Rupture parameter sensitivity of low frequency ground motion response spectra using synthetic scenarios in North Chile

This research performs a sensitivity analysis of response spectrum values for various physical earthquake parameters, which are used to generate synthetic seismograms consistent with the expected seismicity in north Chile. Sensitivity analyses are based on the earthquake scenario and slip distribution model of the 2014, Mw 8.1 Pisagua earthquake, and seven other physically plausible interplate events for north Chile. A finite-fault rupture model, and slip distribution of the Pisagua earthquake, were obtained using inversion of InSAR and GPS data. Three other rupture models based on previous studies of interplate locking for north Chile and capable of generating Mw 8.3–8.6 earthquakes with an estimated maximum slip of 9.2 m, were incorporated in the analyses. Also, four additional scenarios with moment magnitudes in the range Mw 8.6–8.9 were generated by concatenating these physical scenarios into larger rupture areas within the north segment. Using these scenarios, synthetic ground motions were built at four observation sites: Pisagua, Iquique, Tocopilla, and Calama. Response sensitivity was studied for three key rupture parameters: mean rupture velocity, slip rise-time, and rupture directivity. Responses selected were peak ground displacement (PGD), spectral pseudo-velocities, Sv, and spectral displacements, Sd. First and second order variations of PGD, Sv, and Sd relative to the source parameters were computed and used together with a Taylor series expansion to propagate uncertainty into the responses as a function of vr and rise-time tr. To study the effect of rupture directivity, three different foci locations were considered for each scenario: north, south, and at the centroid of the slip model. Response PGD values show no clear trends with rupture velocity, vr; however, the variability increases as the system period increases. The effect of the slip rise-time is significant, and as tr increases, the spectral responses tend to decrease, suggesting that shorter slip rise-times lead to higher seismic demands in long period structures. The results obtained for the directivity analysis suggest that two factors control the expected waveforms and spectral responses: first, the direction of the rupture relative to the location of each site, and the hypocentral distance.