High-resolution Seismic Velocity Research Articles

Study on Nonlinear Elastic Behavior in Diurnal Surface Velocity Variations Caused by Temperature Using Distributed Acoustic Sensing (DAS)

High temporal resolution seismic velocity variations are crucial for detecting potential small precursor signals before hazardous geological events, thus aiding in the prediction of catastrophic geological events. Therefore, the study of the mechanisms underlying high temporal resolution seismic velocity relative variations (dv/v) is of great importance. Using a Distributed Acoustic Sensing (DAS) array, we obtain diurnal dv/v variation curves through coda wave interferometry (CWI) and carefully analyze potential environmental factors that might affect velocity. We find that as the thermal strain rate decreases, dv/v shows a significant increasing trend, while as the thermal strain rate increases, dv/v shows a decreasing trend. In ultrasonic laboratory experiments on rocks, we observe similar patterns during thermal cycling of the rocks. Based on previous studies, we believe that this phenomenon is due to the inherent nonlinear elastic properties of rocks. Using higher-order nonlinear elastic parameters, we successfully reconstruct the trends in near-surface dv/v variations and analyze the underlying physical mechanisms. We recommend that observations of seismic velocity variations be interpreted within the framework of nonlinear elasticity.

Imaging Complex Structures of the Los Angeles Basin via Adjoint-State Travel-Time Tomography

ABSTRACT In this study, we present high-resolution seismic velocity models for the Los Angeles basin (LAB) and its adjacent area using adjoint-state travel-time tomography, fitting an extensive database of P- and S-wave travel times accumulated from 1980 to 2021. We select 151,193 first P-wave travel times and 149,997 first S-wave travel times from local earthquakes archived in the Southern California Earthquake Data Center to determine the velocity models, with earthquake locations updated at each iteration. With seismic stations spaced more than 3.5 km apart, our dataset has limited resolution in the uppermost 1–2 km. However, starting from three different initial models, our VP models, which are optimally imaged between 3 and 15 km, show similar velocity heterogeneity and provide a better fit to the observed first travel-time data compared to the Community Velocity Model-Harvard 15.1.0 and Community Velocity Model-Southern California Earthquake Center 4.26. Our models provide a detailed delineation of the subsurface structure beneath the LAB, revealing significant velocity variations across active faults, a 10-km-thick sequence of sedimentary rocks within the basin, and a distinct basin margin marked by transitions from low to high-velocities. In addition, these models highlight basement structures with elevated VP and VS located at depths of 9 to 12 km and beyond. Specifically, beneath the northeastern part of the basin, the models demonstrate improved accuracy and reliability in reflecting the linear relationship between VP and VS in mafic rocks. The accurate delineation of the basin’s structure provided by our models could also offer robust constraints for seismic response modeling and seismic hazard assessment in the region.

A high-resolution seismic velocity model for East Asia using full-waveform tomography: Constraints on India-Asia collisional tectonics

East Asia's complex tectonic history makes it a unique place for plate-tectonic studies. While the crust and mantle structures beneath this region have been extensively investigated by receiver function and many different kinds of tomography studies, discrepancies among seismic velocity models have complicated our understanding of East Asia's tectonic evolution. Here, we use full-waveform adjoint tomography to study the seismic P- and S-wave velocity structures of the crust and mantle beneath East Asia. We use a large waveform dataset from 141 earthquakes recorded by 4640 broadband seismic stations. Our inversion optimizes the normalized correlation coefficient between synthetic and observed seismograms within individual time windows of different seismic phases, from P arrivals to the slowest surface waves. This approach allows us to fit regional multipathed waveforms and provides very well-resolved seismic P- and S-wave velocity structures from the crust to depths of about 800 km. Furthermore, we are able to obtain this resolution over most of East Asia allowing us to compare the amplitude and shapes of anomalies across the entire region. The observed structures in our new full-waveform tomography model (FWEA23) correlate well with tectonic units, revealing sharp contrasts across tectonic boundaries. Our model reveals narrow and strong linear fast anomalies associated with subduction zones in the western Pacific and Southeast Asia. Compared to these oceanic slabs, we observe different and more complicated mantle structures beneath the India-Asia continental collision zone. While our model spans a wide region in East Asia, we focus our interpretations on the seismic velocity anomalies beneath Tibet and surrounding regions, as our model provides significant implications for India-Asia continental collision tectonics. Our analysis suggests that cratonic Indian lithosphere collided with Eurasian plate around 25–20 Ma, subsequently underthrusting beneath Tibet horizontally with little deformation. This resulted in thickening, destabilization, and delamination of the rheologically weaker and less buoyant mantle lithosphere beneath north-central Tibet. Moreover, our model reveals isolated lithospheric fragments within the mantle transition zone and lower mantle beneath southern Tibet and northern India, suggesting that non-cratonic Greater India continental lithosphere has broken off and sunk into the deeper mantle. The lack of continuity among these lithospheric fragments suggests a convective destabilization process, distinct from typical oceanic plate subduction. Our results emphasize the important role of rheological properties of continental lithospheres in shaping the dynamics of continental collision.

