Seismic Attenuation Tomography Research Articles

Seismic attenuation tomography of the Northeast Japan arc: Insight into the 2011 Tohoku earthquake (Mw9.0) and subduction dynamics

Detailed three-dimensional (3-D) P and S wave attenuation (Qp and Qs) models of the crust and upper mantle under the entire Northeast Japan (Tohoku) arc from the Japan Trench to the Japan Sea coast are determined, for the first time, using a large number of high-quality t* data measured precisely from P and S wave spectra of local earthquakes. The suboceanic earthquakes used in this work are relocated precisely using sP depth phases. Our results reveal a prominent landward dipping high-Q zone representing the subducting Pacific slab, a landward dipping intermediate- to high-Q zone in the mantle wedge between the Pacific coast and the volcanic front, and significant low-Q anomalies in the crust and mantle wedge between the volcanic front and the Japan Sea coast. Prominent high-Q patches surrounded by low-Q anomalies are revealed in the interplate megathrust zone under the Tohoku fore arc where the great 2011 Tohoku-oki earthquake (Mw 9.0) occurred. The high-Q patches in the megathrust zone generally exhibit large coseismic slips of megathrust earthquakes and large slip deficit on the plate interface. We think that these high-Q patches represent asperities in the megathrust zone, whereas the low-Q anomalies reflect weakly coupled areas. We also find that the hypocenters of the 2011 Tohoku-oki interplate earthquakes (Mw > 7.0) are located in areas where Qp, Qs, and Qp/Qs change abruptly. These results suggest that structural heterogeneities in the megathrust zone control the interplate seismic coupling and the nucleation of megathrust earthquakes.

New insights into Mt. Vesuvius hydrothermal system and its dynamic based on a critical review of seismic tomography and geochemical features

<p>The seismic velocity and attenuation tomography images, calculated inverting respectively P-wave travel times and amplitude spectra of local VT quakes at Mt. Vesuvius have been reviewed and graphically represented using a new software recently developed using Mathematica<span><sup>8TM</sup></span>. The 3-D plots of the interpolated velocity and attenuation fields obtained through this software evidence low-velocity volumes associated with high attenuation anomalies in the depth range from about 1 km to 3 km below the sea level. The heterogeneity in the distribution of the velocity and attenuation values increases in the volume centred around the crater axis and laterally extended about 4 km, where the geochemical interpretation of the data from fumarole emissions reveals the presence of a hydrothermal system with temperatures as high as 400-450°C roughly in the same depth range (1.5 km to 4 km). The zone where the hydrothermal system is space-confined possibly hosted the residual magma erupted by Mt. Vesuvius during the recent eruptions, and is the site where most of the seismic energy release has occurred since the last 1944 eruption.</p>

Three-dimensional upper crustal structure of the geothermal system in Tarutung (North Sumatra, Indonesia) revealed by seismic attenuation tomography

The geothermal potential in Tarutung is controlled by both the Sumatra Fault system and young arc volcanism. In this study we use the spatial distribution of seismic attenuation, calculated from local earthquake recordings, to image the 3-D seismic attenuation of the area and relate it with the temperature anomalies and the fluid distribution of the subsurface. A temporary seismic network of 42 stations was deployed around Tarutung and Sarulla (south of Tarutung) for a period of 10 months starting in 2011 May. Within this period, the network recorded 2586 local events. A high-quality subset of 229 events recorded by at least 10 stations was used for the attenuation inversion (tomography). Path-average attenuation (t(p)*) was calculated by using a spectral inversion method. The spread function, the contour lines of the model resolution matrix and the recovery test results show that our 3-D attenuation model (Q(p)) has good resolution around the Tarutung Basin and along the Sarulla graben. High attenuation (low Q(p)) related to the geothermal system is found in the northeast of the Tarutung Basin suggesting fluid pathways from below the Sumatra Fault. The upper part of the studied geothermal system in the Tarutung district seems to be mainly controlled by the fault structure rather than by magmatic activities. In the southwest of the Tarutung Basin, the high attenuation zone is associated with the Martimbang volcano. In the Sarulla region, a low-Q(p) anomaly is found along the graben within the vicinity of the Hopong caldera.

