Earthquake Report: M 7.4 Drakes Passage

Yesterday my team was bringing to fruition weeks of planning (with lots of this preparation taking place in the last week) for a retirement celebration for Nick and my boss, Rick Wilson.

Rick retired in Dec 2024 after 34 years and 4 months at the California Geological Survey (CGS). This Division was actually called the Division of Mines and Geology at the time. Rick had gotten his Master’s Thesis working on the subsurface geology of the southern San Joaquin River Valley.

In 2009 Rick and Kevin Miller (California Governor’s Office of Emergency Services) published 110 Tsunami Inundation Maps for Emergency Planning for the coast of California. I actually contributed to the maps in the Humboldt Bay region.

Nick, my coworker, also worked with Rick before he was hired to the Tsunami Unit at the CGS. Nick contributed to the study of prehistoric tsunami deposits along the coast of California.

Both Nick and I were hired in February 2019 and helped Rick map and publish the next generation Tsunami Hazard Area maps based on Probabilistic Tsunami Hazard Assessment data. The state worked with Hong Kie Thio to generate tsunami models associated with annual return periods (e.g., just like the “100-year” flood, we have a “100-year” tsunami model; we also have 500, 1000, 2500, and 3000-year tsunami models).

Nick, Kate, Tim (other coworkers at the CGS), and I prepared an event that included official recognitions but also gifts and songs written for Rick. Many of the gifts were associated with funny anecdotes about Rick. (much of this was given in the form of a “roast”)

While I spent a week working on a couple photo posters (for people to sign), gifts, etc., Kate spent a week making a tsunami cake! (and gluten free cupcakes)

We were all stressed that the event would proceed successfully. And, it did.

So, I awoke the morning of the event to note that there was a M 7.4 earthquake along a plate boundary fault in the Drakes Passage, south of the tip of South America.

https://earthquake.usgs.gov/earthquakes/eventpage/us7000pwkn/executive

The southern part of western South America is bordered by a subduction zone, a convergent plate boundary, where the Nazca plate dives below the South America plate. Geologists term this “diving down” as “subducting.”

The largest magnitude earthquake was a M 9.5 earthquake in 1960 along the southern part of this subduction zone.

South of this M 9.5 earthquake, the subduction zone may not continue (my colleague Matias pointed this out to me a year or two ago). But, on many maps, the plate boundary fault here is still shown as a subduction zone.

This plate boundary fault turns into a strike-slip fault (a transform plate boundary) called the Shackleton fracture zone.

The M 7.4 and its aftershock pattern suggests that this plate boundary fault is actually active as a convergent plate boundary. So, I will need to look at the notes that I took while talking to Matias. Perhaps this is not a subduction zone, but simply a reverse/thrust system (a crustal fault, not plate boundary). I will try to update this report with more of this information.

So, because I was busy roasting Rick, I did not have the capacity to follow this earthquake in real time. Though I did interact with Elizabeth Vanancore (seismologist who runs the seismic network for Puerto Rico and works on tsunami hazards for PR) about the potential for a tsunami.

It was not until this morning that i noticed that Matias had posted the tide gage observation of a small tsunami generated by this earthquake. I tried downloading these data from the European Union World Sea Levels system but there were errors. So, I used the UNESCO water level data.

I plotted the tide gage data on the interpretive poster.

Speaking of the transition from convergent to strike-slip, we can look at the historical earthquake mechanisms along this boundary. I plot them on the interpretive poster.

This M7.4 earthquake and a 2018 M 6.3 earthquake have thrust (compressional) earthquake mechanisms. To the southeast, the mechanisms are all strike-slip.

Below is my interpretive poster for this earthquake

  • I plot the seismicity from the past month, with diameter representing magnitude (see legend). I include earthquake epicenters from 1925-2025 with magnitudes M ≥ 3.0 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.
  • A review of the basic base map variations and data that I use for the interpretive posters can be found on the Earthquake Reports page. I have improved these posters over time and some of this background information applies to the older posters.
  • Some basic fundamentals of earthquake geology and plate tectonics can be found on the Earthquake Plate Tectonic Fundamentals page.

