More than 2000 instrumentally recorded earthquakes occurring in the Iran region during the period 1918–2004 have been relocated and reassessed, with special attention to focal depth, using an advanced technique for 1-D earthquake location. A careful review of starting depths, association of teleseismic depth phases, and the effects of reading errors on these phases are made and, when necessary, waveforms have been examined to better constrain EHB focal depths. Uncertainties in EHB epicentres are on the order of 10–15 km in the Iran region, owing to the Earth's lateral heterogeneity and uneven station distribution. Uncertainties of reviewed EHB focal depth estimates are on the order of 10 km, as compared to about 4 km for long-period P and SH body-waveform inversions. Nevertheless, these EHB depth estimates are sufficiently accurate to resolve robust differences in focal depth distribution throughout the Iran region and, within their errors, show patterns that are in agreement with the smaller number of earthquakes whose depths have been confirmed by body-wave modelling or local seismic networks. The importance of this result is that future earthquakes with apparently anomalous depths can easily be identified, and checked, if necessary. Most earthquakes in the Iranian continental lithosphere occur in the upper crust, with the crustal shortening produced by continental collision accommodated entirely by thickening and distributed deformation. In the Zagros Mountains nearly all earthquakes are confined to the upper crust (depths <20 km), and there is no evidence for a seismically active subducted slab dipping NE beneath central Iran. By contrast, in southeastern Iran, where the Arabian seafloor is being subducted beneath the Makran coast, low-level earthquake activity occurs in the upper crust as well as to depths of at least 150 km within a northward-dipping subducting slab. Near the Oman Line, a region transitional between the Zagros and the Makran, seismicity extends to depths of up to 30–45 km in the crust, consistent with low-angle thrusting of Arabian basement beneath central Iran. In north-central Iran, along the Alborz mountain belt, seismic activity occurs primarily in the upper crust but with some infrequent events in the lower crust, particularly in the western part of the belt (the Talesh), where the South Caspian basin underthrusts NW Iran. Earthquakes that occur in a band across the central Caspian, following the Apscheron–Balkhan sill between Azerbaijan and Turkmenistan, have depths in the range 30–100 km, deepening northwards. These are thought to be connected with either incipient or remnant northeast subduction of the South Caspian basin basement beneath the east-west trending Apscheron–Balkhan sill. Curiously, in this region of genuine mantle seismicity, there is no evidence for earthquakes shallower than 30 km.
The 3-D shear velocity structure beneath South India's Dharwar Craton determined from fundamental mode Rayleigh waves phase velocities reveals the existence of anomalously high velocity materials in the depth range of 50–100 km. Tomographic analysis of seismograms recorded on a network of 35 broad-band seismographs shows the uppermost mantle shear wave speeds to be as high as 4.9 km s–1 in the northwestern Dharwar Craton, decreasing both towards the south and the east. Below ∼100 km, the shear wave speed beneath the Dharwar Craton is close to the global average shear wave speed at these depths. Limitations of usable Rayleigh phase periods, however, have restricted the analysis to depths of 120 km, precluding the delineation of the lithosphere–asthenosphere boundary in this region. However, pressure–temperature analysis of xenoliths in the region suggests a lithospheric thickness of at least ∼185 km during the mid-Proterozoic period. The investigations were motivated by a search for seismic indicators in the shallow mantle beneath the distinctly different parts of the Dharwar Craton otherwise distinguished by their lithologies, ages and crustal structure. Since the ages of cratonic crust and of the associated mantle lithosphere around the globe have been found to be broadly similar and their compositions bimodal in time, any distinguishing features of the various parts of the Dharwar shallow mantle could thus shed light on the craton formation process responsible for stabilizing the craton during the Meso- and Neo-Archean.
Abstract : The crust and upper mantle structure of the south Caspian Basin and the Turkmenian Lowlands is enigmatic. From Soviet deep seismic sounding data collected in the 1960's, the crust appears to consists of two layers: a thick sedimentary section (15-25 km) with low P-wave velocity (3.5-4.0 km/s) overlying a 12-18 km thick basaltic lower crust. It has been suggested that this basaltic lower crust is 'oceanic-like' crust and that the south Caspian Basin represents a section of-relic ocean from a Paleozoic - Triassic ocean or a Mesozoic - Paleogene marginal sea. Improved knowledge of the crust and upper mantle velocity structure of the south Caspian Basin is important in a seismic verification context because of the anomalous effect it has on regional seismic waveforms. To investigate the crust and upper mantle structure of the south Caspian Basin, we have installed six three-component seismograph stations within the former Soviet Republics of Turkmenia and Azerbaijan. Our objective is to determine the velocity structure of this region using both body wave receiver function and surface wave modeling techniques. We present receiver function inversion results for four sites and fundamental mode Rayleigh wave observations for two great circle paths across this region.