Imaging fluid-related subduction processes beneath Central Java (Indonesia) using seismic attenuation tomography

We use local earthquake data observed by the amphibious, temporary seismic MERAMEX array to derive spatial variations of seismic attenuation (Qp) in the crust and upper mantle beneath Central Java. The path-averaged attenuation values (t∗) of a high quality subset of 84 local earthquakes were calculated by a spectral inversion technique. These 1929 t∗-values inverted by a least-squares tomographic inversion yield the 3D distribution of the specific attenuation (Qp). Analysis of the model resolution matrix and synthetic recovery tests were used to investigate the confidence of the Qp-model. We notice a prominent zone of increased attenuation beneath and north of the modern volcanic arc at depths down to 15km. Most of this anomaly seems to be related to the Eocene–Miocene Kendeng Basin (mainly in the eastern part of the study area). Enhanced attenuation is also found in the upper crust in the direct vicinity of recent volcanoes pointing towards zones of partial melts, presence of fluids and increased temperatures in the middle to upper crust. The middle and lower crust seems not to be associated with strong heating and the presence of melts throughout the arc. Enhanced attenuation above the subducting slab beneath the marine forearc seems to be due to the presence of fluids.

Three-dimensional seismic attenuation tomography of Egypt

The main purpose of the paper is assessing the three-dimensional (3-D) seismic tomography beneath Egyptto reveal the laws of the tectonic activity, dynamic features of the crust and the upper mantle as well as the thermal structure. Thecoda wave attenuation (Q-1c ) was obtained using the single scattering theory for the central frequencies of interest laid between 1 and 24 Hz. A regionalization of the estimated Q-1c values was performed by means of a generalized inversion technique.The obtained spatial distribution of 3-D attenuation results revealshigh contrasts between East and West Egypt. A remarkable contrast in the attenuation levels was compared with the tectonic structures, geothermal gradient and heat flow features. The highest attenuations are concentrated in the east and north western offshore regionsat central frequency 1.5 Hzthat draw a good matching with the seismic andthermal features of Egypt. Smaller attenuation levels were detected with young sediments of the Nile Valley from South to the northern triangle of Nile Delta basin except seismic active areas. Low or normal attenuation was detected at western desert where there is a stable and simple shelf. We can conclude that the extended highest attenuation joins to the strong seismic sources and geothermal structures at lower frequency and the centralized high attenuation takes place at moderate seismic sources at a higher frequency. The 3D attenuation maps can draw not only tectonic and geothermal structures but also the general geologic structure map.

Pg Attenuation Tomography within the Northern Middle East

Abstract A seismic attenuation map of regional phase Pg is constructed for the northern Middle East using two-station frequency-dependent Q measurements and seismic attenuation tomography. Pg is widely used in source discrimination, but its attenuation behavior is not well understood. We investigate the functional form of the geometrical spreading of Pg for both uniform and more complex seismic velocity structures. We find that there is significant trade-off between attenuation and geometrical spreading of Pg , and we quantify this trade-off to interpret the observed Pg attenuation in the Middle East. The northern Middle East attenuation tomographic images show significant lateral Q Pg variations, which may be caused by lateral variations of crustal rheology if intrinsic attenuation dominates the seismic attenuation mechanism of this region. We observe very large variations in the Q Pg model, such as low Q Pg within the Anatolian plate and high Q Pg within much of the stable Arabian plate. The low Q Pg anomalies generally coincide with active fault systems or Quaternary volcanisms. In order to perform a systematic comparison with Lg propagation, we have also updated the Q Lg map of Zor et al. (2007). These two new maps provide estimates for crustal Q α ( Pg ) and Q β ( Lg ) for the northern Middle East.

High resolution regional seismic attenuation tomography in eastern Tibetan Plateau and adjacent regions

[1] The Q of regional seismic phases Lg and Pg within the crust is assumed as a proxy for crustal Qβ and Qα, which is used as a constraint of crustal rheology. We measure regional-phase Q of the eastern Tibetan Plateau and adjacent areas. This method eliminates contributions from source and site responses and is an improvement on the Two-Station Method (TSM). We have generated tomographic images of crustal attenuation anomalies with resolution as high as 1°. In general we observe low Q in the northernmost portions of the Tibetan Plateau and high Q in the more tectonically stable regions such as the interior of the Qaidam basin. The calculated site responses appear to correlate with topography or sediment thickness. Furthermore the relationship between earthquake magnitudes and calculated source terms suggest that the RTM method effectively removes the source response and may be used as an alternative to source magnitude.

