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Ambient seismic noise or microtremor observations used in spatial auto-correlation (SPAC) array methods consist of a wide frequency range of surface waves from the frequency of about 0.1 Hz to several tens of Hz. The wavelengths (and hence depth sensitivity of such surface waves) allow determination of the site S-wave velocity model from a depth of 1 or 2 m down to a maximum of several kilometres; it is a passive seismic method using only ambient noise as the energy source. Application usually uses a 2D seismic array with a small number of seismometers (generally between 2 and 15) to estimate the phase velocity dispersion curve and hence the S-wave velocity depth profile for the site. A large number of methods have been proposed and used to estimate the dispersion curve; SPAC is the one of the oldest and the most commonly used methods due to its versatility and minimal instrumentation requirements. We show that direct fitting of observed and model SPAC spectra generally gives a superior bandwidth of useable data than does the more common approach of inversion after the intermediate step of constructing an observed dispersion curve. Current case histories demonstrate the method with a range of array types including two-station arrays, L-shaped multi-station arrays, triangular and circular arrays. Array sizes from a few metres to several-km in diameter have been successfully deployed in sites ranging from downtown urban settings to rural and remote desert sites. A fundamental requirement of the method is the ability to average wave propagation over a range of azimuths; this can be achieved with either or both of the wave sources being widely distributed in azimuth, and the use of a 2D array sampling the wave field over a range of azimuths. Several variants of the method extend its applicability to under-sampled data from sparse arrays, the complexity of multiple-mode propagation of energy, and the problem of precise estimation where array geometry departs from an ideal regular array. We find that sparse nested triangular arrays are generally sufficient, and the use of high-density circular arrays is unlikely to be cost-effective in routine applications. We recommend that passive seismic arrays should be the method of first choice when characterizing average S-wave velocity to a depth of 30 m (Vs30) and deeper, with active seismic methods such as multichannel analysis of surface waves (MASW) being a complementary method for use if and when conditions so require. The use of computer inversion methodology allows estimation of not only the S-wave velocity profile but also parameter uncertainties in terms of layer thickness and velocity. The coupling of SPAC methods with horizontal/vertical particle motion spectral ratio analysis generally allows use of lower frequency data, with consequent resolution of deeper layers than is possible with SPAC alone. Considering its non-invasive methodology, logistical flexibility, simplicity, applicability, and stability, the SPAC method and its various modified extensions will play an increasingly important role in site effect evaluation. The paper summarizes the fundamental theory of the SPAC method, reviews recent developments, and offers recommendations for future blind studies.

The tectonic attributes of the fossil suture zone are crucial for reconstructing tectonic evolution, but they remain contentious due to inadequate crustal constraints. This study introduces a high‐resolution crustal structure derived from a cost‐effective short‐period dense seismic array across fossil suture zones between the Xing'an and the Songnen‐Xilinhot blocks in the eastern Central Asian Orogenic Belt. Our imaging reveals clear southward and northward subduction remnants of the fossil subducted slabs in the Solonker suture, along with weak subduction signatures in the Heihe‐Hegenshan suture. Combined with geological evidence, these findings indicate that the bi‐directional subduction remnants were related to the closure of the Paleo‐Asian Ocean in the Solonker suture from the Ordovician to the Early Triassic. Therefore, the identification of the fossil suture zones underscores the utility of dense short‐period seismic arrays in providing cost‐effective constrains on “frozen‐in” crustal information pertinent to tectonic evolution. Plain Language Summary The eastern CAOB, shaped by the closure of the Paleo‐Asian Ocean (PAO), features alternating blocks and suture zones, making it an ideal area for studying fossil suture zones. It includes the Solonker suture and the Heihe‐Hegenshan suture (HHS), where tectonic controversies of these two suture zones have hindered reconstructions of the region's tectonic evolution. This study conducts receiver function imaging to investigate the deep structures of these suture zones. The results reveal remnants of fossil subducted slabs from both southward and northward subduction along the Solonker suture, and a Moho offset associated with the northward subducted slabs beneath the HHS. These findings, combined with geological evidence, suggest that the Solonker suture marks the closure of the main PAO's basin, while the HHS corresponds to the closure of a branch ocean basin. Therefore, the identification of fossil suture zones underscores the effectiveness of dense short‐period seismic arrays in providing cost‐effective constraints on “frozen‐in” crustal information, essential for understanding tectonic evolution. Key Points A dense short‐period seismic array offers significant cost‐effectiveness in providing detailed crustal information of fossil suture zones Remnants of the southward and northward subduction of the Paleo‐Asian Ocean (PAO) were discovered in the Solonker suture The main PAO closed in the Solonker suture, while the Heihe‐Hegenshan suture is related to the closure of a branch ocean basin

