Archaeoseismology and Palaeoseismology in the Alpine ... - Tierra

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point was about 80 cm along the shoreline edge with discernable vertical displacement. Some en echelon faults in the deeper portion of the bay about 200 m from the shore were likewise reported by local fishermen. Fig. 4: Photograph (looking WSW) showing the ground rupture produced along the coastline in Suba, Dimasalang. Note the shoreline that was displaced approximately by 0.8m and the subtle uplift/mound in the foreground of the photo About 25 m from the shore, the trace can be followed along the N48 o W trending fault transecting the sandy beach berm. In the beach terrace, the fault also transected two semi‐concrete houses and numerous wooden fences. The wooden fence was displaced horizontally and vertically (northwest block up) by 20cm and 18cm, respectively ‐ Palanas area In Sta. Cruz, Palanas, the ground rupture also displaced several man‐made structures including semi‐concrete to concrete houses, wooden fences, rice paddy dikes and roads and foot trails. Inside one of the gardens, a displaced foot trail (20 cm horizontal displacement) was observed with well‐formed mole track. Moreover, a semi‐concrete house and a wooden house in the same village were transected by the fault. Southwards and into Nabangig, Palanas in Sitio Matugnaw, a foot trail was transected by the ground rupture. The trail was horizontally displaced by 45 cm with negligible vertical displacement (site r in Fig. 3). In this area, the en echelon faults have a N60 o W trend while the ground rupture is trending N40 o W and about 10 cm wide. Across the creek, a small hill is transected by the ground rupture wherein the fault trace is well manifested by right‐stepping en echelon fractures and mole tracks with an average length of 150 cm (Fig. 5) located between sites r and s in Fig. 3. Fig. 5 also shows a coconut tree that was transected by one of the en echelon. It was noted that the surrounding geomorphology in Matugnaw area is indicative of a fault saddle. However, since there was no manmade or cultural structure transected, there was no horizontal measurement obtained. ‐ Cataingan area Throughout the valley and up to the ridge between Sitio Bogtong and Sito Salag, Pawican of Cataingan municipality, the trace was also observed right‐ stepping tensional gashes. In Sitio Salag, the trace approaches the upstream area of Nabangig River. The 1 st INQUA‐IGCP‐567 International Workshop on Earthquake Archaeology and Palaeoseismology 10 Nabangig River has a very deep gulley and thick vegetation preventing the team to investigate the river vicinities. After crossing the Nabangig River, the trace was found along a pre‐existing scarp approximately 150cm high. A fault outcrop was likewise found along the riverbank of Nabangig River. The fault trace was found again across the asphalt national highway in Gahit, Cataingan about 1km south of Gahit Bridge. Interestingly, this trace widened and became more evident after several weeks. During initial investigation on February 19 th , 2003, the surface rupture in this area was just a hairline crack. However, after two weeks the fissure in this road have widened to about 0.8cm along an EW direction with differential horizontal displacement of about 3cm. Moreover, the en echelons became more and apparent along the asphalt road. Fig. 5: The ground rupture in Matugnaw, Palanas manifested as mole tracks and tensional gashes on grass‐covered hill also cutting through the coconut tree. ‐ Bogtong Island Based on the projection of the trace from San Pedro, Cataingan municipality, the Island of Bogtong (Fig. 1) was likewise investigated to check the effects of the February 2003 earthquake and the possibility that the ground rupture transects the island. There was no clear indication of any presence of a fresh ground rupture based on the investigation and accounts by the residents. However, in the eastern side of the island, the limestone hill has a linear flat area. This feature is most probably a fault scarp considering the trend and location of the trace of PFZ in Masbate Island.
