focal mechanisms and tectonics in the taiwan-philippine ... · j. phys. earth, 26, suppl., s 249-s...

J. Phys. Earth, 26, Suppl., S 249-S 263, 1978

FOCAL MECHANISMS AND TECTONICS IN THE

TAIWAN-PHILIPPINE REGION

T. SENO and K. KURITA

Geophysical Institute, Faculty of Science, University of Tokyo, Tokyo, Japan

(Received June 10, 1978; Revised September 1, 1978)

Seismic activity and focal mechanisms in the vicinity of Taiwan and the Philippinesare studied to elucidate the tectonics in this complex region. Epicentral distribution andvertical profiles of earthquake foci support the idea that the entire Philippines is not a

part of the Eurasian plate, but another block of lithosphere and the relative motion be-tween the Philippine Sea and the Eurasian plates is shared by the subduction along the twoboundaries, west and east of the Philippines. Thrust type of mechanisms are the dominantmode of deformation along the eastern margin of the Philippines; in contrast, no thrusttype of solutions are obtained along the western margin of this islands. Between Taiwanand Luzon, mode of plate consumption is most complex. Seismic activity is mostlyshallow and diffuse in a 200km wide zone. Reverse faultings along the eastern marginof Taiwan, strike-slip faultings off the southeastern coast of Taiwan, and normal faultingsbetween the Manila trench and the North-Luzon trough are the major mode of defor-mation. We believe that the region between Taiwan and Luzon constitutes a left-lateralshear zone due to the gradual transition of site of plate consumption from eastern margin ofTaiwan to east Luzon. Subduction under the west-facing Luzon arc in this region is likelyto be ending now; this might be causing the opening of the sedimentary wedge behind theManila trench. Normal faultings in this study probably indicate the initial phase or

prespreading phase of opening of the area behind the Manila trench.

1. Introduction

The 23 focal mechanism solutions presented in this study are from almost all theshallow focus earthquakes occurring during the period from 1970 to 1975 in the vicinityof Taiwan and the Philippines. These solutions are based on the WWSSN long periodrecords. Although they show that all types of mechanism solutions are found in thiscomplex region, each district of the region has a dominant mode of deformation. Thesefocal mechanisms and distribution of recent seismicity are useful for elucidating the tectonicfeatures in the Taiwan-Philippine region.

The region concerned is made complex by the Manila trench-Luzon trough systemto the west, the Philippine-Mindanao trench system to the east, the geologically activefault system in the Philippine archipelago, and the arc-continent collision at Taiwan

(Fig. 1, LUDWIG et al., 1967; HAYES and LUDWIG, 1967; KATSUMATA and SYKES, 1969;LUDWIG, 1970; WU, 1970, 1978; FITCH, 1970, 1972; CHAI, 1972; BIQ, 1972; KARIG,1973; MURPHY, 1973; SENO, 1977; BOWIN et al., 1978). As was pointed out by KATSU-MATA and SYKES (1969), it is impossible to describe the slip vectors of shallow earthquakesin the entire Ryukyu-Taiwan-Philippine region by only one pole of rotation between two

plates. Blank arrows in Fig. 1 indicate the direction of convergence of the PhilippineSea plate with respect to Eurasia (SENO, 1977). It can be seen that the slip directions of

S 249

S 250 T. SENO and K. KURITA

Fig. 1. Tectonic elements in the vicinity of Taiwan and the Philippines. The inferred baundary of

the Philippine block (SENO, 1977) is indicated by the line of trench axis and by the broken line. Triangles show the active volcanic centers. Slip vectors of shallow earthquakes along the eastern margin of the Philippines (FITCH, 1972) are indicated by the thin arrows, and the direction of the relative motion between the Philippine Sea and Eurasia (SENO, 1977) are indicated by the blank arrows.

shallow events along the eastern margin of the Philippines (thin arrows in Fig. 1) deviatein a counterclock wise sense from the computed ones. This implies that the Philippinesis not a part of the Eurasian plate but forms another block of lithosphere (SENO, 1977).We herein call this block of lithosphere the Philippine block; the inferred boundary ofthis block (SENO, 1977) is indicated in Fig. 1. The southwestern portion of the boundary,speculative when inferred, is evidenced by the existence of subduction zones which borderthe islands from the Sulu and Celebes basins (MURPHY, 1975; HAMILTON, 1977). Therelative motion between the Philippine Sea and the Eurasian plates may be shared by themotion along the west and east sides of the block (SENO, 1977). The main purpose ofthis present study is to refine and correct this earlier investigation and delineate tectonicfeatures in this complex region on the basis of seismicity and focal mechanisms.

