Natural Hazards 1 (1988) 27-44. © 1988 by Kluwer Academic Publishers.
27
Seismic Hazard Analysis for The Philippines S. S. SU Manila Observatory, P.O. Box 1231, Manila, Philippines D404 (Received: 2 June 1987; revised: 24 August 1987) Abstract. Two sample probabilistic hazard maps for the Philippine Region are compiled. In these are shown
various levels of expected horizontal ground acceleration for some given annual probability of exceedence, namely, for 0.1% a.p.e, and for 0.01% a.p.e. Such hazard maps are needed by structural engineers for compiling seismic zoning maps. The hazard maps are derived from source-zone or seismogenic maps, which, in turn, are compiled from seismographic, geologic, and geotectonic data. Much weight is put on geotectonic data rather than on seismographic data. The former lends support to extrapolating to much longer periods of 'exposure time' or longer periods of recurrence. Key words. Hazard analysis, risk analysis, seismic zoning, probabilistic.
1. Introduction Seismologists make a distinction between seismic hazard and seismic risk. Seismic hazard is defined as the probability of occurrence of a specified level of ground shaking within a specified period of time. Seismic risk, on the other hand, is defined as the expected (or probable) building damage (or loss of or injury to life), given the probability that specified levels of ground shaking occur.
2. Background Structural engineers in the Philippines, in designing buildings according to some code, such as the Unified Building Code, would like to see the existence of a seismic hazard map. Such a map would enable them to assign a seismic zone number or seismic safety factor to each building they design depending upon its site location anywhere in the Philippines. Two kinds of quasi-hazard maps have heretofore been compiled and used but they contain inherent shortcomings that a strictly probabilistic hazard map avoids. The first is a map based solely on recorded seismicity. This kind of map focuses on epicenter concentrations and fails to generalize to likely areas of future seismicity. Furthermore, it fails to specify the type of hazardous ground motion. The second type of map is based on recorded maximum intensity of ground motion but still fails to generalize to future areas of hazardous ground shaking. Besides, such a map tends to overstate the hazard in regions where the frequency of occurrence is quite low compared to other more seismically active areas. A strictly probabilistic hazard map, it will be seen, avoids all these shortcomings of the two previous types of maps.
28
s.s. Su
The first set of probabilistic hazard maps for the Philippines was introduced by this author in September of 1979 as a result of a pilot project of the Southeast Asia Seismological Program undertaken by the UNDP in cooperation with the UNESCO. It was envisioned that eventually a more detailed and definitive set of maps would be compiled. As a continuance of this work, seismic source zones were revised in 1985 (Su, 1985) and again in 1986. This last revision is incorporated into this paper and contains some radical changes from the 1985 version (which will be pointed out later on) and thus paved the way for finalizing the hazard maps that are presented here.
3. Method The probabilistic approach to seismic hazard analysis that is adopted here, as it was in the U.S. and Canada, follows the methodology of Cornell (1968), and of Algermissen et al. (1982). According to this approach, one takes into account three distinct regions: first, the source or seismogenic region; second, the site where hazard is estimated; and third, the intervening region between the source and the site. The biggest and most difficult step is the detailed specification of the source zones. The resulting map of source zones has to be based not only on seismographic (instrumental) and macroseismic data (such as felt intensity) but also on geology and geotectonics. It will be seen later that the map of the source zones and the final hazard maps will bear striking similarities. Epicenters in seismogenic regions are treated statistically both spatially and temporally: that is, they are given a random distribution both in space and in time. In the spatial distribution of epicenters, one assumes that within a given source or fault line, an earthquake is equally likely to occur at one point as at any other point. Thus, the probability density is a constant, i.e., evenly distributed within the source. To find the cumulative probability one integrates over the entire source area or the entire fault length. In the temporal distribution of earthquakes, the Poisson model has been chosen here, as is frequently done, because of its simplicity in concept and in hazard estimation. Other models, such as the Markov model, could have been used. In addition to the random distribution of epicenters in space and time within the seismogenic region, there is also the magnitude-frequency distribution or the magnitude probability function which is statistically derived from earthquake frequency data. This distribution gives the probability of an earthquake falling within a certain magnitude range.
4. Scope of the Data for This Study The earthquake data that provided the basis for the present hazard analysis cover a period of twenty years: from 1964 to 1983. They include epicenters from latitude 0° to latitude 24°, and from longitude 116 to 130°E. Despite this short period of the seismographic database, it is the backing of the geologic and geotectonic data that
SEISMIC HAZARDANALYSISFOR THE PHILIPPINES
29
lends support to extrapolations to future 'exposure times' of the order of 500 years or more.
