Update as of 03 December 2023

What is happening in Surigao del Sur and its vicinity?

At 10:37 PM Philippine Standard Time (PST) on 02 December 2023 (Saturday), a major earthquake with magnitude (Mw) 7.4 shook the Province of Surigao del Sur and a large portion of Mindanao. The earthquake has an epicenter located 29 kilometers northeast of Hinatuan, Surigao del Sur, and has an estimated depth of 26 kilometers (Figure 1). The earthquake was associated with the subduction along the Philippine Trench. As of 1:00 PM PST on 03 December 2023, the DOST-PHIVOLCS Philippine Seismic Network (PSN) has recorded a total of 819 aftershocks ranging from Ms 1.4 to Ms 6.2.

Using the PHIVOLCS Earthquake Intensity Scale (PEIS), the highest reported intensity is PEIS VII (Destructive) which was felt in Tandag City, Surigao del Sur based on the DOST-PHIVOLCS Earthquake Information 3 (Annex 1). At PEIS VII, many people are frightened and may run outdoors. Heavy objects and 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, fish ponds, road surfaces, or concrete hollow block walls. Limited liquefaction, lateral spreading, and landslides are observed. Trees are shaken strongly.

Have strong to great earthquakes affected Surigao del Sur, and its vicinity in the past?

Yes. At least six (6) significant earthquakes with magnitudes ranging from M 5.7 to M 8.0 affected Surigao del Sur, and nearby provinces in Eastern Mindanao between 1917 and 2017 based on the historical records (SEASEE, 1985) and DOST-PHIVOLCS Earthquake Catalog. Some major earthquakes (M>7) that affected the eastern portion of Mindanao were the 1921 M 7.5, 1929 M 7.2, and the 1952 M 7.6, all of which were confirmed to be tsunamigenic. The last damaging earthquakes that affected Surigao del Sur and its vicinity were the M 7.1 and M 7.5 events that occurred on 17 May 1992.

Why do earthquakes occur in the vicinity of Surigao del Sur?

Surigao del Sur and neighboring provinces experience earthquakes as they are situated in one of the seismically active regions in the Archipelago. The presence of active faults and trenches, such as the Philippine Fault and its segments, as well as the Philippine Trench, plays a significant role in generating seismic events. Additionally, there are local faults in proximity, some potentially buried by recent deposits capable of generating minor to strong earthquakes.

What can we expect from the current earthquake activity?

We can expect moderate to strong aftershocks in the epicentral area. These aftershocks could persist for several days to months, and some may also be felt in nearby provinces.

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 in mountainous or hilly areas. Liquefaction effects, manifested by subsidence, ground fissures, sand boils, lateral spreads, and/or flotation of buoyant structures may affect low-lying, water-saturated, and sandy areas near water bodies. Active river channels and coastal areas may be vulnerable to inundation of seawater due to the combination of liquefaction-induced subsidence, and changes in sea level due to sea level disturbance or tsunami.

Did this recent earthquake event trigger a tsunami?

Yes. DOST-PHIVOLCS sea level monitoring stations confirmed the occurrence of a tsunami in Hinatuan and Bislig City in Surigao del Sur, Dapa in Surigao del Norte, and Mati in Davao Oriental. However, the tsunami that occurred was not destructive. The last recorded tsunami wave arrival in the Philippines occurred at 02:52 AM (PST), 03 December 2023 at the Hinatuan-Bislig Bay Station on Mawes Island in Hinatuan. This means that the tsunami threat associated with this earthquake has now largely passed the Philippines. Consequently, DOST-PHIVOLCS has canceled all Tsunami Warnings issued for this earthquake event.

Can this earthquake indicate volcanic activity?

No. The Mw 7.4 earthquake was determined to be tectonic in origin and the nearest active volcano is the Leonard Kniaseff Volcano, located approximately 110 kilometers south of Hinatuan, Surigao del Sur. However, as part of DOST-PHIVOLCS’ procedure for moderate to major earthquakes occurring near active volcanoes, the Institute will closely monitor earthquake events that may be associated with Leonard Kniaseff Volcano and other nearby active volcanoes.

What should be done by the affected communities?

Communities should be prepared for moderate to strong aftershocks. In case of a strongly felt earthquake, it is recommended that people protect themselves by doing the “drop, cover, and hold.” For coastal communities, be aware of and monitor the natural signs of an approaching local tsunami - strong ground shaking (SHAKE), sudden and unusual changes in the sea level (DROP), and unusual sound (ROAR). If any of these is felt, observed, or heard, immediately move to high grounds or away from the shoreline.

People should be cautious of structures visibly weakened or having signs of damage caused by the Mw 7.4 earthquake. 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 urged to inspect buildings and infrastructures to determine their structural integrity and recommend appropriate actions. Structurally compromised buildings should not be reoccupied until certified safe by structural engineers. In homes and offices, heavy furniture and appliances should be strapped to the walls, and hanging objects securely fastened to prevent them from causing injuries.

Mountain/hill slopes should be checked for tension/incipient cracks that may have resulted from strong ground shaking. Tension cracks may render slopes more susceptible to landslides, especially during heavy rainfall and/or aftershocks. Areas that are identified as prone to landslides should be avoided.

During earthquake events, rumors that may cause panic are easily spread. Avoid sharing messages from unconfirmed and unreliable sources, and only rely on information from DOST-PHIVOLCS and respective 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 123 seismic stations spread across the Philippines. The closest seismic stations to Hinatuan, Surigao del Sur are the staff-controlled seismic stations in Bislig, Surigao City, Davao City, and Kidapawan City, and remote-telemetered seismic stations in Tandag City in Surigao del Sur, General Luna in Surigao del Norte, Talacogon in Agusan del Sur, Butuan City, Cateel in Davao Oriental, Laak in Compostela Valley, Valencia in Bukidnon, Loreto in Dinagat Island, and Mati City. These stations are augmented by the three-station volcano monitoring network at Leonard Kniaseff.

DOST-PHIVOLCS will deploy a Quick Response Team (QRT) to the provinces of Surigao del Sur and Davao Oriental. The QRT aims 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 these provinces.

Please visit our website at www.phivolcs.dost.gov.ph, Facebook (/PHIVOLCS), and Twitter (@phivolcs_dost) 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 social media accounts.