Earthquake Report: Venezuela

Busy week!

We just had a M 7.3 earthquake in northern Venezuela. Sadly, this large earthquake has the potential to be quite damaging to people and their belongings (buildings, infrastructure).

https://earthquake.usgs.gov/earthquakes/eventpage/us1000gez7/executive

The northeastern part of Venezuela lies a large strike-slip plate boundary fault, the El Pilar fault. This fault is rather complicated as it strikes through the region. There are thrust faults and normal faults forming ocean basins and mountains along strike.

Many of the earthquakes along this fault system are strike-slip earthquakes (e.g. the 1997.07.09 M 7.0 earthquake which is just to the southwest of today’s temblor. However, today’s earthquake broke my immediate expectations for strike-slip tectonics. There is a south vergent (dipping to the north) thrust fault system that strikes (is oriented) east-west along the Península de Paria, just north of highway 9, east of Carupano, Venezuela. Audenard et al. (2000, 2006) compiled a Quaternary Fault database for Venezuela, which helps us interpret today’s earthquake. I suspect that this earthquake occurred on this thrust fault system. I bet those that work in this area even know the name of this fault.

However, looking at the epicenter and the location of the thrust fault, this is probably not on this thrust fault. When I initially wrote this report, the depth was much shallower. Currently, the hypocentral (3-D location) depth is 123 km, so cannot be on that thrust fault.

The best alternative might be the subduction zone associated with the Lesser Antilles.

GPS data support the hypothesis that the El Pilar fault is accumulating strike-slip strain, but there is a paucity of evidence that there is active convergence across the thrust fault. However, there does appear to be some small amount of contraction (Reinoza, et a.,. 2015).

Below is my interpretive poster for this earthquake

I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I include earthquake epicenters from 1918-2018 with magnitudes M ≥ 6.0 in one version.
I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes.

  • I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely.
  • I also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.
  • I include the slab contours plotted (Hayes et al., 2012), which are contours that represent the depth to the subduction zone fault. These are mostly based upon seismicity. The depths of the earthquakes have considerable error and do not all occur along the subduction zone faults, so these slab contours are simply the best estimate for the location of the fault. However, the slab contours are only in the southwestern portion of this map.

    I include some inset figures. Some of the same figures are located in different places on the larger scale map below.

  • In the upper left corner is a map from Pindel and Kennan (2009) that shows the plate tectonic boundaries of the Caribbean and northern South America.
  • In the lower right corner is another map that shows teh regional tectonics (Levander et al., 2006).
  • In the upper right corner is a larger scale map showing the faulting in the region surrounding today’s M 7.3 earthquake (Audenard et al., 2000).
  • Here is the map with a month’s seismicity plotted, along with USGS earthquakes M ≥ 6.0.


Other Report Pages

Some Relevant Discussion and Figures

  • Here is the Pindell and Kennan (2009) map.


    • Present day tectonic map of the Caribbean region.

  • Here is the Pindell and Kennan (2009) figure that shows how the large strike-slip plate boundary on teh north side of Venezuela grew from the west over time.


    • Motion histories of: North (NA) and South America (SA) relative to Indo-Atlantic hot spot (IAHS) Mu¨ller et al. (1993) reference frame (grey lines; NA wrt IAHS and SA wrt IAHS); hot spots relative to North America (dashed black line; IAHS wrt NA); Caribbean relative to North America (heaviest black line; Car wrt NA), as summarized from former relative positions of the Caribbean Trench (lighter black lines). Also shown: Cayman Trough (grey outline); Cenozoic convergence between the Americas (inset upper right; P88 ¼ Pindell et al. 1988; M99 ¼ Mu¨ller et al. 1999); seismic tomographic profile of van der Hilst (1990) (inset, lower right).

  • Here is the Audemard (2000) map showing the many faults in this region.


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  • This is a map from Reinoza et al. (2015) where they present their geodetic analysis (analysis of the deformation of the earth). These authors use GPS data to evaluate the potential activity of the El Pilar fault.


    • Location map of the active faults in northeastern Venezuela [Audemard et al., 2000] showing distribution of the GNSS stations: yellow squares, green circles, and red triangles are GNSS sites on which the acquisition campaigns were carried out in 2003, 2005, and 2013 respectively; the blue star corresponds to the cGNSS CUMA station of REMOS-IGVSB Network. We show the epicenter location of 1929 and 1997 events with their respective proposed ruptures (orange lines) [Audemard, 2007]. (top right) The inset box shows a schematic geodynamic map of the southeastern Caribbean [Audemard, 1999b; Audemard et al., 2000; Weber et al., 2001]. Legend: BF = Boconó Fault, EPF = El Pilar Fault, OAF = Oca Ancón fault, SMBF = Santa Marta Bucaramanga Fault, and SSF = San Sebastian Fault.

