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We construct a new shear velocity model for the San Gabriel, Chino and San Bernardino basins located in the northern Los Angeles area using ambient noise correlation between dense linear nodal arrays, broadband stations, and accelerometers. We observe Rayleigh and Love waves in the correlation of vertical (Z) and transverse (T) components, respectively. By combining Hilbert and Wavelet transforms, we obtain the separated fundamental and first higher mode of the Rayleigh wave dispersion curves based on their distinct particle motion polarization. Basin depths constrained by receiver functions, gravity, and borehole data are incorporated into the prior model. Our 3D shear wave velocity model covers the upper 3–5 km of the crust in the San Gabriel, Chino and San Bernardino basin area. The Vs model is in agreement with the geological and geophysical cross‐sections from other studies, but discrepancies exist between our model and a Southern California Earthquake Center community velocity model. Our shear wave velocity model shows good consistency with the CVMS 4.26 in the San Gabriel basin, but predicts a deeper and slower sedimentary basin in the San Bernardino and Chino basins than the community model. Plain Language Summary: Sedimentary basins northeast of Los Angeles can potentially be a low‐velocity channel that focuses earthquake energy from the San Andreas fault into the Los Angeles region. To better understand the focusing effect, we build a new velocity model of this area using a new seismic data set. With the cross‐correlation technique, we extract the travel time information between two stations from the ambient noise, and together with the basin depths based on gravity and receiver functions build a 3D shear wave velocity model. Many geological features, like sedimentary basins and faults, are captured in our velocity model. Compared to the community velocity model, our model predicts a deeper sedimentary basin with lower seismic velocities, indicating the focusing effect of the sedimentary basins northeast of Los Angeles might be underestimated. Key Points: We construct a 3D Vs model in San Gabriel and San Bernardino basins using ambient noise correlation between dense array nodal stationsWe separated the Rayleigh wave fundamental mode and first higher mode in dispersion analysis based on the Rayleigh wave particle motionOur Vs model predicts deeper and slower sedimentary basins than the Southern California Earthquake Center CVMS model, yet is consistent with geological and drilling data [ABSTRACT FROM AUTHOR]

The Shimian area of Sichuan sits at the junction of the Bayan Har block, Sichuan-Yunnan rhombic block, and Yangtze block, where several faults intersect. This region features intense tectonic activity and frequent earthquakes. In this study, we used local seismic waveform data recorded using dense arrays deployed in the Shimian area to obtain the shear wave splitting parameters at 55 seismic stations and thereby determine the crustal anisotropic characteristics of the region. We then analyzed the crustal stress pattern and tectonic setting and explored their relationship in the study area. Although some stations returned a polarization direction of NNW-SSE, a dominant polarization direction of NW-SE was obtained for the fast shear wave at most seismic stations in the study area. The polarization directions of the fast shear wave were highly consistent throughout the study area. This orientation was in accordance with the direction of the regional principal compressive stress and parallel to the trend of the Xianshuihe and Daliangshan faults. The distribution of crustal anisotropy in this area was affected by the regional tectonic stress field and the fault structures. The mean delay time between fast and slow shear waves was 3.83 ms/km, slightly greater than the values obtained in other regions of Sichuan. This indicates that the crustal media in our study area had a high anisotropic strength and also reveals the influence of tectonic complexity resulting from the intersection of multiple faults on the strength of seismic anisotropy.

Shear‐wave velocity (Vs) structures can reveal the shallow sediment thickness and deep tectonic features of buried faults and geological units. They are important for reducing seismic and geological disasters in urban areas. Based on ambient noise data from the Tongzhou dense array (919 seismographic stations), we obtain a fine shallow‐deep (0–5 km) 3D Vs model, by jointly inverting the phase‐velocity dispersions of Rayleigh waves, including short‐period (0.3–2 s) multimode dispersions using the frequency‐Bessel transform method and the long‐period (2–6 s) fundamental‐mode dispersions using the fast marching method. Our results show that the Vs inhomogeneities agree well with the distribution of geological units. We use the 1 km/s isodepth of Vs as the reference thickness of quaternary sediments. Fengbo sag (FBS) and Dachang sag (DCS), which mainly show low velocity and density, have thick sediment thicknesses (approximately 550–600 and 320–420 m, respectively). The NE high‐velocity belt in Daxing high (DXH) has a thinner sediment thickness (∼230 m). Thus, FBS and DCS have a greater risk of earthquake hazards owing to the strong amplification effects of ground motion. Additionally, Vs distribution in the FBS, DCS, DXH, and Yanshan Fold Belt are spatially related to the medium density and buried faults (Nanyuan‐Tongxian, Daxing, and Xiadian faults). We infer that the Vs structures are associated with the controlling effects of these large normal faults and inhomogeneous strata density. The discontinuity of the NE high‐velocity belt in DXH probably results from the intense tectonic activity of Nankou‐Sunhe fault. Plain Language Summary: Quaternary sediments are usually fertile, and many metropolitan cities with dense populations are built on sedimentary basins. However, some large faults covered by thick quaternary sediments may negatively influence urban safety. S‐wave velocity (Vs) can provide useful information to better understand the sediment thickness and tectonic features of buried faults and geological units. We obtained a fine 3D Vs structure using ambient noise from a dense array (919 seismic stations) in Tongzhou. Our results show that the Fengbo and Dachang sags, mainly characterized by low velocities, have relatively thick sediment thicknesses (approximately 550–600 and 320–420 m, respectively). The Yanshan Fold Belt varies from the high‐velocity feature at 0–0.6 km to the low‐velocity feature at 0.6–5 km. With high velocity, the west portion of Daxing has a thinner sediment thickness (∼230 m). Four normal buried faults and media density probably affected the distribution of the Vs distribution. Key Points: An improved Vs model in Tongzhou is obtained by jointly inverting the dispersions from frequency‐Bessel transform and fast marching methodsFour buried normal faults and medium density in the study area have probably affected the distribution of shear‐wave velocityThe thickness of quaternary sediments revealed by 1 km/s of shear‐wave velocity is closely related to the four normal faults [ABSTRACT FROM AUTHOR]

