One-dimensional Velocity Model Research Articles

Image Reconstruction in Seismic and Medical Tomography

Tomography has been used for more than [Formula: see text] in medical science and its success is well known. One of the most important reasons for the success of medical tomography compared to seismic tomography is the data-acquisition geometry. A data-acquisition geometry similar to the one used in medical tomography was designed and tested with synthetic geological models. Geophysical-type data-acquisition geometries were also applied to the geological models, and the resulting images were compared to the medical images. The reconstructed image obtained in the medical case resembled the synthetic geological model and had higher resolution than the geophysical image. Secondary attention is drawn to the determination of the differences between the low- and high-velocity zones in the final image reconstruction of the tomographic inversion process. Therefore, the step-like high- and low-velocity embedded layer models were tested with different spreading systems. The geological models were also tested with one-dimensional (1-D) initial velocity models to understand the effect of the initial velocity model on the final image reconstruction. In the high-velocity layer case, the tomographic process is quite successful in image reconstruction, but cannot achieve the same success in the low-velocity layer case.

Shear wave velocity and crustal thickness in the Pannonian Basin from receiver function inversions at four permanent stations in Hungary

Receiver functions of teleseismic waveforms recorded at four Hungarian permanent broadband stations have been analyzed using semilinearized and stochastic inversion methods to estimate the crustal thickness and S wave velocity structure in the Pannonian Basin. The results of both inversion methods agree well with the crustal thicknesses obtained by previous seismic refraction and reflection studies in the regions which are densely covered with seismic lines (28 and 27 km in westernmost and southern Hungary, respectively) and suggest a thicker crust compared to what was known before beneath the Transdanubian and Northern Ranges (34 and 33 km, respectively). The comparison of the one-dimensional shear wave velocity models derived by the different inversion methods shows that, in case of simple, smoothly varying structures, the results do not differ significantly and can be regarded as absolute velocities. Otherwise, the recovered velocity gradients agree, but there are differences in the absolute velocity values. The back-azimuthal variations of both radial and tangential receiver functions are interpreted as dipping structure and as waves sampling different geological areas. The signature of the deep structure on low-frequency receiver functions suggests a strong velocity contrast at the 670-km discontinuity. The vanishing 410-km boundary may be attributed to the remnant of a subducted oceanic slab with increased Poisson’s ratio in the transition zone.

High-resolution aftershock observations in the source region of the 2004 mid-Niigata Prefecture Earthquake

Abstract We deployed an extremely dense temporal seismic network in the source region of the 2004 mid-Niigata Prefecture Earthquake (thrust fault) on October 23, 2004, Japan. The seismic network consisted of 145 temporary seismic stations within a 30 km squared and had been kept within approximately a month after the mainshock. High accurate hypocenters of 708 events were determined by inverting the arrival times using double-difference earthquake location algorithm. The aftershocks along the mainshock (M w = 6.6) and the largest aftershock (M w = 6.3) rupture zones are distributed on two 60° westward-dipping planes, located approximately 5 km apart. Conversely, the Oct. 27 aftershock (M w = 5.8) occurred on an eastward dipping plane with a dip angle of 25° that was conjugate to the mainshock fault plane. Most of aftershocks at both northeastern and southwestern edges occurred at shallow depths with eastward-dipping planes. Epicenters of aftershocks in the southwestern region are aligned along N15°E, and rotate approximately 20° counterclockwise from the strike of the mainshock fault. This rotation of the aftershock alignments coincides with the rotation of anticline axes in the southwestern area of the source region. Furthermore, distributions of station corrections for a one-dimensional velocity model suggest that the seismic velocity at the western side of the Muikamachi-fault is lower than that at the eastern side. It is also inferred that the velocity structures in the hangingwall vary along the fault strike. The average velocity in the mainshock rupture area is higher than the periphery in the hangingwall, especially compared with the southwestern side of the hypocenter.

