Pn velocity anisotropy in a continental upper mantle

Summary. Earlier interpretations of P _n travel-times from the extensive quarry-blast observation scheme in western Germany — now supplemented by explosion data from the 1972 Rhinegraben experiment — have been checked and enhanced using the new MOZAIC time-term method. The large data set (762 travel times) continues to require a considerable anisotropy of upper-mantle P velocity. The resulting estimates of the overall velocity variation — probably 0.50–0.60 km/s about a mean value of 8.05 km/s, that is, 6 to 7 per cent anisotropy — and of the direction of the maximum velocity (close to 20° E of north) are reasonably reliable. However, the detailed form of the anisotropy is obscured by various limitations of the data.
These results allow a realistic assessment of the resolving power of refraction-based studies of velocity anisotropy in the lithosphere. It is concluded that though such studies are probably adequate if the measurement of in situ anisotropy is required within the context of a generalized discussion of lithospheric dynamics they are not appropriate if a detailed specification of the anisotropy is desired.     

Travel times of Pn -waves in the Aegean and surrounding area

Summary. Based on accurately located 23 very shallow earthquakes ( h = 1–14 km) in northern and central Greece by portable networks of seismic stations and by the joint epicentre method, the travel times of the P_n -waves from the foci of these earthquakes to the sites of 54 permanent stations in the Balkan region have been determined. The travel times of P_n -waves in the central and eastern part of the area (eastern Greece, south-eastern Yugoslavia, the Aegean Sea, Bulgaria, southern Romania, western Turkey) fit a straight line very well with the P_n velocity equal to 7.9 ± 0.1 km s^-1. On the contrary, the travel times of P_n -waves to stations in the western part of the area (Albania, western Greece) do not fit this curve because the P_n -waves travelling to these stations are delayed by more than 1 s due to the thicker crust under the Dinarides–Hellenides mountain range. Time delays for P_n -waves have been calculated for each permanent station in the Balkan area with respect to the mean travel-time curve of these waves in the central and eastern part of the area. Corrections of the travel times for these delays contribute very much to the improvement of the accuracy in the location of the shallow earthquakes in the Aegean and surrounding area.     

Three-dimensional VP and VP /VS models of the upper crust in the Friuli area (northeastern Italy)

3-D images of P velocity and P - to S -velocity ratio have been produced for the upper crust of the Friuli area (northeastern Italy) using local earthquake tomography. The data consist of 2565 P and 930 S arrival times of high quality. The best-fitting V _P and V _P / V _S 1-D models were computed before the 3-D inversion. V _P was measured on two rock samples representative of the investigated upper layers of the Friuli crust. The tomographic V _P model was used for modelling the gravity anomalies, by converting the velocity values into densities along three vertical cross-sections. The computed gravity anomalies were optimized with respect to the observed gravity anomalies. The crust investigated is characterized by sharp lateral and deep V _P and V _P / V _S anomalies that are associated with the complex geological structure. High V _P / V _S values are associated with highly fractured zones related to the main faulting pattern. The relocated seismicity is generally associated with sharp variations in the V _P / V _S anomalies. The V _P images show a high-velocity body below 6 km depth in the central part of the Friuli area, marked also by strong V _P / V _S heterogeneities, and this is interpreted as a tectonic wedge. Comparison with the distribution of earthquakes supports the hypothesis that the tectonic wedge controls most of the seismicity and can be considered to be the main seismogenic zone in the Friuli area.     

The lithosphere–asthenosphere boundary beneath the western United States

S receiver functions from 67 broad-band seismic stations in the western United States clearly reveal the existence of a mantle discontinuity with velocity reduction downward, which we interpret as the lithosphere–asthenosphere boundary (LAB). The average depth of the LAB is ∼70 km. The boundary is relatively sharp with an overall sharpness of less than 20 km. The boundary is more prominent south of the Mendocino Triple Junction, where the Farallon Plate has completely subducted. This may indicate partial melts at the base of the lithosphere caused by the upwelling of the asthenospheric flow through the slab window. A double low velocity zone is observed at base of the lithosphere beneath southern Sierra Nevada, implying a second melting zone at a depth of ∼100 km, well correlated with previous studies of lithospheric delamination in the area.     

A rain on snow climatology and temporal analysis for the eastern United States

ABSTRACT

Rain-on-snow (ROS) has the potential to produce devastating floods by enhancing runoff from snowmelt. Although a common phenomenon across the eastern United States, little research has focused on ROS in this region. This study used a gridded observational snow dataset from 1960–2009 to establish a comprehensive seasonal climatology of ROS for this region. Additionally, different rain and snow thresholds were compared while considering temporal trends in ROS occurrence at four grid cells representing individual locations. Results show most ROS events occur in MAM (March-April-May). ROS events identified with rainfall >1 cm are more frequent near the east coast and events identified with >1 cm snow loss are more common in higher latitudes and/or elevations. Decreasing trends in DJF (December-January-February) ROS events were identified near the coastal areas, with increasing trends in the northern portion of the domain. Significant decreasing trends in MAM ROS are likewise present on a regional scale. Factors playing a role in snowpack depth and rainfall, such as movement of storm tracks in this region, should be considered with future work to discern mechanisms causing the changes in ROS frequency.     

Stacking for the frequencies and Qs of 0S0 and 1S0

Summary. Using nine IDA records for the Indonesian earthquake of 1977 August 19, we have formed an optimal linear combination of the records and have measured the frequency and Q of ₀ S ₀ and ₁ S ₀. The frequency was measured using the moment ratio method. The attenuation was measured by the minimum width method and by the time-lapse method. The frequency and attenuation were measured simultaneously by varying them to obtain a best fit to the data. A 2000-hr stack, the sum of nine individual records, for ₀ S ₀ gave a frequency of 0.814664 mHz±4 ppm. The values for the Q of ₀ S ₀ for the three different methods of measurement were 5600,5833 and 5700, respectively. The error in the estimates of Q ^-1 is about 5 per cent for the minimum power method. For ₁ S ₀ a 300-hr stack yielded a frequency of 1.63151 mHz±30 ppm. The values of Q for this mode were 1960, 1800 and 1850, respectively, with an error in Q ^-1 of about 12 per cent for the minimum power method.