13 May 2017

The Question Everyone Wants an Answer to: Are There Fault Lines in Batangas?

Image credit:  Esurance.com.

In the midst of that earthquake-infested month that was April of 2017, hundreds of people were arriving at this web site from search engines having used the search string “fault lines in Batangas” or something similar. The earthquakes on the fourth and eighth of the month were quite nerve-wracking; and those affected naturally wanted to know what was going on.

Before we proceed, and just to make sure that everyone is on the same page, let us look at a description of what a fault is. The United States Geological Survey (USGS) says that a fault is “a fracture or zone of fractures between two blocks of rock. Faults allow the blocks to move relative to each other. This movement may occur rapidly, in the form of an earthquake – or may occur slowly, in the form of creep... During an earthquake, the rock on one side of the fault suddenly slips with respect to the other. The fault surface can be horizontal or vertical or some arbitrary angle in between.1

To proceed with the question, are there fault lines in Batangas? First, the good news. To date, there appears to be just one known fault in the province. In a radio interview, Phivolcs Science and Research Specialist Erlinton Olevere said after the series of quakes in Batangas that there appeared to be an unnamed fault somewhere in the Mabini peninsula2. What he probably meant was the Calumpang Peninsula, where the town of Mabini is located.

In a separate radio interview, Phivolcs Director Renato Solidum clarified that the institute had known about this fault in Mabini for a while; and that it has not moved significantly in recent times3.

Now consider this excerpt below from the Phivolcs Active Faults and Liquefaction Susceptibility Map of Region IV-A, which for the purposes of this web site has been cropped to show mostly the Province of Batangas. The map does not even reflect the unnamed fault line at the Calumpang Peninsula. Other than this unnamed fault, there is no KNOWN active fault over land in Batangas.

Part of the Active Faults and Liquefaction Susceptibility Map of Region IV-A.
[To download the full Phivolcs Active Faults and Liquefaction Susceptibility Map of Region IV-A, click here.]

The only known fault in the province is offshore, called the Lubang Fault and represented in the map by a thin dash line to indicate that the fault line’s known location is approximate. The fault line is underwater and estimated to start off the tip of the Calatagan Peninsula and runs across Balayan and Batangas Bays all the way to just off the City of Calapan in Oriental Mindoro.

Other than this, the other fault systems that are close to but not in Batangas territory are the southern tails of the West Valley Fault (which reaches down to Cavite) and the Infanta Fault (which reaches down to Lucena City in Quezon).  Both fault lines are represented in the map above as broken red lines.

Now, let us proceed to the not-so-good news.

While Phivolcs has, since 2003, been mapping the Philippine Fault with the help of Kyoto University and that this mapping is 90% complete4, it is not as though these faults ever present themselves to the scientists to be counted. In fact, the 2013 magnitude 7.2 earthquake in Bohol was caused by the movement of a heretofore undiscovered fault5. What this earthquake tells us is that sometimes, a fault only presents itself by way of a significant, and in the case of Bohol, deadly movement.

Next, while no fault lines have been mapped over Batangas’ land territory, the West Valley and Infanta faults reach down to Cavite and Lucena, which are both in geologic terms right next door to Batangas. If and when these two faults cause significant earthquakes, suffice it to say that Batangas will know.

Finally, what the 4 April earthquake off Tingloy, caused by a movement of the Lubang fault, tells us all is that a fault movement does not have to be on Batangas landmass for it to cause great distress to all of us who live in the province.

In fact, the July 1990 Luzon magnitude 7.8 earthquake, with epicenter placed at Rizal in Nueva Ecija6, was felt as at least Intensity 6 in Lipa City roughly 170 kilometers away. Then there was the 1994 magnitude 7.1 Mindoro quake which was caused by the movement of the Aglubang River fault in northern Mindoro. The tremor caused a tsunami along with landslides on Verde Island7. The main quake along with its numerous significant aftershocks were definitely also felt in Batangas.

While it is somewhat comforting to know that there is just one known fault line over Batangas landmass, the simple truth is that if and when there is significant movement of faults elsewhere, particularly those that cause shallow earthquakes that are felt farther out from the epicenter, this will at the very least also cause significant shaking in Batangas.

In fact, the most damage caused by the June 1990 Luzon quake was in Baguio City, roughly 115 kilometers from the epicenter in Nueva Ecija.

Notes and references:
1 “What is a fault and what are the different types? USGS FAQs,” online at the USGS web site.
2 “Batangas tremors spawned by unnamed fault in Mabini town —PHIVOLCS specialist,” online at GMA News Online.
3 “Walang Bagong Fault Line sa Batangas – Phivolcs,” online at the DWIZ web site.
4 “Philippine Fault Zone Maps,” online at the Phivolcs web site.
5 “2013 Bohol earthquake,” Wikipedia.
6 “1990 Luzon earthquake,” Wikipedia.
7 “1994 Mindoro earthquake,” Wikipedia.

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