Richard J Stead

Chapter 12 Source Effects From Broad Area Network Calibration of Regional Distance Coda Waves

Abstract We have applied regional coda techniques to a network of 64 stations across central and east Asia to isolate source effects for bands from 0.03 to 8 Hz. The heterogeneity of the study region required us to determine two‐dimensional (2‐D) path and transfer function corrections. The importance of the 2‐D path corrections increased with frequency and distance, and for continental paths, became critical beyond 500 km for 1 Hz data. We propose a new spreading model for coda amplitudes, termed the extended Street‐Herrmann (ESH) model, to which attenuation can be added, facilitating the use of tomographic techniques for path correction. The 2‐D transfer function varied between continents and oceans, as well as within continents in areas of poor Lg propagation, reflecting differing excitation of Lg and Sn coda. We also demonstrate the use of empirically determined coda shapes, or type curves , to measure coda amplitudes, adding precision and flexibility for source regions of special interest. We applied these techniques to 112,000 records from 35,000 events, magnitudes 2–7, depths 0–50 km, between latitudes 0° and 60° and longitudes 60° and 150°. The resulting coda source spectra were used to derive moments and, for the better recorded events, corner frequencies, allowing computation of apparent stress for just under 6700 earthquakes. Preliminary apparent stress results ranged from 10 −2 to 1 MPa and showed some regional variation. For example, stress increased from south to north across the Tian Shan, perhaps reflecting deformation in varying crustal rheology or effects of prior slip history. Low stress observed in Tibet could be an artifact of under correction for high attenuation; however, the correlation also could be physical. Low stress observed in oceanic regions is inconsistent with local studies and indicates that upgrades to the coda methodology to more explicitly account for mixed Lg and Sn coda will be needed. The regional network coda results should be further tested by comparing to ground‐truth spectra obtained by applying coda techniques to data from local scale networks within the study region.

Modeling the generation of infrasound from earthquakes

Earthquakes can generate complex seismo-acoustic wavefields, consisting of seismic waves, epicenter-coupled infrasound, and secondary infrasound. We report on the development of a numerical seismo-acoustic model for the generation of infrasound from earthquakes. We model the generation of seismic waves using a 3D finite difference algorithm that accounts for the earthquake moment tensor, source time function, depth, and local geology. The resultant acceleration-time histories (on a 2D grid at the surface) provide the initial conditions for modeling the near-field infrasonic pressure wave using the Rayleigh integral. Finally, we propagate the near-field source pressure through the Ground-to-Space atmospheric model using a time-domain parabolic equation technique. The modeling is applied to an earthquake of MW 4.6, that occurred on January 3, 2011 in Circleville, Utah; the ensuing predictions are in good agreement with observations made at the Utah network of infrasonic arrays, which are unique and indicate...