Seismic heterogeneous structure in the lowermost mantle beneath the southwestern Pacific

Abstract

The P and S wave velocity structure of the D″ layer beneath the southwestern Pacific was investigated by using short‐period data from 12 deep events in the Tonga‐Fiji region recorded by the J‐Array and the Hi‐net (two large‐aperture seismic arrays) in Japan. Reflected wave beam forming (RWB) and a migration method were used to extract weak signals originating from heterogeneities in the lowermost mantle. In order to acquire high resolution a double‐array method was applied to the data. The results of the RWB method indicate that seismic energy is reflected at discontinuities near the depths of 2520 and 2650 km, which have a negative P wave velocity contrast of 1% at the most. In addition, there is a positive seismic discontinuity at a depth of 2800 km. In the case of the S wave, reflected energy is produced almost at the same depth (2550 km depth). An apparent depth shift (50 km) of the discontinuity at the depth of 2850 km may indicate that the S wave velocity reduction in the lowermost mantle is ∼2–3 times stronger than that of P. A two‐dimensional cross section, constructed with the RWB method, suggests that the observed discontinuities can be characterized as intermittent lateral heterogeneities whose lateral extent is a few hundred kilometers. The migration shows weak evidence of scattering objects which belong to the seismic discontinuities detected by the RWB method. These anomalous structures may represent a part of hot plume generated beneath the southwestern Pacific in the lowermost mantle.

New insights into theP- andS-wave velocity structure of the D″ discontinuity beneath the Cocos plate

“…In what follows we describe more advanced array processing approaches to resolve smaller scale D″ structure from these PdP arrivals. We will compare two array methods (Kito et al 2004; Thomas et al 2004, 2004b) to understand D″ structures more comprehensively and to figure out possible bias caused by different array methods.…”

“…Double‐array stacking method (DAM) (Kito et al 2004) is applied to the P ‐ and S ‐wave data set discussed above. The DAM uses source and receiver arrays simultaneously to increase the SNR of coherent arrivals and reduce amplitudes of phases generated by heterogeneities close to the individual sources (which arrive coherently at the receiver array).…”

“…Therefore, the traveltime residual must be corrected properly before applying DAM. For more details on the method see Kito et al (2004) and Krüger et al (1996). To use several sources simultaneously the sources must be normalized which is done by source deconvolution as described above, which results in a simple impulse‐like source–time function for each event.…”

“…Recent applications of seismic array methods to high‐quality data from seismic arrays and networks targeting the deep mantle find small‐scale structures at the CMB on scale‐lengths from a few tens to a few hundred kilometres with strong variations both laterally and vertically (e.g. see Weber 1994; Lay & Young 1996; Earle & Shearer 1997; Revenaugh & Meyer 1997; Scherbaum et al 1997; Vidale & Hedlin 1998; Castle et al 2000; Persh et al 2001; Kito & Krüger 2001; Rost & Revenaugh 2001, 2003; Braña & Helffrich 2004; Kito et al 2004; Thomas et al 2004b; Rost et al 2005). The densification and availability of permanent and temporary array deployments (Rost & Garnero 2004) allows increased application of stacking algorithms that enhance the signal‐to‐noise ratio (SNR) of coherent arrivals over incoherent noise (e.g.…”

“…Reasoner & Revenaugh 1999; Thomas et al 2004b; Hutko et al 2006). Here, we apply two different array methods (Kito et al 2004; Thomas et al 2004, 2004b) to Californian P ‐ and S ‐wave recordings from South American earthquakes, in an effort to better understand possible biases in the different array approaches and search for evidence for both P ‐ and S ‐wave velocity discontinuities beneath the Cocos plate. We use core reflected ScS and PcP phases and their precursors, SdS and PdP due to the reflection off the D″ discontinuity (Fig.…”