Full waveform inversion of OBS data of a 350 km long profile in the Pisagua/Iquique earthquake region

Abstract:

The Pisagua/Iquique earthquake with a magnitude of M_W 8.1 occurred in a large seismic gap along the northern Chilean margin in 2014. The full segment had ruptured in 1877 and the southern part remains in a locked state. With the PICTURES (Pisagua/Iquique Crustal Tomography to Understand the Region of the Earthquake Source) seismic experiment we intend to gain insight into why the southern segment had not ruptured during the earthquake. The 2016 experiment consisted of 69 ocean bottom (OBS) and 50 onshore seismic stations and multiple seismic profiles were recorded.

Along an extended north-south line of 350 km length, located approximately 65 km east of the trench, 20 OBS stations were deployed and covered by 1,125 airgun shots. We want to study lateral changes in the crustal structure of the marine forearc across the boundary associated with the rupture zone. In a first step travel time inversion was applied to the OBS data. To gain more insight into the elastic parameter distribution along this boundary, we choose a subpart of the model for full waveform inversion (FWI) which is 25 km deep and 330 km long. It includes 12 OBS stations and more than 1,000 shots with a spacing of 300 m.

We choose a simulation time of 20 s with a time step of 1 ms for modeling and a spatial discretization of 20 m. We use hydrophone data and apply minimal preprocessing, including bandpass filtering and a 3D-to-2D transformation. The lowest frequency included is 3 Hz. For inversion we simulate synthetic seismograms by applying visco-acoustic modeling. Only inversion for the P-wave velocity is carried out, while the density and attenuation models remain unchanged. We suppress updates in the water column and at OBS positions. Additionally, we amplify updates at greater depth and utilize horizontal spatial smoothing. We use a workflow with increasing time- and offset-windows to improve the convergence of the inversion.

The data fit is improved significantly during inversion and a good agreement of the inverted source time functions is achieved. Final P-wave velocity models show a prominent structure in 5-10 km depth with a dip in the region of the gravity change. Further north the reflector cannot be observed as clearly. Furthermore, the shallow structure is updated and provides much more detail compared to models derived from travel times only.