Modelo de velocidades sísmicas de Cuba Oriental

El presente estudio determina un modelo unidimensional de velocidades de ondas sismicas muy util en la rutina diaria de analisis de datos para la localizacion geografica de los eventos sismicos y como modelo inicial para los estudios de tomografia sismica 3D. Para determinar el modelo de velocidades se utilizo el catalogo cubano de terremotos desde 1998 hasta el 2004. La zona escogida para el estudio se encuentra ubicada en la parte oriental de la isla de Cuba, especificamente entre los 19,80-21,00 latitud norte y los 74,33-77,00 longitud oeste. De un total de 209 eventos locales, fueron seleccionados 117 eventos con tiempo residual (RMS) entre 0-1 segundo, angulo residual de cobertura de estaciones sismologicas (GAP) entre 0-180 grados y registrados por 3 o mas estaciones. Se obtuvo un modelo de velocidades compuesto fundamentalmente por tres capas; una capa superficial de un espesor de 1 km con una velocidad promedio de la onda P de 3,6 km/s, una capa intermedia de 6 km de espesor con una velocidad promedio de 5,8 km/s y una capa inferior de 13 km de espesor con una velocidad promedio de 6,9 km/s. Los resultados sugieren una corteza predominantemente de transicion con una espesor promedio de 20 km.

Path-specific Velocity Structure of Western China from Surface-wave Dispersion

We develop one-dimensional (1-D) path-specific velocity models in western China using new Rayleigh and Love wave group and phase velocity dispersion measurements for 20 events in the region. The earthquakes were grouped into three geographic clusters from which we compute the average phase and group velocity dispersion. We invert the average dispersion curves simultaneously for 1-D shear-velocity models appropriate for the three central Asian paths, using three previous shear-velocity models as initial models. The models are validated by forward modeling waveforms of recent events. The crustal thickness beneath western China in the vicinity of the Lop Nor test site is 50–60 km and our velocity models are consistent with major geologic features (e.g., basins and mountain ranges) and previous structural models for this region.

Analysis of Seismic Activity in the Crust from Earthquake Relocation in the Central Tien Shan

Using P and S wave arrivals from local and regional earthquakes recorded by a temporary broadband seismic network, we relocated the earthquakes occurring in the central Tien Shan and adjacent areas during 1997~1998, based on one-dimensional and three-dimensional velocity models. Seismic activity within crust was analyzed according to the source parameters of these relocated earthquakes. Earthquake distribution indicates that most range-bounding faults within the mountain belt are active and they have played a significant role in tectonic activity of the crust of the Tien Shan. However, seismic activity along the Talas-Fergana fault shows different features: the activity on its southeast segment and other faults in the southwest Tien Shan is less active, while its northwest segment, probably activated by adjacent faults, is more active. In addition, seismic activity seems to penetrate from the margins of the mountain belt into foreland basins on the north and south. The earthquakes at 30~40km depths indicate that brittle failure can take place in the mid-lower crust beneath the central Tien Shan and they are probably related to thermal perturbation near the crust-mantle boundary. This means that dynamic processes such as small-scale mantle convection and asthenosphere upwelling are still ongoing beneath the central Tien Shan.

One-Dimensional qP-Wave Velocity Model of the Upper Crust for the West Bohemia/Vogtland Earthquake Swarm Region

The western part of the Bohemian Massif (West Bohemia/Vogtland region) is characteristic in the relatively frequent recurrence of intraplate earthquake swarms and in other manifestations of past-to-recent geodynamic activity. In this study we derived 1D anisotropic qP-wave model of the upper crust in the seismogenic West Bohemia/Vogtland region by means of joint inversion of two independent data sets - travel times from controlled shots and arrival times from local earthquakes extracted from the WEBNET seismograms. We derived also simple 1-D P-wave and S-wave isotropic models. Reasons for deriving these models were: (a) only simplified crustal velocity models, homogeneous half-space or 1D isotropic layered models of this region, have been derived up to now and (b) a significant effective anisotropy of the upper crust in the region which was indicated recently by S-wave splitting. Both our anisotropic qP-wave and isotropic P-and S-wave velocity models are constrained by four layers with the constant velocity gradient. Weak anisotropy for P-waves is assumed. The isotropic model is represented by 9 parameters and the anisotropic one is represented by 24 parameters. A new robust and effective optimization algorithm - isometric algorithm - was used for the joint inversion. A two-step inversion algorithm was used. During the first step the isotropic P- and S-wave velocity model was derived. In the second step, it was used as a background model and the parameters of anisotropy were sought.

