Global glacier melt continues

Glaciers around the globe continue to melt at high rates. Tentative figures for the year 2007, of the World Glacier Monitoring Service at the University of Zurich, Switzerland, indicate a further loss of average ice thickness of roughly 0.67 meter water equivalent (m w.e.). Some glaciers in the European Alps lost up to 2.5 m w.e.

The new still tentative data of more than 80 glaciers confirm the global trend of fast ice loss since 1980. Glaciers with long-term observation series (30 glaciers in 9 mountain ranges) have experienced a reduction in total thickness of more than 11 m w.e. until 2007. The average annual ice loss during 1980-1999 was roughly 0.3 m w.e. per year. Since 2000, this rate has increased to about 0.7 m w.e. per year.

Michael Zemp, glaciologist and research associate of the WGMS, said: �The average ice loss in 2007 was not as extreme as in 2006, but there were large differences between mountain ranges. Glaciers in the European Alps lost up to 2.5 meters water equivalent of ice, whereas maritime glaciers in Scandinavia were able to gain more than a meter in thickness. However, 2007 is now the sixth year of this century in which the average ice loss of the reference glaciers has exceeded half a meter. This has resulted in a more than doubling of the melt rates of the 1980s and 90s.�

For the observation period 2007, dramatic ice losses were reported from glaciers in the European Alps, such as of the Hintereisferner (-1.8 m w.e.) or the Sonnblickkess (-2.2 m w.e.) in Austria, the Sarennes (-2.5 m w.e.) in France, the Cares�r (-2.8 m w.e.) in Italy, or of the Silvretta (-1.3 m w.e.) and Gries (-1.7 m w.e.) in Switzerland. In Norway, many maritime glaciers were able to gain mass, e.g. the Nigardsbreen (+1.0 m w.e.) or the �lfotbreen (+1.3 m w.e.), although the glaciers further inland have continued to shrink, e.g. the Hellstugubreen or the Gr�subreen (both with -0.7 m w.e.).

All mass balance programmes in South American reported negative values ranging from -0.1 m w.e. at the Echaurren Norte in Chile to -2.2 m w.e. at the Ritacuba Negro in Columbia. In North America some positive values were reported from the North Cascade Mountains and the Juneau Ice Field together with a continued ice loss from the glaciers in the Kenai Mountains and the Alaskan Range as well as from Canada’s Coast Mountains and High Arctic.

Measuring unit ‘water equivalent':

Glaciologists express the annual mass balance, i.e. the gain or loss in thickness, of a glacier in ‘meter water equivalent’ (m w.e.). This standardized unit takes the different densities of change measurements in ice, firn and snow into account (see Photos 1 and 2). One meter of ice thickness corresponds to about 0.9 m w.e.

Research links seismic slip and tremor, with implications for subduction zone

In the last decade, scientists have recorded regular episodes of tectonic plates slowly, quietly slipping past each other in western Washington and British Columbia over periods of two weeks or more, releasing as much energy as a magnitude 6 earthquake.

The slip events coincide with regular occurrences of what scientists call nonvolcanic tremor, which showed up clearly on seismometers but for which the origins were uncertain.

Now researchers from Italy and the University of Washington have concluded that both phenomena are signs of the same processes taking place about 25 miles deep at what is believed to be the interface between the Juan de Fuca and North American tectonic plates.

“We are now more confident that the tremor and the slip are both products of the same slip process,” said Kenneth Creager, a UW professor of Earth and space sciences and a co-author of a paper describing the research being published Jan. 30 in Science.

The findings could have major implications for megathrust earthquakes in the Cascadia subduction zone, an area along the West Coast from northern California to southern British Columbia. Megathrust events are huge earthquakes, often in the range of magnitude 9, that occur in areas where one tectonic plate is forced beneath another.

The slow slip events appear to be building stress on the megathrust fault, where the Juan de Fuca plate is sliding beneath the North American plate, with the two locked together most of the time. That pressure is relieved when the plates slip during megathrust earthquakes such as one determined to have occurred off the coast of Washington on Jan. 26, 1700, estimated at magnitude 9.2. That quake was similar to the great Sumatra-Andaman Islands earthquake the day after Christmas in 2004, which also measured 9.2 and triggered a devastating Indian Ocean tsunami.

In such events, the plates are locked together for hundreds of years and then slip past each other by sliding 50 feet or more during a megathrust earthquake.

“The same amount of slip must also occur onshore along the Washington coast,” Creager said. “While megathrust earthquakes account for most of the plate motion offshore, and perhaps slightly onshore, episodic tremor and slip harmlessly accommodates much of the plate motion that is taking place on plate interface just west of the Puget Sound region’s major population centers.”

The paper’s lead author is Mario La Rocca of Italy’s National Institute of Geophysics and Volcanology Vesuvius Observatory. Other authors are Danilo Galluzzo, also of Italy’s geophysics and volcanology institute, and Steve Malone, John Vidale, Justin Sweet and Aaron Wech of the UW.

Slip events occur on the interface between tectonic plates, but previous research has suggested that nonvolcanic tremor occurs in a broad range of depths from the plate boundary to 15 miles above it. The new research indicates the tremor is at the plate boundary, in essentially the same place as the slip.

