Seismogram Modeling Research Articles

Two-stage rupture of the 1999Mw6.0 Southern Lake Baikal earthquake

SUMMARY The 1999 February 25, 1858 hr, Southern Lake Baikal earthquake is evidently a two-stage rupture when compared with its prominent foreshock (mb 4.4) at 0537 hr on the same day and a prominent nearby earthquake (mb 5.1) on 2000 May 31. The foreshock and 2000 May 31 earthquake have similar waveforms recorded at short-period array stations at Δ= 21–79°. The main shock P wave train (including the surface reflections pP and sP) appears most similar to that from the 2000 May 31 earthquake after a shift of ∼8.5 s from the P wave first arrival. A model of two-stage rupture with a smaller stage followed 8.5 s later by a larger rupture fits many aspects of the seismograms of the main shock, whereas the 2000 May 31 seismograms can be fitted with a single source. The main shock first-stage rupture has moment ∼17 per cent of that of the second-stage rupture. Their hypocentres are at depth ∼15 ± 2 km, from modelling pP and sP phases using a crustal model of a low-wave speed sedimentary basin at the epicentral region. The hypocentres of the two stages are the same to within the resolution of the modelling. The 2000 May 31 hypocentre is at depth ∼13 km in the same model. Model seismograms at stations to the northwest and southeast have different time intervals between initial P and the surface reflections: the longer P–pP interval for the northwest stations can be explained by a thicker sedimentary basin at pP bounce points northwest of the epicentre. The two-stage rupture of the main shock could be responsible for the wide range of depths (7–21 km) and mb (4.9–6.1) published in the International Seismological Centre catalogue.

Local seismic quantification of gas hydrates and BSR characterization from multi-frequency OBS data at northern Hydrate Ridge

High-resolution multi-frequency and multi-component seismic data were acquired at northern Hydrate Ridge in the accretionary complex of the Cascadia subduction zone to quantify the amount of hydrate and free gas in the sediment. We present a detailed local analysis of four component (4C) ocean bottom seismometer (OBS) data and show the importance of multi-frequency and shear wave data for determining hydrate reservoir properties. A detailed model of the elastic parameters at the bottom simulating reflector (BSR) is developed by using synthetic seismogram modelling. The main focus in this study is an amplitude-versus-offset (AVO) analysis of shear waves, which originate by PS-conversion at the BSR in 73 m below the seafloor (bsf). The AVO analysis enables the determination of the shear wave velocity above the BSR. A velocity of 400 m/s indicates that the presence of gas hydrate in the pore space significantly increases the shear modulus of the sediment above the BSR. Information about the attenuation and the shape of the BSR transition zone is obtained from the frequency-dependent reflection amplitudes of the BSR. The BSR is shown to be a gradual type transition zone of 1.5–2.5 m thickness. Average Q factors of Q p = 150 for P-waves and of Q s = 35 for S-waves are determined within the gas hydrate stability zone (GHSZ). The low Q s factor points to a pronounced attenuation of S-waves in the uppermost sediments. From rock physics modelling, the hydrate concentration is estimated to vary locally between 3–12% of the pore space. Below the BSR, free gas concentrations of 0.5% and 8% are determined for homogeneous and patchy distributed gas, respectively.