2. Seismicity

Figures 2 and 3 show the spatial distribution of shallow focus and intermediate and

Focal Mechanisms and Tectonics in the Taiwan-Philippine Region S 251

Fig. 2. Epicentral distribution of recent (1964-1976) shallow focus earthquakes (mb〓5.0, depths〓

100km) based on the USGS data.

Fig. 3. Epicentral distribution of recent (1964-1976) intermediate and deep focus earthquakes

(mb〓5.0, depths>100km) based on the USGS data.


deep focus epicenters, respectively, based on the USGS data, occurring during the period

from 1964 to 1976 in the area of the present study. Nearly all the shallow activity in the

vicinity of the Philippines is confined to a narrow zone around the Philippine archipelago

(Fig. 2). North of Luzon, the activity is diffuse in a 200km wide zone. South of Min-

danao, there are two zones of activity extending in a SSE direction to the west and north-

east of Halmahera. Deep focus epicenters are distributed under the area from north of

Mindanao to the Celebes Sea (Fig. 3); their depths are up to 650km. This deep activity

disappears abruptly at about 10°N north of Mindanao. South of Mindanao, as will be

shown later, the deep activity can be joined with the zone of shallow activity extending

to the west of Halmahera. The shallow activity along the Philippine trench continues to

the zone of activity at the northeast of Halmahera (Fig. 2), as does the topography of the

trench, and not to the zone at the west of Halmahera, thus we can say that the Benioff

zone under the Celebes Sea, which extends northward to the north of Mindanao, is the

subducted lithosphere which had existed at the west of Halmahera, contrary to the view of

FITCH (1970) who considered this slab as the subducted Philippine Sea plate. This point

will be discussed later (Fig. 4). Then, it can be said that the seismic activity around the

Philippines which indicates the present relative motion of the Philippine block with respect

to the adjacent basins is mostly shallower than 200km. We call the shallow activity

along the west and east margin of the Philippines the western and eastern seismic zone,

respectively. The western seismic zone may continue farther southward to the north of

Sulawesi as inferred from the embryonic subduction zone there (HAMILTON, 1974, 1977;

SILVER and MOORE, 1978).

Profiles of earthquake foci may provide us with some characteristic features of sub-

duction along the Philippine block. Figure 4 shows the vertical profiles of earthquake

foci along the latitudinal lines south of 22°N. In section A (22°-18°N), activity is mostly

shallower than 100km and diffuse in a zone wider than 200km. In section B (18°-15°

N), two seismic zones, eastern one and western one, appear. The western one, less

active than the eastern one, appears to dip eastward. In section C (15°-12°N), the

eastern zone shifts to the east according to the swing of the topography of Luzon. This

zone is dipping to the west and the western one to the east. In this section, we can also

see the activity between the two zones; this inland activity may be associated with the

Philippine fault system. In section D (12°-10°N), we can also see the two inclined seismic

Fig. 4. Profiles of the seismicity around the Philippines. The foci are projected onto vertical planes

along the latitudinal lines.


zones, though the western one is not very active. In section E (10°-6°N), deep activity

whose depth is from 350 to 650 km suddenly appears. At a first glance, this activity

appears to join with the eastern zone of shallow activity. We, however, do not take this

view as will be seen in the next profile. In section F (5°-2°N), we can see two zones of

shallow activity which are corresponding to the two seismic zones extending to the west

and northeast of Halmahera in Fig. 2. The eastern seismic zone, which appears to dip

steeply in the deeper part, is the continuation of the shallow activity along the Philippine

trench as can be seen in Fig. 2. The westerly dipping slab-like zone in this profile can be

joined with the western zone of shallow activity and not with the eastern one. Thus, also

in section E, the deep activity, which is the northward continuation of the deep activity in

section F, cannot be joined with the shallow activity along the Philippine trench. This

leads us to conclude that the westerly dipping deep activity under the Philippines indicates

the subducted slab which had existed at the west of Halmahera and not the subducted

Philippine Sea plate.