    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 showing the plate tectonic boundaries (from the GEM).
  • In the lower right corner is a map that shows the M 7.7 earthquake intensity using the modified Mercalli intensity scale. Earthquake intensity is a measure of how strongly the Earth shakes during an earthquake, so gets smaller the further away one is from the earthquake epicenter. The map colors represent a model of what the intensity may be. The USGS has a system called “Did You Feel It?” (DYFI) where people enter their observations from the earthquake and the USGS calculates what the intensity was for that person. The dots with yellow labels show what people actually felt in those different locations.
  • Above the intensity map is a plot that shows the same intensity (both modeled and reported) data as displayed on the map. Note how the intensity gets smaller with distance from the earthquake.
  • In the upper right corner is a larger scale map showing the aftershocks. The rupture length is about 110km.
  • To the left of the aftershock map is a plot from Wells and Coppersmith (1994). The plot on the left shows the relations between earthquake subsurface rupture lengths (horizontal axis) relative to the earthquake magnitude (vertical axis). On the right are the correlation lines for these scaling relations. Below the plots is a table based on these rupture length/magnitude scaling relations. The aftershock pattern, the USGS finite fault slip model (shown to the left), and these scaling relations fit each other rather well.
  • In the left center is a plate tectonic map from Galindo_Zaldivar et al. (2004).
  • Here is the map with a month’s seismicity plotted.


Other Report Pages

  • https://earthquakeinsights.substack.com/p/unusual-m74-earthquake-strikes-south

Shaking Intensity

  • Here is a figure that shows a more detailed comparison between the modeled intensity and the reported intensity. Both data use the same color scale, the Modified Mercalli Intensity Scale (MMI). More about this can be found here. The colors and contours on the map are results from the USGS modeled intensity. The DYFI data are plotted as colored dots (color = MMI, diameter = number of reports).
  • In the upper panel is the USGS Did You Feel It reports map, showing reports as colored dots using the MMI color scale. Underlain on this map are colored areas showing the USGS modeled estimate for shaking intensity (MMI scale).
  • In the lower panel is a plot showing MMI intensity (vertical axis) relative to distance from the earthquake (horizontal axis). The models are represented by the green and orange lines. The DYFI data are plotted as light blue dots. The mean and median (different types of “average”) are plotted as orange and purple dots. Note how well the reports fit the green line (the model that represents how MMI works based on quakes in California).
  • Below the map and the lower plot is the USGS MMI Intensity scale, which lists the level of damage for each level of intensity, along with approximate measures of how strongly the ground shakes at these intensities, showing levels in acceleration (Peak Ground Acceleration, PGA) and velocity (Peak Ground Velocity, PGV).


    Some Relevant Discussion and Figures

    • This is an early map of the plate boundaries in the region (Forsyth, 1975) and it largely holds true today, over 50 years later!


      • Summary of inferred plate motions in the South Atlantic. Heavy arrows indicate direction of motion of plates relative to a coordinate system fixed with respect to South America. Light arrows indicate observed sense of motion on plate boundaries. Double lines are spreading centers, single lines are transform faults, and hatched lines are subduction zones. Lines are dashed where location or nature of boundary is uncertain.

  • Here is the physiographic map and the map that shows historic seismicity. Note how the earthquakes get deeper from east to west along the Scotia subduction zone (as the South America plate subducts westward beneath the Sandwich plate, the megathrust fault gets deeper. The Civile et al. (2012) paper is particularly relevant to the strike-slip faults on the southern boundary of the Scotia plate. I include their figure caption below as a blockquote.
  • The lower panel has seismicity plotted with color representing depth. One may observe that the plate boundary along the South Sandwich trench show that the earthquakes deepen from east to west (representing a subduction zone).
  • The rest of these plate boundary faults are shallower, representing either transform or spreading ridge plate boundary faults. These data support the hypothesis that there is no subduction south of the M9.5 earthquake.