Dominant Higher Surface-wave Modes and Possible Inversion Pitfalls

Dominant higher modes of surface waves are known to be generated at sites with large stiffness contrasts and∕or reversals between layers. At many engineering sites, quite often all modal energy is transferred to successively higher modes with increasing frequency. These ‘jumps’ may occur at low [Formula: see text] to high [Formula: see text] frequencies, and, more than once over the recorded bandwidth, but the common features are that the jumps are in amounts exceeding 50% of the phase velocity, and dispersion does not return to the fundamental mode at high frequency. A nonlinear geophone spacing helps to resolve these modes more uniquely. Full-wavefield synthetic seismogram modelling reinforces the effect to be due to a steep, nonlinear stiffness gradient in the uppermost [Formula: see text] of soil. If a high-velocity substrate at depth is not present, the ‘effective’ phase velocity of the higher modes can exceed that of the maximum shear-wave velocity of the model, where leaky modes persist. When this happens, conventional modal dispersion modelling cannot be applied in the inversion. If an artificial stiff layer is added at depth, jumps across several modal boundaries can occur, which is also generated in a Gibson half-space, both at very low and high frequencies. This is a major pitfall, where mode-misidentification, especially at low frequency, can lead to errors in the estimated shear wave velocity models of over 50%. Guided P-waves also manifest as large dispersion discontinuities, but usually at higher frequency and phase velocities, with buried explosive sources. Although Poisson’s ratio has a strong influence of the generation of—and frequencies of transitions to—dominant higher modes, the higher-mode phase velocities themselves are relatively independent of shallow P-wave velocity.

Lehmann discontinuity beneath North America: No role for seismic anisotropy

The Lehmann discontinuity in the upper mantle is often interpreted as a boundary separating anisotropic and isotropic media. We demonstrate, however, that seismic anisotropy plays, if any, a minor role in the origin of the Lehmann discontinuity. Our data are obtained with S receiver function technique from recordings of the MOMA seismograph array in the eastern USA. Part of the data is processed with a new modification of this technique. We observe Sp phase converted from the Lehmann discontinuity at a depth around 200 km with comparable amplitudes and similar polarities in two azimuths differing by about 90°. Contrary to the observations, the polarity in the synthetic seismograms for models with azimuthal anisotropy is opposite for the azimuths differing by 90°. Radial anisotropy with a slow vertical direction results in the Sp phase with the polarity opposite to the observed one.

The northern Walker Lane refraction experiment: Pn arrivals and the northern Sierra Nevada root

In May 2002, we collected a new crustal refraction profile from Battle Mountain, Nevada across western Nevada, the Reno area, Lake Tahoe, and the northern Sierra Nevada Mountains to Auburn, CA. Mine blasts and earthquakes were recorded by 199 Texan instruments extending across this more than 450-km-long transect. The use of large mine blasts and the ultra-portable Texan recorders kept the field costs of this profile to less than US$10,000. The seismic sources at the eastern end were mining blasts at Barrick's GoldStrike mine. The GoldStrike mine produced several ripple-fired blasts using 8000–44,000 kg of ANFO each, a daily occurrence. First arrivals from the larger GoldStrike blasts are obvious to distances of 300 km in the raw records. First arrivals from a quarry blast west of the survey near Watsonville, CA, located by the Northern California Seismic Network with a magnitude of 2.2, can be picked across the recording array to distances of 600 km. The Watsonville blast provides a western source, nearly reversing the GoldStrike blasts. A small earthquake near Bridgeport, CA. also produced pickable P-wave arrivals across the transect, providing fan-shot data. Arrivals from M5 events in the Mariana and Kuril Islands also appear in the records. This refraction survey observes an unexpectedly deep crustal root under the northern Sierra Nevada range, over 50 km in thickness and possibly centered west of the topographic crest. Pn delays of 4–6 s support this interpretation. At Battle Mountain, Nevada, we observe anomalously thin crust over a limited region perhaps only 150 km wide, with a Moho depth of 19–23 km. Pn crossover distances of less than 80 km support this anomaly, which is surrounded by observations of more normal, 30-km-thick crust. A 10-km-thick and high-velocity lower-crustal “pillow” is an alternative hypothesis, but unlikely due to the lack of volcanics west of Battle Mountain. Large mine and quarry blasts prove very effective crustal refraction sources when recorded with a dense receiver array, even over distances exceeding 600 km. New elastic synthetic seismogram modeling suggests that Pn can be strong as a first arrival, easing the modeling and interpretation of crustal refraction data. Fast eikonal computations of first-arrival time can match pickable Pn arrival times.

