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Markikina Fault

Multiple large earthquakes in the past 1500 years on a fault in metropolitan Manila, The

Philippines

(published in the Bulletin of Seismological Society of America, 2000, vol. 90, p. 73-85)

Alan R. Nelson

Stephen F. PersoniusGeologic Hazards Team, Central Region U.S. Geological Survey, MS 966PO Box 25046,

Denver, Colorado 80225, USA Rolly E. Rimando

Raymundo S. Punongbayan

Norman Tugol

Hannah Mirabueno

Ariel RasdasPhilippine Institute of Volcanology and Seismology,

University of the Philippines, Diliman, Quezon City,The Philippines

Figure 1

ABSTRACT

The first 14C-based paleoseismic study of an active fault in The Philippines shows that a right-

lateral fault on the northeast edge of metropolitan Manila poses a greater seismic hazard thanpreviously thought. Stratigraphic, soil, and 14C data from exposures across the northern part of

the west Marikina Valley fault indicate a recurrence interval of 200-400 years for magnitude 6-7

earthquakes on the fault. Stratigraphic relations among faults and silty hillslope colluvium,gravelly stream-channel alluvium, and cobbly debris-flow deposits exposed in trenches-and three

reddish, clay-rich B soil horizons developed on those deposits-record three surface-faulting
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events. AMS 14C ages on detrital charcoal constrain the entire stratigraphic sequence to the past

1300-1700 years, and clay-rich soils point to earthquake recurrence intervals of >200 years.

Minimal soil development and modern 14C ages from colluvium overlying a faulted debris-flowdeposit in a nearby stream exposure show that the most recent, fourth faulting event is historic,

probably dating from the past 200 years.

INTRODUCTION

Recent assessment of the earthquake hazard posed by crustal faults in cities such as Los Angelesand Seattle, and the economic and human loss resulting from recent damaging earthquakes in

Northridge, California, and Kobe, Japan highlight the need for evaluating potentially active

crustal faults in urban areas. Manila--with a metropolitan population of about 10 million--issimilarly subject to earthquakes on nearby crustal faults, as well as earthquakes on more distant

plate-boundary faults (Fig. 1A). The city has been heavily damaged by earthquakes at least six

times in the past 400 years, but the specific sources for the earthquakes are uncertain. The

Marikina Valley fault system, on the northeastern edge of the Manila metropolitan area (Metro

Manila, Figs.Fig. 1AandFig. 2A), is a likely source one or more of these earthquakes.Determining the rate of recurrence of large earthquakes on these shallow crustal faults is critical

for estimating the strength and probability of future earthquake ground motions in themetropolitan area.

Figure 2

Over the past two decades, detailed stratigraphic studies of sediments displaced by recentfaulting have become a standard means of reconstructing the magnitude and recurrence of past

earthquakes on surface-rupturing faults. Unfortunately, such fault assessment techniques have

rarely been applied in many areas where they might yield maximum benefits--the rapidlygrowing urban areas of developing countries. In The Philippines, interest in seismic hazardassessment was stimulated by the 1990 magnitude 7.8 earthquake in northern Luzon (Fig. 1A).

Scarps raised during this earthquake were trenched by Daligdig, Nakata and others to determine

the history of earthquakes on that branch of the Philippine fault system. In 1991 PHIVOLCS

staff used a trench across the east Marikina Valley fault (Fig. 2A) to show recent lateral andthrust (?) movement on that fault, but because no radiocarbon-datable material was found

earthquake recurrence on the fault could not be determined.
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In 1995-96 we made the most successful attempt to determine earthquake recurrence on a

hazardous fault in The Philippines-the westernmost fault in the Marikina Valley system, whichlies only 10 km east of central Manila (Fig. 2A). Stratigraphic, soils, and 14C data obtained from

trenches and stream bank exposures at the Maislap trench site show that a northern splay of the

west Marikina Valley fault (Fig. 2A) has produced at least four large earthquakes in the past1300-1700 years. Such a history suggests a rate of slip several times greater than might beexpected from a fault with less than 30 km of physiographic expression located more than 60 km

from active plate boundaries (Fig. 1A).

Figure 3

Figure 4

CONCLUSIONS: MAGNITUDE AND FREQUENCY OF LARGE EARTHQUAKES ON

THE MARIKINA VALLEY FAULT SYSTEM

Based on our interpretation of the exposures at the Maislap site (Figs.3and4), at least four

large, surface-rupturing earthquakes have occurred on the northeastern splay of the west

Marikina Valley fault since A.D. 600. Because the four surface-faulting events occurred during a

period of time that is short relative to the precision of 14C dating and because all ages are ondetrital (transported) charcoal fragments that are older (or in a few cases younger) than the times

of faulting, we cannot accurately determine the time of each faulting event or determine specific

earthquake recurrence intervals. Nevertheless, the three soils with argillic B horizons in trench 1

(Fig. 3) each took at least 200 years to develop and 14C ages constrain the age of the entiresequence to less than 1300-1700 years ago (Fig. 4), making it unlikely that any of the soils

required more than 400 years to develop. This information combined with our estimate of a

historic faulting event (D) yield an average recurrence interval of as little as 200 years (minimumestimated time needed to form the three argillic horizons, 600 years, divided by 3 earthquakes)and as much as 421 years (the interval A.D. 600 to A.D. 1863 divided by 3) for large earthquakes

on this part of the fault. However, both historic and detailed prehistoric records of large

earthquakes illustrate that recurrence on individual faults is commonly clustered. For this reason,
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a range 200-400 years rather than an average recurrence of 310 years should be used in assessing

the potential for future recurrence of large earthquakes on the Marikina Valley fault system.

