Initiation of unstable slips–microearthquakes by elastic impulses

A series of laboratory experiments have been carried out with a model of two granite blocks under biaxial compression loading. The experiments are mainly intended for assessing the possibilities of partially releasing the accumulated potential energy. The model was subjected to calibrated mechanical impacts (strokes) which induced elastic impulses. The mechanical stresses, strains, and acoustic emission were recorded. The strokes caused both large slips releasing the stresses down to their initial level and small slips which reduced the stresses by 5–8%. The small slips mostly occurred after the precursory emergence of the low frequency oscillations having low amplitudes. Before the large slips, the stages of speeding-up of the relative motion of the sides of the block contact was observed, similar to those emerging before the natural slips unrelated to the strokes. This feature was not universal: in some cases, the model recovered to the stationary state of the block contact without a large slip. All the slips occurred with a time delay after the stroke. The time delay was shorter when the energy of the blow was higher. With the shorter time delays, the small slip is more likely to occur. The energy of the impacts was by three orders of magnitude lower than the energy accumulated by the model, which points to the triggering mechanism of slip initiation. The series of strokes resulting in the small displacements partially reduced the accumulated energy and prevented the emergence of large motions such as the stick-slip events. If after a series of such blows a large sliding event still occurred, its energy was higher than in the slips unrelated to the impacts. The experiments revealed the difficulties in solving the problem of earthquake hazard reduction by elastic impacts.     

Asymmetrical pulses, the periodicity and synchronization of low frequency microseisms

Seismic records of eight IRIS broadband stations were analyzed at distances of 70 to 7160 km from the magnitude 8.3 Hokkaido earthquake of September 25, 2003. The stations situated in the subduction zone recorded asymmetrical microseismic pulses lasting 3–10 min a few days before the earthquake. No such pulses were observed in the records of the stations situated outside the subduction zone. Similar pulses were also recorded before the magnitude 7.8 Kronotskii, Kamchatka earthquake of 1997. The pulses are hypothesized to have been caused by creeping movements. Synchronous oscillations of microseismic noise with periods of 1–3 h were recorded as far as 3000 km from the Hokkaido earthquake a few days before it occurred. The noise coherence measure increased for stations closer to the epicenter. The question of the source of this coherence remains open. These effects belong to the class of those occurring in dissipative metastable systems; parts of the terrestrial lithosphere during the precursory periods of seismic catastrophes seem to be such systems.     

Multi-scale temporal features of the 14 July 2000 meter- wavelength dynamic radio spectrum compared with TRACE data

The 14 July 2000 (`Bastille Day') eruptive and geoeffective flare event was observed by the digital IZMIRAN radio spectrograph in the frequency range of 25–270 MHz. This instrument allowed the analysis of various features of the dynamic radio spectrum and their comparison with other observational data, in particular with development of a spectacular EUV post-eruption arcade recorded aboard the Transition Region and Coronal Explorer (TRACE). (1) A compressed multi-hour radio spectrum shows that the event caused a conspicuous weakening of the pre-existing noise storm. This phenomenon was perhaps caused by interaction of a large halo coronal mass ejection (CME), recorded by the the Large Angle and Spectroscopic Coronagraph (LASCO) aboard the Solar and Heliospheric Observatory (SOHO), with emitting coronal structures. (2) Several type II bands are present at the initial and maximum phases of the flare event. The frequency drifts of the clearest bands correspond to the estimated shock wave speed of 1100–2300 km s⁻¹ that is comparable with the CME speed observed in the sky plane. (3) Significant broadband enhancements of the metric radio emission took place around of 10:24–10:27 UT coinciding with sharp development of the EUV arcade in the northeast direction. It appears to correspond to the intensification of the electron acceleration in a process of post-eruption loop formation. (4) The high-resolution radio spectrum revealed a superposition of numerous type III-like bursts and/or pulsations with a time scale ranging from a few seconds to several tens of seconds. These features can be attributed particularly to successive formation of new loops of the arcade and corresponding temporal fragmentation of the electron acceleration in the course of the post-eruption reconnection. In summary, the analysis demonstrates the correspondence between the multi-scale temporal features of the metric radio emission and such phenomena as the CME and post-eruption EUV arcade. Some spectra, images, and movies illustrating the event are presented also on the accompanying CD-ROM. Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1023/A:1014224004946     

Study of variable stars associated with maser sources: G025.65+1.05

We report variation of K-band infrared(IR) emission in the vicinity of the G025.65+1.05 water and methanol maser source. New observational data were obtained with the 2.5 m telescope at the Caucasian Mountain Observatory(CMO) of Moscow State University on 2017–09–21 during a strong water maser flare. We found that the IR source situated close to the maser position had decreased brightness in comparison to archive data. This source is associated with a massive young stellar object(MYSO) corresponding to the compact IR source IRAS 18316–0602(RAFGL 7009 S). A similar decrease in K-brightness of the IR source close to the maser position was observed in March 2011 when the water maser experienced a period of increased activity. The dips in MYSO brightness can be related to the maser flare phases. Maser flares that are concurrent with dips in the IR emission can be explained if the lower IR radiation field enables a more efficient sink for the pumping cycle by allowing IR photons to escape the maser region.     