We image the shallow seismic structure across the Southern San Andreas Fault (SSAF) using signals from freight trains and trucks recorded by a dense nodal array, with a linear component perpendicular to SSAF and two 2D subarrays centered on the Banning Fault and Mission Creek Fault (MCF). Particle motion analysis in the frequency band 2–5 Hz shows that the examined traffic sources can be approximated as moving single‐ or multi‐point sources that primarily induce Rayleigh waves. Using several techniques, we resolve strong lateral variations of Rayleigh wave velocities and Q‐values across the SSAF, including 35% velocity reduction across MCF toward the northeast and strong attenuation around the two fault strands. We further resolve 10% mass density reduction and 45% shear modulus decrease across the MCF. These findings suggest that the MCF is currently the main strand of the SSAF in the area with important implications for seismic hazard assessments. Plain Language Summary Imaging the internal structure of fault zones is essential for understanding earthquake properties and processes. Here we utilize seismic data generated by trains and trucks in the Coachella valley and recorded by a dense seismic array to image the subsurface structure of two main strands of the Southern San Andreas Fault (SSAF). Several types of analyses allow us to resolve seismic velocities, attenuation coefficients, and mass density across the entire San Andreas Fault zone. The results show a clear contrast in physical properties across the Mission Creek strand of the SSAF, highlighting the presence of a bimaterial fault interface and suggesting that it is the main active strand of SSAF. The research opens up possibilities for using common rail and road traffic signals to derive high resolution imaging results of subsurface seismic properties at other locations. Key Points We detect frequent seismic signals from rail and road traffic in a dense array across the southern San Andreas fault zone We use the traffic signals to image shallow structural properties across the Banning and Mission Creek fault strands The resolved velocity and density contrasts across the Mission Creek fault suggest it is the main active strand of the Southern San Andreas Fault in the area

Understanding glaciers structural heterogeneity is crucial for assessing their fate. Yet, places where structure changes are strong, such as crevasses fields, are often inaccessible for direct instrumentation. To overcome this limitation, we introduce an innovative technique that transforms seismic sources, here generated by crevasses, into virtual receivers using source‐to‐receiver spatial reciprocity. We demonstrate that phase interference patterns between well‐localized seismic sources can be leveraged to retrieve phase velocity maps using Seismic Michelson Interferometry. The obtained phase velocity exhibits sensitivity to changes in glacier structure, offering insights into the origins of mechanical property changes, with spatial resolution surpassing traditional methods by a factor of five. In particular, we observe sharp variations in phase velocity related to strongly damaged subsurface areas indicating a complex 3‐D medium. Applying this method more systematically and in other contexts will enhance our understanding of the structure of glaciers and other seismogenic environments. Key Points We transform seismic sources from crevasses into virtual receivers using source‐to‐receiver spatial reciprocity We derive phase velocity maps in previously inaccessible areas with a resolution five times larger than traditional approaches We retrieve the influence of glacier geometry and structural heterogeneity on the glacier mechanical properties

