Since March 6, the Philippine Institute of Volcanology and Seismology (Phivolcs) has detected a series of “light” to “moderate” earthquakes that struck the same area in the province of Davao de Oro.
Among this series of earthquakes was the 5.9-magnitude earthquake that struck at 2:02 p.m. and was followed by the 5.6-magnitude tremor at 4:47 p.m., both on March 7.
Both quakes originated in New Bataan, Davao de Oro.
Shallower temblors hit the same area hours earlier as Phivolcs recorded moderate quakes on March 6, measuring 4.9-magnitude and 5.3-magnitude that occurred at 12:49 a.m. and 4:43 a.m., respectively.
Phivolcs said 952 earthquakes with magnitudes ranging from 1.5 to 5.9 were recorded as of 7 a.m., Wednesday, March 8.
It noted that 18 of these earthquakes were felt.
Phivolcs said that aftershocks are expected in the epicentral area and the occurrences of strong earthquakes are not discounted. These may persist for days to weeks and some may be felt.
‘Swarm’ or ‘series’ of quakes?
According to Phivolcs, the earthquakes in Davao de Oro are “series” of events, and not “swarm.”
“Swarm if earthquakes, of almost the same magnitude, occur distinctly along a single fault or generator. In the case of New Bataan, the adjacent active faults are causing the sequence of events with the same magnitude,” it explained.
Phivolcs pointed out that Davao de Oro is one of the seismically active regions in the country because of the presence of active faults that include the East Compostela Valley, West Compostela Valley, Central Compostela Valley, Nabunturan, Caraga River, and Mati Segments of the Philippine Fault and the Central Mindanao Fault.
There are other nearby local faults, some of which may be covered by recent deposits, that could be sources of small-to strong-magnitude earthquakes, it added.
‘Strong to great’ quakes also hit Davao de Oro in the past
Citing historical records, Phivolcs said at least 11 “strong to great” earthquakes ranging from magnitude 6.0 to magnitude 8.3 have occurred in and around Davao de Oro from 1891 to the present.
“The most damaging earthquake that affected Davao de Oro was the 21 June 1893 M7.3 Monkayo earthquake along the Philippine Fault, which produced numerous landslides, liquefaction, and structural damages. This earthquake generated long and wide cracks that may be attributed to a surface rupture along the Central Compostela Valley segment of the Philippine Fault,” it said.
Phivolcs noted that the 6.0-magnitude quake on Feb. 1, 2023 caused tension cracks, landslides, and structural damages, with maximum ground shaking intensity of VI, classified as “very strong.”