Search

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]

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]

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]

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]

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]

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 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]

Dense-array (>64 channel) electroencephalography (EEG) systems are increasingly being used in simultaneous EEG-functional magnetic resonance imaging (fMRI) studies. However, with increasing channel count, dense-array EEG caps can induce more severe signal dropout in the MRI images than conventional systems due to the radiofrequency shielding effect of the denser wire bundle. This study investigates the influence of a 256-channel EEG cap on MRI image quality and detection sensitivity of blood oxygen level dependent fMRI signal. A theoretical model is first established to describe the impact of the EEG cap on anatomic signal, noise, signal-to-noise ratio, and contrast-to-noise ratio of blood oxygen level dependent signal. Seven subjects were scanned to measure and compare the T2*-weighted image quality and fMRI detection sensitivity with and without the EEG cap using an auditory/visual/sensorimotor task. The results show that the dense-array EEG cap can substantially reduce the anatomic signal in the brain areas (visual cortex) near the conducting wires (average percent decrease ≈ 38%). However, the image signal-to-noise ratio with and without the EEG cap was comparable (percent decrease < 8%, not statistically significant), and there was no statistically significant difference in the extent of blood oxygen level dependent activation. This suggests that the ability to detect fMRI signal is nearly unaffected by dense-array EEG caps in simultaneous EEG-fMRI experiments. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]