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April 10, 2010

Low-Frequency Earthquake Studies

The recently discovered phenomena of non-volcanic tremor (NVT) and slow aseismic slip are often coupled and recurrent, in which case they are called Episodic Tremor and Slip. Current research suggests that non-volcanic tremor and slow slip are both members of a family of unusually slow earthquakes. These slow earthquakes occur in diverse tectonic environments and appear to have the same mechanism as ordinary earthquakes, but they differ from normal earthquakes in their source location and moment-duration scaling. Unlike ordinary earthquakes, which grow explosively in size with increasing duration, slow earthquakes, whether large or small, grow at a constant rate. However, many questions remain unanswered as to the underlying cause of these events, their impact on general seismicity, their distribution and periodicity along strike, and their impact on the depth extent of the eventual megathrust rupture.

NVT episodes may be comprised of a sequence of identifiable, repeating low-frequency earthquakes (LFEs). Monica Maceira, Charlotte Rowe, and Dale Anderson (Geophysics, EES-17); and collaborator Gregory Beroza (Stanford University) have combined waveform cross-correlation and 3D signal polarization analysis to identify LFEs in non-volcanic tremor using the subspace detector method. The subspace detection method takes a matrix of master event waveforms and uses a singular value decomposition to build a set of basis vectors that, in some linear combination, can reproduce the master waveforms. Operating on the assumption that any waveform coming from the same source region and having a similar source mechanism can be characterized as some linear combination of their basis vectors, the scientists explored the utility of the method to search for LFEs in an hour-long segment of continuous seismic data recorded at several Japan Meteorological Agency seismic stations. Their application of the subspace detector using independent detection methods - cross-correlation and polarization analysis - confirms that LFEs are largely restricted to previously identified times, although they found a few additional events.

Polarization analysis: (a) 24 hours of data for the East component of station TBEH, with associated linearity, circularity and azimuth of the principal eigenvector, and (b) the hour (1700h-1800h) on which their subspace analysis is focused. For clarity in Figures 3a and Figure 3b, linearity and circularity are displayed as a Log10 density function over 4 s time windows, rather than scatter plots. Eigenvector azimuth is shown as Log10 density of the azimuth in two-degree bins.

Subspace detection shows promise for tremor because waveform matching is not needed so long as the templates used provide a suite of waveforms spanning the space of expected signal variation. The study show clearly defined changes in polarization observed during the LFE activity. This result indicates that signal polarization may also have potential for tremor detection, characterization, and monitoring.

The Institute of Geophysics and Planetary Physics (IGPP) program administered by LANL and the University of California, DOE National Nuclear Security Administration (NNSA), and the National Science Foundation supported the work.

"Identification of Low-frequency Earthquakes in Non-volcanic Tremor Using the Subspace Detector Method", Geophysical Research Letters 37, L06303 (2010); doi:10.1029/2009GL041876.

April 7, 2010

Discovery of a Linked Alternating Pattern in Warming Trends of the Arctic and Antarctic Oceans

Understanding the relationship between climate changes in the Arctic and Antarctic regions is essential for scientists to predict the dynamics of Earth's climate system. Polar regions are critical for global human-caused (anthropogenic) climate change due to possible melting of the polar ice sheets and subsequent sea level rise. The positive sea ice/snow albedo-temperature feedback may cause the polar regions to be more susceptible to external climate forcing and to unforced natural variability. An open ocean absorbs more solar radiation and allows more heat to be transferred to the atmosphere than an ocean covered by sea ice. The Arctic has warmed rapidly starting in the 1970s. Anthropogenic climate change is a long-term (centennial) trend superimposed on a natural variability that occurs on multi-decadal time-scales. Only by systematically understanding these two separate features, can scientists develop more precise climate forecasts to guide energy policy.

Petr Chylek (Space and Remote Sensing, ISR-2), Manvendra Dubey (Earth Systems Observations, EES-14), and collaborators from the British Meteorological Office and Dalhousie University analyzed century-long worldwide temperature records to distinguish human-induced warming from natural variability. The scientists discovered anti-correlations between temperature trends in the Arctic and Antarctic. When the Arctic warms, the Antarctic cools, and vice versa (Figure 3). This is the first description of an alternating, seesaw pattern in warming trends of the Arctic and Antarctic oceans during the 20th century. However, similar bi-polar seesaw patterns have been observed in Greenland and Antarctic ice core data. This indicates that polar seesaw patterns similar to the one observed during the 20th century may have existed during the past centuries and millennia.

De-trended Arctic (blue) and Antarctic (red) temperature time series smoothed by a 11 year running average (thin lines) or 17 year running average (thick lines).

The strong alternating pattern of the multidecadal temperature anomalies in the Arctic and Antarctic regions suggests a common cause. The complete cycle of the 20th century residual de-trended temperature occurred over approximately 70 years. The most likely component of the climate system whose state can persist for decades is the ocean. The researchers conclude that these temperature oscillations are related to the sloshing back and forth of the Atlantic Ocean, which redistributes the heat between the two poles, as measured by the Atlantic Multi-decadal Oscillation (AMO). The researchers suggest that the growing anthropogenic warming is now in phase with the AMO, and the interaction is exacerbating the Arctic warming in this decade. They believe that the intense Arctic warming since the 1970s arises from an additive combination of the general global warming trend with the warming phase of the multidecadal climate oscillation, while in Antarctica the cooling phase of the multidecadal oscillation opposes the general warming trend. The new observational finding challenges current coupled climate models, which are unable to predict these features. This result underscores the need for modelers and observationalists to work together to improve climate forecasts. Rising human greenhouse gases increase risks of irreversible changes in Earth's climate system and underscore the urgency of resolving these problems.

Cross-correlation coefficient between individual 5-degree wide latitudinal zones using 11 year (above the diagonal) or 17 year (below the diagonal) averaging of the de-trended temperature data. The dark blue indicates high anti-correlation between the two ends of the polar regions.

The DOE Office of Science, Office of Biological and Environmental Research, LANL Laboratory Directed Research and Development, and the LANL branch of the Institute of Geophysics and Planetary Physics funded different aspects of the LANL work. This research supports LANL's Energy Security mission.

"The 20th Century Bipolar Seesaw of the Arctic and Antarctic Surface Air Temperatures," Geophysical Research Letters, in press, doi:10.1029/2010GL042793.

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