Analysis of Short-Period Rayleigh Waves Recorded in the Bohemian Massif Area During Celebration 2000 Experiment

The aim of this paper is to show the application of short-period surface waves recorded during deep seismic sounding experiment for constraining shallow velocity structure of the crust. Phase velocity of fundamental mode Rayleigh waves, observed along the CELEBRATION 2000 experiment profile CEL09, were obtained by a p-ω method and has been subsequently inverted for one-dimensional shear velocity models for the top 2 km. Multiple filter technique applied to one shot gather was used to carry out a joint inversion of phase and group velocity data and to provide γR data to be used for Qβ inversion. Validity of obtained VS and Qβ models was confirmed by the reflectivity method. Noticeably, no clear dispersive wawes were observed in the Tepla-Barrandian Unit. Quasi-2D model based on the individual 1D VS models is well correlated with the surface geology. Lower VS are observed in the Saxothuringian Zone in comparison to the Moldanubian Zone. In the vicinity of the Central Bohemian and Moldanubian Plutons, the near-surface VS values are relatively low, but below 1 km depth, they are higher than in surrounding areas. We interpret it as the result of the weathering and cracks within the granitoid rocks.

Detailed structure of the upper mantle discontinuities around the Japan subduction zone imaged by receiver function analyses

Abstract High-resolution receiver function (RF) images of the upper mantle structure beneath the Japan Islands are obtained by RF analysis of the P-wave coda portions of 389 teleseismic events observed at 138 high-density broadband stations. We construct RFs through frequency-domain division using a water level of 0.01 and Gaussian low-pass filters of 1.0, 0.5, 0.3, and 0.1 Hz, and produce depth-migrated RFs using the one-dimensional IASP91 velocity model. The RF images clearly resolve the subducting Pacific Plate to a depth of 200 km, and reveal a local 30 km elevation of the 410 km discontinuity within the subducting plate. The 660 km discontinuity is also found to exhibit a broad 50 km depression under the influence of the stagnating slab. From analysis of the frequency dependence of the RFs, the thickness of the 410 km discontinuity is inferred to vary on a relatively local scale, whereas the 660 km transition is sharp throughout the Japan Islands.

Estimation of site amplifications from shear-wave velocity profiles in Yeşilyurt and Avcılar, Istanbul, by frequency–wavenumber analysis of microtremors

During the Mw = 7.4 Izmit earthquake of 17 August 1999, the Yesilyurt district of Istanbul underwent damage despite the epicentral distance of 90 km. At Avcilar (20 km west of Yesilyurt), the ground motion was even stronger and has caused heavy damage and fatalities. We investigate whether the observed ground motions can be explained by theoretical site amplifications calculated from one-dimensional (1-D) shear-wave velocity models. For this purpose, microtremors recorded with sensor-arrays set up at two sites were analyzed to obtain phase velocity dispersion curves using both the conventional and the Capon frequency–wavenumber (f–k) methods. At the Yesilyurt site, the conventional f–k method offered reliable phase velocity estimations whereas the Capon method showed scatter in the estimations. At the Avcilar site, on the other hand, the Capon method provided a higher resolution than the conventional method and hence, allowed estimation of wavelengths up to seven times the array size. At the Yesilyurt site, the shallow shear-wave velocity profile that is correlated with the lithology obtained from boreholes yields a ground motion amplification factor of about 3 at the frequency of 1 Hz. At the Avcilar site, the phase velocity dispersion curve is comparable with the one previously obtained using the spatial autocorrelation method. The site amplification factors calculated from the 1-D shear-wave velocity model are around 2–3 at the frequencies of 0.4, 1.2 and 2.3 Hz, which are about 2–3 times smaller than the amplifications obtained from reference-site techniques using weak/strong motion records of earthquakes. We suggest that the discrepancy may be caused by a 2- or 3-D effect introduced by surface and/or bedrock topography not accounted for by the horizontally stratified model considered here.

Precise hypocentre relocation of microearthquakes in a high-temperature geothermal field: the Torfajökull central volcano, Iceland

The Torfaj¨okull volcanic system is one of approximately 30 active volcanoes comprising the neovolcanic zones of Iceland. The central volcano of the system is the largest silicic centre in Iceland with a caldera of approximately 12 km diameter. Its high-temperature geothermal system is one of the most powerful in Iceland. Torfaj¨okull is a source of persistent seismicity, where both high- and low-frequency earthquakes occur. To study this microseismicity in detail, a temporary array of 20 broad-band seismic stations was deployed between 2002 June and November. These temporary stations were embedded in the permanent South Iceland Lowland (SIL) network and data from nine adjacent SIL stationswere included in this study.Aminimum one-dimensional (1-D) velocity model with station corrections was computed for earthquake relocation by inverting manually picked P- and S-wave arrival times from events occurring in the Torfaj¨okull volcanic centre and its surroundings. High-frequency earthquakes from the Torfaj¨okull volcanic centre were then relocated calculating a non-linear, probabilistic solution to the earthquake location problem. Subsequently, we correlated the waveforms of these 121 events (∼2000 observations) to define linked events, calculated the relative traveltime difference between event pairs and solved for the hypocentral separation between these events. The resulting high-resolution pattern shows a tighter clustering in epicentre and focal depth when compared with original locations. Earthquakes are mainly located beneath the caldera with hypocentres between 1 and 6 km depth and lie almost exclusively within the geothermal system. A sharp cut-off in seismicity at 3 km suggests either that there is a marked temperature increase or that this is a structural boundary. No seismic activity was observed in the fissure swarms to the northeast (NE) and southwest (SW) of the volcanic centre.