In these profiles, active volcanic centers are also shown. Their distribution appears to

be associated with the distribution of the Benioff zones, though more close examination of

chemical composition of volcanic products may be needed to correlate the volcanic ac-

tivity with the plate subduction (e.g., HATHERTON and DICKINSON, 1969).

Seismic activity along the geologically active Philippine fault system in the archipel-

ago is not distinct in Figs. 2 and 4. We do not agree with the view of FITCH (1972),

MURPHY (1973), and HAMILTON (1977), that this fault system plays an important role in

the interplate deformation between the Philippine Sea and Eurasia, as was discussed by

SENO (1977).

Depth of seismic zone may provide important imformation on the rate of plate sub-

duction; we plot the deepest point of the eastern and western seismic zones around the

Philippines versus latitude (Fig. 5). It should be noted that we do not use the deep ac-

tivity under the Philippines in this plot. The depth of eastern seismic zone shoals to the

north surprisingly linearly. If the difference in depth of the seismic zone is due to the

difference in rate of plate convergence, we can locate the pole of rotation of the Philip-

Fig. 5. Distribution of seismic activity with depth as a function of latitude.


pine Sea plate relative to the Philippine block at about 21°N. This pole position is

consistent with the slip directions of the shallow events along the eastern seismic zone (see

Fig. 1 and also SENO, 1977). The western seismic zone being less active than the eastern

one, the depth-latitude relation had to be inferred. But, it appears to shoal to the south.

The arithmetic sum of the depths of the two seismic zones is almost constant (250-350

km), but it looks to shoal to the north slightly. It is interesting to note that the deepest

point of the Ryukyu seismic zone is 250km (KATSUMATA and SYKES, 1969). Thus the

difference in depth between the Ryukyu and Philippine seismic zones is consistent with

the convergence rate of the Philippine Sea plate with respect to the Eurasian plate, if and

only if we take the arithmetic sum of the depths of the two seismic zones around the

Philippines. This supports the idea that the both boundaries of the Philippine block

share the relative motion between the plates.

Between Taiwan and Luzon is a hiatus of subduction zone; we discuss the mode of

deformation in this area in a later section.

3. Focal Mechanisms

The 23 focal mechanisms presented in Fig. 6 are from shallow focus earthquakes oc-curring during the period from January 1970 to December 1975. The events that yieldedreliable focal mechanism solutions were selected from a list of all shallow events reportedto the USGS. Nearly all the focal mechanisms came from the events with body-wave

Table 1. Parameters for Taiwan-Philippine shallow earthquakes from 1970 to 1975.


Fig. 7. Compilation of focal mechanism solutions south of 17°N. Number attached with the

solution denotes earthquake in Table 1. Solutions F 35 and F 37 are from FITCH (1970) .

magnitude greater than 5.6. The mechanisms are shown as equal area projections ofthe lower hemisphere of the focal sphere; their parameters are presented in Table 1.To determine the nodal planes of the mechanisms, we used the first motion of long-periodP-wave and the polarization angle of long-period S-wave recorded by the seismographs

of WWSSN. In some cases, P-wave data are not sufficient to determine nodal planes; insome of such cases, S-wave data were used effectively to determine the nodal planes usingthe computer program written by HIRASAWA (1970). Mechanism solutions are rankedaccording to the reliability of their nodal plane solutions (Table 1, see foot notes).


