2003 February 15 Ms6.2 Masbate Earthquake

Masbate Earthquake
REPORT OF INVESTIGATION
15 FEBRUARY 2003

The Earthquake

A strong earthquake with Ms6.2 struck the province of Masbate at 7:01 in the evening of 15 February 2003. Preliminary determination of epicenter indicated that the event was generated along the Masbate Segment of the Philippine Fault Zone (PFZ) in central Philippines. The epicenter was located offshore of Magcaraguit Island (12.2ºN, 123.8ºE) and about 22 kilometers deep, which is approximately 28 km east of Masbate City. Initial reports from nearby stations implied that the earthquake was felt all over the island of Masbate including the nearby provinces of Bicol, Leyte, Panay, Cebu, Negros and Romblon (Figure 1).

The focal mechanism solution released by PHIVOLCS suggests a dominantly left-lateral slip along a vertical NW-SE fault (Figure 2). This left-lateral slip correlates well with the known structure in the area as observed in Dimasalang, Palanas and Cataingan along its mapped trace. Figure 3 shows the historical earthquakes along the Masbate Segment of the Philippine Fault Zone that dates back in the year 1800’s. Magnitues 6.5 and 7.0 earthquakes were reported on 16 August 1869 and on 19 October 1897 on the island respectively (Lanuza, 1994). Another reported earthquake struck the province of Masbate on 23 October 1877 between 1:00 and 2:00 in the afternoon (SEASEE Vol. IV). Other reported earthquakes for the island of Masbate are as follows: 1915 (Ms 6.6); 1970 (Ms 6.3); and 1971 (Ms 6.3). In 07 May 1998, a magnitude Ms 5.7 quake was felt at intensity VI all over the City of Masbate and the municipalities of Mobo and Uson. This event was preceded by a magnitude Ms 5.1 with reported intensity VI at Masbate City (Torrevillas et. al., 1998).

A Quick Response Team (QRT) was deployed to Masbate on 18 February 2003 based on the extent of the preliminary reported damages submitted by the Masbate Seismic Station (Dela Cruz and Torrevillas, 2003, unpublished internal report). The QRT’s primary purpose was to evaluate the extent of damages due to the earthquakes in the province of Masbate especially those municipalities located near the epicenter. The team was also tasked to verify and map the reported ground rupture, conduct intensity survey, disseminate correct information regarding the event, and install additional seismographs in order to monitor aftershocks of the Ms 6.2 earthquake. This paper reports the overall findings of the QRT during its 10-day investigation in Masbate. All intensity reports in this paper are according to the Philippine Earthquake Intensity Scale (PEIS) as shown in Figure 4.

Foreshock and Aftershock Analysis

The 7:01 February 15, 2003, Ms6.2 earthquake that hit Masbate has numerous foreshocks and aftershocks. The main shock was preceded by a moderately strong foreshock with a magnitude of Ms5.2 located at 12.2ºN and 123.7ºE with a depth of 30 km (Figure 5a). The moment tensor solution indicates strike-slip faulting with strike=55, dip=78, slip= -175 with Mo=5.3x 10¹⁷ Nm equivalent to a seismic moment magnitude Mw 5.8 (Figure 5b). This foreshock was felt over a wide region at the following PEIS intensities:

Intensity VI	Dimasalang, Masbate Palanas, Masbate Magcaraguit Island, Dimasalang, Masbate Deagan Island, Dimasalang, Masbate Uson, Masbate
Intensity V	Masbate City, Masbate Batuan, Masbate Burgos, Masbate
Intensity IV	Baleno, Masbate Aroroy, Masbate Legazpi City, Bicol Talisay, Masbate
Intensity III	Milagros, Masbate San Jacinto, Masbate Borabantique, Masbate Kumawit, Monreal, Masbate Palo, Leyte Bulusan, Sorsogon Mandaue City, Cebu
Intensity II	New Washington, Aklan
Intensity I	Mandaon, Masbate

