The Guinayangan Republic

The 1973 Ragay Gulf earthquake produced an on-shore surface rupture approximately 30 km in length along the Guinayangan segment of the Philippine fault in southern Luzon Island. Through geologic mapping and paleoseismic trenching, we have characterized the amount of coseismic offsets, the average recurrence in-terval, and the slip rate of the segment.

The coseismic offsets we identified in the field were fairly constant along the fault, ranging from 1 to 2 m. Paleoseismic trenching at the Capuluan Tulon site exposed strati-graphic evidence for three or possibly four surface-rupturing events after the deposition of strata datedat AD 410–535. The average recurrence interval wascalculated to be 360–780 years, which is close to that for the Digdig fault, thesourcefault of the1990centralLuzon earthquake. The slip rate, based on the calcu-lated recurrence interval and offsets during the 1973earthquake, has been calculated to be 2.1–4.4 mm/yr.This rate is significantly smaller than the geodetic slipand creep rates of 20–25 mm/yr estimated for thePhilippine fault on the islands of Masbate and Leyte.The slip rate deficit may be explained by the possi-bilities of underestimation of the recurrence interval due to possible missing paleoseismic events within thestratigraphic records, the occurrence of larger earth-quakes in the past, and the aseismic fault creep be-tween the surface-rupturing earthquakes.

Tectonic setting and epicenters of surface-rupturingearthquakes on the Philippine fault since 1970 (stars).

Fault trace of the Guinayangan segment of the Philippine fault cutting across the Bondoc Peninsula. The fault trace in the Ragay Gulf is from . The star indicates the epicenter of the 1973 earthquake.

From the coastline of the Calauag Bay southeast-ward to Mambaling we were able to interpret 1:15,000-scale color aerial photographs taken in 2010 by the National Mapping and Resource Information Agency (NAMRIA) of the Philippine government. For the rest of the area, aerial photographs were not available, and we relied on 1:50,000-scale topographic maps, shaded relief maps from SRTM 3-arc-seconds data, and Google Earth images. We conducted field investigations in Au-gust 2009, July 2010, and March 2011. We identified the fault trace locations based on tectonic geomorphic features and sought eyewitness accounts on the surface rupture. At several localities, we identified rows of coconut trees that were systematically offset left-laterally. We measured the offsets using tape measures by projecting the trend of the alignment of the trees to the fault line. As the mountainous area in the central portion of the surface rupture has very few settlements, accessibility was limited. Thus,  we surveyed only along the main road west of Guinayangan. In order to identify geologic evidence of past surface-rupturing earthquakes, we excavated a trench at Capuluan Tulon.

Locations of the surface rupture associated with the 1973 Ragay Gulf earthquake and field observation points. Base map is 1:50,000-scale topographic map sheets “Lopez” and “Liboro” published by the NationalMapping and Resource Information Agency of the Philippine government. The contour interval is 20 m.

We will describe the location and co seismic displacements identified in the field from north to south. Near its north western end, the surface rupture extends across the alluvial low land along the Calauag River. Here, it is difficult to identify the exact fault trace location based on tectonic geomorphic features due to rapid fluvial sedimentation and erosion. 

Field photographs of offset features related to the1973 Ragay Gulf earthquake. Red horizontal arrows indicate left-lateral offsets of rows of coconut trees. The red verticalarrow in (e) indicates the splitting of a coconut tree by the co seismic rupture.

At Sumilang, the surface rupture off set the railway tracks of the Philippine National Railways by 1.85 m left-laterally. Although the tracks have been repaired, Pileno Romero(born in 1939) showed us the exact location of the surface rupture. About 100 m northwest of the offset rail-way, we identified a systematic offset of aligned coconut trees across a shallow and less than 5-m-wide depression trending N50◦W . 

Ruben Labhawan has learned from the former settlers of the area that the depression follows the location of the open cracks that appeared during the 1973 earthquake. Four rows of coconut trees that are almost perpendicular to the depression are offset left-laterally at 0.8 m, 0.8 m, 0.9 m, and 0.9 m. At Loc.3 in San Roque, there is a N55 W-trending 200 m. long, northeasr facing scarp less than 3m. high. This scarp faces away from the modern northwest-flowing Calauag River, suggesting that it is tectonic in origin. Gregorio Oserin (born in 1931), who has lived next to the scarp, identified open cracks along the scarp immediately after the earthquake. There are pressure ridges northeast of the fault trace at Mambaling and Yaganak. 

At the south western base of the ridge at Mambaling, Domingo Lobioso (born in 1959) showed us an approximately30-cm-high, west-facing scarp that appeared during the earthquake. At the western base of  the ridge at Yaganak, local people documented that 1m.wide open cracks appeared during the earthquake. Left-lateral displacements of 1.1 m were reported at two localities in Yaganak. For a distance of about 15 km between Yaganak and Sintones, the fault traverses a mountainous terrain. Near the main road west of Guinayangan, local people at Dungawan Paalyunan took us to a locality where open cracks appeared during the earthquake.