High-resolution seismic velocity structure using induced inter-event interferometry at the geothermal sites, Geysers, Eastern California

The possibility of retrieving Rayleigh wave empirical Green's functions (EGFs) using virtual seismometers provides an opportunity to calculate high-resolution velocity models without a set of dense stations and/or expensive seismic imaging/monitoring. The high-resolution tomographic map can give an overview of the fracturing within the saturated rock. Theoretically, the cross-correlations of earthquakes contain significant information about the EGFs between event-pairs. In this work, we developed an application of virtual seismometers on a very small scale in superficial layers where anthropogenic seismicity occurred. To test this method's capabilities, we applied it to investigate the NW-part of The Geysers (TG) geothermal field. We used 1276 micro-earthquakes, occurred in a cuboid with the horizontal side of ∼1 × ∼2 km and the vertical edge at a depth from 0.9 to 2.2 km, to obtain the high-resolution (100 × 100 m) velocity structure. A group of strict criteria was imposed separately for single event waveform and event pairs to stabilize the tomographic results. After retrieving the inter-event EGF signal and calculating the Rayleigh wave group velocity dispersion curves, we obtained the group velocity maps for each horizontal layer. The tomographic maps as a function of depth indicated a low-velocity anomaly related to existing inactive faults, topography variation of the top of the steam zone, and low-velocity anomalies possibly stimulated by fluid injection. The topography of the top of the steam varies from ∼1.0 km to 1.3 km of maps, and also anomalies connected with fluid migration appear at the depth of injection wells (1.5–1.9 km).

Upper-plate conduits linked to plate boundary that hosts slow earthquakes

In shallow subduction zones, fluid behavior impacts various geodynamic processes capable of regulating slip behaviors and forming mud volcanoes. However, evidence of structures that control the fluid transfer within an overriding plate is limited and the physical properties at the source faults of slow earthquakes are poorly understood. Here we present high-resolution seismic velocity models and reflection images of the Hyuga-nada area, Japan, where the Kyushu-Palau ridge subducts. We image distinct kilometer-wide columns in the upper plate with reduced velocities that extend vertically from the seafloor down to 10–13 km depth. We interpret the low-velocity columns as damaged zones caused by seamount subduction and suggest that they serve as conduits, facilitating vertical fluid migration from the plate boundary. The lateral variation in upper-plate velocity and seismic reflectivity along the plate boundary correlates with the distribution of slow earthquakes, indicating that the upper-plate drainage system controls the complex pattern of seismic slip at subduction faults.

Geothermal and structural features of La Palma island (Canary Islands) imaged by ambient noise tomography

La Palma island is located in the NW of the Canary Islands and is one of the most volcanically active of the archipelago, therefore the existence of geothermal resources on the island is highly probable. The main objective of this work is to detect velocity anomalies potentially related to active geothermal reservoirs on La Palma island, by achieving a high-resolution seismic velocity model of the first few kilometres of the crust using Ambient Noise Tomography (ANT). The obtained ANT model is merged with a recent local earthquake tomography model. Our findings reveal two high-velocity zones in the island’s northern and southern parts, that could be related to a plutonic intrusion and old oceanic crust materials. Conversely, four low-velocity zones are imaged in the southern part of the island. Two of them can be related to hydrothermal alteration zones located beneath the Cumbre Vieja volcanic complex. This hypothesis is reinforced by comparing the S-wave velocity model with the seismicity recorded during the pre-eruptive phase of the 2021 Tajogaite eruption, which revealed an aseismic volume coinciding with these low-velocity zones. Another low-velocity zone is observed in the southern part of the island, which we interpret as highly fractured rocks which could favour the ascent of hot fluids. A last low-velocity zone is observed in the central part of the island and associated with loose deposits generated by the Aridane valley mega landslide.

