Step-wise relocation of ISC earthquake hypocenters for linearized tomographic imaging of slab structure

Hilst, R. D. van der; Engdahl, E. R.

doi:10.1016/0031-9201(92)90116-d

Cited by 30 publications

(16 citation statements)

References 44 publications

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“…It is important to point out that the EHB procedure cannot entirely remove the effects of the Earth's lateral heterogeneity on teleseismic earthquake location. van der Hilst & Engdahl (1992) and Bijwaard et al (1998) have shown that in or near subducted lithosphere, where aspherical variations in seismic wave velocities are large (i.e. on the order of 5–10 per cent,) the lateral variations in seismic velocity, the uneven spatial distribution of seismograph stations, and the specific choice of seismic data used to determine the earthquake hypocentre can easily combine to produce bias in teleseismic earthquake epicentres of up to several tens of kilometres.…”

Section: Methodsmentioning

confidence: 99%

Relocation and assessment of seismicity in the Iran region

Engdahl¹,

Jackson²,

Myers

et al. 2006

Geophysical Journal International

273

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S U M M A R YMore 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.

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Section: Methodsmentioning

confidence: 99%

Relocation and assessment of seismicity in the Iran region

Engdahl¹,

Jackson²,

Myers

et al. 2006

Geophysical Journal International

273

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“…However, the ISC does not consider S arrival times in the relocation process. We should note that van der Ililst and Engdahl [1992], van der Ililst et al [1993], and Engdahl et al [1996 ] have followed a similar procedure of earthquake relocation and phase reassociation based upon the IASP91 model. For this purpose the IASP91 velocity model [Kennett and is used because it is thought to better represent the mantle discontinuities at 410 km and 660 km.…”

Section: Methodsmentioning

confidence: 99%

Lateral variations in mantle velocity structure and discontinuities determined from P, PP, S, SS, and SS — S_dS travel time residuals

Vasco¹,

Johnson²,

Pulliam³

1995

J. Geophys. Res.

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On the basis of P, PP, S, SS arrival times and SS ‐ S410S, SS ‐ S660S differential times, we construct models of mantle P and S velocity structure and boundary topography of the 410‐km and 660‐km discontinuities. Events from the catalog of the International Seismological Centre (ISC) are relocated relative to the International Association of Seismology and Physics of the Earth's Interior 1991 (IASP91) velocity model using both P and S arrival times. The arrival times are corrected for ellipticity and the PP and SS residuals are corrected for the topography at the bounce point. The cap‐averaged PP ‐ P and SS ‐ S differential time residuals, plotted at the PP and SS surface reflection points, form broad coherent patterns. The geographic distribution of the cap averaged residuals agrees quite well with PP ‐ P and SS‐S differential time residuals derived from long period Global Digital Seismograph Network (GDSN) data. A robust lp inversion scheme is used to infer global mantle structure. Synthetic tests indicate that for regions well sampled by SS ‐ S410S and SS ‐ S660S differential times, the velocity estimates are not seriously contaminated by the topography of the 410‐ and 660‐km discontinuities. However, estimates of boundary deflections may be influenced by extensive P and S velocity variations of 3% or greater. We find the 410‐km discontinuity to be depressed by as much as 24 km beneath North America. Conversely, the discontinuity is deflected upward underneath Eurasia. In some regions the topography of the 660‐km discontinuity is quite distinct from that of the 410‐km discontinuity, but the two appear to be positively correlated. A series of depressions are found at several intersections of the 660‐km discontinuity with known subduction zones. The elevated topography in the 410‐km discontinuity beneath Europe is underlain by a trough in the 660‐km discontinuity. A number of subduction zones are characterized by a thinning of the transition zone. Negative P and S velocity anomalies, underlying back‐arc basins and tectonically active continental regions, encircle the Pacific. Where they are resolved, the stable continental cratons are systematically positive velocity features that extend below 200 km. With the inclusion of PP and SS travel time residuals we are better able to constrain midmantle structure. Most notably, in the depth range 35–660 km beneath the Northwest Pacific we observe high P velocity. Where they are resolved, mid‐ocean ridges are most clearly imaged as low velocity features in the S model. The northern portion of the Mid‐Atlantic Ridge is underlain by negative S velocity anomalies. In the Pacific, the East Pacific Rise is an extensive low S velocity anomaly.

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“…At well‐monitored subduction zones local catalogs provide an independent test of the slab surfaces. At depths exceeding network aperture the hypocenters can be biased toward the trench, due to the fast travel times in a high‐velocity slab [ McLaren and Frolich , 1985; van der Hilst and Engdahl , 1992]. For similar reasons, local catalogs can be more accurate than teleseismic catalogs for events shallower than 200 km if the network geometry is favorable.…”

Section: Robustness and Uncertaintymentioning

confidence: 99%

Global compilation of variations in slab depth beneath arc volcanoes and implications

Syracuse

Abers

2006

Geochem Geophys Geosyst

551

572

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[1] The location and motion of subducting plates relative to volcanic arcs provide a first-order constraint on theories of arc magmagenesis. We compile volcano-specific subduction parameters for 33,000 km of the global arc system at 839 volcanic centers, measuring the depth to the top of the slab (H) beneath each volcano. The compilation also includes estimates of slab strike and dip, incoming plate velocity, and age, all available in accompanying auxiliary material. The slab geometry is contoured from the top surface of Wadati-Benioff zones (WBZs) for a variety of teleseismic and local seismicity catalogs, which provides a reference surface for evaluating the distribution of seismicity within subducting plates. The WBZ thickness exceeds that expected from hypocentral errors in a manner correlating with plate age, indicating that old plates have thicker regions in which earthquakes can occur. When averaged over 500-km-long arc segments, H ranges from 72 to 173 km with a global average of 105 km, increasing by 20 km when hypocentral error effects are taken into account. These depths correlate poorly with most subduction parameters, but significant correlations exist between H and slab dip (correlation coefficient is 0.54 for 45 arc segments). The dip correlation can be explained if the melting region is displaced from the WadatiBenioff zone by a constant-thickness boundary layer. For the north Pacific, H varies inversely with descent rate; this trend may reflect the manner in which wedge thermal structure affects arc location. Over short distances some arc segments exhibit abrupt variations in arc location but not slab geometry, indicating that upper-plate tectonic processes also exert control on H. These along-strike trends in H also correlate with geochemical proxies for the degree of melting, at least in one test case. Thus slab geometry and kinematics provide an important control on the melting that produces arc volcanoes.

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Step-wise relocation of ISC earthquake hypocenters for linearized tomographic imaging of slab structure

Cited by 30 publications

References 44 publications

Relocation and assessment of seismicity in the Iran region

Relocation and assessment of seismicity in the Iran region

Lateral variations in mantle velocity structure and discontinuities determined from P, PP, S, SS, and SS — S_dS travel time residuals

Global compilation of variations in slab depth beneath arc volcanoes and implications

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Step-wise relocation of ISC earthquake hypocenters for linearized tomographic imaging of slab structure

Cited by 30 publications

References 44 publications

Relocation and assessment of seismicity in the Iran region

Relocation and assessment of seismicity in the Iran region

Lateral variations in mantle velocity structure and discontinuities determined from P, PP, S, SS, and SS — SdS travel time residuals

Global compilation of variations in slab depth beneath arc volcanoes and implications

Contact Info

Product

Resources

About

Lateral variations in mantle velocity structure and discontinuities determined from P, PP, S, SS, and SS — S_dS travel time residuals