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Title: Seismotectonics of the southern subduction Chilean margin revealed by recent aftershock sequences
Author: Agurto Detzel, Hans
ISNI:       0000 0004 2729 4547
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2012
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Subduction margins, as in the case of south-central Chile, are active seismotectonic environments and locus of the world largest earthquakes. In this thesis, two segments of the south-central Chilean subduction margin are studied: (A) the southernmost portion, at the termination of the Nazca-South America convergence (~46ºS), and (B) the segment located between 34º-38ºS, where the Mw 8.8 Maule Earthquake took place in 2010. Analysis of data from a local seismic network deployed in 2004-2005 in area A, indicates low levels of background seismicity with magnitudes ranging 0-3.4 Ml. The seismicity corresponds to shallow crustal events, mostly occurring within the upper 10 km. A third of the seismicity is associated to volcanic activity present in the area, while scarce seismicity is associated with a large strike-slip fault, the Liquiñe-Ofqui Fault System (LOFS), that intersects the region along the arc in a N-S-trend. In 2007, this region was affected by a seismic sequence with a peak of activity associated with a Mw 6.2 earthquake in April that year. A local seismic network was deployed after this main event in order to study its sequence of aftershocks, which provided a unique opportunity to characterise seismotectonically this area that usually lacks intermediate magnitude seismicity, including the calculation of a new local velocity model, accurate aftershock locations and computation of focal mechanisms. The results show P-wave velocities of ~5 km/s for the upper 5 km in accordance with the geology of the area, and low S-wave velocities for the upper 3 km of crust due to rock fracturing and the presence of fluids. An average Vp/Vs ratio of 1.76 was calculated for the region. The alignment of most of the aftershocks within the LOFS plus obtained focal mechanisms, indicate that this sequence had tectonic origin related to the re-activation of the LOFS. Further, a maximum seismogenic depth of about 15 km was determined for the entire region. Regarding area B, affected by a large megathrust earthquake in 2010, the study of moment tensor solutions for the sequence of aftershocks provided new insight into the distribution of postseismic activity relative to co-seismic slip and the release of seismic afterslip. Thrust aftershocks dominate the postseismic activity, but also normal faulting was detected in the outer-rise area and in the overriding plate near the coastline. The largest seismically released afterslip is located between the two main patches of co-seismic slip. Large aftershocks (M>4) occur along the megathrust interface, in zones of intermediate co-seismic slip associated to stress introduced on dislocation tips with high co-seismic slip contrast. On the other hand, smaller events (M<4) tend to occur in areas of large co-seismic slip, and might indicate a more diffuse distribution within the damage zone of the megathrust plane. It is likely that these smaller events are associated to secondary processes (fluid release, re-activation of secondary structures). Although belonging to the same subduction margin, the seismotectonics and earthquake patterns of the two areas investigated here show different underlying tectonic regimes. For the northern area, locus of the 2010 Mw 8.8 Chile earthquake, inter-plate thrust seismicity is dominant both in term of quantity of events and moment release. Conversely, the southern area presents only shallow intra-plate crustal seismicity mainly occurring in the arc, where Quaternary volcanism and the LOFS are present.
Supervisor: Rietbrock, Andreas; Kusznir, Nick Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QE Geology