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COMPILATION OF DAMAGING EARTHQUAKES OF THE PHILIPPINES
June 1999 Earthquakes in Agusan del Sur, Philippines

On June 7, 1999, a Ms 5.1 earthquake occurred at 3:45 PM (local time) and damaged the town of Bayugan in Agusan del Sur. Using the PHIVOLCS seismic network, its epicenter was determined to be at 8.575 N lat, 125.754 E long or about 20 km south of the town of Bayugan. Its depth is estimated at 7 km. After two days in June 9, an aftershock occurred and damaged the town of Talacogon also in Agusan del Sur. The epicentral areas of both events are located in the Agusan Valley region, previously called by Maso (1911) as lying along one of the “unstable portion of the line of fracture ...” due to the succession of damaging earthquakes that occurred in the area in the latter part of 19th century. Agusan River meanders through the valley floor at a general NW-trend. Mountain ranges bound the valley at its west and east sides. In the towns of Veruela and Talacogon, many swamps and lakes are found; the largest of which is Lake Lumao located west of Talacogon. The northern part of the valley belongs to Butuan City and the province of Agusan del Norte. Most part of the valley, however, belongs to the province of Agusan del Sur. The two Agusan provinces became distinct from each other from a presidential decree in 1969. Prior to this, there was only one Agusan province.
The earthquake caused damages in the town of Bayugan in the province of Agusan del Sur. Bayugan has a population of 89,999 (National Statistics Office, 1996) distributed over 14 barangays. From among 9,752 dwelling units (National Census and Statistics Office, 1980), only 32 are classified as for commercial, industrial and/or agricultural purposes. About 98% of the total units were classified as “housing units”. Hence by 1995, the town of Bayugan may be described as small with few commercial buildings with most of its structures built for housing purposes. PHIVOLCS scientists who visited the town right after the earthquake noted that Bayugan has developed since then and by the time of the earthquake, had its own commercial buildings, elementary and high schools, a church, a funeral home and a municipal building (Roberto Tiglao, PHIVOLCS, personal communication, 1999). Most of these structures sustained damages from the earthquake (unpublished Quick Response Team reports, 1999). Talacogon, meanwhile, is slightly less well-developed than Bayugan.

 

Tectonic Setting and Seismicity

The affected areas are on the Agusan Valley region where the southern segment of the Philippine Fault passes through (Figure 1). The fault can be traced from the east side of the Malimono Ridge or west Lake Mainit and traverses the Agusan Valley floor in a NW direction. Another fault west of the ridge parallels the main trace and becomes indistinct in the town of Cabadbaran. Meanwhile, a branch of the main fault swerves into the southeast towards the shore town of Lianga. The town of Bayugan is at the junction where the SE-trending splay of the NW-trending Philippine Fault branches off towards Lianga. A plot of earthquakes with Ms > 4.0 from 1960 to 1997 shows that seismicity in the epicentral area that originates mainly from the activity of the Philippine Fault Zone (PFZ) is not so intense (Figure 2). The plot shows that most events are found along the west Lake Mainit fault segment. Seismicity is dispersed throughout the valley area; most of these have small magnitudes and shallow depths. Seismicity in the region is lower as compared to the offshore area between the PFZ and the Philippine Trench, which is also a source of significant seismic activity. Large earthquakes had affected the Agusan Valley area in the past and had also caused damage. Those that caused damages were the earthquakes in 1879, 1912 and 1990. Data show that previous earthquakes that originated from the Philippine Trench had not exacted that much damage to the Agusan Valley region.

