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A family of repeating low-frequency earthquakes at the downdip edge of tremor and slip

ABSTRACT

We analyze an isolated Low-Frequency Earthquake (LFE) family located at the downdip edge of the main episodic tremor and slip (ETS) zone beneath western Washington State. The 9000 individual LFEs from this repeating family cluster into 198 swarms that recur roughly every week. Cumulative LFE seismic moment for each swarm correlates strongly with the time until the next swarm, suggesting that these LFE swarms are time-predictable. Precise double-difference relative locations for 700 individual LFEs within this family show a distribution that is approximately 2 km long and 500 m wide, elongated parallel to the relative plate convergence direction. The distribution of locations (<300 m vertical spread) lies within a few hundred meters of two different plate interface models and has a similar dip. Peak-to-peak LFE S-wave amplitudes range from 0.2 to 18 nm. Individual LFEs exhibit a trend of increasing magnitude during swarms, with smaller events at the beginning and the largest events towards the end. The largest LFEs cluster in a small area (300 m radius) coincident with maximum LFE density. We propose that the less-concentrated smaller LFEs act to unlock this patch core, allowing it to fully rupture in the largest LFEs, usually towards the end of a swarm. We interpret the patch responsible for producing these LFEs as a subducted seamount on the downgoing Juan de Fuca (JdF) plate. LFE locking efficiency (slip estimated during 5 years from summing LFE seismic moment divided by plate-rate-determined slip) is at most 20% and is highly concentrated in two ~50 m radius locations in the larger patch core. Estimated individual LFE stress drops range from 1 to 20 kPa, but could also be significantly larger.

 
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Posted by on September 2, 2014 in Blog

 

Regional study of the archean to proterozoic crust at the Sudbury Neutrino Observatory (SNO+), Ontario: Predicting the geoneutrino flux

Abstract

The SNO+ detector that is currently under construction in Ontario, Canada, will be a new kiloton scale liquid scintillation detector with the capability of recording geoneutrino events that can be used to constrain the strength of the Earth's radiogenic power, and in turn, to test compositional models of the bulk silicate Earth (BSE). We constructed a detailed 3-D model of the regional crust centered at SNO+ from compiled geological, geophysical and geochemical information. Crustal cross sections obtained from refraction and reflection seismic surveys were used to characterize the crust and assign uncertainties to its structure. The average Moho depth in the study area is 42.3 ± 2.6 km. The upper crust was divided into seven dominant lithologic units on the basis of regional geology. The abundances of U and Th and their uncertainties in each upper crustal lithologic unit were determined from analyses of representative outcrop samples. The average chemical compositions of the middle and lower crust beneath the SNO+ region were determined by coupling local seismic velocity profiles with a global compilation of the chemical compositions of amphibolite and granulite facies rocks. Monte Carlo simulations were used to predict the geoneutrino signal originating from the regional crust at SNO+ and to track asymmetrical uncertainties of U and Th abundances. The total regional crust contribution of the geoneutrino signal at SNO+ is predicted to be image TNU (a Terrestrial Neutrino Unit is one geoneutrino event per 1032 target protons per year), with the Huronian Supergroup near SNO+ dominantly contributing image TNU to this total. Future systematically sampling of this regional unit and denser seismic surveys will better model its composition and structure, and thus reduce the uncertainty on geoneutrino signal at SNO+. The bulk crustal geoneutrino signal at SNO+ is estimated to be image TNU, which is lower than that predicted in a global scale reference model that uses an average composition of the global upper continental crust [Huang et al., 2013], due to the fact that Archean to Proterozoic Canadian Shield has lower U and Th concentrations. Finally, without accounting for uncertainties on the signal from continental lithospheric mantle and convecting mantle, the total geoneutrino signal at SNO+ is predicted to be image TNU

 
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Posted by on September 2, 2014 in Blog

 

Changing global diets is vital to reducing climate change, researchers say

Healthier diets and reducing food waste are part of a combination of solutions needed to ensure food security and avoid dangerous climate change, say the team behind a new study.
 