    • (top panel) Physiographic map of the Scotia Arc, with the main geological provinces discussed in the text. Bathymetric contours from satellite-derived data (Smith and Sandwell, 1997). Box shows the present-day plate tectonic sketch for the Scotia Sea and surrounding regions. NSR, North Scotia Ridge; SSR, South Scotia Ridge; ESR, East Scotia Ridge; SFZ, Shackleton Fracture Zone; BD, Bruce Deep; SSP, South Shetland plate; sSP, South Sandwich plate. (bottom panel) Distribution of earthquakes in the Scotia Arc region. Epicenters and focal depths are obtained from EHB Bulletin (Engdahl et al., 1998). EHB shallow (red), intermediate (orange and yellow), and deep (blue), earthquakes are shown as circles. Smaller black circles represent earthquakes located by ISC (http://www.isc.ac.uk).

    • This map from Schellart et al. (2023) shows the main fault systems, their relative motions, and their relative slip rates in mm/year).


      • Map showing the tectonic setting of the Scotia Sea region including the plate boundaries, major faults and structures. Also shown are a number of absolute velocities, namely plate velocities (vP in orange), subducting plate velocities at the trench (vSP in blue) and trench velocities at the trench (vT in green) in cm/yr. These velocities are placed in the moving Indo-Atlantic hotspot reference frame of O’Neill et al. (2005) making use of the relative plate motion model of Kreemer et al. (2014). The location of the MFf is based on Lodolo et al. (2003). Shear rates at the transform plate boundaries and spreading rate at the East Scotia Ridge are also given (in black in cm/yr). Note that structures in grey are no longer active. Also note that trench accretion/erosion rates have been ignored when calculating the trench velocities. ASSA–Ancestral South Sandwich Arc, BaB–Baker Bank, BrB–Bruce Bank, BuB–Burdwood Bank, CH–Cape Horn, CSS–Central Scotia Sea, DaB–Davis Bank, DB–Dove Basin, DiB–Discovery Bank, ESR–East Scotia Ridge, ESS–East Scotia Sea, JB–Jane Basin, JBk–Jane Bank, MFf–Magallanes-Fagnano fault, NSR–North Scotia Ridge, OnaB–Ona Basin, PB–Powell Basin, PiB–Pirie Bank, PrB–Protector Basin, SB–ScanBasin, Sfz–Shackleton fracture zone, SGI–South Georgia Island, SGm–South Georgia microcontinent, Shp–South Shetland plate, SoB–Southern Bank, SOm–South Orkney microcontinent, SR–Shag Rocks, SSA–South Sandwich Arc, SSp–South Sandwich plate, SSR–South Scotia Ridge, SSsz–South Shetland subduction zone, TR–Terror Rise, WSR–West Scotia Ridge, WSS–West Scotia Sea.

    • This map from Schellart et al. (2023) shows the main fault systems, their relative motions, and their relative slip rates in mm/year).


      • New plate tectonic reconstructions of the southern South America-Antarctic Peninsula-Weddell Sea-Scotia region from the late Early Cretaceous to middle Eocene (110–40 Ma). Important features include an Antarctica-South America plate boundary located south of South Orkney microcontinent (SOm), the position of the Rocas Verdes basin and its arc, which later becomes Cape Horn microcontinent (CHm), along the Pacific side of southern South America, and the formation of a new, westward-dipping, subduction zone through the SIPS (Subduction Invasion Polarity Switch) mechanism (see main text). CHm moves southeastward along the western margin of South America, accommodated by major strike-slip faults, and rotates anticlockwise some 70◦ after closure of the Rocas Verdes basin. The reconstructions show the location of the plate boundaries (thick black lines), continental crust (in light grey), oceanic crust (in darker grey), and present-day coastal outlines (dotted grey lines). Also shown are a number of main seafloor magnetic anomalies in the Weddell Sea domain (from Müller et al. (2018)). The reconstructions are placed in an Antarctica-fixed reference frame. South American plate motion relative to the Antarctic plate is based on rotation parameters from Matthews et al. (2016). No attempt has been made to reconstruct the tectonic plates in the Pacific domain. Reconstructions are shown at: (a) 110 Ma, (b) 100 Ma, (c) 90 Ma, (d) 80 Ma, (e) 70 Ma, (f) 60 Ma, (g) 50 Ma, and (h) 40 Ma. BurB–Burdwood Bank, D-B-S–Undifferentiated continental crustal material from Davis Bank, Baker Bank and Shag Rocks, MFf–Magallanes- Fagnano fault, SGm–South Georgia microcontinent, T-P-B-DC–Undifferentiated continental crustal material from Terror Rise, Pirie Bank, Bruce Bank, Discovery Bank and Central Scotia Sea.