Internal structure imaging of the Earth, along with determining basin structure, can aid in evaluating potential seismic hazards. However, the high operating cost limits the current geophysical exploration methods; moreover, it is difficult to apply these techniques over a large area, which limits our understanding of the Quaternary structure and the development of earthquake prevention science. A combination of dense array observation technology and ambient noise surface wave tomography is being rapidly developed as a high-resolution urban detection method. Here, we report the ambient noise imaging results of a high-density array experiment. In the ambient noise surface wave tomography method (e.g., surface wave tomography; Eikonal tomography), the signal is assumed to be a single mode. However, several multimode signals were detected in this dataset. With the use of traditional methods to measure the dispersion, mode confusion occurs and the extracted dispersion curve jumps. To solve this problem, by combining the advantages of phase-matched filtering and dispersion compensation, we realized the automatic pickup of fundamental group velocity using reference phase velocity. From this, a Rayleigh wave group velocity map was obtained. The regional average phase velocity information was included in the inversion steps to reduce the uncertainty in the inversion of shear wave velocity. Finally, an S-wave velocity structure model was obtained within a depth of 500 m. The velocity structure was roughly layered and grew with depth. In the depth range of 240–320 m, obvious decreases in the S-wave velocity were observed. Compared with geothermal drilling data, this was speculated to be the reflection of a water-rich (confined water) sand layer. This study provides a technical approach for and a processing example of a high-density array, and its velocity model can be used as a reference for urban subsurface structure, underground space utilization, and earthquake disaster prevention and control. [ABSTRACT FROM AUTHOR]

Urban structure imaging using noise-based techniques has rapidly developed in recent years. Given the complexity of the cross-correlation function in high-frequency signals, here, the beamforming (BF) method was used to analyze one data set taken from a dense array in the Jizhong Depression and obtain multi-mode dispersion curves. Multi-mode surface waves improved inversion stability, reduced non-uniqueness, and yielded a one-dimensional shear wave (S-wave) velocity model. Interpolation yielded a high-resolution three-dimensional (3D) S-wave velocity model for the study area. The model shows that velocity gradually changed in the horizontal direction and greatly increased in the vertical direction, which is largely consistent with changes in the sedimentary environment related to the continuous subsidence of the Jizhong Depression since the Quaternary. A low-velocity anomaly at a depth of ~300–400 m was revealed and determined to be caused by either a deep-buried ancient river course or low-lying area. This study demonstrates the potential of the BF method for processing dense array data sets of urban exploration. The high-resolution 3D S-wave velocity model provides a new reference for studying the Quaternary structure of the Jizhong Depression, as well as groundwater resources, urban infrastructure, and underground spaces. [ABSTRACT FROM AUTHOR]

Abstract Shear‐wave velocity (Vs) structures can reveal the shallow sediment thickness and deep tectonic features of buried faults and geological units. They are important for reducing seismic and geological disasters in urban areas. Based on ambient noise data from the Tongzhou dense array (919 seismographic stations), we obtain a fine shallow‐deep (0–5 km) 3D Vs model, by jointly inverting the phase‐velocity dispersions of Rayleigh waves, including short‐period (0.3–2 s) multimode dispersions using the frequency‐Bessel transform method and the long‐period (2–6 s) fundamental‐mode dispersions using the fast marching method. Our results show that the Vs inhomogeneities agree well with the distribution of geological units. We use the 1 km/s isodepth of Vs as the reference thickness of quaternary sediments. Fengbo sag (FBS) and Dachang sag (DCS), which mainly show low velocity and density, have thick sediment thicknesses (approximately 550–600 and 320–420 m, respectively). The NE high‐velocity belt in Daxing high (DXH) has a thinner sediment thickness (∼230 m). Thus, FBS and DCS have a greater risk of earthquake hazards owing to the strong amplification effects of ground motion. Additionally, Vs distribution in the FBS, DCS, DXH, and Yanshan Fold Belt are spatially related to the medium density and buried faults (Nanyuan‐Tongxian, Daxing, and Xiadian faults). We infer that the Vs structures are associated with the controlling effects of these large normal faults and inhomogeneous strata density. The discontinuity of the NE high‐velocity belt in DXH probably results from the intense tectonic activity of Nankou‐Sunhe fault.

To understand the shallow structure and complex sedimentary environment of the 2017 Jinghe Ms6.6 earthquake focal area, we used one mouth of continuous sesimic data from a dense seismic array of 208 short period stations around the earthquake focal area, and applied the ambient noise tomography (ANT) method to image the three-dimensional Shear wave velocity structure at the depth less than 4 km. The shear wave velocity shown clear lateral variations and vertical variations from the surface to the deeper regions and has a tight correlation with surface geological and tectonic features in the study area. Obvious low-velocity anomalies has been presented throughout most of the Jinghe depression, whereas the Borokonu Mountains presented high-velocity anomalies. The thickness of the Cenozoic sedimentary basement in the study area is approximately 1–4 km, and the distribution of thickness is highly uneven. The crystalline basement in the study area has strong bending deformation, and the non-uniform Cenozoic sediments are related to the strong bending deformation of the crystalline basement. The Kusongmuchik piedmont fault is a high-angle thrust fault cutting through the base. There are also many medium low-angle faults, which do not penetrate the surface, which has indicated that they are in a concealed state at present. The results have provided a shallow high-resolution 3D velocity model that can be used in the simulation of strong ground motion and for evaluating potential seismic hazards.

Abstract The feasibility of using a dense seismic array with an airgun source to study the quality factors of shallow media is verified. Data were obtained from 37 stations in the dense seismic array located in Binchuan, Yunnan Province, China, and the amplitude–distance attenuation method and the coda normalization method were applied to calculate the S-wave quality factors in the area. The amplitude–distance attenuation method yielded $${Q}_{s}^{-1}$$ Q s - 1 =0.0260 $$\pm$$ ± 0.0103, and the frequency-dependent $${Q}_{s}^{-1}$$ Q s - 1 calculated by the coda normalization method can be expressed by the power law $${Q}_{s}^{-1}(f)\cong 0.0554{f}^{-0.5643}$$ Q s - 1 ( f ) ≅ 0.0554 f - 0.5643 . The consistency between the results of these two methods shows that a dense seismic array with an airgun source can be used to study the attenuation characteristics of shallow media. The amplitudes at some points deviate substantially from the fitted curve and thus have a certain influence on the fitting results; hence, we must select high-precision data for the calculation. Given the topography, we speculate that the anomalous stations located on the edge of the Binchuan Basin and in the western hilly area are due to the edge effect of the basin and the weak attenuation of the hilly area and that the anomalous station located in the northern Binchuan depocenter is attributable to local site factors. Compared with the $${Q}_{s}^{-1}$$ Q s - 1 estimated by previous studies, the $${Q}_{s}^{-1}$$ Q s - 1 in the Binchuan area is found to lie between those of the hard soil and sedimentary rock and is similar to the $${Q}_{s}^{-1}$$ Q s - 1 in the North China Basin, corresponding to the shallow velocity structure in this area. Graphical Abstract

