RELOCATION OF LARGE EARTHQUAKES ALONG
THE PHILIPPINE FAULT ZONE AND THEIR FAULTPLANES
Rey M. Lumbang ∗MEE12608

Supervisor:

Nobuo Hurukawa ∗∗ABSTRACTWe relocated large magnitude (Mw ≥ 7.0) earthquakes that have occurred in the Philippine Fault Zone
(PFZ) from 1900 to 2012 using the modified joint hypocenter determination (MJHD) method. Inorder to obtain their precise hypocenters and identify fault planes of the large earthquakes, we used
P-wave arrival times at stations worldwide. We confirmed that seven earthquakes (Mw ≥ 7.0) amongten relocated large earthquakes have occurred along the PFZ, they are 1911 (Mw 7.7), 1937 (Mw 7.4),
1941 (Mw 7.0), 1947 (Mw 7.0), 1948 (Mw 7.0), 1973 (Mw 7.4) and 1990 (Mw 7.7) earthquakes. Dueto the limited number of data we were unable to relocate the 1901 (Mw 7.5) earthquake. We haveconfirmed that the August 30, 1924 (Mw 7.1) earthquake does not belong to the PFZ, althoughprevious studies suggested that this event had been generated by the PFZ. Referring to the epicentersof aftershocks, damaged distributions and intensity reports, we estimated the fault plane of each
earthquake. In the northern PFZ, the 1990, 1937, 1901, and 1973 ruptured all part of the PFZ. In thecentral PFZ, only three Mw 7.0 earthquakes in 1941, 1947 and 1948 occurred. Furthermore, five 6.0≤ M < 7.0 earthquakes including the 2003 Masbate earthquake occurred along the PFZ. In thesouthern PFZ, we have only the 1911 Agusan Valley earthquake in the northern Mindanao Island.

Keywords: Philippine Fault Zone (PFZ), Modified Joint Hypocenter Determination (MJHD) Method,
Relocation of Hypocenters.
1. INTRODUCTION
The Philippines is located on the western rim of the Pacific Ocean. It is also situated between the twoconverging major tectonic plates, the Philippine Sea plate and the Sundaland plate. The angularvelocity rate measured and estimated by Mid-Ocean Ridge Velocity (MORVEL) is 96 mm/yr (DeMets
et al., 2010). The Philippines is also bounded by many trenches.
The Philippine Fault Zone is situated on the borders of two opposing movements of thePhilippine Trench and the Manila Trench (Acharya, 1980). The Philippine Fault Zone has a total
length of 1200 km and transects the whole Philippine archipelago from north western part of theLuzon region down to the southern part of the Mindanao region. The Philippine Fault Zone is a major
left lateral strike slip type of faulting (Allen, 1962; Acharya, 1980; Acharya and Aggarwal, 1980). The
average displacement rate of the PFZ is about 20 to 25 mm/yr (Duquesnoy et al., 1994).
Since the country is bounded by the numerous trenches and an active inland faults, thePhilippines is considered highly vulnerable to large earthquake hazards and other earthquake related
disasters that affect many citizens and also have effects on economic development. This paper will be
helpful in developing for disaster mitigation measures. The Modified Joint Hypocenter Determination
method (Hurukawa, 1995) is one powerful tool to locate the precise hypocenters of earthquakes anddetermine the fault planes and this will lessen the number of lives being taken if a strong earthquake∗

Philippine Institute of Volcanology and Seismology (PHIVOLCS), Department of Science and Technology.
International Institute of Seismology and Earthquake Engineering (IISEE) / Building Research Institute, Japan.

∗∗

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occurs and if government officials, engineers will have knowledge about earthquake location and theirfault planes, this will lead them to have an effective and well-planned disaster mitigation program.
2. METHODOLOGY
We used the Modified Joint Hypocenter Determination (MJHD) method in relocation of large
earthquakes along the Philippine Fault zone. The MJHD method was developed by Hurukawa andImoto (1990, 1992) and Hurukawa (1995) from the Joint Hypocenter Determination (JHD) method.
The JHD method (Douglas, 1967; Freedman, 1967; Dewey, 1972), calculates hypocenters of a groupof earthquakes and station corrections (SCs) simultaneously. SCs remove the effects of lateralheterogeneity and reflect a travel-time difference between the assumed velocity structure and theactual one. The JHD method uses SCs that remove the effects of lateral heterogeneity.

