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A PRIMER ON THE 12 APRIL 2017 MAGNITUDE 6.0 LANAO DEL SUR EARTHQUAKE PDF Print
Wednesday, 12 April 2017 17:53

12 April 2017

What is happening in Lanao del Sur?

At 5:21 AM on 12 April 2017, Wednesday, a moderate earthquake of magnitude 6.0 shook the island of Mindanao. The epicenter is located 13 km northwest of Wao, Lanao del Sur at a depth of 1 km.  The earthquake was generated by the movement of a northwest trending active fault in the area. Small-magnitude earthquakes followed afterwards, and as of 3:00 PM of 12 April 2017, 73 aftershocks have been recorded by PHIVOLCS seismic monitoring network.

Based on preliminary intensity reports, the strongest ground shaking was felt at PHIVOLCS Earthquake Intensity Scale (PEIS) VII (Destructive) in Wao, Lanao del Sur and Kalilangan, Bukidnon. Furthermore, it was felt at PEIS VI (Very Strong) in Amai Manabilang, Lanao del Sur;  PEIS V (Strong) in Pangantukan, Bukidnon, and Banisilan, Cotabato.  It was also felt at PEIS IV in Cagayan de Oro City, Iligan City, Cotabato City, Gingoog City in Misamis Oriental, Matalam in North Cotabato, Valencia and Maramag in Bukidnon, and Davao City. This was felt at PEIS III to I in Kabacan and Arakan in North Cotabato, Quezon and Don Carlos in Bukidnon, Pikit in Cotabato, Lebak in Sultan Kudarat, Kidapawan City, Koronadal City, and Mambajao in Camiguin. Damages to some buildings and roads due to strong ground shaking were reported near the epicentral area. Landslides were also reported in Amai Manabilang (Bumbaran). At the time of writing, there are no reports of casualties.

Large magnitude earthquake has affected Lanao del Sur in the past!

On April 1, 1955, a magnitude 7.5 earthquake was experienced in Lanao area. This earthquake, which is considered to be one of the most damaging historical earthquakes in Lanao del Sur, resulted to about 400 deaths. Strong ground shaking was also felt during this event, resulting to significant damage to infrastructures including a mosque in Tugaya municipality near Lake Lanao. Several houses also collapsed and wharfs were damaged in Zamboanga City and Pagadian City. Liquefaction and landslides were also documented.

Aside from this, several small to moderate earthquakes have also been recorded within the province, with intensities ranging from PEIS IV to VII.

Why do earthquakes occur in Lanao del Sur?

Central Mindanao, including Lanao del Sur, is one of the seismically active areas in the country because of the presence of the western extension of the Mindanao Fault (Cotabato-Sindangan Fault), an active fault that runs from Sarangani province to northwest of Zamboanga Peninsula. Cotabato Trench is also a major source of earthquakes which can affect the region. In addition, there exist other local active faults, which can be sources of small- to moderate-magnitude earthquakes.

Can these present earthquakes indicate volcanic activity?

No. The origin of the 12 April 2017 magnitude 6.0 earthquake and its subsequent aftershocks are clearly tectonic.

What can we expect from the current earthquake activity?

The current available seismic information suggests that the magnitude 6.0 earthquake on 12 April 2017 is the main shock, which caused the strong ground shaking.  The succeeding small-magnitude earthquakes are the aftershocks.  The aftershocks may continue to occur 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 in mountainous or hilly areas. Liquefaction, manifested by sand boils or lateral spread may affect low-lying, water-logged, sandy areas at the banks of rivers.  Tsunami is not expected because the source of the 12 April 2017 event is on land. The tsunami threat for Lanao del Sur would come from the movement of the Cotabato Trench, located south of the province. However, Lake Lanao may be disturbed and may result to a phenomenon called seiche. Seiche is a temporary oscillation in an enclosed body of water such as lakes caused by strong ground shaking. This phenomenon was observed during the 1955 Magnitude 7.5 Lanao Earthquake.

