HR: 09:00h
AN: S31B-05    [PDF]
TI: On PKiKP Recorded at Short Distance on the Warramunga Array and the High Frequency PKiKP Coda
AU: * Poupinet, G
EM: poupinet@rses.anu.edu.au
AF: Research School of Earth Sciences, Australian National University, Mills Road, Canberra, ACT 0200 Australia
AU: * Poupinet, G
EM: poupinet@rses.anu.edu.au
AF: LGIT, CNRS and UJF, BP 53, Grenoble, 38041 France
AU: Kennett, B L
EM: brian@rses.anu.edu.au
AF: Research School of Earth Sciences, Australian National University, Mills Road, Canberra, ACT 0200 Australia
AB: PKiKP is the P-wave reflected on the inner core boundary (ICB). PKiKP is frequently observed on the Warramunga seismic array in Australia for events of magnitude larger than 6 in the distance range 15-45 degrees. In the frame of the Comprehensive Test Ban Treaty, the Warramunga array has been upgraded with a high dynamic range (24 bits) recording system and broadband sensors. This improvement permits to detect PKiKP on single traces, without stacking. PKiKP is a sharp arrival in the frequency band 2 Hz to 5 Hz. Its waveform is sometimes showing some complexity: it can be preceded by a small onset or can exhibit double or triple pulses, not observed on PcP. In this frequency band, the amplitude of PKiKP is larger than the event's coda. The coda of PKiKP is quickly decreasing within the first 20 seconds. We do not observe the slow (60 s.) building up of amplitude in the PKiKP coda observed by Vidale et Earle (2000) on LASA data which lead them to infer strong kilometer-scale heterogeneites beneath the ICB. We filter the signal in 5 different bandpasses. In each bandpass, we compute the average of the maxima of the 24 traces in successive 2 seconds moving windows. This simple processing exhibits the decay of the coda as a function of time and frequency along the entire seismogram. For reflected phases like PcP or ScP, usually the coda decreases rapidly after the phase arrival time and returns to its level before the arrival of the phase. For the S phase, the amplitude of the coda remains higher after the S arrival time than before: a coda offset is generated at the time of S. The same occurs for PKiKP. PKiKP is followed by a coda with an amplitude about half that of PKiKP, predominantly in the frequency range 3 Hz -6 Hz. This high frequency PKiKP coda seems to ring for a very long duration. Unfortunately, for the events of magnitude 6-7 available, the sensitivity level of the recording system is reached near 1200-1300 s and the coda amplitude becomes smaller than the recorder intrinsic noise level. The amplitude of the coda increases later, around 1800 s, due to PKKP scattering. The ICB seems to be a complex interface which (1) reflects high frequency PKiKP, (2) transmits low frequency PKIKP and (3) generates a high frequency PKiKP coda lasting more than 200 seconds. We present an hypothetical velocity model that may behave like the ICB and attempt to relate it to the mechanism of growth of the inner core.
DE: 7200 SEISMOLOGY
DE: 7207 Core and mantle
SC: Seismology [S]
MN: 2003 Fall Meeting