The Valley Fault System & Seismic Gaps:why the National Structural Code of the

Philippines should be revised

The Valley Fault System & Seismic Gaps:why the National Structural Code of the

Philippines should be revised

DR. RAMON D. QUEBRALStructural Geologist/

Engineering GeologistAMH (Philippines), Inc.

Technical Workshop on Holistic Responses to the Need forResilient Low-Cost HousingESSC-ASEP-ESCAP-Hilti FoundationOracle Hotel, Katipunan Avenue, QCApril 2-3, 2014

VALLEY FAULT SYSTEM

CRITERIA FOR FAULT ACTIVITY

seismic activity historical accounts geological evidence

seismic activity historical accounts geological evidence

CRITERIA FOR FAULT ACTIVITY (VFS)

young morphological features along the fault the presence of a superposed drainage (Pasig River) on the

Guadalupe-Diliman Plateau which is underlain by theQuaternary Diliman Tuff

the presence of fault planes in the Quaternary Diliman Tuff the presence of deformation in man-made structures (e.g.

roads, houses) in the Muntinlupa-Taguig-Pasig, QC. Alabang,San Pedro area

deformation in alluvial material in trenches excavated duringa paleoseismic study conducted by PHIVOLCS (Daligdig et al,1997)

Carbon-14 dating (Daligdig et al, 1997)

young morphological features along the fault the presence of a superposed drainage (Pasig River) on the

Guadalupe-Diliman Plateau which is underlain by theQuaternary Diliman Tuff

the presence of fault planes in the Quaternary Diliman Tuff the presence of deformation in man-made structures (e.g.

roads, houses) in the Muntinlupa-Taguig-Pasig, QC. Alabang,San Pedro area

deformation in alluvial material in trenches excavated duringa paleoseismic study conducted by PHIVOLCS (Daligdig et al,1997)

Carbon-14 dating (Daligdig et al, 1997)

SEISMIC GAPS

The Valley Fault System is not associatedwith any seismic activity. However, there is geologic evidence to

show that the faults are definitely active. It is therefore associated with a seismic gap.

The Valley Fault System is not associatedwith any seismic activity. However, there is geologic evidence to

show that the faults are definitely active. It is therefore associated with a seismic gap.

TYPE A FAULTS

The National Structural Code of thePhilippines defines Seismic Source Type A asfaults that are capable of producing largemagnitude events and that have a high rateof seismic activity. Under this definition, the Valley Fault

System is not technically a Type A Fault. If a PSHA is conducted, it might be

concluded that there is no risk at all.

The National Structural Code of thePhilippines defines Seismic Source Type A asfaults that are capable of producing largemagnitude events and that have a high rateof seismic activity. Under this definition, the Valley Fault

System is not technically a Type A Fault. If a PSHA is conducted, it might be

concluded that there is no risk at all.

MMEIRS

This runs counter to the intent of the MetroManila Earthquake Impact Reduction Study(MMEIRS) of MMDA-JICA-PHIVOLCS. In this case, risk is actually higher despite

the absence of seismic activity becausestress is accumulating and is not beingreleased.

This runs counter to the intent of the MetroManila Earthquake Impact Reduction Study(MMEIRS) of MMDA-JICA-PHIVOLCS. In this case, risk is actually higher despite

the absence of seismic activity becausestress is accumulating and is not beingreleased.

CREEPING FAULTS

That the West Valley Fault is creeping ormoving aseismically is not a guarantee thatit will not move catastrophically. The Anatolian Fault in Turkey, for example,

crept before and after movingcatastrophically.

That the West Valley Fault is creeping ormoving aseismically is not a guarantee thatit will not move catastrophically. The Anatolian Fault in Turkey, for example,

crept before and after movingcatastrophically.

SUGGESTED DEFINITION

The code should therefore take intoconsideration the concept of seismic gaps. It is recommended that Type A Faults be

redefined simply as faults that are capableof producing large magnitude events. This is regardless of whether they are

associated with a high rate of seismicactivity or not.

The code should therefore take intoconsideration the concept of seismic gaps. It is recommended that Type A Faults be

redefined simply as faults that are capableof producing large magnitude events. This is regardless of whether they are

associated with a high rate of seismicactivity or not.

OTHER SEISMIC GAPS

It should be noted that certain segments ofthe Philippines Fault might also beassociated with seismic gaps (e.g. Infanta,Davao). They are candidate sites for major

earthquakes inspite of their seismicquiescence.

It should be noted that certain segments ofthe Philippines Fault might also beassociated with seismic gaps (e.g. Infanta,Davao). They are candidate sites for major

earthquakes inspite of their seismicquiescence.

IMPLICATIONS

Risk is actually higher inspite of absence ofseismic activity. Structures might end up being under-

designed.

Risk is actually higher inspite of absence ofseismic activity. Structures might end up being under-

designed.

IMPLICATIONS

High rise buildings are often seismicallydesigned. It is the low- and medium-rise buildings

which are vulnerable. During theGuinsaugon & Bohol earthquakes, most ofthe damaged structures were low-lyingstructures. Not being high rise does notexempt them from seismic loading.

High rise buildings are often seismicallydesigned. It is the low- and medium-rise buildings

which are vulnerable. During theGuinsaugon & Bohol earthquakes, most ofthe damaged structures were low-lyingstructures. Not being high rise does notexempt them from seismic loading.

RECOMMENDATIONS

Redefine Type A Faults in the NSCP. Even low cost houses should be made

resilient to earthquakes. It does not have tobe expensive. For example, providingconcrete walls w/ bamboo as reinforcementinstead of steel bars might be a bettersolution than having unreinforced walls.

Redefine Type A Faults in the NSCP. Even low cost houses should be made

resilient to earthquakes. It does not have tobe expensive. For example, providingconcrete walls w/ bamboo as reinforcementinstead of steel bars might be a bettersolution than having unreinforced walls.

Thank you.

End of Presentation

Thank you.