In this work, an advanced machine learning technique named diffusion maps is applied for array-based earthquake-explosion discrimination. We rely on prior work that utilizes the diffusion map-based discrimination approach for data collected from a single seismometer. The discrimination task is an essential component of the Comprehensive Nuclear-Test-Ban Treaty verification regime and since many of the International Monitoring System (IMS) stations consist of arrays, the extension to array based processing is of interest. The proposed method includes a pre-processing step, which constructs time–frequency representations of the P-wave and S-wave seismograms followed by a non-linear dimensionality reduction step. Discrimination is performed in the low-dimensional space. The performance of the presented algorithm is demonstrated on a data set from Southern Israel, recorded at the IMS seismic array of Mt. Meron (MMAI). We show that the diffusion maps-based approach enables to enhance the discrimination capabilities of seismic arrays, even when processing low-magnitude events.

We observe high‐frequency scatterers consistent with the interpretation of a tabular high‐velocity structure under the Indian Ocean as an ancient subducted slab. We use a previously rarely used raypath, P waves scattered in the slab into PKP waves (P∗${\\mathrm{P}}^{\\ast }$ PKP), from 12 earthquakes and explosions in five locations recorded on the antique LASA (Large Aperture Seismic Array) located in Montana, United States. The scatterers concentrate in the mantle transition zone and ∼${\\sim} $ 1,500 km depths, in the locations where the fast anomalies in the tomography broaden and strengthen. Our inference that the slab lingers in the upper‐ and mid‐mantle despite subducting and detaching more than 130 million years ago suggests that models of slabs sinking into the mantle may have to accommodate such long‐term stagnation. Plain Language Summary Through study of a novel raypath of seismic waves identified on a large, high‐quality seismographic array, we find evidence of an ancient subducted slab residing beneath the Indian Ocean. Our investigation reveals the presence of high‐frequency scatterers nestled approximately 500 and 1,500 km deep. Notably, these scatterers locate within fast anomalies observed in tomographic imaging. Despite having undergone subduction and detachment over 130 million years ago, this slab remains suspended mid‐mantle. Such longevity challenges prevailing models of subduction dynamics by indicating protracted stagnation of subducted lithospheric material within the Earth's mantle. Key Points A unique raypath P*PKP is utilized to detect heterogeneities in the mantle Strong scattering coincides with a hypothesized ancient subducted slab beneath the Indian Ocean This Southeast Indian Slab may have stagnated more than 100 Ma, which is unusually long

Two-dimensional dense seismic ambient noise array techniques have been widely used to image and monitor subsurface structure characterization in complex urban environments. It does not have limitations in the layout under the limitation of urban space, which is more suitable for 3D S-velocity imaging. In traditional ambient seismic noise tomography, the narrowband filtering (NBF) method has many possible dispersion branches. Aliases would appear in the dispersive image, and the dispersion curve inversion also depends on the initial model. To obtain high-accuracy 3D S-velocity imaging in urban seismology, we developed a robust workflow of data processing and S-velocity tomography for 2D dense ambient noise arrays. Firstly, differing from the NBF method, we adopt the continuous wavelet transform (CWT) as an alternative method to measure the phase velocity from the interstation noise cross-correlation function (NCF) without 2π ambiguity. Then, we proposed the sequential dispersion curve inversion (DCI) strategy, which combines the Dix linear inversion and preconditioned fast descent (PFD) method to invert the S-velocity structure without prior information. Finally, the 3D S-velocity model is generated by the 3D spatial interpolation. The proposed workflow is applied to the 2D dense ambient seismic array dataset in Changchun City. The quality evaluation methods include residual iteration error, horizontal-to-vertical spectral ratio (HVSR) map, and electrical resistivity tomography (ERT). All tests indicate that the developed workflow provides a reliable 3D S-velocity model, which offers a reference for urban subsurface space exploration.

