I am sticking by that assessment.
But Reuters notes that the use of explosives to seal the oil gusher could cause a landslide:
“I think this situation has taught us from the start to have a backup,” retired U.S. Coast Guard Admiral Thad Allen, the top official overseeing the spill response, told reporters this week. …
If both relief wells fail, Allen said BP has a plan to fabricate a new pipeline, place it on the seabed and hook it up to the leaking wellhead. …
[Donald Van Nieuwenhuise, director of petroleum geoscience programs at the University of Houston] said BP also could possibly detonate a bomb — conventional or nuclear — in the well to try to stop the flow.
But a blast could damage seabed oil and gas pipelines or cause an undersea mudslide…
“The big danger with any kind of implosion method is that it’s a wild card, and you don’t know what other kinds of problems it would bring,” Nieuwenhuise said.
Landslides can cause big waves. Indeed, the largest wave ever recorded was caused by a landslide. Specifically, the biggest wave ever recorded was 1,740 feet high (technically, it was smaller; but it’s splash was that high):
Headland beside Lituya Glacier that was swept the giant 1740 ft wave
Photo by Byron Hale.
The biggest wave on record occurred in Lituya Bay on the southern coast of Alaska in 1958. An earthquake measuring 8.3 on the Richter scale hit the area and shook loose an estimated 40 million cubic yards of dirt and glacier from a mountainside at the head of the bay. When the debris hit the water, a massive 1,720-foot wave was created and washed over the headland.
How did the scientists know the wave was so incredibly enormous? Simple. To measure the height of the wave, scientists found the high-water mark — the line where the water reached its highest point on land. This probably is not the biggest wave ever, just the biggest documented. Three fishing boats witnessed the Lituya Bay event. Unfortunately, two people on one of the boats were killed. Incredibly, the other two boats rode the waves and their occupants survived.
That was from a landslide which came down into the water from above.
But as Florida Oil Spill Law – my favorite Gulf oil news aggregator – notes:
Undersea Landslide to Blame for Deadly Wave; Scientists: Conditions for Tsunami May Exist in U.S., ABC News, July 18, 2000:
[D]iscoveries are drawing our attention to other causes of tsunamis, besides the traditional tectonic earthquake, [Eddie Bernard, director of the National Oceanic and Atmospheric Administration's Pacific Marine Environmental Laboratory] said. The more we learn about possible causes, the better we can know when to issue warnings.
Offshore areas most prone to landslides are those where eons of runoff sediments from rivers have created terraces built of loose materials, he said.
When it breaks loose, the material drops with the speed of a snow avalanche, displacing the water below and leaving a void the water fills with a bump that spawns the localized tsunami, Bernard said. …
Tsunamis move at 500 to 600 mph in deep ocean waters but slow and get taller as they reach shallow offshore waters.
The Gulf of Mexico is rarely affected by earthquakes. Any slight movement is likely to displace loose sediments that have not moved in a very long of time.
BP’s blown-out well sits in the path of the Mississippi River Delta, around 50 miles offshore. Louisiana State University geologists estimate between “2.79 trillion and 3.45 trillion tons of sediment have been stored in the delta since the end of the last glacial maximum.”
The last point – the amount of sediment in the Delta – is almost certainly irrelevant, as the spill site is likely not close enough to the Delta to be effected by the bulk of the loose sediment. As the following satellite image taken in early May by NASA’s MODIS Rapid Response Team on May shows, the spill site (white dot) is not directly adjacent to the Delta (green haze), and only a very small portion of Delta sediments even come close:
The [origin of the] Mississippi Canyon … has generally been attributed to channel entrenchment of the Mississippi River during low stands of sea level and erosion of the more distal parts by turbidity currents or submarine gravity flows…. During the interval from Illinoian to late Pleistocene (25,000-27,000 years B.P.), the Mississippi River deposited a series of fluvial and deltaic deposits of approximately 1,000 m. There is no evidence that a submarine canyon existed in the vicinity of the present feature during this time interval. Approximately 25,000 years ago, a C-14-dated horizon was truncated by the initial formation of the submarine canyon. Samples dated by C-14, obtained near the base of the canyon fill, show that by 20,000 years B.P. [this stands for "before present", not "British Petroleum"],canyon fill had commenced. Thus, this major submarine trough had, at most, 7,000 years in which to remove 1,500 to 2,000 km.3 of material. It is highly probable, therefore, that the canyon originated from massive shelf-edge slope failure on an unstable continental margin. A series of successive failures, each one creating an upslope instability that triggered the next failure, caused an elongate trough to form that excavated the canyon to a depth of 1,220 m. below present sea level. Once the canyon has formed, its steep side walls continued to be unstable and sediments slumped into the canyon axis, forming the initial canyon fill. This phase is well documented: the lowermost sediment fill is composed of displaced material similar to that now found on the canyon rim. Large scars from side-wall failures can also be easily mapped on the seismic data. From 20,000 years to approximately 5,000 years B.P., a series of late Wisconsin and Holocene delta lobes formed and were responsible for the remainder of the fill of the canyon. During the past 5,000 years only a thin deep-water pelagic drape has been deposited within the canyon. Maps have been constructed that depict the various horizons, and the geometry of these horizons verify this mode of formation.
In other words, huge underwater landslides in the recent geologic past formed the present features in the Mississippi Canyon. And it is easy to see from images of the spill site that the spill site is located in a fairly steep canyon which – if disturbed – could conceivably slide down:
Production in the deepwater province is centered in turbidite sands recently deposited from the Mississippi delta.
Salt is the dominant structural element of the ultra-deepwater Gulf of Mexico petroleum system. Large horizontal salt sheets, driven by the huge Plio-Pleistocene to Oligocene sediment dump of the Mississippi, Rio Grande and other Gulf Coast Rivers, dominate the slope to the Sigsbee escarpment. Salt movement is recorded by large, stepped, counter-regional growth faults and down-to-the-basin fault systems soling into evacuated salt surfaces. Horizontal velocities of salt movement to the south are in the several cm/year range, making this supposedly passive margin as tectonically active as most plate boundaries.
Porosities over 30 percent and permeabilities greater than one darcy in deepwater turbidite reservoirs have been commonly cited. Compaction and diagenesis of deepwater reservoir sands are minimal because of relatively recent and rapid sedimentation. Sands at almost 20,000 feet in the auger field (Garden Banks 426) still retain a porosity of 26% and a permeability of almost 350mdarcies. Pliocene and Pleistocene turbidite sands in the Green Canyon 205 field have reported porosities ranging from 28 to 32% with permeabilities between 400 mdarcies and 3 darcies.
See also this.
So what’s the bottom line?
In the absence of explosives, an underwater landslide at the spill site is unlikely, and a slide-induced tsunami even less likely.
Hopefully, the relief wells will work, and explosives will be unnecessary.
But if explosives – either nuclear or conventional – are used, explosives experts must consult with the top geologists specializing in the Mississippi Canyon region to determine optimal safety precautions so as to minimize the chance of a slide
Because the explosives would be placed thousands of feet under the sea floor (to seal the oil gusher near the bottom of the well), the chance of a landslide are arguably not that great.
But because the stakes are so high – and because at least some have raised the possibility that seismic waves from a nuclear blast at the leak site could rupture other oil wells in the Gulf – all possible measures should be used to minimize the negative effects from the use of explosives.