Summary. Kingston, Jamaica, the capital of the Caribbean island nation of Jamaica is prone to infrequent but devastating earthquakes and tsunamis, yet the ...
Assessing Geohazards Near Kingston Jamaica: New Results from Chirp Seismic Imaging Matthew J. Hornbach1, Lyndon Brown2, Paul Mann1, Cliff Frohlich1and Kathy Ellins1, 2 The University of Texas at Austin Institute for Geophysics The Jackson School of Geosciences 2
The University of The West Indies, Mona, Jamaica Earthquake Unit 1. Summary Kingston, Jamaica, the capital of the Caribbean island nation of Jamaica is prone to infrequent but devastating earthquakes and tsunamis, yet the locations of the faults responsible for generating these geohazards are poorly known. During the past few hundred years, at least two earthquakes have triggered severe liquefaction across Kingston and generated tsunami within the Harbor, resulting in significant destruction. The goal of our study is to (1) determine the location of faults in Kingston Harbour, (2) assess whether these faults are active, (3) determine whether harbor faults triggered local tsunami reported during the 1692 and 1907 Earthquakes, and (4) from this, constrain the risk of future geohazards across the region. Here, we focus our analysis on recent results from our January 2011 chirp seismic imaging survey of Kingston Harbour. These data reveal a complex fault system extending across the harbor and evidence for multiple slide and liquefaction events. Using sea-level curves as a means of constraining age, we suggest these faults are active. Our study suggest east Kingston where the Cement Factory, twp power plants, a fuel-depot, Highway A4—the only east-west thoroughfare extending east of Kingston, The Norman Manley Airport, and the town of Port Royal are all prone to significant damage during the next earthquake in this region. 2. Introduction Kingston Jamaica rests precariously at the western terminus of the Enriquillo Plantain Garden Fault (EPGF)—the same fault that ruptured during the January 12th, 2010 Haiti earthquake, destroying Port-au-Prince and killing a quarter-million people (Figure 1 inset). Like Port-au-Prince Haiti, Kingston Jamaica experiences a significant earthquake every few hundred years; however, the exact frequency and location of large earthquakes across Jamaica remains unclear. It has now been more than 100 years since Jamaica was struck by a large (>Mw 6.5) earthquake. Recent GPS studies suggest the EPGF is capable of generating a Mw 7.2-7.3 earthquake—an event with more energy than the Mw 7.0 2010 Haiti event (DeMets and Wiggins-Grandison, 2007). Observations of ground fissures and tsunami formation reported within Kingston Harbour during both the 1692 and 1907 earthquakes indicate seafloor deformation or active faulting occurred within the harbor during these earthquakes. Since the 1907 earthquake, Kingston has undergone significant industrial, urban, and commercial development; much of it on reclaimed land built into Kingston Harbour (Figure 1). Man-made development around Kingston during
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the past few centuries makes it very difficult to assess the location of possible active or blind thrust faults extending below the city. Our study, funded by SEG Geoscientists without Borders, aims to provide scientific information that will help prepare the ~1 million inhabitants of Kingston (and the 2 million additional Jamaicans dependent on Kingston’s welfare) for future earthquakes. Specifically, the primary goals of this study are to determine if active faults exist beneath Kingston Harbour, determine the age and rate of deformation along these faults, assess the source of reported tsunamis that formed in Kingston Harbour during both the 1907 and 1692 earthquakes, and from this constrain future geohazard risks across the region. Working with our colleagues at Jamaica’s Office of Disaster Preparedness and Emergency Management, our study is updating the geohazard profile for Kingston and helping Jamaica prepare for future geohazards. Currently, we are halfway through our two year research campaign. In January, with the help and support of the Coast Guard Division of the Jamaica Defense Force, we completed our initial Chirp sub-bottom imaging field campaign in Kingston Harbour. Here, we present results from our chirp study which reveals evidence of faulting as well as recent slope failure and liquefaction adjacent to the city. Figure 1. . (Inset) Location of Kingston, Jamaica with respect to the Enriquillo Plantain-Garden Fault (Red line). The blue box shows the location of main part of the Figure. (Figure) Google Earth image of Kingston Jamaica. Red lines marks the approximate location of a proposed faults imaged in the harbor. 3. Methods We spent two weeks in January collecting single-channel chirp data in Kingston Harbour using a 3.5 kHz center-frequency portable chirp system developed by Knudsen Engineering. The system was mounted on a small (~18 ft long) Coast Guard vessel operated by the Jamaica Defense Force. With this system, we collect more than 150 kilometers of high quality 2D chirp profiles both inside and outside the harbor (Figure 2A). Depth penetration was excellent at most locations, sometimes exceeding 20 m below the seafloor with subsurface reflection clearly imaged (Figure 2B). 4. Results: Faults in Kingston Harbour Our chirp study reveals a complex network of faults extends south of Kingston. Along several of these faults we observe progressively rotated beds consistent with normal or transtensional faulting. One of these faults runs parallel to Long Mountain and through eastern Kingston. An additional fault extends northeast-southwest through the western third of the harbor and has a strike coincident with shoreline near the airport. This
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Figure 2. (A) Mapview image of Kingston Harbour generated using high resolution Space Shuttle Radar Tomography Mission (STRM) data. Black lines show the location of chirp profiles collected in the harbor. The Green line shows the location of the chirp line in figure 2B. (B) north-south chirp seismic line collected across the eastern corner of Kingston Harbour that shows progressively tilted beds, recent slumping, and liquefaction.
