The structural setting of the Green Canyon seismic event: a review of possible mechanisms, from basement to sea floor Frank J. Peel BHP Billiton Petroleum, Houston USA

The views expressed here represent the personal opinions of the author and should not be taken as representing the technical or legal opinion of BHP Billiton

Location of the GC344 earthquake and conjecture as to its mechanism The USGS report a magnitude 5.2 earthquake in Green Canyon Block Thursday, February 09, 2006 at 10:14:17 PM local time at epicenter

Setting of the Green Canyon seismogenic event

Political boundaries USA Mexico Cuba

Green Canyon protraction area USA Mexico Cuba

Where precisely, and how accurately The earthquake’s epicenter was located at °N, °W. This places it on the boundary of OCS blocks GC344/345. The lateral uncertainty is quoted as +/-10.6km (i.e. plus or minus a couple of OCS blocks). The reported hypocentral depth of the earthquake was very shallow, occurring at 5km (3.1 miles) beneath the surface. No error margin was quoted for the depth but the range of depths quoted for similar events suggests error range at least 0-15km depth.

How bad is magnitude 5.2? This is big enough to feel but not big enough to be panic-worthy! Magnitude 5.2 quoted by the USGS is the earthquake moment magnitude, which isn’t the same as the surface intensity measured by the Richter Scale. But for reference, 5.2 on the Richter Scale is near the top end of minor damage. Richter scale 2.5 to 5.4 Often felt, but only causes minor damage. 30,000 of these happen per year globally Richter scale 5.5 to 6.0 Slight damage to buildings and other structures. 500 per year globally

How big was the rupture area? This chart shows that a 5.2 event would normally have a rupture area of 5 to 20 square km. – an area the order of magnitude of one OCS block. But this depends on depth and other factors.

Gross setting Atwater Fold Belt Walker Ridge Salt Pillows Salt withdrawal basins Perdido Fold Belt

Tahiti Holstein Knotty Head K2 Mad Dog Puma Shenzi Atlantis Distance from major deep water fields Neptune 30 miles 15 miles 45 miles

USGS epicenter location relative to OCS blocks and nearby wells Epicenter Location in GC 344 Error margin +/- 10.6km Allegheney Field Scout well Pathfinder well Yorrick well Bison Field

5.2 Magnitude Earthquake in Green Canyon 344 (cgg_abdg Trace 14855) Reported Focus at 3.1 miles (16,368’) (4.2 miles NNW of Scout well) M7 M12 N S Error margin +/10.6km lateral Unknown vertical

Possible locations of the seismogenic event

Plio-Pleistocene Miocene Oligocene Cretaceous Paleocene- Eocene Oceanic Basement NWSE Continental Basement Western Atwater Fold Belt regional depth line (v=10h) Allochthonous salt

Stretched continental Stretched Oceanic Continent/ocean boundary Atwater Fold Belt Salt pillows and withdrawal basins Regional section v:h = 2:1 Earthquake origin area projected in to this line of section

Possible locations of the seismogenic event: 1. Intra-basement movement?

Plate tectonics and the evolution of the Gulf of Mexico NOAA image

subduction Present day plates and movement subduction strike slip and oblique rifting The extent of the strike-slip margin is growing at the expense of the subduction margin. In the gelogically recent past, the whole western margin of Mexico was dominated by subduction Deformation zone

Middle Jurassic South America North America Yucatan Mexico Pindell and Kennan 2001 GOM plate tectonic model

Start of Late Jurassic intracontinental stretching, subsidence South America North America Yucatan Mexico Red shaded areas = tectonically active

Tithonian Breakup and spreading of GoM and Proto-Caribbean South America North America Yucatan Mexico Red shaded areas = tectonically active

Early Cretaceous (Valanginian) Completion of Yucatan movement: reorganization of proto-Caribbean spreading Strike-slip pull-apart rifting in Mexico from Pindell and Kennan, 2001, reprinted with permission Caribbean South America Cuba North America Yucatan Mexico Red shaded areas = tectonically active

Aptian Completion of Yucatan movement: reorganization of proto-Caribbean spreading start of subduction of proto-Caribbean Caribbean South America Cuba North America Yucatan Mexico GoM Red shaded areas = tectonically active

Albian Continued spreading of proto-Caribbean, and subduction of its western part Compression on western Mexico margin Caribbean South America Cuba North America Yucatan Mexico GoM Red shaded areas = tectonically active

Campanian Continued spreading of proto-Caribbean, and subduction of its western part Compression throughout Mexico South America Cuba North America Yucatan Mexico GoM Red shaded areas = tectonically active

Late Paleocene Continued spreading of proto-Caribbean, and subduction of its western part Compression throughout Mexico South America North America Yucatan Mexico Cuba Caribbean GoM Red shaded areas = tectonically active

