Active Faults & Other Active Faults & Other

Earthquake Source ZonesEarthquake Source Zones

Mariton V. BornasMariton V. BornasGeology & Geophysics R&D DivisionGeology & Geophysics R&D Division

Philippine Institute of Volcanology & SeismologyPhilippine Institute of Volcanology & Seismology

ACTIVE FAULTS

•FAULTING is the cause of TECTONIC EARTHQUAKES.

•3 types of faults are called NORMAL, THRUST or

REVERSE and STRIKE-SLIP.

FAULT: a fracture, fissure, or a zone of weakness where ground movement or displacement has occurred or may occur again.

WHAT IS A FAULT?

BEFORE FAULTING

REVERSE

NORMAL FAULT

STRIKE-SLIP

BEFORE FAULTING

REVERSE

NORMAL FAULT

STRIKE-SLIP

TYPES OF FAULT

Normal FaultNormal Fault

Thrust or Reverse FaultThrust or Reverse Fault

Thrust fault of the 1999 Chichi Earthquake, TaiwanThrust fault of the 1999 Chichi Earthquake, Taiwan

Thrust fault of the 1999 Chichi Earthquake, TaiwanThrust fault of the 1999 Chichi Earthquake, Taiwan

StrikeStrike--slip Faultslip Fault

Nojima Fault of the 1995 HanshinNojima Fault of the 1995 Hanshin--Awaji (Kobe) Earthquake, JapanAwaji (Kobe) Earthquake, Japan

LeftLeft--laterally displaced pilapils (rice paddy dikes) in Imugan, Nuevalaterally displaced pilapils (rice paddy dikes) in Imugan, Nueva Vizcaya Vizcaya

along the Digdig Fault rupture of the 1990 Northern Luzon Earthqalong the Digdig Fault rupture of the 1990 Northern Luzon Earthquake.uake.

““Mole tracksMole tracks”” defining the Digdig Fault rupture afterdefining the Digdig Fault rupture after

the 1990 Northern Luzon Earthquake.the 1990 Northern Luzon Earthquake.

Aerial view of the San Andreas Fault Aerial view of the San Andreas Fault

System, the most famous strikeSystem, the most famous strike--slip slip

fault in the world. The San Andreas fault in the world. The San Andreas

Fault marks the boundary between Fault marks the boundary between

the Pacific Sea Plate and the North the Pacific Sea Plate and the North

American PlateAmerican Plate

A fault is defined as an ACTIVE FAULT when it has moved within the last 10,000 years (Holocene Period).

WHAT IS AN ACTIVE FAULT?

BASES OF MOVEMENT:

1. historical and contemporary seismicity (earthquake activity)

2. fault slip (displacement) based on displaced rock or

soil units of known age (paleoseismology)3. displaced landforms (active fault geomorphology)

Active Faults &

Trenches

Philippine seismicity

from 1600s to present

DOST

PHIVOLCS

B: Triangular facet H: Offset streamC: Fault sarplet I: Shutter ridgeD: Sag pond J: Beheaded streamE: Pressure ridge K: Wind gapF: Fault saddle L: Offset piedmont lineG: Graben M: Offset alluvial terrace

Smith & Wesson, 1975)

Active StrikeActive Strike--slip Fault Geomorphologyslip Fault Geomorphology

Pressure ridges

bench

offset streams

sagpond

Gabaldon Fault

Gabaldon Fault

Digdig Fault 1990 rupture

Digdig Fault 1990 rupture

Sample airphoto interpretation of the Gabaldon Fault, Dingalan, Aurora

A weak to violent shaking of the ground produced by the sudden movement of rock materials below the earth’s surface (FAULTING).

WHAT IS AN EARTHQUAKE

•based on relative ground shaking effect to people

and structures; generally

higher near the epicenter•(Reported in Roman

Numerals e.g. I, II..IV..IX)

•based on instrumentally derived data; measure of the

amount of total energy

released at the earthquake’s point of origin

•(Reported in Arabic numerals e.g. 3.5, 7.2, etc)

1. Intensity 2. Magnitude

Two Earthquake Descriptions:Two Earthquake Descriptions:

ACTIVE FAULTS & OTHER EARTHQUAKE

GENERATORS OF THE PHILIPPINES

Most Destructive

Earthquakes in the Philippines

Recent Destructive Earthquakes

• 1968 Casiguran – Ms 7.3

• 1973 Ragay Gulf – Ms 7.0

• 1976 Moro Gulf – Ms 7.9

• 1983 Laoag – Ms 6.5

• 1990 Luzon – Ms 7.8

• 1990 Bohol – Ms 6.8

• 1990 Panay – Ms 7.1

• 1994 Mindoro – Ms 7.1

• 2002 Sultan Kudarat – Ms 6.8

• 2003 Masbate – Ms 6.2

Earthquake Generators in

the Philippines

Earthquake Generators in the Philippines

PHILIPPINE FAULT ZONE:

•1,300 km-long fault system that stretches N-S

from Ilocos to eastern Mindanao

•Left-laeral strike-slip

•Average slip rate: 1.5 to 3.6 cms/year

•M7.8 July 16, 1990 Luzon Earthquake along

the Digdig Fault segment, the M7.3 March 17,

1970 Ragay Gulf Earthquake, and the M6.2

February 15, 2003 Masbate Earthquake

ACTIVE FAULTS IN THE PHILIPPINES

VALLEY FAULT SYSTEM (VFS):

•Originally the Marikina Fault, a pair of

segmented faults that forms the Marikina

River Valley and transects the eastern

margins of Metro Manila

•Consists of the West Valley Fault (WVF)

and the East Valley Fault (EVF)

•Right-lateral strike-slip fault.

ACTIVE FAULTS IN THE PHILIPPINES

Trenches as Earthquake Generators

The Philippine Trench

Zone where the Philippine Sea Plate (PSP) subducts beneath the Philippine Mobile Belt (PMB) between offshore Bicol Peninsula and southeast Mindanao. Convergence between the PSP and the PMB occurs at the rate of 8 cms/year

East Luzon Trough

Zone where the PSP subducts beneath the PMB offshore of northeastern Luzon.

The Manila Trench

Zone where the South China Sea Plate (SCSP) subducts beneath thePMB between Taiwan and Mindoro.

PHILIPPINE SUBDUCTION ZONES

Negros Trench

Zone where the Sulu Sea Plate (SSP) subducts beneath the PMB offshore of Panay and Negros Island.

Sulu Trench

Zone where the SSP subducts beneath the PMB offshore of Zamboanga Peninsula and Sulu Archipelago.

Cotabato Trench

Zone where the Celebes Sea Plate subducts beneath the PMB offshore of the Cotabato and Saranggani Provinces.

PHILIPPINE SUBDUCTION ZONES

Metro Manila and the

VALLEY FAULT

SYSTEM (VFS)

Surface Geology of Metro Manila

East Valley Fault

West Valley Fault

Source: HIGP, Univ. Hawaii

West Valley Fault

Manila Bay

The Worst Case

Scenario Earthquakes

Model MCharacteristi

cs08 – West Valley Fault

7.2

Severe Damage

13 –Manila Trench

7.9

Tsunami

West

Valley

Fault

Manila

Trench

Worst-case Scenario Earthquakes for Metro

Manila

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