[OPINION] Of intensities, magnitudes and the Rizal Day Bangui earthquake

In the late morning of December 30, 2024, a moderate magnitude earthquake with a shallow focal depth (23 km) struck off the coast of Bangui, Ilocos Norte, in northern Philippines. The Philippine Institute of Volcanology and Seismology (Phivolcs) calculated a magnitude 5.6, but observers in the vicinity of the epicenter reported experiences that appear to indicate a more elevated degree of shaking.

In the city of Laoag, located about 50 aerial kilometers to the south of the epicenter, effects of the earthquake included sprawled grocery items fallen from shelves, spilled coffee on tabletops, overflowing aquarium tanks, visually observable swaying of buildings and nauseated feeling of some people at rest. In the town of Sarrat, located about 10 aerial kilometers southeast of Laoag City, fallen brick debris from the walls of a Spanish-era church were reported.

Energy released versus degree of ground shaking

The population centers of both Laoag and Sarrat are built on the banks of the mighty Laoag River, which drains mountain materials eroded from the western flank of the northern segment of the Cordillera Mountain Range.

Over time, gravel and sand deposit at and near the banks of the river. Riverbanks closer to the mountain range receive relatively larger-sized erosional products like gravel, while in areas located farther away like Sarrat and Laoag, fluvial erosion further breaks down coarse materials to finer fragments, allowing sand and silt to deposit instead. A subsurface laden with sand and silt is structurally weak. When shaken by an earthquake, these materials become even weaker, especially when these are saturated with water such as at riverbanks, along coastlines, or after a rain.

The intensity of an earthquake is a qualitative measure of the degree by which the ground shakes during the tremor. It is expressed in Roman numerals usually ranging from I (one) to X (10). Different countries adopt different intensity scales. In the Philippines, the Phivolcs Earthquake Intensity Scale (PEIS) is adopted. It ranges from Intensity I (scarcely perceptible) to Intensity X (completely devastating).

During the December 30 Bangui earthquake, the highest reported intensity was V (strong), while the highest instrumentally recorded intensity was VI (very strong). At Intensity V, people at sleep are awakened, hanging objects swing violently, liquids spill from open containers. At Intensity VI, massive objects such as heavy furniture shift, church bells ring, some people may lose balance (nauseated feeling).

Intensity is a function of many factors, including distance from the epicenter, depth of focus and condition of the ground at the affected site. In general, intensity is higher when the site is closer to the epicenter, the depth of focus is shallower, and the ground is weaker. It is therefore a relative measure, which may change from one place to another.

On the other hand, magnitude is a measure of the energy released by the movement of a fault that generates the earthquake. It is expressed in Arabic numerals in decimal numbers such as 5.6. Unlike intensity, magnitude is an absolute measure, which does not change as a function of distance, focal depth or ground condition.

The distinction between intensity and magnitude maybe likened to the strength of the light perceived from a bulb at different distances. An observer closer to the bulb will perceive a brighter emission of light than one observing from a greater distance. The perceived “brightness” is intensity, a relative measure (brighter-closer; dimmer-farther), while the bulb’s fixed wattage is magnitude, an absolute value (e.g. 10 watts, independent of observer distance).

Earthquake resilience lessons

Some earthquake resilience lessons can be learned from the December 30 tremor in Bangui. While the magnitude was moderate, stronger-than-expected intensities were felt in certain areas where the ground shaking appeared to be amplified even at farther site-to-epicenter distance, mainly as a result of relatively weak ground conditions.

To mitigate earthquake hazards, sites should be assessed following sound earthquake knowledge and in accordance with existing construction codes. In particular, population centers and areas for infrastructure development should undergo thorough site-specific earthquake risk assessment, in addition to standard code-prescribed evaluation methods. Results from such site-specific studies should be used as input parameters in the design of infrastructure to be built, or in retrofitting procedures of already-built up areas.

Wishing everyone an earthquake-resilient and a Happy New Year! – Rappler.com

Mario A. Aurelio, PhD is a professor at the National Institute of Geological Sciences-University of the Philippines, and head of the Structural Geology and Tectonics Laboratory of UP NIGS. He mentors students interested in the study of earthquakes. With reports from Engineer Semantha Chesca Aurelio of the Association of Structural Engineers of the Philippines, Laoag City Chapter.