Intensity surveys revealed that this foreshock was notably strong and caused damages in the epicentral area. Some residents of affected structures straddling the fault in the municipality of Palanas, Masbate reported that their houses were damaged by the foreshock manifested by visible cracks on walls and floors and as fissures in fields and gardens. In Brgy. Sta. Cruz, Palanas, the road was reportedly fissured during this foreshock. According to interviewees, the same fissures later became wider and larger during the Ms6.2 7:01 pm earthquake. In the island of Magcaraguit, Dimasalang, the non-reinforced walls of a chapel collapsed. In Deagan, people and animals panicked during this earthquake. In Deagan island, Dimasalang, interviewees described the booming sound they heard preceding the quake to be very strong and "as if its origin is just there". In Uson, Masbate, many appliances like televisions and cabinets toppled down during the shaking that accompanied the foreshock, which prompted the residents to bolt their appliances to the wall in preparation of the eventuality of another earthquake.

Figure 5a.

Figure 5a. Map showing the location of the largest foreshock that occured 5 hours before the main event. Solid circles are the plotted foreshock events.

As of February 24, 2003, a total of 187 aftershocks were recorded in Masbate Seismic Station (MMP). Some of these aftershocks were felt at intensity II or III along the ground rupture area, in the islands Deagan and Magcaraguit, and as far as the southern part of Ticao Island. A strong aftershock was felt in Magcaraguit and Deagan islands and in Dimasalang and Palanas on February 18, 2003 at Intensity IV. Some of the moderately strong aftershocks were recorded in other permanent PHIVOLCS seismic stations and hypocentral parameters were located. Preliminary determination indicated that these aftershocks are located 15-40 km southeast of Masbate City. This implied the continuous activities along the Philippine Fault that ruptured in the southeast portion of the Masbate Island in the vicinity of Dimasalang and Palanas. Table 1 shows the list of aftershocks plotted in Figure 5.

In Magcaraguit and Deagan Islands of the municipality of Dimasalang, residents reported an average of 2 felt aftershocks everyday. In these islands and in places near the ground rupture, aftershock events were observed to be always preceded by rumbling sounds that vary in intensity depending on the size of the event (e.g. louder booming sounds preceded bigger aftershocks). In Ticao Island, reports regarding felt aftershocks were in lesser frequency but were likewise preceded by booming or truck-like sounds. Please see Figure 5 for the location of aftershock events with resolved epicenters.

In order to detect aftershocks that are very local to the Masbate region and beyond the detection capability of the existing nearby PHIVOLCS permanent stations, a total of two temporary short-period seismographs were installed in the area. One of these instruments was installed

in Cataingan, Masbate (N12º00’10.1"; E123º59’53.9’’) that operated from l9-24 February 2003. The other station was placed in Batuan Poblacion, Batuan, Masbate in Ticao Island (N12º25.33’00"; E123º46.765') and operated from 20-24 February 2003. Cataingan station (CAT) is near the southern termination of the rupture while Batuan station (BAT) is located near the projected extension of the fault southwest of Ticao Island (Figure 6). From 22-23 February, CAT station recorded 5 events while the BAT station recorded 20 quakes. During this very short period of observation, it was noted that more earthquakes were recorded to the north of the mapped rupture. The location of the plotted earthquakes and the results of the installation of the temporary network showed that possible adjustments of the fault were confined along the observed ground rupture and extend up to the possible offshore extension of the said ground rupture, west of Ticao Island.

Figure 6.

Figure 6. Map showing the location of CAT and BAT stations. These were temporary stations installed during the QRT investigation.

Considering the magnitude of the 15 February 2003 quake, the felt aftershocks were anticipated to be observed up to about a month after the mainshock. Thus, everyone was advised to take proper precautions especially when entering buildings and houses that suffered minor to severe damages.

