Source Characteristics of Earthquakes, Explosions and Afterevents

Abstract

Broad-band seismic data available on the magnetic tape library at Berkeley have been used in a study comparing P, and Rayleigh waves from earthquakes, explosions and explosion afterevents, both collapse and earthquake types. Eighty-one events equidistant from Berkeley at 500-550 km provided the data for wave amplitude and spectra comparisons. Explosions exhibit characteristic low Rayleigh wave (12-s period) generation, for given P,, amplitude, relative to earthquakes as well as explosion afterevents. This discriminant, for events recorded at Berkeley, is well developed down to local magnitude M,, 4.0 or less (m; R 3.6) and shows no indication of convergence, although scatter increases, for smaller events. Rayleigh wave spectra for explosions and collapses are quite similar, the similarity continuing up to frequencies of 0.5 Hz for proximate events. However, they differ significantly from spectra of shallow earthquakes in the same source region. This implies the explosions, including those large ones with appreciable surface faulting, Love wave generation, and aftershock sequences, are sources with small time and space dimensions, relative to shallow earthquakes at the same site. Extensive fault displacements near large explosions is proposed as passive medium response to strong shaking and Love wave generation with Rayleigh wave asymmetry is suggested to occur through processes acting in the relatively small high stress explosive source region.

Source mechanism and mb?Msanalysis of aftershocks of the San Fernando earthquake

“…Since the smaller events have flat spectra for frequencies less than 1-2 Hz, their mb-M, values should fall along a curve of slope 1 extrapolated to lower magnitudes. McEvilly (1974), andPeppin (1972) suggest that on an mb-& plot, explosions fall along a line of slope 1 for 3 G mb G 6. If so, Fig.…”

A near-regional explosion source model for tuff

“…Large SV-waves are predicted for a point force, but not for a source of dilatation. In addition, McEvilly & Peppin (1972) and Peppin & McEvilly (1974) find that collapses are the easiest events to separate from explosions using their body wave-surface wave discriminant. If collapses were dominantly sources of dilatation, we would expect them to be difficult to distinguish from explosions using a body wave-surface wave discriminant.…”

“…The plan of the paper is as follows: we will discuss first general considerations which limit our choice of a source model; subject to these limitations we will construct a source model from close-in observations of the megaton explosions JORUM (1969 and HANDLEY (1970; finally we will construct scaling curves which satisfy the data I have collected. 1 Construction of a source model: relevant data and general considerations Of the large amount of data I have investigated, there are five classes of observations that appear to be crucial in the construction of a source model for explosions: (1) the ratio between the body-and surface-wave magnitude (or amplitudes at near-regional distances) is consistently higher (by a factor of 5-20) for explosions than for earthquakes of the same body-wave magnitude (SIPRI 1968;McEvilly & Peppin 1972;Peppin & McEvilly 1974), (2) at near-regional distances, 0.8-3.0 Hz P-wave amplitudes scale linearly (unit slope) with 12-s Rayleigh wave amplitude and with yield, from ca. 5 to ll00kton (Springer & Hannon 1973, Fig.…”

Inelastic processes in seismic wave generation by underground explosions

“…Israelson, Slunga, and Dahlman (1974) reported an after shock sequence within a 4-hour period following a large underground nuclear explosion at the southern end of Novaya Zemlya. McEvilly and Peppin (1972) found that the Rayleigh i.o P n amplitude relations for aftershocks are different from those for explosions at the Nevada Test Site.…”