More than 2,000 earthquakes so far have now been associated with the Batangas “earthquake swarm” that started with a 5.5-magnitude temblor last April 4 and culminated in a 6-magnitude main shock on Saturday.
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, a week after the first quake in the swarm hit.
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 1.6 to 6 magnitude.
“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.
“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 is constituted of 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 by smaller readjustments, whereas the Batangas earthquake swarm was 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 5 to 6 magnitude.
Solidum said earthquake swarms could last from several days up to weeks.