Philippine Standard Time
 

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Update as of 17 December 2019

 

What is happening in Davao del Sur and vicinity?

At 02:11 PM Philippine Standard Time (PST) of 15 December 2019 (Sunday), a strong Magnitude (M) 6.9 earthquake shook the province of Davao del Sur and vicinity. The epicenter of the earthquake is located 9 kilometers northwest of Matanao, Davao del Sur at a depth of 3 kilometers. This earthquake follows the recent earthquake events (the M5.9  on 9 July 2019, the M6.3 on 16 October 2019, the M6.6 and M6.1 on 29 October, and M6.5 on 31 October) in the area. After this, a total of 530 small to moderate magnitude earthquakes have been recorded by the DOST-PHIVOLCS Philippine Seismic Network as of 1:00 PM PST of 16 December 2019.

 

Using the PHIVOLCS Earthquake Intensity Scale (PEIS), the preliminary intensity reports* are summarized below.

 

Have major to moderate magnitude earthquakes affected Davao del Sur in the past?

Recent earthquake events that affected Regions XI and XII are the M5.9  on 9 July 2019, the M6.3 on 16 October 2019,  the M6.6 and M6.1 on 29 October, and M6.5 on 31 October . The maximum intensity of the M6.9 event was felt at PEIS VII (see table) and shaking was felt as far as 300 kilometers away from the epicenter.

At PEIS VII, most people are frightened and run outdoors. People find it difficult to stand in upper floors. Heavy objects and furniture overturn or topple. Trees are shaken strongly. Big church bells may ring. Old or poorly-built structures suffer considerably damage. Some well-built structures are slightly damaged. Some cracks may appear on dikes, fish ponds, road surface, or concrete hollow block walls. When the epicenter is located in the floodplain, liquefaction effects (e.g. sand boils, lateral spreading, etc.), and landslides are frequently observed.

At least five onshore earthquakes with intensities ranging from V to VIII on the Rossi-Forrel Scale have occurred in Davao Region and vicinity between 1908 and 1970 based on the PHIVOLCS Earthquake Catalog, and the report by the Southeast Asia Association of Seismology and Earthquake Engineering (SEASEE).

 

Why do earthquakes occur in Davao del Sur?

Davao Region (Region XI) in southern Mindanao, is one of the seismically active regions in the country because of the presence of several active faults that include the Tangbulan Fault, Central Digos Fault and Cotabato Fault System (CFS): Makilala-Malungon, Makilala, M’lang, North Columbio, South Columbio, Balabag. There are other nearby local faults, some of which may be covered by recent deposits, that could be sources of small to strong magnitude earthquakes.

 

Can this earthquake indicate volcanic activity?

No. Although the nearest active volcanoes from the epicenter are Matutum Volcano (~44 km) and Parker Volcano (~78 km), the M6.9 event and succeeding earthquakes are tectonic in origin. In addition, dedicated volcano monitoring networks currently operate on these and nearby volcanoes for observation of any abnormal seismic activity.

 

What can we expect from the current earthquake activity?

We expect small- to moderate-magnitude earthquakes to occur in the epicentral area which may continue for several days to weeks, some of which may be felt. Figure 1 shows earthquake plots as of 12:50 PM PST on 16 December 2019. Although the occurrence of another earthquake higher than M6.9 cannot be discounted, the probability of it coming from the same source area is low.

 

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 spreading may affect low-lying, water-saturated and sandy areas near river banks and shore lines.

 

Can this recent earthquake event trigger a destructive tsunami?

No. The epicenter of the earthquake is inland. Based on the current Active Faults and Trenches Map of DOST-PHIVOLCS, the tsunami threat could be sourced from the movement of the Cotabato Trench, located west of Davao Region or the Philippine Trench located east, or other offshore active faults. Other potential sources of tsunami that may affect the coastal communities of southern Mindanao are the Sangihe, Halmahera and North Sulawesi Trenches located farther south offshore of Mindanao.

 

What should be done by the affected communities?

People are reminded to be cautious of structures visibly weakened or having signs of damage caused by the series of strong earthquakes in July and October 2019, as these may be further damaged by succeeding earthquakes. In 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 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 earthquakes 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 104 seismic stations spread across the Philippines. Twenty-six of these seismic stations are located in Mindanao, nine of which are staffed-controlled and are located in Davao City, Kidapawan City, Cotabato City, Cagayan De Oro City, Bislig City, Surigao City, Dipolog City, Zamboanga City, and General Santos City. DOST-PHIVOLCS also has 17 remote-telemetered seismic stations located in Don Marcelino in Davao del Sur, Mati City and Cateel in Davao Oriental, Laak in Compostela Valley, Pikit in Cotabato, Bacolod in Lanao del Norte, Marawi City, Valencia in Bukidnon, Bagumbayan in Sultan Kudarat, Talacogon in Agusan del Sur, Butuan City, General Luna in Surigao del Norte, Tandag in Surigao del Sur, Loreto in Dinagat, Pagadian City and Ipil in Zamboanga Peninsula, and Bongao in Tawi-Tawi. Data from the seismic stations are used to determine the location, magnitude and other characteristics of earthquakes. The closest seismic stations to Davao del Sur are the Kidapawan City Seismic Station (KCP-staff-controlled) in Kidapawan City (Cotabato), and remote-telemetered seismic station in Don Marcelino (DDMP-Davao del Sur).

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 a Quick Response Team to Davao del Sur whose main tasks is 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.

 

Figure 1. Seismicity plot of the M6.9 earthquake and the succeeding aftershocks.

 

Download the Primer: http://bit.ly/DavaoDelSurEarthquakePrimer