The Philippine Institute of Volcanology and Seismology (Phivolcs) continued to detect more aftershocks, two days after a 7.0-magnitude earthquake jolted northwestern Luzon on Wednesday morning, July 27.
As of 7 a.m., Friday, July 29, Phivolcs had recorded 1,059 aftershocks, with magnitudes ranging between 1.5 and 5.0.
Of the 1,059 aftershocks, Phivolcs plotted 263 earthquake events close to the epicenter of the main shock in Abra province.
However, only 26 of the recorded aftershocks were felt.
Citing the 1985 report of the Southeast Asia Association of Seismology and Earthquake Engineering, Phivolcs said at least 40 historical earthquakes were reported between 1589 and 1985 that significantly affected Abra, Ilocos Norte, and nearby provinces.
“Ten of these earthquakes had magnitudes ranging from magnitude 3.8 to magnitude 5.6, that reported to have affected Abra province and vicinity,” it added.
Two major earthquake events have occurred in September 1862 (6.2-magnitude) and Sept. 12, 1877 (5.6-magnitude).
Phivolcs explained that northwestern Luzon is one of the seismically active regions in the country because of the presence of active faults that include the northern segments of the Philippine Fault, Abra River Fault, West Ilocos Fault System, and Naglibacan Fault.
“There are other nearby local faults, some of which may now be covered by recent deposits, and offshore active faults that are potential sources of minor to strong earthquakes,” it added.