suggests the Palisadoes sand-spit where Norman Manley Airport is built is structurally controlled. Adjacent to these faults, we observe rotated beds at depths less than 25 meters below sea-level. It is therefore likely that these sediments were deposited and rotated within the last 10,000 years when sea level was rising, and therefore, that these faults may be active. These observations support the existence of an active normal fault in the harbor with a strike direction approximately parallel to Long Mountain (Figures 1, 2, 3) and a possibly active transtensional fault within the Harbor.. Thus our study indicates these active faults extend onto land through densely populated or important industrial/commercial areas of eastern Kingston. 5. Results: Evidence for Recent Slumping and Liquifaction in the Harbor Chirp subbottom profiles also clearly reveal evidence of recent slope failure and liquefaction in the Harbor (Figure 2). Slide debris at several sites is not overlain by any sediments, suggesting these slide occurred in the recent past. The location of at least one of these fresh slides is directly adjacent to the coast where a local tsunami was reported during the 1907 Earthquake. We therefore hypothesize that this slide may have generated the reported tsunami. We also observe chaotic seismic reflectors in the subsurface along the southeast half of the harbor, adjacent to the only road to Norman Manley International Airport (Figure 2). This road extends along the narrow Palisadoes sand spit. The chaotic reflectors are adjacent to the road at the base of a steep slope, and their seismic character is similar to chaotic reflectors observed near Port Royal in areas where known liquefaction occurred during both the 1907 and 1692 earthquakes.
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Figure 3. Gridded Bathymetric image showing the depth to the Basement reflector (shown in figure 2B). Sharp contacts in this basement reflector clearly exist. The sharp contact to the northeast is coincident with a clearly visible fault scarp that runs on land in front of Long Mountain. The fault extending NE-SW is coincident with the Palisadoes sand spit that runs adjacent to the Airport.
5. Conclusions Our analysis of recently collected chirp data in Kingston Harbour reveals evidence for geologically recent faulting, slumping, and liquefaction across the harbor. Reactivation of these faults beneath Long Mountain represents a potentially serious geohazard, as they extend through densely populated neighborhoods and industrial areas that rest on alluvium. Our observation of a shallow recently deposited submarine slide in the northeast corner of the harbor supports the idea that at least one historic tsunami (1907) generated in Kingston Harbor was caused by a submarine slide. Our analysis suggests liquefaction and slope failure frequently occurs along the Palisadoes during earthquakes and that eastern Kingston and the Norman Manley Airport may be particularly vulnerable to future earthquakes. Future Coring studies will help place better constraints on the timing and frequency of faulting and slope failure across this region. 6. Acknowledgements. This research was funded by SEG’s Geoscientists Without Borders.
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EDITED REFERENCES Note: This reference list is a copy-edited version of the reference list submitted by the author. Reference lists for the 2011 SEG Technical Program Expanded Abstracts have been copy edited so that references provided with the online metadata for each paper will achieve a high degree of linking to cited sources that appear on the Web. REFERENCES
Calais, E., A. Freed, G. Mattioli, F. Amelung, S. Jónsson, P. Jansma, S.-H. Hong, T. Dixon, C. Prépetit, and R. Momplaisir, 2010, Transpressional rupture of an unmapped fault during the 2010 Haiti earthquake: Nature Geoscience, 3, 794–799, doi:10.1038/ngeo992. DeMets, C., and M. Wiggins-Grandison, 2007, Deformation of Jamaica and motion of the Gonâve microplate from GPS and seismic data: Geophysical Journal International, 168, 362–378, doi:10.1111/j.1365-246X.2006.03236.x. Hayes, G. P., R. W. Briggs, A. Sladen, E. J. Fielding, C. Prentice, K. Hudnut, P. Mann, F. W. Taylor, A. J. Crone, R. Gold, T. Ito, and M. Simons, 2010, Complex rupture during the 12 January 2010 Haiti earthquake: Nature Geoscience, 3, 800–805, doi:10.1038/ngeo977. Hornbach, M. J., N. Braudy, R. W. Briggs, M.-H. Cormier, M. B. Davis, J. B. Diebold, N. Dieudonne, R. Douilly, C. Frohlich, S. P. S. Gulick, H. E. Johnson III, P. Mann, C. McHugh, K. Ryan-Mishkin, C. S. Prentice, L. Seeber, C. C. Sorlien, M. S. Steckler, S. J. Symithe, F. W. Taylor, and J. Templeton, 2010, High tsunami frequency as a result of combined strike-slip faulting and coastal landslides: Nature Geoscience, 3, 783–788, doi:10.1038/ngeo975. Hornbach, M. J., P. Mann, C. Frohlich, K. Ellins, and L. Brown, Assessing geohazards near Kingston Jamaica: Initial results from chirp profiling: The Leading Edge, 30, doi:10.1190/1.3575287. Sloane, H. 1694, A letter from Hans Sloane M.D. and S.R.S. with several accounts of Earthquakes in Peru, October 20th 1687; and at Jamaica, February the 19th, 1688, and June 7th 1692: Philosophical Transactions, 18, Royal Philosophical Society of London, 77–100.
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