Widespread Anomaly Research Articles

The Timing and Source of Long-Run Returns Following Repurchases

This paper investigates the timing and source of anomalous positive long-run abnormal returns following repurchase authorizations. Returns between program authorization and completion announcements are indistinguishable from 0. Abnormal returns occur onlyaftercompletion announcements. Long-run returns are largely attributable to announcement returns at subsequent authorizations and takeover attempts; that is, anomalous post-authorization returns are not persistent drifts but rather step functions. These findings have important implications for prior papers examining this most persistent and widespread anomaly. Further, our results serve to refocus the search for a rational explanation for the anomaly on subsequent repurchase announcements and takeover bids.

Environmental upheavals of the Ediacaran period and the Cambrian “explosion” of animal life

The second half of the Ediacaran period began with a large impact – the Acraman impact in South Australia, which was accompanied by a negative δ13Ccarb anomaly and an extinction-radiation event involving acritarchs. A few million years later (∼570 Ma?) there was a second, deeper and longer-lived world-wide δ13Ccarb anomaly (the Shuram anomaly) which coincides with extinction of the acanthomorphic acritarchs. Wide distribution of the Shuram event is exemplified by stratigraphic sections from South Australia, Oman, southern California and South China. The widespread anomaly has been tentatively attributed to a marine impact. During recovery from the Shuram event the enigmatic Ediacaran biota achieved its zenith, only to be extirpated and replaced by a polyphyletic assemblage of shelly animals in what is known as the Cambrian “explosion”. This extinction-radiation cycle was preceded by glaciation, another δ13Ccarb excursion and the highest 87Sr/86Sr values known from marine carbonates. These high Sr ratios have been linked to weathering of extensive tracts of continental crust that were elevated during amalgamation of the supercontinent Gondwana. Introduction of essential nutrients to the oceans would have promoted biological production of oxygen and provided P and Ca for the important skeletonization that characterizes the Cambrian “explosion” and caused a quantum leap in the preservation potential of animal remains. Turbulent events of the last 50 million years of Precambrian time include three glaciations, two large impacts and a massive orogenic episode. These dramatic environmental upheavals are held responsible for three consecutive extinction-radiation cycles that culminated in the appearance of a diverse array of shelly fossils. Various lines of evidence suggest that the metazoans have deep roots so that they too may have been subjected to the environmental pressures of the late Ediacaran period clearly illustrated by acritarchs and the Ediacaran biota but the long-lived diversity of the metazoan population was “suddenly” revealed by the acquisition of biomineralization.

The Timing and Source of Long-Run Returns Following Repurchases

This paper investigates the timing and source of anomalous positive long-run abnormal returns following repurchase authorizations. Returns between program authorization and completion announcements are indistinguishable from zero. Abnormal returns occur only after completion announcements. Long-run returns are largely attributable to announcement returns at subsequent authorizations and takeover attempts, i.e., anomalous post-authorization returns are not persistent drifts but rather step functions. These findings have important implications for prior papers examining this most persistent and widespread anomaly. Further, our results serve to refocus the search for a rational explanation for the anomaly on subsequent repurchase announcements and takeover bids.

3‐D Tomography of Velocity Structure in the Upper Crust beneath the Tianchi Volcanic Area of Changbaishan

AbstractWe have obtained tomographic images of P‐ and S‐wave velocity structures and VP/VS perturbation at different depths benenth the Changbaishan Tianchi volcanic region by inversion of three‐dimensional (3‐D) fine velocity structure in the upper crust of the region. The inversion is based on reflected waves from Moho obtained by a 3‐D seismic transmission experiment in the study area. The result shows that the heterogeneity of the velocity structure in the Tianchi volcanic region and its surrounding areas is closely related to the geological structure and existence of the magma system in the area. The distribution of obvious P‐wave low‐velocity anomalous bodies and high VP/VS perturbation image suggests that the media are in a “soft and high‐temperature” state. This conclusion is consistent with the results of wide‐spread geothermal anomaly and low Q‐value structure obtained by other studies.

Assessment of mammographic film processor performance in a hospital and mobile screening unit.

In contrast to the majority of mammographic breast screening programmes, film processing at this centre occurs on site in both hospital and mobile trailer units. Initial (1989) quality control (QC) sensitometric tests revealed a large variation in film processor performance in the mobile unit. The clinical significance of these variations was assessed and acceptance limits for processor performance determined. Abnormal mammograms were used as reference material and copied using high definition 35 mm film over a range of exposure settings. The copies were than matched with QC film density variation from the mobile unit. All films were subsequently ranked for spatial and contrast resolution. Optimal values for processing time of 2 min (equivalent to film transit time 3 min and developer time 46 s) and temperature of 36 degrees C were obtained. The widespread anomaly of reporting film transit time as processing time is highlighted. Use of mammogram copies as a means of measuring the influence of film processor variation is advocated. Careful monitoring of the mobile unit film processor performance has produced stable quality comparable with the hospital based unit. The advantages of on site film processing are outlined. The addition of a sensitometric step wedge to all mammography film stock as a means of assessing image quality is recommended.

Geochemical gold prospecting at Vinsanmaa, northern Finland

At Vinsanmaa, Rovaniemi rural commune, northern Finland, there is a mineralized quartzite vein in albite diabase, which has been investigated by Outokumpu Co. The vein averages 3 g/t Au. In the regional till-geochemical mapping done by the Geochemical Department of the Geological Survey of Finland a clear and widespread Au anomaly was found in till about 1 km SE of the vein. The investigations were made step by step. In the first two stages the samples were taken with light percussion drill, in the third stage with hydraulic hammer and finally with diamond drilling. Geophysical measurements were also carried out. The fine fraction of till and weathered bedrock and also the diamond drill core samples were analyzed for Co, Cu, Mn, Ni, Pb, Zn and Ag by flame atomic absorption and Au and Pd by flameless atomic absorption. The results show that the widespread anomaly cannot have been caused by the previously known vein. The Au occurs in rocks in a very fine-grained form and has clearly been enriched into the fine fraction of weathered bedrock. Only in one diamond drill hole anomalous Au contents were found. The highest Au content is in the upper part of altered diabase. The results indicate clearly that in the Vinsanmaa area there are no other Au showings like the vein already known. The widespread Au anomaly in till resembles the contents in bedrock and has been caused principally by weathering of diabase rock.