Geofisica pura e applicata

We analyze foreshock activity in California and compare observations with simulated catalogs based on a branching aftershock-triggering model. We first examine foreshock occurrence patterns for isolated $$M \ge 5$$ earthquakes in southern California from 1981 to 2011 and in northern California from 1984 to 2009. Among the 64 $$M \ge 5$$ mainshocks, excluding 3 swarms and 3 doubles, 53 % of the rest are preceded by at least one foreshock within 30 days and 5 km. Foreshock occurrence appears correlated with mainshock faulting type and depth. Foreshock area is correlated with the magnitude of the largest foreshock and the number of foreshocks, however, it is not correlated with mainshock magnitude. We then examine the occurrence pattern of all seismicity clusters without a minimum magnitude requirement, and the possibility that they are “foreshocks” of larger mainshocks. Only about 30 % of the small clusters lead to a larger cluster. About 66 % of the larger clusters have foreshock activities, and the spatial distribution pattern is similar to $$M \ge 5$$ mainshocks, with lower occurrence rates in the Transverse Range and central California and higher occurrence rates in the Eastern California Shear Zone and the Bay Area. These results suggest that foreshock occurrence is largely controlled by the regional tectonic stress field and fault zone properties. In special cases, foreshock occurrence may be useful for short-term forecasting; however, foreshock properties are not reliably predictive of the magnitude of the eventual “mainshock”. Comparison with simulated catalogs suggest that the “swarmy” features and foreshock occurrence rate in the observed catalogs are not well reproduced from common statistical models of earthquake triggering.

Der Tagesgang von Temperatur und Luftfeuchtigkeit in San Salvador zeigt deutliche jahreszeitliche Verschiedenheiten, die wesentlich durch Strahlungsgenuss, Sonnenauf- und Untergangszeiten und ein tageszeitliches mittelräumiges Windsystem bestimmt ist.

As Antarctica is almost entirely covered by thick ice shields impeding in situ measurements, information about upper crustal structures and sedimentary basins is still sparse and the analysis of the gravity anomalies offers new insights. Isostatic gravity anomalies are often used to investigate upper crust structures. However, compensating masses significantly reduce the gravity effect of unknown sedimentary and upper crustal structures. To separate these effects, we apply so-called decompensative corrections to the isostatic anomalies for the Antarctic continent, which reach values of up to ± 70 mGal. The obtained decompensative anomalies well correspond to the known sedimentary basins, such as in the areas of the Filchner-Ronne Ice Shelf and Lambert Graben, and also suggest the existence of other large sedimentary deposits both in West and East Antarctica, which are not or only sparsely mapped by existing seismic surveys, e.g. in coastal Dronning Maud Land and Enderby Land. A dipole-like structure exists at the Transantarctic Mountains and the Wilkes Subglacial Basin, suggesting the presence of isostatic disturbances linked to the dynamic uplift of the Transantarctic Mountains and thick sedimentary accumulations in the east. Extended positive anomalies in East Antarctica are likely related to the old and dense cratonic crust as well as to isostatic disturbances caused by the transition from local to regional compensation around the Lambert Graben.

At first the results of the investigations on mother-of-pearl clounds are summarized. In the second section fit methods are suggested in order to further research.

Eine Abschätzung der im Gewitter zum Umsatz kommenden Energiebeträge zeigt, daß die in den elektrischen Phänomenen verbrauchte Energie im Gesamt-Energie-Umsatz nicht ins Gewicht fällt. Zieht man jedoch den Vergleich nur zu dem Teilbetrag des mechanisch-thermodynamischen Energieumsatzes, der sich über den normalen Energie-Umsatz des Wetter- bezw. Austauschgeschehens erhebt, und nimmt versuchsweise für diesen den aus dem Niederschlag allein abgeleiteten Betrag an, so ergibt sich, daß die elektrischen Phänomene etwa 1/4 dieses Betrages beanspruchen. — Die Abschätzung des elektrischen Energie-Umsatzes erlaubt es, die mittlere in einem Erdblitz umgesetzte Energie abzuleiten: Sie ergibt sich zu rund 2000 kWh. Eine noch nicht lösbare Diskrepanz ergibt sich für die Feldstärke unterhalb der Gewitterwolken.

Consideration of the behaviour of elastic bodies shows that it is not possible for a set of overcoring measurements that are made within isolated blocks to show residual strains or stresses that have a non-zero average, unless the size of the equilibrium volume over which the residual stresses balance is both considerably larger than the volume of the overcore and smaller than the size of the isolated block. Since some results have been reported that do not match these constraints, non-elastic behaviour must have occurred during overcoring. A possible explanation, is that oriented microcracks are somehow opened by the overcoring. In some cases, stress fields induced by microcracking near the overcoring cut may explain measured strain changes far from the cut. Consideration of various reported measurements in terms of relative sizes of overcoring and equilibrium volumes shows that care is necessary when interpreting residual stress measurements.

—Als Modell eines mechanischen Empfängers, der im Boden eingebettet ist, dient eine Kugelschale mit lineartranslatorisch beweglichem Gehänge. Das umgebende Medium erfüllt homogen den Vollraum und ist isotrop elastisch. Der Kontakt mit dem eingebetteten System wird entweder als haftend oder gleitend vorausgesetzt. Es ist diskutiert worden, wie sich der Empfänger in einer longitudinalen Planwelle von beliebiger Wellenform bewegt, die sich parallel zur Gehängetranslation ausbreitet. Ferner wurde die (unbekannte) Wellenform aus der Seismogrammform oder der Gestellbewegung explizit berechnet. Es ergeben sich beachtliche mathematische und physikalische Vereinfachungen, wenn das Gehänge vernachlässigbar leicht und die Gestellmasse gleich der «verdrängten Mediummasse» ist. Schliesslich wird der Zusammenhang dieses Vorgangs mit der Empfängerbewegung infolge Kräften besprochen, die nicht von der Welle herrühren und am Gestell und Gehänge gleichzeitig angreifen können. Es wurde der Kraftverlauf gesucht, der einer vorgegebenen Wellenform hinsichtlich der erzwungenen Gestellbewegung (und des Seismogramms) gleichwertig ist, und umgekehrt die zu einer bestimmten Kraftfunktion äquivalente Wellenform angegeben.

A study was made of the maximum amount of energy which might be absorbed by the atmosphere down to 80 km during a sudden ionospheric disturbance. Assuming that the ionization is caused by Lyman α radiation from the sun, and that all oxygen present is ionized, the energy absorbed by the sunlit hemisphere is 5.65·1027 ergs.