In Fig. 7, mechanism solutions in the area south of 17 °N are plotted on the bathy-

metric chart (U.S. NAVAL OCEANOGRAPHIC OFFICE, 1969). Two solutions in the earlier

period by FITCH (1970) are also incorporated into this figure (F35 and F37).

Along the Philippine trench, thrust type of mechanisms are obtained under the land-

ward slope of the trench and the edge of the continental shelf (Nos. 16, 20, F35, F37).

Two normal faultings are distributed at the trench axis (Nos. 15, 22) and one at the axis

of a canyon extending from the southernmost Luzon to the trench (No. 17). Farther

north, along the eastern margin of Luzon are found two thrust type solutions (Nos. 2, 10).

Topography of the trench does not develop well there; however, a steep submarine scarp

and a shallow trough extend along the east coast of Luzon, and join with the Philippine

trench at about 13 °N. We believe that this scarp and trough represent an incipient stage

of subduction zone as is suggested by KARIG (1973). One strike-slip faulting is obtained

where the scarp along the east Luzon swings to the east (No. 3). The aftershock dis-

tribution of this event is elongated in an E-W direction; this indicates that the E-W trend-

ing nodal plane is a slip plane. The E-W trending scarp must play a role of transform fault

here as was pointed out by KARIG (1973); this event can be interpreted to indicate a

sinistral shear motion along this transform boundary.

Thrust type solutions along the eastern margin of the Philippines are consistent with

underthrusting of the Philippine Sea plate beneath the islands from the east. As was

mentioned earlier, the slip direction of these shallow events deviates from the NW-SE

direction of the relative motion between the Philippine Sea and the Eurasian plates.

The difference in the sense of motion implies that the Philippine block is moving to the

northwest with respect to Eaurasia (SENO, 1977).

In contrast to the eastern margin of the Philippines, no thrusts are obtained along the

western margin of the islands. Three strike-slip faultings and one normal faulting are

obtained from Mindoro to Panay (Nos. 1, 9, 18 and No. 21). Though we can see an

inclined seismic zone here (Fig. 4), it is not easy to conclude that these events represent

the interplate deformation between the Philippine block and the adjacent basins. If

these events are interpreted to indicate interplate deformation, the northwestward trending

nodal planes of the strike-slip faultings are consistent with the northwestward migration

of the Philippine block with respect to Eurasia. It is, however, also possible to interprete

these events as indicating the intraplate deformation caused by the E-W compressional

tectonic force due to the subduction of the Philippine Sea plate under the islands from the

east. Seismic activity along the western margin of the Philippines is less active than along

the eastern margin (Fig. 2) and no thrust type of mechanisms are obtained there; this may

provide a line of evidence for the idea that the subduction of the marginal basins under

the Philippine block from the west is now ceasing and arc polarity reversal is currently

taking place from the western boundary to the eastern one (e.g., BOWIN et al., 1978). This

point will be discussed later.

South of Panay, no focal mechanism solution was obtained for the period treated in

this study. However, a large earthquake (Ms=7.8) occurred on August 16, 1976 in the

region of Moro gulf, northern Celebes Sea, south of Mindanao. Focal mechanism of

this event indicates an eastward subduction of the Celebes basin under the Mindanao

(STEWART and COHN, 1977). The relative motion of the Celebes basin to Mindanao

does not necessarily coincide with the one inferred between the Philippine block and

Eurasia. This implies that the Celebes basin is likely to be detached from Eurasia.


Fig. 8. Compilation of focal mechanism solutions north of 17°N. Number attached with the solu-

tion denotes earthquake in Table 1. Solutions KS3 and KS10 are from KATSUMATA and SYKES

(1969).

The idea of dormant arcs in the marginal basins of southeast Asia (MURPHY, 1975;

HAMILTON, 1977) supports this possibility.

