Local Earthquake Travel-time Research Articles

Seismic structure of Kuwait

SUMMARY We have used data from the Kuwait National Seismic Network (KNSN) to estimate the seismic structure of Kuwait using a limited amount of seismic data. First, we made surface wave dispersion measurements and calculated receiver functions from the relatively small amount of data available from the broad-band station, KBD. Models were derived from the joint inversion of teleseismic receiver functions and Rayleigh and Love fundamental mode surface wave group velocity dispersion. While both surface waves and receiver functions by themselves can be used to estimate lithospheric structure, we have successfully combined the two to reduce non-uniqueness in estimates based on the individual data sets. The resulting KUW1 model features a thick (8 km) sedimentary cover and crustal thickness of 45 km. Crustal velocities below the sedimentary cover are consistent with global averages for stable platforms. We infer upper-mantle velocities (7.84 km s ‐1 P-wave velocity; 4.40 km s ‐1 S-wave velocity) that are slightly lower than expected for a stable platform. In comparison with other crustal structure estimates for the Arabian platform to the west, the crust is thicker and the mantle is slower in Kuwait. This is consistent with the overall tectonic trends of the region that find increasing crustal thickness between the divergent plate boundary at the Red Sea and the convergent plate boundary at the Zagros Mts, as well as slow mantle velocities beneath this nearby orogenic zone. The resulting model fits the traveltimes of regional phases (Pn, Pg, Sn and Lg). Independent inversion of local earthquake traveltimes recorded by KNSN (allowing for event hypocentre relocation) results in a remarkably similar velocity structure, providing confidence that the joint inversion of receiver functions and surface wave group velocities can impose accurate constraints on crustal structure for local event location and network operations. Relocation of events in Kuwait improves the clustering of events and results in shallower hypocentres.

Including anisotropy in 3-D velocity inversion and application to Marlborough, New Zealand

SUMMARY Two types of seismic velocity heterogeneity are often observed in a given region: pronounced patterns of anisotropy shown by local earthquake shear wave splitting and seismic velocity structure imaged by local earthquake traveltimes. We seek to combine these types of observations by including anisotropy in 3-D velocity inversion. The fast polarization directions from local earthquake shear wave splitting are used to define an initial anisotropy model. The ray paths are estimated for each of the shear wave splitting observations and the 3-D initial model is determined via an averaging process and a coherence analysis. The initial magnitude of anisotropy at a node is dependent on the number of adjacent ray paths and the consistency of the shear wave splitting anisotropy measurements. The traveltime inversion is parametrized with an isotropic component and two azimuthal anisotropy parameters for each node. The ray paths are defined with approximate 3-D ray tracing, and for each point along a ray path, the velocity is defined as a function of the ray path azimuth at that point. A blended approach is used to damp the perturbations to the magnitude of anisotropy and/or perturbations to the fast direction. Tests with synthetic traveltime data show that the anisotropic velocity inversion method is reliable. Application to the Marlborough region shows variations in anisotropy in the brittle crust, ductile lower crust, mantle and subducted slab. The patterns are consistent with the shear wave splitting observations, but show some additional features. The maximum anisotropy is approximately 12 per cent oriented northeast, between the Awatere and Wairau faults. This high anisotropy may reflect deformation and extensive fracturing in a region of high total shear strain. In the region of interaction of the ductile lower crust with the shallow subducting slab, moderate anisotropy with east–west orientation is imaged. This is consistent with ductile deformation roughly parallel to the plate velocity.

Seismological constraints on the crustal structure beneath the Zagros Mountain belt (Iran)

