What is happening in Masbate and vicinity?
At 2:10 AM Philippine Standard Time (PST) on 16 February 2023 (Thursday), a major earthquake with magnitude (Mw) 6.0 shook the Province of Masbate. The earthquake has an epicenter located 11 kilometers southwest of Batuan, Masbate and an estimated depth of 10 kilometers (Figure 1). As of 1:00 PM PST on 16 February 2023, the DOST-PHIVOLCS Philippine Seismic Network (PSN) has recorded a total of 135 aftershocks ranging from M 1.5 to M 4.2.
Using the PHIVOLCS Earthquake Intensity Scale (PEIS), the highest reported intensity is PEIS VII (Destructive) based on the DOST-PHIVOLCS Earthquake Information 2 (Annex 1). At PEIS VII, most people get frightened and run outdoors. People find it difficult to stand on the upper floors. Trees are shaken strongly. Big church bells may ring. Old or poorly-built structures suffer considerable damage. Heavy objects and furniture overturn or topple. Some well-built structures are slightly damaged. Some cracks may appear on dikes, fish ponds, road surfaces, or concrete hollow block walls. Liquefaction effects (e.g. subsidence, sand boils, lateral spreads, etc.) in low-lying areas and earthquake-induced landslides on sloping terrains are observed near the epicenter.
Have strong to great earthquakes affected Masbate and vicinity in the past?
Yes. At least 12 earthquakes with magnitudes ranging from M6.0 to M7.0 affected Masbate Province and vicinity between 1869 and 2022 based on the Southeast Asia Association of Seismology and Earthquake Engineering (SEASEE) Report and DOST-PHIVOLCS Earthquake Catalog. The last strong earthquake that occurred in the province was a M6.6 event on 18 August 2020. This earthquake generated an approximately 24-km long surface rupture. The maximum ground shaking intensity was felt at PEIS VII (Destructive) within the epicentral area. It was felt as far as Sipalay City at PEIS I (Scarcely Perceptible), which is ~300 kilometers away from the epicenter.
Why do earthquakes occur in Masbate and vicinity?
Earthquakes occur in Masbate because it is one of the seismically active regions in the country and has active faults that include the Masbate and Sibuyan Sea Segments of the Philippine Fault (PF), and potentially active faults that include the Uson Fault and Southern Masbate Fault. There are other nearby local faults, some of which may be covered by recent deposits, and offshore active faults that could be sources of minor to strong earthquakes.
What can we expect from the current earthquake activity?
We can expect aftershocks to occur in the epicentral area, but occurrences of moderate aftershocks are not discounted. These may continue for several days to weeks, some of which may be felt.
Figure 1. Plot of the 16 February 2023 Magnitude 6.0 Masbate Earthquake and aftershocks as of 16 February 2023, 1:00 PM.
Minor earthquakes: M3 to 3.9; Light earthquakes: M4 to 4.9; Moderate earthquakes: M5 to 5.9; Strong earthquakes: M6 to 6.9; Major earthquakes: M7 to 7.9; Great earthquakes: M8.0 and above.
Aside from strong ground shaking, what other seismic hazards may be expected?
Earthquake-induced landslides, rock falls, and other types of mass movement may occur on sloping terrains, specifically on 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, areas adjacent to active river channels and coasts may be vulnerable to inundation of seawater due to the combination of liquefaction-induced subsidence, and changes in sea level.
Can this recent earthquake event trigger a destructive tsunami?
No. Although the epicenter of this earthquake is offshore, no destructive tsunami waves were generated because there was no significant vertical displacement of the seafloor. Tsunamis are often produced by earthquakes generated by active trenches and offshore faults or faults with significant vertical movements or mass movement related to earthquake-induced landslides near bodies of water. 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?
Communities should prepare for aftershocks. 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 and appliances should be strapped to the walls, and hanging objects securely fastened to prevent these from causing injuries.
People should be cautious of structures visibly weakened or having signs of damage caused by the Mw 6.0 earthquake event, as these may further be damaged by succeeding earthquakes and injure building occupants. It is best to contact the concerned Municipal/City Engineering Office for inspection and advice. Civil engineers from the local government, and other organizations are strongly enjoined to inspect buildings and infrastructure to determine their structural integrity and recommend appropriate actions. 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, especially during heavy rainfall or aftershocks. Areas that may be affected by impending landslides should be avoided.
During earthquake events, rumors that may cause panic are easily spread. Please avoid sharing messages from unconfirmed and unreliable sources, and only rely on information from DOST-PHIVOLCS and pertinent Disaster Risk Reduction and Management Offices (DRRMOs).
What is the role of DOST-PHIVOLCS?
DOST-PHIVOLCS conducts seismic monitoring and provides earthquake information. In addition, DOST-PHIVOLCS also operates and maintains a network of 116 seismic stations spread across the Philippines. The closest seismic stations to Masbate are the staff-controlled seismic stations in Masbate City in Masbate, and Roxas City in Capiz; and remote-telemetered seismic stations in Cadiz City in Negros Occidental, Catarman in Northern Samar, Borongan in Eastern Samar, Ormoc City in Leyte, and Medellin in Cebu. These stations are augmented by the Mayon Volcano Observatory, Bulusan Volcano Observatory and the Iriga Volcano Station.
DOST-PHIVOLCS will deploy a Quick Response Team (QRT) to Masbate and vicinity whose main tasks are to assess impacts and hazards, and conduct information dissemination campaigns to allay the fears of the public. Furthermore, DOST-PHIVOLCS works hand-in-hand with other government agencies in mitigating the damaging effects of the earthquake events in Masbate.
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 or through our official Facebook account.
References:
DOST-PHIVOLCS (2003) - 15 February 2003 M6.2 Masbate Earthquake
DOST-PHIVOLCS (2020) - 18 August 2020 M6.6 Masbate Earthquake
Download PDF copy: Primer on the 16 February 2023 Magnitude 6.0 Masbate Earthquake