Intensity Survey

Results of damage assessment and intensity survey conducted by the team throughout the province of Masbate suggest that the Ms=6.2 earthquake at 7:01 in the evening was felt at Intensity VII and VIII within the epicentral area. Isoseismal distribution based on PEIS would be as follows:

Intensity VIII	Sta. Cruz, Palanas, Masbate* Suba, Dimasalang, Masbate*
Intensity VII	Dimasalang, Masbate Brgy. Poblacion, Palanas, Masbate Uson, Masbate Nabangig, Palanas, Masbate Nipa, Palanas, Masbate Deagan, Dimasalang, Masbate Magcaraguit, Dimasalang, Masbate Talisay, San Fernando, Masbate Nipa, San Fernando, Masbate Pinamughaan, San Fernando, Masbate Buyo, San Fernando, Masbate
Intensity VI	Batuan Poblacion, Batuan, Masbate Burgos (Lagundi), Batuan, Masbate Bacolod, Milagros, Masbate Cataingan, Masbate Dapdap, Uson, Masbate Armenia, Uson, Masbate Canvañez (Aricomo), Batuan, Masbate Poblacion, San Fernando, Masbate
Intensity V	Masbate City Sto. Niño (Borabantique), Monreal, Masbate Placer, Masbate Pio V. Corpuz, Masbate San Jacinto, Masbate Putoson, Baleno, Masbate Poblacion, Aroroy, Masbate Ambulong, Aroroy, Masbate Irosin, Sorsogon Legazpi City, Albay
Intensity IV	Palo, Leyte Naga City, Bicol Roxas City, Panay
Intensity III	Metro Cebu Lagta, Baleno, Masbate Cagara, Baleno, Masbate Poblacion, Baleno, Masbate Poblacion, Mandaon, Masbate
Intensity II	Luya, Aroroy, Masbate
Intensity I	Lezo, Aklan Kalibo, Aklan

*along the ground rupture

The above observations clearly indicated that intense ground shaking was concentrated along and near the epicenter and the ground rupture (Figure 7). Figure 8 shows few of the interviewees that gave valuable information regarding their observations during the earthquake. Felt and observed intensities decreased accordingly northeastward and southwestward relative to the epicentral location with apparent elongation along location of the PFZ-Masbate Segment (Figure 7).

Figure 8.1

Figure 8.2

Figure 8. Photos of some of the interviewees in Nabangig (upper photo) and Sta. Cruz (lower photo) in Palanas Municipality.

Ground Rupture

The ground rupture was verified and mapped through field investigations. The total length of the rupture onland is approximately 18km transecting several barangays of Dimasalang, Palanas and Cataingan. The ground rupture was characterized mostly by right-stepping en echelon faults with a general trend of ~N3OW to ~N4OW and had a maximum opening of 20 cm (Figure 9). The maximum horizontal displacement along the fault was 47cm in Brgy. Sta. Cruz, Palanas (Figure 10) while the maximum vertical displacement (23cm) was found in Brgy. Suba, Dimasalang (Figure 11). The average horizontal and vertical displacements mapped along the ground rupture were 15 cm and 5 cm, respectively.

Figure 9.2

Figure 9. The observed ground rupture in Brgy. Sta Cruz, Palanas typically manifested by an echelon fissures as shown in the left and above photos

Figure 10. Photo showing a pilapil (rice paddy dike) transected by the ground rupture in Sta. Cruz, Palanas. Photo is looking SW.

Figure 11.1

Figure 11. The observed ground rupture in Dimasalang displacing a line of vegetation (right, looking NW) and a residential lot (top, looking SE) horizontally and vertically.

On the other hand, the average width of the fault zone measured was about 75 cm and the widest measurement (153 cm) was found in Brgy. Sta. Cruz, Palanas. The ground rupture mapped during the 10-day investigation was traced from Brgy. Suba, Dimasalang to Sitio Burabod, Brgy. Pawican, Cataingan. The February 2003 ground rupture, more or less, followed the old trace/location of the active fault with about 3m localized deviations in some areas. Maximum PEIS intensity of Intensity VIII was observed in some areas along the ground rapture wherein several houses were totally damaged due to significant horizontal and vertical displacements (Figure 12). A displaced coconut tree (Figure 13) and the ground rupture manifestation into the seashore (Figure 14) were observed in Matugnaw, Palanas and Suba, Dimasalang, respectively.

Figure 12. Damaged houses transected by the ground rupture in Bry. Suba, Dimasalang (above photo, looking SE), and in Brgy. Sta. Cruz, Palanas (right photo, looking SW)

Figure 13. Displaced coconut tree found in Bry. Matugnaw, Palanas (left photo, looking SE), Also manifested along the ground rupture in this area were mole tracks (right photo, looking NW).
Figure 14. The Masbate 2003 ground rupture as it entered the sea transecting the shorelines and coral reefs fronting Bry. Suba, Dimasalang (photo looking SW)

To monitor possible post-earthquake deformation along the February 2003 ground rupture, temporary benchmarks were established in Palanas municipality at barangays Sta. Cruz and Nabangig (Figure 15).