SUMMARY The Zagros Mountain belt of western Iran results from the collision of the Arabian and Central Iran continental blocks. The stage of the collision is unclear and the crustal structure of the Zagros is rather poorly known. In this study we investigate the velocity structure of the crust and upper mantle beneath the Ghir region located in the Central Zagros using data collected by a temporary local seismological network including a broad-band instrument. The structures of the sedimentary cover and the upper crystalline crust are estimated from the inversion of P and S traveltimes of local earthquakes recorded on a dense seismological network. The upper crust consists of an ∼11 km thick sedimentary layer (V p ∼ 4.70 km s −1 ) above a ∼8 km thick upper crystalline crust (V p ∼ 5.85 km s −1 ). The velocity of the lower crust and the depth of the Moho are found using receiver function analysis of teleseismic earthquakes. The lower crystalline crust is unusually slow (V p ∼ 6.5 km s −1 ) and ∼27 km thick. The upper bound for the total crustal thickness beneath the Ghir region is 46 ± 2 km. A comparison of the thickness of the crystalline crust of the Zagros with available information for the thickness of the crystalline crust of the Arabian Platform shows that, at present, the Zagros has a thinner crust. The current crustal thickness beneath the Zagros is comparable to the pre-collision crustal thickness of the Arabian Platform, suggesting that the Zagros is now in a very early stage of continental collision.

A method for simultaneous velocity and density inversion and its application to exploration of subsurface structure beneath Izu-Oshima volcano, Japan

We have developed a method for three-dimensional simultaneous velocity and density inversion using traveltimes of local earthquakes and gravity data. The purpose of this method is to constrain the velocity inversion and increase the spatial resolution of shallow velocity structures by introducing additional gravity data. The gravity data contributes to the P- and S-wave velocity models by imposing constraints between seismic velocities and density. The constraint curve is constructed so as to fit the data for porous rock samples, and deviations from the curve are taken into account in the inversion. The constraint is imposed at only the first layer, because density structure is well resolved at shallower parts and it is difficult to determine uniquely at greater depths. Synthetic inversion tests indicate that gravity data can improve the resolution of the velocity models for this layer. The method is applied to investigate the subsurface structure of Izu-Oshima volcano, Japan and velocity structures with high spatial resolution are obtained. The additional gravity data contribute primarily to improvement of the S-wave velocity model. At 0.25 km depth, a high velocity anomaly due to caldera-filling lava flows is observed. At 1.25 and 2.5 km depths, high velocity intrusive bodies are detected. A NW-SE trending high velocity belt at 1.25 km depth is interpreted as being caused by repeated intrusion of dikes.

Sequential inversion of local earthquake traveltimes and gravity anomaly-the example of the western Alps

We present a joint analysis of gravity anomaly and seismic arrival time data recorded in the western Alps. Seismological data were collected by a network of 126 permanent and temporary stations implemented in 1996. A set of ∼550 local events has been recorded. Gravity data result from the addition of two new gravity surveys to an existing data base. A published velocity model obtained by local earthquake tomography (LET), was used to construct an initial 3-D gravity model, using a linear velocity-density relationship (Birch's law). While the synthetic Bouguer anomaly field calculated for this model has the same shape and wavelength as the observed anomaly, its amplitude is strongly underestimated. To derive a crustal velocity-density model that accounts for both types of observations, we performed a sequential inversion of seismological and gravity data. The variance reduction of the arrival time data for the final sequential model was comparable to the variance reduction obtained by simple LET. Moreover, the sequential model explained ∼90 per cent of the observed gravity anomaly. The main features of our model compared with the LET model are: (1) an important broadening of the high-velocity anomaly associated with the high-velocity high-density Ivrea Body, (2) a 10 km thick low-velocity zone beneath the nappes of Digne and Castellane and (3) a high-velocity zone at more than 25 km depth under the internal zone of the range.

What triggers Koyna region earthquakes? Preliminary results from seismic tomography digital array

