FAST BACKWARD: Greatest quakes in PH history

That the Philippines is situated along the ‘Ring of Fire’, a 40,000-km horseshoe-shaped area within the Pacific Ocean where huge number of earthquakes and volcanic activities happen, is a given. Geologic movements have formed this precarious region to the disadvantage of all the countries created within its territory.

In recent memory, three tremors come to mind. On August 2, 1968, the Casiguran, Aurora Province, quake measuring 7.3 magnitude in the Richter scale, brought down the Ruby Tower situated in Binondo, Manila, killing at least 207 people.

Twenty-two years later, a 7.8 temblor, with its epicenter in Nueva Ecija, devastated northern and central Luzon. Hardest hit among the ruined cities was Baguio, where the Hyatt Terraces Hotel was crushed beyond repair. Overall, the number of casualties in the destruction was around 1,621 people. The quake struck on July 16, 1990.

On October 15, 2013, Bohol became the recipient of a 7.2 magnitude tremor that registered numerous subsidences, collapsed structures, and cracked highways. Worse, edifices declared as national treasures and landmarks, mostly old churches, succumbed to the disaster. Official estimates placed the casualties at 222 dead, eight missing and 976 injured.

Just recently, on February 10, 2017, Surigao City and its environs bore the brunt of a 6.5 magnitude earthquake that left in its wake hundreds of buildings and residences destroyed, many of them declared inhabitable and dangerous. Fortunately, only eight persons died as a result. This occurred just five months after a 6.5-magntiude temblor rocked Davao Oriental.

The Surigao tremor was triggered by the Mindanao fault zone, which stretches 320 kilometers from Sruigao City to Mati City, in Davao Oriental. The Mati and Monkayo segments were responsible for another huge earthquake that hit Davao region over a century ago.

The most devastating quake to hit the Philippines happened in Moro Gulf, in Mindanao, on August 17, 1976, which was as high as 8.0 magnitude. The geologic movement resulted in a tsunami, the most disastrous in Philippine history that resulted in the official death count of over 8,000 people, killed or missing, excluding the 10,000 injured and the 90,000 rendered homeless.

But this quartet of destructive quakes that occurred in the last 50 years was not just the strongest on record. Two these, according to the Philippine Institute of Volcanology and Seismology (Phivolcs) and other seismological institutions outside the country, occurred in Davao region and were considered as two “of the greatest earthquakes in Philippine history.”

The June 21, 1893, earthquake that hit Monkayo, Compostela Valley Province, landed in many peer-reviewed publications. American geologist Clarence R. Allen, in his article “Circum-Pacific Faulting in the Philippines-Taiwan Region”, which came out in the Journal of Geophysical Research (November 1962) wrote:

“One of the greatest earthquakes in Philippine history is that of June 21, 1863, in eastern Mindanao, which was clearly associated with widespread subsidences in the swamps of the Agusan River and with possible extensive faulting near the present town of Monkayo. And the largest instrumentally recorded earthquake of the Philippines region occurred on April 14, 1924, along the projected trace of the Philippine fault south of Mindanao.”

The last statement refers to the 8.3 earthquake that shook Mati, the epicenter, on April 14, 1924. This occurred three years only after Manay, Davao Oriental, was struck by a 7.5 magnitude temblor on November 11, 1921. Philvocs, in its website advisory, explains the impact of an earthquake with an intensity 8 strength:

“People find it difficult to stand even outdoors. Many well-built buildings are considerably damaged. Concrete dikes and foundation of bridges are destroyed by ground settling or toppling. Railway tracks are bent or broken. Tombstones may be displaced, twisted or overturned. Utility posts, towers and monuments may tilt or topple. Water and sewer pipes may be bent, twisted or broken. Liquefaction and lateral spreading cause man- made structure to sink, tilt or topple. Numerous landslides and rockfalls occur in mountainous and hilly areas. Boulders are thrown out from their positions particularly near the epicenter. Fissures and faults rapture may be observed. Trees are violently shaken. Water splash or stop over dikes or banks of rivers.”

In the case of the Manay quake, which had its epicenter in the southern part of the Philippine Deep, Fr. Miguel Saderra-Maso, S.J. (1865-1939), a seismologist at the Manila Observatory, recorded his observation about the incident.

“The most unusual effect of the second earthquake was tide-wave [tsunami] which invaded the few bays with low lands there existing, Manay being the one which sustained greater loss in structures and crops.”

The same tsunami was noticed as far as Balut Island in Sarangani Bay, Davao Occidental.

Born in Gerona, Spain, Fr. Maso joined the Jesuits in 1882 and later the Manila Observatory. His vital contributions to Philippine seismology are La sismología en Filipinas (1895),  the first work on the seismology in the country, and the 1913 the first study of seismotectonics of the Philippines. While assigned in the country, he worked on seismology and terrestrial magnetism, installed seismographic stations in Butuan and other areas, established meteorological and geomagnetic stations, and studied the occurrence of earthquakes and volcanic eruptions.