Prominent Low-velocity Zone Research Articles

Deep crustal structure across northeastern Tibet from P receiver functions

The uplift mechanism and deep crustal deformation of the northeast of the Tibetan plateau in response to the Cenozoic India-Asian collision are still poorly constrained. Previous studies linked the uplift mechanism of the northeast of Tibet to the southward underthrusting of the Asian mantle lithosphere and the northward subduction of the Indian plate; yet, the details of crustal properties and thickening mechanism beneath the northeast of Tibet remain controversial. To shed light on the uplift mechanism of the northeast of the Tibetan plateau, we present a detailed image of the crustal structure beneath northeastern Tibet using teleseismic P receiver functions from 54 temporary broadband seismic stations. The Moho beneath the Kunlun Shan reaches ∼66 km but abruptly shallows to ∼40 km at the conjunction of the Kunlun and Qaidam, marking the boundary of northern and central Tibet. The Moho has a convex shape beneath the Qaidam basin and gently undulates northward beneath the Qilian Shan, and substantially shallows to ∼48–50 km under the Alashan block without any Moho offset. The undulating Moho and the absence of any Moho disturbance suggest that crustal thickening and shortening is the main driving mechanism for the northeastern plateau uplift. The average Vp/Vs ratio (∼1.75) beneath northeastern Tibet is indicative of a felsic to intermediate composition without a prominent low-velocity zone or low-viscosity channel in the middle to lower crust. Thus, the moderate crustal Vp/Vs ratio excludes the crustal channel flow as a primary uplift mechanism, implying that the horizontal crustal shortening and vertical thickening is responsible for creating the thick crust of northeastern Tibet.

Seismic Velocity Structure Beneath the Tofino Forearc Basin Using Full Waveform Inversion

Given the effects of steep dips and large lateral variations in seismic velocity beneath the Vancouver Island continental shelf, seismic processing and travel time inversion are inadequate to obtain a detailed velocity model of the subsurface. Therefore, seismic full waveform inversion is applied to multichannel seismic reflection data to obtain a high-resolution velocity model beneath the Tofino fore-arc basin under the continental shelf off Vancouver Island margin. Seismic velocities obtained in this study help in understanding the shallow shelf sediment structures, as well as the deeper structures associated with accreted terranes, such as Pacific Rim and Crescent terranes. Shallow high velocities, as large as ∼5 km/s, were modeled in the mid-shelf region at ∼1.5–2.0 km depth. These coincide with an anticlinal structure in the seismic data, and possibly indicate the shallowest occurrence of the volcanic Crescent terrane. In general, seismic velocities increase landward, indicating sediment over-consolidation related to the compressional regime associated with the ongoing subduction of the Juan de Fuca plate and the emplacement of Pacific Rim and Crescent accreted terranes. Seismic velocities show a sharp increase about 10 km west of Vancouver Island, possibly indicating an underlying transition to the Pacific Rim terrane. A prominent low velocity zone extending over 10 km is observed in the velocity model at 800–900 m below the seafloor. This possibly indicates the presence of a high porosity layer associated with lithology changes. Alternatively, this may indicate fluid over-pressure or over-pressured gas in this potential hydrocarbon environment.

Water, Hydrous Melting, and Teleseismic Signature of the Mantle Transition Zone

Recent geophysical and petrological observations indicate the presence of water and hydrous melts in and around the mantle transition zone (MTZ), for example, prominent low-velocity zones detected by seismological methods. Experimental data and computational predictions describe the influence of water on elastic properties of mantle minerals. Using thermodynamic relationships and published databases, we calculated seismic velocities and densities of mantle rocks in and around the MTZ in the presence of water for a plausible range of mantle potential temperatures. We then computed synthetic receiver functions to explore the influence of different water distribution patterns on the teleseismic signature. The results may improve our understanding and interpretation of seismic observations of the MTZ.

