https://orcid.org/0000-0003-2570-2036
Abstract
The core mantle boundary (CMB) features the most dramatic contrast in the physical properties within the Earth and plays a fundamental role in the understanding of the dynamic evolution of the Earth’s interior. Seismic core phases such as PKKP sample large area of the lowermost mantle and the uppermost core, thus providing valuable information of the velocity structures on both sides of the CMB. Diffraction Waves Well Beyond Cutoff Distance () is one branch of the triplicated PKKP that can be observed beyond its ray theoretical cutoff distance as a result of diffraction along the CMB. The travel time and slowness of the diffracted (denoted as ) can be used to constrain the P‐wave velocities at the lowermost mantle, thus have been investigated in numerous studies. Previous results (Rost and Garnero, 2006) suggest that most of the observations of the waves are in the epicentral distance range of 95°–105° (minor arc convention) ( diffraction length less than 10°). However, high‐frequency (∼1 Hz) synthetic seismograms show that the waveforms could be observable at distance down to 65°, which indicates that the signals could be detected at distances less than 95° in observations. To explore the distance ranges in which is observable, we collected global three‐component broadband waveforms from 246 events with source depth deeper than 100 km and magnitude above M 6 from 2007 to 2017 available at the Incorporated Research Institutions for Seismology Data Management Center. We analyzed the slowness, polarization, and amplitude of the candidate signals, and found 95 events with clear signals, with nearly 60% of the events show diffraction lengths greater than 10°, and the longest diffraction distance is beyond 20°. These newly identified waves would substantially augment the dataset of core phases for improvements of the CMB velocity models.
