Research paperThe 1900 Mw 7.6 earthquake offshore north–central Venezuela: Is La Tortuga or San Sebastián the source fault?
Introduction
The large and destructive October 29th, 1900 earthquake struck a large region of continental north–central Venezuela, and some leeward Antilles islands lying north of that coast, early that morning, just before dawn, around 4:30–4:45 am local time (Centeno Graü, 1949) or 09:11 am GMT (Pacheco and Sykes, 1992). Its intensity center has been placed offshore (e.g., Centeno Graü, 1949, Fiedler, 1961, Jacubowicz and Larotta, 1974, Lugo, 1984, Grases, 1990, Pacheco and Sykes, 1992, Grases et al., 1999, Audemard, 2002, Hernández, 2009a, Hernández, 2009b, Hernández and Schmitz, 2010, Fig 1). Since the active tectonic framework offshore this region of Venezuela is still poorly known (see Audemard et al., 2000; as a reference), regardless of the significant past efforts of acoustic seismic imaging of the continental shelf and offshore regions in central Venezuela carried out by Schubert (1982) and Schubert and Krause (1984), a debate about the source of this offshore event at the brink between historical and instrumental seismicity, has grown with time. In spite of being the largest crustal earthquake in Venezuela, being its magnitude estimated from two far-distant seismograms (at Pasadena station-California, USA and Kew Observatory-England; the first two instrumental records of their kind for any given Venezuelan earthquake, after Fiedler, 1988), at Ms = 7.7 (Abe and Noguchi, 1983) and Mw = 7.6 (Pacheco and Sykes, 1992), Hernández and Schmitz (2010) from a reassessment of the macroseismic intensities for several historical earthquakes affecting Caracas (1641, 1766, 1812, 1900 and 1967 AD), conclude that the 1900 event reached a M = 7.7 and thus must lie farther away than the smaller (M = 7.5) 1812 event because the latter one happens to be the most destructive for Caracas in historical and instrumental times. In that sense, Hernández, 2009a, Hernández, 2009b and Hernández and Schmitz (2010) assigned the 1900 earthquake to La Tortuga fault (LTF), thus validating the intensity centers proposed by Centeno Graü, 1949, Fiedler, 1961, Jacubowicz and Larotta (1974) and Grases (1990), as well as the instrumental relocation obtained by Pacheco and Sykes (1992), placing the event at 11° N and 66° W, west of La Tortuga island. On the contrary, Lugo (1984), after reassessing the macroseismic intensity data for that event, pulls it closer to both Caracas and shore (Fig. 1), adscribing it to the San Sebastián fault (SSF). Audemard (2002) does alike from applying the concept of seismic gap to the Venezuelan catalog of instrumental seismicity 1910–April 2002. Instead of providing an epicenter, this author proposes the extent of the surface rupture that may have generated the 1900 event. This rupture would have largely filled the gap along the easternmost portion of the SSF, lying between the ruptures of the 1812 earthquake to the west and the 1853 event on the El Pilar fault (EPF) to the east. The latter event epicenter should have been on the EPF segment bordering the Cariaco trough on the south Audemard, 2002, Audemard, 2007. This contribution pretends to bring additional supporting evidence to a source on the SSF for the 1900 earthquake, as well as arguments against a source on the LTF.
Section snippets
Tectonic framework
Northern Venezuela lies in the interaction zone between the South America and Caribbean Plate, while western Venezuela and northern Colombia cover several interplaying tectonic blocks or microplates (refer to Audemard, 2014 for further details; Fig. 2). The Caribbean-South America Plate boundary from Colombia to Trinidad is over a 100-km-wide active compressional right-lateral strike-slip zone onshore and offshore northern Venezuela (Audemard, 1993, Singer and Audemard, 1997, Pindell et al.,
Offshore active tectonics
The tectonic structure and style of the Cariaco trough, an active pull-apart basin at the dextral stepover between the right-lateral strike-slip SSF and EPF (e.g., Schubert, 1982, Audemard et al., 2000, Escalona et al., 2011), and surrounding shelves (e.g., Morelock et al., 1972, Espinosa and Daza, 1985, Goddard, 1986) have deserved considerable attention over the years. Conversely, the submarine extent of the Moron Fault Zone –MFZ-, as defined by Schubert and Krause (1984), has received little
High-resolution shallow seismic profiling
The shallow seismic data used in this study were collected during the VARgas Seismic (VARS) and LIToral (LIT) expeditions, which took place in September and October 2007, respectively, onboard of R/V Guaiqueri II, the research vessel of the Oceanographic Institute of the Universidad de Oriente in Cumaná. A first-order grid of high-resolution seismic reflection profiles (Fig. 5) was shot with a RCMG's “Centipede” multi-electrode sparker operated at 300 or 400 J. The sparker produces a
The San Sebastian fault (SSF)
As mentioned earlier, the SSF essentially extends offshore along and near the coastline of North–central Venezuela, except for a very short stretch along the southern foothill of Cerro Machado at the International Airport Simón Bolívar at San Sebastián de Maiquetía. Here the fault cuts outcropping Pliocene and Pleistocene marine sediments, which were described by Weisbord (1957) and Gibson-Smith (1971). This is by far the most conspicuous geologic evidence of the Quaternary activity of the SSF.
The La Tortuga fault (LTF)
La Tortuga fault (LTF) takes its name from the island and bank (platform or rise) of equal name, located at the edge of the northern rim of the Cariaco trough or basin. This roughly east–west striking fault bounds the La Tortuga platform on its southern side. It is as well the bounding fault of the northernmost part of the Cariaco basin. Using a small set of N–S trending acoustic lines, Schubert and Krause (1984) extends it to the west for over 220 km until connecting it to the Oca-Chirinos
Discussion
The pioneering exploration of Schubert by acoustic profiling, among other methods and explorers, revealed the occurrence of Quaternary deformation affecting offshore regions in North–central Venezuela. From previous works and our own investigations, we can confidently state that the San Sebastián fault (SSF) is a prominent structural feature offshore North–central Venezuela, regardless of the depth of investigation considered. This fault can be identified from seismic velocity modeling, such as
Acknowledgments
We are thankful to FUNVISIS for its long-lived unconditional support, to Universidad de Oriente (UDO) and CAMUDOCA for providing the R/V Guaiquerí II at affordable academic price, and to its crew for being always ever-ready and highly performing. This research has been funded by several sources and is also a contribution to Projects: FONACIT-ECOS Nord PI-2003000090 (French code V04U01) and PI-2009000818 (French code V10U01) and GEODINOS (FONACIT 2002000478) and TSUNAMI (FONACIT 2013000361). We
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