The 2007 Pisco earthquake (Mw=8.0), Central Peru: Preliminary Field Investigations and Seismotectonic Context
Abstract
This epicentral area of the 2007 Pisco earthquake marks a major transition in the characteristics of the Nazca subduction zone: 1) the megathrust dip angle is shallower (15-20) to the north than to the south (25-30); 2) megathrust earthquakes have distinctly smaller magnitudes and are more fragmented to the north; 3) the distance between the trench and the coastline changes abruptly from ~180km to the north to ~80km to the south. These variations are likely related to the oblique subduction of the Nazca ridge - a major bathymetric high - beneath the continental margin. The effect of the subduction of the ridge is obvious in the morphology and tectonics of the forearc, in particular, around the Paracas Peninsula where late Pliocene marine formations are uplifted and the forearc tectonic regime changes from compression to extension. The geometry of the coastline reflects the sweeping of ridge beneath the margin. The coastline also seems to relate to the mode of slip along the subduction interface: modeling of the available interseismic GPS data shows that the plate interface was locked at depth shallower than about 50km, with the downdip end of the Locked Fault Zone (LFZ) corresponding approximately to the coastline. However, the resolution of GPS data is not sufficient to test the idea that the coastline morphology mirrors in detail the variation of the downdip edge of the LFZ. We investigated evidence for uplift or subsidence along the coast and found that the coastline didn't experience any significant vertical displacement compared to the tide range (~40cm). The coastline approximately corresponds to a pivot line marking the transition from coastal uplift in the south to subsidence in the north, as the distance from the trench increases. This model is consistent with the co-seismic slip distribution inferred from waveform modeling, and with the distribution of aftershocks which suggests that the subduction interface ruptured mainly updip of the coastline. To place further constraints on the coseismic slip distribution, we have collected data on the spatial extent of Tsunami waves which hit the coast both south and north of the Paracas peninsula. Finally, our field surveys have also revealed evidence for active faulting of the forearc. In particular, the production of coseismic pressure ridges, with up to 50cm of vertical throw suggests that the east dipping Puente Huamani thrust fault system was reactivated over a distance of about 20km during this event. However, we didn't find evidence for reactivation of any of the normal faults on the Paracas Peninsula, although some had been reactivated by the 2006 Pisco earthquake (Mw6.5). Thus, the structure and deformation of the Peruvian forearc and coastline seems to contain important information on lateral variations of seismic and geodetic coupling along the subduction zone.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- Bibcode:
- 2007AGUFM.T33E..02A
- Keywords:
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- 8002 Continental neotectonics (8107);
- 8104 Continental margins: convergent;
- 8123 Dynamics: seismotectonics;
- 8170 Subduction zone processes (1031;
- 3060;
- 3613;
- 8413);
- 8175 Tectonics and landscape evolution