Mapping P-wave azimuthal anisotropy of the New Madrid seismic zone

We determine a detailed tomographic model of 3-D P-wave azimuthal anisotropy in the crust and upper mantle beneath the New Madrid seismic zone (NMSZ) using a large number of high-quality arrival-time data of 877 local earthquakes and relative travel-time data of 1511 teleseismic events recorded at 62 USArray stations. The tomographic inversion is conducted by using a limited-memory Broyden–Fletcher–Goldfarb–Shanno algorithm for bound constrained optimization. Because damping regularization is not required in this inversion scheme, the obtained results are less affected by human subjectivity. A prominent low-velocity zone with NE-SW fast velocity directions of azimuthal anisotropy down to ~160 km depth beneath the NMSZ is imaged, which may reflect upwelling mantle materials probably triggered by passage of the Bermuda hotspot from the southwest of the Mississippi embayment. Our results also reveal a subhorizontal high-velocity anomaly around the Moho discontinuity with a width of ~150–200 km, confirming the existence and defining the geometry of a lens-shaped high-density rift pillow near the Moho, which may result from the mantle upwelling. NW-SE fast velocity directions of azimuthal anisotropy appear in the lower crust beneath the Mississippi embayment. This result may reflect recent weakening of a detachment fault above the rift pillow, which drives recurrently sinking of the rift pillow and, as a consequence, causing large repeating intraplate earthquakes in the NMSZ.

Time-lapse seismic tomography of an underground mining zone

In this study, we determined P-wave tomographic images of the Yongshaba deposit, an underground mining zone in Guizhou Province, China, by inverting arrival-time data of micro-seismic events and blasts recorded by a passive seismic array consisting of 28 sensors during January to April 2014 using an event location technique and a travel-time tomography method that can handle complex seismic discontinuities. The damping parameter used in the damped least-squares method is determined with the help of trade-off curve. To reveal internal P-wave velocity changes of the study area under severe mining activities, a time-lapse data partition scheme is used. To assess the human influence on the underground structure, we introduce a parameter to measure the relative velocity changes in different periods. The resolution of the tomographic images and the robustness of the obtained features are examined by conducting a series of checkerboard resolution tests and a restoring resolution test. Our tomographic results obtained from the entire data set reveal a prominent low-velocity (low-V) zone and many obvious high-velocity (high-V) zones. These features match well with the geological setting and the excavation plan carried out in the mine, according to in-site surveys. The low-V zone may reflect empty volumes, stress releases and rock breaking and cracking caused by mining activities, whereas the high-V zones are probably the consequences of local stress concentrations caused by regional stress redistribution. The time-lapse tomographic images may reveal the process of stress concentrations caused by rock breaking due to the continuing excavations in the entire observation period, indicating that the mining activities influenced the rock property and caused complex changes of the underground structure.

Seismic structure beneath the Tengchong volcanic area (southwest China) from receiver function analysis

Tengchong is a young volcanic area on the collision boundary between the Indian and Euro-Asian plates of the southeastern Tibetan margin. Holocene volcanoes are concentrated in the Tengchong basin, where they align an N-S trending string-like cluster. To study the magma activity and its relation with the volcanoes, we deployed a passive seismic observation across the volcanic area in northern Tengchong. Using tele-seismic data and receiver function technique, we determined the S-wave velocity structure beneath nine temporary stations. Results show that the Tengchong basin is underlain by prominent low-velocity zones that are associated with the magma chambers of the volcanoes. In the north, a small and less pronounced magma chamber lies beneath two crater lakes, with a depth range of 9–16 km and a lateral width of <8 km. To the south, an interconnected magma chamber is found between the Dayingshan (DYS) volcano and the Dakongshan (DKS) volcanic cluster, with a depth range of 6–15 km and a lateral width of <12 km. In the south, the Laoguipo (LGP) volcano is characterized by anomalous low velocities throughout the upper-mid crust. Combined with other studies, we infer that the DYS volcano shares the same magma chamber with the DKS volcanic cluster, whereas the heat flow beneath the LGP volcano belongs to another thermal system, probably relating to the magma activity beneath the Rehai geothermal field in the south or affected by the intersection between the Tengchong volcanic fault zone and the Dayingjiang fault zone. In addition, mantle intrusion has resulted in the Moho elevation beneath the DKS volcanic cluster, and the thick transition zones on the crust-mantle boundary imply a possible penetration of the heat flow from the uppermost mantle into the lower crust.

Lithospheric radial anisotropy beneath the Gulf of Mexico

The Lithosphere–Asthenosphere Boundary (LAB), where a layer of low viscosity asthenosphere decouples with the upper plate motion, plays an essential role in plate tectonics. Most dynamic modeling assumes that the shear velocity can be used as a surrogate for viscosity which provides key information about mantle flow. Here, we derive a shear velocity model for the LAB structure beneath the Gulf of Mexico allowing a detailed comparison with that beneath the Pacific (PAC) and Atlantic (ATL). Our study takes advantage of the USArray data from the March 25th, 2013 Guatemala earthquake at a depth of 200 km. Such data is unique in that we can observe a direct upward traveling lid arrival which remains the first arrival ahead of the triplications beyond 18°. This extra feature in conjunction with upper-mantle triplication sampling allows good depth control of the LAB and a new upper-mantle seismic model ATM, a modification of ATL, to be developed. ATM has a prominent low velocity zone similar to the structure beneath the western Atlantic. The model contains strong radial anisotropy in the lid where VSH is about 6% faster than VSV. This anisotropic feature ends at the bottom of the lithosphere at about the depth of 175 km in contrast to the Pacific where it extends to over 300 km. Another important feature of ATM is the weaker velocity gradient from the depth of 175 to 350 km compared to Pacific models, which may be related to differences in mantle flow.

