Subsea Structures May Be in Seismic Danger Zone part 2

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Gulf of Mexico Subsea Structures May Be in Seismic Danger Zone

Posted on August 27, 2010 by Steve Drinkard

BP's Atlantis - World's Deepest Moored Semisubmersible

Part 2:  Large 2006 Earthquakes of 6.0 and 5.2 Magnitude Strike the Gulf of Mexico–Unidentified Oil Rig Feels Shaking — Epicenter Relocated in Green Canyon

In 2003, the Mineral Management Service ordered an assessment of seismic activity in the Gulf of Mexico (GOM), which we discussed in Part 1 of this series.  There was concern then whether oil production assets appeared to be in the path of increasing seismic risks. Now, within approximately three years of that study, two more larger earthquakes strike in the GOM.  Let’s take a look at the history of those 2006 earthquakes.

The larger of the two earthquakes hit in September 2006.  It shook a broad area and received significant public notice.  The slightly smaller, but more threatening of the two occurred in February 2006. Its epicenter was in oil rich Green Canyon.  It was the one that stirred the most attention from the oil industry. The concern remains identifying the extent of the threat posed by seismic events in the GOM oil producing region. Will continued development risk increased danger to our environment?

On September 11, 2006 the news-wires were reporting a 6.0 magnitude earthquake that had struck in the Gulf of Mexico the day before.  The U. S. Geological Survey (USGS) summary of the event reported a strong earthquake located 250 miles south-southwest of Apalachicola, Florida at 8:56 a.m. MDT on September 10, 2006.  The earthquake shook hard enough to knock items from shelves and created seismic disturbances of oscillating waves appearing in swimming pools in parts of Florida.  Numerous cities in Florida felt the shaking. The strength of the event was substantial for the GOM.  USGS received reports that it was “felt” in Alabama, Florida, Georgia, Kentucky, Louisiana, Mississippi, North Carolina, South Carolina, Tennessee and Texas.  Other areas outside of the United States that reported feeling the earthquake were The Bahamas and Cancun and Merida, Mexico. There were no reports of serious damage or casualties.

USGS Tectonic Summary

The official USGS report place the earthquake beneath the GOM far away from the nearest active plate boundary. Thus, the event was not located along a recognized fault line.  This phenomenon forms some measure of mystery over the cause of quakes in the Gulf.  The “midplate” earthquakes are much less common than “earthquakes occurring on faults near plate boundaries.”  It is thought that the most likely explanation for this earthquake is “long-term tectonic stresses” originating from forces applied at the plate boundary (located in the Atlantic Ocean).  At least one geologist with extensive experience in the Gulf of Mexico has departed from the establishment’s causal theories.  We shall discuss his beliefs later in the series.  The epicenter location is designated with a star in the map below.

Zoomed In Map of Earthquake Epicenter 9-10-06

The USGS says this earthquake was the largest of more than a dozen seismic events instrumentally recorded in the GOM in the past three decades.  It was also the most widely felt.

5.2 Magnitude Earthquake Shakes Unidentified Oil Rig

On February 10, 2006 a 5.2 magnitude earthquake struck in the oil producing Green Canyon region of the Gulf.  According to a USGS study the location of this earthquake stirred the interest of the petroleum industry as indicated in the following excerpt:

The offshore southern Louisiana seismic event has attracted much interest in the U.S. Gulf Coast energy industry, because of its location in an area of dense infrastructure associated with the production of petroleum (fig. 1). The event was felt onboard one offshore production platform (Rijken and Leverette, 2007) and at scattered locations along the U.S. Gulf Coast (U.S. GeologicalSurvey, 2008).

The location of the exact epicenter has attracted an extraordinary round of scientific inquiry.  Several studies have been done and published.  The USGS study referenced here was conducted to relocate the epicenter from it original setting. The task took into account data from private sources to supplement its own seismic readings.  While the exact point of the event remains elusive,  one private report puts an unidentified oil rig platform around 9.4 miles from the Green Canyon event.  The rig apparently escaped without damage to its SeaStar Tension Leg

Atlantis Rig – Located In Green Canyon

Platform (TLP) construction.  The crew onboard the unidentified rig reported feeling the vibrations of the earthquake. The Atlantis, owned by BP, was located near the original epicenter designated by USGS, although not yet in production.  It is the deepest moored semi-submersible platform in the world located in over 7,070 feet of water. At the time of this earthquake, the Atlantis had ocean-bottom node array of seismic exploration monitors taking readings. This information was incorporated into USGS relocation of the epicenter.  One presumes that the exact location of the epicenter becomes more important if it rests upon a highly valued oil prospect, where the building of subsea structures just might prove dangerous to life and environment. For whatever the reason, there was significant effort made to review the USGS initial finding.  The seismic data gathered presented some unusual seismic information, which proved challenging for the geologists.  The best that could be done was to establish a “preferred” location depicted in the map below:

The gray lines depicted in the map show the extensive subsea energy structures that may be threatened by continued seismic activity in the region.  Earthquakes are not reliably predictable under the best geological knowledge.  The difficulty here is evident from the 30 pages of evaluation of the various seismic wave data.  Some of the data was provided from monitoring equipment owned by BP which was operating around the Atlantis seafloor area.  Comparative information was also available from the CGG Green Canyon Phase VIII Multi-Client Survey.  The preferred epicenter was ultimately established 26 km north of the original center point.  This had the effect of moving the epicenter further away from BP’s Atlantis.   However, the accuracy of the determination remains subject to great doubt.  The source for the earthquake was left largely undecided with early speculation of a “landslide” loosing favor and the existence of an unidentified “fault” unknown.  The concluding remarks make this observation:

It is noteworthy, however, that the revised epicenter of the Green Canyon event lies near the USGS/NEIC epicenters of two earlier earthquakes – those of June 30, 1994 (mb 4.2) and December 9, 2000 (mb 4.2; Ms 4.3) (fig. 3).  As with the epicenter of the Green Canyon event, the epicenters of the earlier events are likely not determined with high accuracy. Nonetheless, extrapolating from the times of the three events, it appears that a region with linear dimensions of several tens of kilometers may be experiencing an earthquake of greater than magnitude 4 one or two times a decade, on average. If the earthquakes in this region are more numerous still at lower magnitudes, in keeping with most seismic regions worldwide, one might expect 10 or 20 shocks of magnitude 3 and greater in an average decade.

The widely held view among geologists is that there are no fault lines in the Gulf of Mexico to explain the source of seismic events.  Yet these scientists seem to be saying that the linear area, extrapolated from timing and location of earthquake events, may form a line of several tens of kilometers along which earthquakes are to be expected. Is this a fault line 100 to 200 miles long or what?  Does it increase the threat to oil production in the area?  What design changes or precautions should be taken?  Of course, any such discussion involves cost and adversely affects corporate profits. These seem to be appropriate questions.  My search has found no evidence of proposed precautionary measures even being discussed.  There has been no change in the Zone 0 seismic risk rating of the region.  There has been no study that I have found of what affect on seismicity the extraction of oil and gas may be causing in the Gulf.  I have been unable to find any new studies to expand these findings or test the existing structures.

Is there a coming major earthquake in the future of the Gulf of Mexico?  Are we likely to have an underwater Haiti?  There is cause for concern given the seismic events and USGS studies that have been discussed in Part 1 and 2 of this series.