DISCUSSION Based on detailed field mapping, the February 15, 2003 ground rupture has a classic manifestation of left‐ lateral right‐stepping en echelon faults. From Suba in Dimasalang down to San Pedro in Cataingan, ground rupture features are dominantly right‐stepping en echelon faults along 34 o NNW orientation with well‐ formed mole tracks in some areas. It has a general trend of N30 o W to N60 o W that followed the pre‐ existing active fault trace (with deviation in some places) associated with shear stress and reflective of the type of local materials. Maximum horizontal displacement is 163cm with an average of 40cm. The fissures and ground rupture were obviously wider northwards approaching the epicentral area offshore to the west of Magcaraguit Island. Except for the observed maximum horizontal displacement in Nabangig, the horizontal displacement southeastwards and, the size of each en echelon fractures as well as the width of fault zone from Dimasalang to Cataingan has an apparent decreasing trend. The total length of the mapped onshore ground rupture is approximately 23 km (Fig. 3). If the distribution of aftershocks would suggest to define the extent of faulting for large earthquakes (Wilson, 1936; Benioff, 1955) as well small events (Buwalda and St. Amand, 1955; Richter, 1955; Brown and Vedder; 1967; McEvilly et al., 1967; Liebermann and Pomeroy, 1970), then the aftershock distribution of the February 2003 event would roughly delineate a rupture length of approximately 54km. More than half of the rupture is apparently offshore. In terms of the relationship between the length of ground rupture and earthquake magnitude (Bonilla et al., 1984; Wells and Coppersmith, 1994), assuming the February 2003 ground rupture is symmetrical relative to its epicenter, the total ground rupture length would likewise be around 54 km. If such number is considered, the total magnitude for the 2003 event is M7.1 using the empirical relations established by Wells and Coppersmith (1994). This magnitude is much greater than the calculated magnitude both by PHIVOLCS (2003) and USGS (2003) of M6.2. Mapping the ground rupture and correlating it to seismicity evidently show that the Masbate Fault could produce major earthquakes. Furthermore, through the detailed mapping of the 2003 event, the future ground rupture hazards can be minimized along the Masbate fault, if not be avoided totally. Considering the width of the fault zone, structures to be built in the future is recommended to be constructed at least 2m away from the trace. For high risk and extremely costly structures like nuclear reactors or dams, however, at least 20m should be considered as the setback distance taking into account the observed deviations of the ground rupture from the pre‐existing active fault trace. Acknowledgements: We are indebted to the following institutions and individuals for the coordination, help and support during the mapping activities: The provincial and 1 st INQUA‐IGCP‐567 International Workshop on Earthquake Archaeology and Palaeoseismology 11 municipal government of Masbate Island and their constituents. This work was partly supported by the Philippine National Disaster Coordinating Council Calamity Fund 2003 and the Seafloor Geodesy Project of Nagoya University References Allen, C.R., (1962). Circum Pacific faulting in the Philippines‐ Taiwan region, J. Geophys. Res. 85, 3239‐3250. Arante, R.A., Besana, G.M., Dela Cruz, R., Lumbang, R., Maximo, R.P.R., Papiona, K.L., Peñarubia, H., Punongbayan, B.J.T., and Torrevillas, L. (2003). Preliminary Report: 15 February 2003 Masbate Earthquake Investigation. PHIVOLCS Internal Report, 1‐9. Barrier, E., Huchon, P., Aurelio M. (1991). Philippine Fault: A key for Philippine kinematics. Geology, 19, 32‐35. Benioff, H. (1955). Mechanism and strain characteristics of the White Wolf fault as indicated by the aftershock sequence. Bull. 171. Calif. Div. Mines, 199‐202. Besana, G.M. and M. Ando (2005) The central Philippine Fault Zone: location of great earthquakes, slow events, and creep activity. EPS, 57, pp.1‐8. Besana, G.M., Punongbayan, B.J.T., Peñarubia, H., Maximo, R.P.R., Arante, R.A., Papiona, K.L., Torrevillas, L., Dela Cruz, R. and Lumbang, R. (2003). The 15 February 2003 Masbate Earthquake, PHIVOLCS Quick Response Team (QRT) Special Report No.5, 1‐28. Bonilla, M.G., Mark, R.K., and Lienkaemper, J.J. (1984). Statistical relations among earthquake magnitude, surface rupture length, and surface fault displacement. Bull. Seis. Soc. Am., 74, 2379‐2411. Brown, R.D., Jr. and Vedder, J.G. (1967). The Parkfield‐ Cholame, California earthquakes of June‐August 1966; surface tectonic fractures along the San Andreas fault, U.S. Geol. Survey Prof. Paper 579 pages. Buwalda, J.P. and St. Amand, P. (1955). Geological effects of the Arvin‐Tehachapi earthquake, Bull. 171, Calif. State Div. Mines, 41‐56. Fitch, T.J. (1972). Plate convergence, transcurrent faults and internal deformation adjacent to Southeast Asia and the Western Pacific. Jour. Geophys. Res. 77, 23, 4432‐4460. Liebermann, R.B. and Pomeroy, P.W. (1970). Source dimensions of small earthquakes as determined from the size of the aftershock zone. McEvilly, T.V., Bakun, W.H. and Casaday, K.B. (1967). The Parkfield, California earthquake of 1966. Bull. Seis. Soc. Am., 57. 