The Ningdu basin, located in southern Jiangxi province of southwest China, is one of the Mesozoic basin groups which has exploration prospects for geothermal energy. A study on the detailed velocity structure of the Ningdu basin can provide important information for geothermal resource exploration. In this study, we deployed a dense seismic array in the Ningdu basin to investigate the 3D velocity structure and discuss implications for geothermal exploration and geological evolution. Based on the dense seismic array including 35 short-period (5 s-100 ​Hz) seismometers with an average interstation distance of ∼5 ​km, Rayleigh surface wave dispersion curves were extracted from the continuous ambient noise data for surface wave tomographic inversion. Group velocity tomography was conducted and the 3D S-wave velocity structure was inverted by the neighborhood algorithm. The results revealed obvious low-velocity anomalies in the center of the basin, consistent with the low-velocity Cretaceous sedimentary rocks. The basement and basin-controlling fault can also be depicted by the S-wave velocity anomalies. The obvious seismic interface is about 2 ​km depth in the basin center and decreases to 700 ​m depth near the basin boundary, suggesting spatial thickness variations of the Cretaceous sediment. The fault features of the S-wave velocity profile coincide with the geological cognition of the western boundary basin-controlling fault, which may provide possible upwelling channels for geothermal fluid. This study suggests that seismic tomography with a dense array is an effective method and can play an important role in the detailed investigations of sedimentary basins.

The difference in local sediment thickness and soil properties has a significant impact on the spatial variation mechanism of seismic ground motion in the engineering scale. Due to the scarcity of observation data of dense arrays, the existing theoretical studies are mostly developed by numerical simulation methods, including human factors and a large number of assumptions. In view of this, based on the multistation observation records of the Luxian MS 6.0 earthquake and Yibin MS 5.1 earthquake obtained using a Zigong dense array, the study quantitatively analyzes the spatial characteristics of ground motion in heterogeneous soil sites by integrating a theoretical model with numerical analysis. In this study, many popular approaches including root-mean-square acceleration, horizontal-to-vertical spectral ratio (HVSR) of microtremor and strong motion records, and lagged coherency are comprehensively utilized to make the conclusion accurate and reliable. The results show that local soil conditions could affect the attenuation of coherence function with distance. The station-pairs with similar HVSR characteristics generally present a higher coherence level when the difference of the interstation distance is less than 100 m. In addition, the coherency function between stations will be greatly reduced when the H/V spectral ratio characteristics differ greatly, which is also obvious in the low-frequency part below 5 Hz. Finally, a lagged coherency model that considers the influence of heterogeneous soil is constructed in this study. The model has a definite physical meaning and can better represent the spatial variation of ground motion at nonbedrock sites.

The feasibility of using a dense seismic array with an airgun source to study the quality factors of shallow media is verified. Data were obtained from 37 stations in the dense seismic array located in Binchuan, Yunnan Province, China, and the amplitude–distance attenuation method and the coda normalization method were applied to calculate the S-wave quality factors in the area. The amplitude–distance attenuation method yielded Q s - 1 =0.0260 ± 0.0103, and the frequency-dependent Q s - 1 calculated by the coda normalization method can be expressed by the power law Q s - 1 (f) ≅ 0.0554 f - 0.5643 . The consistency between the results of these two methods shows that a dense seismic array with an airgun source can be used to study the attenuation characteristics of shallow media. The amplitudes at some points deviate substantially from the fitted curve and thus have a certain influence on the fitting results; hence, we must select high-precision data for the calculation. Given the topography, we speculate that the anomalous stations located on the edge of the Binchuan Basin and in the western hilly area are due to the edge effect of the basin and the weak attenuation of the hilly area and that the anomalous station located in the northern Binchuan depocenter is attributable to local site factors. Compared with the Q s - 1 estimated by previous studies, the Q s - 1 in the Binchuan area is found to lie between those of the hard soil and sedimentary rock and is similar to the Q s - 1 in the North China Basin, corresponding to the shallow velocity structure in this area. [ABSTRACT FROM AUTHOR]

The 3D S wave velocity of shallow structures, especially the quaternary sediments at 0–1 km near the surface, is an important issue of concern in urban planning and construction for seismic hazard assessment and disaster mitigation. Due to the facility and less dependence on the site environment, noise‐based technique is an ideal way to acquire the fine structure of urban sedimentary basin. Based on the dense array composed of more than 900 stations deployed in Tongzhou at a local scale of 20 × 40 km2, we prove that the lateral variation of the phase velocity of multimode surface waves can be estimated directly with sufficient accuracy by beamforming seismic noise with moving subarrays. Rayleigh wave phase velocity maps, at frequencies between 0.3 and 2.5 Hz for the fundamental mode as well as 0.8 and 3.0 Hz for the first overtone, are obtained. The 3D S wave velocity model at 0–1‐km depth with lateral resolution of 1 km is then established by inverting phase velocity maps of the two modes. The sediment thickness is delineated by the impedance interface given by microtremor H/V (horizontal‐to‐vertical) spectral ratio. The proposed model is in good agreement with the distribution of tectonic units. The sediment thickness of Daxing high and two sags around Gantang and Xiadian are, respectively, 100–400 and 400–600 m, which correlates well with the 1 km/s isosurface of S wave velocity. The model presents some evidence on the extension of Daxing fault along NE direction. Plain Language Summary: Large cities are usually built on sedimentary basins which are prone to capture and amplify seismic energy with the result of larger damage. The main factors to determine the site amplification are the depth to basement and S wave velocity of the sedimentary layer. The combination of beamforming (BF) and microtremor H/V is suggested to build the high‐resolution 3D S wave velocity model of the basin. We prove that the lateral variation of the Rayleigh wave phase velocity of the fundamental mode as well as the first overtone can be estimated directly with sufficient accuracy by BF seismic noise, without tomographic inversion. The 3D S wave velocity model can thereby be established with high resolution by inverting the phase velocity maps of multimode surface waves. The depth to basement is delineated by the impedance interface given by microtremor H/V. The 3D S wave velocity and the depth to basement of Tongzhou, the subcenter of Beijing, is established using the data at a dense array composed of more than 900 stations using suggested schemes. The 3D S wave velocity model could provide the basic information for seismic hazard assessment and disaster mitigation, therefore fulfilling the requirements in urban planning and construction. Key Points: We prove that the 2D phase velocity maps of the multimode Rayleigh waves can be extracted directly by beamforming the ambient noiseA 3D Vs model of Tongzhou is established by inverting the phase velocity maps of the fundamental and the first overtone of Rayleigh wavesThe sediment thickness is delineated by the interface with strong impedance contrast obtained by H/V spectral ratio [ABSTRACT FROM AUTHOR]