3D density model of the Central Andes

We developed a 3D density model of the continental crust, the subducted plate and the upper mantle of the Central Andes between 20–29°S and 74–61°W through the forward modelling of Bouguer anomaly. The goal of this contribution is to gain insight on the lithospheric structure integrating the available information (geophysical, geologic, petrologic, and geochemical) in a single model. The geometry of our model is defined and constrained by hypocentre location, reflection and refraction on and offshore seismic lines, travel time and attenuation tomography, receiver function analysis, magnetotelluric studies, thermal models and balanced structural cross-sections. The densities allocated to the different bodies are calculated considering petrologic and geochemical data and pressure and temperature conditions. The model consists of 31 parallel E–W vertical planes, where the continental crust comprises distinct bodies, which represent the different morphotectonic units of the Central Andes. We include a partial melting zone at midcrustal depths under the Altiplano-Puna (low-velocity zone) and consider the presence of a rheologically strong block beneath the Salar de Atacama basin, according to recent seismic studies. Contour maps of the depth of the continental Moho, the thickness of the lower crust and the depth to the bottom of the lithosphere below South America are produced. The possible percentage of partial melt in the Central Andes low-velocity zone is estimated. The residual anomaly is calculated by subtracting from the Bouguer anomaly the gravimetric effect of the modelled subducted slab and of the modelled Moho. Isostatic anomalies are calculated from regional and local isostatic Mohos calculated with and without internal loads, derived from our gravity model, which are then compared to the modelled continental Moho. This study contributes to a more detailed knowledge of the lithospheric structure of this region of the Andes and provides an integrated 3D density model, which may be used by the geoscientific community as a tool that can help to understand and interpret the geodynamic features and processes acting along the Central Andes.

Seismic Attenuation Tomography by Envelope Peak Instantaneous Frequency

This letter describes an attenuation tomography method via variations in the envelope peak instantaneous frequencies (EPIFs) of seismic signals. For a linear frequency-attenuation model, assuming that the propagating wavelet can be modeled by a Gaussian function with constant phase, an imaging formula using EPIF variation is derived. In tomography, the simultaneous iterative reconstruction technique (SIRT) with travel-time weighting is proposed for tomographic inversion. Numerical simulations of both noise-free and noise-added data show the validity of attenuation tomography from EPIF variation. SIRT with travel-time weighting yields a better reconstruction and lower residual.

Seismic attenuation tomography of the Mariana subduction system: Implications for thermal structure, volatile distribution, and slow spreading dynamics

The anelastic structure of a subduction zone can place first‐order constraints on variations in temperature and volatile content. We investigate seismic attenuation across the western Pacific Mariana subduction system using data from the 2003–2004 Mariana Subduction Factory Imaging Experiment. This 11‐month experiment consisted of 20 broadband stations deployed on the arc islands and 58 semibroadband ocean bottom seismographs deployed across the fore arc, island arc, and back‐arc spreading center. We compute amplitude spectra for P and S arrivals from local earthquakes and invert for the path‐averaged attenuation for each waveform along with the seismic moment and corner frequency for each earthquake. Additionally, we investigate earthquake source parameter assumptions and frequency‐dependent exponents (α) ranging from 0 to 0.6. Tomographic inversion of nearly 3000 t* estimates (at α = 0.27) for 2‐D QP−1 and QP/QS structure shows a ∼75 km wide columnar‐shaped high‐attenuation anomaly with QP ∼ 43–60 beneath the spreading center that extends from the uppermost mantle to ∼100 km depth. A weaker high‐attenuation region (QP ∼ 56–70) occurs at depths of 50–100 km beneath the volcanic arc, and the high‐attenuation regions are connected at depths of 75–125 km. The subducting Pacific plate is characterized by low attenuation at depths greater than 100 km, but high attenuation is found in the plate between 50 and 100 km depth. The fore arc shows high attenuation near the volcanic arc and beneath the serpentinite seamounts in the outer fore arc. QS structure is less well resolved than QP because of a smaller data set, but QP/QS ratios are significantly less than 2 throughout the study region. As temperatures estimated from QS−1 are unusually high, we interpret the arc and wedge core anomalies as regions of high temperature with enhanced Q−1 due to hydration and/or melt, the slab and fore‐arc anomalies as indicative of slab‐derived fluids and/or large‐scale serpentinization, and the columnar‐shaped high QP−1 anomaly directly beneath the back‐arc spreading center as indicative of a narrow region of dynamic upwelling and melt production beneath the slow spreading ridge axis.