In Fig. 8, the focal mechanism solutions south of 17°N are plotted on the bathymetric

chart. Two solutions of the events in the earlier period by KATSUMATA and SYKES (1969)

are also incorporated into this figure (KS3, KS10). Main features of mode of deformation

in this area are as follows: a thrusting between the Ryukyus and Taiwan (No. 7), high

angle reverse faultings along the east margin of Taiwan (Nos. 8, 12), strike-slip faultings

off southeast Taiwan (Nos. 4, 6, 14), and normal faultings in the area between the Manila

trench and the North Luzon trough (Nos. 5, 11, 19, 23, KS3, KS10). During the period

before 1970, large events of strike-slip faultings occurred at the junction between Taiwan

and the Ryukyus, of which tectonic significance has been in dispute (WU, 1970, 1978;

SUDO, 1972). The most prominent features of the present focal mechanisms, which differ

from the earlier investigations (KATSUMATA and SYKES, 1969; Wu, 1970; SUDO, 1972),are the normal faultings in the area between the Manila trench and the North Luzontrough and the strike-slip faultings off southeast Taiwan.

The region from the junction between Ryukyu arc and Taiwan to north Luzon is one


of the most complex parts of the Philippine Sea-Eurasian plate boundary (KARIG, 1973;

MURPHY, 1973; BIQ, 1972; CHAT, 1972; SENO, 1977; BOWIN et al., 1978). Taiwan is a

hiatus of subduction zone and has been a place of collision between the west-facing Luzon

arc and the continent of Asia since Pliocene time (CHAT, 1972; KARIG, 1973; BOWIN

et al., 1978; Wu, 1978). The Ryukyu island arc changes its trend in the vicinity of Taiwan

and abuts against the crustal block of northeast Taiwan (Fig. 9, BOWIN et al., 1978, Fig. 1).

South of Taiwan, there are the west-facing Luzon arc, the Manila trench and the North

Luzon trough (forearc basin), and the volcanic islands. The portion between the Manila

trench and the North Luzon trough is a topographic bulge interpreted as an accretionary

wedge (BOWIN et al., 1978). The events of normal faultings are distributed under this

sedimentary wedge. Their epicenters (International Seismological Centre) are located

certainly eastward of the trench axis within the error of epicentral determination (Fig. 8)

and theirfo cal depths are ranging from 8 to 42km. Thus these normal fault type of mech-

anisms present a quite difficult problem of why the extensional feature is possible under

the accretionary wedge of subduction zone. We shall discuss this problem later.

Though distribution of epicenters in the vicinity of Taiwan is not presented in this

study, recent study of seismic activity (TSAI et al., 1978) and the earlier investigations (Wu,

1970; HSU, 1971; SUDO, 1972; KATSUMATA and SYKES, 1969) show that the activity is

mainly along the east margin of Taiwan and at the junction between Taiwan and the

Ryukyus. The latter activity is most intense and presents a northward dipping tongue-

like slab whose depth is up to 140km (TSAI et al., 1978). A thrust type of event at the

Ryukyu-Taiwan junction (No. 7, Fig. 8) probably indicates underthrusting of the Philip-

pine block under Asia. The activity along the east Taiwan is shallow, mostly within

40km depth, and confined in an area that has a 100-200km width in the E-W direction;

this seismic area is truncated by the Longitudinal Valley fault at the west end (TSAI et al.,

1978). Reverse faultings along the eastern margin of Taiwan (Nos. 8, 12) show that dip-

slip motion occurs along the Longitudinal Valley fault in addition to the sinistral shear

motion as evidenced geologically and seismologically (ALLEN, 1962; HSU, 1962; KANEKO,

1970).

Between southern Taiwan and north Luzon, seismic activity is shallow and makes a

diffuse zone of 100-200km width in the E-W direction (Fig. 2, and TSAI et al., 1978).

