Engineering, geological lessons from the magnitude 7.2 Bohol earthquake: An on-site investigation

Abstract

The magnitude 7.2-strong earthquake that hit Bohol, although causing deaths and almost indescribable damage to public and private infrastructure, also provided a rare opportunity to better understand this geological hazard for engineering design purposes.

This paper focuses on the aftermath of that earthquake to comprehend the mechanisms operating during the event. New geological knowledge from such a rare incident could further improve the science and engineering practice of constructing infrastructure to save lives during future earthquakes.

Geologically, the event showed that a pre-existing fault is not always a requisite for the occurrence of an earthquake. Geological speculations immediately after the earthquake attributed the event to changing seismogenic sources, e.g., the “old” East Bohol Fault near Tagbilaran City. Failing to find it there, another possible fault east of Carmen town was suspected; then a “West Bohol Fault” between Bohol and Cebu; or possibly a “blind fault” that is hidden somewhere in the island.

These vain efforts to look for the culprit fault are based on the premise that an old fault must always be present nearby in order to cause a lethal earthquake. Eventually, exhaustive field search found a new fault onshore in Inabanga town that residents testified to have been formed at the time of the Oct. 15, 2013 earthquake. Hence, it could not be previously mapped by state seismologists. Thus, the Bohol earthquake proves that a pre-existing fault is not necessary to cause an earthquake.

Site inspection confirmed that the Inabanga Fault is only a five-kilometer-long new ground fracture. It is prominently exposed in a sparsely vegetated area that cursory mapping could have hardly missed, if this existed prior to the Oct. 15 earthquake. The US Geological Survey (USGS) pinpointed the earthquake shallow hypocenter to be 12 kms deep below its epicenter about 5.0 kms southeast of the town of Sagbayan and about 15 kms south of the new Inabanga Fault. This is a thrust type fault, the type that produces shallow and most dangerous of earthquakes. Seismic waves from the breaking of the rocks at the hypocenter generated the deadly ground shaking that completely destroyed houses, churches and buildings as far as 40 to 50 kms away from the Inabanga Fault. In contrast, local houses made of “poor” materials and immediately adjacent to the new fault were not damaged.

These “unexpected” findings call for the review of some time-honored geologic notions vis-à-vis the manner of dealing with earthquakes in the engineering design of buildings. Most significant of these are: 

• A pre-existing surface fault or one hidden beneath the ground surface is not always a requisite for an earthquake to occur. New faults can be generated by an earthquake anywhere, anytime on the earth’s continuously evolving crust.

• Seismic waves that generated the ground shaking emanated from the focus, neither from the epicenter nor from the trace of the fault at the surface.

• The energy released by the breaking rocks in the Sagbayan hypocenter was absorbed by the rocks of adjacent regions, which could initiate new pressure build-up leading to future earthquakes in regions not recently affected.

• Ground shaking, hence, the extent of damage, does not necessarily decrease with distance from the fault or the epicenter.

In the engineering design of buildings, the long-held structural principle that the degree of damage decreases with distance from the epicenter or from the nearest fault trace also needs to be revisited.  The Bohol earthquake aftermath puts to question the blanket application of this “rule-of-thumb.” In particular along the west and southwest coasts of Bohol, the towns of Loon and Maribojoc, at distances as far as 50 kms from the new fault, suffered the greatest damage to infrastructure and most number of fatalities. In contrast, the number of damaged structures in the towns east and southeast of the epicenter decreased away from the epicenter, with the exception of Carmen and Sagbayan, in the middle of the island.

These apparent differences in the effects of the earthquake seemingly not consistent with the “rule of thumb” can be explained by correlating the damage not merely with distance from the fault but more importantly with the geomechanical properties of the soil/rock foundations, which explains the varying intensities of shaking of the variable surface geology of Bohol, i.e., loose marls, hard limestones, deeply weathered shales, sandstones, thick residual soils, soft alluvial deltas and loose coastal sand deposits. Thus, seismic design for safer infrastructure can be further improved by considering the following findings:

• It is not the fault that kills. What does is the building that collapses if it shakes resonantly with the ground seismic shaking, i.e., when the two shake in the same direction with each other.

• Most importantly, the softness or hardness of the soils or rock foundation controls the intensity of the seismic shaking, which can either amplify or reduce the shaking of a building even hundreds of kilometers away from the fault.

• Instead of a fault map, a ground shaking potential map would be a more useful guide for the engineering design of a safe building.

• The right time to institute remedial measures is now, i.e., immediately after the earthquake, even in areas that were not affected.

The public needs to be made aware that there are engineering solutions to make buildings safe in an earthquake. Sensationalized reports, e.g., “earthquake was equal to 32 atomic bombs,” instill fear that make people feel helpless and could react by doing nothing. Treating this new fault in the manner that the Marikina Fault has been characterized, i.e., that buildings near faults are most vulnerable, without considering the geotechnical nuances of their foundations, gives a false sense of security to those areas that were not affected. Re-education to reassure the public that a seismically engineered building, anchored on the empirical understanding of the local geology, leads to proper design and use of appropriate construction methods and materials.

The ultimate objective of post-earthquake reconstruction must be to design buildings commensurate to the ground shaking potential of the geological materials of their foundations. Earthquake-resistant buildings save lives. The science and practice of earthquake protection should not be left to one discipline alone. It is a multi-disciplinary endeavor among geologists, seismologists, structural engineers and construction specialists.  

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This paper was presented at the Philippine Mine Safety National Convention in November 2013 and at the annual convention of Philippine geologists in December 2013.