The 1909 Benavente earthquake occurred on 23 April at 17:39:36 local in the Santarém District of the Central Region, Portugal. The estimated moment magnitude 6.0–6.3 earthquake had a maximum Mercalli intensity of IX ("Violent"). It nearly destroyed the town of Benavente, killing 60 and injuring 75 people as a result.
Geology.
The northeast-trending Lower Tagus Valley of mainland Portugal covers a 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 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 value of 6.0–6.3 on the moment magnitude scale. The reassessed magnitude is significantly lower 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 which was relatively large for its magnitude; in comparison, 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. This suggests the earthquake rupture along the seismogenic structure near the surface were distributed along branch faults or dipped at very shallow angles. If surface ruptures were to occur, it was likely they were destroyed due to flooding in the valley. Surface fissures and sand volcanoes were observed in the ground.
Impact.
The earthquake was assiged a maximum Mercalli-Cancani intensity of X near the epicenter, over . Liquefaction occurred in the Tagus and Sorraia river plains. At Lisbon, away, the shaking was felt VI ("Strong"). Intensity VI was also felt in the cities of Setúbal and Évora, causing some damage. Shaking intensity V was felt in Spain. A reassessment of the maximum intensity on the Modified Mercalli intensity scale in 1956 revised the maximum intensity to IX ("Violent").
The towns of Samora Correia and Muge sustained the heaviest damage occurred. Nearly 90 percent of these 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. Forty percent of buildings in Benavente were razed or had to be torn down. Another 40 perent needed extensive renovation. Only 46 of the 207 structures in Santo Estevao survived the shock.
References.
 ;templatestyles src="Reflist/styles.css" / ;