5. Source Zones 5.1. Procedure
The procedure consists of two parts. The first part is a rough delineation of the source zone boundaries based on seismographic data. The second part is a more refined or detailed delineation of the zone boundaries by recourse to geotectonic data. 5.2. Part I Seismographic data (epicenter, focal depth, magnitude) include those of 3872 earthquakes in the Philippine region from 1964 to 1983. These are shown in the epicentral maps of Figures 1 and 2, taken from Su (1985). From this database, two contour maps were prepared: one for energy flux and one for the b-values of the magnitudefrequency relation (Figures 3 and 4). From the overlay of these four maps a general rough-approximation map of the seismogenic zones was prepared. This is shown in Figure 5 (taken from Su, 1985).
5.3. Part H Based on published literature on the geology and tectonics of the Philippines, the zones of Part I are re-defined and subdivided. As mentioned above (in 'background'), the 1985 version of the source zones (Su, 1985) has been revised. The revised versions are shown in Figures 6 and 7. The main feature of the revision is that faults (Zones 9 to 18) are now treated separately from nonfaults (Zones 1 to 8). Other details of the revision will be described as each zone is taken up. 5.3.1. Zone 1. Zone 1, the East Luzon subduction zone, has a different subdivision from its 1985 version. The former 1D, the transform fault, is now Zone 17. The present 1D lies east of 1C. IA and 1C are at present seismically much more active than 1B and 1D (Lewis and Hayes, 1983). Focal mechanisms for Zone 1 are predominantly of the thrust type while those of Zone 4 (which is contiguous to 1A and 1C) are generally of the strike-slip type (Lewis and Hayes, 1983, citing Cardwell et al., 1980; Seno and Kurita, 1978; Fitch, 1972; Katsumata and Sykes, 1969). Negative free-air gravity anomaly in 1A and 1C is interpreted as indicating crustal downwarping and a precursor to rupture (Lewis and Hayes, 1983). High seismicity, focal mechanism solutions, and seismic sounding profiles show Recent (Pleistocene) to Present underthrusting in 1C (Cardwell et al., 1980). However, the absence of active volcanism, and the lack of a well-defined Benioff zone and of earthquakes deeper than 200 km show that current subduction is young.
S. S. SU
3O
II O°
,,116 °
120 °.
1220
126°
124 °
•
v
leO"
i ,
v
Jonuory 1 9 6 4 -
;:. 4
.' .:.
N
2~
•
"
•. "
u
December
Oepth of 0 - 71
1
1983
Focus (kin) Unknown
M b : 4.0-5.9 •
..~.
130`0
u
":t';~"
,~&
.... .'
M L : 3.0-5.7
:
"Z M b = 6.0-7.9
~o,
,so
,6'
,g
12`0
I0 °
8° •
o •
•
•
6~ "," °i .o
•
• !
oO •
4°
• o,
•
• °o
• Oog,. ,.
I •
.
,- :, ,-'" , -.o.'. t o~,,,
,, :.
• °ml s.Oo oo ,o 6 , , , , ..~./.. A.~. " ; "~r-"-" ."
::,-'**°, .,,, • o ;oo :..'-~i • ""i..';':-."
:~;~,:
I
Fig. 1.
I
I
h
I
I
°
I
I
Seismicity map (depth = 0-70km, 1964-1983).
o
"-.
".:"•
.... _-.. o~
, I
,o, •
• ~,, "..:,'.~. ~.-.: .: ",. t~.'~..~. -.-. t -:..;:4 .-., :" , ' ~ . . • .a. . . ,.-....L~ ; ~( .,.
•. , - ' , , -
|
o
• ¢.:
o
l
I
"
.' •
31
SEISMIC HAZARD ANALYSIS FOR THE PHILIPPINES It~ °
118 °
120 °
122 °
128 °
126 °
1~4 °
130°[
N
Interme~e Jnm~y
Focus
1964-
• • .°
I~:~.lx~ Diplh
1983
of
FOCus (kin)
71 - 3 0 0 id b : 4 , 0
~o"
,.. "o" •
**
**
°.
-S.9
Id t • 3 . O - S , 7
.'ll
i l
b • S.O-7.9
:~
u
, ~, I - - q
,¢
.? 'h
•.
II
•,
.a
• .~.
I0
°
¢.. Y . . r
•,
..
.
I" , • . . ,
•
.
.~j
•
•
.. •
..
,.
.~.:;
.
..
•
1#
-
,
.
., ~ . j . " : . . ' ~ . ~ . . : , ,
•
•
a,.
•
;,
•
.
I"
".
".."',~.'z:
"
..." ,•,
• ,~,., ~ . . : • , ~, ~ .. : ~'.~,~
.
. •
"
.
,. .:, ~ . .
l.:.
. _~l;
" --,'l.,_ ,r."
:::..,
:..
,..~
::
, •
,.,
•
i
Fig. 2.
