Body Wave Arrival Times Research Articles

A new algorithm for three‐dimensional joint inversion of body wave and surface wave data and its application to the Southern California plate boundary region

AbstractWe introduce a new algorithm for joint inversion of body wave and surface wave data to get better 3‐D P wave (Vp) and S wave (Vs) velocity models by taking advantage of the complementary strengths of each data set. Our joint inversion algorithm uses a one‐step inversion of surface wave traveltime measurements at different periods for 3‐D Vs and Vp models without constructing the intermediate phase or group velocity maps. This allows a more straightforward modeling of surface wave traveltime data with the body wave arrival times. We take into consideration the sensitivity of surface wave data with respect to Vp in addition to its large sensitivity to Vs, which means both models are constrained by two different data types. The method is applied to determine 3‐D crustal Vp and Vs models using body wave and Rayleigh wave data in the Southern California plate boundary region, which has previously been studied with both double‐difference tomography method using body wave arrival times and ambient noise tomography method with Rayleigh and Love wave group velocity dispersion measurements. Our approach creates self‐consistent and unique models with no prominent gaps, with Rayleigh wave data resolving shallow and large‐scale features and body wave data constraining relatively deeper structures where their ray coverage is good. The velocity model from the joint inversion is consistent with local geological structures and produces better fits to observed seismic waveforms than the current Southern California Earthquake Center (SCEC) model.

Magma sources in the mantle wedge beneath the volcanoes of the Klyuchevskoy group and Kizimen based on seismic tomography modeling

This study presents a 3D model of the P and S seismic velocities above the Kamchatkan slab obtained as a result of tomographic inversion of arrival times of body waves from deep seismicity in the subduction zone. Various tests performed have shown limitations of the spatial resolution of the model and provided arguments for the reliability of the major structures used in the interpretation. In the uppermost layer down to 20 km depth, the model reveals strong low-velocity anomalies coinciding with Holocene volcanoes of the Klyuchevskoy group and Kizimen. In the seismogenic zone at depths from 80 to 150 km, we observe a low-velocity anomaly, which probably reflects the presence of the relatively thick oceanic crust sinking together with the subducting slab. This anomaly may also represent a zone of phase transitions, melting, and release of fluids from the slab. In the cross sections, we observe vertical and inclined low-velocity anomalies connecting the slab with the volcanic groups that probably represent the paths of ascending fluids and melts, which feed the volcanoes. In the case of Kizimen, we observe a single conduit connecting the volcano with the slab transformation area at 100 km depth. Beneath the Klyuchevskoy group, we identify several linear inclined patterns having different dipping angles. This may show that the volcanoes of the group are fed from different segments of the slab and might be one of the reasons for the diversity of lava compositions in the volcanoes of the Klyuchevskoy group.

The fate of the Indian lithosphere beneath western Tibet: Upper mantle elastic wave speed structure from a joint teleseismic and regional body wave tomographic study

We investigate the fate of the Indian lithosphere following its descent beneath western Tibet by means of tomographic imaging based on arrival times of body waves from regional and teleseismic sources recorded by a portable network deployed in the region from 2007 to 2011. We use a non-linear iterative algorithm that simultaneously models absolute, regional, and relative teleseismic arrival times to obtain a 3-D velocity structure in a spherical segment that extends from 26°N to 37°N, from 76°E to 89°E, and from the surface to 430km depth. We find that variations in P and S wave speeds in the upper mantle are similar, and identify a number of prominent fast anomalies beneath western Tibet and the adjacent Himalayas. We associate these fast anomalies with the mantle lithosphere of India that is likely colder and hence faster than the ambient mantle. Resolution tests confirm the ability of our dataset to resolve their shapes in the upper 300km, and the lack of significant downward smearing of these features. We interpret the presence of faster material below 300km as being consistent with former Indian lithosphere having reached these depths. There are two main fast anomalies in our model. One resembles a ∼100km wide sub-vertical column located directly beneath the India-Asia plate boundary. The other anomaly is thinner, and has the shape of a dipping slab that spans the north-south width of the Lhasa block. It dips towards the NE, starting near the Indus-Yarlung suture and ending north of the Bangong-Nujiang Suture at depths in excess of 300km. Another finding of our study is the absence of major fast anomalies west of ∼80°E, which our resolution tests show to be significant. Our results do not support the notion of a continuous body of formerly Indian lithosphere being presently underthrust northward, and extending all the way to the northern boundary of the plateau. Rather, shapes of fast anomalies in western Tibet suggest colder material beneath the northernmost Himalaya and the southernmost part of the plateau. The presence of multiple relatively small-scale anomalies, rather than a single large fast body aligned with the India-Asia boundary, rules out subduction of the intact Indian lithosphere. Our preferred scenario for the process that currently removes the mantle lithosphere of India from the collision zone envisages viscous behavior, with gravity-driven instabilities leading to the formation of multiple drips of different size. To reconcile our findings with evidence of past underthrusting of Indian lithosphere beneath Tibet we postulate a geologically recent change in the mode of lithosphere removal. Our scenario includes initial underthrusting, subsequent gravitational destabilization, and removal of the mantle lithosphere of Indian plate. At present the mantle lithosphere of India that continues to underthrust southern Tibet develops a drip-like instability that is seen as a fast columnar feature in our model.

