Scott D. Davis

Teleseismic b values; Or, much ado about 1.0

In this paper we investigate the value of b in the Gutenberg-Richter relation for four teleseismic catalogs of earthquakes: Abes historical catalog, the Harvard Centroid Moment Tensor (CMT) catalog, the catalog of the International Seismological Centre (ISC), and the Blacknest catalog. An unfortunate result is that b differs by 30% or more when determined in different magnitude ranges, in different catalogs, or using different methods. For global catalogs separated into shallow, intermediate, and deep earthquake groups, all values determined for b lie between 0.72 and 1.34. We can identify no systematic global variation of b with depth. For teleseismic catalogs it is difficult to believe measured geographic variations in b because systematic errors cause problems of earthquake detection, earthquake location, aftershock identification, and magnitude determination. However, some variations in b are so persistent and large that they must be real. For deep earthquakes in Tonga-Fiji, for example, various measurements of b He between 1.06 and 1.57, comparable to b for shallow earthquakes, whereas measurements of b for deep earthquakes in the rest of the world are much lower, between 0.53 and 0.96. For shallow earthquakes in the Harvard CMT catalog, earthquakes with thrust and strike slip focal mechanisms have significantly lower b values (0.86 and 0.77) than earthquakes with normal faulting mechanisms (1.06). When we separate the ISC catalog into primary events (mainshocks and earthquakes with no aftershocks or foreshocks) and secondary events (aftershocks and foreshocks), we observe that b for secondary events is nearly always significantly higher than b for mainshocks. However, we show that the difference has no physical significance, as it arises simply from the act of choosing mainshocks as the largest earthquake in a foreshock-mainshock-aftershock sequence. When we correct for this systematic effect by comparing the real catalogs to identical catalogs with randomly reassigned magnitudes, we find that b for secondary events in the real catalog is actually lower than expected. Thus among aftershocks large earthquakes are relatively more common than expected, perhaps because the mainshock rupture loads asperities in adjacent regions.

How well constrained are well-constrained T, B, and P axes in moment tensor catalogs?

The T, B and P axes of earthquake moment tensors (MT) are often used to evaluate regional stress directions and other tectonic parameters; we here undertake three comparisons to assess the uncertainty in the orientations of these axes. These are (1) a direct comparison of common MT in the Harvard, U.S. Geological Survey, or Earthquake Research Institute (ERI) catalogs; (2) a comparison of MT slip vectors and plate motion vectors in several tectonically straightforward regions; and (3) an analysis of the axial variability in the Harvard and ERI catalogs implied by the reported uncertainties in individual MT components. All three comparisons indicate that there is considerable variability within the catalog concerning the axial orientation of MT, but all suggest that axis orientations of the majority of Harvard MT have uncertainties of 15° or less. For compensated linear vector dipole (CLVD) components among the three catalogs, the correlation is very low. For the Harvard catalog, three statistics are especially useful for selecting better constrained MT; these are (1) the relative error Erel, which is the ratio of the scalar moments of the reported error tensor and of the MT itself; (2)ƒCLVD, a measure of the strength of the CLVD component; and (3) nfree, the number of MT elements not fixed at zero in the inversion. For selecting better constrained MT, the appropriate statistical cutoffs chosen depend on the problem of interest, the data available, arid personal preference. However, for analysis of shallow earthquakes we have used Erel ≤ 0.15, ƒCLVD ≤ 0.20, and nfree = 6. While this eliminates 53% of the catalog, our calculations suggest that nearly all the remaining events have T, B, and P axes with azimuth and inclination angle uncertainties of 5°–10° or less.