Seismic detection of sublithospheric plume head residue beneath the Pitcairn hot-spot chain

We study array seismograms of French nuclear explosions from two islands in French Polynesia and use these to constrain the structure in the upper mantle beneath the islands. Seismograms of nuclear explosions on the hot-spot-related Fangataufa atoll show discrete, large-amplitude P-coda phases which are not observed in recordings of explosions on Mururoa, an atoll ∼40 km to the north. The source for these P-coda phases is located beneath the Fangataufa atoll, indicating that structural heterogeneities are present in the oceanic upper mantle in this region on very small scale lengths. Synthetic seismograms for models of the upper mantle beneath Fangataufa require a layer with P-wave velocity elevated by ∼10% between depths of 51 and 85 km with a sharp termination in the north, possibly at the Austral Fracture Zone, to match these P-coda phases. Few mineralogic scenarios exist that can explain this structure, and the properties of this layer imply that extensive enrichment in garnet occurs in this depth range. The garnet-enriched layer is likely of similar origin to the well-known xenolithic ‘garnet megacryst’ suite found in kimberlitic regions. We propose a model for the formation of the Fangataufa and Mururoa atolls involving garnet-enriched zones being generated at depth through magmatic processes at the Pitcairn plume head. Thus, the initiation of hot-spots could produce complex geochemical and structural heterogeneities at depth in the suboceanic mantle.

A strong correlation between induced peak dynamic Coulomb stress change from the 1992 M7.3 Landers, California, earthquake and the hypocenter of the 1999 M7.1 Hector Mine, California, earthquake

The 1992 M7.3 Landers earthquake may have played a role in triggering the 1999 M7.1 Hector Mine earthquake as suggested by their close spatial (∼20 km) proximity. Current investigations of triggering by static stress changes produce differing conclusions when small variations in parameter values are employed. Here I test the hypothesis that large‐amplitude dynamic stress changes, induced by the Landers rupture, acted to promote the Hector Mine earthquake. I use a flat layer reflectivity method to model the Landers earthquake displacement seismograms. By requiring agreement between the model seismograms and data, I can constrain the Landers main shock parameters and velocity model. A similar reflectivity method is used to compute the evolution of stress changes. I find a strong positive correlation between the Hector Mine hypocenter and regions of large (>4 MPa) dynamic Coulomb stress changes (peak Δσf(t)) induced by the Landers main shock. A positive correlation is also found with large dynamic normal and shear stress changes. Uncertainties in peak Δσf(t) (1.3 MPa) are only 28% of the median value (4.6 MPa) determined from an extensive set (160) of model parameters. Therefore the correlation with dynamic stresses is robust to a range of Hector Mine main shock parameters, as well as to variations in the friction and Skempton's coefficients used in the calculations. These results imply dynamic stress changes may be an important part of earthquake trigging, such that large‐amplitude stress changes alter the properties of an existing fault in a way that promotes fault failure.

Modelling and imaging the Moho transition: the case of the southern Urals

Synthetic seismic modelling is used to test different possible structures and forming processes of the lower crust and Moho. One and two dimensional synthetic seismic modelling of the Moho reflection is carried out to constrain the internal structure of a crust–mantle transition. A reflectivity algorithm was used to calculate the synthetic seismograms for models of the crust–mantle transition consisting of gradient zones and layered sequences. The synthetic seismograms for models consisting of laterally variable velocity structure were calculated by an explicit finite difference wave equation algorithm. The laterally heterogeneous transition models can be achieved by assuming a Moho with different degrees of variable topography or laterally discontinuous layering. The effects of a low velocity surface cover on the seismic signature of deep signals is also analysed. The seismic modelling coupled with stacked images of wide-angle seismic reflection data constrain the case history of the Moho transition beneath the southern Urals. A smooth gradational crust–mantle transition (gradient velocity–depth function) does not predict the seismic features observed in the shot records. The coda of the PmP and the amplitude behaviour of this phase with offset are modelled with a 6 km thick transition zone which consists of approximately 600 m thick, laterally variable layers with a bimodal velocity distribution. The horizontal component stacks of the wide-angle data feature arcuate events suggesting boudin like structures or a boundary with topographic relief. High amplitude sub-Moho reflections support that the structural complexity of the crust–mantle transition can be followed to the upper mantle suggesting eastward dipping structures indicating, either, remnants of an old subduction zone, trapped crustal material within the mantle, or active crust–mantle interactions.