Topographic map of the Capuluan Tulon trenchsite constructed using a total station. The contour intervalis 20 cm, and the elevation is relative to the lowest pointwithin the surveyed area. The trench site and offset coconuttree lines are also shown.

From Sintones southeast ward to the Ragay Gulf coast, the fault trace follows a linear depression. At Sintones, the fault trace is marked by a linear topographic boundary between elongated hills on the northeast and the alluvial plain of the Capuluan River. Several local residents documented that open cracks about 30 cm wide appeared along the base of the hills. At Loc. 7, aligned coconut trees are offset left-laterally at 1.3 m, 1.3 m, 1.6 m, and1.6 m. At Capuluan Tulon, Emanuel Orbe (born in 1962) took us where he observed open cracks after the earthquake. In this area, rows of coconut trees are systematically offset left-laterally at 1.4 m, 1.5 m., and 1.2 m. Open cracks trending N55 W with widths of 30cm are still present on the ground. At Capuluan Central, there is 50 m. wide depression, and its north-eastern side is bounded by the fault.

At Loc. 10, the surface rupture cuts across the settlements of Capuluan Central. The surface rupture location is marked by a narrow linear valley trending N55◦W. Coconut treelines are displaced left laterally at 1.8 m, 1.7 m, 1.6 m, and 1.5 m. There is also a coconut tree that was split by the rupture.

Near the coastline of the Ragay Gulf at Cabong  Norte, there were several eyewitness accounts of the occurrence of a surface break along a small creek. A3.4-m left-lateral offset of beach line and seaward continuation of the rupture on the shallow sea floor were re-ported after the earthquake . The offshore extension of the Guinayangan segment in the Ragay Gulf was mapped by seismic profiling . They identified NW-trending submarine faults thatcut probable Holocene sediments for a distance of 15km from shoreline. Distinct truncation of submarine strata and near-vertical faults suggest that these are predominantly strike-slip faults.

Paleoseismic Trenching

In order to determine the recurrence interval of surface-rupturing earthquakes on the Guinayangan segment of the Philippine fault, we excavated a trench at Capuluan Tulon in March 2011. The trench was excavatedby a backhoe across a gentle southwest-facing scarp with a direction of N40◦E, almost perpendicular to the faulttrace. The trench was 13 m long with the max-imum depth of 2.5 m. The slopes of the trench wallswere greater than 80◦. During the observation period, the southern wall collapsed and we had to abandon the wall. 

At the final stage of the logging, we dug further around the fault zone to examine the deeper deformational struc-tures. We collected shell fragments and bulk humic soil samples for radiocarbon dating. As is true with many cases of paleoseismic trenching in humid tropicalcountries, we were not able to find charcoals at this site.The samples were dated using the AMS method at PaleoLabo Co., Ltd., Japan. At the trench site, rows of coconut trees were systematically off set left-laterally at1.8 m, 1.5 m, 1.6 m, and 1.6 m. 

Stratigraphy

We divided the strata exposed on the trench wall into10 stratigraphic units (unit 10 to unit 90 in descending order) based on lithology, color, and texture. Thebase of our stratigraphic sequence is massive yellowish light-brown clay, unit 90. This unit is exposed only on the upthrown side of the fault zone and contains numerous near-vertical dark-brown stripes, which are probably due to bio turbation. Shell fragments less than 1 cm in diameter are scattered throughout this unit. A shell fragment sampled from the south wall yielded a14C age of 1605±20 yBP (AD 410–535) (Fig. 6, Table 1). Over-lying unit 90 is dark-brown clay, unit 80. 

Log of part of the north wall of the trench at the Capuluan Tulon site. The triangle indicates the location of ashell sample as projected from the south wall. Rectangles indicate where bulk soil samples were taken. Grid interval is1 m.

This is a key horizon to identify south westward warping of strata on the up thrown side. This unit is also exposed only on the up thrown side of the fault and becomes darker or more humic towards the southwest. The upper boundary of this unit is distinct throughout the trench wall, but its lower boundary against unit 90 is gradual and less distinct. We interpret that unit 80 is paleosol developed from unit 90. A bulk soil sample from the uppermost part of unit 80 yielded a 14C age of 830±20 yBP (AD 1170–1260).