This paper examines the relationship between seismogenic thickness, lithosphere structure and rheology in central and northeastern Asia. We accurately determine earthquake depth distributions which reveal important rheological variations in the lower crust. These variations exert a fundamental control on the active tectonics and the morphological evolution of the continents. We consider 323 earthquakes across the Tibetan Plateau, the Tien Shan and their forelands as well as the Baikal Rift, NE Siberia and the Laptev Sea and present the source parameters of 94 of these here for the first time. These parameters have been determined through body wave inversion, the identification of depth phases or the modelling of regional waveforms. Lower crustal earthquakes are found to be restricted to the forelands in areas undergoing shortening, and to locations where rifting coincides with abrupt changes in lithosphere thickness, such as the NE Baikal Rift and W Laptev Sea. The lower crust in these areas is seismogenic at temperatures that may be as high as 600°C, suggesting that it is anhydrous, and is likely to have great long-term strength. Lower crustal earthquakes are therefore a useful proxy indicating strong lithosphere in places that are too small in areal extent for this to be confirmed independently by estimating effective elastic thickness from gravity–topography relations. The variation in crustal rheology indicated by the distribution of lower-crustal earthquakes has many implications ranging from the support of mountain belts and the formation of steep mountain fronts, to the localization and orientation of rifting. In combination, these processes can also be responsible for the separation of the front of the thin-skinned mountain belts from their hinterlands when continents separate.
We present results of a Rayleigh and Love wave group velocity dispersion study of the Indo‐Eurasian collision zone. Group velocity dispersion curves are measured and combined to produce dispersion maps for 10–70 s period Rayleigh waves from 4054 paths and for 15–70 s Love waves from 1946 paths. Group velocity maps benefit from the inclusion of data recorded at a large number of stations within India, an advantage over previous global studies. This has the largest impact at short periods as a result of the improved path length distribution. Synthetic tests are used to estimate resolution, which ranges from 3° to 5° on the continents for Rayleigh wave maps and from 5° to 7.5° for Love wave maps. Group velocities correspond well with known geological and tectonic features and show good correlation with sediment thickness at short periods. The cratons of the Indian Shield can be distinguished in the short‐period and midperiod group velocities. Group velocities are slow across Tibet until 70 s whereas the cratonic cores of the Indian Shield appear as a high velocity anomaly at 70 s. Dispersion curves extracted from the Rayleigh wave group velocity maps are inverted for shear wave velocity as a function of depth for profiles across India and Tibet. The relationship between shear velocity contours and the Moho indicated by receiver function studies has been used to obtain a first‐order estimate of crustal thickness across the collision zone. Results suggest a slow Tibetan midcrust and low sub‐Moho velocities beneath the central and northeastern Tibetan Plateau.
This paper combines observations of seismicity, gravity, topography and thermal and velocity structures to investigate the rheological properties of the lithosphere in the Lake Baikal region. We examine the seismogenic thickness (Ts) using 25 earthquakes of Mw 5.1–7.1, whose full source parameters have been determined by inversion of teleseismic waveforms, 13 of which are presented here for the first time. These 25 events, plus six others (Mw 5.0–5.8) whose depths are well constrained, show that moderate earthquakes occur at depths up to ∼30 km in the northeast Baikal rift. Based on the teleseismic waveform modelling results and published relocations of microearthquakes using regional networks, we conclude that the mantle is not a significant source of seismicity in the Baikal region. Using the admittance between free-air gravity and topography, we estimate the effective elastic thickness (Te) in the region to be between 5 and 20 km. Nowhere do the data require that Te > Ts, consistent with the simple interpretation that the long-term strength of the lithosphere resides in its seismogenic layer. A weak mantle in the Baikal region can be explained by its high temperature, which we estimate by combining local geotherm estimates with the regional upper mantle velocity structure, obtained from fundamental and higher-mode surface waves. Geotherms are fitted to pressure and temperature estimates from mantle nodules at four sites, both within and outside the Siberian shield. In order to constrain the temperatures at the Moho, we estimated crustal thicknesses using teleseismic receiver functions. Moho temperatures are estimated to exceed ∼550°C beneath the Siberian shield and are higher in the more recently deformed mountain belts to the south. Based on a reassessment of oceanic geotherms and seismicity, it seems likely, therefore, that the mantle in the Baikal region is too hot to be a source of long-term strength. This is consistent with the recent suggestion that the distribution of mantle seismicity in both the oceans and the continents is dependent on temperature alone. Finally, we note that results from S-wave tomography studies, combined with the observed locations of rift-related earthquakes, lead us to suspect that the frequently published position of the edge to the Siberian shield at the surface provides a poor description of that same boundary at depth.