Estimates of the magnitude of prehistoric earthquakes are commonly based on fault rupture

lengths and/or fault displacements. Because we cannot estimate the lateral component of fault

displacement from our three fault-perpendicular exposures, our study at the Maislap site focusedon earthquake recurrence rather than on estimating magnitude from faulting-event displacement.

Many more trenches and offset landforms would have been required to measure fault

displacements, and it is not clear whether the stratigraphy at the site would have been suitable forsuch measurements. Landforms show that recent displacement on the west Marikina Valley fault

is primarily right lateral, although we do not know the ratio of vertical to lateral displacement

during faulting events. If the ratio is similar to ratios (1:2 to 1:8) measured along scarps on the

DigDig fault in northern Luzon soon after the magnitude 7.8 1990 earthquake and the AglubangRiver fault in Mindoro during the magnitude 7.1 1994 earthquake (Fig. 1A), vertical separations

of 20-40 cm that probably occurred during individual surface-faulting events on fault strands in

trench 1 (Fig. 3) and the stream exposure suggest lateral slip of about 1-2 m during each event.

Such lateral displacements are consistent with earthquakes of magnitude 6-7.

Empirical relations between rupture lengths and magnitudes of historic earthquakes in similartectonic environments also argue for earthquakes of magnitude 6-7 on the Marikina Valley faultsystem. The ~30-km-long northern half of the west Marikina Valley fault, whose trace is marked

by young, fault-related landforms, corresponds with earthquakes of this size. Although there is a

possibility that a much longer section of the fault ruptured during earthquakes as large asmagnitude 7.5, landforms suggesting repeated rupture of the west Marikina Valley fault

southward beyond the Pasig River have yet to be identified (Fig. 1B). The geomorphically

expressed trace of the east Marikina Valley fault is even shorter (18 km long), so the chance of

an earthquake larger than magnitude 7 on the faults of the Marikina Valley system seems small.

ACKNOWLEDGMENTS

This project was conceived by S. T. Algermissen as part of the U.S. Geological Survey's

Worldwide Earthquake Risk Management program. Algermissen also obtained funding for theproject from the U.S. Agency for International Development--The Philippines (Technical

Resources Project No. 492-0432). R.S. Punongbayan obtained further funding from the

Philippine Department of Public Works and the insurance industry in Manila (primarily backhoe

support, PHIVOLCS salaries, and supplies). We thank David Nelson and Jos Garzon of U.S.AID in Manila for being strong advocates of our work. Takashi Nakata (Hiroshima University,

Japan) identified the Maislap trench site during earlier geomorphic mapping projects and

encouraged us to trench it. Chris Newhall (USGS, Vancouver, Washington), and Donald Wells,

Andy Thomas, and Tom Bullard (all with Geomatrix Consultants, San Francisco) gave valuableadvice and loaned us equipment. Lee-Ann Bradley prepared Figures 1-5 and Carol Prentice and

Michael Machette provided helpful reviews. We especially thank Rodolfo Alito and his family

for two months of generous hospitality, for permission to dig up their rice paddies, and for hiringand supervising laborers at the site.
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Figure 1 Tectonic setting of the Marikina Valley fault system (MVFS) in central Luzon, ThePhilippines. In A, subduction zone trenches are shown by white barbed lines and other faults

with high rates of Quaternary activity by heavy black lines. White dots show locations of recent

earthquakes on the Philippine fault in Luzon (magnitude 7.8; 1990) and the Aglubang River faultin Mindoro (magnitude 7.1; 1994). Inset B shows how the Marikina Valley pull-apart basin

(MVPB) may have been formed through extension caused by clockwise rotation (dashed circle)

and shearing of this part of central Luzon, which is caught between two active left-lateral strike-slip faults--the Philippine fault and the Lubang fault. A zone of extension and young volcanism

has also influenced the structural development of the valley.


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Figure 2 A) Map showing the locations of the east and west Marikina Valley faults, which strikethrough the metropolitan Manila area (Metro Manila, light shading) of central Luzon. The

Maislap trench site is on a northeast-trending splay of the west Marikina Valley fault in thenorthern part of the Marikina Valley. B) Map of the trench site, showing fault traces and trench

and stream exposures.

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Figure 4. Diagram showing relations among stratigraphic units and timing of surface-faulting

earthquakes at the Maislap site. Triangles show 14C-dated charcoal samples as in Figure 3; 14C

ages are calibrated and expressed as time intervals in cal yr AD (approximate calendar yearsA.D.) at two standard deviations and rounded to the nearest 10 years. Large numbers correspond

with groups of stream-channel deposits (groups 1, 2, and 3) displaced during faulting events A,

B, and C, respectively. The dashed lines bounding the horizontal bars for events B, C, and D areintended to show that evidence of these faulting events may be present in the exposures reachedby the dashed lines, but that the specific faulting event that produced the evidence could not be

associated with a particular event. Erosional unconformities bounding some deposits are inferred

from truncated bedding and soil horizons.


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