The Lower Tagus Valley Fault, Portugal, has long been associated with the damaging earthquakes that affected the Greater Lisbon Area in historical times. These include a poorly documented earthquake that occurred in 1344, the relatively well-documented 1531 earthquake, and the most recent M6.0 1909 earthquake. In this work, we use a 0.5 m resolution LiDAR-based digital elevation model and a 0.5 cm resolution digital surface model based on UAV photogrammetry to accurately locate the fault scarps in the northernmost portion of the western fault strand and to select sites to perform paleoseimolological investigations. The paleoseismological and geochronological analysis performed in the Alviela trench site document the fault activity in the last 3000 years, including two earthquakes during historical times. We performed ground motion scenarios for 20 km, 40 km, and 60 km ruptures including the trench site. The ground motion fields obtained for the 40 km and 60 km ruptures are in agreement with most macroseismic intensity data available for the 1531 earthquake, implying a magnitude in the range M6.8–7.4. However, the degree of deformation preserved in the trench suggests a value closer to the lower magnitude bound. The intensity level observed in Lisbon in 1531 (IX) is lower than the modeled intensities for all considered scenarios and could be related to a particularly high level of vulnerability of the building stock.
<p>SE Iberia is a tectonically active area with an important history of destructive earthquakes. Some of these earthquakes have been associated with known active faults, but the seismic source of most of them remains unclear. The majority of these earthquakes happened long before instrumental record began, so we can only study them through paleoseismology and/or historical records. In some cases, due to current soil usage, paleoseismic studies are extremely difficult to perform and researchers can only rely on historical records. Such is the case of the 1804 Dal&#237;as earthquake.</p><p>In this communication our objective is double. First, we present a methodology which can be useful to constrain the seismic source of historical earthquakes for which only intensity data are available. And second, we apply this methodology to the 1804 Dal&#237;as earthquake in order to constrain its seismic source, which remains unclear up to this day. Our proposed methodology is a combination of Gasperini et al. (1999, 2010)&#8217;s and de Pro-D&#237;az et al. (2022)&#8217;s methods. Our methodology searches for the faults that are most plausible candidates for the earthquake rupture, then builds seismic scenarios for each candidate rupture and finally compares these scenarios with the observed intensity field in order to find the candidate with the best fit. Seismic scenarios are built using OpenQuake and ArcGIS software (although QGIS can be used as well). The candidate that generates the simulation which better resembles the observed intensity field is considered the best candidate and the one closest to the actual earthquake source.</p><p>For the 1804 Dal&#237;as earthquake, we consider different ruptures along the Loma del Viento Fault (LVF) and Llano del &#193;guila Fault (LLAF) traces as candidate ruptures, including some combined ruptures along the two faults. Our results show that there are two almost equally best candidates: a full rupture of the whole inland extension of the LVF, and a combined rupture of this fault and the LLAF.</p>
Historical earthquakes are of major importance in the analysis of seismic hazards, in particular for stable continental regions. In this article, we propose a methodology that uses seismic scenarios to provide constraints on the location of the seismic source of historical earthquakes. Our methodology involves generating seismic scenarios for the proposed seismic sources and comparing the results to the observed intensity field of the earthquake. To avoid the bias related to strongly heterogeneously distributed datasets, we focus on data points that are useful in discriminating between competing ruptures. These data are identified by the spatial patterns of residuals between seismic scenarios produced for each source. We apply this methodology to a test event—the 1999 Athens earthquake—for which both the magnitude and location are constrained by independent data, and to the 1909 Benavente earthquake, for which the magnitude is constrained by seismological studies, but the location is uncertain due to the very poor azimuthal coverage available. Within its application limits, the proposed methodology was capable of identifying the source of the Athens earthquake amongst different ruptures located few kilometers apart. The analysis performed for the 1909 Benavente earthquake suggests that the eastern strand of the lower Tagus Valley fault zone is the most likely seismic source for earthquake, amongst those proposed in the literature.
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