Stress/strain changes and triggered seismicity at The Geysers, California

The principal results of this study of remotely triggered seismicity in The Geysers geothermal field are the demonstration that triggering (initiation of earthquake failure) depends on a critical strain threshold and that the threshold level increases with decreasing frequency, or, equivalently, depends on strain rate. This threshold function derives from (1) analyses of dynamic strains associated with surface waves of the triggering earthquakes, (2) statistically measured aftershock zone dimensions, and (3) analytic functional representations of strains associated with power production and tides. The threshold is also consistent with triggering by static strain changes and implies that both static and dynamic strains may cause aftershocks. The observation that triggered seismicity probably occurs in addition to background activity also provides an important constraint on the triggering process. Assuming the physical processes underlying earthquake nucleation to be the same, Gomberg [this issue] discusses seismicity triggered by the MW 7.3 Landers earthquake, its constraints on the variability of triggering thresholds with site, and the implications of time delays between triggering and triggered earthquakes. Our results enable us to reject the hypothesis that dynamic strains simply nudge prestressed faults over a Coulomb failure threshold sooner than they would have otherwise. We interpret the rate-dependent triggering threshold as evidence of several competing processes with different time constants, the faster one(s) facilitating failure and the other(s) inhibiting it. Such competition is a common feature of theories of slip instability. All these results, not surprisingly, imply that to understand earthquake triggering one must consider not only simple failure criteria requiring exceedence of some constant threshold but also the requirements for generating instabilities.

Remotely triggered microearthquakes at the Geysers Geothermal Field, California

Microearthquake (MEQ) activity recorded on a local network at The Geysers geothermal field has been observed to increase dramatically within seconds to minutes after the arrival of surface waves from large regional earthquakes. This behavior was observed not only following the 1992 Landers earthquake, for which remotely triggered seismicity (at The Geysers and elsewhere) has been documented, but also for the following events: Gulf of Alaska (Ms 7.6, March 6, 1988); Loma Prieta (Ms 7.1, October 17, 1989); two events within the Gorda Plate (Ms 6.9 on July 13, 1991 and Ms 7.1 on August 17, 1991); Petrolia ( Ms 7.0, April 25, 1992); and Northridge (M s 6.6, January 17, 1994). The exact onset times of most swarms are difficult to pinpoint, but all start within minutes of the passage of seismic waves and in at least one case (Gulf of Alaska) the onset appears to coincide with the expected arrival of the surface waves. These swarms generally last for a few hours at most, and decay with time in a manner similar to aftershock sequences. We propose that the dynamic stresses associated with seismic waves emanating from these earthquakes may trigger the onset of these swarms. The Landers event may have been particularly efficient at generating remotely triggered swarms throughout the western U.S. because of its size and shallow focal depth, which would promote large surface waves. Our data demonstrate, however, that the phenomenon is not unique to the Landers event.

Mars Orbiter Camera observations of the Martian south polar cap in 1999–2000

The spring-summer recession of the south polar cap of Mars in 1999–2000 has been investigated using the wide-angle cameras of the Mars Orbiter Camera experiment on Mars Global Surveyor. The 1999–2000 regression closely resembles the recession observed by Viking orbiters in 1977. Although the rates of recession are quite similar, the current recession was slightly ahead of that in 1977 throughout spring. The rapid development of dark terrain, dubbed “cryptic terrain” by Hugh Kieffer, in one sector of the south cap between LS = 198° and 223° is documented. The emergence and separation of the Mountains of Mitchel has also been recorded in detail, culminating with its complete disappearance at solstice. The 2000 residual cap is almost identical to that in 1977; this observation does not resolve the mystery of the very different appearance of the residual cap in 1972. The albedos in the residual cap region and in bright, seasonal frost increase rapidly to high values at around solstice and then level out until the seasonal frost disappears. Observations of more Martian years will be needed to constrain the amount of interannual variability and its relation to other phenomena such as dust storms.

Coordination of behavioral hierarchies during environmental transitions in Caenorhabditis elegans

For animals inhabiting multiple environments, the ability to select appropriate behaviors is crucial as their adaptability is often context dependent. Caenorhabditis elegans uses distinct gaits to move on land and in water. Gait transitions can potentially coordinate behaviors associated with distinct environments. We investigated whether land and water differentially affect the behavioral repertoire of C. elegans. Swimming worms interrupted foraging, feeding, egg-laying and defecation. Exogenous dopamine induced bouts of these land-associated behaviors in water. Our finding that worms do not drink fluid while immersed may explain why higher drug doses are required in water than on land to elicit the same effects. C. elegans is a valid model to study behavioral hierarchies and how environmental pressures alter their balance.

