APPLICATION OF THE MODIFIED JOINTHYPOCENTER DETERMINATION METHOD TO THE
1990 AND 1996 BOHOL EARTHQUAKES IN THE
CENTRAL PHILIPPINES AND CORRELATING GPSRESULTS

Nolan C. Evangelista
MEE10507

Supervisor:

Nobuo Hurukawa
Fumiaki Kimata

ABSTRACTThe relocation of hypocenters was studied using the method of Modified Joint Hypocenter
Determination (MJHD) to improve the location of hypocenters. We conducted a case study of two
events that occurred in the central Philippines, which were the 1990 and the 1996 Bohol earthquakes inthe southeast and the northwest of Bohol, respectively. This study is to unlock the query of theabsolute location of hypocenters, fault planes and the temporal change of seismicity. Seismic phasedata were obtained by the International Seismological Centre (ISC) and the Philippine Institute ofVolcanology and Seismology (PHIVOLCS). The period from 24 hrs right after the mainshock up to
one week period of aftershocks was targeted to identify the fault planes. We identified the fault planesof the 1990 Bohol earthquakes which consisted of two M 6 events with pure reverse faults. The fault
plane of the first mainshock of Mw 6.7 was located east of the mainshock and dipped southeastward,
while that of the second mainshock with Mw 6.6, which occurred 30 minutes later, was located west of
the first mainshock and dipped northwestward. We also found that the fault plane of the 1996
northwest Bohol earthquake (Mw 5.2) dipped northwestward.
The results of the MJHD relocation have a constraint to the aftershocks in the studied areaand showed the lineaments of the existing faults such as strike slip and reverse faulting. In addition to
the hypocenter relocation, we analyzed the GPS data surveyed in the Central Philippines. The result
indicated that SOLK station in Bohol Island moves northward relative to Manila, which is inagreement with the P-axis of the 1996 northwest Bohol earthquake. It was consistent with the fault
mechanisms of the earthquake. All of these findings are useful for seismic hazard assessment in the
study area.
Keywords: Relocation of Hypocenter, Modified Joint Hypocenter Determination, 1990 Bohol
Earthquake, 1996 Bohol Earthquake, GPS
1. INTRODUCTION
The Philippines is situated in the west margin of the Pacific. Much of its activity is related to
the westward subduction of the Philippine Sea Plate along the boundary of the Philippine Trench(Acharya & Aggarwal, 1980). The eastward subduction of the Eurasian Plate along the Manila Trenchand Negros Trench has shown complexity and has contributed seismicity in the western Philippines.
Philippine Institute of Volcanology and Seismology.
Building Research Institute.
Research Center for Seismology,Volcanology and Disaster Mitigation, Nagoya University.
1This compressional regime played a major reaction that processes different tectonic structure inside thePhilippine Mobile Belt. It does exhibit a mixture of faulting such as reverse fault and strike slip fault.
Thus, it is necessary to enhance the country’s disaster mitigation approach to counter seismic hazards
by earthquakes and secondary effects such as tsunami.
We considered a wider area first from 8.5°N- 12.0°N Latitude / 121.0°E-128.0°E Longitude,
to reckon the general features of seismic activity in the studied region (Figure 1). The relocation
focused on the vicinity of Bohol and nearby Islands from 8.6°N-11.3°N Latitude / 123.0°E-125.2°E
Longitude. In 1981, an earthquake of M 5.4 occurred as a reverse fault (global CMT (GCMT)) in thesoutheast of Bohol. Then 9 years later on February 8, 1990 a strong Mw 6.7 occurred, in just 30
minutes later on the same date, another large event of Mw 6.6 took place still on the southeast of theisland (or second mainshock?). All those succeeding events from 1981 fall into a reverse type of
faulting (GCMT). By year 2000 along this area southeastof Bohol, repeated earthquakes were triggered bydifferent fault mechanism of strike slip on the edge of theisland, which were parallel to the 1981 and 1990 seismic
activities. In another venue of seismic activity with samecharacteristics such as a reverse faulting, locatednorthwest of Bohol, southwest of Cebu City, a moment
magnitude Mw 5.2 occurred in 1996.
Figure 1. Shallow earthquakes by ISC
2. METHODOLOGY
The concept of relocation of hypocenters was the generalization from Geiger in 1912. Additionalparameter was added for station correction by Douglas (1967) and Freedman (1967). Station
corrections remove the effects of lateral heterogeneity and reflect a travel-time difference between theassumed velocity structure and the actual one. This method, joint hypocenter determination, calculates
hypocenters of a group of earthquakes and station corrections simultaneously, as express in equation(1):(O  C ) ij  (t ij  To j )  Tij 

t ij

x j

dx j 

t ij

y j

dy j 

t ij

z j

dz j  dTo j  dSi . . . . .1.) . (Where (O  C ) ij is the travel time residual of the j-th event at i-th station , O is the observed travel
time. C is the calculated travel time, t ij the arrival time of j-th event at i-th station, Tij the calculated
travel time of the j-th event at the i-th station, dS i correction to the station correction at the i-th station,