Seismic structure of basalt flows from surface seismic data, borehole measurements, and synthetic seismogram modeling

We use the wide‐angle wavefield to constrain estimates of the seismic velocity and thickness of basalt flows overlying sediments. Wide angle means the seismic wavefield recorded at offsets beyond the emergence of the direct wave. This wide‐angle wavefield contains arrivals that are returned from within and below the basalt flows, including the diving wave through the basalts as the first arrival and P‐wave reflections from the base of the basalts and from subbasalt structures. The velocity structure of basalt flows can be determined to first order from traveltime information by ray tracing the basalt turning rays and the wide‐angle base‐basalt reflection. This can be refined by using the amplitude variation with offset (AVO) of the basalt diving wave. Synthetic seismogram models with varying flow thicknesses and velocity gradients demonstrate the sensitivity to the velocity structure of the basalt diving wave and of reflections from the base of the basalt layer and below. The diving‐wave amplitudes of the models containing velocity gradients show a local amplitude minimum followed by a maximum at a greater range if the basalt thickness exceeds one wavelength and beyond that an exponential amplitude decay. The offset at which the maximum occurs can be used to determine the basalt thickness. The velocity gradient within the basalt can be determined from the slope of the exponential amplitude decay. The amplitudes of subbasalt reflections can be used to determine seismic velocities of the overburden and the impedance contrast at the reflector. Combining wide‐angle traveltimes and amplitudes of the basalt diving wave and subbasalt reflections enables us to obtain a more detailed velocity profile than is possible with the NMO velocities of small‐offset reflections. This paper concentrates on the subbasalt problem, but the results are more generally applicable to situations where high‐velocity bodies overlie a low‐velocity target, such as subsalt structures.

Crustal structure of the northern Reykjanes Ridge and Reykjanes Peninsula, southwest Iceland

Results from the Reykjanes‐Iceland Seismic Experiment (RISE) show that the thickness of zero‐age crust decreases from 21 km in southwest Iceland to 11 km at 62°40′N on the Reykjanes Ridge. This implies a decrease in mantle potential temperature of ∼130°C, with increasing distance from the center of the Iceland mantle plume, along this 250 km transect of the plate boundary. The crust thins off‐axis at 63°N, from 12.7 km thick at 0 Ma to 9.8 km at 5 Ma, most likely due to a ∼40°C change in asthenospheric mantle temperature between these times. This provides evidence for the passage of a pulse of hotter asthenospheric mantle material beneath the present spreading center. A reflective body, the top of which lies at 9–11 km depth, is identified in the lower crust just west of the tip of the Reykjanes Peninsula. Synthetic seismogram modeling of the wide‐angle reflections from this body suggests that it corresponds to a zone of high‐velocity (≥7.5 km s−1), high‐magnesium rocks in the lower crust. The P to S wave velocity ratio beneath the peninsula is 1.78, implying that crustal temperatures are below the solidus. Gravity modeling shows the RISE models to be consistent with the observed gravity field. Mantle densities are lower beneath the ridge axis than beneath older crust, consistent with lithospheric cooling with age.

