All Saint's Day Earthquake of 1755

In Lisbon, Portugal, on the morning of November 1, 1755, parishioners were gathering for All Saints’ Day mass and servants across the city prepared holiday meals at home over cooking fires (Trethewey, 2020). The city was bright and alive with festivities, beautiful in its historical construction. 300 km away, however, the plates that made up the Azores-Gibraltar transform fault slipped, releasing energy of a magnitude that was equivalent to 32,000 Hiroshima bombs (Trethewey, 2020). Historical records estimate that the resulting earthquake was felt at around 9:40 a.m., at a moment magnitude of 8.5-9.0 (Barkan et al., 2009). 

Lisbon, on the whole, was not a city prepared for an earthquake of that magnitude, or really any magnitude, for that matter (Trethewey, 2020). The three violent shocks that spanned a ten-minute period caused cathedrals to crumble and neighborhoods to be buried under their own unstable construction (Trethewey, 2020). As the winding medieval streets were buried under crumbling buildings, residents of the city fled to the only place they considered safe–the waterfront (Trethewey, 2020). What awaited them there was a tsunami 6m tall, wiping out the hundreds who had considered the waterline a safe haven from the shaking within the city (Trethewey, 2020). Where the tsunami did not reach, fire did; cooking hearths and candles that had been knocked over in the ten-minute period of three violent shakes had consolidated into greater flames and began to spread across the ruins of the city that had turned to kindling for the flames (Trethewey, 2020). The fires would burn for five straight days (cite - NISEE). 

Following the earthquake, reports of aftershocks would total 250 shocks spanning a 6 month period (Solares and Arroyo, 2004). While Lisbon arguably suffered the most during the primary event, the aftereffects of the seismic event would be felt in locations across Europe; in Spain, a survey was sent out following a decree by the King of Spain, Fernando VI of Borbon, in order to glean citizens’ perspective of the destruction (Solares and Arroyo, 2004). 

Figure 1. Map of the Azores-Gibraltar transform fault (Solares and Arroyo, 2004).

The majority of the destruction in Lisbon could be attributed to the ill-preparedness of the city for an event of such a magnitude. Following the event, however, the reign of Marquis de Pombal brought about modern thinking that would revamp the city and cultivate safer construction strategies and codes (Trethewey, 2020). Pombal’s design included building codes that required firewalls and wooden frames–Pombalina cages–that would sway with seismic forces and be able to withstand small to medium earthquakes (Trethewey, 2020). His modern designs, which took into account scientific understanding of seismology, architecture, and disaster planning, also included a grid system to expedite evacuation in future emergencies and improve overall transportation, garbage collection, and sewage (Trethewey, 2020). Drains were implemented under city streets to counteract flooding, and timber pilings were installed into the city’s sandy soil to secure building foundations (Trethewey, 2020). 

Estimates of the losses in Lisbon span various magnitudes, as discrepancies between historical records make determining deaths difficult. One estimate was a death toll of 60,000 in Lisbon alone (cite - britannica), whereas another tallied only 10,000 (Solares and Arroyo, 2004). Estimates of deaths due to the resulting tsunami tend to sit around 2,000 (Solares and Arroyo, 2004). Nevertheless, the numbers still demonstrated a sizeable percentage of the city’s overall population in 1755, 275,000 (cite - NISEE). Economic losses were also hard to pinpoint, but one destroyed warehouse was estimated to contain 1.5% of Portugal’s GDP in Brazilian diamonds (Trethewey, 2020). 

Figure 2. Seismicity map and shade relief of the Azores-Gibraltar transform fault. The represented time period spans 1915 to 2000. The white triangle is located at the location of the 1755 All Saints’ Day earthquake (Solares and Arroyo, 2004). 

Estimates of the affected area also vary, as the epicenter of the event was located in the Atlantic Ocean, in the Horseshoe Abyssal Plain (Solares and Arroyo, 2004). The estimated total area affected has been determined to be between 10-15 million km2, with Lisbon being the most affected by the quake (Solares and Arroyo, 2004).

Figure 3. Intensity of earthquake (le - li) with increasing distance (km) from the epicenter (Solares and Arroyo, 2004). 

Figure 4. Differences between observed and calculated intensities of the All Saints’ Day earthquake of 1755. 

Had construction of the city been more prepared for such a natural disaster, the losses sustained might not have been so detrimental. The recovery efforts that were undertaken during Pombal’s reign demonstrated an understanding of disaster planning, but the city today remains susceptible to future quakes, due to such a high concentration of foreign workers and tourists in the area that lack an understanding of proper disaster protocol (Trethewey, 2020). 

Scientific advancements have pointed towards better preparedness, with the implementation of submarine fiber-optic cables that could be integrated with seismic sensors to give a few extra seconds of warning to the residents of Lisbon before tremors are felt (Trethewey, 2020). 

The Great Earthquake of 1755 – Lisbon's Nightmare | Documentary

Video 1. Start at: 10:00. End at: 13:40. Summary of underwater seismometers and underwater earthquake reading. Examines causes and effects of underwater earthquakes. 

Works Cited

Barkan, Roy, et al. “Far field tsunami simulations of the 1755 lisbon earthquake: Implications for tsunami
hazard to the U.S. East Coast and the Caribbean.” Marine Geology, vol. 264, no. 1–2, Aug. 2009, pp.
109–122, https://doi.org/10.1016/j.margeo.2008.10.010.

Encyclopedia Britannica. “Lisbon Earthquake of 1755.” Encyclopædia Britannica, Encyclopædia Britannica,
inc., 26 Aug. 2024, www.britannica.com/event/Lisbon-earthquake-of-1755.

Kozak, Jan T., and Charles D. James. “Historical Depictions of the 1755 Lisbon Earthquake.” National
Information Service for Earthquake Engineering, Nov. 1998.

Martínez Solares, J.M., and A. López Arroyo. “The great historical 1755 earthquake. effects and damage in
Spain.” Journal of Seismology, vol. 8, no. 2, Apr. 2004, pp. 275–294,
https://doi.org/10.1023/b:jose.0000021365.94606.03.

Trethewey, Laura. “The Earthquake That Brought Enlightenment.” Hakai Magazine, 1 Sept. 2020.