The Taiwan orogenic belt is formed by the strong convergence between the Philippine Sea Plate and the Eurasian Plate. The detailed mountain building process is still under debated largely due to the poor constraint of deep crustal structures, particularly the geometry at the Moho-depth. Here the Moho-refracted P waves are identified from the seismic data recorded by a dense seismic array (Formosa Array) in northern Taiwan. Although the refracted seismic energy is often weak at each individual station, the waveform similarity recorded at the nearby stations provides a reliable constraint for estimating the apparent velocity recorded by the dense seismic array. The forward modeling of the observed Moho-refracted P waves shows a larger crustal thickness (~ 52 km) beneath the Backbone Ranges than beneath the adjacent Hsuehshan Ranges (~ 36 km). Such a result is not only confirming the Moho variations along a few of the NW-SE profiles from the previous studies, but also showing the strong Moho variation is well extended along the NE-SW direction. The large change in the crustal thickness across the Taiwan orogeny strongly indicate that the orogenic deformation in Taiwan might extend beyond the shallow crust, possibly involving in the deep crust and upper mantle. The Taiwan orogeny may not be reaching to the isostatic equilibrium yet.

We applied ambient noise tomography to continuous data recorded by a dense seismic array deployed on the volcanic island of Lipari in the southern Tyrrhenian Sea. Since most of Lipari's seismicity occurs offshore and is not evenly distributed, this technique allowed us to obtain the first high‐resolution images beneath the island down to ∼2.5 km depth. Results show a complex seismic structure related to the various ages and compositions of the volcanic products characteristic of the different regions of the island. High shear wave velocities are found in western Lipari, where active hydrothermal vents and N‐S faults are mapped. Low wave speeds are revealed beneath southern and northeastern Lipari, where more recent volcanic activity developed along N‐S dike‐like structures that are aligned with rhyolitic vents. We suggest these dikes likely represent the probable pathways of future volcanic eruptions. Plain Language Summary Recordings of Earth's background seismic noise from an array of tightly spaced instruments allow us to decipher the rocks beneath Lipari, a 37‐km2 active volcanic island in the southern Tyrrhenian Sea. In 2018, for the first time, 48 compact seismic instruments were installed on the island providing continuous recordings for about 1 month that are now used to produce the first three‐dimensional model of the volcano down to ∼3 km depth. The high‐resolution images of Lipari show complex features that are related to the different types and ages of the volcanic rocks. Active CO2‐rich fumaroles and faults found at the Earth's surface are imaged down to depths between 0.5 and 3 km. Beneath southern and northeastern Lipari, conduits of the younger N‐S aligned vents are revealed in the images and they may represent the shallower feeding system of future volcanic eruptions. With the new information from seismic imaging, we are able to better understand the three‐dimensional architecture of the volcanic system and faults, which combined with surface observations will allow us to more accurately determine the potential for future activity and estimate eruptive style and minimum volumes of the eruptive products. Key Points Dense nodal array probes the underground plumbing system of active volcanic islands Ambient noise tomography reveals the complexity of the spatio‐temporal relationship between volcanic, structural, and hydrothermal features High and low shear wave velocities are linked to different eruptive epochs of volcanic edifices in the southern Tyrrhenian Sea

The seismological properties of Earth’s inner core are key to understanding its composition, dynamics and growth history. Within the inner core, fine-scale heterogeneity has previously been identified from backscattering of high-frequency compressional waves. Here we use historical earthquake and explosion data from the Large Aperture Seismic Array, USA, between 1969 and 1975 to build a 3D map of heterogeneity from the inner-core boundary to 500 km depth and determine the geographical distribution of the scatterers across the 40% of the inner core that is visible to the array. Our model has two regions of strong scattering, one beneath eastern Asia and the other beneath South America, both located where past local surveys have identified scattering. We suggest that these loci of strong, fine-scale heterogeneities may be related to random alignments of small, inner-core crystals due to fast freezing. These areas, which have been identified as having high attenuation and lie beneath colder areas of the core–mantle boundary, potentially provide constraints on the dynamics of the inner core and the motions in the outer core, with downwelling in the mantle and outer core possibly associated with strong scattering and inner-core heterogeneity. Two regions of fine-scale heterogeneity in Earth’s inner core may be due to the random alignment of fast-freezing crystals associated with downwelling in the mantle and outer core, according to a 3D map of inner-core seismic data.