted Valdivia Fracture Zone is reactivated by fluid release - The seismogenic zone is free of seismicity; events occur at faults and volcanoes

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.