The Fine Structure of a Seismogenic Zone Revealed by Aftershocks of the 2014 Iquique Mw 8.1 Earthquake
Final Report Abstract
The main objective of the project was to further our understanding of the relationships between great earthquakes, the structure of erosional convergent margins and postseismic relaxation processes using the earthquake sequence along the Northern Chilean margin using late aftershocks from two years of OBS and land data of the 2014 Mw 8.1 Iquique earthquake. This sequence provides an opportunity to image the aftershock activity in great detail and resolve the structure of marine forearc. We used data from two long-term marine deployments of 14 OBS stations and stations from the permanent land stations. The first 18 months of the reporting period were focused on solving technical issues to detect and locate the dense aftershock sequence, necessary because of the high number of events and stations. We applied automated P and S-picking algorithms to determine phase picks. All automated picks on the OBS stations were manually revised. This phase resulted in a high-resolution local event catalogue. The aftershock seismicity in the upper crust is related to normal-faulting (observed by MCS data) close to the updip end of the 2014 Iquique earthquake. This observation is interpreted to be related to extension from erosional processes close to the plate interface during the postseismic phase. Most focal mechanisms at the seismogenic updip end indicate thrust faulting with one focal plane oriented subparallel to slab dip. In the last year of the project, we focussed on the estimation of slip from repeating events using 13 years of land data, allowing to observe slip in-situ in the marine forearc. The additional benefit of the two years of OBS data is that this allows assigning highresolution locations to the repeating events, even when they occurred before OBS deployment. Although the OBS were installed two years, this allows assigning accurate hypocenter locations to events before or after the OBS deployment. The estimation of high-resolution locations effectively enables us to distinguish seismic slip between the upper plate and plate boundary for the 13 years of the observation period. To our knowledge, this was not done previously since long-term seismicity land network, and OBS data are needed. Upper plater repeating earthquakes are spatially correlated to large extensional faults in the upper plate imaged by MCS data and multibeam bathymetry. The along-strike aligned upper plate repeater sequences suggest accelerating cumulative slip in the overriding plate following two strong plate interface events ten days before the mainshock. For other times, the slip partitioning in the upper crust and interface is more balanced. These observations might indicate that gradual unlocking of the plate interface forces the reverse reactivation of upper plate extensional faults causing subduction erosion. In general, we could archive the objectives from the DFG proposal. In fact, the aftershock seismicity months after the mainshock is still significant and contributes to a better understanding of subduction earthquake processes. This project, together with two years of OBS data, provided an exceptional starting point for the PhD study.
Publications
- (2019) Measuring tectonic seafloor deformation and strain-build up with acoustic direct-path ranging. Open Access Journal of Geodynamics, 124. pp. 14-24
Petersen, F., Kopp, H., Lange, D., Hannemann, K. and Urlaub, M.
(See online at https://doi.org/10.1016/j.jog.2019.01.002) - (2021) Analysis of seismic and aseismic deformation using shorelinecrossing observations. Open Access (PhD thesis), Christian-Albrechts-Universität, Kiel, Germany, XI, 217 pp.
Petersen, F.
- (2021) Relationship between subduction erosion and the up-dip limit of the 2014 Mw 8.1 Iquique earthquake. Open Access Geophysical Research Letters, 48 (9). e2020GL092207
Petersen, F., Lange, D., Ma, B., Grevemeyer, I., Geersen, J., Klaeschen, D., Contreras- Reyes, E., Barrientos, S., Tréhu, A. M., Vera, E. and Kopp, H.
(See online at https://doi.org/10.1029/2020GL092207)