High-resolution seismic velocity analysis by sign-based weighted semblance

Building an accurate velocity model plays a vital role in routine seismic imaging workflows. Normal-moveout (NMO)-based seismic velocity analysis is a popular method for making the velocity models. However, traditional velocity analysis methodologies are not generally capable of handling amplitude variations across moveout curves, specifically polarity reversals caused by amplitude-variation-with-offset (AVO) anomalies. I have developed an NMO-based velocity analysis approach that circumvents this shortcoming by modifying the conventional semblance (CS) function to include polarity and amplitude correction terms computed using correlation coefficients of seismic traces in the velocity analysis scanning window with a reference trace. Thus, the proposed method workflow is suitable for any class of AVO effects. The approach is evaluated with four synthetic data examples of different conditions and field data consisting of a common-midpoint gather. Lateral resolution enhancement using the proposed method workflow is evaluated by comparison between the results from the workflow and the results obtained by the application of CS and three semblance-based velocity analysis algorithms developed to circumvent the challenges associated with amplitude variations across moveout curves, caused by seismic attenuation and class II AVO anomalies. According to the obtained results, the proposed method is superior to all of the presented workflows in handling such anomalies.

Subslab heterogeneity and giant megathrust earthquakes

The nucleation and rupture processes of giant megathrust earthquakes (M ≥ 9.0) in subduction zones are still controversial. Most previous studies have focused on the subducting plate interface, and the structure beneath the subducting slab and its influence on earthquake generation remain unclear. Here, we present high-resolution seismic velocity tomography beneath six regions where giant earthquakes have occurred. Subslab low-velocity (slow) anomalies are revealed, which may reflect hot mantle upwelling. The giant earthquake hypocentres are generally located above the edges of the slow anomalies or above the gaps between them. Large coseismic slips of the giant earthquakes mainly occurred above gaps between the slow anomalies. We suggest that differential buoyancy force between the slow anomalies and their gaps may be an important factor for earthquake nucleation, and the rupture extent of a giant earthquake may be constrained by the slow anomalies. Hence, it is necessary to conduct seismic tomography to investigate the detailed subslab structure, which may help to pinpoint the potential location and damage zone of a future giant earthquake. Mantle heterogeneity beneath subducting plates may influence giant megathrust earthquakes, according to seismic tomography of the subslab structure beneath six megathrusts that have ruptured in M ≥ 9.0 earthquakes.

Distributions of gas hydrate and free gas accumulations associated with upward fluid flow in the Sanriku-Oki forearc basin, northeast Japan

Abstract We applied automated velocity analysis to a 3D seismic data volume to obtain a high-resolution seismic velocity model that we used to investigate the influence of fluid behavior on the subsurface distribution of gas in the Sanriku-Oki forearc basin, northeast Japan. We identified free gas accumulations as zones of low P-wave velocity separated from overlying high-velocity gas hydrates by the clear seismic boundary of the bottom-simulating reflection (BSR) between them. We then used conductive modeling to map upward heat flow in our study area from the depth of the BSR derived from the velocity model. The estimated heat flow demonstrates that upward fluid flux has considerable influences on the distributions of both gas hydrate and free gas. The areas of high heat flow (representing high fluid flux) correspond to underlying permeable geological features such as gas chimneys, faults, and the edges of porous slumps, suggesting that these features provide fluid migration pathways from Eocene to Oligocene source rocks to free gas and hydrate accumulations in overlying sedimentary rocks. The techniques we have employed can contribute to the high-resolution mapping of gas hydrate and free gas accumulations in similar deep-water reservoirs off the eastern continental margin of Japan and at tectonically similar plate subduction margins in other regions.

AusArray: Toward updatable, high-resolution seismic velocity models of the Australian lithosphere

In order to improve exploration success under cover the UNCOVER initiative identified high resolution 3D seismic velocity characterization of the Australian plate as a high priority. To achieve thi...

