Lunar seismicity and tectonics

The renewed global interest in lunar exploration, with multiple robotic and crewed landings planned in the coming decades, presents a unique opportunity to pursue scientific objectives that are difficult or infeasible on Earth. Among these objectives, the detection of gravitational waves (GWs) from a lunar platform stands out as a particularly promising frontier in fundamental physics. While Earth-based GW observatories have made groundbreaking discoveries, their sensitivity is fundamentally limited by environmental noise, including persistent seismic disturbances. In contrast, the Moon offers an exceptionally quiet seismic environment three orders of magnitude smaller than on Earth, which is largely free from atmospheric and anthropogenic noise sources, as originally demonstrated by data from the Apollo Passive Seismic Experiment.In this study, we revisit the Apollo-era seismic data to conduct a comprehensive assessment of the lunar seismic noise environment. Particularly, we revisit the transfer function of the Apollo instruments by using the existing catalog of the calibration signals. Using the newly developed transfer function (Specifically,) we perform a statistical analysis of background noise levels recorded at multiple Apollo seismic stations, characterizing the amplitude distribution, spectral content, and long-term variability of ambient seismic noise. Regional differences among the landing sites are examined to identify geophysical or environmental factors contributing to noise variability. We further analyze temporal trends in noise characteristics in relation to local time, surface temperature cycles, and tidal stresses, aiming to determine optimal observational windows for future ultra-sensitive measurements.In addition to persistent background noise, we assess the impact of transient seismic events, which may also pose challenges for GW detection. We compile and analyze the statistical properties, such as occurrence rates, amplitudes, and durations, of the three main classes of seismic events observed on the Moon: deep moonquakes, shallow moonquakes, and meteoroid impacts. Their respective contributions to the lunar seismic signal environment are evaluated, with particular attention to their potential to mask or mimic GW signals in the relevant frequency ranges. Here, careful consideration is given to the deep moonquakes, as their frequent seismic activity defines the seismic floor level for potential future GW detectors. We synthesize deep moonquake seismograms to construct a representative template of this signal class. These synthetic waveforms are then used to simulate the response of a prospective lunar strainmeter, allowing us to explore the signature of deep moonquake noise in a gravitational wave measurement context. We discuss the data analysis strategies developed to distinguish this seismic noise from a true GW signal in order to remove it, emphasizing the importance of signal discrimination methods in future lunar GW observatories.The results of our analysis provide updated and quantified constraints on the lunar seismic noise floor, as well as on the likelihood and characteristics of transient disturbances. These findings are essential for informing the design, site selection, and operational strategies of future lunar gravitational wave observatories. We conclude by discussing the implications of the current seismic environment for the detectability of various classes of GW sources, including low-frequency signals that may be inaccessible to Earth-based detectors.

We probe the present‐day stresses in the lunar interior by examining the slip directions of moonquakes in the A01 nest. In this nest, some deep moonquakes appear to slip “backwards,” in the opposite direction to other events. We assess whether these changes in slip direction result from a spatial variation in the tectonic stress or from a temporal variation in the tidal stress. To test these two options, we first show that a dominant tectonic stress implies deep moonquakes can only slip in one direction: forwards and backwards, while a dominant tidal stress could allow moonquakes to slip in more directions: any combination of forwards, backwards, left, and right. Then we look for the number of slip directions; we separate the deep moonquake waveforms into slip directions using a principal component analysis technique. We find two slip directions present in the A01 deep moonquake nest. The moonquakes slip in a variety of directions as time evolves. This observation implies that the tidal stresses drive deep moonquakes. Additionally, these results place a new constraint on the magnitude of the tectonic stresses at depth; they must be smaller than the modeled tidal stress of ∼0.1 MPa.

While deep moonquakes are seismic events commonly observed on the Moon, their source mechanism is still unexplained. The two main issues are poorly constrained source parameters and incompatibilities between the thermal profiles suggested by many studies and the apparent need for brittle properties at these depths. In this study, we reinvestigated the deep moonquake data to reestimate its source parameters and uncover the characteristics of deep moonquake faults that differ from those on Earth. We first improve the estimation of source parameters through spectral analysis using “new” broadband seismic records made by combining those of the Apollo long‐ and short‐period seismometers. We use the broader frequency band of the combined spectra to estimate corner frequencies and DC values of spectra, which are important parameters to constrain the source parameters. We further use the spectral features to estimate seismic moments and stress drops for more than 100 deep moonquake events from three different source regions. This study revealed that deep moonquake faults are extremely smooth compared to terrestrial faults. Second, we reevaluate the brittle‐ductile transition temperature that is consistent with the obtained source parameters. We show that the source parameters imply that the tidal stress is the main source of the stress glut causing deep moonquakes and the large strain rate from tides makes the brittle‐ductile transition temperature higher. Higher transition temperatures open a new possibility to construct a thermal model that is consistent with deep moonquake occurrence and pressure condition and thereby improve our understandings of the deep moonquake source mechanism.

