Archaeoseismology and Palaeoseismology in the Alpine ... - Tierra

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1 st INQUA‐IGCP‐567 International Workshop on Earthquake Archaeology and Palaeoseismology COSEISMIC OFFSET OF THE CAMINO DE CRUCES CONFIRMS THE PEDRO MIGUEL FAULT AS THE CAUSE OF THE AD 1621 PANAMÁ VIEJO EARTHQUAKE 32 E.M. Gath (1) and T.K. Rockwell (1) (1) Earth Consultants International, 1642 E. Fourth Street, Santa Ana, CA 92701 USA. gath@earthconsultants.com Abstract: We completed a study of the Pedro Miguel fault where it crosses the ca 1520 Camino de Cruces in Central Panama. Using tectonic geomorphic mapping, stream bank exposures, and hand‐excavated trenches, we have demonstrated that the fault has generated a 2.8‐3.0 meter displacement of the Trail, as well as of all fault‐crossing geomorphic features in the local area. A unique cobblestone pavement used for the Trail, combined with the geomorphology, makes pre‐ and post‐event reconstruction of fault slip highly accurate. Fault exposures demonstrate that the youngest alluvial deposits are offset, and the fault projects into the surface topsoil. We conclude that the Pedro Miguel fault last ruptured May 2, 1621 and caused the devastating Panamá Viejo earthquake. Key words: Panamá, paleoseismic, archeoseismic, Cruces As part of the seismic hazard investigation for the Panamá Canal Expansion Project’s design studies, we completed detailed paleoseismic investigations of the Pedro Miguel, and several other faults (Fig. 1). Our studies of the presumed inactive, but unexplored, Pedro Miguel fault involved logging of over 55 trenches, including three locations where we excavated the fault in 3‐D to determine slip and timing in past earthquakes, and fault slip kinematics. These studies revealed the fault as having experienced at least three, 2‐3 meter displacement events within the last 1500 years. As such, the fault not only poses a shaking hazard to the Panamá Canal structures, but it also crosses through the proposed footprint of Borinquen Dam. This new earthen embankment dam is 7 km long and 10.6 meters high, is segmented into four parts, and will contain the new approach channel from Gatún Lake to the new Pacific set of locks. One key piece of data needed for the seismic hazard model was the age of the last displacement event. Based on the apparent youthfulness of offset soils and surface features, we proposed that the latest rupture of the Pedro Miguel fault might have been the earthquake that severely damaged many of the stone and adobe buildings in Panamá Viejo on May 2, 1621. The Panamá region has had other earthquakes between 1535 and 1850 (including in 1541, 1750, 1799 or 1800, 1849? and 1855), but only the historically recorded earthquake in 1621 seems strong enough to correlate with the measured surface fault displacement. However, extensive reworking of charcoal in the fluvial sediments prevented us from definitively proving a historical rupture within the principal trenching area. Fortunately, the Camino de Cruces, a 1.5 meter wide, cobblestone paved trail (Fig. 2), was built from Panamá Viejo to the town of Cruces on the Chagres River, near the present town of Gamboa, in the early 1500s to transport goods via mule pack trains. From Cruces, the goods and passengers would be loaded onto boats that navigated the Chagres River to its mouth by Fort San Lorenzo. These pack trains consisted of hundreds and even thousands of Cruces # # Site Panama Viejo # Fig. 1: Active fault map of Central Panamá showing the study site (modified from ECI, 2009). mules. During the California Gold Rush, thousands of “49’ers” used the Trail to traverse the Isthmus. It was in nearly continuous use until the mid‐ 1800s when the Panamá Railroad was completed. The Trail crosses the northern extent of the Pedro Miguel fault and provides an excellent archeoseismic piercing line to determine whether the Pedro Miguel fault ruptured in 1621. We conducted a detailed geologic and geomorphic field study of this area to prove whether or not the Trail is offset at the fault, which would be a key factor in general acceptance of the Pedro Miguel fault as active, and its role in generating the 1621 earthquake.
The site reconnaissance successfully located the Pedro Miguel fault where it crosses the Trail (Fig. 3). At this location, the Trail is abruptly terminated into a projecting sidewall of the stream terrace riser along the toe of which the Trail had been constructed. During the site investigation, we made several other observations that support the theory of a youthful fault offset of the Trail. These observations are listed below from south to north, and illustrated on Fig. 4: The rebuilt Trail immediately adjacent to the offset had been relocated to the east and north to smooth out the abrupt 3‐meter, 90‐degree jog in the Trail. The Trail followed along on the first elevated terrace of the stream, and the terrace tread/canyon wall riser is offset 3‐4 meters. Detailed surveying of the Trail cobbles reveals a segment of the old Trail abruptly severed at the fault, with remnant Trail cobbles preserved under fault‐scarp‐derived colluvium. Right at the fault, the stream is diverted right‐ laterally over 30 m, forming an incised horseshoe meander. The south‐facing stream cut exposes several soil‐ filled fault fissures that flare to the surface. The terrace formed between the river meanders has a low (
Page 1: Abstracts Volume Archaeoseismology
Page 4 and 5: Edita e imprime: Sección de Public
Page 7 and 8: 1 st INQUA‐IGCP‐567 Internation
Page 9 and 10: the importance of performing absolu
Page 11 and 12: indicative of strike‐slip faultin
Page 13 and 14: DISCUSSION Based on detailed field
Page 15 and 16: Table 1: Source parameters used in
Page 17 and 18: elations. However, the attenuation
Page 19 and 20: mostly concentrated on the DST in I
Page 21 and 22: continuous support. N. Shalev, G. G
Page 23 and 24: The tumulus initial morphology show
Page 27 and 28: our understanding and prediction of
Page 29 and 30: spans, with a threefold increase fr
Page 31 and 32: Nevertheless the lack of research,
Page 33: earthquake. In uncertain cases, as
Page 43 and 44: Table 1: Surface faulting extent re
Page 47 and 48: The graben has a markedly asymmetri
Page 53 and 54: are a valuable addition to the rout
Page 55 and 56: to 400 m) the calculated values for
Page 57 and 58: RISK ANALYSIS The created hazard ma
Page 59 and 60: 290 m water depth the basin accumul
Page 61 and 62: CONCLUSIONS From the palaeostress a
Page 63 and 64: Fig. 2: Geological map of the study
Page 71 and 72: however, the massive 1995 earthquak
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Page 79 and 80: The Shepran Cave is located on the
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1 st INQUA‐IGCP‐567 Internation
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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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Archaeoseismology and Palaeoseismology in the Alpine ... - Tierra

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