Group A, Poster #175, Ground Motions

3D ground motion simulations for the 1755 Lisbon earthquake

Anupam Patel, Kim B. Olsen, Te-Yang Yeh, & Susana Custodio
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Poster Presentation

2023 SCEC Annual Meeting, Poster #175, SCEC Contribution #13190 VIEW PDF
This study focuses on simulating a significant hazard scenario for Portugal: a potential recurrence of the 1755 Lisbon earthquake, renowned for its destructive impact on Lisbon and other coastal areas in SW Portugal. Various offshore tectonic structures have been proposed as potential sources for the event, but despite its historical significance, the fault(s) and magnitude involved in the 1755 event remain subject of debate. Toward a better understanding of the causative fault(s) and magnitude for the 1755 event, we have carried out simulations of 0-1 Hz wave propagation for 30+ hypothetical earthquake scenarios on several fault structures proposed for the Lisbon 1755 earthquake. The simula...tions use the discontinuous-mesh 4th-order staggered grid finite difference method AWP-ODC with support for surface topography and kinematic finite-fault source descriptions. We use the recent PRISM3D reference model for Iberia (Arroucau et al., 2021) for our scenario simulations, validated against data recorded from a 2018 M4.3 event near Lisbon. In our validation, we tested whether the addition of a shallow geotechnical layer and small-scale heterogeneities improved the fit to data. We find that a geotechnical layer with a tapering depth of about 350 m appears to provide the best-fitting results. In addition, small-scale heterogeneities with standard deviation of σ= 5 percent relative to the background model, a vertical correlation length of 1000 m, and horizontal-to-vertical anisotropy of the correlation length of 5 improves the fit between the synthetic and observed waveforms. Finally, anelastic attenuation for S-waves using QS = 0.1 VS (VS in m/s) provides the optimal waveform fit to the Mw4.3 event. The study reveals that most of the single fault source for the 1755 Lisbon earthquake is insufficient to match the MMI pattern from the earthquake, except Mw8.3 on Guadalqavir Bank Fault, which gives a similar MMI pattern. On the other hand, the wavefield from a Mw8.3 event on the Marques de Pombal Fault combined with that from a (likely triggered) Mw6.5 earthquake on the Lower Tagus Valley Fault provides a reasonable fit to isoseismal data from the earthquake.