Detection and identification of interesting events in single‐station seismic data with little prior knowledge and under tight time constraints is a typical scenario in planetary seismology. The Apollo lunar seismic data, with the only confirmed events recorded on any extraterrestrial body yet, provide a valuable test case. Here we present the application of a stochastic event detector and classifier to the data of station Apollo 16. Based on a single‐waveform example for each event class and some hours of background noise, the system is trained to recognize deep moonquakes, impacts, and shallow moonquakes and performs reliably over 3 years of data. The algorithm's demonstrated ability to detect rare events and flag previously undefined signal classes as new event types is of particular interest in the analysis of the first seismic recordings from a completely new environment. We are able to classify more than 50% of previously unclassified lunar events, and additionally find over 200 new events not listed in the current lunar event catalog. These events include deep moonquakes as well as impacts and could be used to update studies on temporal variations in event rate or deep moonquakes stacks used in phase picking for localization. No unambiguous new shallow moonquake was detected, but application to data of the other Apollo stations has the potential for additional new discoveries 40 years after the data were recorded. Besides, the classification system could be useful for future seismometer missions to other planets, e.g., the InSight mission to Mars.

Over the years since the Apollo era, the current lunar interior structure has been investigated using seismic, gravity, and magnetic field data. The Apollo seismic network recorded about 1,800 meteoroid impacts, 28 energetic shallow moonquakes (with body wave magnitudes up to five and hypocenters about 100 km below the surface), and about 7,000 extremely weak deep moonquakes that were located about halfway to the center of the Moon (e.g., Wieczorek 2009). The deep moonquakes are very enigmatic in that their occurrences are correlated with the tides raised by the Earth, they involve very low stress drops (less than 1 bar), they appear to originate from about 300 “nests” that are repeatedly activated, and almost all of these nests are located on the Moon’s nearside hemisphere (Nakamura 2003; Bulow et al. 2007; Qin et al. 2012). The nearside distribution of the deep moonquakes could perhaps indicate that the farside hemisphere is seismically inactive. Such a hypothesis is possible, especially if one considers that almost all of the Moon’s mare basalts erupted on its nearside hemisphere. Alternatively, it is possible that the deepest interior of the Moon attenuates seismic signals originating from farside events. In support of this hypothesis, seismic arrivals along ray paths passing through regions deeper than 700 km radius are not detected, suggesting mantle in this region hot enough to be seismically attenuating and possibly partially molten (Nakamura 2005). One scenario that could be consistent with the above observations is that numerous magma-filled fractures currently located in the deep mantle are capable of relieving the small stresses induced by Earth-raised tides. The Lunar Laser Ranging mission (Williams et al. 2001) and recent Gravity Recovery and Interior Laboratory (GRAIL) mission (Zuber et al. 2012) provide us the gravity measurements. Especially the Love numbers from the GRAIL mission are unprecedentedly accurate. These Love numbers have been used to invert the bulk solid-body “dissipation factor” Q (Williams et al. 2001) as well as the tidal tomography (Zhong et al. 2012). The Q number is around 30, which is compatible to that of the Earth andMars. This Q number infers a core–mantle boundary (CMB) temperature 1,400 C (Nimmo et al. 2012). This temperature is below the harzburgite mantle solidus while higher than ilmenite-mixed harzburgite solidus (Zhang et al. 2013a, see Fig. 1). Joint inversion of seismic and gravitational data (Khan et al. 2006) suggests a CMB temperature range from 1,170 C to 1,500 C, consistent with that inferred from the Q facter. To gain a partial melt, the lunar deep mantle has to be chemically heterogeneous (either ilmenite rich or water rich (Evans et al. 2014)). The surface and orbit magnetic records have also been used to explore the present-day mantle temperature (Sonett 1982). The electrical conductivity profile of the Moon has been estimated by using electromagnetic-sounding techniques, whereby the relationship between simultaneous time variations in the ambient magnetic field as measured from orbit and the resulting field measured on the surface was analyzed (Wieczorek 2009; Hood et al. 1982). This electrical conductivity profile suggests the lunar mantle temperature from 1,100 C to 1,250 C (see Fig. 1). The lunar mantle is energetically interacting with its core. A number of seismic and gravity studies identify the lunar core with a radius of about 250–430 km (Wieczorek 2009; Weber et al. 2011; Garcia et al. 2011). The core is believed to have liquid outer and solid inner regions based on seismic

