Edgar Kausel

Variations of Upper Mantle Structure under the Pacific Ocean

The inversion of Rayleigh wave dispersion data for the Pacific Ocean shows that lithospheric thickness increases systematically with age. The lid to the low-velocity channel is very thin or absent near the ridge crest; the low-velocity channel may be absent in the oldest parts of the ocean.

A seismological study of the 1835 seismic gap in south-central Chile

We study the possible seismic gap in the Concepcion–Constitucion region of south-central Chile and the nature of the M = 7.8 earthquake of January 1939. From 1 March to 31 May 1996 a seismic network of 26 short period digital instruments was deployed in this area. We located 379 hypocenters with rms travel time residuals of less than 0.50 s using an approximate velocity distribution. Using the VELEST program, we improved the velocity model and located 240 high precision hypocenters with residuals less than 0.2 s. The large majority of earthquakes occurred along the Wadati–Benioff zone along the upper part of the downgoing slab under central Chile. A few shallow events were recorded near the chain of active volcanos on the Andes; these events are similar to those of Las Melozas near Santiago. A few events took place at the boundary between the coastal ranges and the central valley. Well constrained fault plane solutions could be computed for 32 of the 240 well located events. Most of the earthquakes located on the Wadati–Benioff zone had “slab-pull” fault mechanism due to tensional stresses sub-parallel to the downgoing slab. This “slab-pull” mechanism is the same as that of eight earthquakes of magnitude around 6 that are listed in the CMT catalog of Harvard University for the period 1980–1998. This is also the mechanism inferred for the large 1939 Chilean earthquake. A very small number of events in the Benioff zone had “slab-push” mechanisms, that is events whose pressureaxis is aligned with the slab. These events are found in double layered Wadati–Benioff zones, such as in northern Chile or Japan. Our spatial resolution is not good enough to detect the presence of a double layer, but we suspect there may be one.

The Andean subduction zone between 22 and 25°S (northern Chile): precise geometry and state of stress

Abstract One year of seismicity recorded by a local network is used to obtain more precision about the geometry and the stress regime of the Andean subduction between 22 and 25°S in the northern Chile seismic gap. A sharp image of the Wadati-Benioff Zone (WBZ) is obtained down to 270 km in depth. A seismically quasi-quiescent zone is observed in the WBZ below the volcanic arc, between 150 and 210 km in depth. Hypocentres of distant intermediate depth earthquakes located with the local network are compared with worldwide seismic network hypocentres in order to evaluate the accuracy of the WBZ image at depth greater than 100–150 km in depth. No shallow microearthquakes have been observed in the continental crust but some seismic activity is likely to occur locally at the deep root of the Atacama Fault. The stress field and the characteristics of faulting along the subducted slab are investigated. Underthrusting and localized reverse faulting earthquakes define the seismically coupled plate interface from 20 to 50 km in depth (Locked zone). Downdip, intra-slab normal faulting prevails (Tensile zone), but some strike-slip faulting is observed. A transition between normal faulting with variable fault azimuth and normal faulting with nearly homogeneous NNW- to NW-oriented fault plane is found at about 80 km in depth. It is found that the stress axes σ 1 and σ 3 in the Locked zone are oriented in the convergence direction (75–80°E). Downdip, in the tensile zone, σ 3 has a mean azimuth 60–65°E. There, the slab is hence submitted to a tensional force (slab pull) oblique relatively to the convergence. The transition between seismic underthrusting and intraplate normal faulting downdip occurs at the depth where the continental Moho encounters the Wadati-Benioff Zone, suggesting that a relationship exists between seismic coupling and the presence of continental crust at the plate interface. The pre-seismic state of this segment of the Andean subduction zone is confirmed by the occurrence of strong earthquakes located by the global network around the presumed rupture area and by the stress regime found along the Wadati-Benioff Zone.

The Ms = 8 tensional earthquake of 9 December 1950 of northern Chile and its relation to the seismic potential of the region

Abstract The only known great ( M s = 8) intermediate depth earthquake localized downdip of the main thrust zone of the Chilean subduction zone occurred landward of Antofagasta on 9 December 1950. In this paper we determine the source parameters and rupture process of this shock by modeling long-period body waves. The source mechanism corresponds to a downdip tensional intraplate event rupturing along a nearly vertical plane with a seismic moment of M 0 = 1 × 10 28 dyn cm, of strike 350°, dip 88°, slip 270°, M w = 7.9 and a stress drop of about 100 bar. The source time function consists of two subevents, the second being responsible for 70% of the total moment release. The unusually large magnitude ( M s = 8) of this intermediate depth event suggests a rupture through the entire lithosphere. The spatial and temporal stress regime in this region is discussed. The simplest interpretation suggests that a large thrust earthquake should follow the 1950 tensional shock. Considering that the historical record of the region does not show large earthquakes, a ‘slow’ earthquake can be postulated as an alternative mechanism to unload the thrust zone. A weakly coupled subduction zone—within an otherwise strongly coupled region as evidenced by great earthquakes to the north and south—or the existence of creep are not consistent with the occurrence of a large tensional earthquake in the subducting lithosphere downdip of the thrust zone. The study of focal mechanisms of the outer rise earthquakes would add more information which would help us to infer the present state of stress in the thrust region.