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Posted by on August 31, 2014 in Blog

 

Antarctic sea level rising faster than global rate

A new study of satellite data from the last 19 years reveals that fresh water from melting glaciers has caused the sea level around the coast of Antarctica to rise by 2cm more than the global average of 6cm. Researchers detected the rapid rise in sea-level by studying satellite scans of a region that spans more than a million square kilometers. The melting of the Antarctic ice sheet and the thinning of floating ice shelves has contributed an excess of around 350 gigatonnes of freshwater to the surrounding ocean.
 
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Posted by on August 31, 2014 in Blog

 

Wine only protects against cardiovascular disease in people who exercise, study finds

Wine only protects against cardiovascular disease (CVD) in people who exercise, according to results from the a study. Evidence suggesting that mild to moderate consumption of wine protects against cardiovascular disease has been accumulating since the early 1990s. In particular, retrospective studies have found that wine increases levels of HDL, the "good" cholesterol. But until now there has been no long-term, prospective, randomised study comparing the effects of red and white wine on HDL cholesterol and other markers of atherosclerosis.
 
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Posted by on August 31, 2014 in Blog

 

Drinking tea reduces non-cardiovascular mortality by 24 percent, study finds

Drinking tea reduces non-cardiovascular mortality by 24 percent, reveals a study in 131,000 people. "Tea has antioxidants which may provide survival benefits. Tea drinkers also have healthier lifestyles so does tea drinking reflect a particular person profile or is it tea, per se, that improves outcomes -- for me that remains an open question. Pending the answer to that question, I think that you could fairly honestly recommend tea drinking rather than coffee drinking and even rather than not drinking anything at all," one researcher said.
 
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Posted by on August 31, 2014 in Blog

 

A geodetic plate motion and global strain rate model

Abstract

We present a new global model of plate motions and strain rates in plate boundary zones constrained by horizontal geodetic velocities. This Global Strain Rate Model (GSRM v.2.1) is a vast improvement over its predecessor both in terms of amount of data input as in an increase in spatial model resolution by factor of ˜2.5 in areas with dense data coverage. We determined 6739 velocities from time-series of (mostly) continuous GPS measurements; i.e., by far the largest global velocity solution to date. We transformed 15772 velocities from 233 (mostly) published studies onto our core solution to obtain 22511 velocities in the same reference frame. Care is taken to not use velocities from stations (or time periods) that are affected by transient phenomena; i.e., this data-set consists of velocities best representing the interseismic plate velocity. About 14% of the Earth is allowed to deform in 145,086 deforming grid cells (0.25º longitude by 0.2º latitude in dimension). The remainder of the Earth’s surface is modeled as rigid spherical caps representing 50 tectonic plates. For 36 plates we present new GPS-derived angular velocities. For all the plates that can be compared with the most recent geologic plate motion model, we find that the difference in angular velocity is significant. The rigid-body rotations are used as boundary conditions in the strain rate calculations. The strain rate field is modeled using the Haines and Holt method, which uses splines to obtain an self-consistent interpolated velocity gradient tensor field, from which strain rates, vorticity rates, and expected velocities are derived. We also present expected faulting orientations in areas with significant vorticity, and update the no-net rotation reference frame associated with our global velocity gradient field. Finally, we present a global map of recurrence times for Mw=7.5 characteristic earthquakes.

 
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Posted by on August 30, 2014 in Blog

 

Pioneer strategy for creating new materials

Making something new is never easy. Scientists constantly theorize about new materials, but when the material is manufactured it doesn't always work as expected. To create a new strategy for designing materials, scientists combined two different approaches at two different facilities to synthesize new materials. This new strategy gives faster feedback on what growth schemes are best, thus shortening the timeframe to manufacture a new, stable material for energy transport and conversion applications.
 
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Posted by on August 29, 2014 in Blog

 

New biodiversity metric defined by researchers

To understand how the repeated climatic shifts over the last 120,000 years may have influenced today's patterns of genetic diversity, a team of researchers developed a new biodiversity metric called 'phylogeographic endemism.'
 
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Posted by on August 29, 2014 in Blog

 

Reducing water scarcity possible by 2050

Water scarcity is not a problem just for the developing world. Increased water-recycling and improved irrigation techniques are among many strategies identified in North America as key to successfully reducing global water scarcity. But despite what some may see as an insurmountable problem, it is possible to turn the situation around and significantly reduce water scarcity in just over 35 years, according to researchers.
 
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Posted by on August 29, 2014 in Blog