Ambient noise surface wave tomography of marginal seas in east Asia

We conducted ambient noise tomography in east Asia, including the Chinese coastal provinces, Korea Peninsular, Japan, Taiwan Island, and marginal seas in between. We retrieved Rayleigh Green’s functions from inter-station correlations of 12 months of continuous waveform data at 573 broadband stations in the region. We obtained group and phase velocity dispersion curves and dispersion maps for periods from 10 to 70 s and inverted for 3D Vs model of the crust and uppermost mantle. Moho and lithosphere thickness were derived from the 3D model. We observed three prominent low velocity zones in the upper mantle, two in the accretionary wedges above the Pacific and Philippine subduction slabs and one beneath the Changbai Mountain region. The crust and lithosphere are generally thin in the region. The velocity anomalies, crustal thickness, and lithosphere thickness all show a similar trend in NNE-SSW direction. The lithosphere shows a striking sausage”-type structure with alternating thickness. The crust thickness and lithosphere thickness both decrease progressively from NW to SE direction, which coincides with the distribution of episodic magmatism in SE China. We propose that the subduction of paleo-Pacific slab and its rollback were mainly responsible for the crustal and lithosphere extension and the mantle lithosphere removal in east Asia.

Two crustal flowing channels and volcanic magma migration underneath the SE margin of the Tibetan Plateau as revealed by surface wave tomography

The SE margin of the Tibetan Plateau is an important area to develop a better understanding of the plateau uplift and the Indian-Eurasian continental collision dynamics. Previous studies have reported widespread low-velocity anomalies beneath this region, particularly in the Tengchong volcanic field (TCVF). However, the spatial distribution and dynamic processes of these low-velocity anomalies are not well constrained. In this study, a 3-D S-wave velocity structure model of the crust and upper mantle (10–120km) in the region is constructed by the inversion of surface wave dispersion data. A two-step inversion procedure is adopted to generate the S-wave velocity structure images. The measured phase velocities and inverted S-wave velocities jointly show a large-scale low-velocity anomaly distributed in the crust, consistent with the view that the region is the passageway of the eastward migration of Tibetan Plateau material. Two crustal flowing channels are clearly observed at depths of ∼20km and ∼30km, which connect and rotate clockwise around the Eastern Himalaya Syntaxis. Beneath the TCVF, there are two prominent low-velocity anomaly zones at depths of ∼15–25km and ∼50–80km, which indicate the existence of magma chambers. One of the crustal flowing channels is connected with the magma chamber of the TCVF, and the other has a short branch north of Kunming toward the Mile-Shizong fault at a depth of 20km. Based on the distribution of the S-wave velocities under the TCVF, a dynamic model of the Tengchong volcano magma system is proposed to explain the migration patterns of the volcanic material.

Deflation and inflation of a large magma body beneath Uturuncu volcano, Bolivia? Insights from InSAR data, surface lineaments and stress modelling

The voluminous ash-flow eruptive products in the Central Andes imply that there are major magma bodies; however, how these magma bodies develop and change in time and space are not understood. In this study, we analyse the deformation activity of the Uturuncu Volcano, SW Bolivia, from 2003 to 2009 using a satellite radar interferometry (InSAR) data set. We find that the strength and the pattern of the present deformation can be explained by a pressurized source, such as an inflating flat-topped magma body at ∼22 ± 9 km depth below the surface. Furthermore, we examine the optical remote sensing data to perform a lineament analysis, which shows in a geographic information system (GIS) that a girdle of river streams and faults encircle the volcano at radial distance of approximately 15 km. Using numerical stress models, we locate a magma body beneath the volcano and find that the lineaments are best explained by a deflating flat-topped magma body at approximately 18 ± 2 km depth, which is consistent with the InSAR study. Thus, both the independent analysis of InSAR and lineament data suggest the presence of a horizontally extended, flat-topped magma body beneath Uturuncu. The location depth is in agreement with, or just above, a prominent seismic low velocity zone. Consequently, although the sign of deformation caused by the herein constrained magma body differs, the similar geometry and similar location suggest them to be similar, possibly indicating longevity of a magma storage region.

