Update as of 20 August 2023
What is happening in Ragay, Camarines Sur and vicinity?
Ragay, Camarines Sur is currently experiencing an earthquake swarm which started on 19 August 2023 (Figure 1). An earthquake swarm is defined as a short-lived sequence of minor to light earthquakes, showing no identifiable large event or main shock. As of 3:00 PM on 20 August 2023, 45 earthquakes were plotted, 34 of which were felt. The highest magnitude recorded by the Philippine Seismic Network for this earthquake swarm is MW 4.4 which was felt at PHIVOLCS Earthquake Intensity Scale (PEIS) V (Strong) in the municipality of Ragay, Camarines Sur. At PEIS V, ground shaking is generally felt by most people. Indoors, strong shaking is felt throughout the building, hanging objects swing violently, and small, light and unstable objects may fall or overturn. Outside, standing vehicles rock noticeably, and shaking of leaves and twigs of trees are observed.
What can we expect from the current earthquake swarm?
An earthquake swarm may vary in duration and outcome. One scenario is a short-duration burst of earthquake activity that may last for a few days, similar to the two earthquake swarms that occurred in Camarines Sur last 13 December 1993 and 14 to 16 October 2021. Another possible scenario that should not be discounted is that the earthquakes may be a precursor to a larger magnitude earthquake.
Have strong to major earthquakes affected Ragay, Camarines Sur and vicinity in the past?
Yes, there have been at least seven significant earthquakes, with magnitudes ranging from 6.0 to 7.4, that have impacted Ragay, Camarines Sur, and its vicinity from 1811 to the present. This information is based on reports from the Southeast Asia Association of Seismology and Earthquake Engineering (SEASEE) and the PHIVOLCS Earthquake Catalog. The most damaging earthquake that affected Ragay, Camarines Sur was the 17 March 1973 M7.0 Ragay Gulf Earthquake generated by the Guinayangan Segment of the Philippine Fault. This earthquake was felt at a maximum intensity of VIII (Very Destructive) and caused numerous landslides, ground rupture, liquefaction, tsunami and structural damages (Morante, 1974).
Why do earthquakes occur in Camarines Sur and vicinity?
Camarines Sur is a seismically active region in the country because of the presence of active faults that include the Legaspi Lineament and offshore segments of the Philippine Fault in Ragay Gulf. Other local faults in close proximity might exist, and some of them could potentially be concealed by recent deposits. These concealed faults have the potential to generate minor to strong earthquakes. According to earthquake data, the ongoing cluster of earthquakes is caused by a strike-slip fault, a fault with dominant horizontal movement (Figure 1).
Aside from strong ground shaking, what other seismic hazards may be expected?
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.
Can these recent earthquakes trigger a destructive tsunami?
No, the epicenters of these earthquakes are situated inland, and tsunamis are typically produced by earthquakes that occur along active trenches, offshore faults with significant vertical movements, or mass movement such as earthquake-induced landslides offshore or near bodies of water. However, localized sea-level disturbances may be observed as a result of extreme ground shaking resonating along bays.
Can these earthquakes indicate volcanic activity?
No, the earthquake swarm is clearly tectonic in origin. The closest volcano, Labo Volcano, is approximately 15 kilometers away, and is considered potentially active. The nearest active volcano is Isarog Volcano, situated around 60 kilometers away. Nevertheless, as part of DOST-PHIVOLCS monitoring procedures for moderate to large earthquakes occurring near active volcanoes, the institute will closely monitor earthquake events in relation to any activity that may be associated with the volcanoes.
What should be done by the affected communities?
In case of a strongly felt earthquake, it is recommended that people protect themselves by doing the standard practice of “drop, cover, and hold.” This practice has been widely recognized for its ability to reduce the risk of injury during earthquakes.
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 series of earthquakes as these may further damage the structures and injure the building occupants. It is best to contact the 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 repeated ground shaking. Tension cracks may render slopes more susceptible to landslides, especially during heavy rainfall or earthquakes. 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 operates and maintains a network of 123 seismic stations across the Philippines. The closest seismic stations to Ragay, Camarines Sur is the staff-controlled seismic station in Guinayangan, Quezon; and satellite-telemetered seismic stations located in San Jose, Camarines Sur, Jose Panganiban in Camarines Norte, and San Andres and Alabat Island in Quezon. These seismic stations are augmented by the Iriga Volcano Station in Camarines Sur. Data from the seismic network will be used for monitoring earthquakes.
Furthermore, DOST-PHIVOLCS works hand-in-hand with other government agencies in mitigating the damaging effects of earthquakes.
Please visit our website at www.phivolcs.dost.gov.ph, Facebook (/PHIVOLCS), and X (formerly 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 Facebook Messenger.
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