HR: 1340h
AN: T43A-1316    [Abstracts]
TI: A Temporal Change Of Inner Core Travel Times Confirmed By High-Quality Waveform Doublets
AU: * Zhang, J
EM: jian@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 201A Seismology Building 61 Route 9W, Palisades, NY 10964 United States
AU: Richards, P G
EM: richards@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 201A Seismology Building 61 Route 9W, Palisades, NY 10964 United States
AU: Waldhauser, F
EM: felixw@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 201A Seismology Building 61 Route 9W, Palisades, NY 10964 United States
AU: Song, X
EM: xsong@uiuc.edu
AF: Department of Geology, University of Illinois at Urbana-Champaign, 245 Natural History Building 1301 W. Green St., Urbana, IL 61801 United States
AB: The first claim of temporal change of differential travel times between PKP(DF) waves (traversing the inner core) and PKP(BC) waves (avoiding the inner core) was made by Song and Richards (1996) and interpreted as evidence of inner core super-rotation. This observed/apparent time dependence of the BC-DF times was instead proposed by Souriau and Poupinet (1997; 1998; 2000) to be an artifact of systematic earthquake mislocations. Li and Richards (2003) showed how to avoid such artifacts by use of BC-DF measurements made on a waveform doublet (two earthquakes at essentially the same location, as evidenced by their highly similar waveforms) for events separated by several years, but they found only one example in the South Sandwich Islands (SSI), recorded at just one station (COL). Here we report on three additional doublets, each showing a change of differential BC-DF travel times at several stations and always with the same trend: signals of the later event travel faster on paths from SSI to Alaska and Canada. From high-quality waveform data recorded at seismographic stations of the International Monitoring System, the U.S. Atomic Energy Detection System, the Alaska Seismic Network, and station INK in Canada, we have found a change of differential travel times at 50 stations for one of these doublets; and at 5 stations for each of the other two. The observed changes in BC-DF times lie in the range 0.08 to 0.18 s over 10 years. For the best-recorded doublet (events on 1993 Dec 01 and 2003 Sep 06), waveform similarity restricts hypocenter separation to be less than 2 km. From array data at ILAR we measure the rate of change of BC-DF times with respect to epicentral distance as 0.013 s/km --- far too small a value to allow plausible mislocations to cause the observed changes in travel time, and leading us to conclude that the changes are indeed temporal.
DE: 3924 High-pressure behavior
DE: 1213 Earth's interior--dynamics (8115, 8120)
DE: 1507 Core processes (8115)
DE: 1015 Composition of the core
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting