Tectonic deformation of the Andes and the configuration of the subducted slab in central Peru; results from a microseismic experiment

Abstract

SUMMARY A microearthquake survey was conducted in the central Andes of Peru, east of the city of Lima, to study the seismicity and style of tectonic deformation of the Peruvian Andes. Although most of the stations forming the temporary seismographic network were located on the high Andes, the vast majority of the microearthquakes recorded occurred to the east of the mountain belt: on the Huaytapallana fault in the Eastern Cordillera and beneath the western margin of the sub‐Andes. Thus the sub‐Andes appear to be the physiographic province subject to the most intense seismic deformation. Focal depths of the crustal events in this region range generally from 15 to 35 km and some events beneath the sub‐Andes appear to be as deep as 40‐50 km. The fault‐plane solutions of events in the sub‐Andean margin show thrust faulting on steep planes oriented roughly north‐south, similar to that observed in teleseismic earthquakes studied using body wave modelling. The Huaytapallana fault in the Cordillera Oriental also shows relatively high seismicity along a NE‐SW trend that agrees with the fault scarp and the east‐dipping nodal plane of two large earthquakes that occurred on this fault on 1969 July 24 and October 1. Microearthquakes of intermediate depth recorded during the experiment show a flat seismic zone about 25 km thick at a depth of about 100 km. This agrees with recent observations showing that beneath Peru the slab first dips at an angle of about 30° to a depth of 100 km and then flattens following a quasi‐horizontal trajectory. Fault‐plane solutions of intermediate‐depth microearthquakes have horizontal T axes oriented east‐west suggesting slab pull is the dominant force in the downgoing slab.

Seismic structure in southern Peru: evidence for a smooth contortion between flat and normal subduction of the Nazca Plate

“…There is a general predominance (69 per cent) of normal faulting events below ∼80 km depth, as would be expected for earthquakes which occur in the oceanic lithosphere, as these are typically attributed to bending of the slab and/or slab pull (e.g. Schneider & Sacks 1987;Suárez et al 1990). The decreased seismicity northwest of the PG line that was noted for the ISC catalogue data above is also evident here.…”

“…• W, may imply focusing of stress related to the changing slab geometry (Suárez et al 1990). Its shallower depth range, ∼15-45 km above the estimated slab surface at ∼100 km [from teleseismic receiver functions (Phillips & Clayton 2014)], suggests possible upwarping of the slab as a flexural response to resubduction downdip and/or lateral contortion.…”

Mesozoic–Cenozoic Evolution of the Western Margin of South America: Case Study of the Peruvian Andes

“…Near Huancayo, the Huaytapallana fault system uplifted this basement next to the Paleozoic sediments. This fault system is seismically active [4,82,83]. The Paleozoic and Mesozoic strata overlying the crystalline basement are affected by large-scale folding and thrusting.…”

“…In the segments of northern and central Peru, as well as central Chile, the Nazca plate descends at shallow angles to a depth of 100 km with little or no asthenospheric mantle between the plates. As a consequence these regions lack volcanic activity [1][2][3][4][5][6]. The structure of the Andes themselves also undergoes significant changes along strike.…”

Earthquake Hazard Along the Western Coast of South America Inferred From Conditional Probabilities

“…Nazca plate seems to be of complicated tectonic structure (Bilek, 2010) and is subducting underneath the South America plate. The seismic activity is mostly concentrated along the coasts of the Pacific Ocean, where the subduction process takes place and reverse faulting dominates the tectonic regime (Suarez et al, 1990). The relative velocity of the convergence is about 9.3 cm/yr (Casaverde and Vargas, 1984), while there were referred velocities like 8.5 cm/yr (Quezada, 1997) and 9.0 cm/yr (Dewey and Lamb, 1992).The main characteristic of the area is the generation of large to great earthquakes like the widely known event of 1960 with moment magnitude Mw=9.5.…”