  • The tectonic setting for this region from Nerlich et al. (2013). I include their figure caption below as a blockquote.


    • View of Scotia Sea consisting of the Scotia and Sandwich plates, located in between the Antarctic Peninsula and South America (see also insert map, where the red dot indicates the center of the top view map). The region is framed by transform boundaries in the north (North Scotia Ridge (NSR)), south (South Scotia Ridge (SSR)), west (Shackelton Fracture Zone (SFZ)), and the South Sandwich subduction zone (SSSZ) in the east. Other features are Shag Rocks (SR), South Georgia (SG), and the South Orkney Microcontinent (SOM). Flowlines displaying motion paths of different continental fragments are shown in green. Active spreading (East Scotia Ridge (ESR)) exists in the East Scotia Sea. Extinct spreading ridges are found in theWest Scotia Sea (West Scotia Ridge (WSR)), Central Scotia Sea (Central Scotia Ridge (CSR) which remains controversial; see Subsection 2.2) and in the Protector (Pro), Dove (Dov), and Discovery Basins (Dis), respectively, which are bounded by presumably — as some discussion on their origin persists — South American continental fragments (Terror Rise (TR), Pirie Bank (PB), Discovery Bank (DB)). Extinct ridges are also found on the boundary between the Antarctic plate and former Phoenix plate as well as in the Powell Basin (Pow). Location of the dredge sample with Pacific mantle type signature (Pearce et al., 2001) [see discussion] is marked by a triangle.

  • Here is the kinematic tectonic reconstruction from Nerlich et al. (2013). The panel begins in the upper left at 50 million years ago. I include their figure caption below as a blockquote.


    • Tectonic reconstructions corresponding to 50 (a), 41 (b), 32 (c), 21 (d), 7 (e) and 0 Ma (f); labels as in Fig. 1; present-day bathymetry is shown (i.e. no age-masking). White/grey arrows indicate active/inactive spreading. 50 Ma ago, South America and Antarctica are connected by a coherent bridge of continental fragments. At 41 Ma, separation between South America and Antarctica and subduction behind Discovery Bank and South Georgia has started, as suggested by Barker (2001). Protector Basin has opened. 32 Ma ago, active spreading occurs along the West Scotia Ridge but has ceased in Dove Basin. At 21 Ma, further subduction behind Discovery Bank leads to opening of Discovery Basin. Moreover, subduction behind South Georgia is about to cause back-arc spreading in the Central Scotia Sea. Powell Basin is already fully open, placing the South Orkney Microcontinent to its present position relative to Antarctica. Seafloor spreading lasts along the West Scotia Ridge. 7 Ma ago spreading occurs along the West and East Scotia Ridges and ceases along the Central Scotia Ridge.

  • Nerlich et al. (2013) used asthenospheric flow estimates, their kinematic history model, and a dynamic topography deconvoloution method (using 4DPlates; Clark et al., 2012) to model the age of the oceanic lithosphere in this region. I include their figure caption below as a blockquote.


    • Synthetic age-grid based on the reconstruction model as shown in Fig. 2.

