antipodal seismicity

[keel@bildad.astr.ua.edu (William Keel)]

The notion of a lareg impact producing significant seismic effects at its
antipodes due to focussing of seismic waves originated from observations
that the largest impact sites on the Moon and Mercury have what sure
looks like seismically disturbed regions opposite them (and their crust
stays put over long times so we can tell this). Not to imply that this
is related to the Deccan traps (BTW, trap is Dutch for staircase, so maybe
that's the language of origin), but the phenomenon seems real enough.
A quick stab at the Astrophysics Data System abstract search turned
up the following abstract on the matter.

Bill Keel               Astronomy, University of Alabama
Title:              Seismic effects from major basin formations on the
                    moon and Mercury
Authors:            SCHULTZ, P. H.; GAULT, D. E.
Affiliation:        AB(NASA, Ames Research Center, Space Sciences Div.,
                    Moffett Field, Calif.)
Journal:            The Moon, vol. 12, Feb. 1975, p. 159-177.
Publication Date:   02/1975
Origin:             STI
Category:           Solar Physics
NASA/STI Keywords:  LUNAR MARIA, MERCURY (PLANET), P WAVES, PLANETARY
                    SURFACES, SEISMOLOGY, ANTIPODES, IMPACT DAMAGE,
                    LUNAR CRATERS, MOONQUAKES, SPALLATION, TERRAIN
                    ANALYSIS
Bibliographic Code: 1975Moon...12..159S

Abstract:

Grooved and hilly terrains occur at the antipode of major basins on the moon
(Imbrium, Orientale) and Mercury (Caloris). Such terrains may represent 
extensive landslides and surface disruption produced by impact-generated 
P-waves and antipodal convergence of surface waves. Order-of-magnitude 
calculations for an Imbrium-size impact on the moon indicate P-wave-induced 
surface displacements of 10 m at the basin antipode that would arrive prior 
to secondary ejecta. Comparable surface waves would arrive subsequent to 
secondary ejecta impacts beyond 1000 km and would increase in magnitude as 
they converge at the antipode. Other seismically induced surface features 
include: subdued, furrowed crater walls produced by landslides and concomitant 
secondary impacts; emplacement and leveling of light plains units owing to 
seismically induced 'fluidization' of slide material; and perhaps the 
production and enhancement of deep-seated fractures that led to the 
concentration of farside lunar maria in the Apollo-Ingenii region.