Minimum 1D Velocity Model from Local Earthquake Data in the Provence Region, South-Eastern France

We computed a one-dimensional (1D) velocity model and station corrections refering to the Provence region (South-eastern France) by inverting P-wave arrival times recorded on an eight-station local seismic network. Using this velocity model and the program HYPOELLIPSE (Lahr, 1989), we relocated a set of 108 local events. The quality comparison between previous earthquake locations and new relocated shows a good improvement.

Inferring upper-mantle temperatures from seismic velocities

Constraints on mantle temperatures are fundamental for a better understanding of the dynamics of the Earth. We evaluate how well upper-mantle thermal structure can be inferred from seismic velocities. Our forward calculation of seismic velocities and density includes uncertainties in anharmonic and anelastic parameters for various compositional models. The sensitivity of V P ( V S) to temperature along a 1300 °C adiabat decreases with depth from −0.75±0.15% (1.3±0.30%) per 100 °C at 200 km, to −0.23±0.05% (−0.45±0.10%) per 100 °C at 800 km, with an additional reduction in the wadsleyite stability zone. Shear anelasticity introduces a significant non-linear dependence of seismic velocities on temperature in the upper-mantle. This means that seismic anomalies with similar amplitudes but different signs correspond to temperature anomalies of a different magnitude, and absolute seismic velocities are required for making a thermal interpretation. With depth, the importance of anelasticity decreases. Temperature derivatives of bulk sound velocity ( V φ ) and density ( ρ) are a factor two to five lower than those of V P,S and do not depend strongly on temperature. Common mantle compositions cause velocity anomalies of about 1% throughout the upper-mantle. Above 400 km depth, the effect of composition on V P and V S is secondary to the effect of temperature, but it gains importance with increasing depth. For most upper-mantle compositions and depths, the relative sensitivity of V P, V S and V φ to temperature and composition is not different enough to distinguish the two factors from combined velocity models. But ∂ln ρ/ ∂ln V S allows a separation of thermal anomalies from compositional anomalies for iron-depleted and subducted material. Spherically symmetric reference profiles underlie most seismic velocity models. However, small differences between common one-dimensional velocity models translate into disparate thermal interpretations. A better understanding of seismic uncertainties and the physical interpretation of reference models is necessary to interpret absolute velocities. Assuming seismic structure is well resolved and composition known, uncertainties in inferred thermal structure are ±100 °C above 400 km depth and ±250 °C in the shallow lower-mantle. This makes an inversion of seismic velocity models for temperature feasible, above 400 km depth. At larger depths, testing seismic velocities calculated for a proposed thermal and compositional structure against observations is a more promising interpretation approach.

A Crustal Velocity Model for Locating Earthquakes in Monterey Bay, California

The seismicity of the Monterey Bay displays a sparse distribution of events with a majority in northern Monterey Bay, on the San Gregorio fault. The paucity of near-shore and offshore seismic recording instruments and the use of velocity models from inland regions for earthquake hypocentral locations and focal mechanisms have led to uncertainties and inaccuracies for seismic events on major faults that crosscut the Bay. New three-component seismic data were acquired during 1997–1999 by the Monterey Bay Aquarium Research Institute (MBARI) Margin Seismology project using ocean-bottom digital corehole and standard seismometers, and coastal digital RefTek instruments obtained from IRIS-PASSCAL by the University of California, Santa Cruz. We have analyzed phases from earthquakes in the Monterey Bay vicinity located by these instruments and used them to supplement the adjacent coastal stations of the permanent Northern California Seismic Network. A new one-dimensional velocity model for the region requires slow velocities from 2 to 6 km that we attribute to sheared granites observed in the Salinian Block in the center of Monterey Bay. Velocities in the 10–16 km layer are consistent with continental crustal velocities. A sharp increase in velocity at ∼16 km suggests a boundary that results from underplating of oceanic crust. This underplated zone appears to extend to a depth of ∼27–30 km where we observe normal upper mantle velocities near 8.0 km/sec. New details from the ocean-bottom corehole and coastal RefTek instruments for events along the SGF and Monterey Bay fault zones hint at local fine-scale structures and have implications for tectonic history and plate reconstruction interpretation.

