EARTHQUAKE SOURCE PARAMETERS FORSUBDUCTION ZONE EVENTS CAUSING TSUNAMISIN AND AROUND THE PHILIPPINES
Joan Cruz SALCEDO ∗MEE09186

Supervisor:Tatsuhiko HARA ∗∗ABSTRACT
We have made a set of earthquake source parameters for events which can occur in subduction zones
surrounding the Philippines and cause large tsunamis and damages. The earthquake source
parameters consist of location of fault (longitude, latitude, depth), fault length, fault width, strike angle,
dip angle, rake angle, and slip amount, and the maximum plausible earthquake magnitude for eachfault segmentation. To set these parameters, first we collated tsunami databases and reviewed thehistorical tsunamigenic events with other studies such as re-evaluation of historical records andmagnitudes. Then, we reviewed seismicity data and Global Centroid Moment Tensor solutions. We
followed the segmentation set by Bautista, and set the maximum plausible magnitude, fault width andslip amount following the scaling laws. We also utilized some geophysical studies such as slab dipmodels, geometry of subducting slabs, and recent studies on Global Positioning System (GPS) andplate motions.
Combining all the above-mentioned information, we identified six source regions (ManilaTrench, Negros Trench, Sulu Trench, Cotabato Trench, East Luzon Trough, and the PhilippineTrench) with 17 segments surrounding the Philippines. Then, we set the earthquake source parameters
for these 17 segments.
These sets of earthquake source parameters can be used as input data to tsunami simulation,
the results of which will be used for further tsunami hazard assessment studies that are now underimplementation.
Keywords: Tsunami, Source Parameters, Subduction, Trench.

1. INTRODUCTION

Due to the present geographic and tectonic setting of the Philippines, it is prone to various natural
hazards on the earth such as hydro-meteorological and geologic hazards. Earthquakes cause hazards
that affect the Philippine community such as ground rupture, ground shaking, landslides, and tsunami.
There are also some of the most active volcanoes of the world in the Philippines. Historical andinstrumental data show evidences of their activities. Human-induced hazards and environmental
hazards are also causing disasters in the Philippines.
The Philippines is located in an active seismic source zone. Both inland and offshore
earthquakes are generated in the Philippine seismic source regions. Based upon the factors such as
being an archipelago, seismically active nature due to active earthquake sources, along withoccurrence of past tsunamis, we could say that the Philippine region is very prone to tsunami andtsunami hazards.
∗Philippine Institute of Volcanology and Seismology – Department of Science and Technology(PHIVOLCS-DOST), Philippines.
∗∗Chief Research Scientist, International Institute of Seismology and Earthquake Engineering – Building
Research Institute (IISEE-BRI), Japan.