Tidal stress is a periodic stress acting globally on the Earth, driven primarily bythe gravitational forcing of the Moon and the Sun. Understanding how tidalstress can trigger seismic events is essential for constraining tectonicenvironments that are sensitive to small, periodic stress perturbations.Here we investigate tidal triggering on stable sliding, velocity-weakening (VW)rate-and-state frictional (RSF) faults using a spring-block framework. We firstapply idealized step-like and boxcar normal stress perturbations todemonstrate a resonance-like amplification of slip rate when the perturbationduration approaches the intrinsic RSF time scale. Building on this observation,we perform non-dimensional analyses and numerical simulations withsinusoidal tidal-like perturbations to identify the key parameters controllingtidal triggering and their admissible ranges. We further characterize thetriggered events through observable quantities, including radiation efficiencyand tidal phase. Our results show that resonance effects allow tidal stress to trigger bothregular periodic and complex temporal slip events on otherwise stable slidingVW faults. The triggering behavior is primarily controlled by two non-dimensional parameters: the normalized perturbation period and thenormalized perturbation amplitude. Increasing the normalized period shiftsevent timing from peak tidal stress toward the maximum stress rate, whereasincreasing the normalized amplitude promotes a transition from slow to fastslip events. The parameter space permitting tidally triggered slip eventssuggests that the RSF parameter,$a\sigma$, which characterizes theinstantaneous frictional strength of an interface, should not exceed tens tohundreds of kilopascals, and that the characteristic slip distance for frictionalweakening is likely on the order of micrometers.

Evidence from numerical modeling of the M2 tide, supplemented by geological data, reveals marked changes in the tidal and sedimentary regimes of Cook Strait over the last postglacial rise in sea level. Approximately 18,000 years B.P., when sea level was approximately −113 m, the strait was reduced to a narrow bay by the emergence of a land bridge connecting the North and South islands of New Zealand. High sediment supply and low, tidally induced bottom stress encouraged widespread deposition of fine sediment except at the bay mouth (the Narrows) where localized high stress resulted in the transport of sediment to the deep ocean via Cook Strait canyons. At 17,000 years B.P. (approximately −88 m), the bay reached maximum size and was near resonance with the oceanic tide resulting in high tidal amplitudes, bottom stress, and sediment movement at the Narrows and bay head. The land bridge was breached and Cook Strait was formed about 15,000 years B.P. (approximately −75 m). Both the Narrows and breach were subject to high bottom stress, whereas the intervening area was a zone of convergent low stress and fine sediment deposition. By 11,000 years B.P. (approximately −40 m) Cook Strait was well established and had a tidal response similar to the modern strait. The rising sea levels inundated remnants of the land bridge to form the broad continental shelf approaches to Cook Strait. Today, overall tidal stress is reduced, and mud deposition prevails except on the inner shelf where periodic forcing by wind‐induced currents maintains a sandy substrate, and in the Narrows where a tidal phase difference of approximately 120° maintains a zone of high sediment movement.

Introduction:In the 1960s, the Apollo missions deployed seismometers on the near side of the Moon, and the obtained moonquake data enabled the seismic study of internal structure of the Moon. The lunar seismic waveforms are dominated by strong coda waves caused by scattering in the near surface, which masks the reflected and converted waves from the discontinuities inside the Moon. The Double Array Stacking method is an important technique to enhance the reflected and converted waves. However, its performance would be affected by many factors including signal to noise ratio, arrival time errors, differences in amplitude and polarity of waveforms caused by different station azimuth, and source mechanisms, thus needs to be further analyzed.Method:In this study, we use the pseudo-spectral and finite-difference hybrid method on staggered grid to simulate wave propagation of deep moonquakes, considering near surface scattering, to analyze the influence of the above factors on the stacking results of reflected and converted phases from outer core of the Moon.Results and discussion:Our results indicate that coda and noises would greatly reduce the stacking energy but have no significant impact on the peak position of the stacked waveforms. The arrival time errors have a significant influence on the vertical component of ScP phase and the radial component of the ScS phase, but have little influence on the transverse component of ScS phase. The difference in amplitude also leads to differences in stacking energy, but the stacking image shows good consistency in the shape. When the polarities of seismic phases are relatively consistent, the peak of the stacking energy comes from reflected phases from the core mantle boundary of the Moon. This research may be helpful to improve the detection accuracy of the internal structure of the Moon and provide a solid reference for the deployment of future seismic stations, which is of great significance for understanding the formation and evolution process of the Moon.

The precise measurement of earth strain