Focal mechanism solutions of earthquakes from 1960 to present are shown in Figure 4. The data were derived from the Harvard University and the National Earthquake Information Center (NEIC) of the U. S. Geological Survey (USGS). In the offshore area east of Mindanao, most mechanisms show underthrusting possibly due to subduction-related activities. Offshore from the west Malimono segment, the mechanism of a shallow earthquake on May 4, 1993 showed thrust faulting (Figure 4). In the Lake Mainit area, a Ms 5.5 event that occurred on March 27, 1990 showed a left-lateral strike-slip faulting along a NW-trending fault line. This quake caused damage to Butuan City and the towns of Jabonga, Santiago and Cabadbaran, all in Agusan del Norte (Oanes and Salugsugan, 1990). There were also minor landslides at Diwata Range. Another event that showed strike-slip faulting occurred southeast of Lake Mainit at Diwata Range on May 1, 1979. After a few hours, this event was followed by an aftershock that showed normal faulting. Bautista (1996) related such type of mechanism to extension leading to the formation of a pull-apart basin. No damage was reported for these events. An earthquake with a normal mechanism occurred on August 12, 1989 near the Agusan del Sur town of Esperanza. Likewise, no damage was reported. Meanwhile, the fault swerves to the SE from the western Diwata Foothills area towards the seashore town of Lianga. Very near the town of Lianga, an earthquake in November 22, 1978 showed normal faulting. Three events (August 27, 1983, two on November 2, 1984) in the southeast part of the valley showed left-lateral strike-slip movement possibly related to movement of splays of the Philippine Fault.

 

Sites, Sizes and Effects of Past Large Earthquakes in the Area

The review of historical earthquakes by Bautista (1996) and further refined by Bautista and Oike (2000) showed no earthquake event that may be related to the activity of the PFZ along the Agusan Valley segment prior to 1879. This could either be an artifact of lack of earthquake records, a true lack of strong earthquakes along this segment of the PFZ or miscalculation of earthquake size and epicenter due to sparse historical earthquake acocunts.

On July 1, 1879, a damaging earthquake affected the west Lake Mainit fault segment of the Philippine Fault Zone. The high intensity area covers the towns of Jabonga in Agusan del Norte, Mainit town in Surigao del Norte and a place simply called “Surigao” possibly referring to present-day Surigao City (Letter of Fr. Luengo, July 8, 1879). In the lakeshore towns of Jabonga and Mainit, hills and rocks slid down and the areas were described as highly disturbed. Numerous, large fissures opened to as much as 3.6 m wide along the lakeshore area. Some points along the lake disappeared while large landslides occurred. It caused a great fright among the few inhabitants in the area. At least 12 large aftershocks were counted within 24 hour of the main shock. Another place where significant damage to the environment was documented was in the town of Anao-oan (site of present day San Francisco) in the province of Surigao del Norte. In this place, there were 40 houses made of nipa. Fourteen tilted by as much as 25-30o while the rest collapsed. Two bridges were rendered useless. There were conspicuous landslides along the steep mountains near the shore. At Bilaa point, there was a report that a “fissure divided the promontory into two” (Letter of Fr. Luengo, July 8, 1879). While the reports are scarce, it is possible that the fissure described here is a ground rupture. This could be confirmed by detailed field investigations from the west Lake Mainit region northwards to the shoretown of San Francisco. Liquefaction was also observed in this town with the emission of NW-SE trending sandboils. The earthquake also created a potable water spring. Buildings sustained vertical cracks while floors tilted and windows and doors jammed. A mangrove area sank by 300 m at Bilan Port, near Bilaa Point. There were also numerous fissures, some measuring 4-6 cm wide. Some of these fissures later on closed. Movable objects were thrown down and two statues fell. A thick-walled church made of coralline limestone and with galvanized iron roof sustained both horizontal and vertical cracks (Letter of Fr. Luengo, July 8, 1879). Unlike Jabonga, there were 40 events felt within a 24-hour period. The direction of motion is SSE-NNW (Centeno y Garcia, November 1, 1879). Bautista (1999) inferred the epicenter to be at 9.350 N lat and 125.600 E. long and its Ms to be 6.9 using felt are-Ms relations.

On July 11, 1912, a Ms 7.5 earthquake affected the NE Mindanao region. Damage and high intensity were experienced by towns (La Paz, Bunauan, Veruela and Talacogon) in the Agusan Valley area where intense ground shaking, liquefaction, widespread landslides and river/lake seiches occurred. Accuracy of location during the turn of the century is in the order of +/- 5o due to the few number of participating global stations during this time. While its epicenter was instrumentally located in the east Lake Mainit area, intensity data suggest that its epicenter was probably along the valley area where the fault passes through.

