Phivolcs computer simulation shows scope of damage of an Intensity 7 earthquake in Cebu City downtown

‘QUAKE-PROOF OLD BUILDINGS’

Tumuluak

Many old buildings  downtown  would collapse and thousands of people could get injured, trapped or killed  if a “very strong earthquake” of Intensity 7   hits Cebu City.

This scenario is guiding City Hall in issuing advisories to owners and contractors to retrofit structures built before 1992, the year the Philippine National Building Code was enacted.

A computer simulation was made showing the impact of a powerful quake on a downtown street  using software developed by the Philippine Institute of Volcanology and Seismology (Phivolcs).

The results were explained to reporters in  a briefing last week by Cebu City  Councilor Dave Tumulak, head of the Cebu City  Disaster Risk Reduction and Management Council.

Intensity 7 is what residents in Cebu City and Tagiblaran City, Bohol felt during the devastating earthquake of  October 2013 where the epicenter was in Sagbayan, Bohol province.

That quake  destroyed  over a dozen century-old churches in Bohol and shut down the Cebu Palace of Justice and Cebu City Medical Center, where cracks rendered the buildings unfit for occupancy.

Data for the simulation exercse was based on the profile of buildings on a national road  in downtown Cebu City.

“On a normal working and school weekday, if an Intensity 7 earthquake would hit Cebu City, the number of occupants that would be affected in the said street could reach 11,000 to 15,000,” said Tumulak.

He asked reporters not to name the street “to avoid causing panic”.

A satellite photo of downtown Cebu City was flashed in the briefing.

Tumulak  described  the road  as  670 meters long with  nearly 50 buildings, including schools.

Based on their data, 32 buildings there were built before the 1992 passage of the National Building Code by Congress.

Five were built after 1992 and 12 were built in the 1900s.

“We’re requesting all building administrators and structural engineers to retrofit their structures especially those buildings before 1992,” he said.

He said letters will be sent to  building owners and administrators through the Office of the Building Official (OBO).

“Aside from retrofitting, building owners should also strengthen the disaster preparedness of their people,” Tumulak said.

Cebu city representatives joined a one-week training in Iriga City last February to learn how to use the Phivolcs software.  A month after training, they started developing the simulation for Cebu City, which they focused on one road so far.

Based on Phivolcs data, Cebu City has one known faultline in a  mountain barangay. The simulation was based on a possible earthquake from that fault.

The  Rapid Earthquake Damage Assessment System (REDAS) was developed by Phivolcs to plot the extent of damage in an area that would result from various  intensities of earthquakes.

Cebu city staff  had to feed information on the number of buildings along the national road in downtown Cebu City, the number of people who enter and stay in each building, and passersby – commuters, motorists, and pedestrians.

Tumulak said they plan to simulate how an Intensity 7  earthquake would affect other Cebu city streets, especially in the urban area.

Harold Alcontin, head of the council’s research and planning division, said the simulation was not meant to cause panic, but to help people prepare for a massive earthquake.

“The simulation is just to prepare them (so that) in case there’s an earthquake, they would know what will happen. It can also help them plan their evacuation. Where should they go? Where should they exit?” he said.

DIFFERENT MEASURES
Intensity and magnitude  are two different measures of the impact of an earthquake.

Intensity measures how the earthquake shaking is felt by people.

Magnitude is the size of the earthquake based on the amount of energy released at the source.

The 2013 Bohol-Cebu earthquake had a magnitude of 7.2 with an epicenter in Sagbayan, Bohol.

But the intensity in Cebu City and Tagbilaran, Bohol were both rated 7.

Under the Modified Mercalli Intensity Scale,  an intensity 7  shaking is “very strong”.

There would be “considerable damage in ordinary substantial buildings with partial collapse” while poorly built structures would sustain great damage. Columns, monuments and walls would fall and heavy furniture would be overturned.

Phivolcs director Renato Solidum has been encouraging local government units to use REDAS since the Negros Oriental earthquake in 2012 and the Bohol earthquake in 2013.

In a forum in Cebu City in April last year, Solidum said imagining and simulating an earthquake will help local government units  prepare for the damage wrought by an earthquake.

He said that Cebu province should take a closer look at how prepared it would be to withstand a powerful tremor.

In he 2013 Bohol earthquake, 13 people were killed in Cebu and 89 were injured.

The belfry of  the Basilica del Sto. Niño belfry collapsed while the Cebu Metropolitan Cathedral sustained cracks on its facade and belfry. The Cebu provincial government reported over a million pesos worth of damage to school buildings, public markets, roads and bridges.