I
I
I
I
." ,l.,..-,..,
:..,
i
I
I
7. "~,.:.,.... ...~,. :,
,
i
Seismicity map (depth = 71-300km, 1964-1983).
.'"
I
..
~....,
•
,,'. ,..f"
I
.
.". l,,.:-'.,
, •
i
•
..
• ;,~:'."
•'. '....:'..~.., •
• • •
.,~',,
~
l.a ..'.~
, •
"
. ,'o.z~.'a,
.
" " "-;i"
I
I
,
32 Iltl e
S. S. SU I
Ill*
1|2 e
I|0° i
124°
1
i
IZ6* •
121lee
i,i
v
Seismic Flux Jonuory 1964 - Oecmnber 1983 SOisl1~icity Contours in Units of I0 IB I[rgs k n n Z y r I
b N
~
/ q
,a'
,6'
~e
,m
• •
b"
,\
!
3
'=C~"
s
,I
Fig. 3.
Energy flux map (contours numbered in 1015ergs km -2 year -1).
33
SEISMIC HAZARD ANALYSIS FOR THE PHILIPPINES 116 °
118 e
120 °
122 ° 75
N
85
.80
~, \
124 ° 70
\
126 °
128 ° E
.65
\
N 20°
18 °
,6*
.70
"~14
14 ¢
.85
o
.80
.8(~
12"
.85
.75
!12°
%.65
8c
8°
.70
G°
6°
.75
4°
4°
.65
.69
.70
.71
2°
2°
116 °
I15 °
120°
Fig. 4.
I Z 2°
124 °
b-values, 1964~1983.
126°
128° E
S. S. SU
34 120"
IZ5=E
J
N 20"
Its*
d
to*
o -D C' c~ o
5
I 120°
Fig. 5.
125=E
Preliminary seismogenie zones.
5.3.2. Zones 2 and 3. The transcurrent Philippine Fault, which was the former Zone 3 of the 1985 version, is now Zone 9. The present Zone 2 and Zone 3 comprise the Philippine Trench (2B and 3B) and its Quaternary volcanic forearc (2A and 3A) (Divis, 1980). North latitude 12° separates Zone 2 from Zone 3. Zone 2 has a poorly developed Benioff zone that dips westward to a depth of about 100 km (Cardwell et al., 1980). Zone 3 has a better developed Benioff zone that extends to a depth of about 200 km south of Samar, as shown by intermediate earthquakes. Considerably deeper earthquakes are found east of Mindanao and southward to Talaud Island. The relative shallowness of the dipping lithosphere in Zones 2 and 3, together with the lack of significant Quaternary volcanism in Eastern Mindanao and Samar (Lewis and Hayes, 1983), coupled with the evidence from seismic reflection profiles showing
35
SEISMIC H A Z A R D ANALYSIS F O R T H E PHILIPPINES
SOURCE ZONES Nos. I fo B (FAULTS • FINITE-WIDTH SOURCE ZONES
-25
ARE IN A SEPARATE MAP)
20
15
Io -5
115 l
Fig. 6.
I
120 I
I
I
I
I
125 i
l
=
I
l
130 I
=
=
=
I
Source zones Nos. 1 to 8 (faults = Finite-Width Source Zones are in a Separate Map).
no well-developed accretionary prism in the forearc region (Hamilton, 1979; Karig, 1975; Karig and Sharman, 1975) strongly suggest that the present subduction episode may have begun only in Quaternary time, and is still propagating southward to the east of Talaud and Halmahera islands (Cardwell et al., 1980; Murphy, 1973). 5.3.3. Z o n e 4. Zone 4 is the double forearc associated with the Manila Trench which lies west of Luzon (Lewis and Hayes, 1983; Cardwell et al., 1980; Karig, 1973). 4B and 4D are the 'inner' volcanic forearc while 4A and 4C make up the 'outer' nonvolcanic forearc.