P-wave tomography and dynamics of the crust and upper mantle beneath western Tibet

The continental collision between the Indian and Asian plates plays a key role in the geologic and tectonic evolution of the Tibetan plateau. In this article we present high-resolution tomographic images of the crust and upper mantle derived from a large number of high-quality seismic data from the ANTILOPE project in western Tibet. Both local and distant earthquakes were used in this study and 35,115 P-wave arrival times were manually picked from the original seismograms. Geological and geochemical results suggested that the subducting Indian plate has reached northward to the Lhasa terrane, whereas our new tomography shows that the Indian plate is currently sub-horizontal and underthrusting to the Jinsha river suture at depths of ~100 to ~250km, suggesting that the subduction process has evolved over time. The Asian plate is also imaged clearly from the surface to a depth of ~100km by our tomography, and it is located under the Tarim Basin north of the Altyn Tagh Fault. There is no obvious evidence to show that the Asian plate has subducted beneath western Tibet. The Indian and Asian plates are separated by a prominent low-velocity zone under northern Tibet. We attribute the low-velocity zone to mantle upwelling, which may account for the warm crust and upper mantle beneath that region, and thus explain the different features of magmatism between southern and northern Tibet. But the upwelling may not penetrate through the whole crust. We propose a revised geodynamic model and suggest that the high-velocity zones under Lhasa terrane may reflect a cold crust which has interrupted the crustal flow under the westernmost Tibetan plateau.

Seismic structure of the Tengchong volcanic area southwest China from local earthquake tomography

We performed a local earthquake tomography to image the crust of the Tengchong volcanic area in southwestern China, using P-wave arrival data from a temporary network and permanent stations. The objective is to determine the magma sources of the volcanic area and the seismic structure of two M>7 earthquakes in the Longling area south of Tengchong. The result reveals a correlation between the velocity structures and the volcanic activities from late Miocene to Holocene. A prominent low-velocity zone is observed beneath the central volcanic area, that is cored at 10–20km depths and extends down to 30km depth, with a lateral extent of 20–30km. We infer that this unusual low-velocity zone represents a magma source of the heat flows for the volcanic eruption during Pleistocene and Holocene. Close to it, a high-velocity zone appears beneath the eastern volcanic area and it is likely to reflect solidified magma intrusions and high-density remnants within the cooled volcanic channel prior to Pleistocene. In the M>7 earthquake area, the upper crustal structure shows evident variations across the Longling fault and the Nu River fault. Bounded by the two fault zones, the Gaoligong metamorphic belt and the Baoshan block are imaged by high velocities that are consistent with their high strength in the structures; on the contrary, low velocities are imaged in the area between the Longling fault and the Nu River fault, implying a decrease of the strength probably caused by igneous intrusions. It was the lateral variation in the rheologic structure that led to the uneven stress accumulations across these fault zones and created tectonic conditions for the seismic ruptures of the two M>7 earthquakes in the Longling area.

A synoptic view of the distribution and connectivity of the mid‐crustal low velocity zone beneath Tibet

Based on 1–2 years of continuous observations of seismic ambient noise data obtained at more than 600 stations in and around Tibet, Rayleigh wave phase velocity maps are constructed from 10 s to 60 s period. A 3‐D Vsv model of the crust and uppermost mantle is derived from these maps. The 3‐D model exhibits significant apparently inter‐connected low shear velocity features across most of the Tibetan middle crust at depths between 20 and 40 km. These low velocity zones (LVZs) do not conform to surface faults and, significantly, are most prominent near the periphery of Tibet. The observations support the internal deformation model in which strain is dispersed in the deeper crust into broad ductile shear zones, rather than being localized horizontally near the edges of rigid blocks. The prominent LVZs are coincident with strong mid‐crustal radial anisotropy in western and central Tibet and probably result at least partially from anisotropic minerals aligned by deformation, which mitigates the need to invoke partial melt to explain the observations. Irrespective of their cause in partial melt or mineral alignment, mid‐crustal LVZs reflect deformation and their amplification near the periphery of Tibet provides new information about the mode of deformation across Tibet.