The cause for prolific seismicity in the Koyna region is a geological enigma. Attempts have been made to link occurrence of these earthquakes with tectonic strain as well as the nearby reservoirs. With a view to providing reliable seismological database for studying the earth structure and the earthquake process in the Koyna region, a state of the art digital seismic network was deployed for twenty months during 1996–97. We present preliminary results from this experiment covering an area of 60 × 80 km2 with twenty seismic stations. Hypocentral locations of more than 400 earthquakes confined to 11×25 km2 reveal fragmentation in the seismicity pattern — a NE — SW segment has a dip towards NW at approximately 45°, whilst the other two segments show a near vertical trend. These seismic segments have a close linkage with the Western Ghat escarpment and the Warna fault. Ninety per cent of the seismicity is confined within the depth range of 3–10 km. The depth distribution of earthquakes delimits the seismogenic zone with its base at 10 km indicating a transition from an unstable to stable frictional sliding regime. The lack of shallow seismicity between 0 and 3 km indicates a mature fault system with well-developed gouge zones, which inhibit shallow earthquake nucleation. Local earthquake travel time inversion for P- and S-waves show ≈ 2% higher velocity in the seismogenic crust (0–10 km) beneath the epicentral tract relative to a lower velocity (2–3%) in the adjoining region. The high P- and S-wave velocity in the seismogenic crust argues against the presence of high pressure fluid zones and suggests its possible linkage with denser lithology. The zone of high velocity has been traced to deeper depths (≈ 70 km) through teleseismic tomography. The results reveal segmented and matured seismogenic fault systems in the Koyna region where seismicity is possibly controlled by strain build up due to competent lithology in the seismic zone with a deep crustal root.

Velocity structure around the Baikal rift zone from teleseismic and local earthquake traveltimes and geodynamic implications

We present new results on the velocity structure of the Baikal rift zone, Asia, deduced from a comparative teleseismic and local tomography analysis. The aim of this paper is to better identify the role of deep mantle processes versus that of far-field tectonic effects on the occurrence of extensional tectonics within a continental plate. We use 36000 traveltimes of P-refracted waves from the ISC catalogues and Pg and Pn traveltimes of 578 earthquakes recorded by the Russian regional network to determine a velocity model by the use of local and teleseismic inversion procedures. The models show that some velocity patterns are continuous from the surface down to at least 400 km. Among them, a narrow negative anomaly goes through Mongolia and follows the southern and eastern margins of the Siberian craton: this structure is interpreted as a thin mantle plume rising beneath the rift axis. However, our results do not evidence any wide asthenospheric upwarp at this place. Other velocity anomalies observed near the surface are not deeply rooted. In particular, a negative anomaly is observed at shallow levels (48 km) beneath the northern third of Lake Baikal, which is disconnected from deeper structures. It may be explained by the existence of underplated magmatic material at the bottom of the crust. By comparing the geometry of deep-rooted anomalies to the present-day stress field patterns, we conclude that the sub-lithospheric mantle dynamics is not the main factor controlling extensional processes in the Baikal rift. However, it does contribute to a thermal weakening of the lithosphere along a mechanical discontinuity bounding the Siberian shield. We finally conclude that three favourable conditions are gathered in the Baikal area to generate extension: far-field extensional stress field, mechanical inherited lithospheric weakness and heat supply. Further studies should help to precise the genetic link between these three factors.

Three dimensional velocity structure and relocated aftershocks for the 1985 Constantine, Algeria (MS = 5.9) earthquake

Local earthquake travel-time data were inverted to obtain a three dimensional tomographic image of the region centered on the 1985 Constantine earthquake. The resulting velocity model was then used to relocate the events. The tomographic data set consisted of P and S waves travel-times from 653 carefully selected aftershocks of this moderate size earthquake, recorded at 10 temporary stations. A three-dimensional P-wave velocity image to a depth of 12 km was obtained by Thurber's method. At shallower depth, the velocity contrasts reflected the differences in tectonic units. Velocities lower than 4 km/s corresponded to recent deposits, velocities higher than 5 km/s to the Constantine Neritic and the Tellian nappes. The relocation of the aftershocks indicates that most of the seismicity occured where the velocity exceeded 5.5 km/s. The aftershock distribution accurately defined the three segments involved in the main shock and led to a better understanding of the rupture process.

