Marian Ivan

Non quasi-Hemispherical Seismological Pattern of the Earth’s Uppermost Inner Core

We assembled a database consisting of 5,404 PKIKP/PKiKP observations from 555 events, where PKIKP is the phase sampling the inner core (IC) and PKiKP is the phase reflected at the inner core boundary (ICB). Around 138° distances, their differential arrival times and amplitude ratio are mostly sensitive to the seismic velocity and attenuation structure in the uppermost IC (UIC), respectively. Our observations do not support a large-scale anisotropy in the UIC, but do not exclude its presence in some restricted areas. A robust inversion for the isotropic P-wave velocity perturbations shows a higher velocity cap with a radius of ~60°, approximately centered beneath the Northern Sumatra, with a local low velocity zone beneath the central Indian Ocean. The rest of the UIC, including the Northern part of Eurasia and of the Atlantic Ocean, exhibits mostly lower velocity. Amplitude ratio values of PKIKIP/PKiKP (observed vs. computed) from 548 high signal-to-noise (>5) recordings show a large variance, suggesting only a faint correlation between higher velocity and lower attenuation in the UIC. Our results provide better constraints to the models invoking a heat transfer in the UIC, with a complex temperature pattern near ICB.

On: Gravity and magnetic fields of polygonal prisms and application to magnetic terrain corrections; discussion and reply

Testing the position of the field point relative to the mass center of the prism (resulting from the computation of sm) can be avoided in Plouff’s equation (3) by assuming g=γρ∑L=1nsPA|z2|-|z1|+z2…,where z1 and z2 are the vertical coordinates (not their absolute values) of the top and bottom, respectively, of the prism. An absolute value is necessary for the computation of the angle; i.e., sPA=sgn(x1y2-x2y1)cos-1[|x1x2+y1y2|/(r1r2)]=π/2sgn(x1y2-x2y1)-tan-1[(x1x2+y1y2)/(x1y2-x2y1)].