On the Use of Calibration Explosions at the Former Semipalatinsk Test Site for Compiling a Travel-time Model of the Crust and Upper Mantle

Two chemical calibration explosions, conducted at the former Semipalatinsk nuclear test site in 1998 with charges of 25 tons and 100 tons TNT, have been used for developing travel-time curves and generalized one-dimensional velocity models of the crust and upper mantle of the platform region of Kazakhstan. The explosions were recorded by a number of digital seismic stations, located in Kazakhstan at distances ranging from 0 to 720 km. The travel-time tables developed in this paper cover the phases P, Pn,Pg, S, Sn, Lg in a range of 0-740 km and the velocity models apply to the crust down to 44 km depth and to the mantle down to 120 km. A comparison of the compiled travel-time tables with existing travel-time tables of CSE and IASPEI91 is presented.

Variability of Site Response in Seattle, Washington

Ground motion from local earthquakes and the SHIPS (Seismic Hazards Investigation in Puget Sound) experiment is used to estimate site amplification factors in Seattle. Earthquake and SHIPS records are analyzed by two methods: (1) spectral ratios relative to a nearby site on Tertiary sandstone, and (2) a source/site spectral inversion technique. Our results show site amplifications between 3 and 4 below 5 Hz for West Seattle relative to Tertiary rock. These values are approximately 30% lower than amplification in the Duwamish Valley on artificial fill, but significantly higher than the calculated range of 2 to 2.5 below 5 Hz for the till-covered hills east of downtown Seattle. Although spectral amplitudes are only 30% higher in the Duwamish Valley compared to West Seattle, the duration of long-period ground motion is significantly greater on the artificial fill sites. Using a three-dimensional displacement response spectrum measure that includes the effects of ground-motion duration, values in the Duwamish Valley are 2 to 3 times greater than West Seattle. These calculations and estimates of site response as a function of receiver azimuth point out the importance of trapped surface-wave energy within the shallow, low-velocity, sedimentary layers of the Duwamish Valley. One-dimensional velocity models yield spectral amplification factors close to the observations for till sites east of downtown Seattle and the Duwamish Valley, but underpredict amplifications by a factor of 2 in West Seattle. A two-dimensional finite-difference model does equally well for the till sites and the Duwamish Valley and also yields duration estimates consistent with the observations for the Duwamish Valley. The two-dimensional model, however, still underpredicts amplification in West Seattle by up to a factor of 2. This discrepancy is attributed to 3D effects, including basin-edge–induced surface waves and basin-geometry–focusing effects, caused by the proximity of the Seattle thrust fault and the sediment-filled Seattle basin.

Constrained Joint Location of New Madrid Seismic Zone Earthquakes

A subset of 1961 earthquakes recorded during 1976-1993 by stations of the Central Mississippi Valley Seismic Network (CMVSN) was relocated using the JHD technique with constraints. These constraints were derived from the analysis of a subset of 246 earthquakes recorded simultaneously by the CMVSN and a portable network (PANDA) in 1989-1992. Because the PANDA stations targeted events in the central arm of the New Madrid seismic zone, only CMVSN events that occurred in the vicinity of this area were relocated. In addition, the stations used in the relocation were either within the embayment or outside of it but close to its edge. The one-dimensional velocity model used in the relocation includes a low-velocity layer corresponding to the unconsolidated embayment sediments. The constrained relocation has two major steps. The first one involved the joint location of the CMVSN/PANDA subset, with PANDA earthquake locations and station corrections fixed. The second step involved the joint location of the CMVSN subset with some locations and some station corrections fixed (as determined in the first step). To reduce the errors in the locations, only those earthquakes with maximum azimuthal gaps of less than 180° and final RMS residuals of 0.3 s or less in the last iteration were relocated. This decreased the number of earthquakes to 1015. To estimate the location errors, the earthquakes having fixed locations (90 events) were relocated using a single-event location program and the station corrections generated in the second step. Comparison of the new and original locations show average differences (in absolute value) in epicenter and depth of 0.6 and 1.0 km, respectively, although for some earthquakes the depth differences are much higher. Comparison of the JHD locations with the original catalog locations shows that, on average, the absolute values of the differences in epicenters and depths are equal to 0.9 and 1.7 km, respectively. The differences between the two sets of locations are fairly random. Although the new locations represent a substantial improvement over the original ones, particularly in depth, the former do not have the same quality as the PANDA JHD locations. An important result of our study is that the JHD technique detected the presence of arrival times corresponding to S -to- P converted waves that were misidentified as direct S -waves. This conversion is corroborated by comparison of PANDA and CMVSN seismograms for common events. The JHD S-wave station corrections compensated for this misidentification.