Figure 15. Temporary stakes installed across the ground rupture to monitor and measure possible post-earthquake deformation (photo looking SW)

Considering the following facts and observations:

the Masbate Segment is a left-lateral fault,
its epicentral location is west of Magcaraguit Island,
the distribution of observed aftershocks up to Talisay in Ticao Island,
and the observed concentration of high intensities, the projected northern extension of the ground rupture associated to the February 15, 2003 earthquake is deduced to be up to the Brgy. Talisay in western Ticao Island. Thus, this would indicate a ground rupture with a total length of at least 50km (Figure 16).

Damage on vertical and Horizontal Infrastructure

Large magnitude earthquakes determine the structural integrity of solid structures built on earth. Based on the initial survey conducted by the QRT during its field investigations, some school buildings, roads, bridges and river flood control projects performed poorly. Moreover, they were deemed structurally unsafe and hazardous to life and property after experiencing the strong ground shaking of the 15 February 2003 quake.

During this event, engineered structures proximal to the fault trace sustained the worst damage. A road section of the Masbate-Cataingan Road was intersected by the fault near the Dimasalang-Palanas boundary (Figure 17). In this area the road was damaged as lateral longitudinal cracks were formed along the fault producing buckled and cracked section in this road. On the other hand, the Nipa Bridge along the Masbate- Cataingan Road (km post 57+607), located less than 2km from the ground rupture suffered significant structural damages. In this bridge, at least one of its columns showed concrete spalling at mid-height with striking vertical misalignment on both horizontal directions (Figure 18). Displacement at the bridge deck with respect to the bridge approach was also noticeable along with the yawning deck joints. Moreover, the slope protection grouted riprap at Nabangig Bridge located along the Masbate-Cataingan Road (km 62+560) and the Cantil River Control in Brgy. Poblacion in Palanas, Masbate were also severely damaged. The riprap structures in these areas suffered numerous large cracks as their foundation failed most probably due to compaction and slumping.

Figure 18.1 Figure 18.2

Figure 18. The Nipa Bridge in Dimasalang (left photo)
and the riprap structures (right photo) that
were damaged during the February quake.

Furthermore, several school buildings at Masbate National Comprehensive High School suffered severe shear cracks and column-wall joint failure (Figure 19). In the same structures, some longitudinal and transversal fractures along the length of the beam and of the column were likewise observed. The Provincial Health Office's Administration building's middle concrete roof beam reveals a possible longitudinal rupture. The same failure characteristic was observed on at least two school buildings in Jose Zurbito Sr. Elementary School (also known as Jose Masbate South Elem. School) in Masbate City (Figure 20).

	Figure 19. School buildings at Masbate National Comprehensive High School that suffered severe shear cracks and column wall joint failure
	Figure 20. A collapsed concrete wall of a Marcos-type school building in Dimasalang, Masbate.

It was also noted during the investigation that very little attention was given to non-engineered houses or facilities (Figure 21). Based on ocular investigations, many concrete or semi-concrete houses in Palanas and Dimasalang suffered severe damages. Since the owners of these houses are quite unsure about the condition of their residence, the assessment of these houses is urgently needed. Local engineers both private and government practitioners are enjoined to take the necessary steps to help owners assess the condition of their damaged houses and facilities immediately after the quake.

Figure 21. During the earthquake, this non-engineered fence of a residential house (upper-right, looking NW) dismembered and a parking garage (lower-right, looking South) collapsed causing severe damage on parked vehicle. Both photos were located in Bry. Poblacion, Dimasalang.

Figure 21.2

Other Previous Significant Events

From the intensity surveys and ground rupture mapping activities, several significant earthquakes that affected the Masbate area were likewise obtained. These events were either experienced by the interviewees to be weaker and/or stronger than the 7:01 15 February 2003 Ms 6.2 event and with observable ground rupture along the Dimasalang-Cataingan area.

