Face to face with earthquakes
Earthquakes pass by in a flash and leave trails of destruction as swiftly as they come. In track lingo, Usain Bolt could beat most of them in a dash but most likely not in a 400-meter run.
You don’t have 11 hours to deal with the presence of an earthquake, so unlike the recent hostage crisis, police may not have time to smash laminated-glass bus windows with crude hammers just to wait for minutes of eternity to figure out what to do next. “Veteran” primetime TV news anchors could not spare a moment to ask ground reporters where snipers were holed out, while the hostage-taker enjoyed free information watching television feeds from inside his bus of hostages.
The day an earthquake confronts you, there will be no time for rescuers to revisit the comic talent of Charlie Chaplin and television anchors to imitate the amusing humor of Mr. Bean. This is not a promise but a guarantee. Except for such earthquakes, such as the one in Sumatra in December 2004, which recorded abnormally long durations of ground shaking, many of the most destructive earthquakes on the planet took just a few seconds to make their presence felt.
The most recent large earthquake that shook urban centers in the Philippines was the Luzon earthquake of 1990. Within seconds, it swept across and ravaged the cities of Baguio, Dagupan and Cabanatuan before dusk on a morbid July 16. It recorded a surface-wave magnitude of 7.8 and originated from a focus 25 kilometers below the surface of the earth.
Structural engineers like me, who merely use earthquake data for use in the design of structures, are essentially laymen in earthquake engineering, let alone the science of seismology (which, in this country, is the turf of seismologists at Phivolcs) itself. Nevertheless, certain common knowledge about earthquakes, particularly in the context of building structures, could help increase useful understanding of very basic facts about earthquakes without any attempt to present a sloppy doctoral dissertation.
Earthquakes are caused by movements of so-called tectonic plates, which practically divide the surface of the earth between boundaries. This tells us that the outermost surface of the earth is not as solid as we normally think. The Philippines sits between at least two nearby trenches (which are like surface ruptures): the Manila trench by the South China Sea to the west, and the Philippine trench near the Lamon Bay area by the Pacific Ocean to the east.
Intermediate fissures, generally known as fault lines, dot the Philippines from north to south, which means practically every place is not spared from a potentially earthquake-generating fault nearby. But two fortunate exceptions are Palawan and that area somewhere south of Basilan, where the level of seismicity (susceptibility to earthquakes) is relatively low. Nice place to dwell in peace, you would say. Maybe, but in the case of Basilan, you could use some extra provisions to shield you from bullets courtesy of the unresolved Mindanao conflict.
In general, the Philippines shares nearly the same seismicity as Japan, California and India, among many others. Hong Kong, Singapore, and Dubai, for instance, have relatively low seismicity, which explains in part why those territories can have tall, slender buildings standing like needles.
The source of an earthquake is called the “focus,” which could be several kilometers underground. The vertical projection of the focus to the surface of the earth is called the “epicenter.” In general, shallow earthquakes are those that have foci of less than 35 kilometers below the surface of the earth and are generally destructive even at relatively lower magnitudes. Earthquake magnitude is the measure of energy (read: strength) released by an earthquake and has been traditionally measured using the Richter scale.
Media have often interchanged the use of “magnitude” and “intensity” in earthquake reporting, although improved understanding of the distinction between these two terminologies has been apparent in recent years. As opposed to magnitude, intensity is a relatively crude measure, based on human observation of the level of damage or effect of an earthquake in a specific location. As such, the intensity scale is used to associate an earthquake with, say, the extent to which people in sleep feel it, the fright associated with the experience, the level of damage to structures, and so on. Giuseppe Mercalli invented the Mercalli Intensity Scale in 1902. Phivolcs adapts its own intensity scale here in the Philippines.
Magnitude can be a positive real number (e.g., 6.9, 7.0, 7.2, 8.1), whereas intensity is represented by a Roman number (e.g., I, II, III up to X), although representations using positive integers (i.e., 1, 2, 3, up to 10) may not be a mortal sin (ask Phivolcs). Thus, a report saying “intensity 7.1” may, in fact, mean “magnitude 7.1” unless there is a typographical error somewhere. Given this, a magnitude 6 is a magnitude 6 earthquake wherever you may be, but the intensities that one and the same earthquake could generate from place to place could differ (e.g., intensity 3 in Manila, intensity 4 in Lucena, intensity 6 in Cebu, etc).
Expression of earthquake magnitude is logarithmic to the base-10, which means that every unit increment rise in magnitude (e.g., 7.0 to 8.0, 6.4 to 7.4, etc.) represents an increase of 10 times the amount of energy released (read: strength) by an earthquake!
Anyone who is interested in drawing some analogy between storm signal warnings and earthquake magnitude and intensity is facing sleepless nights. While technology is capable of storm-signal prediction, it can only measure earthquake magnitude and intensity after an earthquake has left all of us eating dust, figuring out ways to rebuild our cities, and counting bodies that could pile up by the minutes. Now, that would be some scene right out of the disaster movies we watch in Dolby Digital or DTS 5.1 surround systems.
Nevertheless, there are available mitigating measures that we can take to cushion the impact of an earthquake event; otherwise, we end up watching ourselves swept by something faster than a cheetah in the long haul, more destructive than Desert Storm by a long shot, and elusive enough to keep our “veteran” TV newscasters wondering how come they were not even able to ask this “mysterious guest” intelligent questions they learned from their nannies in the days when they were just starting to know who, for heaven’s sake, was their papa or their mama.
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(Note: This article is intended only for laymen’s basic information. Seismologists and earthquake engineers are the best source of knowledge for a highly technical understanding of earthquakes. Hence, the use of layman’s language is given overriding priority over more accurate terminologies that would have been necessary for technical correctness.)