12. SETTING OF EARTHQUAKE SOURCE PARAMETERS
2.1 Aim of this study
The setting of earthquake source parameters is one of the most important components of tsunami
numerical modeling. This study identified some tsunami sources in subduction zones around thePhilippines that might generate large tsunamis, possibly imposing damages on the Philippines. Thenwe produced a set of parameters of these earthquake sources as input data to tsunami simulation, theresults of which will be effective for further tsunami hazard assessment studies.
2.2 Review of Tsunami Databases
Earthquake and Tsunami catalogues were collated to review the past tsunamigenic earthquakes that
affected the Philippines and to identify the earthquake source regions. Three catalogues are
considered: they are the NOAA-WDC Tsunami Event Database (National Geophysical Data
Center-National
OceanicandAtmosphericAdministration(NGDC-NOAA,
http://www.ngdc.noaa.gov), Catalogue of Damaging Earthquakes in the World (Through 2008)
maintained by the International Institute of Seismology and Earthquake Engineering (IISEE-Utsu,
iisee.kenken.go.jp/utsu/index_eng.html), and the Philippine Tsunamis and Seiches, 1589 to 2004
(Bautista et al. in press). For source location both for NGDC and IISEE-Utsu, we considered 4o to 22olatitude and 116o to 129o longitude. For depth and magnitude, if no values are selected, the assumed
minimum and maximum values are assigned. For Bautista et al. catalogue, no parameters are set as
the catalogue contains only the Philippine tsunamis and seiches.
Each event was carefully evaluated to assess the validity and to identify the source
regions. A paper by Berninghausen (1969) was also reviewed to validate some historical records.
His paper was later discussed by Cox (1970) since Berninghausen (1969) did not use most of theexisting catalogues at that time. The Catalogue of the Philippine Earthquake (South East Asia
Association of Seismology and Earthquake Engineering, SEASEE, 1985) was also used to get
information on the earthquake effects especially those events with conflicting locations, the accounts
from this database was used to expertly judge the most possible
location of events.
Works by Abe (1981) and Abe and Noguchi (1983)
on magnitudes of large shallow earthquakes from 1904 to 1980
and 1897 to 1912, respectively were considered in this database
analysis to set the most plausible magnitude for events in late 19thand early 20th centuries.
Many factors were considered in the database
analysis; the final summary includes only the events that will beused for this study. Some of the events that were eliminated aretsunami events caused by volcanoes, those that are considered
doubtful by some authors, and events with inland earthquake
source. In summary, a total of 72 events were considered after
eliminating some events based on the above-mentioned criteria.
As for the preliminary list of the events, it includes
more than one set of earthquake parameters for many events.
Thus, we have chosen the most plausible parameters for each
event for further analysis. Basically, we refer to the bathymetry
Figure 1. Map of epicenters forof the area to set the plausible location. If available, we also refer
the final list of events
to information on intensities and damages. We also considered
considered for each earthquake
consistency among the reviewed catalogues. We chose thesource segments analysis
highest magnitude sometimes. Figure 1 is the map of epicentersof the events in the final list used for each earthquake source segment analysis.

22.3 Earthquake Source Characterization
2.3.1 Earthquake Source Region
Earthquake source regions were identified based on the database reviewed in the previous section,seismicity, and bathymetry. Figure 2 shows the source regions for which we set earthquake source
parameters in this study. This study concentrates on the subduction zones and their interplate thrust
events that can generate tsunamis and cause damages to the Philippines. We follow the segmentation
given by Bautista (DOST-GIA Tsunami Risk Mitigation Project 2007). Table 1 shows the names ofthe trenches, their codes, and the number of segments.
Table 1. List of earthquake sources, its codes, andnumber of segments
Name of theCode
Number ofEarthquake Source
Segments
Manila Trench
MT
4Negros Trench
NT2
Sulu Trench
ST2
Cotabato Trench
CT2
East Luzon Trough
ELT1
Philippine Trench
PT6
Total number of17
segments

Figure 2. Earthquake source regions
2.3.2 Earthquake Source Parameterization
The following earthquake source parameters are considered in this study: location of the fault(longitude, latitude, top depth), fault length, fault width, strike angle, dip angle, rake angle, and slip
amount. In addition, the maximum credible earthquake magnitude is also estimated.
Aside from the historical tsunami review, a survey of earthquake source parameters for
instrumentally recorded events was done by reviewing recent earthquake databases. We downloaded
seismicity data from the International Seismological Center (ISC) (http://www.isc.ac.uk). The data
covers the region of 4o to 22o in latitude and 116o to 129o in longitude, period of January 1964 to April
2008, with magnitude 2.0 to 7.8, and depth range of 0 to 700 km; the total number of the events is19,133. The SEIS-PC (Nakamura and Ishikawa 1997) was used for seismicity analysis. We also
downloaded data from the Global CMT (Centroid Moment Tensor) Solution database(http://www.globalcmt.org/CMTsearch.html). The search parameters are as follows: the period
January 1976 to May 2010, 0 to 10 in magnitude, depth of 0 to 100 km, and for the area, the regionincludes the computed area of each segment. We selected events with thrust mechanisms following
Frohlich and Apperson (1992).
Location of the fault (longitude and latitude) and Depth
The location of the earthquake source is estimated based on the bathymetry, seismicity, CMT solutions,and previous studies such as segmentation by Bautista (DOST-GIA Tsunami Risk Mitigation Project2007). The corner location of the fault is estimated based on the above-mentioned data.
Thesource region was specified by the corner location, the length, and the width of the fault.
The depth of the earthquake source is based on historical and recent seismicity of theregion. Cross-sections of seismicity are studied for each segment. In this study, we provide a table
for the case when the depth of the top of the fault is 0 km. When the depth of the bottom of the fault
plane exceeds a certain value (say, 60 km), it will be necessary to change the width, since significant

