Intraplate Earthquakes Near Lisbon, Portugal and the 1755 Conundrum
The high intensities (MMI 9-10) of the 1755 earthquake in and around Lisbon – at an epicentral distance of ~250km as per tsunami-travel-time constraints – have supported very large magnitude estimates, but the search for a suitable source has been inconclusive till now. Ever since 1949, when B. Gutenberg and C.F. Richter suggested “a magnitude between 83/4 and 9” on the basis of “the enormous area perceptibly shaken”, magnitude estimates remained at that level (M8.7 in Johnston, 1996, M8.5 in Martinez-Solares and Lopez-Arroyo, 2004, M>9 in Oliveira, 2008) despite the fact that the scrutiny of historical sources led to a substantial reduction of the estimated felt area. The robust methods for the analysis of historical macroseismic data developed in the last years of the 20th Century (e.g., Bakun and Wentworth, 1997) were not applied to the vast dataset available for this iconic earthquake until recently. When this was done, the outcome was significantly lower, at M7.7+/-0.5 (Fonseca, 2020). Explaining the destruction of Lisbon became an even bigger challenge.
To address this conundrum, I use the distribution of intensity outliers among the 1206 intensity datapoints – 3.6% of the dataset, deviating above 2 rms residuals, all above MMI 8 – to argue that the easiest explanation for the damage distribution is a multiple rupture over spatially distributed active faults, onshore as well as offshore. I assign the extreme damage in Lisbon to the occurrence of a nearby intraplate earthquake associated with the Lower Tagus Valley Fault System (LTVFS), within a few minutes from the mainshock. I present new results derived from GNSS and PSInSAR data that document the ongoing deformation of the crust around Lisbon, showing that the intraplate activity of the region is stronger than previously assumed. Finally, I discuss the implications for the seismic and tsunami hazard of the Portuguese capital.
Session: Tectonics and Seismicity of Intraplate Regions I
Type: Oral
Room: Cedar
Date: 4/20/2022
Presentation Time: 08:45 AM Pacific
Presenting Author: Joao F. D. Fonseca
Student Presenter: No
Additional Authors
Joao Fonseca Presenting Author Corresponding Author jfonseca@tecnico.ulisboa.pt Instituto Superior Tecnico |
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Intraplate Earthquakes Near Lisbon, Portugal and the 1755 Conundrum
Category
Tectonics and Seismicity of Intraplate Regions
Description