Exposed at the base of the trench wall on the down thrown side is 90-cm-thick yellowish pale-brownclay, unit 70. Near the south western edge of the trench, yellowish-brown well-sorted coarse sand, unit 75, is ex-posed. Overlying unit 70 is 20-cm-thick brown clay, unit 60. The upper boundary of this unit is fairly distinct, but its lower boundary against unit 70 is less distinct. We interpret that unit 60 is paleosol developed from unit 70.The upper boundary of unit 60 can be traced northeast to N8, where it seems to be truncated by a fault strand(F4). Unit 50 is yellowish light-brown clay, and its facies are almost the same as those of unit 70. Unit 50 contains thin lenses of yellowish-white clay. Northeast of the fault zone, units 50, 60, and 70 cannot be differentiated. Overlying unit 50 is dark-brown clay, unit 40, which we interpret as paleosol developed from unit 50.Unit 40 can be confidently traced across the fault zone as far northeast as around N8.5. The upper and lower boundaries of unit 40 are irregular, but the thickness of the unit,15 cm, is almost constant. A bulk soil sample from the top most part of unit 40 yielded a 14C age of 815±20 yBP(AD 1185–1265). Unit 30 is light-brown silt to clay with patchy black soil from the upper units probably due to bioturbation. This unit occasionally contains modern co-conut tree roots. Southwest of the fault  zone, unit 30 is almost horizontal and 30–40 cm thick. Across the fault zone, this unit thins out northeastward. Unit 20 is well developed paleosol composed of black to blackish-gray silt to clay characterized by a blocky texture.

Coconut tree roots are abundant down to this unit. This unit exhibits a uniform thickness of  30 cm throughout the trench wall A bulk soil sample from the lowermost part of unit 20 yielded a 14C ageof395±20yBP (AD 1445–1500,1505–1510, 1600–1615). Immediately below the ground surface is modern soil composed of brown silt to clay characterized by a blocky texture, unit 10. It is 50 cm thick at the southwestern edge of the trench and gradually thins to the northeast.

Deformational Features
A distinct shear zone appeared between N7.1 and 8.1with five fault strands, named F1 to F5 . In addition to stratigraphic offsets by the fault strands, the strata are warped into a monocline down to the southwest. These fault strands dip greater than 70◦.The westernmost fault, F1, clearly offsets the top of unit 80 with 16 cm of stratigraphic separation measured Journalof DisasterResearchVol.10No.1,2015 8 along the fault.

The extension of F1 within unit 70 is invisible. F1 does not cut the top of unit 70 and is interpreted to terminate upward within unit 70. F2 cuts all the stratigraphic horizons exposed on the trench wall. The stratigraphic offsets by F2 are 16 cm (top of unit 80),10 cm (top of unit 60), 5 cm (top of unit 50), 5 cm(top of unit 40), 13 cm (top of unit 30), and 13 cm (topof unit 20). The fault is invisible within unit 10, and the ground surface is flat across the fault. F3 is identified based on a10 cm offset of both the top and base of unit 80. Although invisible within unit 70, F3 may merge upward into F2. F4 branches upward from F3 near the trench bottom and dips steeply to the southwest. Unit 80 is sharply offset 20 cm by F4. Neither the top nor the base of unit 60 can be traced across the possible upward extension of F4, suggesting that the unit is truncated byF4. However, unit 40 is not cut by F4, suggesting that F4 terminates upward within unit 50. F5 is the eastern most fault strand. The fault is clearly identified by the off set of unit 80; the lower boundary of unit 80 is offset 20 cm. The thickness of unit 80 changes from 40 cm (southwest)to 30 cm (northeast) across the fault, suggesting horizontal displacement. This strand is also invisible within the over lying strata, and unit 40 is not offset by the fault.  

In addition to the discernible stratigraphicoff sets by the fault strands,the strata are warped into a west-facing monocline. West of the fault zone, all the strata are almost flat where as the strataon the northeast block increase their dip towards the fault zone. 

Paleoseismic Events 

Because of the massive sediments and poor preservation of datable material, the stratigraphy at the Capuluan Tulon site is far from ideal to identify paleoseismic events and determine their ages. Nevertheless, we can point out geologic evidence of past surface rupturing earth quakes and estimate their average recurrence interval.

Event 1

The stratigraphic boundary between units 10and 20 is offset 13 cm up-on-the-northeast by F2, indicating that a surface-rupturing earthquake occurred during (or after) the deposition of modern soil, unit 10.We cannot directly date unit 10, but a bulk soil sample from unit 20 was dated at AD 1445–1615. In the past 400 years for which a written historical earthquake catalogue is available, the 1973 Ragay Gulf earthquake has been the only large earthquake on the Guinayangan segment of the Philippine fault. We therefore interpret that the faulting event marked by the offset of unit 10 corresponds to the 1973 earthquake.

