Non-volcanic tremor driven by large transient shear stresses

Non-impulsive seismic radiation or 'tremor' has long been observed at volcanoes and more recently around subduction zones. Although the number of observations of non-volcanic tremor is steadily increasing, the causative mechanism remains unclear. Some have attributed non-volcanic tremor to the movement of fluids, while its coincidence with geodetically observed slow-slip events at regular intervals has led others to consider slip on the plate interface as its cause. Low-frequency earthquakes in Japan, which are believed to make up at least part of non-volcanic tremor, have focal mechanisms and locations that are consistent with tremor being generated by shear slip on the subduction interface. In Cascadia, however, tremor locations appear to be more distributed in depth than in Japan, making them harder to reconcile with a plate interface shear-slip model. Here we identify bursts of tremor that radiated from the Cascadia subduction zone near Vancouver Island, Canada, during the strongest shaking from the moment magnitude M(w) = 7.8, 2002 Denali, Alaska, earthquake. Tremor occurs when the Love wave displacements are to the southwest (the direction of plate convergence of the overriding plate), implying that the Love waves trigger the tremor. We show that these displacements correspond to shear stresses of approximately 40 kPa on the plate interface, which suggests that the effective stress on the plate interface is very low. These observations indicate that tremor and possibly slow slip can be instantaneously induced by shear stress increases on the subduction interface-effectively a frictional failure response to the driving stress.     

Dynamics of seismogenic volcanic extrusion at Mount St Helens in 2004-05

The 2004-05 eruption of Mount St Helens exhibited sustained, near-equilibrium behaviour characterized by relatively steady extrusion of a solid dacite plug and nearly periodic shallow earthquakes. Here we present a diverse data set to support our hypothesis that these earthquakes resulted from stick-slip motion along the margins of the plug as it was forced incrementally upwards by ascending, solidifying, gas-poor magma. We formalize this hypothesis with a dynamical model that reveals a strong analogy between behaviour of the magma-plug system and that of a variably damped oscillator. Modelled stick-slip oscillations have properties that help constrain the balance of forces governing the earthquakes and eruption, and they imply that magma pressure never deviated much from the steady equilibrium pressure. We infer that the volcano was probably poised in a near-eruptive equilibrium state long before the onset of the 2004-05 eruption.     

At least two genes reside within a large intron of the dunce gene of Drosophila

The dunce locus of Drosophila melanogaster is considered to house a gene involved in memory, because flies carrying lesions at the locus have shortened memory of several different conditioned behaviours. Our recent partial characterization of the gene at the molecular level, along with prior genetic and biochemical evidence, recently provides compelling evidence that the gene codes for the enzyme cAMP phosphodiesterase. The observation that the gene encodes at least six overlapping poly(A)+ RNA molecules ranging in size from 4.2 to 9.5 kilobases (kb) (ref. 8), suggests that the gene is extraordinarily complex. Here we provide the sequence of a dunce complementary DNA clone and the corresponding genomic coding regions which show that the organization of the gene is elaborate. The cDNA clone defines dunce exons which are separated by a large intron of 79 kb. More importantly, at least two other genes are shown to reside within the large intron, including the well-defined glue protein gene, Sgs-4. The location of dunce exons relative to the molecular breakpoints of chromosomal aberrations with defined cytological positions indicates that the dunce gene extends over more than five polytene chromosome bands.     

Kuhn reconstructed: Incommensurability without relativism

The standard reading of Kuhn's philosophy attributes to him the view that the incommensurability of rival theories and theory-ladenness of observation make rational debate about competing paradigms nearly impossible. If this reflects his real view, then he has claimed something prima facie absurd, and easily refuted with historical counter-examples. It is not the incommensurability thesis per se that is easily refutable, but Kuhn's gestelt interpretation of it. The gestalt interpretation, moreover misrepresents his more fundamental ideas on paradigms, and is in itself confused. The incommensurability thesis can be explained and defended without invoking gestalts, and this reconstructed view can be used to show that familiar criticisms, such as those of Davidson and Laudan, and unwelcome endorsements, such as that of Barnes, which are based on the assumption that Kuhn must be an extreme relativist, are all directed at views that he need not, and should not, hold.     

A meiotic gene conversion gradient opposite to the direction of transcription.

Genetic recombination involves classical crossing-over and gene conversion (aberrant segregation). In fungi that produce an ascus containing four spores, a gene conversion event is manifested as 3:1 or 1:3 (or more rarely 4:0 or 0:4) segregations, in contrast to the normal mendelian 2:2 segregation. Polarity is one of the properties of gene conversion; in almost all cases the frequency of conversion exhibits a gradient across the gene monitored. The frequency of conversion is usually independent of the specific allele used as a marker, but dependent on its location. An interpretation of conversion polarity is that it is caused by the existence of specific initiation sites for meiotic recombination, located at the high end of the polarity gradient. Here we show that the polarity gradient for the HIS2 gene of Saccharomyces cerevisiae is high at the 3' end of the gene, implying that the promoter of HIS2 is not the initiation site.