1957
This event is very memorable for an interviewee in Batuan, Ticao Island and some other interviewees in Deagan Island and Palanas because it is stronger than the 7:01 February 15, 2003. The aftershocks of the 1954 event were observed to be more frequent and lasted for at least a month compared to the 2003 mainshock. In Magsaysay, Masbate people in the streets lost balance and fell due to intense ground shaking. People along the rupture area in Brgy. Matugnaw, Palanas, reported co-seismic ground fissures in the same zone that ruptured during the February 2003 earthquake.
1967
In San Fernando, Ticao Island, interviewees talked of a strong earthquake during the 1960s at the time of Mayor Romero. After strong ground shaking, water fountaining was observed in Sitio Magsasaka, San Fernando Poblacion, Masbate that reached a height of about 16m. The fountaining phenomena can be associated to either liquefaction or localized disturbance of an aquifer. During this 1967 quake, ground rupture was likewise observed in Palanas.
1973
This event was remembered by residents of Kumawit, Monreal of Ticao Island to be more intense than the 7pm February 15, 2003 earthquake. The 1973 event caused large fragments of the limestone cliffs to fall into the sea in the northern point of Ticao Island.
1979
An interviewee from Magcaraguit Island remembered the 1979 earthquake to have caused stronger ground shaking in the island compared to the 2003 event. The felt aftershocks were greater in number than the felt aftershocks during the 2003 event and were described to have occurred "incessantly night and day". There was no tsunami report in Magcaraguit Island.
1989
This earthquake caused many objects to fall in Sto. Niño (then Borabantique), Monreal in Ticao Island with associated ground shaking stronger than the February 2003 7pm earthquake.
1998
Remembered by Kumawit, Monreal inhabitants with stronger ground shaking compared to the February 15, 2003 7pm event with accompanying booming sound. An interviewee from Brgy. Nabangig, Palanas observed that during this earthquake, boulders crumbled down from the mountain. It also produced ground rupture in the same area but with much wider ground rupture compared to the ground rupture produced by the February 2003 earthquake. Almost all of the felt aftershocks in Palanas during this quake were accompanied by rumbling sounds.

Considering where significant events were fairly remembered, the northern segment of the Masbate Fault: PFZ implied higher rates of activity (Fig. 22). Moreover,its apparent from the above observations that M6 event in this segment of PFZ produces a relatively large movement of ground rupture.

Tsunami, Rockslides, Lanslides, and Liquefaction

Tsunami is a wave or series of waves generated when a significant displacement occurred under water due to underwater faulting with large vertical component, underwater landslides and underwater volcanic eruption. Rock slides and landslides, on the other hand, are slope failures that occurred along mountain or hill slopes during intense ground shaking. Ground failure can likewise occur in flat regions especially when intense ground shaking is experienced by areas underlain by water saturated loose sediment (e.g. along shorelines, deltas and riverbanks).

During the February 2003 event, the occurrence of these kinds of phenomena were very few or almost none. Reports of unusual wave height and activity were located only near and around the northwestern part of Ticao Island. Since observed displacement along the ground rupture showed very minimal and localized vertical displacement, the tsunami in northwest Ticao Island can be attributed most probably to local underwater landslides. Onland landslides and rockslides were not observed. Minor rockfalls were observed in Sitio Kumawit, Monreal in the northern part of Ticao Island as well as in Hamoraon and Cudao islands located near Magcaraguit and Deagan Islands in Dimasalang. Fragments of limestone exposed along vertical cliffs bordering the shoreline collapsed in these above-listed islands. Liquefaction on the other hand, was also very minimal. Although there were some reports of ground failure along and near the shorelines, local compaction of the underlying basal foundation cannot be discounted as the main cause of such failure considering the prevailing construction practices in the region. Taking into account the intensity of the felt ground shaking near and along the ground rupture area, it is very notable that effects of liquefaction, landslides and rockslides were very minimal to almost non-existent. Such striking and fortunate incident can probably be directly attributed to the very short duration of the February 15, 2003 quake. Based from the interviews, the ground shaking during the 2pm and 7pm events on February 15, 2003 lasted only for about 4sec and 7sec, respectively.

