The Letter of a White Giant

To the people of Guinayangan Quezon
My name is Earl Carter, the proud son of Guinayangan-native Rosalinda Manalo Carter.  A few months ago I was fortunate enough to be able to visit Guinayangan for the first time with my mother. My only regret is that I didn't visit sooner, also that I wasn't able to stay longer.  It is such a beautiful place, a great experience for me, the one that I will remember forever and hopefully I will enjoy it again. 

I would like to use this as an opportunity to thank everyone in Guinayangan, especially to my family and newfound friends. I thank them for their hospitality, for being so friendly and courteous, and for welcoming me in as one of their own.  Some of the local kids even gave me a new nickname:  "White Giant" and "American Giant". My friends here in the U.S. have decided to make the nickname permanent--they thank the kids for that one. 

While walking near a fishport with my mother, a group of young children started following us, they were curious and giggling. They are amazed at how white my legs (my father is of Irish descent and my complexion is very light) and how big my hands and feet were.  Some of them even measured their hands against mine and then laughed, which made me and my mother laughed too.

 I got the chance to meet all my cousins and aunts and other relatives. Sadly I did not make it there in time to meet my grandmother, Lucia Manalo, who was brought home to heaven before I arrived.  I enjoyed the conversations I had with my cousins William, Wilson, Rona and Rochelle, as well as Rochelle's boyfriend Christian.  I thank all of them for their excellent English, since I haven't learned Tagalog yet. Gladly there were no nosebleeding incident happened.

I got the chance to play basketball with my cousin William and Christian and many of the local kids at the basketball court. I am so amazed at how some of the guys were able to play in slippers or even barefoot. They were very fast and agile too. 

I saw my family's coconut farm at Brgy.Dungawan and met friends of the family there. I met a young boy named Martin, who enjoyed coloring the Ninjago drawings that I drew for him.  My mother and I also took a boat ride to the lighthouse built in an island called Parola. My younger brother Neil also visited this place when he was a young boy. He loved swimming there with all the friends he made while visiting Guinayangan. 

We had a family picnic at the Salacan Resort. I enjoyed delicious food cooked by my Tita Norma.  We enjoy the nature and the swimming pool. I shot a lot of family pictures to keep.

I remember when I first arrived in this lovely town; it was about 4:30 in the morning. I was able to take pictures of a beautiful sunrise in a place called Bayside Inn.  Those were the first of many pictures I would take throughout the town during my visit.  Hopefully someday I can return again with my mom, my brother Neil, my sister Joy, and her son Joey (Joey would love to meet his cousin Jason).  In fact, my brother was jealous that I was able to visit Guinayangan  as an adult. Maybe because he was only five years old then when he visited this place, not old enough to really remember the whole experience.

My brother Neil (5 years old then) during his visit in Guinayangan.

I also enjoyed riding the tricycles all over town, even up the hills to the coconut farm.  I met my Tita Lilia and my cousins Bubut, Lengleng, Lala, and Boyet.  I also met many of my mother's friends and their families.  And everyone was so nice to me, welcoming me into their homes and offering all types of delicious goodies.

I saw my Lola's house where my mother was raised in Brgy.Manlayo.  I was there when the schoolchildren went back to class in June. I watched as my Tita Baby supplied ice water and snacks to the kids.  I gave some of the kids some chocolate candies that I brought with me, and they have requested that I bring more when I return.  They watched with curiosity as I made little paper throwing stars for them. I think I made at least 20 of them---they were very much in demand.

 Soon I will return on this lovely place---so please tell the kids that the "White Giant" will return . . .  bringing more chocolates and paper throwing stars. 

Earl Carter

Traysikel Republic

My Ancestors Home Sweet Home

Notice the serenity of the ocean. Here, they're still using basnig for fishing like they used to in Manlayo.

Bumalik sa Labac Naic Cavite ang karamihan ng aking kamaganak noong wala ng mahuling isda at iba pang Marine Life sa atin. Nagbaligtad naman ang naging buhay sa Manlayo. Bumalik sila sa Cavite at naswertihan naman nila ang dagat doon.