To j is the origin time, dx j , dy j , dz j and dTo j are correction of trial hypocenters of j-th event.
However, the approach is unstable if the media are too heterogeneous and there is large trade-off
between station corrections and focal depths. Therefore, Hurukawa and Imoto (1990, 1992), for local
events and Hurukawa (1995), for teleseismic events, modified the method by adding additional
constraints that the station correction is independent of the distance and azimuth from the center of thestudied region to the station. Although actual hypocenters are sacrificed, it can justify the effectivenessof relocation and even those less events are recorded at some stations. These added parameters as
express in equation (2), enable us to relocate the precise location of hypocenters of earthquake which
we applied in this study.
n S i Di 0i 1n S i cos i 0i 1n

 S i sin i 0
i 1n

 S 0.......... .......... .......... ...( 2)i 1i

Where S i is the station correction at i-th station, Di is the distance between the i-th station and thecenter of the region,  i is the azimuth of the i-th station from the center of the region and, n is thenumber of stations.

23. MJHD APPLICATION TO THE 1990 BOHOL EARTHQUAKEThe 1990 Bohol earthquake (Mw 6.7) was located at 1 km depth (ISC). The aftershocks areconcentrated at 33 km and deeper from the crustal depth boundaries (Figure 2). This large gap between
mainshocks and aftershocks cannot directly represent the real activity of these events for someanomaly of depth location of hypocenters. Fault plane solution from GCMT of two possible nodal
planes described no relation of those clusters of aftershocks from the mainshock with respect to depths.
The GCMT solution described the first mainshock (Mw 6.7) as a pure reverse fault (Figure 2). Then
second mainshock (Mw 6.6) occurred 30 minutes later near the first mainshock. In these events people
had experienced the intense ground shaking, observed ground rupture and receding of the sea water
hundreds of meters away from the normal tide mark along the coast (Umbal et al; QRT Report 1990).
This phenomena could prove and describe the assumption that the 1990 Bohol earthquakes were
crustal earthquakes and not beyond the boundary of crustal depths or mantle earthquakes.
First step, we gathered the immediate aftershocks of 24 hrs from the 1st mainshock (Mw6.7).
Fourteen events including one foreshock and 12 aftershocks were used originally from ISC database.
The MJHD results have 12 events and 29 stations as plotted in Figure 3.

Figure 2. Epicentral distribution and depth
cross-sections along A–B and C–D forforeshock, mainshock and immediate
aftershocks located by the ISC. The GCMT
solution of the mainshock (Dziewonski etal. (1981) and later updates) is also shown.
Strike A-B and C-D are perpendicular to thestrike of two nodal planes of the GCMT.
solution.

Figure 3. Relocation results of 24 hrs
and distribution of station

The fault plane of the GCMT
solution agrees with the new relocated eventsof aftershocks. However the mainshock location
was pulled down to deeper position. One
observation of such effects was due toinsufficient number of local stations used in theprocess and this gave a biased location of
hypocenters to deeper location. Only 2 regional
stations of PHIVOLCS were used in theprocess. Probably it needs also another velocity
structure model that can agree with a finer
result. In 24 hrs relocation of aftershocks, thefault plane is still not clear in which slope
makes. It seems that both fault planes shown in
two cross sections A-B and C-D have activity
along the slope. We extended the duration ofaftershocks to one week. Twenty-nine eventsand 43 seismic stations were used. The results
are shown in Figure 4, in which two clusters ofactivities are distributed along the nodal planesin A-B and C-D cross sections.

Figure 4. Relocation results of one week
and distribution of station

3

The hypocenters were well constraint at the depths of 40 km to 60 km location. This activity is muchcomplex than that of a usual fault plane with reverse characteristics of faulting. The west activity
from the two mainshocks (Figure 4) shows hypocenter location migrated nearer to the mainland.
Dipping direction might be changing from its sequence of activity.
In order to study the detail of the aftershock distribution, we split the activity into two;eastern group from Mw 6.7 (Fig.5) and western group from Mw 6.6 (Fig.6). Almost all events werelocated along the nodal plane shown in the C-D cross section in the eastern group.

Figure 5. East activity from the 1st mainshock.

Figure 6. West activity from the 2nd mainshock.

Therefore, the fault plane of the first mainshock is dipping southeastward. On the other hand,earthquakes in the western group were located on the nodal plane in the A-B cross section. Therefore,
the fault plane of the second mainshock is dipping northwestward. Although the west is not so
conclusive because there are only 5 events available in one week time of relocation. However, it
shows different direction from east activity
Furthermore, we relocated longer term from 1964-2008 (Figure 7) and the result establishedmuch constraint location than shorter term (Figure 8). The longer term has established an indicator forthe shallow location of earthquakes between 124°E to 125°E longitude and 9.5°N to 10°N latitude
trending northeast-southwest direction. The approximate length is about 70 km for crustal depth
earthquakes in cross section A-B and approximately 20 km in width in cross section C-D (Figure 8).
Although deeper earthquakes location beyond the crustal boundaries is constraint at the depth of about
40 km to 70 km, actual depth errors will be much larger than standard errors shown in figure 8.