Tomography of the 2011 Iwaki earthquake (M 7.0) and Fukushima nuclear power plant area

Abstract. High-resolution tomographic images of the crust and upper mantle in and around the area of the 2011 Iwaki earthquake (M 7.0) and the Fukushima nuclear power plant are determined by inverting a large number of high-quality arrival times with both the finite-frequency and ray tomography methods. The Iwaki earthquake and its aftershocks mainly occurred in a boundary zone with strong variations in seismic velocity and Poisson's ratio. Prominent low-velocity and high Poisson's ratio zones are revealed under the Iwaki source area and the Fukushima nuclear power plant, which may reflect fluids released from the dehydration of the subducting Pacific slab under Northeast Japan. The 2011 Tohoku-oki earthquake (Mw 9.0) caused static stress transfer in the overriding Okhotsk plate, resulting in the seismicity in the Iwaki source area that significantly increased immediately following the Tohoku-oki mainshock. Our results suggest that the Iwaki earthquake was triggered by the ascending fluids from the Pacific slab dehydration and the stress variation induced by the Tohoku-oki mainshock. The similar structures under the Iwaki source area and the Fukushima nuclear power plant suggest that the security of the nuclear power plant site should be strengthened to withstand potential large earthquakes in the future.

Three dimensional shear wave velocity structure of the crust and upper mantle beneath China from ambient noise surface wave tomography

We determine the three-dimensional shear wave velocity structure of the crust and upper mantle in China using Green’s functions obtained from seismic ambient noise cross-correlation. The data we use are from the China National Seismic Network, global and regional networks and PASSCAL stations in the region. We first acquire cross-correlation seismograms between all possible station pairs. We then measure the Rayleigh wave group and phase dispersion curves using a frequency-time analysis method from 8 s to 60 s. After that, Rayleigh wave group and phase velocity dispersion maps on 1° by 1° spatial grids are obtained at different periods. Finally, we invert these maps for the 3-D shear wave velocity structure of the crust and upper mantle beneath China at each grid node. The inversion results show large-scale structures that correlate well with surface geology. Near the surface, velocities in major basins are anomalously slow, consistent with the thick sediments. East-west contrasts are striking in Moho depth. There is also a fast mid-to-lower crust and mantle lithosphere beneath the major basins surrounding the Tibetan plateau (TP) and Tianshan (Junggar, Tarim, Ordos, and Sichuan). These strong blocks, therefore, appear to play an important role in confining the deformation of the TP and constraining its geometry to form its current triangular shape. In northwest TP in Qiangtang, slow anomalies extend from the crust to the mantle lithosphere. Meanwhile, widespread, a prominent low-velocity zone is observed in the middle crust beneath most of the central, eastern and southeastern Tibetan plateau, consistent with a weak (and perhaps mobile) middle crust.

Ambient noise Love wave tomography in the eastern margin of the Tibetan plateau

We collected continuous waveform data from June to December 2007 recorded by 49 broadband seismic stations in western Sichuan and eastern Tibet, and applied an ambient noise technique to obtain a high-resolution tomography model of the crust beneath the eastern margin of the Tibetan plateau. Cross-correlations of ambient noise data are calculated in one-hour segments and stacked over 7 months. Then Love wave group dispersion curves along approximately 700-station pairs were determined using the multiple-filter analysis method. The study region was divided into a grid of 0.2° × 0.2°, and group velocity distribution maps between 8 and 40 s were obtained. The Love wave group velocity maps show good correlations with surface features. From the group velocity maps we inverted a 3-D model for SH wave velocity. The inferred SH velocity heterogeneities generally agree with variations in SV derived from Rayleigh wave dispersion, discrepancies between them are examined in view of non-uniqueness of the models and radial anisotropy. Both the Rayleigh and Love wave inversions show the significant difference of the crustal structure and reveal a prominent low-velocity zone within the mid-to-lower crust beneath the eastern margin of the Tibetan plateau.

Seismic images of crustal variations beneath the East Anatolian Plateau (Turkey) from teleseismic receiver functions