Dispersion of Love waves from the 2010 Efpalio earthquake in the Corinth Gulf region, Greece

A moderate size earthquake occurred close to the town of Efpalio in the Corinth Gulf region on January 18, 2010. In the present paper, we have used this earthquake to study the dispersion of short-period Love waves along nine paths in a broader vicinity of the Corinth Gulf. The observed group velocities require very low S-wave velocities in the uppermost 2-km depth. This structural feature differs distinctly from most previous models that were derived from arrival times of body waves in the region. Moreover, it was recognized that the observed group velocities were relatively low for western paths, and medium velocities were observed in the central part of the region, and the highest velocities in its eastern part.

Seismicity and structure of Akutan and Makushin Volcanoes, Alaska, using joint body and surface wave tomography

AbstractJoint inversions of seismic data recover models that simultaneously fit multiple constraints while playing upon the strengths of each data type. Here we jointly invert 14 years of local earthquake body wave arrival times from the Alaska Volcano Observatory catalog and Rayleigh wave dispersion curves based upon ambient noise measurements for local Vp, Vs, and hypocentral locations at Akutan and Makushin Volcanoes using a new joint inversion algorithm. The velocity structure and relocated seismicity of both volcanoes are significantly more complex than many other volcanoes studied using similar techniques. Seismicity is distributed among several areas beneath or beyond the flanks of both volcanoes, illuminating a variety of volcanic and tectonic features. The velocity structures of the two volcanoes are exemplified by the presence of narrow high‐Vp features in the near surface, indicating likely current or remnant pathways of magma to the surface. A single broad low‐Vp region beneath each volcano is slightly offset from each summit and centered at approximately 7 km depth, indicating a potential magma chamber, where magma is stored over longer time periods. Differing recovery capabilities of the Vp and Vs data sets indicate that the results of these types of joint inversions must be interpreted carefully.

Seismicity in the Basin and Range Province of Sonora, México, between 2003 and 2011, near the Rupture of the 3 May 1887 Mw 7.5 Earthquake

The regional seismicity in the Basin and Range Province of Sonora, México, near the epicentral region of the 3 May 1887 Mw 7.5 earthquake was studied. Body wave arrival times recorded by the local network RESNES (Red Sísmica del Noreste de Sonora) and two regional broadband stations of the RESBAN (Red Sismológica de Banda Ancha del Golfo de California) network were used to locate the seismic sources originating between 2008 and 2011. Preliminary hypocenter coordinates were first determined with the program Hypoinverse (Klein, 2002) and then relocated the initial coordinates with the source-specific station term method (Lin and Shearer, 2005). The same procedure was used in a previous study (Castro et al., 2010) to locate the earthquakes originating in the same region between 2003 and 2007. I also present in this paper a depurated catalog of events recorded from 2003 to 2011. Most relocated events cluster near the faults that ruptured during the 1887 earthquake. Additionally, I document seismicity along Basin and Range Province normal faults south of the 1887 rupture, such as the Villa Hidalgo and Granados faults and the faults bounding the Bacadéhuachi basin. The region of the largest population center, Hermosillo, appears to be seismically quiescent. This observation is supported by a lack of entries in the catalog of the International Seismological Centre (ISC) or documented historical seismicity in that area.