Refraction Microtremor method for delineation of layers and lenses, and assessing liquefaction potential within an alluvial setting - Morobe Province, Papua New Guinea

Refraction Microtremor (ReMi) is a relatively new method in the geophysics industry. ReMi provides high resolution seismic shear wave velocity models up to 200m depth and as such has the potential of being an efficient method for assessing the soil liquefaction potential in seismically active regions.This paper presents a geophysical investigation carried out as part of a geotechnical feasibility study for a proposed Tailings Storage Facility (TSF) in the Morobe Province of Papua New Guinea. The primary objective of the investigation was to use geophysical methods to obtain subsurface parameters to assess the liquefaction potential within an interbedded and lensed clay/gravel alluvial setting. ReMi together with down hole and cross hole seismic methods were used to generate shear wave velocity information of multiple layers with depth, and in particular to define seismic velocity inversions.ReMi data was acquired using two array setups specifically targeting the top 100m of subsurface material and the top 50m of subsurface material at increased layer resolution. The data was inverted to produce shear wave velocity soundings which were correlated with the cross-hole and down-hole seismic methods, and with bore hole Standard Penetration Tests (SPT). The soundings were compiled to generate high resolutions shear wave velocity sections, analysis of which proved pertinent in defining the interconnectivity of the lensed clay/gravel and shear wave velocity variations for the calculation of liquefaction potential thresholds.

Imaging high-resolution velocity and attenuation structures from walkaway vertical seismic profile data in a carbonate reservoir using visco-acoustic waveform inversion

Frequency domain visco-acoustic waveform inversion is applied to multi-offsets vertical seismic profile data acquired over a carbonates reservoir in an oil field offshore Abu Dhabi in the United Arab Emirates, to produce high resolution seismic velocity and attenuation structures. The starting velocity and attenuation models are obtained from traveltime tomography, and from the centroid frequency-shift method, respectively. Waveform tomography is performed between the frequencies 4 and 50 Hz. To reduce coupling between velocity and attenuation parameters during model updates, a two stage inversion strategy is adopted. During the first stage, only the velocity model is recovered and, during the second stage, velocity and attenuation models are recovered. The density is estimated from empirical correlation derived from sonic and density logs at the borehole location. A judicious selection of the time-damping constant [Formula: see text], used to suppress late arrivals in the Laplace-Fourier domain is critical to mitigate nonlinearities. Prior to the inversion, data preconditioning is done to transform the data in a form convenient for the visco-acoustic wave equation. High resolution final velocity and attenuation models are obtained. Extracted 1D profiles from the final models generally correlate well with the sonic log and estimated clay content from the gamma-ray log. These observations give confidence to the results. A comparison of the synthetic data generated from the final models and the field data shows a high degree of similarity. The high resolution of the results enables us to readily identify layers and provide a geologic interpretation from Quaternary to Late Jurassic. The inverted models show a stack of layers with alternating relatively high and low velocity values associated with relatively high- and low-attenuation values defining anticlines, as we progressed deeper into the subsurface. The two main reservoirs units in the Lower Cretaceous are clearly identified.

Imaging high-resolution seismic velocity from walkaway vertical seismic profile data in a carbonate reservoir using acoustic waveform tomography

High-resolution seismic velocity was obtained using acoustic full-waveform tomography of walkaway vertical seismic profile (VSP) data from an oil field dominated by carbonate rocks, offshore Abu Dhabi in the United Arab Emirates. The data were collected in a deviated borehole with receivers located from 521 to 2742 m depth. The inversion was performed in the frequency domain. The success of the inversion was determined by three important factors: the starting model, the preconditioning of the input data, and the inversion strategy, which included an appropriate selection of a damping term [Formula: see text] in the Laplace–Fourier transformation. The inversion was performed between the frequencies of 4 and 50 Hz, and a logarithmic data residual was used. The extracted 1D velocity profiles from the final high-resolution velocity model correlate well with the sonic log, and estimated vertical incidence VSP velocities. The predicted data obtained by the final velocity model indicate a generally good fit with the field data, thus confirming the success of the inversion. A reverse time migrated section derived by the final velocity model provides additional structural details. The velocity model indicates anomalous zones of low-velocity values that correlate with known locations of hydrocarbon reservoirs.

Characterization of hydrate and gas reservoirs in plate convergent margin by applying rock physics to high-resolution seismic velocity model

