Minimal quake damage due to several factors

In the news item “7.6 quake ‘equivalent to 32 Hiroshima atomic bombs’” (Inquirer, 9/2/12), Director Renato Solidum of the Philippine Institute of Volcanology and Seismology (Phivolcs) was reported to have said that the Aug. 31, 2012 earthquake “would have been strongly felt had its epicenter been on land.” I hope he was not misquoted.

The Luzon earthquake of Aug. 2, 1968, with magnitude 7.3 at 31-kilometer depth (Unesco Mission report, 1969), had its epicenter under the sea off the coast of Casiguran, Aurora. And yet, with a lower magnitude and shallower depth than the earthquake last Aug. 31, the 1968 event did serious damage to structures not only in Isabela and nearby areas, but also in Manila. The Eastern Luzon earthquake of April 7, 1970, with epicenter located off the coast of Baler, Aurora, caused the collapse of P. Guevara Elementary School and severe damage to several other buildings in Manila. It seems the consequences of these earthquake events do not support Solidum’s theory on the Aug. 31 earthquake.

On the other hand, Dr. Alfredo Mahar Lagmay of UP was quoted in the same report as saying that “the Philippines was fortunate that the earthquake did not meet the conditions of a larger-scale disaster: power, proximity and the kind of structures in the affected places.” Lagmay may be correct in attributing minor damage to structures in the isoseismal region (areas where ground shaking was felt), but not because the earthquake did not have disastrous power (energy released at the focus?). A magnitude 7.6 earthquake (which Solidum equated with 32 atomic bombs of the Hiroshima vintage) is not a weak earthquake. The consequential minor damage could probably be because of one or a combination of the following conditions: (1) structures in the epicentral region were earthquake-resistant, (2) the subsoil conditions did not amplify the ground motions and (3) the period of the seismic waves did not resonate with the natural periods of the structures. It may be safe to conclude that no disastrous tsunami was generated because the vertical displacement of the earth’s crust underwater was, indeed, very minimal to generate huge waves.

Looking at the consequences of the earthquake as a blessing and to what Lagmay thought was a matter of luck may be a no-no to many disaster managers as such a view could build in individuals belief of personal invulnerability, making it more difficult, for instance, to evacuate populations to safer grounds.

Since the overall assessment of the earthquake’s consequences was minor damage, Intensity 7 (sic) probably was an overestimation if based only on “most people are frightened and run outdoors.” (People who rarely experience strong earthquakes are easily frightened compared with people like the Japanese who do.)  Evacuation in the coastal areas was a proper response, though, even if no huge waves came.

—ROLANDO G. VALENZUELA,

retired seismologist from Pagasa,

[email protected]