Evaluation of precursory seismic quiescence in sixteen subduction zones using single-link cluster analysis

This paper describes a new method, single-link cluster analysis (SLC), to evaluate percursory quiescence for shallow earthquakes in sixteen subduction zones, using data from the ISC catalog. To define quiescent regions, we divided the catalog into time intervals with a durationT, overlapping byT/2. We considered all earthquakes having magnitudes larger than some magnitudeMmin, lying within a specified distance of a great circle which is approximately coincident with the trench near a subduction zone. Within each time interval we connected or ‘linked’ all earthquakes lying within some cutoff distanced of one another. We then projected all these links onto the great circle, and defined a region to be quiescent if it was not covered by the projection of any links. For this study,T was two years,Mmin wasmb=4.9, and we variedd from 100 to 400 km. We defined an earthquake as ‘following quiescence’ if it occurred within two years following, and within 75 km of a quiescent zone as defined above. The primary conclusion of this study was that earthquakes with surface wave magnitudes 7.2 and greater were about 5–15% more likely to follow quiescence than were the smaller background earthquakes withmb>-4.9. A chi-squared analysis shows that this result is significant at the 99% level. In contrast, earthquakes with surface wave magnitude of 6.7 to 7.1 were no more likely to follow quiescence than were background earthquakes. Of sixteen individual regions, Central America, Japan, and Peru-Chile were the only regions where large earthquakes were more likely to occur following quiescence than were background earthquakes. For a cutoff link length of 300 km, only in Central America was the difference between large earthquakes and background earthquakes significant at the 95% level of significance. For a cutoff link length of 250 km, the significance level exceeded 95% only in Japan. The SLC method is an objective, quantitative method for evaluating large data catalogs, or for monitoring quiescence in regions where quiescence is conjectured to precede large earthquakes.

BK Channel Function Bidirectionally Influences Behavioral Deficits Following Withdrawal from Chronic Ethanol in Caenorhabditis elegans

The severity of withdrawal from chronic ethanol is a driving force for relapse in alcohol dependence. Thus, uncovering molecular changes that can be targeted to lessen withdrawal symptoms is key to breaking the cycle of dependence. Using the model nematode Caenorhabditis elegans, we tested whether one highly conserved ethanol target, the BK potassium channel, may play a major role in alcohol withdrawal. Consistent with a previous report, we found that C. elegans displays behavioral impairment during withdrawal from chronic ethanol that can be reduced by low-dose ethanol. We discovered that the degree of impairment is exacerbated in worms lacking the BK channel, SLO-1, and is alleviated by selective rescue of the BK channel in the nervous system. Conversely, behavioral impairment during withdrawal was dramatically lower in worms with BK channel function enhanced via gain-of-function mutation or overexpression. Consistent with these results, we found that chronic ethanol exposure decreased BK channel expression in a subset of neurons. In addition, we found that a distinct, conserved large-conductance potassium channel, SLO-2, showed the inverse functional relationship, influencing withdrawal behavior via a SLO-1 channel-dependent mechanism. Our findings demonstrate that withdrawal symptoms in C. elegans are mechanistically explained in part by a functional imbalance in the nervous system associated with a reduction in SLO-1 channel expression. Therefore, selective modulation of Slo family ion channel activity may represent a novel therapeutic approach to explore for normalizing behaviors during alcohol withdrawal. ARTICLE SUMMARY People addicted to alcohol maintain maladaptive drinking patterns in part to avoid the terrible symptoms of withdrawal. It is unclear whether any single molecule may be genetically modified to alleviate withdrawal symptoms. Here, we discover that for the nematode C. elegans, upregulating function of the conserved BK potassium channel SLO-1 prevents alcohol withdrawal symptoms. Conversely, downregulating SLO-1 channel function makes withdrawal worse. Moreover, we identify an inverse relation between SLO-1 and a second type of BK channel named SLO-2 in the severity of withdrawal. The BK channel thus represents an attractive molecular target to consider for alleviating alcohol withdrawal symptoms. Statement on data and reagent availability Strains are available upon request or through the Caenorhabditis Genetics Center.