By Engr. Edgar Mana-ay
Being located in the “rim of fire” zone, the Philippines will continuously experience earthquakes, just like the recent intensity 7 that rocked the towns of Kidapawan, Tulunan and Mlang in North Cotabato on the evening of November 11. Metro Manila is regularly conducting earthquake drills at all levels of society, in schools, business establishments like malls and hotels, even in hospitals. This will mentally prepare the populace in case the “big one” occurs at the perceived and inferred Marikina fault rupture which will be in the magnitude of at least 7 or more in the Richter scale as per Phivolcs prediction. The Marikina Fault also known as the West Valley Fault extends from Dona Remedios in Bulacan in the north and runs through the Province of Rizal and Metro Manila Cities and the Provinces of Cavite and Laguna that ends in Canlubang.
It is projected that a quake with a magnitude of 7 to 8 in the Richter scale (as strong or even stronger than the earthquake that devastated Baguio many years ago) is sure to hit a highly urbanized Metro Manila but the bigger question is WHEN exactly this will happen. At this intensity, chimneys, towers and elevated tanks are twisted and may fall, landslides and caving along riverbanks will occur, extensive damage to concrete structures with partial collapse, cracks in wet ground and on steep slopes. Liquefaction may occur on loosely packed, fine-grained sands and silt which are saturated by water resulting in the sinking of buildings and artificial flooding with raging fires caused by broken gas lines and electrical wires.
Almost all earthquakes are of tectonic origin which is a break or rupture along a fault line and this displacement causes the energy release known as an earthquake. The greater the length of the ruptured fault, the greater is the earthquake magnitude and duration! The physical properties of soils and rocks through which the seismic waves travel, as well as their geological structure, also influence ground surface motion. There is very little information on past breakage along active fault lines. In some faults, recurrence is less than a hundred years, in some, many thousands or hundreds of thousands of years passed in between major fault rupture. In the case of the Marikina fault, we don’t have the record as to when was a major fault rupture or breakage in the past that caused a 7 to 8 magnitude earthquake.
As earthquakes occur, the first shock wave recorded is the primary or P Waves whose oscillation occurs to and from the path of the wave. P waves are also called compression waves or longitudinal waves. The next pulses recorded are the S Waves, sometimes referred to as secondary or shake waves. It oscillates at a right angle to the path of propagation. S waves usually have a higher amplitude than P waves. P waves travel faster than S waves (5.95 – 6.75 km/sec as against 2.9 – 4.9 km/sec in the earth’s crust), hence the farther they travel from the focus of an earthquake, the greater is the time interval between them. Thus, the distance of a recording station from the epicenter of an earthquake can be calculated from this time interval. P waves are not as destructive as S waves because they have a smaller amplitude (extent of vibratory movement), and the force their customary vertical motion creates rarely exceeds the force due to gravity. On the other hand, S waves may develop violent, tangential vibrations strong enough to cause great destruction.
If we can predict the occurrence of an earthquake it will have great potential in saving lives and reducing property damage. A good prediction would give the location, time and magnitude of a future earthquake with acceptable accuracy. However, forecasting earthquakes is notoriously difficult because they are so complex. Hundreds of variables are involved and no two fault systems are exactly the same. And so science now is concentrating more on EEW – EARTHQUAKE Early Warning rather than on prediction. EEW systems not only tread in the supposedly taboo or impossible realm of earthquake prediction, but will do it INFALLIBLY, FLAWLESSLY, PERFECT EVERY TIME AND AS DEPENDABLE AS STOP WATCHES! For almost a hundred years, EEW has been declared as pure nonsense, impossible to achieve, this was the worst chapter in science history. It’s just like when science orthodoxy years ago insisted that the sound “barrier” is unbreakable, shunned smallpox vaccines that saved whole societies or mocked the existence of jet streams and the Van Allen radiation belt.
Since most fault lines are identified by Geologists (this writer have worked as part of a team of “fault-finder” in a coal mining operation because fault lines are dangerous in underground tunnels as it is a source of loose materials and water), sensors are emplaced directly on well-known and imminently dangerous fault lines such as the famous and most studied San Andreas fault in California. These sensors instantly detect the very first notice of any large earthquake taking place in the immediate vicinity, the P-waves of the incoming tremble.
Once the P-waves have flipped the switch, however, the race is on towards these cities that lie within the radius of impact. It’s a contest the earthquake must lose since seismic waves travel along the surface some 3-4 km/sec – certainly fast, the speed of an incoming ICBM missile – far less than a snail’s pace compared to humankind’s signaling ability, namely, the speed of light. Homo Sapien will win this one every time. So if a very large magnitude 7.9 earthquake should take place on the San Andreas Fault some 50 km. away from Ontario Airport or the operating rooms at Cedar Sinai Medical Center, or Union Station downtown, the screaming electronic bulletin that an earthquake is definitely on the way will arrive at the command centers of these facilities, along with causing any number of millions of personal cell phones to ping an alarm as well, some 14 seconds before the S-waves of the earthquake arrives on the scene. If the epicenter is further away, the warning is delivered with even more time to spare!
14 seconds may not seem like much, but it’s more than enough to wave off a passenger jet making its approach into a Southland Airport, freeze the doors and wheels of Metro Rail trains throughout Greater LA, and caused surgeon preparing to engage in extremely delicate operations to put the scalpel away for the moment. And, there are tens of thousands of other terminals- including gas, electrical, water, nuclear- and millions of cell phones which will be receiving warnings to LA residents seconds before the next big one strikes.
At present, NASA, the Russian Academy of Sciences, India’s Ministry of Earth Science, Japan’s Tokai Warning System, China’s State Seismological Bureau and dozens of the most esteemed scientific bodies on the planet are engaged in trying to push forward the boundaries on seismic forecasting (EEW). Hopefully, our own PHIVOLCS could join them.
