This document discusses active faults and earthquake source zones in the Philippines. It defines an active fault as one that has moved within the last 10,000 years. The three types of faults are described as normal, thrust/reverse, and strike-slip. Some of the most destructive earthquakes in Philippine history are noted, including the 1990 Luzon earthquake. The Philippine Fault Zone and Valley Fault System are identified as two of the most important active faults. Trenches are also described as earthquake generators where tectonic plates are subducting. The potential impacts of worst case earthquakes on the West Valley Fault and Manila Trench impacting Metro Manila are presented.

1. Active Faults & OtherActive Faults & Other
Earthquake Source ZonesEarthquake Source Zones
Mariton V. BornasMariton V. Bornas
Geology & Geophysics R&D DivisionGeology & Geophysics R&D Division
Philippine Institute of Volcanology & SeismologyPhilippine Institute of Volcanology & Seismology

2. ACTIVE FAULTS

3. •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?

4. BEFORE FAULTING
REVERSE
NORMAL FAULT
STRIKE-SLIP
BEFORE FAULTING
REVERSE
NORMAL FAULT
STRIKE-SLIP
TYPES OF FAULT

5. Normal FaultNormal Fault

6. Thrust or Reverse FaultThrust or Reverse Fault

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

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

9. StrikeStrike--slip Faultslip Fault

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

11. LeftLeft--laterally displaced pilapils (rice paddy dikes) in Imugan, Nuevalaterally displaced pilapils (rice paddy dikes) in Imugan, Nueva VizcayaVizcaya
along the Digdig Fault rupture of the 1990 Northern Luzon Earthqalong the Digdig Fault rupture of the 1990 Northern Luzon Earthquake.uake.

12. ““Mole tracksMole tracks”” defining the Digdig Fault rupture afterdefining the Digdig Fault rupture after
the 1990 Northern Luzon Earthquake.the 1990 Northern Luzon Earthquake.

13. Aerial view of the San Andreas FaultAerial view of the San Andreas Fault
System, the most famous strikeSystem, the most famous strike--slipslip
fault in the world. The San Andreasfault in the world. The San Andreas
Fault marks the boundary betweenFault marks the boundary between
the Pacific Sea Plate and the Norththe Pacific Sea Plate and the North
American PlateAmerican Plate

14. 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)

15. Active Faults &
Trenches
Philippine seismicity
from 1600s to present

16. DOST
PHIVOLCS

18. B: Triangular facet H: Offset stream
C: Fault sarplet I: Shutter ridge
D: Sag pond J: Beheaded stream
E: Pressure ridge K: Wind gap
F: Fault saddle L: Offset piedmont line
G: Graben M: Offset alluvial terrace
Smith & Wesson, 1975)
Active StrikeActive Strike--slip Fault Geomorphologyslip Fault Geomorphology

19. 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

20. 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

21. •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:

22. ACTIVE FAULTS & OTHER
EARTHQUAKE
GENERATORS OF THE
PHILIPPINES

23. Most
Destructive
Earthquakes in
the Philippines

24. 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

25. Earthquake
Generators in
the Philippines

26. Earthquake Generators in the Philippines

27. 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

28. 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

29. Trenches as Earthquake Generators

30. 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 the
PMB between Taiwan and Mindoro.
PHILIPPINE SUBDUCTION ZONES

31. 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

32. Metro Manila
and the
VALLEY
FAULT
SYSTEM (VFS)

33. Surface Geology of Metro Manila
East Valley Fault
West Valley Fault

34. Source: HIGP, Univ. Hawaii
West Valley Fault
Manila Bay

36. The Worst Case
Scenario Earthquakes
Model M
Characteristi
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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