SEISMIC VELOCITY AND ATTENUATION STRUCTURES IN THE TOP 400 KM OF THE EARTH'S INNER CORE ALONG EQUATORIAL PATHS

Abstract

We study seismic velocity and attenuation structures in the top 400 km of the Earth's inner core based on modeling of differential traveltimes, amplitude ratios, and waveforms of the PKiKP-PKlKP phases observed at the epicentral distance range of 120°-141° and the PKPbc-PKIKP phases observed at the distance range of 146° -160° along equatorial paths. Our data are selected from the seismograms recorded in the Global Seismographic Network from 1990 to 2001 and many regional seismic networks. The observed PKiKP-PKIKP and PKPbc-PKIKP phases exhibit distinctive "east-west" hemispheric patterns: (1) At the distance ranges of 131°-141° and 146°-151°, PKIKP phases arrive about 0.3 s earlier than the theoretical arrivals based on the Preliminary Reference Earth Model (PREM) for the PKIKP phases sampling the "eastern hemisphere" (40°E-180°E) of the inner core and about 0.4 s later for those sampling the "western hemisphere" (180°W-40°E). At the distance range of 151°-160°, PKIKP phases arrive about 0.7 s earlier than the predicted arrivals based on PREM for those sampling the eastern hemisphere and about 0.1 s later for those sampling the western hemisphere. (2) Amplitude ratios of the PKIKPIPKiKP phases at the distance range of 131°-141° and of the PKIKPIPKPbc phases at the distance range of 146°-151° are, in general, smaller for the PKIKP phases sampling the eastern hemisphere than for those sampling the western hemisphere. At distances greater than 151°, the PKIKPIPKPbc amplitude ratios become indistinguishable for the two hemispheres. These observations can be best explained by two different types of seismic velocity and attenuation models along equatorial paths, one for each hemisphere, in the top 400 km of the inner core. For the eastern hemisphere, the velocity structure has a velocity increase of 0.748 km/s across the inner core boundary (ICB), a small velocity gradient of 0.0042 (km/s)/100 km in the top 235 km, followed by a steeper velocity gradient of 0.1 (km/s)/100 km extending from 235 km to 375 km, and a velocity gradient of 0.01 (km/s)/100 km in the deeper portion of the inner core; the attenuation structure has an average Q value of 300 in the top 300 km and an average Q value of 600 in the deeper portion of the inner core. For the western hemisphere, the velocity structure has a velocity increase of 0.645 km/s across the ICB and a velocity gradient of 0.049 (km/s)/100 km in the top 375 km; the attenuation structure has an average Q value of 600 in the top 375 km of the inner core. Our results suggest that the inner core hemispheric variations in velocity extend deeper than 375 km below the ICB and the top 235 km of the inner core in the eastern hemisphere is anomalous compared to the rest of the inner core in having a small velocity gradient, high velocity, and high attenuation. Copyright 2006 by the American Geophysical Union.