The December 28, 1908, Messina Straits, southern Italy, earthquake: Waveform modeling of regional seismograms

The 1908 Messina Straits earthquake is one of the most catastrophic events in history. There were 60,000 to more than 100,000 deaths, and the cities of Messina and Reggio Calabria, on the opposite sides of the straits, were almost completely destroyed. During the last decades only low magnitude events occurred in the area. The 1908 earthquake is then crucial for understanding the mode of stress release in the area. We collected and digitized several regional seismograms of this event recorded in central Europe with the aim of studying the source characteristics. In order to separate the path effects, we analyzed recent events with simple and known sources. Owing to the small azimuth range spanned by the available stations, we could not determine a fault plane solution for the Messina Straits event, nor discriminate between the published focal mechanisms. However, the normal fault character of the rupture is confirmed. By inverting the historical P waveforms we derived source time functions, and obtained a seismic moment of 5.38(±2.16)×1019 N m (Mw; = 7.1). This is in good agreement with the results obtained by several authors from the inversion of the historical leveling data. This value is confirmed by the SH wave modeling, which also allowed the assessment of the unilateral northward character of the rupture propagation along an ∼43 km fault. Finally, we applied a simple rupture model in order to derive the slip distribution along the fault. The resulting function is in good correspondence with the geodetic inversions performed by Boschi et al. [1989] and De Natale and Pingue [1991]. In particular, a maximum slip of ∼4 m is located in proximity of the center of the slipped area. The high dislocation patch beneath the Messina harbor, as depicted by De Natale and Pingue, is not confirmed by our analysis and is probably connected to surface collapse of some of the bench marks.

Active faulting in the Gulf of Aqaba: New knowledge from theMW 7.3 earthquake of 22 November 1995

AbstractOn 22 November 1995 the largest earthquake instrumentally recorded in the area, with magnitude MW 7.3, occurred in the Gulf of Aqaba. The main rupture corresponding to the strike-slip mechanism is located within the gulf of Aqaba, which forms the marine extension of the Levantine fault, also known as the Dead Sea fault. The Levantine fault accommodates the strike-slip movement between the African plate and the Arabian plate. The Gulf of Aqaba itself is usually described as the succession of three deep pull-apart basins, elongated in the N-S direction. Concerning historical seismicity, only two large events have been reported for the last 2000 years, but they are still poorly constrained. The seismicity recorded since installation of regional networks in the early 1980s had been characterized by a low background level punctuated by brief swarmlike activity a few months in duration. Three swarms have already been documented in the Gulf of Aqaba in 1983, 1990, and 1993, with magnitudes reaching at most 6.1 (MW). We suggest that the geometry of the rupture for the 1995 event is related to the spatial distribution of these previous swarms. Body-wave modeling of broadband seismograms from the global network, along with the analysis of the aftershock distribution, allow us to propose a well-constrained model for the rupture process. Northward propagation of the rupture has been found. We have demonstrated that three successive subevents are necessary to obtain a good fit between observed and synthetic wave forms. The total seismic moment released was 7.42 × 1019 N-m. The location of the subsevents shows that the three stages of the rupture involve three different segments within the gulf. Substantial surface breakage showing only normal motion (up to 20 cm) affecting beachrock was observed along the Egyptian coast. We show that these ruptures are only a secondary feature and are in no case primary ruptures. The stress tensor derived from striations collected in quaternary sediments shows radial extension. This result supports landsliding of the beach terraces under the action of the earthquake shaking.

Structure of the Cretaceous Kerguelen Volcanic Province (southern Indian Ocean) from wide-angle seismic data.

Travel time inversion and amplitude modelling of OBS wide-angle seismic data recorded in the Enderby Basin, west of the Kerguelen Plateau, show a 10 to 13 km thick crust characteristic of oceanic crust emplaced near a hotspot (velocity reaches 7.4 km s −1 at the bottom of the crust). From this study we infer that the eastern border of the Enderby Basin is part of the Cretaceous Kerguelen Volcanic Province, and that it results of the influence of the Kerguelen hotspot on the spreading axis. Synthetic seismogram modelling confirms that the northern Kerguelen Plateau is probably emplaced over the Kerguelen hotspot in an Iceland-type setting.