Spin-transfer torque (STT) magnetic random access memory (MRAM) is gaining commercial traction in low-density embedded and standalone memories, but the available market opportunities would grow exponentially if STT-MRAM could approach dynamic random access memory (DRAM) bit density. Of course, achieving DRAM density will require, among other things, demonstrating the ability to pattern MRAM bits at an extremely tight pitch. Here, an experimental demonstration of fabrication and electrical testing of a STT-MRAM bit array are reported. By optimizing the hard mask and ion beam etching (IBE) processes, MRAM bit arrays with a full pitch down to 50 nm are fabricated, in which the bits are individually tested using a novel bottom-point-contact method. [ABSTRACT FROM AUTHOR]

Low‐velocity zones (LVZ) in the shallow crust such as basins infilled with unconsolidated sediments could significantly amplify seismic waves and cause intense ground motions. In this study, we estimate the seismic site response in the Binchuan basin across the active Chenghai fault zone in northwest Yunnan Province, Southwest China using data from a large‐aperture (∼8 km) dense linear array of 125 three‐component seismometers deployed in 2018 for ∼1 month. We observe that local earthquakes with larger epicentral distances tend to produce stronger spatial variability in peak ground velocity (PGV) across the array. The PGV values are overall larger in the middle of the array than the two sides, especially in the horizontal directions, which is consistent with both tomographic and traveltime results. Using both nearby and distant earthquakes, we find that horizontal ground motions from 0.2 to 2 Hz at stations inside the LVZ are up to 20 times as large as that outside, and a more localized zone (∼500 m wide) enclosing the fault surface trace is characterized by broader frequency bands and larger factors of amplification. As the distance from the fault increases, amplification factors become smaller while the lower limit of the amplification frequency band gradually increases. Variations in predicted ground motion from one‐dimensional SH‐wave modeling reveal amplification and attenuation effects of the LVZ, suggesting that strong lateral heterogeneity should be considered. Our results demonstrate that based on dense array data, analyzing seismic site response is an alternative method to effectively constrain the shallow subsurface fault structure. Plain Language Summary: Seismic site response refers to the effects of local site conditions on arriving seismic waves and is an important factor for precisely predicting ground motions. It could vary considerably around major crustal faults, where numerous ruptures and many other processes have together generated complex subsurface structures. Here we investigate seismic site response with its spatial variability across the Chenghai fault zone in northwest Yunnan Province, Southwest China using a ∼8‐km‐long dense array of 125 seismometers. We find clear variations in peak ground velocity (PGV) produced from local earthquakes across the array and higher PGV values around the fault. Moreover, the spatial variability in PGV increases with the epicentral distance. From local, regional, and teleseismic earthquake recordings in horizontal directions, we observe that ground motions within the previously identified low‐velocity zone are amplified by factors up to 20 between 0.2 and 2 Hz, and a ∼500‐m‐wide zone near the fault with the strongest amplification effects is newly identified. Predictions using one‐dimensional SH‐wave modeling and the shear‐wave velocity model indicate complicated effects from near‐fault subsurface structures on ground motions. Our results suggest seismic site response analysis based on dense array data can help effectively constrain low‐velocity structures around active fault zones. Key Points: We investigate the variations of ground motions in the Binchuan basin in Northwest Yunnan with a temporary dense arrayThe amplitude observed in the low velocity zone across the Chenghai fault is up to 20 times as large as outside stationsAnalyzing seismic site response based on dense array data could be an effective method to constrain fault zone structure [ABSTRACT FROM AUTHOR]

Urban structure imaging using noise-based techniques has rapidly developed in recent years. Given the complexity of the cross-correlation function in high-frequency signals, here, the beamforming (BF) method was used to analyze one data set taken from a dense array in the Jizhong Depression and obtain multi-mode dispersion curves. Multi-mode surface waves improved inversion stability, reduced non-uniqueness, and yielded a one-dimensional shear wave (S-wave) velocity model. Interpolation yielded a high-resolution three-dimensional (3D) S-wave velocity model for the study area. The model shows that velocity gradually changed in the horizontal direction and greatly increased in the vertical direction, which is largely consistent with changes in the sedimentary environment related to the continuous subsidence of the Jizhong Depression since the Quaternary. A low-velocity anomaly at a depth of ~300–400 m was revealed and determined to be caused by either a deep-buried ancient river course or low-lying area. This study demonstrates the potential of the BF method for processing dense array data sets of urban exploration. The high-resolution 3D S-wave velocity model provides a new reference for studying the Quaternary structure of the Jizhong Depression, as well as groundwater resources, urban infrastructure, and underground spaces.

The spatial patterns of earthquake ground motion amplitudes are commonly represented using a double‐couple model that corresponds to shear slip on a planar fault. While this framework has proven largely successful in explaining low‐frequency seismic recordings, at higher frequencies the wavefield becomes more azimuthally isotropic for reasons that are not yet well understood. Here, we use a dense array of nodal seismometers in Oklahoma to study the radiation patterns of earthquakes in the near‐source region where the effects of wavefield scattering are limited. At these close distances, the radiation pattern is predominantly double couple at low frequencies (<15 Hz). At higher frequencies, the recorded wavefield contains significant isotropic and residual components that cannot be explained as path or site effects, implying complexity in the rupture process or local fault zone structure. These findings demonstrate that earthquake source complexity can drive variability in the ground motions that control seismic hazard. Plain Language Summary: The amplitude of the ground motions produced by an earthquake is not constant in all directions, but instead varies systematically in relation to the orientation of the fault on which the earthquake occurs. Here, we study these directional variations in ground motions, known as the seismic radiation pattern, using a large data set of thousands of closely spaced seismometers recording nearby earthquakes. Our focus is on understanding how the spatial pattern of ground motions depend on their frequency of vibration. We find that at low frequencies, a simplified and widely used four‐lobed model of earthquake ground motions does a good job describing the observed seismic wavefield. At higher frequencies, however, this four‐lobed radiation pattern becomes less clear, deteriorating due to complexity in earthquake source processes and fault zone structure. Understanding the physical mechanisms driving spatial variations in ground motion will help create more accurate earthquake hazard forecasts for communities living near active faults. Key Points: We study the frequency dependence of earthquake radiation patterns in the near‐source region using a dense seismometer arrayAt low frequencies (<15 Hz), radiation patterns show excellent agreement with the double‐couple modelAt high frequencies, the double‐couple radiation pattern deteriorates, likely due to source and fault zone complexity [ABSTRACT FROM AUTHOR]

