05 April 2017

Why the 2017 Tingloy, Batangas Earthquake was Intense Even in Lipa City

Image credit:  Phivolcs

I was lying on the sofa watching television when I felt the first jolt at 8:58 pm on the fourth of April, and typically, because my home is right beside the highway, I wondered if it was because of a bus or a container truck that just passed by. I immediately bolted up when the television and the walls suddenly began to sway. I knew then that there was an earthquake.

It did not seem like a left to right swaying as is typical of most minor earthquakes, but also an up and down undulation of the entire house. I have no way of corroborating this since Phivolcs’ reports are always regrettably sketchy; but that was how it looked like to me. More about this later.

I have lived through enough earthquakes in my life to make me quickly realize that there would be no need to dive under the table, even if the prolonged undulation was still quite unnerving. As soon as the shaking died down, I turned on my computer and went to the Phivolcs web site. In a matter of minutes, the site was reporting a magnitude 5.4 tremor with its epicenter placed at 7 kilometers northwest of the island municipality of Tingloy in Batangas.

Phivolcs subsequently revised this to magnitude 5.5 with epicenter at 6 kilometers northeast of Tingloy, stronger than it was originally reported. Do not discount the 0.1 difference as insignificant as I shall now begin to explain. Just to make sure that everyone is on the same page, I will try to explain the scientific mumbo-jumbo in lay terms as much as I can.

First of all, the Tingloy earthquake was tectonic in origin. This simply means that there was a rupture or breaking of the rocks that compose the earth’s crust. Upon the rupture, energy was released that would subsequently spread out from the focus or source of the rupture in what we would all call an earthquake. This energy can be measured numerically in what is called magnitude. Geologists frequently use what is called the Richter scale to calculate just how strong one earthquake is.

Tulane University Geology professor Stephen A. Nelson explains how even a difference of one (1) in magnitude is significant:
“…each increase in 1 in Magnitude represents a 31 fold increase in the amount of energy released. Thus, a magnitude 7 earthquake releases 31 times more energy than a magnitude 6 earthquake. A magnitude 8 earthquake releases 31 x 31 or 961 times more energy than a magnitude 6 earthquake.1
If the amount of energy released during an earthquake is measured in terms of magnitude, the effects of this release across the landscape is assigned a numerical value called intensity. Intensity varies according to the effects caused by the shaking and is also affected by the sturdiness or strength of the land underneath the surface and distance away from the epicenter.2

Tingloy is 42 kilometers or so from Lipa as the crow flies. Philvocs has placed the strength of the earthquake in Batangas City, roughly 21 kilometers from the island, at Intensity 6. No figure is given for Lipa City, but Intensity 5 is assigned to Malvar, which is just over 51 kilometers from Tingloy.3 While no decimals are assigned to intensity levels, we can safely assume that shaking in Lipa was somewhere between Intensity 5 and Intensity 6.

To fully appreciate the meaning of these numbers, let us refer to the Philvocs Earthquake Intensity Scale. Intensity 5, assigned to Malvar, is described as:
“Strong - Generally felt by most people indoors and outdoors. Many sleeping people are awakened. Some are frightened, some run outdoors. Strong shaking and rocking felt throughout building. Hanging objects swing violently. Dining utensils clatter and clink; some are broken. Small, light and unstable objects may fall or overturn. Liquids spill from filled open containers. Standing vehicles rock noticeably. Shaking of leaves and twigs of trees are noticeable.”
Meanwhile, Intensity 6, assigned to Batangas City, is described as:
“Very Strong - Many people are frightened; many run outdoors. Some people lose their balance. Motorists feel like driving in flat tires. Heavy objects or furniture move or may be shifted. Small church bells may ring. Wall plaster may crack. Very old or poorly built houses and man-made structures are slightly damaged though well-built structures are not affected. Limited rock-falls and rolling boulders occur in hilly to mountainous areas and escarpments. Trees are noticeably shaken.4
Moreover, the earthquake occurred at a depth of only 5 kilometers beneath the surface, which means it was a “shallow earthquake.5” This type of earthquake tends to be stronger and is more destructive than those that occur deeper because the energy is not dissipated and seismic waves do not have far to travel to the surface.6

Personally, what I found so unnerving about last Tuesday’s quake was not just the sideways shaking of the house – caused by what are called “Love” waves – but also the seeming up and down undulation – caused by what are called “Raleigh” waves. It was as though, for the duration of the quake, my entire house was on water instead of terra firma.

I still consider the July 1990 magnitude 7.8 Luzon earthquake as the most horrifying I have ever had to experience, even if its epicenter was somewhere in Nueva Ecija, almost 200 kilometers away. The one that hit at 8:58 at night last Tuesday did not even come close; but it was the strongest that I have had to sit through in the last decade or so.

Notes and references:
1 “Earthquakes and the Earth's Interior,” by Prof. Stephen A. Nelson, online at the Tulane University web site.
2 “What is the difference between Magnitude and Intensity?” online at GNS Science.
3 Distances calculated using Google Earth’s Ruler Tool.
4 “Phivolcs Earthquake Intensity Scale,” online at the Phivolcs web site.
5 “Phivolcs Earthquake Information No. 3, 4 April 2017,” online at the Phivolcs web site.
6 “AP EXPLAINS: Difference between shallow, deep earthquakes,” by Alicia Chang. Online at the Associated Press web site.

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