What should be done by the affected communities?

People are reminded to be cautious of houses and buildings and other structures visibly weakened or with signs of damage by the 12 April 2017 earthquake as these may be further damaged by aftershocks. In case of houses and other buildings with visible damage, it is best to contact the Municipal/City Engineering Office for advice.  Engineers from the local government, other agencies and organizations should inspect houses and other structures to determine their integrity and recommend appropriate actions to concerned affected groups or individuals.  Slopes should be checked for tension cracks that may have resulted from the strong ground shaking. Tension cracks may make slopes more susceptible to landslides. These areas should also be avoided.

The best course of action is preparedness. In case of another felt earthquake, it is recommended that people protect themselves by doing the “duck, cover and hold”.  In houses and offices, cabinets should be strapped to the walls, and appliances be secured to prevent them from toppling and causing injury to persons.

What is the role of PHIVOLCS?

PHIVOLCS operates and maintains a network of 93 seismic stations spread across the Philippines. Eighteen of these seismic stations are located in Mindanao Island, eight of which are staffed-controlled and are located in Cotabato City, Kidapawan City, Cagayan De Oro City, Bislig City, Surigao City, Dipolog City, Zamboanga City, and General Santos City. PHIVOLCS has also ten remote seismic stations located in Pikit in Cotabato, Valencia in Bukidnon, Sultan Kudarat, Talacogon in Agusan del Sur, Butuan City, Don Marcelino in Davao Occidental, Mati and Cateel in Davao Oriental, Pagadian City and Ipil in Zamboanga Peninsula.  Data from these seismic stations are used to determine the locations of earthquakes, as well as the characteristics of the earthquakes generated.

Aside from monitoring the occurrences of earthquakes, PHIVOLCS also provides the public with information on hazards analyses and assessments. As part of its mandate, PHIVOLCS works hand-in-hand with other government agencies in mitigating the damaging effects of earthquakes.

More information about recent earthquakes, volcanic activities and hazard maps can be obtained at PHIVOLCS website at www.phivolcs.dost.gov.ph, Facebook (www.facebook.com/PHIVOLCS) and Twitter (@phivolcs_dost) accounts. Earthquake observations may also be reported to PHIVOLCS at telephone numbers (02) 929-9254 and (02) 426-1468 to 79, local 124 and 125.

 

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PRIMER ON THE APRIL 2017 SERIES OF EARTHQUAKES IN BATANGAS PROVINCE PDF Print
Tuesday, 11 April 2017 14:14

11 April 2017


What is happening at Batangas Province?

A phenomenon called earthquake swarm is currently happening at Batangas.  An earthquake swarm is a burst of earthquake activity clustered in a specific area in a short period of time due to movement of a fault. The Batangas swarm started on 4 April at 8:58 PM, and is still ongoing to date.

Large-magnitude earthquakes have also affected Batangas in the past!

At least six large earthquakes had affected Batangas province – the Mindoro earthquakes of November 15, 1994, April 9, 1942, and May 26, 1889, and the earthquakes of April 26, 1972, October 1, 1869, and September 16, 1852 whose epicenters were located in West Philippine Sea. The 1994 Magnitude 7.1 Mindoro Earthquake was felt at intensity VII in the epicentral areas and may have shaken Batangas at intensities of V to VI or even VII while the 1889 Magnitude 6.4 Earthquake was felt at Intensity VI-VII in Mindoro and Batangas. The 1852 Magnitude 7.6 and 1869 Magnitude 6.6 offshore events were reportedly felt at Intensity VII in and around Batangas. Although there were no accounts of direct damage in Batangas due to the 1994, 1972, and 1942 events, the 1852, 1869 and 1889 events damaged several buildings and infrastructures in the area.

Why do earthquakes occur at Batangas?