Tomographic imaging based on long‐term ambient seismic noise measurements, mainly the phase information from surface waves, has been shown to be a powerful tool for geothermal reservoir imaging and monitoring. In this study, we utilize seismic noise data from a dense nodal array (192 3C nodes within 20 km2) over a ultra‐short observation period (4.7 days) to reconstruct surface waves and determine the high‐resolution (0.2 km) three‐dimensional (3‐D) S wave velocity structure beneath a rural town in Zhejiang, China. We report the advantage of cross‐coherence over cross‐correlation in suppressing pseudo‐arrivals caused by persistent sources. We use ambient noise interferometry to retrieve high quality Rayleigh waves and Love waves. Body waves are also observed on the R‐R component interferograms. We apply phase velocity dispersion measurements on both Rayleigh waves and Love waves and automatically pick more than 23,000 dispersion curves by using a Machine Learning technique. 3‐D surface wave tomographic results after depth inversion indicate low‐velocity anomalies (between −1% and −4%) from the surface to 2 km depth in the central area. Combined with the conductive characteristics observed on resistivity profile, the low‐velocity anomalies are inferred to be a fluid saturated zone of highly fractured rock. Joint interpretation based on horizontal‐to‐vertical spectral ratio (HVSR) measurements, and existing temperature and fluid resistivity records observed in a nearby well, suggests the existence of the high‐temperature geothermal field through the fracture channel. Strong correlation between HVSR measurements and the S wave velocity model highlights the potential of extraction of both amplitude and phase information from ambient noise. Plain Language Summary: The geothermal energy potential beneath our feet is vast and clean. Increased development of geothermal energy provides people a promising solution for the nation and the world as we become ever more concerned about global warming, as well as air pollution. However, the development of national geothermal resources in China is far behind the target. We utilize seismic ambient noise to produce a high‐resolution three‐dimensional (3‐D) tomographic image of the geothermal system beneath a rural town in Zhejiang, China. This technique allows us to detect velocity variations/contrasts due to subsurface inhomogeneity over an ultra‐short time window which offers the opportunity for high temporal‐resolution (several days scale) geothermal reservoir imaging and monitoring/tracking. We identify low‐velocity variations outlining a highly fractured zone from the surface to 2 km depth in the central area. We interpret the low‐velocity/high‐conductivity/high‐temperature fracture zone as a heat transfer channel of the local geothermal system. Key Points: Ambient noise data have been recorded using a dense nodal array (192 3C nodes within 20 km2) over ultra‐short observation period (4.7 days)Both surface waves (Rayleigh and Love waves) and P waves are identified in the cross‐coherence functionsS wave velocity model is consistent with existing geophysical data and suggests the existence of high‐temperature geothermal resources [ABSTRACT FROM AUTHOR]

In this study, we investigate the shallow shear wave velocity structure of the Los Angeles Basin in southern California, using ambient noise correlations between 5 dense arrays and 21 broadband stations from the Southern California Seismic Network (SCSN). We observe clear fundamental mode and first overtone Rayleigh waves in the frequency band 0.25–2.0 Hz, and obtain group velocity maps through tomography. We further derive a 3D shear wave velocity model, covering a large portion of the central LA Basin for the depths shallower than 3 km. We found that the small scale shallow velocity structure heterogeneities are better resolved compared with the SCEC Community velocity models. Our model captures the presence of the Newport‐Inglewood fault by a NW–SE trending high velocity belt. Our model provides more accurate constraints on local ground motion predictions with detailed mapping of structural heterogeneities. Key Points: We derive top 3 km Vs structure of the central Los Angeles Basin using dense industrial arrays correlated with broadband seismic stationsOur model better resolves small scale heterogeneities and better predicts dispersion observations compared with the SCEC CVM modelsOur model illuminates the Newport‐Inglewood fault zone by a NW–SE trending high shear wave velocity belt [ABSTRACT FROM AUTHOR]

High-resolution imaging of fault zone structure and its temporal changes can not only advance our understanding of earthquake physics, but is also critical for better seismic hazard preparation and mitigation. In the past a few years, we deployed multi-scale dense arrays across the Chenghai fault system in Binchuan, Yunnan, China. The first array consisted of 381 intermediate-period three-component seismometers with an average station spacing of ∼2 ​km. The array has been deployed in the field for ∼3 months in 2017 and recorded numerous local and teleseismic earthquakes. Travel time analyses based on teleseismic earthquakes and an airgun source in the region indicated clear signature of low-velocity fault zones in the southern branch of the Chenghai fault system. In 2018 we deployed two other linear arrays using the same instruments with much smaller inter-station spacing, e.g. 30–50 ​m, across the southern branch the Chenghai fault. The profile lengths were 8 and 5 ​km, respectively. Record sections of the airgun source on the two linear arrays clearly marked a low-velocity zone (LVZ) within the southern array but no such signature in the northern array, suggesting along-strike variation of the LVZ. Although the instruments within our dense arrays had an intermediate frequency band, we demonstrated that they were capable of characterizing crustal structure with techniques commonly applied to broadband signals such as receiver functions. To our best knowledge, this was the first time to have multi-scale across-fault dense arrays with three-component seismometers in such apertures. These results lay out the pavement to comprehensively investigate fault zone structures as well as to derive subsurface structural changes using dense arrays and the airgun source.