36
s.s. su
PHILIPPINE FAULTS (AS FINITE-WIDTH SOURCE ZONES)
20
.15
/
.IO
,5 after Philippine Bureau of Mines, 1981. 125
120 I
1
I
I
I
I
[
I
I
I
I
i
I
I,,,
,
Fig. 7. Philippine faults (as Finite-Width Source Zones). (After Philippine Bureau of Mines, 1981.) Seismicity is much higher in 4C than in 4D; also relatively higher in 4A than in 4B. 4B and 4D are marked by presently active volcanoes and Quaternary cones (Cardwell et al., 1980). Focal mechanism solutions of earthquakes in the N o r t h Luzon Ridge (4B) are predominantly of the strike-slip type Some of these are left-lateral and some are right-lateral. 5.3.4. Z o n e 5. Zone 5 is the Manila Trench. It has a Benioff zone that dips 40 ° eastward. The dip angle increases to almost 90 ° as one proceeds southward to Manila
SEISMIC HAZARDANALYSISFOR THE PHILIPPINES
37
Bay. The Benioff zone extends to a depth of about 200 km (Hayes and Lewis, 1984). South of latitude 13°, the Manila Trench changes its trend from north-south to northwest-southeast, and curves towards the Mindoro Strait. This is believed to be due to the collision of the subducting lithosphere with the North-Palawan (Calamian) micro-continental block (Lewis and Hayes, 1984). Seismicity is lower in the trench itself (5A) than in the forearc (5B). Present day volcanic activity in the forearc is an indication of convergence between the South China Sea Plate and Luzon (Hamburger et al., 1983). Convergence rate is estimated at about 10 to 20ram per year (Hayes and Lewis, 1984). 5.3.5. Zone 6. Zone 6 comprises the Negros Trench and the Sulu Trench (6A) and their volcanic forearcs (6B). Seismicity is relatively low in this subduction zone. There are relatively few shallow earthquakes of the thrust type in the Negros Trench. However, seismic reflection profiles show sediments being underthrusted to the east along the Negros Trench (Cardwell et al., 1980; Hamilton, 1979). There is no clearly defined Benioff zone; but there are intermediate earthquakes suggesting that the lithosphere is being subducted eastward (Cardwell et al., 1980). The Sulu Ridge (6B) parallels the Palawan Ridge which has no seismicity at all. The Sulu Ridge is associated with a subduction that occurred from late Cenozoic to Pleistocene (Cardwell et al., 1980). The Negros Trench and the Sulu Trench are considered to be one tectonic unit (Divis, 1980). Kanlaon is an active volcano on Negros Island. 5.3.6. Zones 7 and 8. The former 7A of the 1985 version is now subdivided into 7A (the Cotabato Trench) and 7B (the forearc to the east-northeast) in the present version. The former 7B is now Zone 8 which is subdivided into 8A (western and deeper part of the dipping lithosphere) and 8B (eastern and shallower part of the lithosphere). Zone 7 and Zone 8 cover the same geographic area. Zone 7 is the shallower structure. Zone 8 is the deeper structure which is the northern extension of the Molucca Sea Plate. This plate had buckled and dips both westward and eastward. Its surface expression is the Sangihe Ridge. Zone 8 is the westward dipping portion of the Molucca Sea Plate. It is characterized by intermediate earthquakes (8B) and deep earthquakes (8A), as far down as 680 km (Cardwell et al., 1980). 5.3.7. Faults (Zones 9 to 18).. Following the division of Philippine faults into transcurrent, normal, and thrust faults according to the Philippine Bureau of Mines (1981), three transcurrent, two normal, and three thrust faults are considered very probably seismically active. In addition, two transform faults are added. Thus, a total of ten faults are selected as source zones. In the context of the methodology commonly in use, there are two possible ways of viewing faults. One is to regard them as line sources; the other is to treat them as finite-width sources. In regarding them as line sources, as for example in McGuire's
38
s.S. su
'Frisk' program (1978), additional fault parameters such as rupture length have to be considered. In treating them as finite-width sources, their parameters are no different from those of Zones 1 to 8 earlier described. A test was made to compare the results of treating a fault as a line source and as a finite-width source. The results agree very closely. Thus, as a measure of convenience, faults are considered finite-width sources and included in the same computation used for Zones 1 to 8. 5.3.8. Transcurrent Faults (Zones 9 to 11). Zone 9 is the transcurrent Philippine Fault that extends over 1200 km from Lingayen Gulf in Luzon to the Davao Gulf south of Mindanao (Allen, 1962). Krause (1966) suggests that it extends south of Mindanao along a submarine scarp into the Talaud Islands. Movement is left-lateral strike-slip (Ranneft et al., 1960; Allen, 1962; Rutland, 1968). Morante and Allen (1974) studied the geomorphic effects of the 1973 Ragay Gulf earthquake and found a left-lateral displacement of 3.2 m near the Tayabas Isthmus of Southern Luzon. This was later confirmed by an earthquake focal mechanism solution of Lewis and Hayes (1983). Other earthquakes (1937, 1973, 1975) studied by Acharya and Aggarwal (1980) have focal mechanisms that can be correlated with the left-lateral movement along the fault. Geomorphic features in other parts of Luzon, Masbate, and Leyte likewise indicate left-lateral movements (Allen, 1962). Eight other large earthquakes, namely, of 1893, 1901, 1911, 1924, 1937, 1941, 1947, 1948, had their epicenters along or very near the fault zone (Rowlett and Kelleher, 1976). Thus seismicity gives evidence that the Philippine Fault is presently active, at least in some parts. Additional evidence from geology, such as the sharpness of fault scarps, disrupted soil horizons and stream offsets confirmed that the fault has been active since Quaternary time (Lewis and Hayes, 1983). Certain surface deformations near and along the fault have been associated with historical earthquakes such as those of 1869, 1879, and 1893 (Allen, 1962). In Central Luzon, the fault divides the mountainous Cordillera Central in the north from the lowlands of the Central Valley Basin in the south. Past episodes of intense activity along the fault have been placed in Late-Miocene and Post-Pliocene (Rutland, 1968). Zone 10 is another transcurrent fault, the Tablas Lineament that is traceable for about 350 km from Western Panay northward through Tablas Island to the Tayabas Isthmus (Phil. Bureau of Mines, 1981). Allen (1962) pointed out that it might be conjugate to the Philippine Fault and thus, right-lateral. Zone 11 is the third transcurrent fault, called the Mindanao Fault. It is traceable for about 400 km. from the Davao Gulf northwestward to the Sindangan Valley of Northern Zamboanga (Phil. Bureau of Mines, 1981; Gervacio, 1964). On its northwest end there is a possibility it might be a high-angle thrust; but it is believed to be more probably a transcurrent fault. There is a fourth transcurrent fault, the Ulugan Fault in the Island of Palawan, but it is considered inactive because its seismicity is practically nil.