Event 2

East of F2, paleosol of unit 40 dips more steeply than paleosol of unit 20, and unit 30 in between the two paleosol horizons thins out to the northeast. Unlike the other paleosol horizons, unit 20 developed on topof units 30, 40, and undifferentiated units 50–70 .This suggests that unit 30 and the underlying units dipped to the west more steeply than the topographic slope when the paleosol of unit 20 developed. These observations suggest that there was a faulting event that warped unit 30 and the underlying units south westward. The monoclinal scarp was subsequently eroded to form a gentler topographic slope, and the paleosol of unit 20 developed. The timing of this event is after the deposition of unit 30 and before the formation of unit 20 paleosol. The evidence for this event is weaker than that of the other events be-cause there is no fault strand that terminates upward at the proposed event horizon.

Event 3 

F4 appears to truncate units 60 and 70 but does not offset the base of unit 40, suggesting that F4terminates within unit 50. This observation suggests an older event during the deposition of unit 50. During thisevent,F5 may have also moved since it terminates upward within undifferentiated units 50–70.

Event 4

Unit 80 is deformed significantly more than the over lying units; the unit is offset by all five of the fault strands, with70cm of total stratigraphic separation, and it is warped more steeply than unit 40. F1 clearly offsets unit 80 and terminates upward within unit 70. These observations suggest that there was a faulting event during the deposition of unit 70.

Recurrence Interval and Slip Rate

Schematic diagram illustrating calculations for (a) the shortest possible average recurrence inter-val (assuming four paleoseismic events) and (b) thelongest possible recurrence interval (assuming threepaleoseismic events) after the deposition of unit 90,dated at AD 410–535.

Schematic diagram illustrating calculations for (a) the shortest possible average recurrence inter-val (assuming four paleoseismic events) and (b) thelongest possible recurrence interval (assuming threepaleoseismic events) after the deposition of unit 90,dated at AD 410–535.

We identified evidence for three (Events 1, 3, and 4) and possibly four (Events 1–4) faulting events, including the 1973 earthquake, occurring since the deposition of unit 80. A bulk soil sample from unit 80 was dated at AD 1170–1260. However, a bulk soil sample from unit 40 yielded an almost similar age (AD 1185–1265), raising a question as to the reliability of ages from bulk soil samples. Therefore, we used the age of a shell sample from unit 90 (AD 410–535) to calculate the average recurrence interval of the three and possibly four seismic events. The shortest possible average recurrence interval (assuming four paleoseismic events) would be(1973−535)4 ≈ 360 years and the longest possible interval (assuming three paleo-seismic events) would be(1973−410)2 ≈ 780 years These recurrence intervals determined for the Guinayangan segment are close to that of the Digdig fault (i.e., 500–600 years, determined by trenching),which ruptured during the 1990 earthquake. 

The Guinayangan segment may have ruptured with a shorter recurrence interval than those we calculated, because we used the age of unit 90, not unit 80, for the calculation of the average recurrence intervals.We can estimate the slip rate of the Guinayangan segment of the Philippine fault based on coseismic offset during the 1973 earthquake and calculated recurrence intervals. The mean of the four offset measurements of rows of coconut trees at the trench site is 1.6 m.Using the shortest and longest average recurrence intervals above, we estimate the slip rate of 2.1–4.4 mm/yr based on the assumption of characteristic slip.
This rate is significantly smaller than the GPS-derived slip rate of 22±2 mm/yr on Masbate Island  or the 20 mm/yr creep rate derived from offset cultural features on Leyte Island. The slip rate deficit may be caused by one ora combination of the following: 
1) we underestimatedthe recurrence interval due to possible missing paleoseis-mic events within the stratigraphic record at the Capu-luan Tulon site, 
2) earthquakes with larger coseismic dis-placements have occurred in the past, 
3) the fault creepsaseismically in addition to rupturing moderate- to large-earthquakes, similar to what we recently identified on the Masbate segment.
 

Conclusion

In order to better characterize the coseismic and long-term behavior of the Guinayangan segment of the Philippine fault on southern Luzon Island, we conducted geological and paleoseismological studies of the surface ru-ture associated with the 1973 Ragay Gulf earthquake. The earthquake produced a 30-km long surface rupture on land along the fault trace marked by pronounced tectonic geomorphic features. The coseismic slip was predominantly left lateral, and displacements we identified in the field were fairly constant (1–2 m) along the strike of the fault. Paleoseismic trenching at the Capuluan Tulon site exposed stratigraphic evidence for three or possibly four surface rupturing events after the deposition of unit 80.The average recurrence interval was calculated to be between 360 and 780 years, which was close to that for the Digdig fault, the source fault of the 1990 central Luzon earthquake. 

Based on the calculated recurrence intervaland  coseismic offsets during the 1973 earthquake, we estimated the slip rate of the Guinayangan segment to be 2.1–4.4 mm/yr. This geologic slip rate was significantly lower than the geodetic slip and creep rates estimated forthe Philippine fault on Masbate and Leyte Islands. Our paleoseismic data were derived from only one site, so additional trenching is necessary to document the complete faulting history of the Guinayangan segment.