Figure 8. ISC hypocenters before relocation.

Figure 7. Relocated hypocenters (MJHD method).

4

Also some constraints are observed in theneighboring island where the Philippine Fault
Zone is located northeast of Bohol andindicating some seismic gap along the Philippine
Fault (Figure 8).
The relocated hypocenters are welldistributed in space and along the tectonic line
(Figure 9), but we cannot say that this line is amajor tectonic line in the sense that the dipping
direction are on both sides and seismic activities
are randomly distributed along the tectonic line.
We assumed that this stress release is only due
to compression along the fault.
Figure 9. Relocated hypocenters in the southeast
and northwest of Bohol for long term (1964-2008)
4. MJHD APPLICATION TO THE 1996 BOHOL EARTHQUAKE
A moderate to strong earthquake occurred in the northwest of Bohol with the depths of 50
km (ISC) and the magnitude was Mw 5.2(GCMT). Here we relocated the aftershocks
recorded by PHIVOLCS from May 27, 1996
until August 2, 1996. The first iteration from 12
events, only 7 events were included in the
succeeding iteration. In the final results only
four events were utilized to determine the fault
plane (Figure 10). Therefore, we concluded thatthe nodal plane in cross section A-B is the fault
plane of this event of 1996 that is dipping
north-northwestward.

Figure 10. Relocated hypocentersof the 1996 Northwest Bohol earthquake.
5. GPS RESULTS
We processed and analyzed the GPS data and compared to results of the MJHD relocation.
This relates some vectors results of SOLK and
SOLF stations (Figure 11) generally to north
northwest direction with respect to PIMO in
Manila. In the previous earthquake event,northwest of Bohol, The focal mechanism(GCMT)
showedacompression
tonorthwestward as a reverse fault (Figure 10).
This indicates northward movement of Bohol
Island that is in agreement with the P-axis of the1996 northwest Bohol earthquake. The resultant
vectors of SOLK, and SOLF has a velocity of
19.51 mm/yr and 19.42 mm/yr, respectively.

Figure 11. GPS Vector Results

5Station NOMA on the other hand is trending east southeast direction with a velocity of 21.5
mm/ yr.
At present, we could not generally conclude whether those vectors could represent theregional or local crustal movement in this studied area in the sense that only few stations have beenprocessed. It needs to place some more GPS sites and more campaign periods in order to be able toderive much finer results in the study area.

6. CONCLUSION
The relocation using the Modified Joint Hypocenter Determination (MJHD) method canconstrain precise locations of a group of earthquakes simultaneously. Such precision is necessary toanalyze fault planes of large earthquakes, the tectonic structure and migration process of seismic
activity. Therefore, we have applied the relocation to the 1990 and 1996 Bohol earthquakes in thecentral Philippines and obtained the following results: The 1990 earthquake activities were divided
into two groups. The eastern group located east from the first mainshock (Mw 6.7) indicated that its
fault plane dips southeast, while the western group located west of the second mainshock (Mw 6.6),
which occurred 30 minutes after the first mainshock, indicated that its fault plane dips northwest. The
temporal change of seismicity was also clear and established a well constraint location of events after
the relocation of longer term, which also gave clearer location for the strike slip of faulting and thereverse type of faulting in the southern part of Bohol Island. The fault plane of the 1996 northwest
Bohol earthquake (Mw 5.2) is dipping north northwestward. The fault orientations of these two areas,southeastern and northwestern part of Bohol, are generally trending northeast and southwest.
Some segment in the Philippines Fault Zone was also constraint and gave clearer view of its
seismic activity, of some segment with a seismic gap. Finally, we analyzed the GPS data in and around
Bohol Island relative to Manila. The result indicates northward movement of Bohol Island that is inagreement with the P-axis of the 1996 northwest Bohol earthquake.
ACKNOWLEDGEMENT
My appreciation to Mr. I. Narag and Dr. J. Punongbayan (PHIVOLCS Data Management),Dr. T. Bacolcol (PHIVOLCS-GPS TEAM) and Ms. N. R. Hanifa Gunawan, (Nagoya University) for
guiding me to process the GPS data thank you very much. To Dir. R. Solidum Jr., Dr. B. Bautista, Dr.
K. Okino (Tokyo University) thank you for the advices during my individual study.
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Freedman, H. W., Bull. Seism. Soc. Am., 57, 545-561,1967.
Geiger, L., 1912, (1910 German article) Bulletin of St. Louis University, 8 (1), p. 56-71.
Hurukawa, N. and M. Imoto, M., 1990, J. Seism. Soc. Japan, Ser. 2, 43, 413-429.
Hurukawa, N. and M. Imoto, Geophys. J.Int.(1992) 109, 639-652.
Hurukawa, N., 1995, Geophy. Res. Letters, Vol. 22, No. 23, pp 3159-3162.
Umbal, J.V. et.al, PHIVOLCS QRT TEAM 1990 (Report of Investigation on the February 8, 1990
Earthquake in Bohol Province)6