Seismic Velocity and Attenuation Structure of the East Rift Zone and South Flank of Kilauea Volcano, Hawaii

It has been proposed that a deep magma body beneath the east rift zone (ERZ) of Kilauea volcano, Hawaii, must be present in order to explain the observed deformation of the south flank. From November 1999 to June 2000, 29 IRIS-PASSCAL three-component seismographs were operated across Kilauea9s ERZ and south flank. Using local earthquakes recorded by these stations, supplemented with data from the Hawaiian Volcano Observatory stations, we investigated the deep structure of the ERZ and south flank through two approaches: seismic velocity tomography and seismic attenuation tomography. Results along two north-northwest-south-southeast profiles are examined. The western profile shows a low-velocity region beneath and south of the Hilina Pali, and earthquakes at depth are sandwiched between a high V p / V s , high Q zone and a low V p / V s , low Q zone. The velocity and attenuation differences are interpreted to reflect the contrast in material properties between the overlying volcanic pile and the underlying fluid-rich ocean sediments and fractured, altered ocean crust beneath the south flank thrust fault. The eastern profile shows an anomalous feature with low V p , low V p / V s , high Q p , and low Q s at about 7 km depth beneath the ERZ. The low V p / V s ratio is inconsistent with the presence of partial melt. Instead, this anomaly is attributed to a trapped reservoir of CO 2 . Our results do not support the existence of a sizable molten or partially molten body at depth beneath the ERZ. If the deep magma body exists, it must be relatively small. Based on synthetic resolution tests, an estimated limit on the maximum allowable cross-sectional size of the deep magma body is about 2 by 4 km.

Seismic Attenuation Tomography Using the Successive Linearization Method

AbstractBased on the attenuation tomographical method in time domain, we propose a successive linearation method which is similar to the current travel‐time tomographical methods. We deduce a series of successive linearization formulae and obtain the iterative arithmetic and the executable technique of the seismic attenuation tomography using the successive linearization method. The method feasibility is validated by numerical experiments. As a supplement, multi‐parameters tomography is realized. We have applied the successive linearization method to engineering reconnaissance practice and gained preferable results.

Removal of wavelet dispersion from ground‐penetrating radar data

Wavelet dispersion caused by frequency‐dependent attenuation is a common occurrence in ground‐penetrating radar (GPR) data, and is displayed in the radar image as a characteristic “blurriness” that increases with depth. Correcting for wavelet dispersion is an important step that should be performed before GPR data are used for either qualitative interpretation or the quantitative determination of subsurface electrical properties. Over the bandwidth of a GPR wavelet, the attenuation of electromagnetic waves in many geological materials is approximately linear with frequency. As a result, the change in shape of a radar pulse as it propagates through these materials can be well described using one parameter, Q*, related to the slope of the linear region. Assuming that all subsurface materials can be characterized by some Q* value, the problem of estimating and correcting for wavelet dispersion becomes one of determining Q* in the subsurface and deconvolving its effects using an inverse‐Q filter. We present a method for the estimation of subsurface Q* from reflection GPR data based on a technique developed for seismic attenuation tomography. Essentially, Q* is computed from the downshift in the dominant frequency of the GPR signal with time. Once Q* has been obtained, we propose a damped‐least‐squares inverse‐Q filtering scheme based on a causal, linear model for constant‐Q wave propagation as a means of removing wavelet dispersion. Tests on synthetic and field data indicate that these steps can be very effective at enhancing the resolution of the GPR image.

