Back‐projection stacking of P‐ and S‐waves to determine location and focal mechanism of microseismic events recorded by a surface array

We present an automatic method of processing microseismic data acquired at the surface by a star‐like array. The back‐projection approach allows successive determination of the hypocenter position of each event and of its focal mechanisms. One‐component vertical geophone groups and three‐component accelerometers are employed to monitor both P‐ and S‐waves. Hypocenter coordinates are determined in a grid by back‐projection stacking of the short‐time‐average‐to‐long‐time‐average ratio of absolute amplitudes at vertical components and polarization norm derived from horizontal components of the P‐ and S‐waves, respectively. To make the location process more efficient, calculation is started with a coarse grid and zoomed to the optimum hypocenter using an oct‐tree algorithm. The focal mechanism is then determined by stacking the vertical component seismograms corrected for the theoretical P‐wave polarity of the focal mechanism. The mechanism is resolved in the coordinate space of strike, dip, and rake angles. The method is tested on 34 selected events of a dataset of hydraulic fracture monitoring of a shale gas play in North America. It was found that, by including S‐waves, the vertical accuracy of locations improved by a factor of two and is equal to approximately the horizontal location error. A twofold enhancement of horizontal location accuracy is achieved if a denser array of geophone groups is used instead of the sparse array of three‐component seismometers. The determined focal mechanisms are similar to those obtained by other methods applied to the same dataset.     

Risk‐based seismic design for collapse safety

Risk‐based seismic design, as introduced in this paper, involves the use of different types of analysis in order to satisfy a risk‐based performance objective with a reasonable utilization rate and sufficient reliability. Differentiation of the reliability of design can be achieved by defining different design algorithms depending on the importance of a structure. In general, the proposed design is iterative, where the adjustment of a structure during iterations is the most challenging task. Rather than using automated design algorithms, an attempt has been made to introduce three simple guidelines for adjusting reinforced concrete frames in order to increase their strength and deformation capacity. It is shown that an engineer can design a reinforced concrete frame in a few iterations, for example, by adjusting the structure on the basis of pushover analysis and checking the final design by means of nonlinear dynamic analysis. A possible variant of the risk‐based design algorithm for the collapse safety of reinforced concrete frame buildings is proposed, and its application is demonstrated by means of an example of an eight‐storey reinforced concrete building. Four iterations were required in order to achieve the risk‐based performance objective with a reasonable utilization rate. Copyright © 2016 John Wiley & Sons, Ltd.     

Origin of Coronal Shock Waves

The basic idea of the paper is to present transparently and confront two different views on the origin of large-scale coronal shock waves, one favoring coronal mass ejections (CMEs), and the other one preferring flares. For this purpose, we first review the empirical aspects of the relationship between CMEs, flares, and shocks (as manifested by radio type II bursts and Moreton waves). Then, various physical mechanisms capable of launching MHD shocks are presented. In particular, we describe the shock wave formation caused by a three-dimensional piston, driven either by the CME expansion or by a flare-associated pressure pulse. Bearing in mind this theoretical framework, the observational characteristics of CMEs and flares are revisited to specify advantages and drawbacks of the two shock formation scenarios. Finally, we emphasize the need to document clear examples of flare-ignited large-scale waves to give insight on the relative importance of flare and CME generation mechanisms for type II bursts/Moreton waves.     

The Lyrid Meteor Stream: Orbit and Structure

A filamentary structure in the Lyrid meteor stream based on photographic orbits available in the IAU Meteor database is identified and studied. About 17 Lyrids are found in the database and the stream mean orbit is derived. The shower radiant is compact, of a size 2° × 1.5°. Applying a stricter limiting value for the Southworth-Hawkins D-criterion, two distinct filaments in the stream, on a short and a long period orbit, are separated. To confirm their consistency as filaments, their orbital evolution over 5,000 years is investigated.     

Determination of the reference symmetry axis of a generally anisotropic medium which is approximately transversely isotropic

For a given stiffness tensor (tensor of elastic moduli) of a generally anisotropic medium, we estimate to what extent the medium is transversely isotropic (uniaxial) and determine the direction of its reference symmetry axis expressed in terms of the unit reference symmetry vector. If the medium is exactly transversely isotropic (exactly uniaxial), we obtain the direction of its symmetry axis. We can also calculate the first–order and second–order spatial derivatives of the reference symmetry vector which may be useful in tracing the reference rays for the coupling ray theory. The proposed method is tested using various transversely isotropic (uniaxial) and approximately transversely isotropic (approximately uniaxial) media.     

Detection capability of seismic network based on noise analysis and magnitude of completeness

Assessing the detection threshold of seismic networks becomes of increased importance namely in the context of monitoring induced seismicity due to underground operations. Achieving the maximum possible sensitivity of industrial seismic monitoring is a precondition for successful control of technological procedures. Similarly, the lowest detection threshold is desirable when monitoring the natural seismic activity aimed to imaging the fault structures in 3D and to understanding the ongoing processes in the crust. We compare the application of two different methods to the data of the seismic network WEBNET that monitors the earthquake swarm activity of the West-Bohemia/Vogtland region. First, we evaluate the absolute noise level and its possible non-stationary character that results in hampering the detectability of the seismic network by producing false alarms. This is realized by the statistical analysis of the noise amplitudes using the ratio of 99 and 95 percentiles. Second, the magnitude of completeness is determined for each of the nine stations by analysing the automatic detections of an intensive swarm period from August 2011. The magnitude–frequency distributions of all detected events and events detected at individual stations are compared to determine the magnitude of completeness at a selected completeness level. The resulting magnitude of completeness M _c of most of the stations varies between ?0.9 and ?0.5; an anomalous high M _c of 0.0 is found at the most distant station, which is probably due to inadequate correction for attenuation. We find that while the absolute noise level has no significant influence to the station sensitivity, the noise stationarity correlates with station sensitivity expressed in low magnitude of completeness and vice versa. This qualifies the method of analysing the stationary character of seismic noise as an effective tool for site surveying during the seismic station deployment.