  .svg) | |
| UTC time | 1955-03-31 18:17:13 |
|---|---|
| ISC event | 889384 |
| USGS-ANSS | ComCat |
| Local date | April 1, 1955 |
| Local time | 02:17:13 (UTC+8) |
| Magnitude | 7.4Mw |
| Depth | 35 km (22 mi) |
| Epicenter | 7°40′N123°10′E / 7.66°N 123.16°E /7.66; 123.16 |
| Fault | Cotabato-Sindangan Fault |
| Type | Strike-slip |
| Areas affected | Mindanao |
| Total damage | $5 million (1955 rate) |
| Max. intensity | MMI VIII (Severe) |
| Casualties | 465 deaths 898 injured |
The1955 Lanao earthquake struckLanao del Sur on April 1 at 02:17 local time. The earthquake measuring 7.4 on themoment magnitude scale and assigned a maximum intensity of VIII (Severe) on theModified Mercalli intensity scale was one of the largest to hitMindanao. It caused immense damage around the northern and central parts of Mindanao, as well as the southern reaches ofVisayas. It killed at least 465 individuals and injured a further 898.[1][2]
Both provinces ofLanao del Sur andLanao del Norte run along two main faults, the western extension of the Mindanao Fault (Cotabato–Sindangan) and the northern segment of theCotabato Trench. The two faults were formed by the collision of thePhilippine Sea Plate andSunda Plate. In between the two faults lie thePhilippine Mobile Belt, a region the takes up most of the western regions of the Philippines.[3] It hosts multiple faults and active seismic blocks from southern Luzon to the Cotabato Trench.[4] The Cotabato Trench itself caused two earthquakes in1918 (Mw 8.3) and1976 (Mw 8.0). The last significant earthquake to happen in the same area as the 1955 event was a magnitude 6.0 earthquake on April 12, 2017.[4]
The earthquake struck at dawn. Multiple magnitudes vary from moment magnitudes of 7.4–7.6 by local seismographs. It was assigned an intensity of VIII on theRossi–Forel scale. Most earthquakes that occur along the Cotabato Trench displays a strike-slip mechanism, which may be that of the 1955 earthquake.[5]
Multiple foreshocks preceded the mainshock, the largest measuring 6.3 occurred 6 hours earlier. It was first believed to be a double earthquake with the mainshock, however it was later confirmed to be a foreshock. Aftershocks also struck the area with varying magnitudes.[6]
Most damage was centered aroundLanao Lake, more specifically the city of Dansalan (now known asMarawi). Numerous mosques were damaged or destroyed completely. Local houses and infrastructures collapsed due to the strong acceleration of ground movement. Towns southwest of theLanao Lake were said to have been submerged underwater for a few years due to ground displacement by a few meters in a southward direction, causing a miniature tsunami to that permanently flood the towns. The death toll from ranged from 225 to 465 according to multiple accounts, and injuries were counted near a thousand. InMisamis Occidental, the city ofOzamiz was also greatly affected. The earthquake caused the destruction of both the city's parish church and the southwest bastion of theFuerte de la Concepcion y del Triunfo, a Spanish fort that was built in the 18th-century. Thepresident at the time of the event,Ramon Magsaysay, declared a state of calamity in theprovinces ofMisamis Occidental,Misamis Oriental,Lanao, andSurigao.[7][2][8]
The earthquake was deemed a sign fromGod towards those who sinned against him. This was due to the large religious Muslim and Christian populations that were present in the areas near the epicenter. TheBible andQuran were often cited and repeated during the aftermath by preachers and locals. During the earthquake itself, cries of children were heard along with chanting and prayers.[7]