Investigation of Deep Seismic Reflections Beneath the Accretionary Prism of the Chile Subduction Zone in South-Central Chile
Abstract
Wide aperture, multi-channel seismic data from the 2017 CEVICHE (Crustal Examination from Valdivia to Illapel to Characterize Huge Earthquakes) experiment were used to investigate variation in the characteristics of seismic reflections along-strike of the south-central CSZ (Chile subduction zone) 40-120 km east of the trench. CEVICHE data were acquired from the R/V Marcus G. Langseth using its 15.1-km long, 1212-channel streamer and 6600 in3 tuned airgun array. Strong reflections are found intermittently in the data from around 7 to 10s two-way travel time along 1000 km between 30°S and 44°S. These reflections appear to be located at the expected depth of the plate boundary beneath the continental upper slope and shelf of the CSZ. Many of the deep reflections observed in this time range occur in pairs about 2s apart, suggesting that these reflections originate at the top and bottom of the subducting oceanic crust of the Nazca Plate. Previous studies have suggested that strong reflectivity in the lower forearc crust is caused by sharp porosity contrasts in regions of high strain, representing heavily sheared rocks. In addition to noting where lower crustal reflections are observed, we examine the change in the background noise level along the survey lines to quantify their possible masking effect on signals from the lower crust. Ongoing data analysis will help better understand the relation between observed deep reflections and their locations along the subduction zone, and determine the degree to which irregular signal strength is affected by the geology of the overlying accretionary prism and background acoustic noise. The common occurrence of large earthquakes on the south-central Chile margin makes this region a natural laboratory for studying subduction zone mechanics and the relationship between slip during large earthquakes and geologic structure. This study aims to determine whether there is a correlation between large slips during the 1960 and 2010 earthquakes and lower crustal reflectivity. The relation of reflectivity characteristics and earthquake behavior at the CSZ can serve as an analog when studying quieter subduction zones such as the Cascadia subduction zone, where relatively infrequent large seismic activity makes it difficult to resolve the impact of earthquakes on structure.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.T21F0276Z
- Keywords:
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- 8104 Continental margins: convergent;
- TECTONOPHYSICSDE: 8170 Subduction zone processes;
- TECTONOPHYSICS