Download amphibious experiments spoc (subduction processes off chile)

UNIVER SIDAD DE CONCEPCIÓN
DEPARTAMENTO DE CIENCIAS DE LA TIERRA
10° CONGRESO GEOLÓGICO CHILENO 2003
AMPHIBIOUS EXPERIMENTS SPOC (SUBDUCTION PROCESSES OFF
CHILE) IMAGE 1960 CHILE EARTHQUAKE AREA
KRAWCZYK, C.M. 1, STILLER, M. 1, MECHIE, J. 1, LÜTH, S. 2, WIGGER, P. 2, REICHERT, C. 3,
ONCKEN, O. 1, BATAILLE, K. 4 & SPOC RESEARCH GROUP
1
GFZ Potsdam, Telegrafenberg, D-14473 Potsdam, Germany
email: [email protected], [email protected], [email protected], [email protected]
2
Free University Berlin, Malteserstr. 74-100, D-12249 Berlin, Germany
email: [email protected], [email protected]
3
BGR Hannover, Stilleweg 2, D-30655 Hannover, Germany
email: [email protected]
4
Universidad de Concepcion, Concepcion, Chile
email: [email protected]
INTRODUCTION
Convergent continental margins are the Earth's principal locus of important earthquake hazards.
Some 90% of global seismicity and nearly all interplate megathrust earthquakes with magnitudes
>8 occur in the seismogenic coupling zone between the converging plates. Despite the societal,
economic, and scientific importance associated with the coupling zone, the processes that shape
it and its relation to surface deformation are poorly understood. Seismogenic coupling zones
occupy a limited depth range of convergent plate interfaces between 5 to 10 km depth at the
updip end and 30 to 60 km at the downdip end (e.g. Tichelaar & Ruff 1993). The location of
rupture nucleation and the distribution of slip are probably mainly constrained by stronger
asperities or a variation of the material state that control the strength of the otherwise rather
weak coupling zone and subduction channel (Ruff 1999, Pacheco et al. 1993). The related extent
and degree of seismic coupling plays a major role in the generation mechanism of great
interplate earthquakes (e.g. Hyndman & Wang, 1993).
The vision of our integrated study is a quantitative understanding of megathrust earthquake
seismicity in subduction zones and its relation to processes at depth and at the surface. We have
started with a series of experiments in the area of the 1960 Southern Chile earthquake that are
designed to image the processes operating at the seimogenic plate interface and their effect for
surface deformation. These experiments – the first integrated marine experiment (SPOC:
Subduction Processes Off Chile) completed in early 2002 - are planned in national and
international cooperation within the scope of the SALT project (South American Lithospheric
Transect) under the auspices of the ILP program. A series of field and lab studies will (1) image
a complete seismogenic plate interface from the updip to below the downdip end, in order to (2)
yield key petrophysical and mechanical properties. We will also (3) test the variation of
properties along different segments of a plate interface which are at different stages of the
seismic rupture cycle, and (4) observe and model the surface response to seismic rupture and
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identify the controls of hazard distribution. Here, we report first results of project SPOC
(Subduction Processes Off Chile) and the scope of future plans within project IMTEQ (Interplate
MegaThrust Earthquake Processes).
The southern Chilean convergent margin provides a first test site and natural laboratory for our
studies. Here, the largest instrumentally recorded earthquake occurred in 1960 (Mw = 9.5). It
ruptured the margin starting at 38°S at a hypocentral depth of some 30 km below the continental
forearc towards the south for approximately 1000 km (Cifuentes 1989) with a coseismic slip of
up to 40 m, up to 2 m vertical displacement and a tsunami up to 15 m high that affected the
entire Pacific (Kanamori & Cipar 1974, Plafker & Savage 1970). Recent GPS data reveal this
part of the upper plate to still be in the post-seismic relaxation stage. The unusual width of the
seismogenic coupling zone in this area with a downdip end well inland allows observation of this
part of the plate interface with onshore based experiments, while most of the ruptured surface is
situated beneath the offshore forearc (Diaz Naveaz 1999).