However, since there is a trade-off between location and SC especially when the station
coverage is limited, the solution becomes unstable and unreliable. Hurukawa and Imoto (1990,
1992) overcome this problem in the JHD method with adding constraint as follows. SC isdependent of distance and azimuth from the center of the source region to each station, which
minimizes their absolute values.
3. DATAWe selected large events, which were located near the Philippine Fault Zone (PFZ), by the ISC-GEM(Storchak et al., 2013) from magnitude 6.0 or larger with shallow depths (Z ≤ 60 km) during 19002012. We used data from British Association for the Advancement of Science Society Committee(BAASSC), International Seismological Summary (ISS) and International Seismological Centre (ISC).
To calculate the MJHD hypocenters, two parameters must be defined: the minimum numberof stations (MSTN) that observed each earthquake and the minimum number of events (MEQ)
observed at each station. In this relocation we set the parameters of MSTN and MEQ to 6 and 6,
respectively. Phase data with an absolute value of P-wave travel-times residual (O-C, observed minus
calculated time) of ≥ 12 s were excluded. We only use initial P-wave arrival time which is very
reliable in hypocenter determination. The depth (Z) was fixed to 15 km to reduce the number of
unknowns and can help to the accuracy in relocation of epicenters.
4. RESULTS AND DISCUSSION
We were able to relocate ten M ≥ 7.0 earthquakes for the period after 1911. However, since
travel-time residuals of P-arrival times of the 1901 (M 7.5) earthquake were too large and limited
number of stations only had recorded this event, the 1901 earthquake was excluded from the MJHD
relocation. Furthermore, we relocated 13 events from 6.0 ≤ M < 7.0 earthquakes including the 2003
Masbate (Mw 6.2) earthquake and three immediate aftershocks of the 1990 Luzon earthquake and one
immediate aftershock of the 1973 (Mw 7.4) event.
Seven events among ten earthquakes were relocated on the Philippine Fault Zone (PFZ)(Table 1). Other three events are not events on the PFZ based on our result in MJHD relocation. The
August 30, 1924 (Mw 7.1) and the June 13, 1929 (Mw 7.2), they behave differently among the otherevent in the relocation their mainshocks and aftershocks are scattered around the southern PFZ. The
April 14, 1924 (Mw 8.0) earthquake, which is known as the Mati earthquake, was relocated very near
the Mati segment of the PFZ. The earthquake was describe as violent but not destructive and had
caused more damages on the ground than damages on the infrastructure (SEASEE, 1985). Therefore,
further studies are needed to this earthquake before we decide if this event belongs to the PFZ.
Since we need to determine the fault length corresponding to the earthquake magnitude inorder to establish the history of large earthquakes along the PFZ and to obtain possible seismic gaps,
the fault length of magnitude ≥ 7.0 are estimated by the equation for large earthquakes in Sagaing
Fault in Myanmar (Hurukawa and Maung, 2011).

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Table 1. List of hypocenters of M ≥ 7.0 earthquakes along the Philippine Fault Zone since 1900.
Date(y/m/d)

Time
(h:m)

Origin Time(s)

Latitude(°N)
14.0
09.35 ± 0.40

Z
(km)

