Update as of 26 October 2022
What is happening in Abra and vicinity?
At 10:59 PM Philippine Standard Time (PST) on 25 October 2022 (Tuesday), a strong Magnitude (Mw) 6.4 Earthquake shook the provinces of Northern Luzon. The epicenter of the earthquake is located 5 kilometers northeast of Lagayan, Abra, at a depth of 16 kilometers (Figure 1). As of 7:00 PM PST on 26 October 2022, the DOST-PHIVOLCS Philippine Seismic Network has recorded a total of 528 aftershocks ranging from M 1.4 to M 4.8. The M6.4 Earthquake occurred near the epicentral location of the 27 July 2022 Mw 7.0 Earthquake and its associated aftershocks.
Using the PHIVOLCS Earthquake Intensity Scale (PEIS), the largest reported intensity is at PEIS VII (Destructive) (Annex 1). At PEIS VII, most people are frightened and run outdoors. People find it difficult to stand on the upper floors. Heavy objects or furniture overturn or topple. Big church bells may ring. Old or poorly-built structures suffer considerable damage. Some well-built structures are slightly damaged. Some cracks may appear on dikes, fishponds, road surfaces, or concrete hollow block walls. Limited liquefaction, lateral spreading, and landslides are observed. Trees are shaken strongly.
Have major to moderate magnitude earthquakes affected Abra and vicinity in the past?
At least 40 recorded historical earthquakes were reported in Abra and adjacent provinces between 1589 and 1983 sourced from the Southeast Asia Association of Seismology and Earthquake Engineering (SEASEE)- Series on Seismology Volume IV: A catalogue of Philippine Earthquakes (1985) and the catalog of Bautista and Oike (2000). Two of which are notably strong earthquake events, the September 1862 (M6.2) and September 1877 (M5.6) earthquakes. Recently, the 27 July 2022 M7.0 is the highest magnitude event among instrumentally-recorded earthquakes that have affected the area. The maximum reported intensity during the M7.0 event was PEIS VII and the shaking was felt as far as the Bicol Region.
Why do earthquakes occur in Abra and vicinity?
The Ilocos Region (Region I) in Northwestern Luzon is one of the seismically active regions in the country. The seismicity in Abra and vicinity can be attributed to the presence of several active faults in the region such as the Abra River Fault, West Ilocos Fault System, Bangui Fault, Naglibacan Fault, Manmanoc Fault, as well as the offshore presence of the Manila Trench, west of the province. Other nearby local faults, buried by recent deposits, could also be possible sources for both small and strong magnitude earthquakes.
What can we expect from the current earthquake activity?
Due to the magnitude of the recent earthquake, minor to moderate-magnitude aftershocks are expected to occur in the epicentral area, but occurrences of strong aftershocks are not discounted. These may continue for several days to weeks, some of which may be felt.
Aside from strong ground shaking, what other seismic hazards may be present?
Landslides, rock falls, and other types of mass movement may occur in mountainous or hilly areas. Liquefaction, manifested by subsidence, ground fissures, sand boils, and/or lateral spreads may affect low-lying, water-saturated, and sandy areas near water bodies. In particular, active river channels and coastal areas that may be vulnerable to inundation of seawater due to the combination of liquefaction-induced subsidence, and changes in sea level due to high tide.
Can this recent earthquake event trigger a destructive tsunami?
No. The epicenter of the earthquake is inland, thus significant vertical deformation will be confined inland near the epicenter. Based on the current Active Faults and Trenches Map of DOST-PHIVOLCS, a tsunami threat can occur from the movement of the Manila Trench and other offshore sources of significant submarine movement of the seafloor, located west of Abra and its vicinity, or other offshore active faults. Tsunamis are often produced by earthquakes with significant vertical movements near bodies of water, and earthquakes generated by active trenches and offshore faults. However, localized sea-level disturbances may be observed as a result of extreme ground shaking resonating along bays.
What should be done by the affected communities?
People are reminded to be cautious of structures visibly weakened or having signs of damages or unattended non-structural falling hazards (e.g., ceiling tiles, mounted computer billboards/LCD monitors, parapets, gable walls, large windows, and other items) caused by the 27 July 2022 event, as these may be further damaged by succeeding earthquakes or may eventually fall, injure or kill building occupants. For houses and other buildings with visible damage/s, it is best to contact the concerned Municipal/City Engineering Office for advice. Civil engineers from the local government, other agencies, and organizations are strongly enjoined to inspect buildings and infrastructure to determine their integrity and recommend appropriate actions to the affected population. Structurally compromised buildings should not be reoccupied unless certified safe by structural engineers.
Slopes should be checked for tension/incipient cracks that may have resulted from the strong ground shaking. Tension cracks may render slopes more susceptible to landslides. Such areas should be avoided.
The best course of action is preparedness. In case of another strongly felt earthquake, it is recommended that people protect themselves by doing the “drop, cover, and hold.” In homes and offices, heavy furniture should be strapped to the walls, hanging objects securely fastened, and appliances secured to prevent these from toppling and causing injuries to persons.
During earthquake events, rumors that may cause panic are easily spread. Please avoid sharing messages from unconfirmed and unreliable sources.
What is the role of DOST-PHIVOLCS?
DOST-PHIVOLCS operates and maintains a network of 116 seismic stations spread across the Philippines. The closest seismic stations to Abra are the staff-controlled seismic stations in Pasuquin in Ilocos Norte, Baguio City in Benguet, Sinait in Ilocos Sur, Peñablanca in Cagayan; and satellite-telemetered seismic stations located in Dolores in Abra, Santa in Ilocos Sur, Sagada Mountain Province, Conner in Apayao, Tabuk City in Kalinga, Pamplona and Gonzaga in Cagayan, Aguinaldo in Ifugao, Bolinao and San Manuel in Pangasinan, Cabarroguis in Quirino, Cauayan and Palanan in Isabela, Aritao in Nueva Vizcaya, and Casiguran in Aurora.
Aside from monitoring earthquakes, DOST-PHIVOLCS also provides other services such as hazard analyses, assessments and installing additional instruments for aftershocks monitoring, and downloading data from existing network instruments. DOST-PHIVOLCS works hand-in-hand with other government agencies in mitigating the damaging effects of earthquakes. Furthermore, DOST-PHIVOLCS immediately deployed a Quick Response Team to Abra and vicinity whose main tasks are to assess impacts, hazards, and conduct information dissemination campaigns to allay the fears of the public.
Please visit our website at www.phivolcs.dost.gov.ph, and our Facebook (/PHIVOLCS) and Twitter (@phivolcs_dost) accounts for earthquake information, volcano updates, hazard maps, and other information on earthquakes and volcanoes. Earthquake observations may also be reported to DOST-PHIVOLCS at telephone numbers (02) 8929-9254 and (02) 8426-1468 to 79, local 307 and 308.
Download PDF: https://bit.ly/Primer_25Oct2022_M6-4EQ
