I recently gave a talk for the USGS Earthquake Seminar Series at the USGS Earthquake Science Center in Menlo Park. Here is a web page where I post material from my talk, including some background material. Below I present some links that are limited to the talk.
- Here is the Digital Presentation from this talk about Seismoturbidite Chronostratigraphy and Slope Stability offshore Sumatera, Indonesia: USGS Earthquake Science Center Seminar (175 MB)
Here is a link to the main research cruise page for the CASEIS 16 Lesser Antilles Submarine Paleoseismology cruise. Our goal is to find turbidites so that we might establish a prehistoric record of earthquakes along this subduction zone.
Below is a summary to date of the material that I have prepared to describe our findings from this cruise. These posts are intended to be educational and informational.
Here is a map of historic seismicity in this region from 1900-2016. Here is the kml file I used to create this map.
I include some inset figures.
- In the upper left corner I include a plate tectonic map from Pindell and Kennan (2009).
- In the upper right corner I include a figure from Fueillet et al. (2011) that shows felt regions of the two largest historic earthquakes associated with the subduction zone. These earthquakes are pre-instrumental, so the estimates of size are based upon observations of intensity.
- In the lower left corner is a figure from Hough (2013) that shows the felt reports from the Feb. 8, 1843 earthquake.
- The Lesser Antilles subduction zone has a short record of earthquakes and there is little known about the prehistoric record of earthquakes. Given a record of a few hundred years, and that subduction zone earthquakes typically have recurrence intervals spanning this long to thousands of years, it is important to develop a prehistoric record of earthquakes spanning multiple earthquake cycles. Given that there appear to be cycles of cycles (a.k.a. supercycles, Sieh et al., 2008), this paleoearthquake record needs to span many many earthquake cycles. How long is long enough? Nobody knows because (1) not many fault systems have been studied like this and (2) nobody has observed subduction zones for this long (tens of thousands of years).
- Hough (2013) presented an analysis of felt reports for the 1843 earthquake. This earthquake was probably on the subduction zone fault, but we do not really know since it happened before we had seismometers that would allow us to locate the earthquake. Hough (2013) suggests that the pre-1900 record of historical earthquakes is under-reported and magnitudes are under-estimated. Hough uses felt records to suggest that prior estimates of the magnitude for the 1843 earthquake were too small. Macroseismic effects from the earthquake were used to estimate a magnitude of M 7.5-8 (Bernard and Lambert, 1988). Feuillet et al., (2011) reconsiders the intensities of this event. Feuillet et al. (2011) focus on accounts from the Lesser Antilles and their estimated magnitude is M = 8.5, with an estimated rupture length of 300 km. Hough uses felt reports and compares these records with other subduction zone earthquakes. Below is a plot showing her comparisons. I include the figure caption below the figure as a blockquote.
Intensities versus distance. Intensity values for (red circles) the 8 February 1843 Lesser Antilles earthquake, (gray circles) the 2011 Tohoku-Oki earthquake, and (black circles) the 2010 Maule, Chile earthquake. Values are plotted versus epicentral distance, for the 1843 earthquake assumed to be 17.5 N, 60.5 W; the precise assumed epicenter is in question but does not significantly change the data points at regional distances.
- Feuillet et al (2002) suggest that there is oblique convergence that leads to strain/slip partitioning. They place a forearc sliver fault along the volcanic arc (where the islands are). This figure below shows this shear couple across the subduction zone. I include their figure caption below the figure as a blockquote.
Tectonic model of the Lesser Antilles Arc. 500 m bathymetric contours are from Smith and Sandwell . Structural interpretation of accretionary prism is based on analysis of bathymetric and topographic contour patterns. Black arrows along the trench: NAM/CAR boundary-parallel slip with rates indicated [DeMets et al., 2000]. Large white arrows: NAM/CAR and SAM/CAR motion vectors from DeMets et al.  and Weber et al. , respectively. NAM/CAR GPS relative motion vectors measured at Aves and Sainte Croix are indicated [DeMets et al., 2000]. Black double arrows, local direction of extension deduced from fault geometry and distribution. Half black arrows, slip on oblique or strike slip faults. 1, in light gray, zone of sinistral extensional shear; 2-transition zone; 3-in dark grey, zone of dextral oblique thrusting. White large half arrows indicate sinistral and dextral motion along the trench, respectively.