Advanced techniques in site characterization and mining hazard detection for the underground coal industry

Today's highly productive coal mining systems represent large capital investments. Careful geotechnical planning and design are essential to protect these systems and to help ensure their overall success. Too often, however, the ability of the mine operator to adequately characterize the geotechnical environment and keep up with changing geologic conditions lags the advance rate of the mining equipment. Seismic tomographic imaging, based on the same principles as medical Computer-Aided-Tomography (CAT) Scans, offers the ability to identify changing geologic conditions, loading patterns, and the location of nearby abandoned mine workings. Although seismic tomography has been used for many years in the oil industry, until recently its practicality in the mining industry has been limited because the processes involved interfered with mining productivity and were labor-intensive in terms of data collection and interpretation. New advancements in signal processing have greatly enhanced the speed, resolution, and range of applications of tomographic imaging in underground coal mine settings. Many tomography applications in the mining industry use seismic velocity and/or attenuation tomography within a volume enclosed by the seismic source and receiver array. Although results obtained using this geometry work well in many situations, recent developments in reflection tomography show promise in eliminating the requirement that the target volume be surrounded by sources/receivers. For example, in-seam seismic reflection tomography may now be used to image structures and/or old workings from one general location in the mine (e.g., face areas of mains and panel developments) well ahead of planned developments—largely eliminating the need to probe-hole drill on regular intervals. These new developments have greatly expanded the ability of the mine operator to cost-effectively characterize previously inaccessible areas of the property. This paper gives an overview of the theories and processes involved in seismic tomography applicable to coal mine settings. Recent examples are presented of tomographic imaging applied to a variety of coal mine ground control problems and old works detection in the United States and Europe. The applicability of both seismic refraction and seismic reflection tomography is discussed.

Thermal state of the Campi Flegrei caldera inferred from seismic attenuation tomography

A three-dimensional Qp image of the Campi Flegrei caldera between 0 and 3 km of depth has been inferred by the inversion of P rise time and pulse width data of 87 local earthquakes recorded during the last bradiseismic crisis by a local array deployed in the area by the University of Wisconsin. The availability of both thermal measurements in 5 deep boreholes and of a heat flow surface map of the area allowed us to calibrate the local temperature F vs. Qp relationship. The comparison of Qp, Vp and Vp/Vs images, combined with hydrogeological and geochemical data from deep boreholes, allowed us to distinguish some low- Qp anomalies related to the presence of fluids in the rocks from a deep low- Qp anomaly related to the conductive cooling of a magma reservoir. The deep anomaly is located in the same zone where several authors believe that the volcanic and magmatic activity migrated after the Neapolitan Yellow Tuff eruption. Moreover this anomaly includes the area where the existence of a magma chamber at depth between 4 and 5 km was inferred by an active seismic experiment.

Seismic Neighboring Traces Attenuation Tomography in Time Domain

AbstractIn time domain, seismic amplitude and rise‐time are commonly used for obtaining the attenuation coefficient or the quality factor (Q value) of subsurface rock. Because it is difficult to measure seismic source energy, we have developed a method of seismic neighboring trace attenuation tomography in time domain that can remove the effect of source from the calculation process. Seismic velocity tomography, seismic amplitude attenuation tomography, and rise‐time attenuation tomography have been integrated into a series procedure that is applied to field data obtained in a mine. The results prove that the seismic tomographic method is flexible, practicable, and reliable.

Seismic attenuation tomography in frequency domain and its application to engineering

Seismic tomography is a new geophysical technique being developed to meet the needs of mining and engineering projects. Attenuation tomography, as one kind of seismic tomography technique, is a powerful prospecting tool for determining structure defaults such as fault, broken zone, and hidden voids, but it has been little discussed up to now. Its principle and applications are presented in detail. Attenuation tomography technology includes the following main points. First, on the basis of spectrum analysis to each seismic record, geometric correction is conducted, then common source parameters are introduced into common shot records in order to keep absorption effect of the geological media separate from source spectrum influence. Finally, an equation set is obtained which consists of absorption coefficients, common source parameters and energy field data observed. This technique can be employed together with velocity tomography to provide more evidence for engineering and geological diagnosis. Its applications to exploration of mineral and engineering have led to satisfactory results.