    • Here is a map that shows the regional plate tectonics (Almendros et al., 2020). Their work focused on an oceanic spreading center (divergent plate boundary) called the Bransfield Rift.


      • (a) Map of the Scotia region between South America and Antarctica (see globe inset), showing the main tectonic units and interactions. The red box labeled “b” is the Bransfield Strait area, which is zoomed below. BB: Bransfield Basin; SSB: South Shetland Block; SST: South Shetland Trench. (b) Map of the Bransfield Strait, showing the position of normal faults around the Bransfield Basin, the spreading centers that constitute the Bransfield Rift, and the main volcanic areas (modified from Jim´enez-Morales et al., 2017).

    • Here are some maps from Beniest (2020).
    • This first map shows the geology of this region. The second map includes a plot of the historical earthquakes with symbols representing depth.


      • Lithological map of the Scotia Sea region with major geological features indicated. The main lithology mapped is basalt, including seamounts (in dark green) that vary in diameter from 1 km to 25 km, which are often surrounded by volcaniclastic material and debris. The Central Scotia Sea consists of more andesitic material, but the area is mostly covered with oceanic sediments. The Scotia Sea is surrounded by highs that consist of more felsic material, indicated in purple, with some smaller blocks located further away from the major features. Red lines indicate major extinct and active spreading centres, orange lines indicate former and present trench location, black lines are active and inactive transform-faults, fracture zones and fault zones in general. The geology of the South American and Antarctic continents and the Falkland Islands are taken from the already existing geological maps (Burton-Johnson and Riley, 2015, U.S. Geological Survey, 1999). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)


      Geological map with the moment magnitude and epicentres located on the map. The majority of the earthquakes occur at the South Sandwich subduction zone and along the SSR. Some earthquakes still occur along the extinct spreading centre (WSR and APR) and along the NSR. Earthquakes from USGS (https://earthquake.usgs.gov/earthquakes/search/) and IRIS (http://ds.iris.edu/ds/) seismic databases.

    • Here is a map from Bohoyo et al. (2018) that shows the topography/bathymetry of the region , tectonic structures, and seismicity with colors representing magnitude.


      • Geological map of the Scotia Arc. Bathymetry from GEBCO_2014 including IBCSO v1.0 below 60° S. White square identifies the study region.

    • Here is a larger scale map showing the detailed structures in the plates on either side of the Shackleton Fracture Zone (Bohoyo et al. 2018).


      • (A) Bathymetry map of the Drake Passage with the colour ramp used in the Main Map and main geographical signatures.

    • This map shows magnetic anomaly interpretations associated with the structures in these plate as formed at the spreading ridges (Bohoyo et al. 2018).


      • (B) Bathymetry map in grey ramp including additional geological information as seismicity (depth and magnitude), magnetics (magnetic lineation chrons), tectonics (relationship between plates or major structures and plate motion velocity with Scotia Plate fixed according to Thomas et al., 2003).

    • Here is another map showing the magnetic anomaly data as they represent the plate origins at the spreading centers (Livermore et al., 1994).


      • Bathymetry and location of features in the Scotia Sea. The 2000 and 6000 m contours are shown (after GEBCO), together with a summary of magnetic anomaly isochrons [11]. Heavy lines denote ship tracks as follows: SH73 = R.R.S. Shackleton 1973; SH75 = R.R.S. Shackleton 1975; CD37 = R.R.S. Charles Darwin cruise CD37, 1989.

    • This map shows the gravity data for the region (Livermore et al., 1994).


      • Colour shaded relief image of satellite-derived free-air gravity grid computed from all available GM and ERM passes in the Scotia arc region. A grid spacing of 0.04 ° latitude × 0.05 ° longitude has been used; illumination is from the east

    • Maldonado et al. (2000) studied the tectonic history of this region.
    • The first map shows magnetic anomaly data they used in their study.


