Investigation of scattering in lunar seismic coda

• Published in: Journal of Geophysical Research: Planets Vol. 117; no. E6
• Main Authors: Blanchette-Guertin, J.-F., Johnson, C. L., Lawrence, J. F.
• Format: Journal Article
• Language: English
• Published: Washington, DC Blackwell Publishing Ltd 01-06-2012; American Geophysical Union
• Subjects: coda Q; Decay; Earth sciences; Earth, ocean, space; Exact sciences and technology; Geology; Moon; Physical properties; Planetology; Planets; Remote sensing; Rocks; scattering; Seismology; attenuation; epicentral distance; Moon; exhibits; global; lunar interior; velocity; geology; Modeling; ejecta; moonquakes; frequency; quality; amplitude; depth; waveforms; size distribution; seismograms; energy
• ISSN: 0148-0227; 2169-9097; 2156-2202; 2169-9100
• DOI: 10.1029/2011JE004042

We investigate the intrinsic attenuation and scattering properties of the Moon by parameterizing the coda decay of 369 higher‐quality lunar seismograms from 72 events via their characteristic rise and decay times. We investigate any dependence of the decay times on source type, frequency, and epicentral distance. Intrinsic attenuation, scattering, and possible focusing of energy in a near‐surface, low‐velocity layer all contribute to the coda decay. Although it is not possible to quantify the exact contribution of each of these effects in the seismograms, results suggest that scattering in a near‐surface global layer dominates the records of shallow events (∼0–200 km depth), particularly at frequencies above 2 Hz, and for increasing epicentral distance. We propose that the scattering layer is the megaregolith and that energy from shallow sources encounters more scatterers as it travels longer distances in the layer, increasing the coda decay times. A size distribution of ejecta blocks that has more small‐scale than large‐scale scatterers intensifies this effect for increasing frequencies. Deep moonquakes (700–1100 km depth) exhibit no dependence of the decay time on epicentral distance. We suggest that because of their large depths and small amplitudes, deep moonquakes from any distance sample a similar region near a given receiver. Near‐station structure and geology may also control the decay times of local events, as evidenced by two natural impact records. This study provides constraints and testable hypotheses for waveform modeling of the lunar interior that includes the effects of intense scattering and shallow, low‐velocity layers.
Key Points
Scattering controls the coda decay of high‐frequency shallow seismic events
Near‐station structure and geology control the coda decay of local events
Near‐station structure and geology control the coda decay of deep events