Here we cannot apply the idea of double arc such as discussed in the area south of 17°N;

we recall that the pole of rotation of the Philippine Sea plate with respect to the Philippine

block is located at about 21°N, thus the eastern boundary of the Philippine block must

vanish at the north of Luzon. We propose a model for plate consumption in this area as

illustrated in Fig. 9. We agree with the view that arc polarity reversal, probably in

response to collision at Taiwan, is now taking place between the west-facing Luzon arc and

the new subduction boundary along the east margin of Luzon (KARIG, 1973; KARIG

and WAGEMAN, 1975; BOWIN et al., 1978). This new subduction boundary is likely to

extend farther north along the Palaui ridge (Fig. 8) from the west margin of Luzon, be-

cause a free-air gravity anomaly low and/or a topographic feature which indicate an em-

bryonic subduction zone continues to about 19°30′N along the Palaui ridge (BOWIN et al.,

1978) and because the pole of rotation of the Philippine Sea plate relative to the Philippine

block is located at about 21°N. The rate of subduction along this boundary must taper

off to the north and probably ends at about 21°N. Therefore, the extent of interplate

deformation along the eastern margin of Taiwan must taper off to the south in response

to the gradual increase of the rate of plate consumption to the south along the east Luzon


Fig. 9. Model for palte consumption in the region between Taiwan and Luzon. Thickness of the lines

along the eastern margins of Taiwan and Luzon shows the relative rate of plate convergence along

these boundaries.

subduction zone. This is schematically illustrated in Fig. 9. Some extent of plate con-sumption is probably taking place along the Manila trench in this area. We, however,believe that the subduction along the west-facing Luzon arc in this portion is now endingbecause the collision at Taiwan hinders the subduction of the south China basin underthe Luzon arc as suggested by BOWIN et al. (1978). Thus the major portion of interplatedeformation in this area is described by the gradual southward decrease of plate con-sumption along the eastern margin of Taiwan and, in contrast, by the gradual increasealong the east margin of Luzon. This configuration would produce a left-lateral shearin a NW-SE direction between Taiwan and Luzon (Fig. 9). This type of interplate de-formation is termed as 'transform belt' by ISHIBASHI (1978) who presented a similar type ofdeformation in the Izu peninsula at the northernmost part of the Philippine Sea-Eurasian

plate boundary.Now the mysterious features of seismic activity and focal mechanisms between Taiwan

and Luzon may be understood as follows. The diffuse activity between southern Taiwanand north of Luzon is interpreted as indicating the activity due to the shear zone in thisregion. Three strike-slip events off the southeast Taiwan (Nos. 4, 6, 14) are with north-westward striking nodal planes consistent with the left-lateral shear motion of the trans-


form belt. Segmented ridge and trough system north of Luzon also indicates a left-lateralshear in a NW-SE direction (Fig. 8), contrary to the KARIG'S (1973) interpretation of left-lateral shear in a NE-SW direction.

The tectonic stress regime along the Manila trench is likely to be turning to extensionfrom compression in response to the cessation of subduction along this boundary; thiswould make the opening of the portion between the Manila trench and the North Luzontrough. Normal faultings (Nos. 5, 11, 19, 23, KS3, KS10) may indicate the incipient

phase of spreading in this portion. Seismic profiles in east-west cross-section of this regionshow vertical faults between the accretionary bulge and the trough sediment which in-dicates the uplift of the bulge relative to the trough sediment (R.N. Anderson, personalcommunication, 1978). Thus the region from the bulge to the trough forms a horst-

graben structure, which may indicate an original uplifting phase of opening (R.N. Ander-son, personal communication, 1978). Three heat flow measurements at the northernmostcorner of the Manila trench (ANDERSON et al., 1977) show the values from 1.8 to 2.6, whichare abnormally high compared with that associated with trench. This also supports thespeculation that this is a region of incipient spreading. Normal fault type of mechanisms

presented in this study may provide an important piece of evidence for the initial or pre-spreading phase of opening of marginal basin due to the change of mode of plate con-sumption. Reconnaissance of this region is amply warranted because of its importance toelucidate the diverse behaviour of consuming plate boundary.

We wish to thank S. Uyeda and C.O. Bowin for their critical review of the manuscript. We also wish tothank T. Eguchi for assistance of plotting epicenters and line drawings utilizing computer and R.N. Anderson,Y.B. Tsai, and G.S. Stewart for offering us their data before publication. We benefited with discussion withR.N. Anderson, F.T. Wu, H. Mizutani, Y. Matsubara, R.W. Murphy, and K. Ishibashi.

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