Batangas is one of the seismically active areas in the Philippines.  Instrumental monitoring of earthquakes for the past century has detected many small to large-magnitude earthquakes near Batangas generated by the Manila Trench and Lubang Fault.  The Manila Trench is an earthquake generator located offshore west of Luzon Island, roughly parallel to the Philippine archipelago in the north but veers very close to land at the southern tip of Occidental Mindoro.  Another offshore earthquake generator is Lubang Fault, located between Mindoro Island and Batangas, which is also the locus of small to large-magnitude earthquakes.  Other active faults on land are present in Southern Luzon, such as the Valley Fault System and the Philippine Fault. The current series of earthquakes in Batangas can be attributed to the movement of an unnamed local fault in the vicinity of the Tingloy-Mabini area.

Can these present earthquakes indicate volcanic activity?

No. Although Taal Volcano is located approximately 30 kilometers from Mabini, Batangas, the present network of instruments located in and around Taal Volcano shows no indication of any significant change of monitoring parameters suggesting renewed magmatic activity. However, as part of PHIVOLCS’ monitoring procedures for moderately large earthquakes occurring near active volcanoes, the institute will closely monitor this earthquake events in relation to any activity that may be monitored in Taal Volcano.

Can these recent earthquake events trigger a destructive tsunami?

No. The magnitude is not big enough to generate a destructive tsunami.

Can human activity such as geothermal drilling, blasting, etc.  trigger large earthquakes along active faults?

No. Hypocenters of moderate- to large-magnitude earthquakes along active faults are too deep to be influenced by any human activity.

What can we expect from the current earthquake activity?

Small to moderate magnitude earthquake events can still occur in the following days to weeks.

What should we do?

The best course of action is preparedness – the damaging effects of earthquakes can be minimized if we prepare ourselves for the event.  Because a large-magnitude earthquake, either from active faults in Luzon or the Manila Trench may affect Batangas as a whole, it is always prudent to prepare for such an eventuality.

What can we expect in the event of a large-magnitude/high-intensity earthquake?

Strong ground shaking may cause extensive damage to, or even the collapse of houses, buildings, bridges, and other infrastructures.  Collapsed structures usually account for most of the casualties during a strong earthquake.  Falling objects may also cause injuries.

Aside from strong ground shaking, what other seismic hazards are life threatening?

Landslides, rock falls, and other types of mass movements may occur in mountainous or hilly areas.  Liquefaction manifested by sandboils or lateral spreading may affect low-lying, waterlogged, sandy areas near the coast or at the banks of rivers.

What is the role of PHIVOLCS?

PHIVOLCS operates and maintains a network of 93 seismic stations spread across the Philippines. Twelve of these seismic stations are located in or around Batangas. Four of which are staffed-controlled located in Puerto Galera, Tagaytay, and Lucban and Guinayangan in Quezon and seven are remote seismic stations located in Lubang and San Jose in Mindoro, Busuanga and Cuyo in Palawan, Romblon, Marinduque and San Andres in Quezon.  Data from the seismic stations are used to determine the locations of earthquakes, as well as the characteristics of the earthquakes generated. We also have a volcano monitoring network in and around Taal that could record seismic activity.

Aside from monitoring the occurrences of earthquakes, PHIVOLCS also conducts hazards analyses and assessment, and makes this information available to the public.  PHIVOLCS works hand-in-hand with other government agencies in mitigating the damaging effects of earthquakes.

Please visit our website at http://www.phivolcs.dost.gov.ph for earthquake bulletins, volcano updates, and other information on earthquakes and volcanoes.

 
PAALALA: PDF Print
Saturday, 08 April 2017 10:00

Ang PHIVOLCS-DOST ay hindi nagbibigay ng prediksyon patungkol sa lindol.

Para sa karagdagang impormasyon, bisitahin ang OPISYAL NA PHIVOLCS-DOST WEBSITE:

(www.phivolcs.dost.gov.ph), Facebook (www.facebook.com/PHIVOLCS) at Twitter (@phivolcs_dost).

PAKIUSAP, HUWAG magpadala o mag-forward ng anumang impormasyon na maaaring lalong makadagdag sa pagkalito at pagkatakot ng mga makakabasa nito.

 
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