In this article, we propose a novel type of a wideband 2-D array of cold-electron bolometers (CEBs) sensitive to both polarizations of the incoming optical signal. This array was developed for osservatorio per il lontano infrarosso e le microonde su pallone orientabile (OLIMPO) balloon telescope with a requirement on bandwidth more than 10%. Sensitivity to both polarization components was achieved by a special meander-type arrangement of orthogonal L/2 dipoles. We present theoretical estimations for the optimal arrangement of 2-D arrays of CEBs, which allows achieving maximum efficiency in a wide band with the lowest noise equivalent power (NEP). A pixel layout with an optimal density of CEBs, sensitive to dual polarizations, and having more than 10% FWHM bandwidth at around 350 GHz was proposed and numerically analyzed. The layout consists of a periodic sparse array of orthogonal dipoles sensitive to both polarization components of the incoming signal. The numerical analysis confirmed that the sparse array is the most promising for a combination of high absorbing efficiency and low noise of the system and satisfying the 10% bandwidth requirements for the OLIMPO instrument. Different combinations of series and parallel connections of bolometers were considered for better noise properties to achieve the photon noise-limited level. [ABSTRACT FROM AUTHOR]

Ground motions in the Los Angeles Basin during large earthquakes are modulated by earthquake ruptures, path effects into the basin, basin effects, and local site response. We analyzed the direct effect of shallow basin structures on shaking duration at a period of 2–10 s in the Los Angeles region through modeling small magnitude, shallow, and deep earthquake pairs. The source depth modulates the basin response, particularly the shaking duration, and these features are a function of path effect and not site condition. Three‐dimensional simulations using the CVM‐S4.26.M01 velocity model show good fitting to the initial portion of the waveforms at periods of 5 s and longer but fail to predict the long shaking duration during shallow events, especially at periods less than 5 s. Simulations using CVM‐H do not match the timing of the initial arrivals as well as CVM‐S4.26.M01, and the strong late arrivals in the CVM‐H simulation travel with an apparent velocity slower than observed. A higher‐quality factor than traditionally assumed may produce synthetics with longer durations but is unable to accurately match the amplitude and phase. Beamforming analysis using dense array data further reveals the long duration surface waves have the same back azimuth as the direct arrivals and are generated at the basin edges, while the later coda waves are scattered from off‐azimuth directions, potentially due to strong, sharp boundaries offshore. Improving the description of these shallow basin structures and attenuation model will enhance our capability to predict long‐period ground motions in basins. Key Points: Shallow earthquakes generate long shaking duration in the Los Angeles basin, which is not adequately captured by current 3‐D velocity modelsBeamforming analysis shows that most of the coherent arrivals propagate along great circle paths, excited by the basin edgesImproved descriptions of shallow basin structure and attenuation model are crucial in predicting the observed long shaking duration [ABSTRACT FROM AUTHOR]

Proper classification of nontectonic seismic signals is critical for detecting microearthquakes and developing an improved understanding of ongoing weak ground motions. We use unsupervised machine learning to label five classes of nonstationary seismic noise common in continuous waveforms. Temporal and spectral features describing the data are clustered to identify separable types of emergent and impulsive waveforms. The trained clustering model is used to classify every 1 s of continuous seismic records from a dense seismic array with 10–30 m station spacing. We show that dominate noise signals can be highly localized and vary on length scales of hundreds of meters. The methodology demonstrates the complexity of weak ground motions and improves the standard of analyzing seismic waveforms with a low signal‐to‐noise ratio. Application of this technique will improve the ability to detect genuine microseismic events in noisy environments where seismic sensors record earthquake‐like signals originating from nontectonic sources. Plain Language Summary: Improvements in microseismic detection will advance observations of failure processes on faults subjected to slowly accumulating tectonic stress. Continuous seismic waveforms contain copious variations of nontectonic signals that inhibit the detection of genuine microearthquakes. Developing a framework to identify emergent and impulsive signals originating from natural and anthropogenic activity will advance seismic network monitoring capabilities. We apply unsupervised machine learning techniques to classify multiple types of weak ground motions that occur ubiquitously in continuous seismic records. A trained model is used to label every 1 s of a dense geophone array to provide a high‐resolution description of the anatomy of continuous seismic records. Further methodology developments and application in various environments have high potential for improving the ability to monitor earthquakes and other sources of ground motion. Key Points: Emergent and impulsive signals in continuous seismic waveforms are identified using cluster analysis on a dense array dataAn unsupervised learning model is trained to identify multiple classes of noise using temporal and spectral data featuresA more complete understanding of seismic noise signals will improve the ability to detect genuine microseismic events [ABSTRACT FROM AUTHOR]

To derive high‐resolution fault zone (FZ) structure of the Chenghai fault in Yunnan, southwestern China, we deployed a linear dense array crossing the fault from January to February 2018. The array consisted of 158 short‐period (5 s) three‐component instruments and spanned an aperture of ~8 km with average station spacing of 40–50 m. During the 1‐month deployment, 20 teleseismic earthquakes with moment magnitudes larger than 5.5 and 41 local earthquakes were recorded. We first analyzed the travel times of P and S waves from teleseismic earthquakes to determine the boundaries of the FZ. After correcting the station‐event geometry effects, teleseismic travel time differences between the reference station, and all other stations clearly marked a zone of 3.4 km in width with distinct travel time anomaly, suggesting a low‐velocity zone (LVZ) surrounding the Chenghai fault. We then conducted ambient noise tomography and found that the S wave velocity of the LVZ was reduced by 60% and 40% compared to the northwest and southeast of the LVZ, respectively. Our ambient noise results suggested the LVZ extending to ~1.5 km in depth, consistent with the travel time anomalies of teleseismic earthquakes. Integrating ambient noise tomography with teleseismic travel times in a dense array with such an aperture is an effective approach for resolving FZ structure and depth extent. Plain Language Summary: High‐resolution fault zone (FZ) structure and evolution are critical to understand earthquake physics. As the FZ width is usually on the order of 100 to 1,000 m, resolving the structure requires dense seismic arrays with small interstation spacing. One of the difficulties is to robustly constrain the depth extent by small‐aperture arrays. Here we demonstrate that deployment of a large‐aperture (8 km) array across a fault can robustly resolve the FZ structure and depth extent. Using the data acquired from a linear dense array crossing the Chenghai Fault in northwest Yunnan, China, we show that the low‐velocity zone of the fault extends to 1.5 km from the results of ambient noise tomography, which are consistent with observations of travel times of body waves from teleseismic earthquakes. Such large‐aperture array is an effective tool that enables integration of a variety of methods to robustly constrain high‐resolution FZ structure. Key Points: We deployed a linear dense array of 8 km in length across the Chenghai fault that is critical to constrain the fault zone depthBased on teleseismic travel times and ambient noise tomography, we found a low‐velocity zone of 3.4 km in width associated with the faultAmbient noise tomorgraphy showed that the low‐velocity zone extends to 1.5 km in depth, consistent with teleseismic travel times [ABSTRACT FROM AUTHOR]