Gas hydrates are widely distributed in the Kumano forearc basin, which is located above accretionary prism in the Nankai margin off the Kii peninsula, Japan. Bottom-Simulating Reflector (BSR) at the base of gas hydrate stability zone has been imaged as a strong acoustic impedance contrast on the reflection seismic profiles. In order to better define the accumulations of gas hydrates and free gases, we performed a high-resolution seismic velocity analysis to 3D seismic data using a method of conventional semblance spectra via automatic velocity picking algorithm. The results revealed that gas hydrate-bearing sediments above the BSR and free gas-bearing sediments below the BSR are characterized by P-wave velocities of 1900–2500 m/s and 1000–1800 m/s, respectively. Then, the velocity model was converted into gas hydrate and free gas saturation using rock physics approaches. The results indicated that saturation of gas hydrates ranges from 0% to 45% in the pore space, and highly concentrated around the outer ridge where faults are densely developed. Additionally, concentrations of free gas ranging from 0% to 20% in the pore space are widely distributed below BSRs and are considerably high above ridge structure generated by displacement of large fault splayed from the deep plate boundary décollement. Based on these results, we suggest that the gas hydrates concentrated due to the free gas influx which migrated upward through the steeply dipping strata and faults (or fractures) cutting through the basin. The accumulations of gas and/or hydrates are further controlled by fault movements in the accretionary prism beneath the forearc basin. Therefore, these factors generated by intensive tectonic movements in the plate subduction zone control the distribution and saturation pattern of gas hydrate and free gas formations.

High-resolution seismic velocity analysis as a tool for exploring gas hydrate systems: An example from New Zealand’s southern Hikurangi margin

The existence of free gas and gas hydrate in the pore spaces of marine sediments causes changes in acoustic velocities that overprint the background lithological velocities of the sediments themselves. Much previous work has determined that such velocity overprinting, if sufficiently pronounced, can be resolved with conventional velocity analysis from long-offset, multichannel seismic data. We used 2D seismic data from a gas hydrate province at the southern end of New Zealand’s Hikurangi subduction margin to describe a workflow for high-resolution velocity analysis that delivered detailed velocity models of shallow marine sediments and their coincident gas hydrate systems. The results showed examples of pronounced low-velocity zones caused by free gas ponding beneath the hydrate layer, as well as high-velocity zones related to gas hydrate deposits. For the seismic interpreter of a gas hydrate system, the velocity results represent an extra “layer” for interpretation that provides important information about the distribution of free gas and gas hydrate. By combining the velocity information from the seismic transect with geologic samples of the seafloor and an understanding of sedimentary processes, we have determined that high gas hydrate concentrations preferentially form within coarse-grained sediments at the proximal end of the Hikurangi Channel. Finer grained sediments expected elsewhere along the seismic transect might preclude the deposition of similarly high gas hydrate concentrations away from the channel.

“High resolution seismic imaging of an active fault in the eastern Guadalquivir Basin (Betic Cordillera, Southern Spain)”

We calculated the high resolution seismic velocity, Poisson's ratio, crack density and saturation ratio structures in and around the source areas of the Torreperogil seismic series (October 2012–April 2013). This seismic series, characterized by a large number of low magnitude (below Mw 3.7 or Md 3.9) and very shallow microearthquakes, took place in the Guadalquivir Basin, a large flexural foreland basin with a linear ENE–WSW trending bounded to the north by the Iberian Massif and to the south by the Betic Cordillera and filled from a middle Miocene to Plio–Quaternary sedimentary sequence.In the upper layers of the crust, strong low-velocity anomalies are extensively distributed under the central zone, which together with high Poisson's ratio and crack density values may correspond to rocks which are less likely to fracture, perhaps due to the accumulation of tectonic and seismic stress. 93% of the earthquakes occurred at depths of up to 8km, which could indicate that the base of the seismogenic zone lies at this depth. The seismic series was concentrated in layers of strong structural heterogeneities (in the boundary area between low and high anomalies), which were likely to generate earthquakes due to differential strain accumulation beneath the region. The high velocity areas are also considered to be strong yet brittle parts of the fault zone, which may generate earthquakes (at depths of between 5km and 9km). By contrast, low velocity areas are less prone to fracture, allowing seismic slippage to take place (from 2 to 4km depth).The best estimate of the depth of the main shock (mbLg 3.9) is 7.6km, which could tend to nucleate at the base of the seismogenic zone, at the “fault end” on the boundary between a low velocity zone to the east and a high velocity zone to the west, indicating the fault plane which separates both areas laterally. Assuming that this seismic contrast is one of the main Torreperogil faults it could imply that stress has accumulated in an existing fault zone with lateral heterogeneity in velocity.