3co-seismic slips at such depths are not expected. In this study, when the depth of the bottom of thefault plane exceeds 60 km, we change the width so that the bottom depth is 60 km. Then, werecalculate the slip amount following the scaling law of Papazachos et al. (2004), which is explained
below, and then the corresponding seismic moment and magnitude. This procedure is applicable when
the top depth is changed.
Fault length, Fault width, and Slip amountThe fault length, fault width, and slip amount are computed using scaling laws. For historicalearthquakes, magnitude is used as reference to compute the fault length, fault width, and the amount ofslip. To evaluate the maximum plausible magnitude, length is calculated and then the empiricalequations are used to compute the magnitude, fault width, and the amount of slip.
The empirical equations by Papazachos et al. (2004) are used in this study. The
following are the equations by Papazachos et al. (2004) for dip-slip faults in subduction zones:log L = 0.55M – 2.19
log w = 0.31M – 0.63
log u = 0.64M – 2.78

(1)
(2)
(3)

where M is magnitude, L is fault length (km), w is fault width (km), and u is the slip amount (cm).
All these equations are applicable for events with a magnitude ranging from 6.7 to 9.2.
Strike, Dip, and Rake Angles
The strike is given by the strike of the trench axis.
The dip angle is estimated using seismicity and previous studies such as Jarrard (1986),Bautista (1996), and Bautista et al. (2001). The slab dip model by Lallemand et al. (2005) is used forsegments in Manila Trench, Philippine Trench, and Negros Trench.
The rake angle of the slip vector in compressional tectonics is mainly dependent on theorientation of relative plate motion. Kreemer et al. (2000) showed that the compression between theSunda and Philippine Sea plates is consumed by relatively large trench normal convergence along thePhilippine and Manila trenches and strike slips motion along the Philippine fault. Rangin et al.
(1999) proposed a tectonic set-up of the Philippine mobile belt, where strain between the Sunda andPhilippine Sea plates is released by motions of several blocks between them. Their orientations ofconvergence along the Philippine and Manila trenches do not deviate for the trench normal directions
for most of the segments. Therefore, we set 90o to the values of the rake angles for most of thesegments, assuming a pure thrust fault with a strike parallel to the trench or coast. For the segments
MT1, MT2, and MT3, referring to the result of Rangin et al. (1999), we calculated the rake angles
using the following formula (Michael 1990), λ=tan-1[tan(θ)/cos(δ)], where λ, θ, and δ are the rakeangle, the angle of the relative plate motion with respect to the strike, and the dip angle, respectively.
We also refer to the distribution of rake angles of the CMT solutions if enough solutions are available.
3. RESULTS
Table 2 and 3 show the final lists of the parameters set for each segment. Seventeen segments wereanalysed and finally we assigned set of parameters for each. Two tables are shown below; Table 2shows the set of parameters with reference to the fault length considering the maximum plausible
magnitude, while Table 3 shows the set of parameters with reference to the magnitude of historicalevent that possibly occurred in the segment. Since there is no historical event for PT3, the parameters
for this segment is not shown in Table 3. The modes of the rakes of the thrust events from the Global
CMT catalogue are shown for NT1, ST1, CT1, ELT, and PT2. For CT2, the rake of the GCMTsolution for the August 1976 event (Mw8.0) is shown. Since they do not differ much from 90o, it isreasonable to use 90o for further tsunami simulation using the other earthquake source parameters
given in this table.

4

Table 2. Final list of the parameters set for each segment.
SourceMagnitude

MT1
MT2
MT3
MT4
NT1
NT2
ST1
ST2
CT1
CT2
ELT
PT1
PT2
PT3
PT4
PT5
PT6

8.4
8.4
8.3
8.18.28.1
8.0
8.3
7.9
8.1
8.5
8.3
8.1
7.9
8.2
8.1
8.5

Corner location of
the fault

Length
(km)

Width
(km)

Strike(deg)Dip(deg)Rake(deg)

Slip
Amount(m)longitude Latitude119.2017.75
119.1016.06
119.06
13.93
120.6012.85
121.509.00
122.707.80
121.407.20
119.606.20
124.504.90
123.605.70
124.2018.20
125.1015.00
126.0013.00
126.6011.60
127.0010.40
127.508.20
127.40
6.44