In traditional surface wave tomography based on seismic noise, 2D phase or group velocity distribution is obtained by performing pure‐path inversion after extracting interstation velocities based on the noise cross‐correlation function. In this paper, we show that 2D surface wave phase velocity maps of adequate quality can be obtained directly, without interferometry, by beamforming the ambient noise recorded at array of stations. This method does not require a good azimuthal distribution of the noise sources. The 2D surface wave phase velocity map is obtained by moving the subarrays within a larger dense network of stations. The method is illustrated with seismic noise recorded by over 600 stations of the ChinArray (Phase II). We obtain 2D Rayleigh wave phase velocity maps between 7 and 35 s in Northeastern (NE) Tibetan Plateau and adjacent regions that compare well with results obtained with other methods. The shear wave velocity model is then derived by inverting the phase velocity with depth. The model correlates well with geology and tectonics in NE Tibet. Two clear mid‐to‐low crustal low‐velocity zones are observed at 15‐ to 35‐km depth beneath the Songpan‐Ganzi terrane and Northwestern Qilian Orogen, possibly facilitating lower crustal flow in this key region for the tectonic evolution of NE Tibet. Key Points: We present a beamforming method to extract the phase velocity of the Rayleigh wave directly from the ambient noise, without interferometryA 3D Vs model of NE Tibet is established by inverting the obtained Rayleigh wave velocity mapsMid crustal low velocity zones are observed in the Songpan‐Ganzi Terrane and in NW Qilian Orogen [ABSTRACT FROM AUTHOR]

The North Anatolian Fault (NAF) is one of the most well-known continental strike slip faults, however, the details related to the geodynamic processes and the extent of deformation in the crust remain poorly understood. Within the context of the Dense Array for North Anatolia (DANA) project, a comprehensive data set was gathered from a dense temporary seismic network consisting of 70 stations that were deployed in early May 2012 and operated for 18 months in the Sakarya region and the surroundings. With the aim of further exploring the crustal deformation near the fault mainly caused by the strike slip motion also resulting in the alignment of minerals, the crustal seismic anisotropy beneath the western segment of NAF was investigated by local shear wave splitting analysis. Out of 1371 events, 90 well located earthquakes were extracted with magnitudes >1.4 corresponding to a total of 645 splitting measurements. This method makes use of earthquakes nearby or directly below each station benefiting from the fast polarization direction and the relevant delay time parameters as the main output. Despite the scattered patterns, the fast orientations are dominantly E -W parallel to the fault strike. Delay times between the fast and slow components of the shear wave vary between 0.01 and 0.3 s clearly revealing the existence of crustal anisotropy. In particular, measurements at each station exhibits spatial variations across the fault where it splays into two main branches beneath the study area separating different geologic units. A strong correlation could not be established between small scale faults and azimuthal anisotropy. The measurements presented here constrain crustal anisotropy above the deepest earthquakes at approximately 15 km depth. • The extent of crustal anisotropy beneath the western segment of North Anatolian Fault. • Fast direction and delay time measurements revealed and scattering highlighting this structural complexity of active tectonic units. • Based on the observations, it might be suggested that there is not a strong coupling between mantle and crustal anisotropy. [ABSTRACT FROM AUTHOR]

Karatungk Mine is the second-largest Cu-Ni sulfide mine in China. However, the detailed structure beneath the mine remains unclear. Using continuous waveforms recorded by a dense temporary seismic array, here we apply ambient noise tomography to study the shallow crustal structure of Karatungk Mine down to ~1.3 km depth. We obtain surface-wave dispersions at 0.1–1.5 s by calculating cross-correlation functions, which are inverted for 3D shear-wave structure at the top-most (0–1.3 km) crust by a joint inversion of group and phase dispersions. Our results show that low-velocity zones beneath Y1 ore-hosting intrusion (hereafter called Y1) at 0–0.5 km depth and northwest of the Y2 ore-hosting intrusion (hereafter called Y2) at 0–0.6 km depth are consistent with highly mineralized areas. A relatively high-velocity zone is connected with a weakly mineralized area located to the southeast of Y2 and Y3 (hereafter called Y3) ore-hosting intrusions. Two high-velocity zones, distributed at 0.7–1.3 km depth in the northernmost and southernmost parts of the study area respectively, are interpreted to be igneous rocks related to early magma intrusion. Furthermore, the low-velocity zone at 0.7–1.3 km depth in the middle of the study area may be related to: a possible channel related to initial magma transport; mine strata or a potentially mineralized area. This study demonstrates a new application of dense-array ambient noise tomography to a mining area that may guide future studies of mineralized regions.

The Zhuxi ore deposit, located in Jiangxi province, South China, is the largest tungsten reserve in the world. To better understand the geological structure and distribution of orebodies, we conducted a high resolution three-dimensional P-wave velocity tomography of the uppermost 0.5 km beneath the Zhuxi ore deposit and adjacent area. Our velocity model was derived from 761,653 P-wave first arrivals from 998 control-source shots, recorded by a dense array. As the first 3D P-wave velocity structure of the Zhuxi ore deposit, our model agrees with local topographic and tectonic structures and shows depth-dependent velocity similar to laboratory measurements. The Carboniferous formations hosting the proven orebodies are imaged as high velocities. The high-velocity anomalies extend to a larger area beyond the proven orebodies, and the locations of high−low velocity boundaries are in accordance with the boundaries between the Neoproterozoic formation and the Carboniferous−Triassic formation. Seismic tomography reveals that high-velocity anomalies are closely related to the mineralized areas. Our results are helpful for further evaluating the total reserves and suggest that seismic tomography can be a useful tool for mineral exploration.

