Special report; When the earth shook: Remembering the 7.2 magnitude quake

CEBU, Philippines - What was supposedly a laid-back national non-working holiday in commemoration of Eid al-Adha October 15 of last year made a mark in the lives of Boholanos and Cebuanos.

On that fateful day, central Philippines awoke to the earth’s noisy rumbling and intense shaking.

One of the most destructive earthquakes in recent history at magnitude 7.2, the tremor claimed the lives of over 200, injured a thousand, destroyed roads, bridges and buildings, and crushed historic churches in the region.

An earthquake, also known as seismic activity, tremor or temblor, is any sudden movement of the earth’s crust caused by the release of stress accumulated along geologic faults or by volcanic activity.

The gravity of the October 15 earthquake was likened by seismologists to 32 atomic bomb explosions.

Based on the data from the Philippine Institute of Volcanology and Seismology under the Department of Science and Technology, the October 15 earthquake was most felt in western Bohol.

Intensity VIII (very destructive):

Sagbayan, Catigbian, Loon, Maribojoc, San Isidro, Antequera, Cortes, Clarin, Inabanga & Buenavista (Bohol)

Intensity VII (destructive):

Tagbilaran City; Cebu City; Lapu-Lapu City; Mandaue City; Toledo City; Carcar City; Naga City (Cebu); Danao, Carmen, Bilar, Loboc, Loay, Baclayon, Alburquerque & Dauis, Bohol

Intensity VI (strong, able to cause panic):

Hinigaran, Negros Occidental; Dumaguete City; Siquijor Island, Lila, Dimiao, Valencia, Garcia-Hernandez, Jagna, Duero, Guindulman, Candijay

Intensity V (strong, able to cause panic):

Iloilo City; La Carlota City; Guimaras Island; Camiguin Island; Abuyog, Leyte; Ozamis City; Sibulan, Negros Oriental; Gingoog, Misamis Oriental; Cagayan de Oro City

Intensity IV (moderately strong):

Roxas City; Masbate City; Tacloban City; Bulusan, Sorsogon; TabonTabon, San Pablo & Bato, Leyte; Dipolog City; Bacolod City; Naval, Biliran; Bayawan City; Hinunangan & Baybay, Southern Leyte; San Jose &Patnungon, Antique; Guihulngan, Negros Oriental; Butuan City

Authorities initially thought the East Bohol Fault caused the powerful earthquake, but further assessment led to the discovery of a new, unmapped fault located around 20 to 25 kilometers from the previously identified fault.

PHIVOLCS lineament analysis of digital topography revealed a blind fault at the northeast-southwest-trending structures that cut across the northern portion of the island of Bohol, which covered seven municipalities.

Due to the impact of the tremor, a soil wall was formed in barangay Anonang, Inabanga, Bohol. A gently sloping ground rose to a height of about three meters stretching for over five kilometers.

AFTERSHOCKS

At least 6,000 aftershocks were recorded from October 15, 2013 until May of this year.

PHIVOLCS Central Visayas chief Robinson Giorgio said the agency stopped recording aftershocks last summer after assessing that no significant tremors were felt in the region.

“Aftershocks are expected because the soil is still in its stabilization period after the major movement last year. Once the soil gets stabilized, another stress may occur. But no recent technology has been able to predict the occurrence of an earthquake,” Giorgio explained.

Authorities have monitored an average of 20 aftershocks a day in connection to October 2013 earthquake.

SINKHOLES

Following the most recent strong earthquake, the Mines and Geosciences Bureau of the Department of Environment and Natural Resources 7 has found at least 500 sinkholes in Bohol province alone, revealed chief geologist Josephine Aleta.

Aleta admitted that to this day, because the agency does not have enough manpower, it has not finished assessing all affected areas despite the number of requests from concerned local government units and individuals.

“Apart from granting the requests related to the earthquake, we also have to accomplish our target to finish this year the more detailed geohazard assessment which is already 1:10,000 scale,” she explained.

Nevertheless, MGB 7 has issued Subsidence Threat Advisory to 11 towns in Bohol province.