Regularized seismic velocity inversion based on inverse scattering theory

In this paper, the regularized finite difference contrast source inversion algorithm based on inverse scattering theory is utilized to invert the seismic velocity distribution in the underground medium, and this algorithm is a waveform inversion method in frequency domain based on wave equation. Velocity distribution is updated iteratively by minimizing the cost function using the nonlinear conjugate gradient method. The geophysical inversion problem has the characters of ill-posedness and instability, we handle this problem by adding an additional regularization item based on total variation of inversion parameter, making the inversion problem become a well-posed problem, and the algorithm has the ability of anti-noise interference. In the inversion process we use the frequency-space domain 9-point difference propagation operator under PML absorption boundary condition. Compared with other inversion algorithm, the construction of forward modeling operator and other matrix factorization calculation process can be avoided in each iterative step because the background medium remains unchanged throughout the inversion process, this makes the algorithm more suitable for large-scale three-dimensional inversion. In addition, the MPI parallel computation is applied, by which the efficiency of inversion process is improved greatly. The two-dimensional CSEG model inversion results show that this algorithm can obtain high resolution seismic velocity reconstruction, and provide an accurate velocity information for seismic data processing and interpretation.

High resolution seismic velocity structure around the Yamasaki fault zone of southwest Japan as revealed from travel-time tomography

The Yamasaki fault zone in southwestern Japan currently has a high potential for producing a large damaging earthquake. We carried out a seismic tomographic study to determine detailed crustal structures for the region. The velocity model clearly images a low-velocity and high V p /V s (high Poisson’s ratio) anomaly in the lower crust beneath the Yamasaki fault zone at a depth of ~15–20 km. This anomaly may be associated with the existence of partially-melted minerals. The existence of this anomaly below the fault zone may contribute to changing the long-term stress concentration in the seismogenic zone.

High-resolution seismic velocity structure and azimuthal anisotropy around the 2010 Ms = 7.1 Yushu earthquake, Qinghai, China from 2D tomography

To investigate upper crust anisotropy, a tomography algorithm was reformulated to include lateral variations in both velocity and azimuthal anisotropy, and it was applied to Pg travel time data around the epicenter of the 14 April 2010 Ms=7.1 Yushu earthquake. A high-resolution two-dimensional (horizontal) seismic velocity and azimuthal anisotropy model was obtained using this simple tomography method including both station and event depth corrections. The results show clear anisotropy in the upper crust and it is as important as the velocity variation in explaining the travel time residuals. The most striking result is a high velocity anomaly with large anisotropy at the epicenter on the Yushu–Garze fault. The main rupture originated within this high velocity anomaly and propagated southeastward into a low velocity anomaly with small anisotropy at Yushu. The azimuthal anisotropy shows fast velocity direction along Yushu–Garze fault and larger anisotropy exists in the epicenter than surrounding regions. These results demonstrate a clear example that lateral variation and anisotropy in seismic structures of the upper crust controlled the origination (stress accumulation) and rupture propagation of the 2010 Yushu earthquake and distribution of aftershocks as well. The simple 2D Pg-wave travel time tomography method presented here introduces a new approach utilizing abundant aftershocks data for investigating the rupturing process of a major earthquake and possible fracture distribution around seismogenic zone.

3-D imaging of the upper crust beneath the Denizli geothermal region by local earthquake tomography, western Turkey

The Denizli basin and its surroundings are one of the regions that have high seismic activity and rich geothermal fields in western Anatolia. In the first half of year 2000, an increasing seismic activity was observed around the Denizli basin without a large mainshock occurrence. A temporary seismic network consisting of 28 stations were deployed to observe the seismic activity in the region by The Scientific and Technological Research Council of Turkey-Marmara Research Center-Earth and Marine Science Institute (TÜBİTAK-MAM-EMSI). In this study, one- and three-dimensional V P and V P/ V S structure of the Denizli basin have been determined by using the travel times of the 635 recorded earthquake data. Firstly one-dimensional inversion schema was performed to stabilize initial velocity model and to have more reliable hypocentral locations. Then an iterative and simultaneous three-dimensional inversion procedure was carried out to obtain 3-D high-resolution seismic velocity models. Furthermore to assess the solution quality of our inversion, we conducted a series of resolution tests. We concluded high-resolution 3-D V P and V P/ V S seismic velocity models for the upper 20 km of the crust beneath the Denizli basin and surroundings and interpreted the results in the context of known geologic and tectonic units. The resulting 3-D V P models define the geometry of the basin, sediment thickness, and intrusive magmatic bodies as well as regions of anomalous velocity. The V P/ V S models help to constrain the location and geometry of the faults, zones of weakness, and gas or fluid saturated formations with high pore pressure zones. All tomographic results show that there is a strong correlation between local seismicity and the high V P and V P/ V S anomalies. Consequently, this relationship indicates that the reason of the high seismicity in the Denizli basin depends on activity of hydrothermal system as well as tectonic regime.