Abstract ISAG 2008 Ingles

7th International Symposium on Andean Geodynamics (ISAG 2008, Nice), Extended Abstracts: www-www

0BRelationship between Topography and Seismicity in the Peruvian Andes

V. Manuel Uribe 1, Laurence Audin 2, Hugo Perfettini 3

1 Universidad Nacional Mayor de San Marcos, Av. Venezuela 3400, Lima 1, Peru ([email protected])2 Institut de Recherche pour le Developpement, Teruel 357, Lima 18. ([email protected])3 Institut de Recherche pour le Developpement, Teruel 357, Lima 18. ([email protected])

KEYWORDS: Seismicity, Topography, Inter-seismic period, Lithosphere period, Tectonic yield stresses.

1BAbstract

Seismicity in Peru and the Andean Orogeny has a common origin that is the subduction process between the

Nazca Plate and the South American Plate, even though both of them differ in time scale. During the inter-

seismic period the seismicity has a very complex spatial distribution. We observed that the seismicity correlates

with the topography where both plates encounter in the fore-arc zone. This shows a compressive stage becoming

extensional in the sub-Andean zone. This ends very sharply between both zones, specifically beneath the high

Andes (>2000m). We propose the lithosphere weight related to the high Andes modifies the tectonic yield

stresses state producing an increase in the vertical stress. Also, it produces the tectonic yield stress compensation

generated by the subduction process. As result the latter causes disappearance in the seismicity beneath the high

Andes.

2BIntroduction

The seismic activity in Peru is heterogeneous with a wide-ranging spatial and time distribution. The western

edge of the South American Plate (the Peruvian fore-arc) shows compressive seismic activity which it is a

typical characteristic of active subduction zones. In the sub-Andean zone, the seismicity has a tendency of

compressive faults in the crustal surface and extensional faults in the plates contact (60-300 Km). Between such

zones the Cordillera of the Andes is present, where beneath of it seismicity decrease is noticed or even it

disappears.

What occurs beneath the central high Andes? Is the topography related to the change in the seismicity type found

in the convergence boundary: inverse seismicity in the fore-arc and normal seismicity in the back-arc? Will the

field of stresses affect the seismicity decrease as Bollinger et al. (2004), demonstrated in the Himalayas

Cordillera?

The Cordillera of the Andes shows a well-defined 3D structure, but we took in consideration a 2D first approach

using transversal sections to it.

For comparison of seismic parameters, we used seismic local data set provided by the Peruvian Institute of

Geophysics (IGP) (F. Grange data set) such as focal mechanisms and Tele-seismic data set provided by the

University of Harvard (CMT data set).

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3BData Gathering

Seismicity data were taken from two data sets: the Peruvian Seismic Net for the period 1982-2005 and its 2007

update. We added the corrections made by Engdahl & Villaseñor (2002). The IGP- Engdahl data set has 34088

seismic events with magnitude range between 1-7.7 ML. The second data set was taken from F. Grange (1984),

who implemented a dense temporal net (~43 local stations) for the period 1980 – 1981. We chose 888 events in

total with superficial and intermediate depths (<300 Km) to show clearly the seismic activity between 13°30’S

and 17°30’S, because these geometry variations in this zone represent the slab variant.

We used two data sets to analyze the focal mechanisms. Tavera & Bufom (2001) create a data set used to study

and analyze earthquakes in Peru during 1990-1996. They took as reference 19 characteristic earthquakes in

zones of high seismicity and these correspond to average hypocenter parameters in the main seismogenic zones.

The second data set is the Harvard Centroid-Moment-Tensor (CMT). CMT solution gives the spatial and time

location, Centroid depth, fault rupture orientation (focal mechanism), the seismic moment and the geometry

parameters of the fault.

We used the global seafloor topography provided by Sandwell & Smith (1997) for topography modeling. They

gathered satellite altimeter data and ship depth soundings. The contour interval resolution was 50 m.

Discussion

Bollinger et al. (2004), showed the influence of the topography in the seismicity behavior during the inter-

seismic period in the Himalayas. He analyzed seismic data and modeled the stresses state. Our study tries to

demonstrate whether these parameters have a similar relationship in the Cordillera of the Andes during the inter-

seismic period. The Magnitude of completeness was Mc= 3.9 ML for the IGP data set. We did a decluster

process of the dependent events or aftershocks. We can show clearly the main parameters and spatial distribution

of the earthquakes from the analysis of seismic sections perpendicular to the Peru-Chile Trench. The seismic

activity was separated in three zones: 1) Fore-arc zone, where the focal mechanisms showed a compressive

tendency (feature commonly found in active subduction zones) and this compressive regime dominates the

stresses state. It generates that the first stress component (σ1) has an orientation ENE-WSW to E-W along the

convergence boundary. However, we observed sections with extensional focal mechanisms produced by

tensional stresses in two zones: In the Nazca Plate close to the trench and near the coast. Those close to the

trench are few and are associated to the Nazca Plate bending when goes down the South American Plate. 2) Sub-

Andean zone (<60 Km) and the Amazonic flat terrain probably (this is no sure because of the lack of seismic

data). Here, focal mechanisms show reverse/thrust with orientation E-W and ENE-WSW parallel to the Andes

where the first stress component (σ1) has an orientation E-W similar to the fore-arc zone. This compressive

process is associated to the Brazilian convergence shield beneath the Eastern Cordillera (Suárez et al., 1983). 3)

Intermediate depths (60-300 Km) beneath the sub-Andean zone where normal faulting is observed in focal

mechanisms. This is due to the “detachment” or separation of the oceanic slab by the gravity effect. The slab

subductes toward the inner mantle and its surface (Nazca Plate’s) blends generating tensional stresses. Their axis

are oriented NW-SE.