Active Faults in Uson Area

Information Dissemination Campaigns

Conclusions

The 15 February 2003 event was generated along the Masbate Segment of the PFZ. The Ms6.2 earthquake produced at least 18 km long ground rupture onland with a total length of about 50km. Significant displacements along the Masbate Segment of PFZ caused a maximum intensity of Intensity VIII wherein houses were totally damaged along the ground rupture. Intensity distribution showed that ground shaking and damages were concentrated near and along the epicentral area and the ground rupture, respectively. Severe damages were generally observed on structures like school buildings, bridges and irrigation canals. Minor to severe damages were likewise observed on un-engineered houses and facilities that were manifested as small fissures on floors and walls or even as total collapse of walls and houses. Phenomena like tsunami, landslides, rockslides and liquefaction were not observed except for the tsunami due most probably to underwater landslide and minor rockslides in western part of Ticao Island.

Aftershocks were usually observed near and along epicentral area and are expected to last up to about a month after the mainshock. Based on PHIVOLCS historical data and other previous events gathered through the QRT field investigations, the return period of large magnitude earthquakes along the Masbate Segment of PFZ can be as short as five years or as long as thirty years with an average of about 10 years. Typical large earthquakes along this part of the PFZ usually have magnitudes from Ms 6.0 to Ms 7.0. Furthermore, it is very notable that even earthquakes with less than Ms 6.0 produced ground rupture along the Masbate Segment and caused significant damages near and along the location of the Masbate Segment. Considering the possible locations of the recent significant events along Masbate Segment and the scale of damage suffered by man-made structures, a closer study on these historical events is highly recommended. Moreover, information dissemination about earthquakes should be regularly undertaken together with the implementation of an annual earthquake drill especially in schools and government offices.

At present, the Uson splay of the PFZ in Uson vicinities cannot yet be all together discounted. Further studies will be undertaken to ascertain its level of activity and its location.

Recommendations and Future Activities

To further understand the activity along the Masbate Segment of PFZ, the following activities will be forthwith undertaken.

Detailed mapping, if possible, of the whole February 2003 ground rupture and continuous measurements of horizontal and vertical displacements to monitor possible post-earthquake deformation using the temporary benchmarks established on some selected areas.
Installation of permanent benchmarks along the Masbate Segment wherein future movement will be measured more extensively.
Encourage the local populace to mark the location of the ground rupture as a reminder of the active fault location and to prevent future inhabitants in constructing any structures within the 5m-buffer zone from the active fault or from the deformation zone caused by ground rupture.
More intensive information dissemination campaign in Masbate Island and adjoining islands and provinces to allay fears of future large magnitude earthquakes.
Production and distribution of posters, leaflets and brochures written in Tagalog and in local dialect.
Production of visual aids for lectures that will be conducted in areas where modern equipment are not available.
Establish links with local barangay officials in noting time and date of felt earthquakes in their localities, information that should be immediately relayed to the PHIVOLCS MMP station in Masbate City.
Immediate and thourough study on the structural soundness of all structures, both engineered and non-engineered, by the concerned authorities.
Immediate evacuation of severely damaged buildings and thus, eliminating further usage of damaged facilities and exposure of local inhabitants to possible future collapse.
Active and practical integration of earth science to school curriculum and strict implementation of the annual earthquake drill especially in schools and government facilities.
Active participation of the Disaster Coordinating Councils from all levels in terms of disaster preparedness prior to large events and rapid damage assessment after a large magnitude earthquake like the 15 February 2003 event.
Identification of evacuation sites and evacuation routes so that local inhabitants would know where to go in case of major earthquakes or whenever the need arises that they have to evacuate their premises/residence. Further studies on the historical events along the Masbate Segment of PFZ.

References

Acknowledgements

Quick Response Teams

Figure 23. QRT members conduct lectures on earthquakes for students and teachers on a school ground in Palanas
Figure 24. Lectures undertaken at a government office (DPWH District Office) in Dimasalang.
Figure 25. Earthquake Info-dissemination inside a Barangay Hall in Nabangig, Palanas.
Figure 26. Lectures undertaken in open fields.

2003 February 15 Ms6.2 Masbate Earthquake

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