Disentangling roles of stress, pore pressure and shaking from wavefields recorded during the 2014 Northern Chile earthquake sequence
Final Report Abstract
The combination of the high quality permanent IPOC network with a densified instrumentation in the aftershock period provided a suitable dataset for the investigation of crustal processes associated with great subduction events. Using a variety of seismological methods, we achieved the main aim of our project, i.e. ,the monitoring of possible variations in the subsurface due to a megathrust earthquake. For the study of processes associated with the megathrust event the permanent network proved invaluable. The Pisagua earthquake was by far the strongest perturbation of the crustal system in the analysed time frame in northern Chile. Every observation of temporal changes in properties or dynamics of the system will thus be compared to changes during the main shock in the first place which requires pre-event records as provided mainly by the IPOC network. It was used here to identify repeating earthquakes and investigate the rupture directivity and temporal changes of material properties, emphasizing the need for permanent instrumentation in critical areas. For the earthquake sequence of the 2014 Mw 8.1 mainshock in Northern Chile, which was recorded by the local IPOC network, we (1) identified repeating earthquake sequences and mapped their migration during the foreshock and aftershock phases; (2) detected and quantified variations of seismic velocities by ambient noise methods and coda wave interferometry; (3) calculated stress drops and found weak indications for reduced dynamic stress drop in the rupture area of the mainshock; (4) determined the rupture directivity for a subset of earthquakes, which show a surprisingly consistent orientation with no clear spatial and temporal variations; (5) observed the influence of tidal deformation on seismic velocities. Our main conclusions are: • The velocity structure in the IPOC area shows the strong contrast between the Coastal Cordillera and the segmented Longitudinal Valley which is reflected in the susceptibility to changes of the velocity. • Seasonal variations and co-seismic changes of subsurface velocities are most pronounced along the high velocity rocks of the Coastal Cordillera. • The observation of tidal changes allows for a natural experiment to obtain parameters of the seismic nonlinearity of the subsurface. • Abundant presence of repeating events in the Iquique event sequence. • Temporal and spatial patterns of repeating events reveal an increased occurrence rates updip of the mainshock during the aftershock series. • No signature of a temporally variable influence of fluids in the fault area was found. Technically, we have developed and implemented Python workflows for calculating stress drop and rupture directivity as well as for analysing clusters of repeating events. These can easily be adopted in the future to different seismological data sets. We are aware that the stress drop studies show promise but so far are only limited and preliminary. The very rich seismological data set of the 2014 Northern Chile earthquake sequence has the rare potential to provide more detailed information on the stress state and the fault heterogeneity of a megathrust fault. Analysis of the interferometrically determined velocity changes from the coda of repeating events is ongoing and might yield more information on deep changes, but so far the patterns found are complex and separating surface and deep signals remains challenging.
Publications
- (2017): Estimating Rupture Directivity of Aftershocks of the 2014 Mw 8.1 Iquique Earthquake, Northern Chile, Geophysical Research Abstracts, Vol. 19, EGU2017-15415
J. Folesky, J. Kummerow, F. Tilmann and S. A. Shapiro
- (2017): Rupture Directivity of Aftershocks of the 2014 Mw 8.1 Iquique Earthquake, Northern Chile, using P wave Polarization Analysis, DGG 77. Jahrestagung, Vol. 77, p. 381, SO.B-107
J. Folesky, J. Kummerow, F. Tilmann and S. A. Shapiro
- (2017): Search for repeating events at the plate interface in the seismic sequence of the 2014 Mw 8.1 Iquique earthquake, Chile, Geophysical Research Abstracts, Vol. 19, EGU2017-13953
J. Kummerow, G. Asch, Ch. Sens-Schönfelder, B. Schurr, F. Tilmann and S. A. Shapiro
- (2018): Estimating Rupture Directions from Local Earthquake Data Using the IPOC Observatory in Northern Chile, Seismological Research Letters, Vol. 89, 2A
J. Folesky, J. Kummerow, G. Asch, B. Schurr, C. Sippl, F. Tilmann and S. A. Shapiro
(See online at https://doi.org/10.1785/0220170202) - (2018): Estimating rupture directivity of local earthquake data in central Italy using P-wave polarity stacking, Geophysical Research Abstracts, Vol. 20, EGU2018-8747
J. Folesky, J. Kummerow and S. A. Shapiro
- (2018): Patterns of Rupture Directivity of Subduction Zone Earthquakes in Northern Chile, J. Geophys. Res., Vol. 123, pp. 10785-10796
J. Folesky, J. Kummerow, and S. A. Shapiro
(See online at https://doi.org/10.1029/2018JB016331) - (2019): Multiple Constituents of Solid Earth Tides Observed with Ambient Seismic Noise., DGG 78. Jahrestagung, Vol. 79, p. 166, SO-6.01
T. Eulenfeld and C. Sens-Schönfelder
- (2019): Multiple Constituents of Solid Earth Tides Observed with Ambient Seismic Noise., Seismological Society of America, Annual Meeting
C. Sens-Schönfelder and T. Eulenfeld
- (2019): Probing the in-situ Elastic Nonlinearity of Rocks With Earth Tides and Seismic Noise Phys. Rev. Lett., Vol. 122(13), p. 138501
C. Sens-Schönfelder, and T. Eulenfeld
(See online at https://doi.org/10.1103/PhysRevLett.122.138501) - (2019): Rupture Directivity in the Northern Chilean Subduction Zone , Geophysical Research Abstracts, Vol. 21, EGU2019-5701
J. Folesky, J. Kummerow and S. A. Shapiro
- (2019): Structure of and dynamic processes within the crust of Northern Chile (IPOC area) based on ambient seismic noise correlations and repeating Earthquakes., 25th Latin-American Colloquium of Geosciences
C. Sens-Schönfelder, F. Tilmann, B. Heit and R. Green
- (FU Berlin, 2019): Rupture Propagation Imaging Across Scales: from Large Earthquakes to Microseismic Events
Jonas Folesky