      • Geological setting of the study area. 1, active subduction zone; 2, inactive subduction zone; 3, transpressive fault zone; 4, fault zone; 5, active spreading axis; 6, inactive spreading axis; 7, active extensional zone; 8, active transtensional fault zone; 9, continental crust; 10, oceanic crust; 11, continental-oceanic crust boundary. BS, Brandsfield Strait; PAR, Phoenix/Antarctic Ridge; HFZ, Hero Fracture Zone; SFZ, Shackleton Fracture Zone; SSB, South Shetland Block; SST, South Shetland Trench; SSR, South Scotia Ridge; STC, South Chile Trench; WSR, West Scotia Ridge. Other symbols, magnetic anomalies. Magnetic anomalies from BAS (1985) and Livermore and Woollett (1993).

    • Here are some sketches of the plate boundary through time (Maldonado et al., 2000).


      • Sketches of three tectonic stages of the central Drake Passage during the Cenozoic showing the changing scenario along the Shackleton Fracture Zone. Legend: BS, Brandsfield Strait; PANT, Phoenix/Antarctic/Nazca triple junction; PAR, Phoenix/Antarctic Ridge; PNR, Phoenix/Nazca Ridge; SFZ, Shackleton Fracture Zone; SSR, South Scotia Ridge; SST, South Shetland Trench; STC, South Chile Trench; WSR, West Scotia Ridge.

    • Here are some sketches of the plate boundary through time (Maldonado et al., 2000).


      • Sketches of plate tectonic reconstructions of the central sector of the Drake Passage from the late Miocene to present showing the tectonics along the Shackleton Fracture Zone.

    • Here is a map that shows earthquake mechanisms for historical earthquakes from Thomas et al. (2003)
    • Note how the mechanisms along the Shackleton fracture zone are all strike-slip mechanisms and at earthquake number 16, the mechanisms change to reverse/thrust.


      • Lower focal spheres representing best-fitting focal mechanisms from CMTs and body wave inversion (Pelayo & Wiens 1989). Compressional sectors of focal spheres for events used in our inversion are shaded black; those not used are shaded grey. At the South Sandwich arc, focal spheres are drawn for averages of events over 1 latitude intervals (see text). Relocated epicentres of Engdahl et al. (1998) are shown as grey filled circles (shallow), triangles (intermediate), and stars (deep). Base map shows shaded relief of satellite-derived free-air gravity (Sandwell & Smith 1997). Numbers above focal mechanisms correspond to numbers in Table 1. Red focal mechanisms at the South Sandwich Trench show moment tensor sums.

    • Eagles and Jokat (2014) used relative plate motion and rotation data to create a plate tectonic reconstruction model for this region of the world.
    • Their first figure shows the tectonic fabric as evidenced by gravity data. The lower panel includes labels of the different features they describe in their paper.


      • Drake Passage, the Scotia Sea, and surrounding region. Top: satellite altimetry-derived free-air gravity data (Sandwell and Smith, 2009) with earthquake focal mechanisms (Dziewonski et al., 1981). Bottom: Place names used in text. Light grey shading, shallower than 2 km; mid grey, shallower than 3 km; dark grey, deeper than 6 km. Inset: present
      plate boundaries.

    • This is a figure that shows a summary of the Eagles and Jokat (2014) plate reconstruction model. Each panel represents a specific time in Earth’s past.


      • Simple paleobathymetric reconstructions of Drake Passage based on present-day bathymetry and the tectonic reconstructions in Figs. 6–14. Dark blue: deep (N2.5 km), mid blue: intermediate (2.5–1.0 km), light blue: shallow (b1.0 km), light green: ‘coastal’ (±150 m), green: terrestrial.

    • Here is an animation that I created from maps in Eagles and Jokat (2014) that shows the plates as they moved relative to each other in the past 50 million years.


