By: Engr. Edgar Mana-ay
The recent 6.5-magnitude earthquake that occurred on October 29 and 31 in the southern part of Mindanao caused great concern over the ongoing construction of the P11.2-billion Jalaur River Multi-purpose Project Stage II (JRMP II) dam in Calinog, Iloilo.
The earthquakes in Mindanao devastated the towns of Kidapawan, Tulunan in South Cotabato going towards Magsaysay, Digos in Davao del Sur including Davao City. As of this writing, the Philippine Institute of Volcanology and Seismology (PIVS or Phivolcs) cannot yet ascertain if the second quake of the same intensity and in the same area was an AFTERSHOCK or a MAIN SHOCK from an entirely newly collapsed segment of a fault zone. Generally, an aftershock intensity is less than 50% of the main shock, but in the Cotabato quake, the two quakes, whose interval is only three days, have almost the same intensity.
We must understand and accept that earthquakes are part of geologic processes in the earth that we live in. Living creatures, including man, lives on the skin of the earth called the CRUST with thickness from 30 to 100 km. It floats on a semi-molten mass called the MANTLE. Imagine a Sunkist or Valencia orange, its skin or peel is equivalent to the earth crust but in the case of the earth, its skin or crust is NOT solid but cut by FAULTS into segments called TECTONIC PLATES. These tectonic plates, as they float on the earth’s semi-molten mantle, are constantly rubbing against its other so that when one plate gives way, collapse or sub-duct, an earthquake occurs.
At the area where Jalaur Dam is being built, there is a presence of tectonic faults that are classified as MINOR or MAJOR, ACTIVE or INACTIVE. To be candid and frank about it, most of the faults “identified” and indicated in the maps are mostly inferred. Government agencies cannot even decisively label them as major or minor and active or inactive. Remember that damaging earthquake eight years ago in Negros Oriental whose epicenter was I think in Guihulngan, Phivolcs was surprised that there was such an active fault in that area!
The same as in the 2013 intensity 7 earthquake in Carmen, Bohol. The only best studied MAJOR and ACTIVE fault in the world is the San Andreas fault which runs almost the entire length of the state of California. The San Andreas Fault caused the 8.3 magnitude earthquake in 1906 which leveled the City of San Francisco and the 6.7 quake in 1994 which caused extensive damages in Los Angeles.
Despite the hi-tech instruments in the US, it cannot predict exactly (they said maybe in 2032) WHEN the BIG ONE in magnitude of 8 to 9 on the Richter scale will occur. If this happens (God forbid!), it will cause the following damages: buildings shifted off foundation, rails bent, underground pipes broken, landslides from river banks and slopes, bridges destroyed, few if any concrete structures remain standing, earth slumps and there will be landslip on soft grounds. In other words, it will totally destroy Los Angeles or San Francisco city!
We go back to JRMP II which is mostly financed by a Korean bank, contracted by a Korean construction company and whose technical consultant is also a Korean firm, Dasan Consultant. We give them the benefit of the doubt that Dasan Consultant had performed a satisfactory GEOTECHNICAL INVESTIGATION of the area because it has declared that it is SAFE TO CONSTRUCT THE DAM.
It would have been better if then Gov. Defensor Sr. had provided an experience Ilonggo to look over the shoulder of the Korean Consultants as they performed their work. Geo-tech is a complex geology and drilling investigation to determine ALL the conditions that exist underground such as type/layering of soil up to 400 meters deep, the texture and strength of the soil, water movement underground and many more tests being conducted in the lab. It is by core drilling that faults are identified, and it takes a lot of imagination and experience of a geologist to determine by stratigraphic correlation as to whether there is a fault and if there is, whether it is minor or major, then active or inactive.
Cores retrieve from all drillings are catalogued and kept in a core box and library for easy revisiting when geo-tech doubts and problems surfaces. I understand there is a separate office at the National Irrigation Administration (NIA) in Jaro where the Jalaur Dam people holds office and where they keep all files and cores.
For the Ilongos to have peace of mind, it is incumbent for Governor Defensor Jr. to make public the geo-tech study and recommendations of the Consultant that indeed JRMP II is in a geologically safe area to construct. If it is of any consolation, on record, only two dams in the world had failed – St. Francis Dam in LA in 1928 where 400 people perished and the Vaiont Dam in Italy in l963 where 2,000 died. Both dams did not fail because of an earthquake caused by a fault. The St.Francis Dam failed because the ground below became unstable because of water saturation and in the case of Vaiont Dam, an entire mountainside slipped because of a heavy rain and the presence of a geologic sliding plate.
We therefore appeal to Gov. Defensor Jr. to release the Consultant’s geo-tech study on JRMP II Dam so that we will understand and have peace of mind. For according to Marie Curie: “Nothing in life is to be feared. It is only to be understood.”
Note: The author is a retired Exploration Drilling Manager of PNOC Coal Corporation.