Quasi‐Love phases between Tonga and Hawaii: Observations, simulations, and explanations

Seismograms of some shallow Tonga earthquakes observed at Hawaii contain SV‐polarized phases in the Love wave time window, most prominently on the vertical component. Given the geometry of the observations (Δ ≈ 40–45°), such phases may be explained either as body waves or as mode‐converted surface waves. Detailed synthetic seismogram modeling of representative events reveals several instances where the body wave explanation is inadequate, even when plausible uncertainties in the source mechanism are taken into account. The observed, SV‐polarized phase can instead be generated through Love‐Rayleigh scattering, which requires laterally varying seismic anisotropy along the Tonga‐Hawaii path. Trial‐and‐error forward modeling with simple structures based on the transversely isotropic mid‐Pacific velocity model PA5 of Gaherty et al [1996] obtains velocity structure that yields synthetic seismograms matching the observations. This model, while non unique, suggests first‐order constraints on the lateral variation in anisotropic properties, and associated mantle flow, along the Tonga‐Hawaii path. By examining trade‐offs in model parameters, we conclude that robust features of the model are: (1) a transition from radial to mixed radial and azimuthal anisotropy 3°–5° from Hawaii; (2) the NW‐SE alignment of the axis of azimuthal anisotropy; (3) higher degree of P anisotropy relative to S anisotropy; and (4) the presence of azimuthal anisotropy within upper 200–250 km of the mantle. Taken together, these features imply a disruption of mantle fabric by the processes forming Hawaii‐Emperor volcanic system. A model with anisotropic gradients in both the lithospheric lid and shallow asthenosphere is the simplest extension of our starting model. However, an equivalent data fit can be obtained if the azimuthal‐anisotropy gradients are restricted to line beneath the high‐velocity “lid” of model PA5, so that mantle hot spot flow need not penetrate the lithospheric lid.

Scattering of elastic waves in 2-D composite media II. Waveforms and spectra

The boundary integral representation of the complete seismic wavefield in two-dimensional composite media characterised by the distribution of many cavities (Paper I in this issue) is used to study the waveforms and spectra of the scattered wavefield for three models of media heterogeneity, upon the incidence of P and S plane waves and line sources. First, the case of one circular cavity and S primary waves shows that the scattered wavefield is composed mainly of S waves, and that S-P scattering can be ignored in any frequency range for all forward scattering angles (scattering angle θ measured clockwise with respect to the direction of propagation of the primary wave). The spectra of forward scattering computed for θ ⋍ 0° resemble the spectrum predicted by the Born approximation for acoustic or scalar waves for θ = 0°: of small amplitudes for small values of non-dimensional frequency kd ( k is the wavenumber and d is the cavity diameter), increasing with kd, up to kd ⋍ 2, and becoming constant for larger values of kd. The spectra for backward scattering (θ ⋍ 180°) behave similarly, showing amplitudes as large as those computed for the forward cases. The non-isotropic pattern of scattering predicted by analytical solutions is also confirmed. In the case of P primary waves, P-S scattering appears to be significantly stronger than P-P scattering for most scattering angles, except for θ ⋍ 0° and θ ⋍ 180°. The computation of synthetic seismograms for models with many cavities show scattered waves of low frequency corresponding to wavelengths much larger than the size of the cavities, as well as those of high frequency due to multiple reflections and conversions at the boundaries of the cavities. A cluster of 20 cavities randomly distributed within a small region produces well-defined low frequency waves that appear to be associated with the presence of one low-velocity heterogeneous body, or soft inclusion, represented by the whole cluster. The case of 50 cavities randomly distributed within a horizontally extended region (of narrow thickness) shows coda-like wave arrivals, particularly strong in the horizontal component. Also in this case, nearly horizontally incident plane waves produce low frequency scattered waves of large amplitudes. It appears that while in the long-wavelength limit this model synthesises a coherent wave corresponding to reflection upon a horizontal interface, towards the short-wavelength limit the scattered waves show a rather complex, incoherent pattern immediately after the arrival of the incident wave, as if the region were a transitional zone of effective thickness. The analysis presented in this paper suggests that if the wavelengths are much larger than the size of the cavities, our representation of random media can be used to represent regional heterogeneity in the earth's crust, associated with observed seismic scattering phenomena.