SEISMIC HAZARD ANALYSIS FOR THE PHILIPPINES
39
5.3.9. Normal Faults (Zones 12 and 13). Zone 12 is a normal fault on the northeast side of Mindoro. Zone 13 is the Cotabato Normal Fault on the southwest flank of Mindanao. Another normal fault, the Marikina Fault located east of Manila, is not included here because it is considered inactive. 5.3.10. Thrust Faults (Zones 14 to 16). Zone 14 is the Zambales Thrust Fault on the western part of Luzon, that is, north of Manila Bay. Zone 15 is a thrust fault on the southwest side of Mindoro. Zone 16 is a thrust fault on the western flank of Panay Island. The series of thrust faults along Zamboanga Peninsula are not included here because they are considered to be inactive. 5.3.11. Transform Faults (Zones 17 and 18). Zone 17 is a transform fault at about north latitude 15.5 degrees, linking the East Luzon Trench (Zone 1) with the Philippine Trench (Zones 2 and 3) (Lewis and Hayes, 1983; Hamburger et al., 1983). Confirmatory evidence consist of concentrated seismicity, sharp bathymetric low, and focal mechanism solutions of two earthquakes (Seno and Kurita, 1978; Cardwell et al., 1980). Zone 18 is a transform fault running along the Verde island passage north of Mindoro. Supposedly it links the Manila Trench and the Negros Trench. This interpretation finds confirmation in the intense or concentrated seismicity north of Mindoro (Wolfe and Self, 1983). Its sense of movement is believed to be left-lateral (Lewis and Hayes, 1984).
6. Attenuation To characterize the attenuation properties of the intervening region between the source and the site, the attenuation coefficients of Su (1980) were adopted for the entire Philippines, that is, without distinction of source zones. Valenzuela et al. (1985) made an effort to derive attenuation coefficients for each of the Source Zones 1 to 8. However, since their results were somewhat incomplete and tentative, they are not incorporated in this study.
7. Hazard Evaluation at the Site 7.1. Theory In accordance with the theory given by Algermissen et al. (1982), F(a) is defined as the probability that the acceleration (A) will not exceed a certain level (a), given an earthquake with magnitude (M) greater than a certain threshold M(minimum). 1 - F(a) is then the corresponding probability of exceedence and typically has the shape of an exp ( - a) curve. If v is the annual rate of events with M > M(minimum), then v(1 - F(a)) is the annual number of exceedence n, that is, annual number of
40
s.s. s u
INTENSITIES (g) AT .001 RISK ( RISK s ANNUAL PROBABILITY OF EXCEEDENCE ) .20
05
!.1
i.o~
15
/0
T i.os
~0 .
Fig. 8.
,
i
I
i
i
I
|
i
*
i
Intensities (g) at 0.001 risk (risk = Annual Probability of Exceedence).
events with acceleration exceeding the level a. 1/n is then the return period, R(y), for the acceleration level a. The quantity e x p [ - t v ( l F(a))] or e x p [ - t / R ( y ) ] or exp [ - tn] expresses a probability of nonexceedence for the exposure time t, and this is called F(a, t). The quantity 1 - F(a, t) is the corresponding probability of exceedence. The above reasoning assumes that the F(a)'s follow a Poisson distribution, that is, they are independent of one another for sufficiently large a. In the program, the expected annual number of exceedence n is first calculated from the parameters of the magnitude-frequency relation which are statistically derived
41
SEISMIC HAZ/~.RD ANALYSIS FOR THE PHILIPPINES
INTENSITIES (g) AT .OOOI RISK (RISK = ANNUAL PROBABILITY OF EXCEEDENCE ) -20
-IO
-5
-O 120 I
12s |
I
*
!
