Update as of 19 August 2020
What is happening in Masbate and vicinity?
At 08:03 AM Philippine Standard Time (PST) of 18 Aug 2020 (Tuesday), a strong Magnitude (Mw) 6.6 earthquake shook the province of Masbate and vicinity. The earthquake has an epicenter located 7 kilometers S29°E of Cataingan (Masbate) and a depth of 21 kilometers. As of 4:00 PM, 19 August 2020, 244 aftershocks ranging from M 1.6 to M 5.1 were recorded, 129 of which were plotted, and five were felt (Figure 1).
Using the PHIVOLCS Earthquake Intensity Scale (PEIS), the preliminary intensity reports* are summarized below.
The 18 August 2020 M6.6 earthquake was felt with a maximum ground shaking intensity of PEIS VII (Destructive). At PEIS VII, most people get frightened and run outdoors. People find it difficult to stand on the upper floors. Heavy objects and furniture overturn or topple. Trees are shaken strongly. 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, fish ponds, road surfaces, or concrete hollow block walls. Liquefaction effects (e.g. subsidence, sand boils, lateral spreads, etc.) in low-lying areas and landslides in mountains are observed near the epicenter.
Have major to moderate magnitude earthquakes affected Masbate in the past?
At least 11 earthquakes with magnitudes ranging from M6.0 to M7.0 affected Masbate Province and vicinity between 1869 and 2003 based on the PHIVOLCS Earthquake Catalog and the report by the Southeast Asia Association of Seismology and Earthquake Engineering (SEASEE). The last strong earthquake that occurred in the province was a M6.2 event on 15 February 2003. This earthquake generated an approximately 23-km long surface rupture. The maximum ground shaking intensity was felt at PEIS VIII (Very Destructive) within the epicentral area. It was felt as far as Kalibo, Aklan, which is ~200 kilometers away from the epicenter.
Why do earthquakes occur in Masbate?
Masbate is one of the seismically active regions in the country because of the presence of active faults that include the Masbate segment of the Philippine Fault, and potentially active faults that include Uson Fault and the 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.
Can this earthquake indicate volcanic activity?
No. The nearest active volcano is the Biliran Volcano which is approximately 65 kilometers from the epicenter and this earthquake is tectonic in origin.
What can we expect from the current earthquake activity?
Minor to moderate aftershocks are expected to occur in the epicentral area but occurrences of strong aftershocks cannot be discounted. These may continue for several days to weeks, some of which may be felt.
Aside from strong ground shaking, what other seismic hazards are life-threatening?
Landslides, rock falls, and other types of mass movements may occur on mountainous or hilly areas. Liquefaction, manifested by subsidence, sand boils or lateral spreads may affect low-lying, water-saturated, and sandy areas near river banks and shorelines. In particular, coastal areas are vulnerable to inundation of seawater due to the combination of liquefaction-induced subsidence and changes in sea level during high tide.
Can this recent earthquake event trigger a destructive tsunami?
No. Although the epicenter of the 18 August 2020 M6.6 event is offshore, no destructive tsunami waves were generated because there was no significant vertical displacement of the seafloor. More often, tsunamis are produced by earthquakes with significant vertical movements and from earthquakes generated by active trenches. Localized sea-level disturbances may occur though as a result of submarine landslides.
What should be done by the affected communities?
People are reminded to be cautious of structures visibly weakened, having signs of damage or are partially collapsed by the 18 August 2020 event. The expected aftershocks may also cause further damage to already weakened structures. In the case of houses and other buildings with visible damage, 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 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. In case of another strongly felt earthquake, it is recommended that people protect themselves by doing the “drop, cover and hold.”
Immediate life safety is the priority when evacuation after an earthquake is necessary. It is important for the public to understand that an earthquake evacuation takes priority over a COVID-19 Stay-at-Home order. It is also important that risks of COVID-19 spread among the public during evacuations are managed. At all times, people affected by the earthquake are also strongly advised to wear face masks and to strictly observe physical distancing before, during, and after evacuation.
During earthquake events, rumors and unverified information 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 104 seismic stations spread across the Philippines. Data from the seismic stations are used to determine the location, magnitude and other characteristics of the earthquakes generated.
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.
Aside from monitoring earthquakes, DOST-PHIVOLCS also provides other services such as hazards analyses and assessments. DOST-PHIVOLCS works hand-in-hand with other government agencies in mitigating the damaging effects of earthquakes. Furthermore, DOST-PHIVOLCS immediately deployed PHIVOLCS personnel from Masbate Seismic Station 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.
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