Observations of PKKP Precursors Used to Estimate Small-Scale Topography on the Core-Mantle Boundary

Stacks of global seismic data reveal the time and distance dependence of high-frequency (1-hertz) precursors to the seismic phase PKKP . Synthetic seismogram modeling shows that scattering from random small-scale topography at the PKKP core-mantle – boundary reflection point generates precursory arrivals similar to those seen in the data. Models with a root mean square core-mantle–boundary topography of 250 to 350 meters and correlation length of 7 to 10 kilometers explain the main features of the data. However, a systematic range-dependent misfit between observed and predicted precursor power suggests that inner core scattering may contribute to the precursors.

Faulting parameters of earthquakes in the New Madrid, Missouri, region

The advent of high-resolution digital seismic recording and advances in computer technology enable the combination of traditional regional seismic network observations with direct seismogram modeling to improve estimates of small earthquake faulting geometry, depth, and size. We illustrate a combined modeling approach using observations from three earthquakes that occurred within the environs of the New Madrid Seismic Zone: two Missouri earthquakes from September 26, 1990 and May 4, 1991; and the southern Illinois earthquake of February 5, 1994. We also re-examine the faulting geometry for two events from the 1960s that are inconsistent with the current estimate of the regional stress field. Based on direct modeling of the long-period seismograms associated with these events, we revise earlier estimates of the earthquake parameters for the March 3, 1963 and July 21, 1967 Missouri earthquakes. Comparing the new and revised results with existing earthquake mechanisms in the region, we find that tension-axes are generally aligned in a N-S to NW-SE direction, while the compression-axes trend in a NE to E direction. An interesting exception to this pattern are the March 3, 1963 and two nearby earthquakes that lie within a well-defined 30-km long left step in seismicity near New Madrid.

Subhorizontal Layering in the Lower Crust and its Tectonic Significance in the Narmada-Son-Region, India

—Comparison of deep seismic sounding (DSS) results of different profiles across the Narmada-Son Lineament (NSL), India indicates the anomalous nature of the crust along the Ujjain-Ma han profile. Forward travel time and synthetic seismogram modeling, using normalized record sections of refraction and wide angle reflection data acquired along the Ujjain-Mahan deep seismic sounding profile across NSL, brings into focus the presence of high velocity (7.0–7.3 km/s) subhorizontal layers from a depth of 8–12 km down to Moho. The tectonic implication of such reflections (layering in the crust) is discussed. The two fault zones, reported by earlier workers, flanking the rift might have acted as feeders for the mantle material to intrude into the middle and lower crustal columns.

Färoe‐Iceland Ridge Experiment 1. Crustal structure of northeastern Iceland

Results from the Färoe‐Iceland Ridge Experiment (FIRE) constrain the crustal thickness as 19 km under the Northern Volcanic Zone of Iceland and 35 km under older Tertiary areas of northeastern Iceland. The Moho is defined by strong P wave and S wave reflections. Synthetic seismogram modeling of the Moho reflection indicates mantle velocities of at least 8.0 km/s beneath the Tertiary areas of northeastern Iceland and at least 7.9 km/s beneath the neovolcanic zone. Crustal diving rays resolve the structure of the upper and lower crust. Surface P wave velocities are 1.1–4.0 km/s in Quaternary rocks and are rather higher, 4.4–4.7 km/s, in the Tertiary basalts that outcrop elsewhere. The highest crustal P wave velocities observed directly from diving rays are 7.1 km/s, from rays that turn at 24 km depth. Velocities of 7.35 km/s at the base of the crust are inferred from extrapolation of the lower crustal velocity gradient (0.024 s−1). A Poisson's ratio of approximately 0.27, equivalent to an S wave to P wave travel time ratio of 1.78, is measured throughout the crust east of the neovolcanic zone. The Poisson's ratio and the steep Moho topography (in places up to 30° from the horizontal) indicate that the entire crust outside the neovolcanic zone is cool (<800°C). Gravity data are well matched by a velocity/density conversion of our seismic crustal model and indicate a region of low mantle density beneath the neovolcanic zone, believed to be due to elevated mantle temperatures. The crustal thickness in the neovolcanic zone is consistent with geochemical estimates of the melt generation, placing constraints on the flow within the Iceland mantle plume.