 

Epicenters, Magnitudes and Depths of the June 7 and 9 events

The 3:45 PM event on June 7 was recorded by 20 PHIVOLCS seismic stations as far as Pasuquin Seismic Station in Ilocos Norte. Its epicenter was determined to be at 8.575 N lat, 125.754 E long using eight P-wave readings. Its depth is about seven km. The epicenter is about 15 kms from Bayugan, where the damages were reported, and which is inside the the range of error of PHIVOLCS plotting.

Based on the earthquake’s location, the source is believed to be from the movement of the segment of the Philippine Fault in Agusan Valley. While the magnitude of this quake may be considered as ‘moderate’, its expected intensity at Bayugan town could be as high as Intensity VII for medium soil using the attenuation-relation of Fukushima and Tanaka (1990). In Bayugan, poorly built structures collapsed, posts and foundations of buildings sank or tilted. There were also reports of injuries due to falling debris or objects. People in the epicentral area were frightened and most of them ran out when the earthquake occurred. After two days at 9:05 AM on June 9, a Ms 5.0 earthquake again occurred near the epicentral area. While this appears to be an aftershock of the June 7 event, its location is about 2 km north of the June 7 event. This event caused damages to bridges, roads, schools, a commercial complex, telephone station and municipal hall of the town of Talacogon also in Agusan del Sur. This was also felt in other towns inside Agusan Valley.

A large aftershock occurred in June 9 at 9:05 AM. By using 11 P-wave and 1 S-wave readings, the epicenter was determined to be at 8.604 N lat, 125.648 E long and its depth to be about 14 km. Unlike the June 7 event, this was most severely felt in the town of Talacogon.

 

Focal Mechanism of the June 7 earthquake

The NEIC which routinely publishes focal mechanism solutions of events with greater than Ms 5.5 did not issue solutions for the June 7 as its magnitude was below the Ms 5.5 cut off magnitude. Instead, we used the first motion data of 18 PHIVOLCS seismic stations to determine the solutions for the two events. Figure 4 shows the focal mechanism for this event. It shows a normal faulting with some strike slip component. This correlates with the August 12, 1989 event shown in Figure 3 and this could be interpreted as possibly due to movement along a pull-apart basin common in strike-slip environments.

Intensity Distribution

June 7 event

The most severely affected place is the town of Bayugan where structures collapsed. Based on the Philippine Earthquake Intensity Scale (PEIS) of I to X, the intensity is assessed as Intensity VII in this town. PHIVOLCS main office personnel and the QRT Bayugan team also conducted telephone and personal interviews to assess the intensities at different sites. The following are the results of both the intensity survey and reports of PHIVOLCS stations and as shown in Figure 5:

Bayugan, Agusan del Sur - Intensity VII
San Francisco, Agusan del Sur - Intensity V
Butuan City - Intensity V
Hinatuan, Surigao del Sur - Intensity IV
Lianga, Surigao del Sur - Intensity II
Cagayan de Oro City - Intensity II
Bislig, Surigao del Sur - Intensity II
Camiguin Island - Intensity I

June 9 event

The June 9 event damaged the town of Talacogon.

Talacogon, Agusan del Sur - Intensity VI
Bayugan, Agusan del Sur - Intensity V
Hinatuan, Surigao del Sur - Intensity IV
Butuan City - Intensity IV
Bislig, Surigao del Sur - Intensity IV
San Francisco, Agusan del Sur - Intensity IV
Lianga, Surigao del Sur - Intensity III
Prosperidad, Agusan del Sur - Intensity III

 

Foreshocks and Aftershocks

Figure 7 shows the background seismicity a month before the June 7 event. The events were small, ranging from Ms 3.3 to 4.8 with depths between 7 to 53 km. No earthquake, therefore, may be considered as a foreshock to this event based on the data. Table 1 lists the parameters of the background seismicity a month before the quake. There were no recorded events in the epicentral area.