The researchers used seismometer arrays at Sequim and Lopez Island in Washington state and at Sooke on the southern edge of Canada’s Vancouver Island to record an episodic tremor and slip event in 2004. La Rocca devised a novel method using the different times that specific waves generated by the tremor were detected by seismometers, and the data helped the scientists pinpoint the depth of the tremor. At the same time, GPS measurements recorded the slow plate slippage.

Since they were discovered in the last decade, slow slip and tremor events in western Washington and British Columbia have been recorded on a regular basis about every 15 months. GPS signals indicate slip of about 1 inch during an average episode.

“We are quite confident that each episodic tremor and slip event will increase the stress on the megathrust fault,” Creager said. “If a megathrust earthquake were to begin off the Washington coast, one might expect it to occur during one of these slow slip events.”

But he said the findings demonstrate that much research remains to be done.

“We’re just scratching the surface in understanding how all of this works.”

MIT: Improving oil extraction with new mapping technology

Picture this: an accurate map of a large underground oil reservoir that can guide engineers’ efforts to coax the oil from the vast rocky subsurface into wells where it can be pumped out for storage or transport.

Researchers in MIT’s Department of Civil and Environmental Engineering have developed technology that can generate such a map, which has the potential to significantly increase the amount of oil extracted from reservoirs.

The new technology uses the digital image compression technique of JPEG to create realistic-looking, comprehensive maps of underground oil reservoirs using measurements from scattered oil wells. These maps would be the first to provide enough detail about an oil reservoir to guide oil recovery in the field in real time.

“Our simulation studies indicate that this innovative approach has the potential to improve current reservoir characterization techniques and to provide better predictions of oil-reservoir production. The hope is that better predictions ultimately lead to more efficient operations and increased oil production,” said Behnam Jafarpour, a recent MIT graduate who is now an assistant professor in petroleum engineering at Texas A&M University.

Jafarpour and Dennis McLaughlin, the H.M. King Bhumibol Professor of Water Resource Management at MIT, published a pair of papers describing the technique that will appear in an upcoming issue of the Society of Petroleum Engineering Journal, as well as a third paper that appeared in the June 2008 issue of Computational Geosciences.

The spatial structure in geologic formations makes it possible to compress rock property maps. But JPEG compresses the many pixels in a detailed image down to a few essential pieces of information that require only a small amount of storage. In the oil reservoir characterization application developed by MIT researchers, a similar mechanism is used to provide concise descriptions of reservoir rock properties. The new technique uses oil flow rates and pressure data from oilfield wells to create a realistic image of the subsurface reservoir.

Petroleum extraction is expensive and relatively inefficient – sometimes as little as one-third of the oil in a reservoir is actually recovered through pumping. So engineers rely on enhanced recovery techniques such as water flooding to mobilize the oil. To guide this work, they make real-time predictions of subsurface variables, including oil saturation and pressure, but they’re essentially working blindly. The rock properties needed to make these predictions (for instance fluid conductivity of rock at a particular depth) can’t be seen or measured.
Instead, engineers infer geologic properties indirectly from seismic data and measurements taken at scattered wells.

“In a typical reservoir, millions of pixels are needed to adequately describe the complex subsurface pathways that convey the oil to wells. Unfortunately, the number of seismic and well observations available for estimating these pixel values is typically very limited. The methods we’ve developed extract more information from those limited measurements to provide better descriptions of subsurface pathways and the oil moving through them,” said McLaughlin, lead researcher on the project.

In a 36-month simulated oil-recovery process, McLaughlin and Jarfarpour’s estimation approach accurately captured the main features and trends in fluid conductivity of a reservoir formation, demonstrating that the new technique is robust, accurate and efficient.

“Our next step – already in progress – is to test our idea in real oil reservoirs and evaluate its impact on oil recovery under realistic field settings,” Jafarpour said.

Early warning systems underestimate magnitude of large earthquakes

Scientists seek to create reliable early warning systems that accurately estimate the magnitude of an earthquake within the first seconds of rupture. In this paper published by the Bulletin of the Seismological Society of America, authors S. Murphy of University College Dublin, Ireland and S. Nielsen of the Instituto Nazionale di Geofisica e Vulcanologia, Roma, Italy look at the idea that an earthquake’s final size can be determined during its initiation, rather than something that only becomes apparent at the end of the rupture. They found that, while this may be true over a small range of earthquake sizes, it is unlikely to hold for the larger magnitudes, limiting its applicability for early warning systems.

Alternatively, the authors found that rapid magnitude estimation could be better explained in terms of what seismic stations capture of an earthquake in a few seconds. This section is generally quite large and is dependent on the relative position of the station to the fault. Therefore using a number of seismic stations around an earthquake fault, as is the case in early warning systems, the size of the earthquake can be quickly estimated.

This explanation shows a scaling between ground motion and final earthquake size similar to that observed from seismograms. The authors found that this relationship breaks down for very large earthquakes, i.e. earthquakes with a magnitude greater than 6.5. In these cases, the seismic stations no longer capture the edges of the fault in a few seconds due to the large area of the fault. When this happens, the authors suggest that early warning systems which use the peak ground displacement technique for estimating earthquake size, shall underestimate the size of the earthquake.