Three‐dimensional crustal structure of the Mendocino Triple Junction region from local earthquake travel times

The large‐scale, three‐dimensional geometry of the Mendocino Triple Junction at Cape Mendocino, California, was investigated by inverting nearly 19,000 P wave arrival times from over 1400 local earthquakes to estimate the three‐dimensional velocity structure and hypocentral parameters. A velocity grid 175 km (N‐S) by 125 km (E‐W) centered near Garberville, California, was constructed with 25 km horizontal and 5 fan vertical node spacing. The model was well resolved near Cape Mendocino, where the earthquakes and stations are concentrated. At about 40.6° N latitude a high‐velocity gradient between 6.5 and 7.5 km/s dips gently to the south and east from about 15 km depth near the coast. Relocated hypocenters concentrate below this high gradient which we interpret as the oceanic crust of the subducted Gorda Plate. Therefore the depth to the top of the Gorda Plate near Cape Mendocino is interpreted to be ∼15 km. The Gorda Plate appears intact and dipping ∼8° eastward due to subduction and flexing downward 6°–12° to the south. Both hypocenters and velocity structure suggest that the southern edge of the plate intersects the coastline at 40.3° N latitude and maintains a linear trend 15° south of east to at least 123° W longitude. The top of a large low‐velocity region at 20–30 km depth extends about 50 km N‐S and 75 km E‐W (roughly between Garberville and Covelo) and is located above and south of the southern edge of the Gorda Plate. We interpret this low velocity area to be locally thickened crust (8–10 km) due to either local compressional forces associated with north‐south compression caused by the northward impingement of the rigid Pacific Plate or by underthrusting of the base of the accretionary subduction complex at the southern terminous of the Cascadia Subduction Zone. South of Cape Mendocino and southwest of the Garberville fault, high velocities indicative of oceanic crust are detected at 15 km depth. We interpret this evidence as tectonic underplating of the Pacific oceanic crust beneath the coastal Franciscan Complex, similar to the situation seismically imaged in central coastal California (Page and Brocher, 1994). This geometry indicates that the westernmost portion of the slab window has been filled by tectonic underplating and crustal thickening, while deep‐seated mantle upwelling is confined to an area centered about 75 km north of Clear Lake (Benz et al, 1992; Zandt et al., 1992).

P wave velocity variations in the Coso Region, California, derived from local earthquake travel times

Inversion of 4036 P wave travel time residuals from 429 local earthquakes using a tomographic scheme provides information about three‐dimensional upper crustal velocity variations in the Indian Wells Valley‐Coso region of southeastern California. The residuals are calculated relative to a Coso‐specific velocity model, corrected for station elevation, weighted, and back‐projected along their ray paths through models defined with layers of blocks. Slowness variations in the surface layer reflect local geology, including slow velocities for the sedimentary basins of Indian Wells and Rose valleys and relatively fast velocities for the Sierra Nevada and Argus Mountains. In the depth range of 3–5 km the inversion images an area of reduced compressional velocity in western and northern Indian Wells Valley but finds no major velocity variations beneath the Coso volcanic field to the north. These results are consistent with a recent study of anomalous shear wave attenuation in the Coso region. Between 5 and 10 km depth, low‐velocity areas (7% slow) appear at the southern end of the Coso volcanics, reaching east to the Coso Basin. Numerical tests of the inversion's resolution and sensitivity to noise indicate that these major anomalies are significant and well‐resolved, while other apparent velocity variations in poorly sampled areas are probably artifacts. The seismic data alone are not sufficient to uniquely characterize the physical state of these low‐velocity regions. Because of the Coso region's history of Pleistocene bimodal volcanism, high heat flow, geothermal activity, geodetic deformation, and seismic activity, one possibility is to link the zones of decreased P velocity to contemporary magmatic activity.

Velocity structure of the southeastern Hawaiian Ridge determined by Tau inversion

The Tau Inversion method has been used to invert earthquake travel times from the Hawaiian Ridge to determine velocity structure with limits. The inversion gives a velocity model showing a rapid increase in velocity from 5.9 km/sec to 7.2 km/sec between 4‐km and 7.5 ‐km depth, a monotonic increase to 7.5 km/sec near 11 km, and Moho velocity of 8.0 km/sec at 15 ‐km depth. Three prominent travel time delays, indicating heterogeneous velocity structure, are shown to correspond to the major volcanoes on Hawaii: Kilauea, Mauna Loa and Mauna Kea. Also, from the Moho depth determined by Tau Inversion, we infer that previous estimates of crustal flexure for the Hawaiian Ridge, based on deeper Moho depths (> 15 km), are too large. The Tau method is shown to be useful in delineating heterogeneous velocity structure and determining limits on velocity depth models from local earthquake travel time data.