Figure 8 shows the 51 earthquakes plotted from the June 7 main shock until June 30, 1999. Most of the events clustered around the June 7 main shock. Some, however, are located farther away (at most is 10 km) from the fault trace. The largest aftershock is the June 9 events at 9:05 AM. While some of the aftershocks cluster along the fault trace, some are spread out around the epicentral area. It is difficult to determine whether this was caused by the sparse seismic stations in the area resulting in a dispersed aftershock plot or possibly due to inhomogeneity of the rock units. Table 2 lists the aftershocks of the June 7 event, including their parameters, as detected by the PHIVOLCS seismic network.

 

PHIVOLCS’ response

Since there was no PHIVOLCS station near the epicentral area and there was a need to determine actual intensity in the epicentral, PHIVOLCS conducted intensity survery right after the quake through telephone interviews of residents in Butuan City and the towns of Bayugan, Prosperidad and San Francisco.

During the same day , PHIVOLCS dispatched a Quick Response Team (QRT) composed of personnel from PHIVOLCS seismic stations in Surigao City and Bislig. They were tasked to conduct ocular inspection of reported damages, determine cause of damage, document impacts and to conduct information drive to allay the fears of the local people. Their reports relayed on June 8 and 9 are shown in Appendices ‘A’ and ‘B’. A second QRT team arrived on June 9 to assist the first team.

Two days after the main shock on June 9, a rumor that “an impending Intensity VIII earthquake in Agusan del Sur” caused panic and confusion in the affected area. Classes were suspended as a result. As a result, PHIVOLCS was swamped with calls regarding the veracity of this rumor. In the afternoon of June 9, PHIVOLCS released an “Earthquake Update” that “categorically denied issuing statements that there will be a strong earthquake in the area and that such rumors are ill-founded and do not have any scientific basis”.

 

Conclusion and Recommendations

The area where the June 7 and 9 earthquakes occurred is within a seismically active region. Data show that the two events were not the biggest events that had occurred in this area during historical times. The June 7 event in the Agusan Valley region may be considered as part of a ‘normal’ earthquake activity within a seismically active region. The event, despite its moderate magnitude, became damaging only because of possible site amplification due to thick sedimentary fill in the area and the poor construction prevalent in the locality of Bayugan. The area is also susceptible to liquefaction based on its local geology and as evidenced by past liquefaction in the area (e.g. 1879, 1912 earthquakes). We have also observed that earthquake intensities from previous earthquakes and also during the recent June 7 and 9 events were amplified due to the presence of thick, water-saturated sedimentary deposits along the valley floor.

Our concern right now is that should there be another moderate to large earthquake along this segment of the fault, some areas may experience strong ground shaking or may liquefy causing buildings and other structures to fail. We therefore advise that structures in the area be regularly inspected by structural engineers to evaluate their soundness. Buildings that are yet to be built should be properly anchored by pile driving through the liquefiable layers to counter the possible effects of liquefaction.

The area which sustained the heaviest damage, Bayugan, is about 15 km from the epicenter determined using the current PHIVOLCS stations. Similarly, the June 9 event was also several kilometers away from the instrumental epicenter. We recommend that additional seismic stations in any of the valley towns (e. g. Talacogon, San Francisco, Veruela), in Bukidnon and another in either Cabadbaran, Bayugan or Butuan City be established to improve earthquake determination capability of PHIVOLCS. Site response evaluation studies are recommended to pinpoint the reason why the damages caused by the June 7 and 9 earthquakes were far from the instrumental epicenter. Mapping of the segment of the Philippine Fault should be done to finetune location of faults determined from seismicity and digital terrain data. Information campaign about the possible effects of future earthquakes in the Agusan Valley region should be started in order for local people to be prepared for future earthquakes.

 

Acknowledgements

The contents are quoted mostly from the work of Bautista et al (1999) entitled “Seismic Characteristics of the June 1999 Earthquakes in Agusan del Sur, Philippines: A Preliminary Report”

 

References

Allen, C. R. 1962. Geological Criteria for Evaluating Seismicity. Geological Society of America Bulletin. 86. 1041-1057.

Bautista, Bartolome. 1996. Seismotectonic Implications of Recent Philippine Earthquakes from 1980 to 1994. Unpubl. M. A. thesis. State University of New York at Binghamton, New York, USA. 218 pp.