These include certain areas in Loon (barangays Lintuan, Napo, Nagtuang, Basdacu, Basdio, Sandingan island, Moto Norte, Moto Sur), Tubigon (barangays Bunacan, Poblacion, Buenos Aires), Clarin (Bogtongbod, Buangan, Poblacion, Tontonan, Caboy, Bacane), Maribojoc (Poblacion, Guiwanon, Anislag), Antequera (Bungahan, Poblacion, Tagubaas, Obujan, Canlaas, Mag-aso, Tupas, Quinapon-an), Catigbian (Cambailan, Bagtik, Poblacion, Cambuslot), Balilihan (Boyog Sur, del Carmen West, Cantalid, Sal-ing, del Rosario), Calape (Lauis, Kinabag-an, Lomboy, Talisay, Kahayag), Sagbayan (Poblacion, Canmaya, Sta. Cruz, Ubojan), San Isidro (Poblacion, Caimbang, Cambansag, Abelihan), and Tagbilaran City (Taloto, Poblacion I, Cogon, Dao).

Sinkholes, also called doline or karst terrain, are topographic depressions formed in soluble (limestone) rock by the action of the water, serving to conduct surface water to an underground passage.

A number of houses, public structures including schools have been found to be standing on top or near sink holes. Some of these structures have already been declared unsafe and the occupants have been advised to relocate to safer grounds.

Aleta said there is a need to acquire more ground penetrating radars, which can help state geologists assess geohazards in earthquake-hit Bohol and even Cebu. DENR has only one GPR, she said, adding that the radar costs around P3 to 4 million each.

FUTURE EARTHQUAKES?

From the year 1600 to 1956, PHIVOLCS recorded major tremors in the Visayas region that ranged from magnitude 5 to 7.3.

Most prominent was the one that struck Cebu City, Lapu-Lapu City and Camotes Island in January 28, 1922. The tremor damaged old structures made of coral limestone.

Although earthquakes cannot be predicted, PHIVOLCS national director Renato Solidum presented scenarios that may involve the region should a major tremor be felt.

Scenario 1 is the stress along West Panay Fault that may result to a 7.4 magnitude quake, similar to what happened in 1787.

Under this scenario, intensity 8 will be felt in Panay region near the fault; intensity 7 in other areas of Panay away from the fault zone; intensity 6 in some areas in Panay Guimaras, and Negros; intensity 5 in some parts of Guimaras, Negros, Cebu, Tablas, Romblon, Sibuyan, Mindoro and Masbate; intensity 4 in some areas of Mindoro, Busuanga, Negros, Cebu, Bohol, Leyte, Masbate, Quezon, Batangas.

Scenario 2 is a magnitude 7.7 earthquake that will be caused by movements in the Negros Trench which could lead to intensity 7 shaking in some areas of Negros Occidental, Guimaras, and southern Panay; intensity 6 in some areas of Negros Oriental, Panay, and Guimaras; intensity 5 in some parts of Siquijor, Negros and Cebu.

Scenario 3 is a magnitude 7.5 along the Offshore Thrust Fault of Bohol, which may cause intensity 7 to 8 tremors in southern Bohol; intensity 7 in northern and northwestern areas of Bohol, some areas of Cebu, Siquijor, Leyte, and Negros; intensity 6 in many areas of Cebu and some areas of Negros, Leyte, and northwestern coasts of Mindanao.

Scenario 4 is a seismic movement that may originate at the East Bohol Fault that could cause a magnitude 7 quake. This could lead to shaking at intensity 7 to 8 in most areas of Bohol; intensity 7 in northwestern areas of Bohol; intensity 6 in southern part of Cebu, Cebu City, Mandaue, Lapu-Lapu City, Siquijor island and western part of Southern Leyte; intensiy 5 in Negros.

Scenario 5 is similar to the 1922 quake in Cebu that could lead to shaking that may reach intensity 7 in Mactan and Camotes islands; intensity 6 in northeastern part of Cebu and some areas of Bohol; intensity 5 in some areas of Negros and Bohol; intensity 4 in southern part of Negros.

Scenario 6 is a magnitude 7.5 in central Cebu which may lead to intensity 7 to 8 in most areas of Cebu, Bais, and Dumaguete; intensity 7 in the northern tip of Cebu, northern half and western areas of Bohol and many areas of Negros both eastern and central; intensity 6 in western areas of Negros and Siquijor island; intensity 5 in Leyte, Panay, northwestern Mindanao.

Giorgio, for his part, said that the agency is currently finalizing a study on fault verification in Cebu. In fact, he added, that a submarine survey was done last week.

Authorities are trying to verify the suspected active fault systems in Cebu which consist of four ligaments or areas. Results of the survey, Giorgio said, are expected to be released by the end of the year. — /QSB (FREEMAN)