TheMw= 8.0 Antofagasta (northern Chile) earthquake of 30 July 1995: A precursor to the end of the large 1877 gap

AbstractAn Mw = 8.0 earthquake occurred on 30 July 1995 in the Antofagasta region (northern Chile). The main rupture, corresponding to thrust faulting, developed from 10 to 50 km in depth along the subduction interface between the Nazca and the South American plates. The 1995 earthquake took place just south of the large seismic gap where a great earthquake (M = 9) had occurred in 1877. Most of the 1995 rupture was located within a local network consisting of nine short-period stations that had been previously installed at the southern end of the 1877 gap, and the aftershock sequence could be accurately monitored. Little destruction resulted from the 1995 earthquake in spite of its large size. Ground acceleration in Antofagasta reached 29% of gravity. A tsunami wave, 2 to 2.5 m high, was observed along the coast from Mejillones to Taltal. One strong foreshock (Mw = 6.2) occurred in the 1995 hypocentral region 6 months before the main event.Body-wave modeling of broadband seismograms from the global network, along with the analysis of the aftershock distribution, allows us to propose a well-constrained model for the whole rupture process. Some additional details of the rupture were obtained from an accelerometer record at Antofagasta. The main rupture started as a double even with thrust mechanism below the southern part of the Mejillones peninsula, and it propagated southward in a N200°E direction with an average velocity of 2.8 km/sec. It ended near the trench in normal faulting. The total rupture area and seismic moment are 185 × 90 km2 and 1.2 × 1028 dyne-cm, respectively. The aftershock distribution delineates a well-defined rupture surface along the subduction interface. The distribution of epicenters during the first 20 h of aftershock activity shows a sharp northern boundary beneath the Mejillones peninsula. Hence, the 1995 main rupture did not propagate north of the Mejillones peninsula into the 1877 gap. Aftershocks during the following 2 weeks indicate a growth of the initial rupture zone toward the north. The mechanisms of the strongest aftershocks are similar to that of the mainshock. The down-dip termination of the main rupture corresponds to the maximum depth (50 km) of the region that had been identified as the locked part of the subduction interface from the analysis of the microseismicity recorded by the local network prior to the 1995 event.A well-constrained dislocation model is proposed for the 1995 main rupture, which produces surface displacements in good agreement with available observations of coastal uplift and GPS measurements. The dislocation model, as well as Global Positioning System (GPS) measurements, indicate that the 1995 earthquake generated E-W extension in the coastal region of Antofagasta. The Atacama fault, located 40 to 50 km above the 1995 main rupture, showed small fresh surface ruptures near Sierra Remiendos (70 km to the SSE of Antofagasta) with a maximum vertical offset of 20 cm. This offset corresponds to normal faulting, which is in agreement with the E-W co-seismic extension.The Mejillones peninsula appears to be the surface expression of a barrier that interrupted the propagation of the 1995 rupture to the north into the region of the 1877 gap. Modeling of static stress changes induced by the Antofagasta earthquake indicates an increase in compressive stresses along a direction transverse to the trench immediately to the north of the 1995 rupture surface. Thus, the chances for the reactivation of the 1877 gap after this event are greater now.