Seismic attenuation tomography of the Tonga‐Fiji region using phase pair methods

The anelastic structure of the region surrounding the Tonga slab and Lau back arc spreading center in the southwest Pacific is studied using data from 12 broadband island stations and 30 ocean bottom seismographs. Two differential attenuation methods determine δt* over the frequency band 0.1 to 3.5 Hz for earthquakes in the Tonga slab. The S‐P method measures the difference in spectral decay between P and S waves arriving at the same station. The P‐P method measures the difference in spectral decay for P waves with different paths through the upper mantle. Eight hundred sixty phase pairs are used to invert for two‐dimensional 1/Qα structure using a nonnegative least squares algorithm. A grid search method determines the Qα/Qβ ratio most compatible with both the S‐P and P‐P differential measurements. The highest attenuation (Qα = 90) is found within the upper 100 km beneath the active portions of the Lau Basin extending westward to the Lau Ridge. These regions probably delineate the source region for the back arc spreading center magmas, expected to be within the upper 100 km based on petrological considerations. The high attenuation regions also correlate well with zones of low P wave velocity determined by regional velocity tomography. Somewhat lower attenuation is found beneath the Fiji Plateau than beneath the Lau Basin. The entire back arc is characterized by a gradual decrease in attenuation to a depth of 300 to 400 km. The slab is imaged as a region of low attenuation (Qα > 900) material. A Qα/Qβ ratio of 1.75 provides the best fit between the S‐P and P‐P data sets upon inversion. Spectral stacking shows no frequency dependence within the frequency band analyzed.

Seismic attenuation tomography and its application to rock mass evaluation

We developed two methods of ray-based seismic attenuation tomography. One is amplitude attenuation tomography, which uses amplitude attenuation of the first arrival P-wave. As amplitude is easily influenced by various factors other than viscosity, we removed the effects of velocity structure on wave amplitude, such as transmission loss and diffraction, in the analysis. Decrease of wave frequency during propagation is estimated to obtain the Q-value directly. The other method is pulse broadening tomography, which uses broadening of rise-time or pulse-width of the first arrival P-wave. This method is based on the rise-time principle, which describes pulse broadening with travel time in viscous media. In this method, attenuation information can be obtained by using time data only. The combined use of velocity and attenuation characteristics can be a powerful tool to characterize an in situ rock mass. Therefore we integrated the seismic travel time tomography and the seismic attenuation tomography to a series procedure, and applied it to field data obtained in a mine. The result of travel time tomography agrees with the rock condition estimated by the geological observation at the sites. Faults and fractured zones were detected by the attenuation tomography.

Seismic structure of the northeastern Japan convergent margin: A synthesis

Many studies recently made on the basis of seismic observations have revealed a detailed structure of the crust and the upper mantle beneath the northeastern Japan arc and its relationship to seismic and volcanic activity. Spatial distributions of the depths to the Conrad and the Moho discontinuities, estimated from shallow earthquake data and seismic explosion data, show that both discontinuities are deep in the middle of the land area and shallow toward the coastlines of the Japan Sea and the Pacific Ocean. The Pn velocity has a lateral variation; it is as low as ∼7.5 km/s beneath the land area, while that beneath the Japan Sea and the Pacific Ocean is 8.0–8.2 km/s. It changes abruptly at the transition zones, which are located along the coastlines. Precise structure and location of the subducted Pacific plate beneath the land area is inferred from converted or reflected seismic waves at the top or bottom of the plate. The Pacific plate is composed of a thin (∼5 km) low‐velocity upper layer and a thick high‐velocity lower layer, its total thickness being 80–90 km. The upper plane seismicity of the double seismic zone is confined to the thin low‐velocity upper layer, which probably corresponds to the subducted former oceanic crust. The lower plane seismicity lies at the middle of the high‐velocity lower layer, and the lower half of the plate below it is incapable of generating earthquakes. The shallower portion of the upper surface of the plate beneath the Pacific Ocean, along which major seismicity with low‐angle thrust faultings is actually occurring, is also located by seismic observations on land and in the sea. The Pacific plate subducts at an extremely low angle of ∼5° for the first ∼25‐km depth, and then the dip steepens rather abruptly to ∼30°. Normal‐fault type events at the top of the plate have not been detected in the portion where the downward‐bending is the largest, but have been ditected near the trench axis, where it is rather small. Tomographic inversions for seismic velocity structure clearly delineate the inclined high‐velocity Pacific plate with a thickness of 80–90 km and low‐velocity zones in the crust and the mantle wedge beneath active volcanoes. Seismic attenuation tomography also shows similar zones of low‐Q value beneath active volcanoes, although its spatial resolution is much lower. The low‐velocity zones with 2–6% velocity lows are continuously distributed from the upper crust just beneath active volcanoes to a depth of 100–150 km in the mantle wedge, and are approximately parallel to the dip of the underlying Pacific plate. These low‐velocity zones probably reflect the pathway of magma ascent from a depth in the mantle wedge to the Earth's surface, corresponding to a portion of the subduction‐induced secondary mantle wedge flow.