A sanity check for earthquake recurrence models used in PSHA of slow deforming regions: the case of SW Iberia

Abstract

Abstract. Probabilistic Seismic Hazard Assessment (PSHA) is the most common tool used to decide on the acceptable seismic risk and corresponding mitigation measures. We propose two consistency tests to address the variability of earthquake generation models found in PSHA studies: i) one rule-of-thumb test where the seismic moment release from the model is converted to an average slip on a typical fault and compared with known plate kinematics or GNSS deformation field; ii) using a neotectonic model, the computed deformation is converted into seismic moment release and to a synthetic earthquake catalogue. We apply these tests to the W and SW Iberia slow deforming region, where two earthquake source areas are investigated: 1) the Lower Tagus Valley, one of the largest seismic risk zones of Portugal; and 2) the offshore SW Iberia area, considered to be the source for the 1st November 1755 event (M~8.7). Results show that some of the earthquake source models should be considered as suspicious, given their high/low moment release when compared to the expected values from GNSS observations or neotectonic modelling. In conclusion, PSHA studies in slow deforming regions should include a similar sanity check on their models' evaluation, downgrading the weight of poorly compliant models.

Interseismic Strain Accumulation Near Lisbon (Portugal) From Space Geodesy

“…A block to the West of Lisbon, sampled by three GNSS sites, moves with respect to the surrounding region on both sides with a relative velocity of 0.96 ± 0.20 mm/yr, possibly as the result of small‐scale lateral extrusion. These results support the hypothesis that the Lower Tagus Valley is the locus of an active first‐order crustal fault which drives the observed simple shear surface deformation, while contradicting the widespread view (e.g., Ramalho et al., 2020) that the LTV faults are too slow to contribute with a major parcel to the seismic hazard of the region. The impact of intraplate faults in the seismic hazard of the LMA may therefore be more important than currently assumed.…”

“…(2015), Ramalho et al. (2020) obtained $${\dot{M}}_{0}^{seis}$$ of 8.69 × 10 16 , 1.33 × 10 17 , and 1.77 × 10 17 Nm/yr, respectively. These values compare well with our geodetic estimate of the moment‐rate, especially the central value, suggesting a strong seismic coupling of the area.…”

“…Taking as reference the source area used by Woessner et al (2015) to account for the seismicity of the Lower Tagus Valley (Figure 1b), and using 15 km for seismogenic thickness and 30 GPa for crustal rigidity we estimate a value of 𝐴𝐴 Ṁ𝑀𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔 0 = 1.35 × 10 17 Nm/yr for the scalar geodetic moment-rate (see the Supporting Information). Ramalho et al (2020) reviewed the probabilistic seismic hazard assessment of Woessner et al (2015) and estimated 𝐴𝐴 Ṁ𝑀𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 0 from the Gutenberg-Richter parameters for the same source area, so a direct comparison between the two results can be performed. For the three values of maximum magnitude (7.1, 7.4, and 7.6) considered by Woessner et al (2015), Ramalho et al (2020) obtained 𝐴𝐴 Ṁ𝑀𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 0 of 8.69 × 10 16 , 1.33 × 10 17 , and 1.77 × 10 17 Nm/yr, respectively.…”

“…Ramalho et al (2020) reviewed the probabilistic seismic hazard assessment of Woessner et al (2015) and estimated 𝐴𝐴 Ṁ𝑀𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 0 from the Gutenberg-Richter parameters for the same source area, so a direct comparison between the two results can be performed. For the three values of maximum magnitude (7.1, 7.4, and 7.6) considered by Woessner et al (2015), Ramalho et al (2020) obtained 𝐴𝐴 Ṁ𝑀𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 0 of 8.69 × 10 16 , 1.33 × 10 17 , and 1.77 × 10 17 Nm/yr, respectively. These values compare well with our geodetic estimate of the moment-rate, especially the central value, suggesting a strong seismic coupling of the area.…”

“…On one hand, in view of the modest NW Africa–SW Iberia convergence rate (∼4 mm/yr in a NW–SE direction; Fernandes et al., 2003), it has been argued that most of the cumulated crustal deformation is fully released by 1969‐type offshore earthquakes of the Gulf of Cadiz, implying that intraplate faults account for very small slip‐rates. It follows that destructive intraplate earthquakes are deemed very rare events with limited contribution to the probabilistic hazard (e.g., Ramalho et al., 2020). This view is supported by very low intraplate slip‐rate estimates derived from geological studies (0.005–0.3–0.5 mm/yr; Cabral, 2012).…”