Primer on the Magnitude 6.1 Central Luzon Earthquake

What is happening at Zambales and Pampanga?

At 5:11 PM on 22 April 2019, Monday, a moderate earthquake of Magnitude 6.1 shook the provinces of Zambales, Pampanga and vicinity. The epicenter is located 18 kilometers east of Castillejos, Zambales, on a mountainous area, at a depth of 10 kilometers. Small-magnitude earthquakes followed afterwards, and as of 8:00 AM of 23 April 2019, 421 aftershocks have been recorded by the PHIVOLCS-DOST seismic monitoring network.

Based on preliminary intensity reports, the strongest ground shaking was felt at PHIVOLCS Earthquake Intensity Scale (PEIS) VI (Very Strong) in San Marcelino and Subic in Zambales; Olongapo City; Floridablanca, Lubao, Porac, and Angeles City in Pampanga. Intensity V (Strong) was felt in Tarlac City; Castillejos and San Felipe in Zambales; Magalang, Mexico, and San Fernando in Pampanga; Abucay, Balanga, and Mariveles in Bataan; Malolos, and Obando in Bulacan; Indang in Cavite; Lipa City in Batangas; Makati City, Mandaluyong City, Manila City, Quezon City, Pasay City, San Juan City, Taguig City and Valenzuela City. Intensity IV (Moderately Strong) was felt in Meycauayan, Plaridel, and San Jose Del Monte City in Bulacan; San Rafael in Tarlac; Rosales and Villasis in Pangasinan; Itogon and La Trinidad in Benguet; Kasibu in Nueva Vizcaya; Gabaldon in Nueva Ecija; San Mateo and Antipolo City in Rizal; Bacoor, Imus, and Maragondon in Cavite; Nasugbu in Batangas; Baguio City, Tagaytay City, Caloocan City, Las Piñas City, Marikina City and Pasig City. Furthermore, Intensity III (Weak) was felt in Marilao in Bulacan; Santo Domingo, Gapan City, Cabanatuan City, Palayan City, and Talavera in Nueva Ecija; Maddela in Quirino; Dingalan in Aurora; Lucban in Quezon; Carmona, Dasmariñas, General Trias and Silang in Cavite; San Nicolas and Talisay in Batangas; Calamba City and Santa Cruz in Laguna; and Muntinlupa City. Intensity II (Slightly Felt) was felt in Baler, Aurora. The strong ground shaking near the epicentral area resulted to damages to some buildings, roads, and bridges.

Have large to moderate magnitude earthquakes affected Central Luzon in the past?

At least 18 onshore moderate earthquakes occurred in the vicinity of Zambales and Pampanga in the past. Earthquakes ranging from M5.1 to M5.8 and maximum Intensity VI (Rossi-Forel Scale) ground shaking was generated in the region from 1928 to 1993 based on SEASEE Report and PHIVOLCS Earthquake Catalog. After the eruption of Pinatubo Volcano in 1991, moderate-sized earthquakes occurred as a result of the activity of the volcano.

Why do earthquakes occur in Central Luzon?

Central Luzon, which includes Zambales and Pampanga, is one of the seismically active areas in the country because of the Philippine Fault, Iba Fault, East Zambales Fault, and Manila Trench, which are the main earthquake generators that can affect the area. In addition, there are other nearby local faults, which may be covered by recent deposits, and may be sources of small- to moderate-magnitude earthquakes.

Can this earthquake indicate volcanic activity?

No. Although the nearest active volcano in Central Luzon is the Pinatubo Volcano, the Magnitude 6.1 earthquake was determined to be tectonic in origin. However, as part of PHIVOLCS-DOST 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 to Pinatubo Volcano.

What can we expect from the current earthquake activity?

The current seismic trend indicates that the Magnitude 6.1 earthquake on 22 April 2019 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. However, the probability of an earthquake higher than Magnitude 6.1 to occur from the same source 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 in mountainous or hilly areas. Liquefaction, manifested by sand boils or lateral spread may affect low-lying, water-saturated, sandy/lahar areas near the banks of rivers.

Can this recent earthquake event trigger a destructive tsunami?

No. The magnitude of the earthquake is not big enough to generate a destructive tsunami. Also, the epicenter of the earthquake is inland. The tsunami threat for Zambales and Pampanga would come from the movement of the Manila Trench, located west of the provinces.

What should be done by the affected communities?

People are reminded to be cautious of structures visibly weakened or with signs of damage by the 22 April 2019 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 buildings and other infrastructures to determine their integrity and recommend appropriate actions to concerned affected groups or individuals.

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. These areas should 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 “drop, cover and hold”. In homes and offices, heavy furniture should be strapped to the walls, and appliances be secured to prevent them from toppling and causing injuries injury to persons.

What is the role of PHIVOLCS-DOST?

PHIVOLCS-DOST operates and maintains a network of 101 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 Zambales are the Pinatubo Volcano Observatory (staff-controlled) in Magalang (Pampanga), and remote-telemetered seismic stations in Santa Cruz and Iba (Zambales), Abucay (Bataan) and Dona Remedios Trinidad (Bulacan). PHIVOLCS also has a volcano monitoring network in and around Pinatubo Volcano that could record seismic activity.

Aside from monitoring the occurrences of earthquakes, PHIVOLCS also conducts hazards analyses and assessments, 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 , and our Facebook and Twitter accounts for earthquake bulletins, volcano updates, hazard maps, and other information on earthquakes and volcanoes. Earthquake observations may also be reported to PHIVOLCS at telephone numbers (02) 929-9254 and (02) 426-1468 to 79, local 124 and 125.

Source: https://www.phivolcs.dost.gov.ph/index.php/news/8233-primer-on-the-22-april-2019-magnitude-6-1-central-luzon-earthquake-23-april-2019

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