Publication Date:
2016-06-14
Description:
While the northern and central part of the South American subduction zone has been intensively
studied, the southern part has attracted less attention, whichmay be due to its difficult
accessibility and lower seismic activity. However, the southern part exhibits strong
seismic and tsunamogenic potential with the prominent example of the Mw=9.5 May 22,
1960 Valdivia earthquake. In this study data from an amphibious seismic array (Project
TIPTEQ) is presented. The network reached from the trench to the active magmatic
arc incorporating the Island of Chiloé and the north-south trending Liquiñe-Ofqui fault
zone (LOFZ). 364 local events were observed in an 11-month period from November
2004 until October 2005. The observed seismicity allows to constrain for the first time
the current state of stress of the subducting plate and magmatic arc, as well as the local
seismic velocity structure.
The downgoing Benioff zone is readily identifiable as an eastward dipping plane with an
inclination of ∼30◦. Main seismic activity occurred predominantly in a belt parallel to the
coast of Chiloé Island in a depth range of 12–30 km, which is presumably related to the
plate interface. The down-dip termination of abundant intermediate depth seismicity at
approximately 70 km depth seems to be related to the young age (and high temperature)
of the oceanic plate. A high-quality subset of events was inverted for a 2-D velocity
model. The vp model resolves the sedimentary basins and the downgoing slab. Increased
velocities below the longitudinal valley and the eastern part of Chiloé Island suggest the
existence of a mantle bulge.
Apart from the events in the Benioff Zone, shallow crustal events were observed mainly
in different clusters along the magmatic arc. These crustal clusters of seismicity are related
to the LOFZ, as well as to the volcanoes Chaitén, Michinmahuida and Corcovado.
Seismic activity up to a magnitude of 3.8 Mw reveals the recent activity of the fault zone.
Focal mechanisms for the events along the LOFZ were calculated using a moment tensor
inversion of amplitude spectra for body waves which mostly yield strike-slip mechanisms
indicating a SW-NE striking of σ1 for the LOFZ. Focal mechanism stress inversion indicates
a strike-slip regime along the arc and a thrust regime in the Benioff zone. The
observed deformation – which is also revealed by teleseismic observations – suggests a
confirmation for the proposed northwardmovement of a forearc sliver acting as a detached
continental micro-plate.
Type:
Thesis
,
NonPeerReviewed
Format:
text
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