Tarapacá intermediate‐depth earthquake (Mw 7.7, 2005, northern Chile): A slab‐pull event with horizontal fault plane constrained from seismologic and geodetic observations

[1] A large (Mw 7.7) intermediate-depth earthquake occurred on 13 June 2005 in the Tarapaca region of the northern Chile seismic gap. Source parameters are inferred from teleseismic broadbands, strong motions, GPS and InSAR data. Relocated hypocenter is found at

New Findings on the 1958 Las Melosas Earthquake Sequence, Central Chile: Implications for Seismic Hazard Related to Shallow Crustal Earthquakes in Subduction Zones

98 km depth within the subducting slab. The 21-days aftershock distribution, constrained by a postseismic temporary array, indicates a sub-horizontal fault plane lying between the planes of the double seismic zone and an upper bound of the rupture area of 60 km  30 km. Teleseismic inversion shows a slab-pull down dip extension mechanism on a nearly horizontal plane. Total seismic and geodetic moments are

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

5.5 Â 10 20 N.m, with an averaged slip of 6.5 m from geodesy. The earthquake rupture is peculiar in that the effective velocity is slow, 3.5 Km.s A1 for a high stress-drop, 21 –30 MPa. We propose that rupture was due to the reactivation by hydraulic embrittlement of a inherited major lithospheric fault within the subducting plate. The stress-drop suggests that the region of the slab between planes of the double seismic zone can sustain high stresses. Citation: Peyrat, S., et al. (2006), Tarapaca intermediate-depth earthquake (Mw 7.7, 2005, northern Chile): A slab-pull event with horizontal fault plane constrained from seismologic and geodetic observations, Geophys.

Reply to the comment by R. A. Astini and F. M. Dávila on “The West Andean Thrust, the San Ramón Fault, and the seismic hazard for Santiago, Chile”

On the 4th of September 1958, a sequence of 3 earthquakes of magnitude 6.7–6.9 struck the Andean Main Cordillera at the latitude of Santiago, Central Chile. The quakes were preceded by a magnitude 6.0 foreshock one week earlier. This seismic sequence provided the only documented effects of strong shaking related to shallow earthquakes in a subduction-zone environment in which seismicity is dominated by interplate and intermediate-depth intraplate earthquakes. The 1958 earthquake sequence is reviewed as part of a project of seismic hazard assessment of the densely populated region of Santiago. We reinterpret historical documents and carried out field observations to obtain new intensity estimates, and we estimate ranges of peak acceleration values based on geotechnical back-analyses of earthquake-induced landslides. Estimated peak intensities of 9 and peak accelerations close to 1 g illustrate the significant seismic hazard in areas around active faults in the region and the need to adapt the building codes to these rare but potentially highly destructive types of earthquakes.

Crustal Dynamic Investigations in the Central Andes Using GPS

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.

Interseismic strain accumulation measured by GPS in the seismic gap between Constitución and Concepción in Chile

[1] We have proposed earlier a new tectonic model for the evolution of the Andes mountain belt as a bivergent orogen. Here, to reply to a comment by Astini and Davila [2010], we discuss briefly the protracted diachronic evolution (over tens of million years) by propagating deformation at the large‐ scale (over 10–10 km), its influence on basin formation in the back‐arc region (retroarc foreland basin), and the mechanical implications of the bivergence in the tectonics of the fore‐arc region, particularly the possible effects of the underthrusting of the coastal crustal‐scale rigid block (the Marginal Block) beneath the West Andean Thrust. [2] The comment by Astini and Davila [2010] criticizes our new model presented by Armijo et al. [2010] suggesting that theAndes is a fundamentally bivergent (or doubly vergent) orogen and defends the conventional model of Andean orogeny, which we think obsolete, involving crustal shortening only by development of retroarc thrusts in the back‐arc region (e.g., as discussed, among many others, by Isacks [1988]). Our model is based on the structural study of the San Ramon Fault system and the Principal Cordillera at the front of the western flank of the Andes, which is used to characterize the crustal‐scale West Andean Thrust (WAT), a major fold‐thrust system in the fore‐arc region, synthetic to the subduction zone. Astini and Davila raise cursorily a large number of important questions, which cannot be fully addressed in this reply. [3] To summarize the main argument: (1) Astini and Davila think that according to the critical taper wedge model [e.g., Davis et al., 1983], the dominant thrusting of the Andes cannot have shifted from an initial westward vergence to an eastward vergence, as we propose; (2) they claim that the well‐known eastward (cratonward) migration of thrusting associated with foreland basin formation in Argentina cannot be explained if the initial dominant thrusting of the Andes was in the fore arc, with westward directed vergence; and (3) Astini and Davila argue that no continental block in the fore‐arc region, as the Marginal (or Coastal) Block that we have defined, can be considered as a western foreland of the Andes because there is no well‐ developed western foreland basin. The miscellaneous final remarks by Astini and Davila express their doubts that our model may fit currently accepted models of mass transfer, sediment flow across orogens, and tectonic‐climatic forcing because according to our model, a major topographic slope would have been created to the west of the Andes (a feature that as anyone can check is not hypothetical, but a matter of fact). Last (but not least), the Andes at 33.5°S latitude would not be in an early stage of its evolution (as we claim) because sediments in the Argentinean foreland record its development since more than 20 Ma. Our reply, intended to identify first‐order conflicting issues, is as follows: