Batangas swarm reaches 2k quakes, but only 37 felt

More than 2,000 earthquakes so far have now been associated with the Batangas “earthquake swarm” that started in a Magnitude 5.5 temblor last April 4 and culminated in a Magnitude 6 main shock on Saturday.

And the “swarm” is still ongoing, in “small-sized earthquakes, some of which are felt,” and may possibly go on for weeks, said Philippine Institute of Volcanology and Seismology (Phivolcs) director Renato Solidum Jr., in a phone interview on Tuesday, one week after the first quake in the swarm.

Solidum said as of 5 p.m. on Tuesday, there were already 2,390 earthquakes associated with the swarm, with 37 of them being felt. The quakes range from magnitude 1.6 to magnitude 6.

“An earthquake swarm is a burst of earthquake activity clustered in a specific area in a short period of time due to movement of a fault,” the Phivolcs explained, in a primer uploaded on the Phivolcs website on Tuesday afternoon.

“Swarms aren’t unusual. That happens a lot in the Philippines, essentially minor or small earthquake events constituting the swarm,” Solidum said.

The Batangas swarm constituted small-sized and moderately-sized earthquakes, “part of a series of incremental movements” of an unnamed local offshore fault spanning Tingloy Island and Mabini town in Batangas province,” Solidum said.

The swarm is different from the usual main shock with aftershocks in that the latter would constitute a major or distinct fault movement, followed with smaller readjustments, whereas the earthquake swarms in Batangas were triggered by incremental fault movements, with intervening small events, Solidum said.

“Small to moderate magnitude earthquake events can still occur in the following days to weeks,” the Phivolcs said, of the Batangas swarm. Moderate-sized earthquakes, Solidum said, are within magnitudes 5 to 6. Solidum said earthquake swarms can last from several days up to weeks.

While not discounting the possibility of another earthquake within the range of the Magnitude 6 quake on Saturday, Solidum, however, discounted the possibility of a major earthquake in the swarm, or an earthquake reaching magnitude 7, since major earthquakes are usually associated with long faults, while the unnamed fault along Tingloy and Mabini is “not that long.”

No volcanic activity, tsunami

Too, the Phivolcs debunked rumors that the earthquakes can trigger volcanic activity or a destructive tsunami: “Although Taal Volcano is located approximately 30 kilometers from Mabini, Batangas, the present network of instruments located in and around Taal Volcano shows no indication of any significant change of monitoring parameters suggesting renewed magmatic activity,” the Phivolcs primer read. The earthquake magnitude is also not big enough for a tsunami.

The Phivolcs, however, cautions of collapsing structures and falling objects due to ground shaking, landslides or rockfalls in mountainous or hilly areas, and liquefaction and sandboils in low-lying areas or waterlogged coastlines or riverbanks.

“The best course of action is preparedness – the damaging effects of earthquakes can be minimized if we prepare ourselves for the event.  Because a large-magnitude earthquake, either from active faults in Luzon or the Manila Trench may affect Batangas as a whole, it is always prudent to prepare for such an eventuality,” the Phivolcs said, in its primer.