It Could Happen Here, Too—Phivolcs

>>> Shared from the Original Post at Manila Standard

As a magnitude 7.8 earthquake recently hit Turkey and Syria, the head of the Philippine Institute of Volcanology and Seismology (Phivolcs) on Tuesday said such a magnitude is also likely to happen in the country.

“There’s always this possibility. In fact, it already happened on July 16, 1990… The 1990 Luzon earthquake was magnitude 7.8,” Phivolcs Director Teresito Bacolcol said in a briefing.

Bacolcol added that the country has many active faults that are capable of generating a magnitude 7.8 earthquake.

Based on instrumental records, the 1990 Luzon earthquake was the strongest to hit the country, producing a 125-kilometer-long ground rupture that stretched from Dingalan, Aurora to Kayapa, Nueva Vizcaya.

The quake was caused by strike-slip movements along the northwest segment of the Philippine Fault Zone and its splay, the Digdig Fault.

“We have several active segments in the Philippines. It’s more than a hundred segments, but the longest is the Philippine Fault – 1,200 kilometers from Davao to Luzon,” Bacolcol said.

The Phivolcs chief added that the Philippine Fault moved just last week, causing the magnitude 6 earthquake in Davao de Oro.

He said he thinks that people are more prepared now compared to 20 years ago, regularly participating in quarterly earthquake drills.

“People now are more aware than they were 20 years ago, and especially with the advent of social media. They see the effects of strong earthquakes like what happened in Turkey,” he said.

Participating in earthquake drills helps in developing a culture of preparedness, making people aware of what to do during actual events, he said.

Bacolcol reminded the public to duck, cover and hold in case of earthquakes, and to go out in an orderly manner once the ground shaking stops.

The epicenter of the 7.8-magnitude quake in Turkey was located in the province of Kahramanmaras near the Syrian border, Turkey’s Afad disaster management agency. Another 6.6-magnitude earthquake was measured shortly after in Gaziantep province, Afad said.

Turkey has long been of particular interest to earthquake researchers.

The German Research Center for Geosciences (GFZ) in Potsdam installed measuring equipment in Turkey and has been conducting seismic monitoring in the country since the 1980s.

GFZ records show that the earthquake risk is very high throughout the region around the Sea of Marmara, for example, where Istanbul lies.

“The question is not whether an earthquake will come. The question is when it will come,” Marco Bohnhoff, a seismologist at GFZ and an expert on the region, said in 2019.

Bohnhoff and other experts base this estimate on the occurrence of several strong earthquakes throughout Istanbul’s history, the ongoing continental drift beneath the Sea of Marmara and the existence of an area of the earthquake zone located just outside Istanbul that has long been suspiciously quiet.

Bohnhoff said that there was a lot of evidence suggesting that the plates in this area were “snagged,” causing tension to build up.

Eventually, he suggested, the rock would not be able to withstand the tension anymore and the plates pushing against each other would move in a jolt of several meters in a matter of seconds.

The real danger to buildings, infrastructure and the local population is posed by the earthquake-waves in which the ground moves. How much damage they do depends on how strong the earthquake is.

A good way to keep people safe is earthquake-resistant construction.

Unfortunately, this is very expensive, Bohnhoff said, adding that builders have to ask themselves whether to retrofit old buildings or tear them down and start from scratch.

During the quakes in the Turkish-Syrian border region on Monday morning, more than 1,700 buildings are said to have collapsed in Turkey alone.

“Photos shows that some of the collapsed buildings may have been built prior to modern seismic design code so they might not be appropriately designed and detailed for such a large magnitude earthquake,” said Mohammad Kashani, associate professor of structural and earthquake engineering at the University of Southampton in the UK.

“The combination of large magnitude and shallow depth made this earthquake extremely destructive. We need to investigate the collapsed structures in detail and learn from this devastating event to design our structures and cities to be resilient to such events.”

But it is not just a building’s bones that play a significant role in whether it can withstand an earthquake ― the ground on which it is built matters, too. As a general rule, the firmer the better, Bohnhoff said: “It is best if the subsoil is granite. Dry-laid sediments such as sand or clay are less preferable.”

On soft bedrock, he said, ground motion amplification is more likely to occur, sometimes along with what is called liquefaction. The seismologist compared this process to what happens with wet sand on a beach. If you repeatedly tap the same spot in the sand, water collects in the pit.

“Then the subsurface becomes unstable,” he said. With AFP

The post It could happen here, too—Phivolcs appeared first on Manila Standard.