1909 Benavente Earthquake
Geology
The northeast-trending Lower Tagus Valley of mainland Portugal covers a 3,200 km (1,200 sq mi) area and extends to Lisbon at its southern end. Earthquakes around the valley are usually moderate or small in magnitude with the 1909 shock being the most recent large earthquake. In the last 30 years of the 20th centory, at least 39 earthquakes had been felt; 3 with magnitudes of at least 4.0 while 16 events were between 3.0 and 3.9. The first documented earthquake in the region was on 26 January 1531, estimated at Mw 6.9, which caused serious damage in Lisbon and killed two percent of the population. The study of potentially active faults beneath the valley is obstructed by a thick sedimentary layer especially along the Tagus River's eastern bank.
Earthquake
Reassessment of the earthquake magnitude suggest a magnitude of 6.0 on the moment magnitude scale. The reassessed magnitude is significantly smaller than the previous value of 6.6. The most consistent focal mechanism solution is reverse faulting along a northeast–southwest striking fault plane at an estimated depth of focus of 10 km. The location suggest it occurred in a seismic zone in the Lower Tagus Valley. The earthquake was the largest recorded crustal earthquake in the Iberian peninsula during the 20th century. It was felt for 215,000 km (83,000 sq mi) which was a very wide area for its magnitude; the 1908 Messina earthquake which measured 7.2 had a felt area less than a third of that.
There are no documented surface ruptures associated with the earthquake. It is thought that the earthquake rupture along the seismogenic structure near the surface were distributed along branch faults or dips at very shallow angles. If surface ruptures were to occur, it is likely they were destroyed due to flooding in the valley. Surface fissures and sand volcanoes were observed in the ground.
Impact
The earthquake produced extreme shaking, peaking at X (Extreme) near the epicenter, over an area of 450 km. Liquefaction occurred in the Tagus and Sorraia river plains. At Lisbon, 30 km away, the quake was felt VI (Strong). Intensity VI was also felt in the cities of Setúbal and Évora, causing some damage. Moderate shaking (V) was felt in Spain.
Despite being known as the Benavente earthquake, named after the town, it was in the towns of Samora Correia and Muge were the heaviest damage occurred. Nearly 90% of the towns were destroyed. In Benavente, the foundation of a building detached from its structure and tumbled into the Tagus river. Two churches were severely damaged and unsafe for use. A seminary sustained damage when plasters fell from the front facade.