I
t3o I
*
i
]
i
Fig. 9. Intensities (g) at 0.0001 risk (risk = Annual Probability of Exceedence).
from recorded seismicity. This is calculated for different designated levels of acceleration or intensity. In other words, for each acceleration level a, there is an n(a) which is the expected number of events per year that exceed the specified acceleration. F r o m this, the quantity 1 - exp ( - n ) , which is the annual probability of exceedence, is evaluated. This is the same expression for 1 - F(a, t), shown above, with the exposure time t = 1. Conversely, by pre-specifying different degrees of annual probability ofexceedence, the corresponding expected acceleration or intensity may be evaluated by interpolation.
42
S.S. SU
7.2. Results Two hazard maps are shown in Figures 8 and 9. Figure 8 shows expected acceleration contours (in % g) for annual probability of exceedence 0.001; Figure 9 for annual probability of exceedence 0.0001. In accordance with the expression: 1 - F(a, t) = 1 - exp ( - n t ) which is the probability of exceedence in t years, the acceleration values in the 0.001 map should reach a 5% probability of exceedence in 50 years; 10% probability in 100 years. The acceleration values in the 0.0001 map should reach a 5% probability of exceedence in 500 years; 10% probability in 1000 years. One can detect some similarity between the shape of the contours of Figures 8 and 9 and the shape of the boundaries of the source zones in Figures 6 and 7. This is generally expected. Statistical uncertainty in the acceleration values in the hazard maps (Figures 8 and 9) have been studied by those who use the Cornell method. Acceptable values are plus or minus 20 to 25% of the contour values.
References Acharya, H. K. and Aggawal, Y. P.: 1980, Seismicity and tectonics of the Philippine Islands, J. Geophys. Res. 85, 3239-3250. Algermissen, S. T., Perkins, D. M., Thenhaus, P. C., Hanson, S. L., and Bender, B. L.: 1982, Probabilistic estimates of maximum acceleration and velocity in rock in the contiguous United States, U.S. Geol. Survey Open-file Report 82-1033. Allen, C. R.: 1962, Circum-Pacific faulting in the Philippine-Taiwan region, J. Geophys. Res. 67, 4795~ 4812. Balce, G. R., Magpantay, A. L., and Zanoria, A. S.: 1979, Tectonic scenarios of the Philippines and northern Indonesian region, Paper presented at the ESCAP CCOP-IOC Meeting on the Geology and Tectonics of Eastern Indonesia, Bandung, 9-14 July. Balce, G. R., Encina, R. Y., Momongan, A. L., and Lara, E. D.: 1980, Geology of the Baguio district and its implications on the tectonic development of Luzon Central Cordillera, Geol. Paleontol. Southeast Asia 21, 265-287. Balce, G. R. and Garcia, L. C.: 1985, Seismotectonics of the Philippines, in Southeast Asia Association of Seismology and Earthquake Engineering, Series on Seismology, Vol. 4, pp. 809-843. Ben Avraham, J.: 1978, The evolution of marginal basins and adjacent shelves in east and southeast Asia, Tectonophysics 45, 269-288. Bowin, C., Lu, R. S., Lee, C. S., and Schouten, H.: 1978, Plate convergence and accretion in the TaiwanLuzon region, Bull. Am. Assoc. Petrol. Geol. 62, 1645-1672. Cardwell, R. K., Isacks, B. L., and Karig, D. E.: 1980, The spatial distribution of earthquakes, focal mechanism solutions, and subducted lithosphere in the Philippines and northeastern Indonesian islands, in The Tectonic and Geologic Evolution of the Southeast Asian Seas and Islands (AGU monog. 23), Vol. 1, pp. 1-36. Christian, L. B.: 1964, Post-oligocene tectonic history of the Cagayan Basin, Philippines, Phil. Geologist 18, 114-147. Cornell, C. A.: 1968, Engineering seismic risk analysis, Bull. Seis. Soc. Am. 58, 1583-1606. De Boer, J., Odom, L. A., Ragland, P. C., Snider, F. G., and Tilford, N. R.: 1980, The Bataan Orogen: eastward subduction, tectonic rotations and volcanism in Western Pacific (Philippines), Tectonophysics 63, 251-282. Divis, A. F.: 1980, The petrology and tectonics of recent volcanism in the central Philippine islands, in The Tectonic and Geologic Evolution of Southeast Asian Seas and lslands (AGU monog. 23), Vol. 1, pp. 127-144.