Mw

Source ofhypocenter

15

7.5

47.96 ± 2.57

Longitude
(°E)
122.0
125.86 ± 0.55

1901/12/14

22:57

-

7.7

BAASSCThis study

1911/07/12

04:07

1937/08/20

11:59

18.30 ± 0.48

121.80 ± 0.60

14.74 ± 0.07

15

7.4

This study

1941/11/05

17:38

53.76 ± 0.65

123.60 ± 0.15

12.90 ± 0.11

15

7.0

This study

7.0This study

1947/06/07

18:47

53.11 ± 0.67

124.56 ± 0.14

11.27 ± 0.12

15

1948/09/02

23:34

54.02 ± 0.59

125.06 ± 0.14

10.43 ± 0.10

15

7.0

This study

7.4

This study

7.7This study

1973/03/17

08:30

46.16 ± 0.16

122.78 ± 0.04

13.47 ± 0.03

15

1990/07/16

07:26

34.28 ± 0.13

121.08 ± 0.03

15.73 ± 0.02

15

Z = depth (fixed for earthquakes after 1911).
Mw: Moment magnitude by ISC-GEM (Storchak et al., 2013).
4.1. Fault Planes of M ≥ 7.0 Earthquakes from 1911 to 2012
In this study, large earthquakes with M ≥ 7.0
that have occurred along the PFZ wererelocated. Large earthquakes from 1911 to
2012
were
relocated
simultaneously
mainshocks and their immediate aftershockusing only the initial P wave arrivals. We
divided the PFZ into three regions to explain
the seismic activities of each region andshow the detailed characteristics of the PFZ,
such as the Northern PFZ where large
earthquakes occurred, the Central PFZ where
the M ≤ 7.0 earthquake occurred, and theSouthern PFZ where the largest earthquake
in the Philippines occurred.

Luzon

4.1.1. Northern Philippine Fault Zone
Figure 1. MJHD relocated events in the northern
PFZ. Global CMT solutions are also shown (Global
CMT catalog, 2013). Large red, middle yellow andsmall yellow circles indicate M ≥ 7.0, 6.0 ≤ M < 7.0,
and M < 6.0 earthquakes, respectively. Cross
indicates standard error of an epicenter. Solid lines
indicate the PFZ configuration (Allen, 1962).

The 16 July 1990 Luzon earthquake (Mw7.7) was generated by a left-lateral faulting in
the central Luzon Island. Ground rupture wasobserved along a linear trace extending to a
125 km (Nakata et al., 1996; Tsutsumi, 2005).
The maximum horizontal offset as measured
on the fault was 6 meters. The global
Centroid Moment Tensor (CMT) solution
from the surface waves represents almost pure left lateral strike-slip faulting (Figure 1). The locationof the mainshock and three largest aftershocks was consistent with the 120 ~ 125 km rupture length ofthe previous studies (Yoshida and Abe, 1992).

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The 20 August 1937 (Mw 7.4) earthquake was located by ISS with the coordinates of 122.0°
E and 14.1° N and is near the south end of Alabat Island near the Calauag Bay. This event was
relocated to 121.80°E and 14.74°N. It moves to 0.5° in the north and 0.02° in the east from the firstlocation by ISS and sits on top of the PFZ (Figure 1).
The 17 March 1973 Ragay Gulf earthquake (Mw 7.4) was relocated in the middle of the
Ragay Gulf which is very close to the ISC location (Figure 1). Ground breakages were seen along thesegment of the PFZ, from west coast of Ragay Gulf to Calauag Bay, a stretch of about 30 km. The
fault traces exhibited mole track features with ground fissures arranged in echelon to one another in an
E-W trend. Ground displacement exhibited left lateral (Morante, 1974). The MJHD relocation result
improves the location only by slight number and the expected fault length that corresponds for an Mw
7.4 is 113 km.

4.1.2. Central Philippine Fault Zone
The 05 November 1941 earthquake (Mw7.0) was also generated by the PFZ and thisearthquake was located by ISS at 123.5°E
and 12.5°N. After the relocation the new
hypocenter of this event suggested that thefault plane of this event is main PFZ
segment in the central part of the PFZ.
Leyte
The 07 June 1947 earthquake (Mw7.0) was located by ISS at 124.7°E and11.3°N. The results of MJHD relocation
improves the initial location by ISS andconfirms that this event was generated by
the central region of the PFZ and its fault
plane is the northern part of the Central
Leyte fault segment of the PFZ (Figure 2).
The
02
September
1948
Figure 2. MJHD relocated events in the central PFZ.
earthquake (Mw 7.0) was located by ISS
Symbols are as in Figure 1.
located in the Pacific Ocean about 100 km
east of the PFZ. The ISC-GEM also locatedthis event approximately 50 km near the PFZ. Therefore we relocated this event to determine if thisevent is generated by the PFZ, and our results on MJHD relocation confirms that this event is generated
by the PFZ. Based on our results the fault plane of that this event is the southern part of the Central Leyte
Fault, a segment in the central region of the PFZ. Seven 6.0 ≤ M < 7.0 earthquakes occurred in thisregion (Figure 2).

