GEOLOGICAL SOCIETY OF THE PHILIPPINES

 

Engineering and Geological Lessons from the Bohol M7.2 Earthquake of October 15, 2013

An On-Site Study 
Dr. Arthur Saldivar-Sali, Phd, Msc, Fgs, Dic

Chair/CEO, Geotecnica Corporation (The Rock Engineers)

 

Abstract

The M7.2-strong earthquake that hit Bohol, although causing deaths and almost indescribable damage to civil 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 rare events could further improve the science and engineering practice of protecting infrastructure to save lives during future earthquakes.

Geologically, the Bohol event showed that a pre-existing fault is not always a requisite for the occurrence of an earthquake. Speculations In the days immediately after the earthquake attributed the event to changing seismogenic sources, i.e., the “old” East Bohol Fault near Tagbilaran City; then the old fault east of Carmen; then a West Bohol Fault offshore between Bohol and Cebu; then a “blind fault” that could not be seen. These efforts to look for the culprit fault were based on the premise that an old fault must be present nearby in order to cause a lethal earthquake. Finally, exhaustive field search proved it was caused by a “new fault” onshore in Inabanga northwest of the island, hence, “not previously mapped by state seismologists”.

On-site inspection one week after the earthquake confirmed that the Inabanga Fault is a fresh ground fracture aligned north-northeast. It is prominently exposed in a sparsely vegetated area that cursory mapping could have hardly missed, if this existed prior to the earthquake. The U.S. Geological Survey (USGS) pinpoints the epicenter at “5.0 kilometers southeast of the town of Sagbayan with a shallow focus at a depth of 12 kms”, and about 45 kms south of Inabanga. This indicates a thrust fault, the type that produces the shallow, and most dangerous earthquakes. Seismic waves from this shallow focus generated the deadly ground surface shaking, simultaneously creating the new fault. The direction and mode of collapse starkly shown in majority of the damaged buildings indicate horizontal ground motions along observable trends. In contrast, 45 kms away adjacent to the surface trace of the new fault the houses made of “poor” materials were hardly damaged.

This Bohol earthquake calls 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 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 rocks in the Sagbayan focus was absorbed by and increased stresses in the rocks of adjacent regions, which could initiate new pressure build-up leading to future earthquakes in regions not affected.
  • Ground shaking, hence, the extent of damage, does not automatically decrease with distance from the epicenter.

In the engineering design of buildings, the long held seismologic principle that degree of damage is inversely proportional to the distance from the epicenter or from the nearest fault trace also needs to be revisited. The earthquake aftermath puts to question the blanket application of this rule of thumb. In particular, along the west and southwest coasts of Bohol the number of damaged infrastructure and fatalities increased with distance from the epicenter. 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 not consistent with the “rule of thumb” can be explained by correlating the damage not merely with distance from the epicenter but more importantly with the geomechanical properties, hence, the varying frequencies of shaking of the variable surface geology of Bohol, i.e., loose marls, young limestones, deeply weathered shales, firm 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. It is the building that collapses when it resonates with ground shaking, i.e., both sympathetically shaking together.
  • By correlating the measured direction of failure of the buildings with the regional geologic structures, the direction of ground shaking may be estimated.
  • The right time to institute remedial measures is now, i.e., immediately after the event, even in areas that were not affected.
  • Most importantly, the geomechanical, dynamic characteristics of the ground can either amplify or reduce ground shaking even hundreds of kilometers from the epicenter.

It is important to recognize that there are engineering solutions to make buildings safe. 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 geology, leads to proper design and use of appropriate construction methods and materials.

The ultimate objective of earthquake studies must be to protect buildings, hence, save lives. The science and practice of earthquake protection should not be left to one discipline alone. It is a multi-disciplinary endeavour among geologists, seismologists, structural engineers and construction specialists.


 
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