The produced stresses by the plates convergence and the presence of the Cordillera of the Andes play a key role

in seismicity generation. In the fore-arc and back-arc zones exist continuity in the seismicity where thus there are

the main seismogenic zones in Peru. However, beneath the high Andes exists a sharply disappearance in the

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seismicity following its irregular geometry along the mountain chain (>2000m). Bollinger et al. (2004), showed

the same fact in the Himalayas. In the fore-arc and back-arc (<2000m), the stresses state has the principal

component σ1 in compression which is perpendicular to the Andes. Both zones present lithospheric volumes less

than 41.5 Km thick to 30 Km to the coast (Manrique, 2003), in comparison to the high Andes (>2000m) where

the crustal volume increases (54.7 Km thick to 200 Km to the coast). In the high Andes case, the principal tensor

stress component σ1 is the vertical component (σ2) and σ3 is the compressive stress component in the horizontal.

Because the vertical stresses increase with the lithosphere weight, therefore at greater depths the vertical stress is

larger (Turcotte & Schubert, 2002). So, when it increases in the high Andes as it equals the compressive

horizontal stress, the tectonic process becomes extensional and this stress has a direction almost parallel to the

convergence boundary. Hence, the lithosphere weight (σz) as the high Andes pushing over the stress field

produces the compensation among the compressive horizontal stresses (which generate the seismic activity),

where the two main stresses (vertical stress: topography, and horizontal stress: compression) compete with each

other. To show in a better way how the topography model induces the seismicity decrease, we have quantified

the vertical stress deviation produced over the 2000m level in ~53 MPa. This result is similar to the value

determined by Bollinger et al. (2004), in the Himalayas (~35 MPa). Now, if we take ~35 MPa as the stresses

deviation value, ~1300m would be the height where this variation is produced. This height is in the acceptable

range to produce the seismicity decrease.

Conclusion

The Andean topography is the consequence of stresses applied for the plate tectonics evolution through the time.

High topography controls the seismic activity generated beneath the Andes. In the fore-arc and back-arc zones

(>2000m), the deformation process has a main compressive component with lithospheric volumes much lesser

than the high Andes (>2000m), where the main tensor stress is the vertical component (σ3). Therefore when the

vertical stress (σ3) increases in the high Andes and overcomes the horizontal stress (σ1), the tectonic process

becomes extensional generating a zone where they are similar in magnitude and compete among them. The

lithosphere weight as the high topography that pushes over the field stress, generates a tectonic compensation

which in turn produces the seismicity decrease generated by the compressive tectonic. The necessary stress

deviation to produce this seismicity decrease is ~53 MPa. This value is similar in magnitude to the modeled by

Bollinger et al. (2004), for the Himalayas (~35 MPa).

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a) b)


Figure 1. Comparison between the Himalayan Model (Bollinger et al., 2004) and the Andean Model. a) Microseismicity recorded between April 1995 and April 2000 by Nepal Seismological Center, Department of Mines of Geology. Only events with magnitude above Ml=3.0. the grey band and red line present the assumed location of the locked portion of the fault and the location of the 3500 m contour line (DEM-Gtopo30/USGS), respectively. B) Recorded seismicity between January 1982 and December 2005 by the Peruvian Institute of Geophysics. The black dots represent superior events to 3.9 Ml. The focal mechanisms correspond to the Tavera & Buforn (2001) . The gray section corresponds to the lock part of the Plates and red line present the location of the 2000 m contour line (Sandwell & Smith, 1997)

References:BOLLINGER L., AVOUAC J.P., CATTIN R., AND PANDEY M.R., 2004. Stress buildup in the Himalaya. Journal of Geophysical Research, 109-B11405MANRIQUE M.O., 2003. Estimación del espesor de la corteza continental en el centro y surdel Perú a partir de fases PmP. Compendio de trabajos de investigación CNDGIGP, Lima-IV: 9p.TURCOTTE D. L. & SCHUBERT G., 2002. Geodynamics. Cambridge University Press, 2002, 406 p.ENGDAHL, E.R., AND VILLASEÑOR A. 2002. Global Seismicity: 1900–1999, in W.H.K. Lee, H. Kanamori, P.C. Jennings, and C. Kisslinger (editors), International Handbook ofEarthquake and Engineering Seismology, Part A, Chapter 41, 665–690.TAVERA H. & BUFORN E., 2001. Source Mechanism of Earthquakes in Peru. Journal ofSeismology, 5: 519-539.SMITH W. H., & SANDWELL D.T. 1997. Global sea floor topography from satellite altimetry and ship depth soundings. Science, 277: 1956-1962GRANGE, F. 1984. Etude sismotectonique detaille de la subduction lithospherique au SudPerou, Ph.D. Thesis, IRIGM, Grenoble, France.

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Figure 2. Model of the topographic profile (referencial scale), geologic profile (scale 1:3) and seismic profile (scale 1:1) showing the lithosphere weight effect of the high Andes (>2000 m). The focal mechanisms correspond to the Centroid-Moment-Tensor (CMT).


SUÁREZ O., MOLNAR P. AND BURCHETEL C., 1983. Seismicity, fault plane solutions, depth of faulting and active tectonics of the Andes of Peru, Ecuador and Southern Colombia. Journal of Geophysical Research, 88: 10403-10428.

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Abstract ISAG 2008 Ingles