277
254
238
190
206
174
167
230
135
190
317
230
190
143
206
190
325

91.00
91.16
87.88
77.40
81.01
73.66
71.97
84.00
63.84
77.40
100.00
86.20
77.40
65.95
81.01
77.40
95.00

20
1359
310
20
310
30
45
315
355
200
150
150
165
165
180
135

41
36
40
25
32
32
45
45
25
35
37
33
34
45
36
35
39

79
95
98
90
100
90
129
90
79
92
89
90
95
90
90
90
90

3.72
3.69
3.42
2.63
2.89
2.37
2.26
3.16
1.77
2.63
4.44
3.29
2.63
1.89
2.89
2.63
4.00

Slip
Amount(m)
2.19
2.19
0.67
0.78
0.58
1.21
1.05
3.40

Table 3. List of parameters with reference to magnitude of historical events.
Source M

Length(km)Width(km)MT1
MT2
MT3MT4
NT1NT2
ST1
ST2

25.00
97.72
97.72
32.00
208.9335.48
208.93208.93

13.00
53.21
53.21
16.00
81.66
30.06
81.66
81.66

6.4
7.6
7.6
6.6
8.2
6.8
8.2
8.2

Slip
Amount
(m)
0.79
1.21
1.21
1.00
2.94
0.37
2.94
2.94

SourceM

Length
(km)

Width
(km)

CT1CT2
ELT
PT1
PT2
PT4
PT5
PT6

8.0
8.0
7.2
7.3
7.1
7.6
7.5
8.3

162.18162.18
58.88
66.83
51.88
97.72
86.10
237.14

70.79
70.79
39.99
42.95
37.24
53.21
49.55
87.70

3.1 An example of calculation of seafloor deformation
Here we show an example of seafloor deformation computed
using the earthquake source parameters determined in thisstudy (Figure 3). We followed Okada (1985) in this calculation.
We use MT3 segment and the parameters provided in Table 2for the calculation. The yellow rectangle shows the fault
plane, red contours show uplift, and blue contours show
subsidence.
Figure 3.
of MT3.

Seafloor deformation

54. CONCLUSION
The Philippine archipelago is bounded by various earthquake generators that can also generate
tsunamis. The setting of earthquake source parameters is one of the most important components oftsunami numerical modelling. We collated tsunami databases, both historical and instrumental data andselected events for the analysis of this study. We characterized events by considering the collated
database, previous studies, documented accounts, and bathymetry data. We also collated andreviewed seismicity data and Global Centroid Moment Tensor solutions. We used scaling laws withreference to the segmentation by Bautista, and set the maximum plausible magnitude, fault width andslip amount. We also referred to some studies such as slab dip models, geometry of subducting slabs,
and studies on Global Positioning System (GPS) and plate motions. We identified the tsunami sourcesin subduction zones around the Philippines that generated and might generate large tsunamis, possibly
imposing damages on the Philippines.
Considering all these information we produced a set of earthquake source parameters ofthese earthquake sources as input data to tsunami simulation. The following earthquake sources wereconsidered: the Manila Trench (MT), Negros Trench (NT), Sulu Trench (ST), Cotabato Trench (CT),
East Luzon Trough (ELT), and the Philippine Trench (PT). Each earthquake source was divided intosegments and each segment was given a set of parameters. The total number of segments consideredis 17. The set of parameters include the magnitude, the corner location of the fault, the fault length,fault width, the strike angle, dip angle, rake angle, and the amount of slip. These parameters areimportant inputs for tsunami numerical modelling for the preparation of reliable tsunami hazard maps
which eventually will be utilized in preparing tsunami preparedness plans of various communities inthe Philippines.
AKNOWLEDGEMENT
I would like to acknowledge the valuable comments and supports given by Dr. Shoichi Ando, Dr.
Yushiro Fujii, Dr. Bunichiro Shibazaki, Dr. Leyo Bautista, Dr. Toshiaki Yokoi, Mr. Ishmael Narag, Dr.
Bartolome Bautista, Dr. Nobuo Hurukawa, and Dr. Renato Solidum.
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