      • 50 Ma (chron 22) reconstruction of present-day bathymetry. All motions with respect to present day East Antarctica. See text for a discussion of the uncertainties in this reconstruction. Double lines: mid-ocean ridges (dashed where inferred after Eagles and Scott, in review). Barbed lines: subduction (black barbs). ANP: Antarctic Peninsula, B: Bruce Bank, BTF: Burdwood Transform Fault, Bu: Burdwood Bank, CS: central Scotia Sea, D: Discovery Bank, Da: Davis Bank, ECZ: Endurance Collision Zone, MEB: Maurice Ewing Bank, SG: South Georgia, SO: South Orkney Microcontinent, TF: Tierra del Fuego.

      41 Ma (chron 13) reconstruction of present-day bathymetry. See text for a discussion of the uncertainties in this reconstruction. All motions with respect to present day East Antarctica. P: Pirie Bank, T: Terror Rise.

      33 Ma (chron 13) reconstruction of present-day bathymetry. See text for a discussion of the uncertainties in this reconstruction. All motions with respect to present day East Antarctica. CSP: Central Scotia Plate, MP: Magallanes Plate.

      30 Ma (chron 11) reconstruction of present-day bathymetry. See text for a discussion of the uncertainties in this reconstruction. All motions with respect to present day East Antarctica. lines with white barbs: collision zones.

      26 Ma (chron 8) reconstruction of present-day bathymetry. See text for a discussion of the uncertainties in this reconstruction. All motions with respect to present day East Antarctica. FT: Falkland Trough, Po.: Powell Basin, PP: Pirie Province, SST: South Sandwich Trench, W1, W5: numbered segments of the West Scotia Ridge.

      20 Ma (chron 6) reconstruction of present-day bathymetry. See text for a discussion of the uncertainties in this reconstruction. All motions with respect to present day East Antarctica. SR: Shag Rocks Bank, SSR: South Scotia Ridge,W6: numbered segment of the West Scotia Ridge.

      17 Ma(chron 5C) reconstruction of present-day bathymetry data with plate outlines. See text for a discussion of the uncertainties in this reconstruction. All motions with respect to present day East Antarctica. ESR: East Scotia Ridge, Ja.: Jane Basin, Sc.: Scan Basin.

      10 Ma (chron 5) reconstruction of present-day bathymetry. See text for a discussion of the uncertainties in this reconstruction. All motions with respect to present day East Antarctica. BB: Barker Bank, NE: Northeast Georgia Rise.

      6 Ma (chron 3A) reconstruction of present-day bathymetry. See text for a discussion of the uncertainties in this reconstruction. All motions with respect to present day East Antarctica. SFZ: Shackleton Fracture Zone.

    • Polonia et al. (2017) used “multi-channel” seismic reflection data, gravity data, and structural analyses to investigate the nature of the plate boundary system.
    • They were specifically looking at the subduction zone, to evaluate if tectonic accretion or subduction erosion dominates.
    • We present some of these seismic reflection profiles below.


      • Geodynamic setting of the study area. The Antarctic, Scotia, and South America plates meet together at the triple junction indicated by the black dot at about 52S, located at the seaward projection of the Magallanes-Fagnano fault system. The Chile triple junction, at the Taitao Peninsula, where the Chile ridge is being subducted below the Chile trench, is also indicated. The slip vector [DeMets et al., 1990] is indicated by the black arrows. Circles represent major earthquakes in the studied area: gray circles are from from Febrer [2001] and black circles are from Harvard centroid moment tensor (CMT) catalog (http://earthquake.usgs.gov/regional/neic/). Existing focal mechanism solutions for shallow crustal events are shown. Black triangles are volcanic centers. The star on the seaward projection of the Magallanes-Fagnano fault system represents well A-1x drilled by Empresa Nacional del Petro´leo (ENAP). Antarctic Plate magnetic anomalies (AN) are offset along major fracture zones (dotted lines).

    • This map shows where their seismic reflection profiles are located (Polonia et al., 2017). We present these profiles, north to south, below.