1221‐1244. PHIVOLCS (1999). Historical Earthquakes in the Philippines, PHIVOLCS, Quezon City, 1:5,000,000 map. PHIVOLCS (2000). Distribution of active faults and trenches in the Philippines, Active Faults Mapping Group, PHIVOLCS, Quezon City, 1:2,000,000 map. PHIVOLCS (2003). Preliminary earthquake bulletin no. 2, PHIVOLCS, Quezon City, 1. Rowlett, H. and Kelleher, J.A. (1976). Evolving seismic and tectonic patterns along the Western margin of the Philippine Sea Plate, Jour. Geophys. Res., 81, 3518‐3524. Wells, D.L. and Coppersmith, K.J. (1994). New empirical relations among magnitude, rupture length, rupture area, and surface displacement. Bull. Seis. Soc. Am., 84, 974‐ 1002. Wessel, P., and W.H.F. Smith (1998). New, improved version of generic mapping tools released, EOS Trans. AGU 79(47), 579, 1998. Willis, B. (1937). Geologic observation in the Philippine archipelago, Nat. Resources Council Bull. Manila, Philippines, 13. Wilson, J.T. (1936). Foreshocks and aftershocks of the Nevada earthquake of December 20, 1932, and the Parkfield earthquake of June, 1934. Bull. Seis. Soc. Am., 26, 189‐194.
Page 1: Abstracts Volume Archaeoseismology
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Fig. 2: Geological map of the study
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1 st INQUA‐IGCP‐567 Internation
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however, the massive 1995 earthquak
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mapped on the slopes above Qiryat
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interpreted as either an expression
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The Shepran Cave is located on the
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properties. The mean value of speci
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excavated a deep trench, thus provi
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Fig. 8: Seismic ground shaking may
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elevant qualitative criteria are po
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CONCLUDING REMARKS The exposed exam
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produced by significantly different
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It is possible that a portion of th
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In addition, the Database of histor
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were published by IGME (1983) and E
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The city collapsed and it was aband
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Fig. 4: Slipped lintel in a window
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to overcome shear resistance and in
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Newmark, N.M. (1965). Effects of ea
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associated with various tsunamis (<
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this method, the magnitude of the e
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etween pieces. Fragments have not f
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on the exact date of the earthquake
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THE ARCHAEOLOGICAL ZONE COLOGNE Aft
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affecting the Alcázar was about 50
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archaeoseismology could instead bec
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In the area later occupied by Vilni
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Fig. 2: Assemblage of landforms ass
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Fig. 8: Data and regression for mag
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Fig. 3: Relationships among magnitu
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CONCLUSIONS 1. Spatial distribution
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The stratigraphic sequence (Fig. 4)
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especially in the zones of hanging
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interpretations are based on relati
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Fig. 5: Gray‐scale value laser im
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(~40 cm), F‐ rotated building str
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Index 1 st INQUA‐IGCP‐567 Inter
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