SEISMIC HAZARD ANALYSIS FOR THE PHILIPPINES
43
Divis, A. F.: 1983, The geology and geochemistry of Philippine porphyry copper deposits, in The Tectonic and Geologic Evolution of Southeast Asian Seas and Islands (AGU monog. 27), Vol. 2, pp. 173-216. Fitch, T. and Molnar, P.: 1970. Focal mechanism along inclined earthquake zones in the Indonesia Philippine region, J. Geophys. Res. 75, 1431-1444. Fitch, T.: 1972, Plate convergence, transcurrent faults, and internal deformation adjacent to Southeast Asia and the Western Pacific, J. Geophys. Res. 77, 443~4460. Fuller, M. et al.: 1983. Paleomagnetism of Luzon, in The Tectonic and Geologic Evolution of Southeast Asian Seas and Islands (AGU monog. 27), Vol. 2, pp. 79-95. Gervacio, F. C.: 1964, A Study of the Tectonics of the Philippine Archipelago, Phil. Geologist 20, 51-75. Hamburger, M. W., Cardwell, R. K., and Isacks, B. L.: 1983, Seismotectonics of the Northern Philippine Island arc, in The Tectonic and Geologic Evolution of Southeast Asian Seas and Islands (AGU monog. 27), Vol. 2, pp. 1-22. Hamilton, W.: 1979, Tectonics of the Indonesian region, U.S. Geol. Survey Prof. Paper 1078. Hawkins, J. W. and Evans, C. A.: 1983. Geology of the Zambales Range, Luzon, Philippine Islands: ophiolites derived from an island arc-back arc basin pair, in The Tectonic and Geologic Evolution of Southeast Asian Seas and lslands (AGU monog. 27), Vol. 2. pp. 95-123. Hayes, D. E. and Ludwig, W. J.: 1967, The Manila Trench and West Luzon Trough: II. Gravity and magnetic measurements, Deep Sea Res. 14, 545-569. Hayes, D. E. and Lewis, S. D.: 1984, A geophysical study of the Manila Trench, Luzon, Philippines: I. crustal structure, gravity, and regional tectonic evolution, J. Geophys. Res. 89, 9171-9195. Karig, D. E.: 1973, Plate convergence between the Philippines and the Ryukyu Islands, Marine Geol. 14, 153-168. Karig, D. E.: 1975, Basin genesis in the Philippine Sea, Initial Report of the Deep Sea Drilling Project 31, pp. 857-879. Karig, D. E.: 1983, Accreted terranes in the northern part of the Philippine Archipelago, Tectonics 2, 211-236. Karig, D. E. and Sharman Ili, G. F.: 1975. Subduction and accretion in trenches, Bull. Geol. Soe. Am. 86, 377-389. Karig, D. E. and Wageman, J. M.: 1975, Structure and sediment distribution in the northwest corner of the West Philippine Basin, Initial Report of the Deep-Sea Drilling Project 31, pp. 615-620. Katsumata, M. and Sykes, L. R.: 1969, Seismicity and tectonics of the West Pacific: Izu-MarianaCaroline and Ryukyu-Taiwan regions, J. Geophys. Res. 74, 5923-5948. Krause, D. C.: 1966, Tectonics, marine geology, and bathymetry of the Celebes Sea-Sulu Sea region, Bull. Geol. Soc. Am. 77, 813-832. Lewis, S. D. and Hayes, D. E.: 1983, The tectonics of northward propagating subduction along Eastern Luzon, Philippine Islands, in The Tectonic and Geologic Evolution of Southeast Asian Seas and Islands (AGU monog. 27), Vol. 2, pp. 57-78. Lewis, S. D. and Hayes, D. E.: 1984, A geophysical study of the Manila Trench, Luzon, Philippines: 2. forearc basin structural and stratigraphic evolution, J. Geophys. Res. 89, 9196-9214. Ludwig, W. J., Hayes, D. E., and Erving, J. I.: 1967, The Manila Trench and West Luzon Trough: I. Bathymetry and sediment distribution, Deep-Sea Res. 14, 533-544. Ludwig, W. J.: 1970, The Manila Trench and West Luzon Trough: III. Seismic refraction measurements, Deep-Sea Res. 17, 553-571. McCaffrey, R., Silver, E. A., and Raitt, R. W.: 1980, Crustal structure of the Molucca Sea collision zone, Indonesia, in The Tectonic and Geologic Evolution of Southeast Asian Seas and Islands (AGU monog. 23), Vol. 1, pp. 161-178. McGuire, R. K.: 1976, Fortran computer program for seismic risk analysis, U.S. Geol. Survey Open-file Report 76-67. McGuire, R. K.: 1978, FRISK: computer program for seismic risk analysis using faults as earthquake sources, U.S. Geol. Survey Open-file Report 78-1007. McKenzie, D. P. and Morgan, W. J.: 1969, Evolution of triple junctions, Nature 224, 125-133. Mitchell, A. H. G., Hernandez, F., and de la Cruz, A. P.: 1985, Cenozoic evolution of the Philippine Archipelago, J. Southeast Asian Earth Sei. 1. Moore, G. F.: 1981, Geology of Southern Mindanao, Philippines (abstract), EOS Trans. AGU 62, 1086. Moore, G. F. and Silver, E. A.: 1983, Collision process in the northern Molucca Sea, in The Tectonic and Geologic Evolution of Southeast Asian Seas and Islands (AGU monog. 27), Vol. 2, pp. 360-372.