- Pichot et al. (2012) study the fracture zones as expressed by the Barracuda and Tiburon Ridges. These are somewhat related to some fracture zones that extend from the Mid Atlantic Ridge. These structures may be responsible for segmentation of the subduction zone (earthquakes or volcanism). This is one of the hypotheses that will be tested by conducting this marine paleoseismology research. The Tiburon rise does cause a major inflection of the subduction zone fault, so it is reasonable to think that this may cause segmented rupture along the megathrust. Below are three figures. First is a map that shows bathymetry and gravity data for the region between the Lesser Antilles and the Mid Atlantic Ridge (Pichot et al., 2012). The lower figure pair shows (a) these ridges as they relate to bathymetry and faults in the accretionary prism and (b) these structures as expressed as gravity anomalies (Pichot 2012). Below each figure I include their figure caption as a blockquote.
The Atlantic Ocean between 2°N and 22°N. A) 1×1 min-Satellite free air gravity map (V18.1 from Sandwell and Smith, 2009), with NAm, North America Plate; Car, Caribbean Plate; SAm, South America Plate; Afr, Africa Plate; MAR, Mid-Atlantic-Ridge; AR, Aves Ridge; LA, Lesser Antilles; TB, Tiburon Rise; BR, Barracuda Ridge; RT, Royal Trough; RR, Researcher Ridge; Caribbean structural geology after Pindell and Kennan (2009).B) Bathymetric map (Smith and Sandwell, 1997) with, BAP, Barbados Ridge accretionary prism; ORS, Orinoco river system; ADSF, Amazon Deep Sea Fan, DAP, Demerara abyssal plain.C) Regional map showing Atlantic Ocean and the study area in red box. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fold axes (folds) and overlap (thrusts) (black line) parallel to the front of accretion. In continuation of wrinkles Barracuda and Tiburon, their axes are a sub-parallel towards the axis of wrinkles
Wrinkles Barracuda, Tiburon and Saint Lucia’s characterized by gravimetric anomalies to the positive open. Wrinkles Barracuda and Tiburon are limited respectively to the north and south by the graves of Barracuda and Tiburon. The front of the accretion prism is represented by the black line with triangles. The free air gravity anomaly map is from Sandwell and Smith (2009).
- Bernard, P. and Lambert, J., 1988. Subduction and seismic hazard in the northern Lesser Antilles: Revision of historical seismicity, Bull. Seismol. Soc. Am. v. 78, p. 1965–1983
- Feuillet, N., Manighetti, I., and Tapponier, P., 2002. Arc parallel extension and localization of volcanic complexes in Guadeloupe, Lesser Antilles in Journal of Geophysical Research, v., 107, DOI:10.1029/2001JB000308
- Feuillet, N., Beauducel, F., Tapponnier, P., 2011. Tectonic context of moderate to large historical earthquakes in the Lesser Antilles and mechanical coupling with volcanoes in Journal of Geophysical Research, v. 116, DOI:10.1029/2011JB008443.
- Hough, S., 2013. Missing great earthquakes in Journal of Geophysical Research: Solid Earth, v. 118, p. 1098-1108.
- Pichot, J., 2012. The Barracuda Ridge and Tiburon Rise, East of the Lesser Antilles: origin, evolution and geodynamic implications. [Ph.D. Thesis] l’Université de Bretagne Occidentale 286 pp.
- Pichot, T., Patriar, M., Westbrook, G.K., Nalpas, T., Gutscher M.A., Roest, W.R., Deville, E., Moulin, M., Aslanian, D., and Rabineau, M., 2012. The Cenozoic tectonostratigraphic evolution of the Barracuda Ridge and Tiburon Rise, at the western end of the North America-South America plate boundary zone in Marine Geology, v. 303-306, p. 154-171
- Pindell, J.L. and Kennan, L., 2009. Tectonic Evolution of the Gulf of Mexico, Caribbean and northern South America in the mantle reference fram: an update, in James, K., Antonieta-Lorente, M., and Pindell, J.L., (eds), The geology and evolution of the region between North and South America, Geological Society of London Special Publication.