G

LOBAL

S

EISMIC

F

OCUSING

AT

THE

A

NTIPODE

R. Brian White

Department of Earth and Planetary Sciences; Washington University; St. Louis, MO, USA October 25, 2002

A

BSTRACT

There is a remarkable focusing of seismic body and surface wave energy at the antipode of seismic sou rces. This pheno menon is expla ined and the diffic ulties wit h creating synthetic seismograms a t the antipode are discussed. Examples of antipodal f ocusing are then given and compa red to synthet ics. Applic ations of antipo dal data to studying Earth structure a re then discussed with sp ecial attention given to the inner core. Finally, implications for the focusing of seismic energy from impacts on the Earth and other planets are discussed.

I

NTRODUCTION

The antipode is the point at the opposite side of a planet (

∆

=180

°

). For o ur pur poses, we are int ere ste d in the antip ode of sei smi c sou rce s suc h as ear thq uak es, explo sio ns, or imp act s. For a spherical planet, all great circles paths from a given source pass through the antipode, meaning the antipo de receives seismi c energy from all azimuths. The possibil ity of antipodal focusin g of seismi c energy has been realized since the earliest days of seismology [

Gutenberg and Richter

, 1934], and it was noted that obscure inner core phases such as PKIIKP could be enhanced at the antipode [

Bolt and O'Neill