Seismological evidence for the nucleation of two strong earthquakes

The structure of seismicity before the Kronotskii (1997) and Simushir (2006) earthquakes off Russian shores is studied. Different algorithms are applied to the identification of seismic quiescence, seismicity activation, and clustering of seismic events. It is established that the seismic process developed according to the same scenario in both cases. Seismic quiescence arose a few years before an earthquake near the epicenter in an area ～200 km in size. A 100-km area of subsequent activation included the epicenter. The activation stage ended more than one year before an earthquake. An abrupt rise in the number of clusters of seismic events was observed a few days before an earthquake near its epicenter.     

Composition of the Siberian cratonic mantle: evidence from Udachnaya peridotite xenoliths

Bulk compositions and mineral analyses for forty-one, large, garnet- and spinel-facies peridotite xenoliths from the Udachnaya kimberlite in the central Siberian platform have many similarities to those of well-studied peridotites from the Kaapvaal craton in southern Africa. Coarse Mg-rich lherzolites and harzburgites with equilibration temperatures below 1000 °C are abundant and are believed to form the principal rock type in the Siberian lithosphere. The low-temperature Udachnaya peridotites have an average mg number [Mg/(Mg+Fe)] of 92.6 with a wide dispersion in modal enstatite, ranging to over 40 wt%. High-temperature peridotites are relatively richer in Fe and Ti and are commonly deformed, with porphyroclastic or mosaic-porphyroclastic textures, some of the latter having fluidized enstatite. The Udachnaya peridotites have experienced late-stage metasomatism before, during and after eruption. Garnets and pyroxenes in many of the high-temperature rocks are zoned, probably by reaction with melt prior to eruption. Virtually all the peridotites contain secondary diopside, inhomogeneous on a micron scale, that mantles primary orthopyroxene. It is believed to have crystallized along with lesser amounts of intergranular calcite and monticellite during eruption. Bulk analyses for total Fe in many specimens are higher than whole-rock Fe calculated from the electron probe analyses and the modes. The magnitude of the difference between the two measurements of total Fe correlates with loss-on-ignition, suggesting that Fe has been introduced during serpentinization following eruption. These late metasomatic processes have thus affected some major as well minor and trace element compositions. The similarities in bulk composition of peridotites from Udachnaya and the Kaapvaal are evidence of a common origin. Low-temperature cratonic peridotites differ from oceanic peridotites in having higher mg numbers (>92) and in having relatively high but wide-ranging modal enstatite (Mg/Si = 1.06–1.49 weight fraction). The Udachnaya low-temperature peridotites have an inverse correlation between FeO (calculated from the probe analyses and modes) and SiO₂. This correlation is also present in the Kaapvaal data but is complicated by a greater range in fertility that produces a positive variation of Fe with Si. A negative trend for Fe/Si can be seen within a portion of the Kaapvaal data, that for low-Ca harzburgites, in which the variation in fertility is restricted. The negative trends for Fe/Si can be interpreted as a consequence of either segregation of olivine and orthopyroxene by metamorphic differentiation or partial sorting during cumulate formation. Received: 18 June 1996  / Accepted: 11 February 1997     

Star formation around the H ii region Sh2-235

We present a picture of star formation around the H  ii region Sh2-235 (S235) based upon data on the spatial distribution of young stellar clusters and the distribution and kinematics of molecular gas around S235. We observed ¹³CO (1–0) and CS (2–1) emission toward S235 with the Onsala Space Observatory 20-m telescope and analysed the star density distribution with archival data from the Two Micron All-Sky Survey (2MASS). Dense molecular gas forms a shell-like structure at the southeastern part of S235. The young clusters found with 2MASS data are embedded in this shell. The positional relationship of the clusters, the molecular shell and the H  ii region indicates that expansion of S235 is responsible for the formation of the clusters. The gas distribution in the S235 molecular complex is clumpy, which hampers interpretation exclusively on the basis of the morphology of the star-forming region. We use data on kinematics of molecular gas to support the hypothesis of induced star formation, and distinguish three basic types of molecular gas components. The first type is primordial undisturbed gas of the giant molecular cloud, the second type is gas entrained in motion by expansion of the H  ii region (this is where the embedded clusters were formed) and the third type is a fast-moving gas, which might have been accelerated by winds from the newly formed clusters. The clumpy distribution of molecular gas and its kinematics around the H  ii region implies that the picture of triggered star formation around S235 can be a mixture of at least two possibilities: the 'collect-and-collapse' scenario and the compression of pre-existing dense clumps by the shock wave.