Mid Eocene Subduction of proto-Caribbean Peak of Cuban collision Major compression throughout Mexico Caribbean South America North America Yucatan GoM Cuba Mexico Red shaded areas = tectonically active

Early Oligocene Reorganization of north Caribbean margin Caribbean subduction front continues to migrate eastwards Caribbean South America Cuba North America Yucatan Mexico GoM Red shaded areas = tectonically active

Plate tectonics: a very unlikely candidate There is no active plate boundary in the Gulf of Mexico The last plate movement within the GoM was in Early Cretaceous Last close-by tectonic movement (Cuba) ended in the Paleogene Orogeny in Mexico is currently active but does not extend in to the GoM and does not connect to GC344 The GC seismogenic event cannot be linked to an active plate boundary How about the inactive boundaries?

Where is the Continent-Ocean boundary? Pindell and Kennan (2001) GOM plate tectonic model Stretched continental crust Post-Oxfordian oceanic crust GC344 seismic event

Stretched continental crust COB predicted by Pindell How close was the event to the COB? COB from seismic imaging and magnetics

Earthquake location Depth map of top of basement in Green Canyon from reflection seismic imaging. Note the proximity of the earthquake location to an obvious major basement boundary which probably corresponds to continent-ocean boundary

Mapped basement structure from seismic imaging

Northern GoM magnetics

Historical seismicity 1990 – 1Q2006, superimposed on basement structure (stretched continental?) (stretched oceanic?) Continent/ocean boundary

If it was in intra-basement event, what was the mechanism? Intraplate stress-induced earthquakes. Earthquakes are known to occur away from plate boundaries, caused by high regional stress levels. These are commonly associated with large through- going faults (e.g. the New Madrid event of 1811). The sawtooth map form of the COB in Green Canyon makes it unlikely to be reactivated in this way

Sawtooth map from of the COB

If it was in intra-basement event, what was the mechanism? Differential vertical movement. The composition, density, thickness, rigidity and thermal properties of the basement change across the COB. It is likely that the two blocks will have different vertical movement in response to sediment loading, thermal blanketing, etc.

Basement faulting driven by differential subsidence stretched continental stretched oceanic basement Vertical response to sediment load Vertical response sediment load

Possible locations of the seismogenic event: 1. Intra-basement faulting Summary: Basement faulting is possible A major basement fault lies within the source area Movement is not related to active plate tectonics Probably not induced by intraplate stress May be a response to differential subsidence

Possible locations of the seismogenic event: 2. Sliding on the deep salt detachment

Slip along deep salt as a mechanism? The timing is wrong – folding in the Atwater Fold Belt finished at end Miocene Folds are too old (Finished by end Miocene) Contact between slid sediments and base of salt

Western Atwater Fold Belt regional line present day no vertical exaggeration Pliocene sediments onlap the folds Allochthonous salt Basement salt Upper Miocene Lower Miocene Pleistocene J-lower K K NWSE

Possible locations of the seismogenic event: 2. Sliding on the deep salt detachment Summary: Unlikely mechanism Timing is wrong. Process has been active in the past (Miocene) but there is strong evidence against active movement on this surface

Possible locations of the seismogenic event: 3. Sliding on the Sigsbee salt

Slip along Sigsbee salt as a mechanism? The timing is right – active contraction is seen at the front of the Sigsbee Active folds and thrusts at toe of Sigsbee salt (multiple km of observed contraction) Contact between sliding sediments and base of salt Active growth faults above Sigsbee salt

Welded minibasins on seismic data Welded minibasins can be seen within the reported earthquake source area Movement of salt nappe and overlying sediments Crunch point where the bottom of the withdrawal basin is grinding along the base of the salt

Areas of welded minibasins (red) = potential sticking points on base salt Reported location and error margin welded area = candidate fault rupture

Possible locations of the seismogenic event: 3. Sliding on the Sigsbee salt Summary This is a possible explanation: Strong evidence that movement is occurring at present day Candidate welded area exists within source area

Possible locations of the seismogenic event: 4. near-surface gravity failure (slump)

Bathymetry map from 3D seismic data No steep sustained slopes near event location Area to the west is much steeper

Seafloor shaded relief map No steep sustained slopes near event location Area to the west is much steeper

Seafloor dip Bright color = strong dip No steep sustained slopes near event location Area to the west is much steeper

Seafloor dip map showing faults

Shaded seafloor topography No evidence of major slope failures at sea floor

Possible locations of the seismogenic event: 4. near-surface gravity failure (slump) Summary Unlikely mechanism: No strong surface dips Adjacent area has steeper dips and higher topography No visible evidence of large failures seen at surface

Basement faulting: possible –Not plate tectonic in origin –Probably not induced by intraplate stress –Possibly related to differential subsidence Movement on deep salt: unlikely Sliding on base Sigsbee: possible Near surface gravity failure: unlikely Conclusions