Los temblores de Oaxaca de 1978 (Ms = 7.8), de Colima de 1973 (Ms = 7.5) y de Petatlán de 1979 (Ms = 7.6) ocurrieron a lo largo de la trinchera centroamericana y tienen mecanismos focales similares (fallas inversas de bajo ángulo de buzamiento). El momento del temblor de Colima fue determinado a partir de ondas Rayleigh del manto.La comparación de las ondas del manto indica que el temblor de Oaxaca tiene un momento de 1.5 - 3.0 x 1027 dina,-cm, un poco menor que el de Colima; el momento para el temblor de Petatl:in l..5-2 x 10²⁷ dinas-cm. Usando el área de réplicas como una estimación del área de ruptura, se obtiene que la caída de esfuerzos para el temblor de Oaxaca es de 10-20 bares, similar al de Colima (18 bares); para el temblor de Petatlán se obtiene una caída de esfuerzo de 20 - 30 bares. Registros de instrumentos de período largo de estaciones SRO y WWNSS fueron utilizados para inferir sobre la complejidad de la fuente. Los sismogramas sintéticos se calculan por medio de! programa de Chapman basado en la teoría de rayos según el método de WKBJ. Resultados preliminares sugieren que el terremoto de Oaxaca se inició en una pequeña área (r = 10 km) con una caída de esfuerzos alta (Ʌδ≥1,000 bares) que se expandió a una área mayor con una caída de esfuerzos menor en promedio.
'%3e%3cpath fill='%23012169' d='M0 0v30h60V0z'/%3e%3cpath stroke='%23fff' stroke-width='6' d='m0 0 60 30m0-30L0 30'/%3e%3cpath stroke='%23C8102E' stroke-width='4' d='m0 0 60 30m0-30L0 30' clip-path='url(%23b)'/%3e%3cpath stroke='%23fff' stroke-width='10' d='M30 0v30M0 15h60'/%3e%3cpath stroke='%23C8102E' stroke-width='6' d='M30 0v30M0 15h60'/%3e%3c/g%3e%3c/svg%3e)
'%3e%3ccircle r='20' fill='%23008'/%3e%3ccircle r='17.5' fill='%23fff'/%3e%3ccircle r='3.5' fill='%23008'/%3e%3cg id='d'%3e%3cg id='c'%3e%3cg id='b'%3e%3cg id='a' fill='%23008'%3e%3ccircle r='.875' transform='rotate(7.5 -8.75 133.5)'/%3e%3cpath d='M0 17.5.6 7 0 2l-.6 5L0 17.5z'/%3e%3c/g%3e%3cuse xlink:href='%23a' transform='rotate(15)'/%3e%3c/g%3e%3cuse xlink:href='%23b' transform='rotate(30)'/%3e%3c/g%3e%3cuse xlink:href='%23c' transform='rotate(60)'/%3e%3c/g%3e%3cuse xlink:href='%23d' transform='rotate(120)'/%3e%3cuse xlink:href='%23d' transform='rotate(-120)'/%3e%3c/g%3e%3c/svg%3e)
'%3e%3cg id='e'%3e%3cg id='c' fill='none' stroke='%23fff' stroke-width='2'%3e%3cpath id='b' d='M0 1h26M1 10V5h8v4h8V5h-5M4 9h2m20 0h-5V5h8m0-5v9h8V0m-4 0v9' transform='scale(1.4)'/%3e%3cpath id='a' d='M0 7h9m1 0h9' transform='scale(2.8)'/%3e%3cuse xlink:href='%23a' y='120'/%3e%3cuse xlink:href='%23b' y='145.2'/%3e%3c/g%3e%3cg id='d'%3e%3cuse xlink:href='%23c' x='56'/%3e%3cuse xlink:href='%23c' x='112'/%3e%3cuse xlink:href='%23c' x='168'/%3e%3c/g%3e%3c/g%3e%3cuse xlink:href='%23d' x='168'/%3e%3cuse xlink:href='%23e' x='392'/%3e%3c/g%3e%3cg fill='%23da0000' transform='matrix(45 0 0 45 315 180)'%3e%3cg id='f'%3e%3cpath d='M-.548.836A.912.912 0 0 0 .329-.722 1 1 0 0 1-.548.836'/%3e%3cpath d='M.618.661A.764.764 0 0 0 .422-.74 1 1 0 0 1 .618.661M0 1l-.05-1L0-.787a.31.31 0 0 0 .118.099V-.1l-.04.993zM-.02-.85 0-.831a.144.144 0 0 0 .252-.137A.136.136 0 0 1 0-.925'/%3e%3c/g%3e%3cuse xlink:href='%23f' transform='scale(-1 1)'/%3e%3c/g%3e%3c/svg%3e)