Bautista, Ma. Leonila P. 1996. Determination of the Epicenters and Magnitudes of Philippine Historical Earthquakes (1589 to 1896). Unpubl. M.S. thesis. Kyoto University.

Bautista, Maria Leonila P. and Kazuo Oike. 2000. Estimation of the Epicenters and Magnitudes of Philippine Historical Earthquakes. Tectonophysics. 317. 137-169.

Bautista, Ma. Leonila P., B. C. Bautista, E. Q. Amin and J. C. Salcedo. 1999. Seismic Characteristics of the June 1999 Earthquakes in Agusan del Sur, Philippines: A Preliminary Report. PHIVOLCS Internal Report.

Centeno y Garcia, Jose. 1879. Report dated November 1, 1879 about the July 1, 1879 earthquake. Submitted to the Governor General of the Philippines.

Fukushima, Y and T. Tanaka. 1990. A new attenuation relation for peak horizontal acceleration of strong earthquake ground motion in Japan. Bull. Seism. Soc. Am. 757-783.

Fr. Luengo, S.J. 1979. Letter of Fr. Luengo, S. J. dated July 8, 1879 about the July 1, 1879 earthquake in Surigao.

Maso, Miguel Saderra. 1912. The Earthquake of the Agusan Valley and the Eastern Coast of Mindanao, July 12, 1911. Bull.. Seism. Soc. Am. II. June 1912. 141-142.

National Census and Statistics Office. 1980. Census of Population and Housing: Housing Characteristics of Occupied Dwelling Units by Region, Province, City and Municipality, Philippines. Special Report No. 5. Manila. 1-382.

National Statistics Office. 1996. Census of Population. Report No. 1-P (Caraga): Population by Province, City/Municipality and Barangay. Manila. 1-44.

Oanes, Alejandro and Luisito M. Salugsugan. 1990. Investigation of Agusan del Norte Earthquake on 0647 H March 27, 1990. PHIVOLCS unpubl. Report of Investigation. 2 pp.

Quebral, Ramon D., Manuel Pubellier and Claude Rangin. 1996. The Onset of the Movement on the Philippine Fault in Eastern Mindanao: A Transition from a Collision to a Strike-Slip Environment. Tectonics. 15. 4.713-726.

  Strk1 197
Dip1 50
Rake1 -135
Strk2 75
Dip 2 57
Rake2 -50

 

P-axis Plunge 57                       Azimuth 41

T-axis Plunge 4
            Azimuth 137

Focal mechanism solution of the 07 June 1999 event using first motion data of 18 PHIVOLCS seismic stations.

Isoseismal Map of the 07 June 1999 (Ms=5.1) earthquake in Bayugan, Agusan del Sur. Intensity data based on PHIVOLCS Station reports, investigation and interviews

Tables/Figures 

List of Figures:

 

Tectonic map of study area. Structures based on previous studies (Allen, 1964; Quebral et al, 1996), seismicity, focal mechanism and digital topographic data.

 

Plot of all shallow (0-60 kms) earthquakes from 1600 to 1997. Data from Bautista (1999) for earthquakes earlier that 1897 and from the U.S. Geological Survey (USGS) for earthquakes from 1897 to 1997. Dates of earthquakes that caused damaged are given.

 

Focal Mechanism Solutions of earthquake from 1960 to 1997. Data from Harvard University and U.S. Geological Survey.

 

Plot of background seismicity recorded in the month of May 1999 by the PHIVOLCS Seismic Network. Yellow stars represents the location of epicenters. Ms values ranged from 2.0 to 4.8 while depths were from 1 to 53 kms. Bold black lines represent faults. Thick gray line represent the Philippine Trench.

 

Location of the 07 and 09 events and the ensuing aftershocks recorded by the PHIVOLCS Seismic Network in June 1999. Yellow circles represents locations of epicenters. Ms values of epicenters ranged from 2.2 to 4.8 while depths were from 1 to 39 kms. Bold black lines represents faults while thick gray lines represents the Philippines Trench.