The internal structure of the Moon has been investigated over many years using a variety of seismic methods, such as travel time analysis, receiver functions, and tomography. Here we propose to apply body-wave seismic interferometry to deep moonquakes in order to retrieve zero-offset reflection responses (and thus images) beneath the Apollo stations on the nearside of the Moon from virtual sources colocated with the stations. This method is called deep-moonquake seismic interferometry (DMSI). Our results show a laterally coherent acoustic boundary around 50 km depth beneath all four Apollo stations. We interpret this boundary as the lunar seismic Moho. This depth agrees with Japan Aerospace Exploration Agency's (JAXA) SELenological and Engineering Explorer (SELENE) result and previous travel time analysis at the Apollo 12/14 sites. The deeper part of the image we obtain from DMSI shows laterally incoherent structures. Such lateral inhomogeneity we interpret as representing a zone characterized by strong scattering and constant apparent seismic velocity at our resolution scale (0.2-2.0 Hz).In this study we analyze deep-moonquake seismograms. We apply body-wave SI [e.g., Claerbout, 1968;Wapenaar et al., 2008;Schuster, 2009] via autocorrelation of the first P wave phase to the P wave coda. This allows us to retrieve the zero-offset subsurface reflection response from virtual sources colocated with the Apollo stations. For the sake of shorthand notation, we term this technique deep-moonquake seismic interferometry (DMSI). Obtaining virtual reflection responses of the Moon beneath the Apollo stations obviates the need for active seismic sources, such as explosives and artificial impacts recorded by the Apollo instruments.Our goal is to identify the lunar seismic Moho using the DMSI technique. Knowledge of the crustal thickness is important to the understanding of the evolution of the Moon; it has implications for bulk composition, petrogenesis, and other aspects of lunar evolution. Previous studies using various seismic methods have reported widely NISHITSUJI ET AL.DEEP-MOONQUAKE SEISMIC INTERFEROMETRY 695 PUBLICATIONS
We estimated source parameters of 216 intermediate-depth (65-150 km) earthquakes (M w 4.0-7.0) in the Pacific slab beneath Japan along using Hi-net data. We made determinations of static stress drop, radiated energy and radiation efficiency, along with estimates of the whole path attenuation, to study the source scaling as a function of earthquake size and depth. Our results show that there is a small increase in the values of the ratio of radiated energy to seismic moment, as a function of seismic moment, which is due to an associated slight increase of static stress drop with earthquake size. We also estimated the radiation efficiency for these events using the static stress drops and radiated energies. The values of radiation efficiency are slightly lower compared to shallow crustal earthquakes. These results indicate that dissipative energy processes may be relatively more important for intermediate-depth earthquakes.
Obtaining detailed images of aseismic parts of subducting slabs remains a large challenge for understanding slab dynamics. Hypocenter mapping cannot be used for the purpose due to the absence of seismicity, whereas the use of receiver functions might be compromised by the presence of melt. Global tomography can be used to identify the presence of the slab, but it does not reveal the structure in detail. We have determined how detailed images can be obtained using global-phase seismic interferometry. The method provides high-resolution ([Formula: see text] in depth) pseudo zero-offset (i.e., colocated source and receiver) reflection information. We have applied the method to aseismic zones of the Nazca slab in which initiation of possible slab tearing and plume decapitation was identified by global tomography and electrical conductivity, respectively. We have obtained an image of the Moho and the mantle and found an attenuated area in the image consistent with the presence of an aseismic dipping subducting slab. However, our interpretation was not unambiguous. The results confirmed that the method is useful for imaging aseismic transects of slabs.
Automatic 3D illumination-diagnosis method for large-N arrays: robust data scanner and machine-learning feature provider Chamarczuk, M.; Malinowski, M.; Nishitsuji, Yohei; Thorbecke, Jan Willem; Koivisto, E.; Heinonen, S.; Juurela, S.; Mężyk, M.; Draganov, Deyan ABSTRACTThe main issues related to passive-source reflection imaging with seismic interferometry are inadequate acquisition parameters for sufficient spatial wavefield sampling and vulnerability of surface arrays to the dominant influence of the omni-present surface-wave sources.Additionally, long recordings provide large data volumes that require robust and efficient processing methods. We address these problems by developing a two-step wavefield evaluation and detection method (TWEED) of body waves in recorded ambient noise. TWEED evaluates the spatio-temporal characteristics of noise recordings by simultaneous analysis of adjacent receiver lines. We test our method on synthetic data representing transient ambient-noise sources at the surface and in the deeper subsurface. We discriminate between basic types of seismic events by using three adjacent receiver lines. Subsequently, we apply TWEED to 600 hours of ambient noise acquired with ~1000-receiver array deployed over an active underground mine in Eastern Finland. We demonstrate detection of body-wave events related to mine blasts and other routine mining activities using a representative one-hour noise panel.Using TWEED, we successfully detect 1093 body-wave events in the full data set. To increase the computational efficiency, we use slowness parameters derived from the first step of TWEED as input to a support vector machine (SVM) algorithm. Using this approach, we detect 94 percent of the TWEED-evaluated body-wave events indicating the possibility to limit the illumination analysis to only one step and therefore increase the time efficiency at the price of lower detection rate. However, TWEED on a small volume of the recorded data followed by SVM on the rest of the data could be efficiently used for a quick and robust (real-time) scanning for body-wave energy in large data volumes for subsequent application of seismic interferometry for retrieval of reflections.
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