Imagine this, if the West Valley Fault (WVF), which runs through the eastern section of Metro Manila, moves and generate an estimated 7.2-magnitude earthquake, what do you think may happen?
According to the Philippine Institute of Volcanology and Seismology (Phivolcs), Metro Manila, parts of Bulacan, Rizal, Laguna, Cavite and even parts of Batangas might be affected by an Intensity 8 shaking.
The state seismology bureau estimated that the potential impacts of such “worst-case earthquake scenario” may result to around 33,500 deaths in Metro Manila alone, with 114,000 people seriously injured, 18,000 fire casualties, and widespread damage on properties and infrastructure.
Such seismic scenario—known as the “Big One”—is only one of some scenarios that might happen across the country, Phivolcs said, adding that such scenarios cannot be predicted.
What are ‘earthquake scenarios’
Department of Science and Technology Phivolcs (DOST) Undersecretary and Phivolcs director Dr. Renato Solidum Jr. said earthquake scenarios are assumed sequences of events, which help experts determine not only the hazards but also the possible impacts of an earthquake.
“An earthquake scenario is an assumed hazard scenario describing the estimated ground shaking, for example the intensity, other potential hazards related to the earthquake, and most importantly the impact that can be caused by these specific earthquake events,” he told reporters in a webinar last week.
“An earthquake scenario is also used as a planning tool so that we understand a potential future earthquake, so that we can plan accordingly as to the possible impacts that these earthquakes can bring,” he added.
Worst-case earthquake scenario
On the other hand, Solidum explained that “worst-case earthquake scenario” describes an earthquake that can bring the most severe impact to a region or locality.
“That is why when we released the potential impact of a magnitude 7.2 earthquake in Metro Manila, our friends from the media called it the ‘big one’ because there can be many people that will die and even significant number of buildings that might be destroyed,” he added.
The ‘Big One’
The term “big one” is used in many parts of the world. It is a term used to describe damaging and strong earthquake scenarios. In the Philippines, it is often equated to the worst-case earthquake scenario in the greater Metro Manila area.
But he noted that Phivolcs provides many worst-case earthquakes scenarios for different parts of the country. “Therefore, there can be many big ones. It’s not simply in Manila but there can be other big ones in other parts of the Philippines.”
Metro Manila’s ‘Big One’
According to Solidum, studies indicated that the WVF moved four times in the past 1,400 years, with a movement interval measured between 400 to 600 years. He said that the last major earthquake event generated by the movement of the fault was in 1658.
“So in terms of recurrence interval with the last earthquake event, and if you consider the lower limit of the recurrence interval which is 400, then we can say that a large earthquake can happen from the West Valley Fault in our generation, it can happen in the next generation,” Solidum said.
Other earthquake scenarios
Aside from the earthquake scenario in the metropolis, Solidum said an earthquake scenario of a magnitude 8.2 quake may occur in the Philippine Trench, affecting the nearby areas of Samar, Leyte and Dinagat Islands.
He also explained that movement in the Mati Fault in Mindanao could also generate a scenario of a 7.3-magnitude temblor, which may potentially affect Davao area.
Importance of earthquake scenarios
“We need to take into consideration that we want individuals to imagine the risk that may face properly so that they have an informed decision,” Solidum said.
Earthquake scenarios, he added, can also be used by local government units to formulate appropriate strategies for mitigation, response, and coordination during disaster.
“When distinctive impacts are explained, quantified, individuals and communities can understand disasters can be mitigated.”
Not a prediction
The Phivolcs chief noted that such earthquake scenarios are not predictions.
“Because predictions have exact time, level and exact magnitude. So these are scenarios that can happen. If we are prepared for the worst case scenario, we are prepared for the smaller ones. That is important,” Solidum said.
“Scenarios are tools so that we can imagine the possible disaster that can happen if we do not do anything. If we do something, the scenario will not happen. The scenario is important so that we can prepare better,” he added.