Noong early 70's sila lumipat doon para mangisda. Marami din sa kanila ang nakapagpaaral ng mga anak sa College. Yong iba nilang anak ay nakapag abroad din at nakatapos ng Nursing or BSN. Karamihan sa mga kamaganak ko na bumalik sa pinanggalingan ng aming mga Mahal na Ninuno ay yong mga nawalan ng bahay at mga bangkang pang hanapbuhay. Yan ngayon ang nangyari sa mga kamaganak ko na pinalad naman na makaahon sa hirap noon sa Manlayo.

Ang pag babalik nila sa Labac Naic Cavite ang nagbigay naman ng magandang kapalaran sa kanilang buhay. Yan din ang nakakalungkot ngayon sa atin. Nawala ang mga Marine Life sa ginawang pangaabuso ng mga mangingisda na gumamit ng dinamita. Ang ating mga Coral Reefs ay namatay kasama ang mga itlog ng isda , alimasag, hipon at halos lahat ng buhay sa karagatan ay namatay. Sana'y wala ng mangabuso pa ngayon na unti unti na uling bumabalik ang kayamanan ng ating Mahal ba Dagat.

Pakiusap ko lamang sa ating nga Mahal na Kababayan at Kamaganak na panatilihing Malinis at Maganda ang ating paligid para sa atin na ring kabutihan. Maraming maraming salamat sa inyong lahat dyan na walang patid ang pagtutulong tulong sa ikauunlad ng ating Mahal na Bayan .

 narrated by: Ms. Rosie Carter

republished from: Sa Tabing Dagat 
 (with permission by the author) 

Young Guinayangan 2

Coseismic Displacement and Recurrence Interval of the 1973 Ragay Gulf Earthquake

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).
Although the Philippine fault is one of the fastest-slipping faults on the earth, surface-rupturing earth-quakes on the fault are relatively rare, with only three such events having occurred in the past 50 years, includ-ing the 1973-7.0 Ragay Gulf, 1990-7.7 Central Luzon, and 2003-6.2 Masbate earthquakes.

It is therefore important to document and analyze each sur-face rupture in as much detail as possible. 
The 1990 Central Luzon earthquake was the first earthquake alongthe Philippine fault for which systematic and detailed sur-face rupture mapping was conducted. Detailed sur-face rupture mapping was also conducted after the 2003 Masbate earthquake. 

Although the 1973 Ragay Gulf earthquake occurred only about 40yearsago, there was no detailed description of the surface rupture. General seis-mological and geological characteristics of the earthquake were reported, but a detailed description of the surfacerupture was not provided; coseismic displacements were reported only from three localities along the 30km long on shore surface rupture.
There is also no information on the history of past surface-rupturing earthquakes on the Guinayangan segment of the Philippine fault that was re-activated in 1973. In orderto better evaluatethe seismic risk of the Philippine fault, we have been conducting tectonic geomorphic mapping and paleoseismic trenching along the different segments from Luzon to Mindanao Islands. Since2009, we have surveyed the surface rupture of the 1973 Ragay Gulf earthquake to better characterize its locationand coseismic displacements. We have also excavated apaleoseismic trench across the surface rupture to obtaingeologic evidence of multiple earthquakes in the past.In this paper,we describe the results of the segeological and paleoseismological studies on the surface rupture associated with the 1973 Ragay Gulf earthquake. We show the location and coseismic displacements of the surface rupture in detail, and we discuss the average recurrenceinterval of large earthquakes on this portion of the Philippine fault. This is a companion paper of an offshore faultmappingin the RagayGulf basedon seismic profiling [6].These two papers present a prototype of geological stud-ies of the Philippine fault, about half of which is located underwater.

The 1973 Ragay Gulf Earthquake

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.
The 7.0 Ragay Gulf earthquake of March 17, 1973 claimed 14 lives and caused extensive damage to southern Quezon Province on the Bondoc Peninsula. Although the seismic stations in the Philippines were sparse at that time, the epicentre is thought to have been located at approximately 13.4◦N and 122.8◦E in the Ragay Gulf. The earthquake was felt on most of Luzon Island and the northern Visayas region. Intensity VIII on the Rossi-Forel scale was assigned to the municipalities of Calauag, Lopez, and Guinayangan. The earth-quake caused serious damage to houses, concrete buildings, roads, bridges, and railways.A surface rupture appeared on land along the Guinayangan segment of the Philippine fault. The rupture cuts across the northeastern part of the Bondoc Peninsula from the Ragay Gulf shoreline north-westward to the Calauag Bay shoreline, a distance of 30 km.

