PHIVOLCS Seismic Zones and Active Faults - context for the instrumentation coverage triggers

Republic of the Philippines
DEPARTMENT OF SCIENCE AND TECHNOLOGY
Philippine Institute of Volcanology and Seismology (PHIVOLCS)

Seismic Zones and Active Faults: Context for Earthquake Recording Instrumentation

Explanatory reference compiled from public PHIVOLCS-DOST sources

I. Purpose of this reference

This document is an explanatory reference, not a statute or regulation. It summarizes the seismic-hazard framework that determines which buildings must comply with the DPWH Guidelines and Implementing Rules on Earthquake Recording Instrumentation for Buildings (NBCDO Memorandum Circular No. 1, Series of 2015) and Section 105.2 of the National Structural Code of the Philippines 2010 (NSCP 2010). The coverage triggers in those rules depend on the seismic zone of the building's location and its proximity to active faults as identified by the Philippine Institute of Volcanology and Seismology (PHIVOLCS). It is provided so a reader can interpret those triggers; the authoritative texts are the DPWH guidelines and the NSCP.

II. Seismic zones under NSCP 2010

NSCP 2010 divides the Philippines into two seismic zones for structural-design purposes:

Because almost the whole country lies in Zone 4, the DPWH earthquake-recording-instrumentation requirement (which applies to buildings in Zone 4) reaches the great majority of Philippine cities and municipalities. The zone map is the first-level filter for coverage.

III. Active faults and the proximity trigger

Beyond the zone, coverage keys on distance to an active fault. Under the DPWH Guidelines and Implementing Rules on Earthquake Recording Instrumentation for Buildings, the building types required to install earthquake recording instrumentation are those in cities and municipalities within a 200-kilometer radius of a Type A fault (the fault classification is that of NSCP 2010), as indicated on the active-fault maps issued by PHIVOLCS. For buildings located outside that 200-kilometer radius, only a single accelerograph may be required. The 200-kilometer proximity rule is set by the DPWH guidelines; the Type A/B/C source classification it relies on is defined in NSCP 2010.

NSCP 2010 classifies seismic sources (faults) by type:

Type A

Faults capable of producing large-magnitude events and with a high rate of seismic activity. These are the sources that drive the 200-kilometer proximity trigger.

Type B

All faults other than Types A and C.

Type C

Faults that are not capable of producing large-magnitude earthquakes and that have a relatively low rate of seismic activity.

IV. The Philippine Fault Zone and PHIVOLCS fault mapping

The Philippine Fault Zone (PFZ) is a major, roughly 1,200-kilometer-long left-lateral strike-slip fault system that transects the archipelago from northwestern Luzon to southeastern Mindanao. It is one of the principal sources of large, shallow, and destructive earthquakes in the country.

PHIVOLCS, together with Kyoto University, has since 2003 conducted large-scale mapping of the Philippine Fault, and the great majority of its on-land trace has now been mapped. PHIVOLCS publishes these active-fault maps and makes them available on its website and on request. To determine a specific site's exposure, PHIVOLCS provides the FaultFinder tool, which reports the distance to the nearest active fault together with the fault name, segment name, year of mapping, and map scale used.

V. Strong-motion monitoring: the national network

PHIVOLCS operates the country's earthquake-monitoring networks, including the Philippine Seismic Network and a network of strong-motion accelerographs strategically located in densely populated areas and near active faults. This strong-motion network records the ground motion produced by high-magnitude earthquakes. Building-mounted earthquake recording instruments installed under the DPWH guidelines complement this national network: the DPWH rules provide that data produced by a building's instruments shall be made available to the Building Official or to PHIVOLCS on request.

VI. How to use this with the DPWH guidelines

1. Determine the building's seismic zone. Almost everywhere in the Philippines is Zone 4 (all except Palawan and Tawi-Tawi).
2. Determine proximity to a Type A fault using the PHIVOLCS active-fault maps or the FaultFinder tool. Within 200 kilometers of a Type A fault, the full instrumentation requirement applies; outside it, a single accelerograph may be required.
3. Apply the covered-building types, instrument placement, Seismic Instrumentation Room, equipment specifications, data-retrieval, and certification duties set out in the DPWH Guidelines and Implementing Rules on Earthquake Recording Instrumentation for Buildings (NBCDO MC No. 1, s. 2015) and NSCP 2010 Section 105.2.