Brace for Magnitude 7 quake

Brace for Magnitude 7 quake
Developing a culture of safety is awareness of hazards by everyone in a community.

By Alfredo Mahar Lagmay
Philippine Daily Inquirer
First Posted 17:42:00 01/23/2010

Filed Under: Earthquake, Housing & Urban Planning, Infrastructure, Safety of Citizens



THERE ARE no earthquakes in a perfect world. Too bad we don’t live in such a place, as the staggering scope of the destruction in Port-au-Prince, Haiti, reminds us.

In more ways than one, the geological history of the Philippines is similar to that of the island of Hispaniola, where the sovereign states of Haiti and the Dominican Republic are located.

Like the Philippines, Hispaniola harbors pristine beaches and places where jungles blanket mountains of gold.

Sadly, where gold is found, earthquake faults lurk and spoil the calm of island paradises.

Faults in Haiti

The Enriquillo-Plantain Garden Fault in the southern part of Haiti was responsible for the destruction of its capital on January 12.

One of the two faults that straddle the east-west trending island of Hispaniola, the fault generated 7-magnitude earthquakes in centuries past.

The strongest in historical record was the 7.5-magnitude earthquake that struck Port-au-Prince in 1770.

The recent tremor originated 13 kilometers below the surface and released about 32 megatons of energy, comparable to the simultaneous detonation of 1,000 Nagasaki atom bombs.

Compressive stresses

The faults in Hispaniola were formed because of compressive stresses generated throughout millions of years by the unhampered westward movement of the western Atlantic seafloor underneath the Caribbean.

Hispaniola succumbed to these stresses and broke apart to form fractures that slip from time to time at intervals from a few decades to a few centuries.

When the fracture slips damaging earthquakes are spawned.

As a rule of thumb, the longer the time an active fault does not move, the larger the next earthquake will be along that fault or fault segment.

Unfortunately for Haiti, the last rupture of the Enriquillo-Plantain Garden Fault was in 1860.

Most feared

The Philippine Fault is no different from the active faults that traverse Hispaniola.

Both are left-lateral strike-slip faults, which means that if you stand straddling the rupturing fault during an earthquake, your left foot would move back and your right foot, forward.

Both are also part of the list of the longest known active strike-slip faults in the world along with the San Andreas Fault of California, Denali Fault in Alaska and the North Anatolian Fault of Turkey.

Given that length of fault rupture is directly related to earthquake magnitude, it is no wonder that the Philippine Fault is one of the most feared faults on Earth.

Like how the Enriquillo-Plantain Garden Fault of Haiti formed, faults in the Philippines are the product of the collision of the northwestward- moving Pacific Ocean floor with the South China Sea floor.

Brittle Philippine crust

In the grip of two colliding masses, the brittle Philippine crust cracked and formed many faults, the closest to Metro Manila being the Marikina Valley Fault.

Whenever these faults slip, stored energy is released and earthquakes occur.

This network of faults is responsible for the 5,000 to 7,000 island temblors recorded each year in the Philippine archipelago.

Most of these, however, are unfelt.

Quakes in Manila

Manila has been devastated by at least six large tremors between 1589 and 1864, experiencing heavy ground shaking in 1599, 1601, 1658, 1700, 1766 and 1863, according to a report published in 1985 by the Southeast Asia Association of Seismology and Earthquake Engineering.

The tally of earthquakes with intensity 6 or greater felt in Manila from 1589 to 2000 was placed at 23 in an updated study of historical quakes, with 13 causing significant damage.

The deadliest among these were the 1863 and 1880 earthquakes, which were comparable to the impact of the 1990 Luzon seismic event.

Ruby Tower

One of the latest devastating seismic events that affected Metro Manila was the 1968 Casiguran, Aurora earthquake.

Infamously known as the Ruby Tower earthquake, its name is derived from the six-story apartment building in Binondo, Manila, which collapsed like a house of cards, killing 342 of its more than 600 tenants.

Most residents of Metro Manila, who are at least 25 years old, remember the earthquake of June 16, 1990.

1990 Luzon temblor

Known as the Luzon earthquake, the 7.8-magnitude temblor, which originated in Nueva Ecija, was felt as intensity 6 in Quezon City and as intensity 7 in the low-lying areas of Manila.

Although nearer to the earthquake epicenter, the tremor registered lower in the intensity scale in Quezon City because of its relatively firmer ground composed of adobe.