44
s.s. su
Morante, E. M. and Allen, C. R.: 1974, Displacement on the Philippine fault during the Ragay Gulf Earthquake of 17 March 1973 (abstract), Abstr. Geol. Soc. Am. 5, 744-745. Murauchi, S. et al.: 1973, Structure of the Sulu Sea and the Celebes Sea, J. Geophys. Res. 78, 3437-3447. Murphy, R. E.: 1973, The Manila Trench-West Taiwan Foldbelt: A flipped subduction zone, Bull. Geol. Soc. Malays. 6, 27-42. Philippine Bureau of Mines: 1963, Geological Map of the Philippines. Philippine Bureau of Mines: 1981, Geology and Mineral Resources of the Philippines, Vol. 1. Ranneft, T. S. N., Hopkins, R. M. Jr., Froelich, A. J., and Gwinn, J. W.: 1960, Reconnaissance geology and oil possibilities of Mindanao, Bull. Am. Assoc. Petrol. Geol. 44, 529-568. Rowlett, H. and Kelliher, J.: 1976, Evolving seismic and tectonic patterns along the western margin of the Philippine Sea Plate, J. Geophys. Res. 81, 3518-3524. Rutland, R. W. R.: 1968, A tectonic study of part of the Philippine Fault zone, Quart. J. Geol. Soc. London 123, 293-325. Schweller, W. J. and Karig, D. E.: 1982, Emplacement of the Zambales ophiolite into the West Luzon Margin, Bull. Am. Assoc. Petrol. Geol. Schweller, W. J. et al.: 1983, Original setting and emplacement history of the Zambales ophiolite, Luzon, Philippines, from stratigraphic evidence, in The Tectonic and Geologic Evolution of Southeast Asian Seas and Islands (AGU monog. 27), Vol. 2, pp. 124-138. Seno, T. and Kurita, K.: 1978, Focal mechanism and tectonics in the Taiwan-Philippine region, J. Phys. Earth 26, $249-$263. Su, S. S.: 1980, Attenuation of intensity with epicentral distance in the Philippines, Bull. Seis. Soc. Am. 70, 1287-1291. Su, S. S.: 1985, Seismic source zones of the Philippines, in Southeast Asia Association of Seismology and Earthquake Engineering, Series on Seismology, Vol. 4, pp. 793-808. Sukamto, R. et al.: 1979, The geology and tectonics of Halmahera and its surroundings (abstract), Proc. Meeting on the Geology and Tectonics of Eastern Indonesia. Taylor, B. and Hayes, D. E.: 1980, The tectonic evolution of the South China Sea basin, in The Tectonic and Geologic Evolution of Southeast Asian Seas and Islands (AGU monog. 23), Vol. 1, pp. 89-104. Taylor, B. and Hayes, D. E.: 1983, Origin and history of the South China Sea basin, in The Tectonic and Geologic Evolution of Southeast Asian Seas and Islands (AGU monog. 27), Vol. 2, pp. 23-56. Uyeda, S. and McCabe, R.: 1983, A possible mechanism of episodic spreading of the Philippine Sea, in Accretion Tectonics in the Circum-Pacific Regions M. Hashimoto and S. Uyeda (eds.) (Tokyo), pp. 291-306. Valenzuela, R. G., Garcia, L. C., and Ambubuyog, G. F.: 1985, Assessment of seismic intensity of Philippine historical earthquakes, in Southeast Asia Association of Seismology and Earthquake Engineering, Series on Seismology, Vol. 4, pp. 743-792. Weissel, J. K.: 1980, Evidence for Eocene oceanic crust in the Celebes basin, in The Tectonic and Geologic Evolution of Southeast Asian Seas and Islands (AGU monog. 23), Vol. 1, pp. 37-47. Wolfe, J. A.: 1981, Philippine geochronology, J. Geol. Soc. Philippines 35, 1-30. Wolfe, J. A. and Self, S.: 1983, Structural lineaments and neogene volcanism in southwestern Luzon, in The Tecton& and Geologic Evolution of Southeast Asian Seas and Islands (AGU monog. 27), Vol. 2, pp. 157-172.