      • Shaded relief satellite-derived free-air gravity map [Sandwell and Smith, 1997] with the location of the multichannel seismic and multibeam data sets. Geology on land has been modified from Cunningham [1993]. M-FFS, Magallanes-Fagnano Fault System.

    • The profile offshore of Chile near the southernmost part of the 1960 M 9.5 subduction zone earthquake.
    • Note the profile locations for each seismic reflection profile is designated as a thick black line in the map on the lower right of this figure.


      • (top) Prestack depth-migrated 30-fold MCS line I97-247 and (bottom) the line drawing. The location of the seismic profile is indicated in the inset map and in Figure 2. SVF, subvertical fault

    • The next profile to the south.
    • Note the profile locations for each seismic reflection profile is designated as a thick black line in the map on the lower right of this figure.


      • (top) Prestack depth-migrated 30-fold MCS line I97-251 and (bottom) the line drawing. The location of the seismic profile is indicated in the inset map and in Figure 2.

    • The next profile to the south.
    • Note the profile locations for each seismic reflection profile is designated as a thick black line in the map on the lower right of this figure.


      • (top) Prestack depth-migrated 30-fold MCS line I95-168 and (bottom) the line drawing. The location of the seismic profile is indicated in the inset map and in Figure 2. SVF, subvertical fault.

    • This is the southernmost profile from their paper.


      • (top) Prestack depth-migrated 30-fold MCS line I95-171 and (bottom) line drawing across the continental margin from the Antarctic plate abyssal plain to the fore-arc basin. The location of the seismic profile is indicated in the inset map and in Figure 2. SVF, subvertical fault

    • Maldonado et al. (2000) looked at the tectonics of the oceanic spreading center (divergent plate boundary where oceanic lithosphere is created) adjacent to the Shackleton fracture zone.
    • Some of the data that they used were multi-channel seismic reflection profiles.
    • We saw their plate tectonic map above. This figure shows the location of their seismic reflection profiles. Note the location of SM 07 (perpendicular to the spreading ridge and parallel to the Shackleton fracture zone) and SM 10 (perpendicular to the Shackleton fracture zone).


      • Top: bathymetry predicted from satellite-derived free-air anomaly field (Sandwell and Smith, 1997) and location chart of the ANTPAC 97/98 cruise with B/O Hesperides track lines. Thick lines represent the location of the MCS profiles. Contour interval 250 m (depth in meters). Bottom: simplified map from the GEOSAT gravimetric anomaly map recently released by the US Navy (Sandwell and Smith, 1997). Stars: earthquake epicenters (1973–1978), USGS data base.

    • Here is profile SM 07, perpendicular to the spreading ridge. One may observe that there is a depresion filled with sediment. This depression is the spreading center.


      • Selected segment of MCS profiles showing the structure of the relict spreading center of the WSR (A) and the deeply inclined reflectors of the crust in the proximity of the boundary between the SFZ and the Scotia plate (B). Layers 2 and 3 of the igneous crust and the Moho seismic discontinuity are depicted in the line drawing interpretations. The reflectors (AMC) observed below the rift valley of the WSR (A0) may indicate petrological discontinuities developed in the relict axial magma chamber. See discussion in the text and Figure 2 for location. (Vertical exaggeration: 4.2)

    • Here is profile SM 10, crossing the Shackleton fracture zone. Note the appearance of dipping faults (evidence for reverse/thrust (convergent) faults. This is far to the south of where maps show subduction.


      • Representative fragment of MCS profiles illustrating the structure of the West Scotia Ridge (profile PRSM 07) and Shackleton Fracture Zone (profiles PRSM 06 and PRSM 10). See Figure 2 for location. (Vertical exaggeration: 6.6)

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    References:

    Basic & General References

  • Frisch, W., Meschede, M., Blakey, R., 2011. Plate Tectonics, Springer-Verlag, London, 213 pp.
  • Hayes, G., 2018, Slab2 – A Comprehensive Subduction Zone Geometry Model: U.S. Geological Survey data release, https://doi.org/10.5066/F7PV6JNV.
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