For the most part, the rupture appeared along the fault trace previously identified from aerial photograph interpretations. Surface faulting was predominantly left lateral without conspicuous or systematic vertical movement. The observed horizontal displacements from three localities ranged from 1.1 to3.4 m. In many places, the surface rupture exhibited mole track features with ground fissures arranged en echelon in an E–W direction, which is typical for left-lateral faults and similar to the 1990 central Luzon earthquake rupture. The aftershock distribution within eight days following the main shock were elongated along a 250-km-long section of the Philippine fault.

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.

Location and Displacements  of Surface Rupture 

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. 


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.

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.

Coseismic Displacement and Recurrence Interval of the 1973 Ragay Gulf Earthquake
Southern Luzon, Philippines

Hiroyuki Tsutsumi
-Department of Geophysics, Kyoto University Kitashirakawa-oiwake-cho, Sakyo-ku, Kyoto 606-8502, JapanE-mail: 

Jeffrey S. Perez, Kathleen L. Papiona, Jaime U. Marjes -Philippine Institute of Volcanology and Seismology (PHIVOLCS)C. P. Garcia Avenue, Quezon City 1101, Philippines 
Noelynna T. Ramos -National Institute of Geological Sciences, University of the Philippines, Diliman C. P. Garcia Avenue, Quezon City 1101, Philippines


We thank M. S. Begonia and R. Unating of PHIVOLCS for their logistical support. 
We are grateful to local administrative officialsand residents for sharing their information on surface ruptures. 
The Municipality of Guinayangan kindly let us use a backhoe, which made the paleoseismic investigation possible.  
We thanJosepito Cleofe, the owner of the land at the Capuluan Tulon trench site, for allowing us to conduct the excavation survey
This work was supported by a MEXT/JSPSGrant-in-Aid to Hiroyuki Tsutsumi and by the JICA-JST project “Enhancement of earthquake and volcano monitoring and effective utilization of disaster mitigation information in the Philippines.”

1. C. R. Allen, “Circum-Pacific faulting in the Philippines-Taiwan re-gion,” Journal of Geophysical Research, Vol.67, pp. 4795-4812,1962.
2. T. Nakata, H. Tsutsumi, R. S. Punongbayan, R. E. Rimando, J.A. Daligdig, A. S. Daag, and G. M. Besana, “Surface fault rup-tures of the 1990 Luzon earthquake, Philippines,” Research Centerfor Regional Geography, Hiroshima University, Special PublicationNo.25, p. 86, 1996.
3. PHIVOLCS Quick Response Team, “The 15 February 2003 Mas-bate Earthquake,” PHILVOCS Special Report No.5, p. 30, 2003.
4. E. M. Morante, “The Ragay Gulf earthquake of March 17, 1973,southern Luzon, Philippines,” Journal of the Geological Society of the Philippines, Vol.28, No.2, pp. 1-31, 1974.
5. H. Tsutsumi and J. S. Perez, “Large-scale active fault map of thePhilippine fault based on aerial photograph interpretation,” ActiveFault Research, No.39, pp. 29-37, 2013.
6. H. Yasuda, T. Bacolcol, A. Daag, M. Cahulogan, E. Bariso, E. Mi-tiam, J. Marjes, T. Ueki, T. Kobayashi, and T. Nakata, “Geometryand structure of the Philippine Fault in Ragay Gulf, southern Lu-zon,” Journal of Disaster Research, Vol.10, No.1, 2015 (this issue).
7.  M. L. P. Bautista and K. Oike, “Estimation of the magnitudes andepicenters of Philippine historical earthquakes,” Tectonophysics,Vol.317, pp. 137-169, 2000.
8. H. Tsutsumi, J. A. Daligdig, H. Goto, N. M. Tungol, H. Kondo,T. Nakata, M. Okuno, and N. Sugito, “Timing of surface-rupturingearthquakes on the Philippine fault zone in central Luzon Island,Philippines,” EOS Transactions AGU, Vol.87, p. 52, 2006.
9. T.Bacolcol, E.Barrier, T.Duquesnoy, A. Aguilar, R. Jorgio, R. de laCruz, and M. Lasala, “GPS constraints on Philippine fault slip ratein Masbate Island, central Philippines,” Journal of the GeologicalSociety of the Philippines, Vol.60, No.1-2, pp. 1-7, 2005

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