The crack was found in Pine Island Glacier, which last calved a significant iceberg in 2001; some scientists have speculated recently that it was primed to calve again. But until an Oct. 14 IceBridge flight, no one had seen any evidence of the ice shelf beginning to break apart. Since then, a more detailed look back at satellite imagery seems to show the first signs of the crack in early October. "We are actually now witnessing how it happens and it's very exciting for us. It's part of a natural process, but it’s pretty exciting to be here and actually observe it while it happens."
Gravity pulls the ice in the glacier westward along Antarctica's Hudson Mountains toward the Amundsen Sea. A floating tongue of ice reaches out 30 miles (48 kilometers) into the Amundsen beyond the grounding line, the below-sea-level point where the ice shelf locks onto the continental bedrock. As ice pushes toward the sea from the interior, inevitably the ice shelf will crack and send a large iceberg free. Pine Island Glacier is of particular interest to scientists because it is big and unstable and so is ONE OF THE LARGEST SOURCES OF UNCERTAINTY IN GLOBAL SEAL LEVEL RISE PROJECTIONS. It is one of the most significant science targets in the world of glaciology.
The IceBridge team observed the rift running across the ice shelf for about 18 miles (29 km), using an instrument called the Airborne Topographic Mapper, which uses a technology called lidar (light detection and ranging) that sends out a laser beam that bounces off a surface and back to the device. The lidar instrument measured the rift's shoulders about 820 feet (250 meters) apart at its widest, although the rift stretched about 260 feet (79 meters) wide along most of the crack. The deepest points from the ice shelf surface ranged from 165 to 195 feet (50 to 60 meters). When the iceberg breaks free, it will cover about 340 square miles (880 square kilometers) of surface area. Radar measurements suggested the ice shelf in the region of the rift is about 1,640 feet (500 meters) feet thick, with only about 160 feet of the shelf floating above water and the rest submerged. It is likely that once the iceberg floats away, the leading edge of the ice shelf will have receded farther than at any time since its location was first recorded in the 1940s.
Roger Miller
LARGEST QUAKES -
This morning -
None 5.0 or higher.
Yesterday -
11/2/11 -
5.1 SULAWESI, INDONESIA
5.0 COSTA RICA
6.0 PACIFIC-ANTARCTIC RIDGE
NEW ZEALAND - Newly uncovered details about the earthquake that rocked New Zealand in February may offer grim lessons regarding the potential threat of fault lines running through urban centers. The relatively moderate earthquake that struck the city of Christchurch in February surprised many with its destructive power. The magnitude 6.2 temblor killed more than 180 people and damaged or destroyed more than 100,000 buildings, the deadliest quake to strike New Zealand in 80 years. Much of the damage came from a phenomenon called liquefaction, where soils are shaken and begin to behave as a liquid, undermining buildings and other structures.
"The high intensity of shaking was greater than expected, particularly for a moderate-size earthquake, and the liquefaction-induced damage was extensive and severe." The degree of damage was particularly surprising given the relative preparedness of the city. "Compared to the earthquake that destroyed much of Haiti, the scale of disaster in Christchurch may seem small. Christchurch, however, was constructed using much better technology and engineering practices, raising a very sobering alarm to other major, high density western urban centers."
The Christchurch earthquake followed a larger magnitude 7.1 quake in Darfield, New Zealand, in September 2010 that was less destructive and did not cause any deaths. Both earthquakes ruptured along previously unmapped faults, but the corresponding damage was quite different. The differences seen between the sites helped offer scientists insights as to why the Christchurch earthquake proved so devastating. One key lesson regarding the unexpected intensity of the Christchurch quake may have to do with the city's foundations. Much of Christchurch was once swampland, beach dune sand, estuaries and lagoons that were drained as the area was settled. As a result, large areas beneath the city and its environs are characterized by loose sand, gravel and silt — soil types highly susceptible to liquefaction. Widespread damage induced by liquefaction within the central business district of the city required 1,000 buildings to be demolished.
Another lesson comes from the basin of bedrock that lies under Christchurch: The shape and material of this basin likely amplified ground shaking, trapping and focusing seismic energy within it just as a lens bends light. "Many urban areas are built over soft sediments and in valleys or over basins — for example, the San Francisco Bay Area and Los Angeles Metropolitan. These are urban areas that sit atop geological features that may exaggerate or amplify ground motion, just as Christchurch experienced." Profound changes in building codes are getting evaluated for the next generation of structures in New Zealand, ideas that may influence cities in the United States and the rest of the world that face similar hazards. "One of the major lessons to learn from Christchurch is to make the foundations of these buildings much stronger to reduce damage from liquefaction. However, the most important lesson may be to avoid construction on soft soils where liquefaction is a problem. This is just the beginning for New Zealand. I'm sure we'll see many changes down the road in their construction practices."
VOLCANOES -
CANARY ISLANDS - A series of quakes including one measuring 4.0 on the Richter scale shook Hierro island in Spain's Canaries on Wednesday, three weeks after the nearby undersea volcanic eruption. The 4.0-magnitude quake struck at 0755 GMT in the Atlantic about five kilometres (three miles) northwest of the town of Frontera. It was measured at a depth of 20 kilometres (12 miles).
Experts advised that the possibility of a new volcanic eruption could not be confirmed or ruled out. "It could happen but in principle it is not imminent." No evacuations were under way.The Spanish authorities sent tents, toilets and a canteen to the island to lodge 2,000 people in case of an evacuation. When combined with Red Cross supplies, a total 2,500 evacuees could be sheltered if needed. Some 500 residents and tourists were evacuated from the village of La Restinga after a 4.3-magnitude quake and undersea volcanic eruption October 10 but have since returned home. Restinga remains on red alert for a possible eruption, the highest point on a three-level warning scale. The previous volcanic eruption on the Canary Islands struck on nearby La Palma in 1971.
TROPICAL STORMS -
In the Arabian Sea -
Tropical cyclone 03a (Keila) was located approximiately 55 nm east-northeast of Salalah, Oman. It was expected to dissipate within 12 hours as a significant tropical cyclone over land. There is a small possibility that the remnant of 03a could exit back into the Arabian Sea and reintensify.