Unlike Quezon City, Manila’s subsurface is dominated by unconsolidated river and coastal sand, ground that is more unstable and prone to liquefaction.

Marikina Valley Fault

The name “Marikina Valley Fault” was first coined by Dr. Carlo Arcilla of the National Institute of Geological Sciences in a monograph published in 1983.

Based on his field research in San Mateo, Rizal, Arcilla, who was then only 21 years old, described two fault structures that caused the Marikina Valley to sink relative to Quezon City and the hillsides of San Mateo and Montalban.

At the time, it was not known that the Marikina Fault was actively moving and it took nearly a decade before the Marikina Fault was finally established as active through detailed investigation by the Philippine Institute of Volcanology and Seismology (Phivolcs) and by paleoseismologists of the United States Geological Survey (USGS).

The name “Marikina Valley Fault” was changed to the “Valley Fault System” when the local government complained that the fault’s name might affect the economy of the city.

Tagaytay Ridge

Since the western part of the Marikina Valley Fault System extends up to Tagaytay Ridge, it was deemed unfair to call it by its original name. Hence, the change of term. (See map.)

Some contend, however, that such change in nomenclature is not standard practice in the field of Geology, since the locality of the fault structure is usually attached to the place where it is best exposed and first described.

I believe this is rightfully so, because the name “Valley Fault System” may refer to any valley in the world – imagine calling the “San Andreas Fault” the “Fault.”

Science in its true form does not buckle from political, economic and even religious pressure.

The study by Allan R. Nelson and others of the USGS and Phivolcs, published in the Bulletin of the Seismological Society of America, reports the Marikina Fault as having moved at least two and perhaps four times over the past 1,300 to 1,700 years.

A range of 200 to 400 years is estimated as the recurrence interval of the studied part of the Marikina Valley Fault, with the most recent rupture occurring probably some 200 years ago.

A 6 to 7-magnitude earthquake is expected based on the predicted rupture length of the Marikina Valley Fault, according to Nelson.

The Metro Manila Earthquake Impact Reduction Study, designed to reduce the impact of hazards in the metropolis, predicts a slightly larger 7.2-magnitude event. [See damage in Metro Manila.]

Like branch of tree

The paleoseismic study by Nelson and others was conducted on a splay of the West Marikina Valley Fault.

A fault splay is like the branch of a tree reaching outward from the main trunk.

The main trunk of the Marikina Valley Fault is easily traceable since its surface expression is quite distinct.

Splays of the Marikina Fault, however, are more difficult to find because they are less prominent and are often masked by urban concrete.

High-rise buildings

In some instances, these fault splays crop up when large pits as deep as 20 meters are excavated during the early construction stages of high-rise buildings in areas like Muntinlupa, Fort Bonifacio, Ortigas and Quezon City.

Whenever these faults are seen, it is important that an in-depth study is made to determine the fault’s activity or inactivity and its relationship with the Marikina Valley Fault.

It is irresponsible to dismiss the fault when it is found because the possible consequences are dire.

I would recommend full disclosure by property developers of the underlying geology, including the presence of faults, if any, to prospective- building tenants, a practice observed in the United States.

Culture of safety

To prepare ourselves for any disaster we need to develop a culture of safety.

It is not difficult to demonstrate that we lack this culture. For instance, cyclists take the wrong lane without helmets and reflectors, and search-and-rescue teams don’t know where to find rubber boats to save flood victims.

Developing a culture of safety is awareness of hazards by everyone in a community.

It means active participation of all sectors in disaster-mitigation efforts following the lead of civil authorities, long before catastrophe strikes.

It is not taking short cuts or condoning illegal acquisition of environmental and building permits because business is more important.

A society with a culture of safety invests heavily in knowledge and treats disasters not as rare events but as unresolved problems in developmental planning.

We have to be smart about living with natural hazards because nature will not adjust to people. It is society that must adjust.
*****
(Alfredo Mahar Lagmay is an associate professor at the National Institute of Geological Sciences, University of the Philippines. He holds a Ph.D. in geology from the University of Cambridge and was a visiting scientist at the Geophysics Department of Stanford University.)
*****
Expected damage in Metro Manila from 7.2-magnitude quake

33,500
Number of people killed

18,000
Number of dead from 500 simultaneous fires

114,000
Number of injured

3 million
Number of residents who will flee

175,000
Number of buildings destroyed

30 to 35
Percentage of public buildings damaged

Sources: Metro Manila Earthquake Impact
Reduction Study (2004) and inquirer reports