Minimum 1D velocity models in Central and Southern Italy: a contribution to better constrain hypocentral determinations

We computed one-dimensional ( I D) velocity models and station corrections for Centrai and Southern Italy, in- verting re-picked P-wave alTival times recorded by the Istituto Nazionale di Geofisica seismic network. The re-picked data yield resolved P-wave velocity results and proved to be more suited than bulletin data for de- tailed tomographic studies. Using the improved velocity models, we relocated the most significant earthquakes which occurt.ed in the Apennines in the past 7 years, achieving constrained hypocentral determinations for events within most of the Apenninic belt. The interpretation of the obtained lD velocity models allows us to infer interesting features on the deep structure of the Apennines. Smooth velocity gradients with depth and low P-wave velocities are ob,'ierved beneath the Apennines. We believe that our results are effective to constrain hypocentral locations in Italy and may represent a first step towards more detailed seismotectonic analyses.

Incorporating Socorro magma body reflections into the earthquake location process

When network station spacing is large relative to event depths, constraining focal depths using direct arrivals is difficult. To help overcome this problem, we incorporated the SzS, SzP, and PzP Socorro magma body reflections into the earthquake location process. To find a velocity model appropriate for combining direct and reflected arrivals, we jointly inverted for 75 hypocenters and a one-dimensional velocity model. We used over 1000 direct arrivals and over 400 reflected arrivals, solving for P velocity ( Vp ) and Poisson's ratio ( v ) above the approximate base of the seismogenic zone, Vp and v below the base of the seismogenic zone, depth to the upper surface of the magma body, and station corrections. High-velocity and low-velocity initial models converged to the same solutions without damping. We found that Vp and v may both decrease slightly below about 10-km depth. Magma body depth is near 18.75 km. A close fit between observed and modeled reflected phase arrival times and an even lateral distribution of positive and negative reflected-phase residuals indicate that the magma body upper surface is nearly flat. In the Socorro area, combining direct and reflected phases reduces average focal depth error by more than a factor of 3 and average origin time error by a factor of 2 compared with using only direct phases. This is because reflected arrivals are more sensitive to changes in focal depth than direct arrivals, and combining upgoing (direct) and downgoing (reflected) times reduces the trade-off between focal depth and origin time. For this method to be successful, phases must be carefully identified. To help identify secondary phases, we used relative arrival time curves, plotting phase-minus- P times versus S -minus- P times. Picking reflection pairs (SzP and SzS, SzP and PzP, or SzS and PzP identified on a single seismogram) significantly reduces the chances of phase misidentification. The SzP-SzS and SzP-PzP pairs also help prevent trade-offs between reflector depth and focal depths.

Effects of three-dimensional velocity structure on the seismicity of the 1984 Morgan Hill, California, aftershock sequence

Abstract A three-dimensional velocity model for the area surrounding the 24 April 1984 Morgan Hill earthquake has been developed by simultaneously inverting local earthquake and refraction arrival-time data. This velocity model corresponds well to the surface geology of the region, predominantly showing a low-velocity region associated with the sedimentary sequence to the southwest of the Madrone Springs fault. This low-velocity sequence extends to a depth of approximately 5 km. Previously constructed geologic cross-sections for this region had suggested that these sediments extended to a depth of less than 2 km. Relocation of 2,179 hypocenters with the three-dimensional velocity model did not show great differences when compared to locations done with a one-dimensional velocity model with station corrections. In both cases, the Calaveras fault is steep but has a noticeable dip to the northeast and the seismicity does not intersect the surface along the trace of the Calaveras fault in the southern half of the rupture zone. The greatest differences between the two sets of locations was that the locations using the three-dimensional velocity model were deeper, and fewer earthquakes were located within the low-velocity region. The focal mechanisms were also determined for 946 earthquakes using both the one-dimensional and three-dimensional earth models. Both earth models yield similar focal mechanisms for these earthquakes.