  • Here are the GPS data. The white arrows (vectors) show the observed velocities (motion rate) for the GPS sites shown on the previous map. The black arrows (vectors) show how their model results compare with the observational data.


    • Observed velocities (white arrows) with error ellipses for 66% confidence level and simulated velocities (black arrows) according to the upgrade of displacement-simulation method. All displacements are based on the South America reference frame.

  • Here are some cross sections showing the El Pilar fault, along with some of the thrust faults in the region. Section B is just to the west of where this M 7.3 earthquake happened.


    • Simplified sections across the southeastern Caribbean margin (based on maps and sections by Bellizzia et al. (1976), Stéphan et al. (1980), Campos (1981), Beck (1986), Chevalier (1987); locations in Fig. 1).

Geologic Fundamentals

  • For more on the graphical representation of moment tensors and focal mechnisms, check this IRIS video out:

  • Here is a fantastic infographic from Frisch et al. (2011). This figure shows some examples of earthquakes in different plate tectonic settings, and what their fault plane solutions are. There is a cross section showing these focal mechanisms for a thrust or reverse earthquake. The upper right corner includes my favorite figure of all time. This shows the first motion (up or down) for each of the four quadrants. This figure also shows how the amplitude of the seismic waves are greatest (generally) in the middle of the quadrant and decrease to zero at the nodal planes (the boundary of each quadrant).


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  • Here is another way to look at these beach balls.

  • There are three types of earthquakes, strike-slip, compressional (reverse or thrust, depending upon the dip of the fault), and extensional (normal). Here is are some animations of these three types of earthquake faults. The following three animations are from IRIS.

    • Strike Slip:

    Compressional:

    Extensional:

  • This is an image from the USGS that shows how, when an oceanic plate moves over a hotspot, the volcanoes formed over the hotspot form a series of volcanoes that increase in age in the direction of plate motion. The presumption is that the hotspot is stable and stays in one location. Torsvik et al. (2017) use various methods to evaluate why this is a false presumption for the Hawaii Hotspot.


    • A cutaway view along the Hawaiian island chain showing the inferred mantle plume that has fed the Hawaiian hot spot on the overriding Pacific Plate. The geologic ages of the oldest volcano on each island (Ma = millions of years ago) are progressively older to the northwest, consistent with the hot spot model for the origin of the Hawaiian Ridge-Emperor Seamount Chain. (Modified from image of Joel E. Robinson, USGS, in “This Dynamic Planet” map of Simkin and others, 2006.)

  • Here is a map from Torsvik et al. (2017) that shows the age of volcanic rocks at different locations along the Hawaii-Emperor Seamount Chain.


    • Hawaiian-Emperor Chain. White dots are the locations of radiometrically dated seamounts, atolls and islands, based on compilations of Doubrovine et al. and O’Connor et al. Features encircled with larger white circles are discussed in the text and Fig. 2. Marine gravity anomaly map is from Sandwell and Smith.

    References:

  • Audemard, F.A., Machette, M.N., Cox, J.W., Dart, R.L., and Haller, K.M., 2000. Map and Database of Quaternary Faults in Venezuela and its Offshore Regions, USGS Open File Report 00-018
  • Hayes, G.P., Wald, D.J., and Johnson, R.L., 2012. Slab1.0: A three-dimensional model of global subduction zone geometries in, J. Geophys. Res., 117, B01302, doi:10.1029/2011JB008524
  • Jouanne, F., Audemard, F.A., Beckm, C., Van Welden, A., Ollarves, R., and Reinoz, C., 2011. Present-day deformation along the El Pilar Fault in eastern Venezuela: Evidence of creep along a major transform boundary in Journal of Geodynamics, v. 51., p. 398-410, doi:10.1016/j.jog.2010.11.003
  • Meyer, B., Saltus, R., Chulliat, a., 2017. EMAG2: Earth Magnetic Anomaly Grid (2-arc-minute resolution) Version 3. National Centers for Environmental Information, NOAA. Model. doi:10.7289/V5H70CVX
  • Pindell, J.L. and Kennan, L., 2009. Tectonic evolution of the Gulf of Mexico, Caribbean and northern South America in the mantle reference frame: an update in JAMES, K. H., LORENTE, M. A. & PINDELL, J. L. (eds) The Origin and Evolution of the Caribbean Plate. Geological Society, London, Special Publications, 328, 1–55.
    DOI: 10.1144/SP328.1
  • Reinoza, C., F. Jouanne, F. A. Audemard, M. Schmitz, and C. Beck (2015), Geodetic exploration of strain along the El Pilar Fault in northeastern Venezuela, J. Geophys. Res. Solid Earth, 120, 1993– 2013, doi:10.1002/2014JB011483.


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