The stochastic estimation of coherency is perceived, in some studies, to be significantly influenced by the time window length under consideration. For the engineering purposes, usually, coherency is estimated from the strong motion, i.e., the shear (S-) wave segment of the recorded time series. However, the identification of a purely S-wave dominated window could be challenging in case of complex wave mixing on the seismograms. Moreover, there has been relatively little research on how much variation, in reality, is introduced in the coherency estimates owing to the choice of signal length. Therefore, the debate about the procedure for selecting a representative time window has remained inconclusive: different authors keep their different practices. The current article is an effort to shed light on the sensitivity of the coherency estimates to the choice of various time windows. The research draws on the dataset gathered from a dense seismic-array deployed at the small size, shallow alluvial valley of Koutavos-Argostoli, situated in Cephalonia Island, Greece. The lagged coherency over interstation distance ranges from 5 to 80 m has been estimated from a set of 46 earthquakes having magnitude 2-5 and epicentral distance 3-200 km, considering different lengths of time windows. The findings revealed that the statistics of coherency estimates derived from many events is only weakly sensitive to the selection of time window lengths provided that the segments include the same energetic pulse. [ABSTRACT FROM AUTHOR]

The term “spatial variability of seismic ground motions” denotes the differences in the amplitude and phase content of seismic motions. The effect of such spatial variability on the structural response is still an open issue. In-situ experiments may be helpful in order to answer the questions regarding both the quantification of the spatial variability of the ground motion within the dimensions of a structure as well as the effect on its dynamic response. The goal of the present study is to quantify the variability of the seismic ground motion accelerations in the shallow sedimentary basin of Argostoli, Greece, and thereafter to identify its effect on the linear and non-linear elasto-plastic response of a single degree of freedom system in terms of spectral displacements. Around 400 earthquakes are used, recorded by the 21-element very dense seismological array deployed in Argostoli with inter-station spacing ranging from 5 to 160 meters. The seismic motion variability, evaluated in terms of spectral accelerations, is found to be significant and to increase with inter-station distance and frequency. Thereafter, the amplitude variability in terms of spectral displacements, which is indeed the linear response of a single degree of freedom (SDOF) system with various fundamental periods, is compared with the amplitude variability of a SDOF with non-linear elasto-plastic response. The variability of the maximum top displacement of the linear single degree of freedom system is estimated to be on average 12% with larger variabilities to be observed within two narrow frequency ranges (between 1.5 and 1.7 Hz and between 3 and 4 Hz). Such high variabilities are caused by locally edge-generated diffracted surface waves. The non-linear perfectly elasto-platic structural response of the SDOF system shows that although the variability has the same trends as in the case of linear response, it is almost constantly increased by 5%. [ABSTRACT FROM AUTHOR]

The spatial variation of the earthquake ground motion over short distances can significantly affect the dynamic response of large and extended engineered structures, especially on sites with inhomogeneity in surface geology and geometry. In current practices, such variation is taken into account in terms of coherency, a function of frequency and distance, established on an essentially empirical basis and difficult to extrapolate at different sites. Hence, a better understanding of its physical significance and its relationship with the underlying ground structure is indispensable. A two-dimensional dense array, deployed at the small and shallow Koutavos-Argostoli valley in Cephalonia, Greece, provided an abundance of data to study the stochastic characteristics of seismic ground motions over very short distances. A set of 46 magnitude 2-5 events at epicentral distances 0-200 km has been selected for the analysis. The lagged coherency of the S-wave dominating seismogram was computed for each station-pair within the array and was averaged over various distance intervals for the whole data set. The results indicate a lack of a clear dependence of the average coherency on the magnitude, back azimuth or site-to-source distance of the event. The most striking result concerns the influence of the site geometry: the coherency is systematically lower for the pairs aligned perpendicular to the axis of the valley (2D) compared to those aligned in the parallel direction. This finding is consistent with the dominance of valley-edge generated surface waves propagating from one edge to the other. The averaged coherency estimates are only weakly represented by the existing parametric models, indicating its strong site dependent nature. [ABSTRACT FROM AUTHOR]

The details of magma plumbing beneath active volcanoes remain a major challenge in geochemistry, geophysics, and volcanic hazard evaluation. Here we apply a relatively novel variation of seismic interferometry, which we call Virtual Reflection Seismic Profiling, to produce a high-resolution image of a known crustal magma body. The technique takes advantage of recent advances in both seismic instrumentation (dense arrays) and seismic analysis (seismic interferometry). We have applied this technique to data recently acquired at an iconic volcanic system, Krafla, which lies on the mid-Atlantic ridge as exposed in northern Iceland. What make this particular site exceptional are encounters with rhyolitic magma in two drill holes, K-39 and Iceland Deep Drilling Project-1 (IDDP-1). These known magma bodies represent a unique calibration opportunity for surface geophysical measurements of magma distribution at depth. In this study, we produced a stacked, seismic reflection section by applying common depth point processing techniques to virtual shot gathers derived from interferometry of P waves from microearthquakes generated by tectonic, magmatic, and/or geothermal activity. We observe a strong, coherent reflection on the seismic section at a travel time corresponding to the depth at which magma was encountered in the IDDP-1 wellbore. We interpret this reflection to be from magma or magma-related fluids. Additional coherent reflections may correspond to other components of the magma plumbing beneath Krafla. These results represent a promising new technique for structural imaging with natural sources that can be applied to a wide array of geologic and energy problems that involve natural or induced seismic clusters. [ABSTRACT FROM AUTHOR]

We report on Förster resonance energy transfer in the dense arrays of epitaxial quantum dots (QDs), formed by fractional monolayer CdSe insertions within a ZnSe matrix. In such arrays comprising the QDs of different sizes, the energy transfer can take place between the ground levels of small QDs and the excited levels of large radiating QDs, when these states are in resonance. This mechanism provides directional excitation of a limited number of the large QDs possessing the excited levels. It reveals itself by the shrinkage of photoluminescence (PL) bands and the appearance of the narrow single excitonic lines in micro-PL spectra. The strong shortening of characteristic PL decay times in the energy-donating QDs is observed when the distance between them and the energy-accepting QDs decreases. Photoluminescence excitation spectroscopy demonstrates the switching of the dominant energy transfer mechanism at the energy predicted by theoretical modeling of the excitonic levels in the QD arrays. Our results pave the way for engineering of the architecture of excitonic levels in the QD arrays to realize efficient nano-emitters. [ABSTRACT FROM AUTHOR]

Monolithic Interconnected Modules (MIM) are densely packed arrays of series interconnected photovoltaic (PV) cells that are manufactured on the same semiconductor substrate. This review presents the result of a prospective study whose objective is to provide an overview of the historical development and current state of the art of the MIM technology. The most outstanding works from the conception of the first MIM devices in the late 70s to the most recent ideas to date, including all relevant milestones achieved during these four decades, are reported. This review focuses on MIM devices that are designed for high-irradiance photovoltaic (HIPV) applications, such as concentrator PV (CPV), thermophotovoltaics (TPV), and laser power conversion (LPC), in which the highly dense series interconnection is particularly relevant in order to boost the output voltage without scarifying the receptor photoactive area. [ABSTRACT FROM AUTHOR]