ABSTRACT

Seismological studies of the Earth’s deep interior are commonly based on earthquake data acquired on quasi‐permanent sparse broadband networks on either global or local scales. Stationary broadband instruments are required for an accurate representation of teleseisms at low frequencies and to ensure recordings with high repeatability. Certain first‐order characteristics of teleseismic events such as phase arrival times and relative amplitudes can still be deduced from short‐period instruments or even from low‐frequency geophones. Depending on the recording aperture and density, such data are used to investigate crustal and upper‐mantle structures. However, the application of nodal arrays for studying the lower mantle and the core is still limited. Here, I present the incidental recording of a reflection (PKiKP) from the inner core boundary on 915 4.5‐Hz geophones during a regional 3D active source survey. Despite the high corner frequency of the instruments, the PKiKP wavelet appears clearly and consistently on the majority of the receivers and compares well with recordings from the permanent broadband network. The observed arrival‐time variation across all instruments indicates small‐scale velocity perturbations in the lower mantle. The stacking power of the array enables to image a consistent coda with constant delay across all stations, which is interpreted for vertical layering of the inner core. Receiver‐side crustal imaging is facilitated by the steep incidence of the PKiKP phase. It is concluded that nodal large‐N deployments can and should be used to complement broadband observations for studying deep Earth processes.

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