EXPERIMENT SETUP
The first component of the study, the ship-borne integrated geophysical experiment SPOC with
R/V SONNE, took place in fall and winter 2001/2002 (operated by The Federal Institute for
Geosciences and Natural Resources, in cooperation with GEOMAR and the Berlin-Potsdam
Andes research group, SFB 267). It will yield a near 3D image of the offshore forearc including
the updip parts of the seismogenic coupling zone of the great 1960 Chile-earthquake segment
and the seismic gap segment to the north (Reichert & SPOC Scientific Shipboard Party, 2002).
The offshore profiles reveal that the slope area in the region is overprinted by a faint lineation
pattern with a dominant azimuth of some 120 degrees correlating with onshore structures. The
upper plate is split into many segments with pronounced forearc basins and strikingly narrow
accretionary wedges. The relatively thick trench fill of up to more than 2.000 m seems to be
subducted through a thick subduction channel, thereby suggesting a non-accretionary subduction
mode here.
Between 36° and 39° S, a combined onshore-offshore, active-passive seismic experiment was
carried out linking the marine profiles to the subduction features observed onshore. It comprised:
(1) a 3D-wide angle reflection/refraction component simultaneously recording the airgun pulses
from the R/V SONNE with 32 3-component stations deployed in an array and 50 stations along 3
W-E profiles; (2) recording of explosive shots fired at the ends of the three profiles; and (3) an
additional pilot seismic reflection experiment covering the coastal onshore-offshore transition at
the southern E-W line in order to provide a complete link between the offshore dataset and the
landbased experiments with the first complete high resolution coverage of an entire seismogenic
plate interface (see also Lüth et al., Stiller et al., this volume).
RESULTS
The first results from the near-vertical incidence reflection (NVR) seismic experiment
component (located at 37° 15`S) yields an image of the part of the subduction zone between the
S-America and Nazca-Plate that is located in the offshore-onshore transition zone. The 54 km
long line recorded the offshore profile shot by the R/V SONNE with the airgun array and a series
of shots in the Pacific Ocean, and on land resulting in a 45 km long 2-fold CDP line, and a
single-fold coverage along 72 km profile length. The data processing gives an image of different
reflection bands in the upper and middle crust. On the entire profile, a c. 2-km-thick strong
reflection band is observed between 5 and 10 km depth, which shows almost no dip. On the
western half of the profile, prominent reflections dip eastward from c. 15 km down to c. 30 km
depth. Finally, in the central part of the seismic reflection profile, some relatively weaker
reflections are found between 30 to 45 km depth (SPOC Research Group (onshore), 2003).
Those reflections found between 16-42 km correlate with Wadati-Benioff seismicity and are
interpreted as imaging the top of the downgoing plate. In the central part of the profile, a break in
reflectivity located below the axis of the coastal cordillera coincides with the intersection
between oceanic plate and continental Moho, and also correlates with the downdip end of the
seismogenic plate interface defined by geodetic modelling. These new seismic data provide the
geometry of the subduction zone in the area, and hence we suggest the relocation of the 1960
Chile earthquake at 73°05’ W.
The SPOC Working Group (onshore)
M. Araneda 1, K. Bataille 2, J. Bribach 3, A. Cser 4, C.M. Krawczyk3, S. Lüth 5, S. Martin 6, J.
Mechie 3, L. Rabenstein 5, W. Schnurr 5, M. Stiller 3, P. Wigger 5
1
SEGMI (Santiago, Chile)
2
Universidad de Concepcion (Concepcion, Chile)
3
GeoForschungsZentrum Potsdam (Germany)
4
Universidad de Concepcion (Los Angeles, Chile)
5
Free University Berlin (Germany)
6
Potsdam University (Germany)
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