44.1.3. Southern Philippine Fault Zone

Figure 3. MJHD relocated events in the southern PFZ.
Symbols are as in Figure 1.

The 12 July 1911 Agusan Valley
earthquake (Mw 7.7) was located at the
Pacific Ocean with the coordinates of
127.1°E and 8.5°N by ISS while
ISC-GEM located it on the northeast tipof the Mindanao Island. The MJHD
results relocated this event very near thenorthern part of the Mindanao fault
segment of the Philippine Fault Zone
(Figure 3). Since we have only few data
and limited worldwide seismologicalstations recorded this event and thequality of data is less accurate so we havelarge but acceptable (O-C) residuals. We
could relocate this event from offshore toinland earthquake close to the northernpart of the Mindanao fault segment.

5. EARTHQUAKE HISTORY ALONG THE PHILIPPINE FAULT ZONE

Figure 4. Earthquake history along the Philippine Fault Zone.
Red stars indicate the epicenters of M ≥ 7.0 earthquakes during
1900 – 2012. Ellipses indicate their fault planes.

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To obtain an earthquake history
along the Philippine Fault Zone
(PFZ), we plotted the calculated M≥ 7.0 events (MJHD results) along
the PFZ during the period 1901 to2012 (Figure 4). There are eight M≥ 7.0 occurred along the PFZ since
1900 (Figure 4 and Table 1). The M≥ 7.0 earthquakes are as follows:1901 (Mw 7.5), 1911 (Mw 7.7),
1937 (Mw 7.4), 1941 (Mw 7.0),
1947 (Mw 7.0), 1948 (Mw 7.0),
1973 (Mw 7.4) and 1990 (Mw 7.7).
The northern PFZ, where the recent
large and damaging (1990 Mw 7.7)
and other large earthquakes (1901,
1937 and 1973) had occurred, hadbeen ruptured all parts of the PFZ.
The central PFZ was defined by
Besana and Ando (2005) as locationof large and medium sized events,
correlates well with our relocation
results particularly the 2003 (Mw
6.2), 1947 (Mw 7.0) and 1948 (Mw7.0) events. In the southern PFZ,
only the 1911 (Mw7.7) earthquake

had occurred in the northern part of Mindanao Island and no large earthquake occurred in the southernpart of the Mindanao Island.
6. CONCLUSIONSThe Modified Joint Hypocenter Determination (MJHD) method can constraint precise locations of agroup of earthquakes simultaneously. Therefore, we have applied the relocation of the large
earthquakes that occurred near the Philippine Fault Zone (PFZ) from 1900 to 2012 and identify theirfault planes. We confirmed that seven large earthquakes (Mw ≥ 7.0) among the ten relocated large
earthquakes have occurred along the PFZ, they are the 1911 (Mw 7.7), 1937 (Mw 7.4), 1941 (Mw7.0), 1947 (Mw 7.0), 1948 (Mw 7.0), 1973 (Mw 7.4) and 1990 (Mw 7.7). However, we could notrelocate the 1901 (Mw 7.5) earthquake due to the limited number of precise arrival-time data. We
have also confirmed the August 30, 1924 (Mw 7.1) earthquake does not belong to the PFZ, althoughprevious studies suggested this event was generated by the PFZ. Referring to the epicenters ofaftershocks, surface ruptures, damaged distributions and intensity reports, we estimated their fault
planes of each earthquake.
I will continue to study on historical earthquakes. I will relocate past large earthquakes in thePhilippines by using the Modified Joint Hypocenter Determination (MJHD) method and determine theirprecise location and their fault planes. The results will contribute in the development of hazard
assessments and disaster mitigation in the Philippines.
ACKNOWLEDGEMENTS
We used arrival time data of BAASSC, ISS and ISC. We thank Drs. D Storkchak and A. Villasenor who
provided us with digital ISS data.
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