Abstract We used teleseismic P-wave receiver functions recorded by the Eastern Turkey Seismic Experiment to determine the crustal structure across an active continent–continent collision zone. Moho depth and V p / V s variations in the region are mapped by incorporating crustal multiples and later two-dimsional (2-D) seismic profiles are produced using a common conversion point technique with our crustal V p / V s estimates. Moho depths do not correlate with surface topography and reveal a relatively thin crust consistent with the high plateau being supported by hot asthenosphere near the base of the crust. Under the Arabian plate, the crust is thinnest ( c . 35 km) and exhibits high V p / V s (≥1.8) associated with mafic compositions. In the east, the crust gradually becomes thicker towards the north and exceeds 45 km in the northeastern side whereas in the west, the crust thickens sharply near the Bitlis suture and displays pronounced Moho topography within the Anatolian plate that suggests the presence of multiple fragments. V p / V s variations show an anomalously high V p / V s corridor (≥1.85) along the North Anatolian Fault and near the youngest volcanic units ( c . 3 Ma) and support the presence of partial melt. This corridor is spatially limited from both north and south by low V p / V s regions implying a change in crustal composition. Near the Bitlis suture, a layered V p / V s model points to the source of low V p / V s in the lower crust that may be rich in quartz. Furthermore, the seismic profiles indicate a prominent low velocity zone in the lower crust across a large area beneath the plateau that may act as a decoupling zone between the crust and upper mantle.

The lithosphere of North China Craton from surface wave tomography

Fundamental mode Rayleigh wave group velocity dispersion data were used in a tomographic study to reconstruct the crust and upper mantle shear velocity structure in east Asia. This article expounds the features of lithosphere structure in different sub-regions of North China, and discusses their implications for the tectonic evolution and present tectonics of North China Craton (NCC). The study revealed that strong earthquakes took place in the low velocity zones and along the boundaries between high and low velocities in the middle and lower crust; the active NW-trending Zhangjiakou–Bohai seismic belt corresponds to a prominent low velocity zone in the crust and upper mantle lid. In addition to the well-known structural difference between western and eastern NCC, we are able to resolve clearly the lithospheric features of sub-regions in eastern NCC. The southern North China Basin and north Yellow Sea are characterized by a high velocity crust and upper mantle lid down to 70–80 km depth, while the northern North China Basin has a low velocity crust and the velocity below Moho to about 130 km is rather uniform and low. There is also a general trend of velocity decrease in the upper mantle from west to east. The velocity images bear the imprints of subduction and collision that took place in the periphery of NCC since Mesozoic, and shed some light on the mechanism of Mesozoic lithospheric thinning of eastern NCC.

Ambient noise Rayleigh wave tomography in western Sichuan and eastern Tibet

We perform Rayleigh wave tomography in western Sichuan and eastern Tibet by applying the ambient noise method to broadband data recorded at 49 stations from the Sichuan digital seismic network. Cross-correlations of vertical-component ambient noise data are computed in one-hour segments and stacked over 7 months from June to December 2007. Then Rayleigh wave group dispersion curves from 6 to 48 s were measured using the multiple-filter analysis method. Group velocity maps at periods from 8 to 44 s were inverted. The resulting group velocity maps generally show good correlations with tectonic features. The Sichuan Basin is clearly resolved with lower group velocities at the short periods (8 to 20 s) due to thick sedimentary layers, and a prominent low-velocity zone exhibits in the middle crust beneath the eastern margin of the Tibetan Plateau. The Longmen Shan thrust belt marks not only the rapid lateral velocity variation between the topographically high Tibetan Plateau and low-elevation Sichuan Basin, but also the transition from the thick crust beneath the Tibetan Plateau to the normal continental crust beneath the Sichuan Basin.

VP Structure of Mount St. Helens, Washington, USA, imaged with local earthquake tomography

We present a new P-wave velocity model for Mount St. Helens using local earthquake data recorded by the Pacific Northwest Seismograph Stations and Cascades Volcano Observatory since the 18 May 1980 eruption. These data were augmented with records from a dense array of 19 temporary stations deployed during the second half of 2005. Because the distribution of earthquakes in the study area is concentrated beneath the volcano and within two nearly linear trends, we used a graded inversion scheme to compute a coarse-grid model that focused on the regional structure, followed by a fine-grid inversion to improve spatial resolution directly beneath the volcanic edifice. The coarse-grid model results are largely consistent with earlier geophysical studies of the area; we find high-velocity anomalies NW and NE of the edifice that correspond with igneous intrusions and a prominent low-velocity zone NNW of the edifice that corresponds with the linear zone of high seismicity known as the St. Helens Seismic Zone. This low-velocity zone may continue past Mount St. Helens to the south at depths below 5 km. Directly beneath the edifice, the fine-grid model images a low-velocity zone between about 2 and 3.5 km below sea level that may correspond to a shallow magma storage zone. And although the model resolution is poor below about 6 km, we found low velocities that correspond with the aseismic zone between about 5.5 and 8 km that has previously been modeled as the location of a large magma storage volume.