ISC-GEM: Global Instrumental Earthquake Catalogue (1900–2009), I. Data collection from early instrumental seismological bulletins

In order to produce a new global reference earthquake catalogue based on instrumental data covering the last 100+years of global earthquakes, we collected, digitized and processed an unprecedented amount of printed early instrumental seismological bulletins with fundamental parametric data for relocating and reassessing the magnitude of earthquakes that occurred in the period between 1904 and 1970. This effort was necessary in order to produce an earthquake catalogue with locations and magnitudes as homogeneous as possible. The parametric data obtained and processed during this work fills a large gap in electronic bulletin data availability. This new dataset complements the data publicly available in the International Seismological Centre (ISC) Bulletin starting in 1964. With respect to the amplitude-period data necessary to re-compute magnitude, we searched through the global collection of printed bulletins stored at the ISC and entered relevant station parametric data into the database. As a result, over 110,000 surface and body-wave amplitude-period pairs for re-computing standard magnitudes MS and mb were added to the ISC database. To facilitate earthquake relocation, different sources have been used to retrieve body-wave arrival times. These were entered into the database using optical character recognition methods (International Seismological Summary, 1918–1959) or manually (e.g., British Association for the Advancement of Science, 1913–1917). In total, ∼1,000,000 phase arrival times were added to the ISC database for large earthquakes that occurred in the time interval 1904–1970. The selection of earthquakes for which data was added depends on time period and magnitude: for the early years of last century (until 1917) only very large earthquakes were selected for processing (M⩾7.5), whereas in the periods 1918–1959 and 1960–2009 the magnitude thresholds are 6.25 and 5.5, respectively. Such a selection was mainly dictated by limitations in time and funding. Although the newly available parametric data is only a subset of the station data available in the printed bulletins, its electronic availability will be important for any future study of earthquakes that occurred during the early instrumental period.

Joint Inversion of Body-Wave Arrival Times and Surface-Wave Dispersion for Three-Dimensional Seismic Structure Around SAFOD

We incorporate body-wave arrival time and surface-wave dispersion data into a joint inversion for three-dimensional P-wave and S-wave velocity structure of the crust surrounding the site of the San Andreas Fault Observatory at Depth. The contributions of the two data types to the inversion are controlled by the relative weighting of the respective equations. We find that the trade-off between fitting the two data types, controlled by the weighting, defines a clear optimal solution. Varying the weighting away from the optimal point leads to sharp increases in misfit for one data type with only modest reduction in misfit for the other data type. All the acceptable solutions yield structures with similar primary features, but the smaller-scale features change substantially. When there is a lower relative weight on the surface-wave data, it appears that the solution over-fits the body-wave data, leading to a relatively rough V s model, whereas for the optimal weighting, we obtain a relatively smooth model that is able to fit both the body-wave and surface-wave observations adequately.

Automated Seismic Event Location by Travel-Time Stacking: An Application to Mining Induced Seismicity

Online Material: Additional synthetic test results, plots, and tables. The automated location of seismic events is an important and challenging task in microseismic monitoring applications (e.g., to analyze induced seismicity following oil/geothermal field exploitation and mining operations), where we deal with a large number of seismic events and weak signals characterized by low signal‐to‐noise ratios. Given the large number of seismic events, manual location procedures are time consuming, or not feasible. Standard automated location routines require precise automated picking procedure and phases identification (Gharti et al. , 2010). These methods are, generally, modified versions of the Geiger (1910, 1912) algorithm, based on the minimization of time residuals between theoretical and observed arrival times of body waves (generally first P and S onsets) by iterative inversion algorithms. In the last two decades a large number of picking algorithms have been developed; although P onsets can now be accurately picked, the automatic picking of later seismic phases (including S onsets) is still problematic. Their performance is limited in the presence of noisy data, when picking and phase identification might be difficult. The increasing interest on microseismic monitoring applications pushed the recent development of alternative techniques for automated seismic event location. These methods, similar to migration techniques used in reflection seismology, exploit the full waveforms and do not need any prior phase identification. Some methods are …

Seismogram Analysis of C052198B Earthquake, Minahasa at Observatory Stations in Australia

The earth structure between the Minahasa earthquake, coded as C052198B, and observatory stations in Australia has been investigated through analysis in the time domain of three-component seismograms. The synthetic seismograms are constructed from an earth model, the CMT solution of the earthquake and station locations. The calculation is based on the GEMINI method, and the corner frequency is set at 20 mHz. Using deconvolutions of the station instruments responses, the measured and synthetic seismogram can be compared with the same units. The seismogram comparison indicates discre-pancies between recorded data and synthetic seismograms calculated from the anisotropic PREM model, in the travel times of Rayleigh and Love surface wave, as well as the S and core reflected ScS and ScS 2 body waves. Discrepancies of Love wave travel time and the number of oscillations are explained by changes in the crustal velocity model and by setting the positive gradient of b h in the upper mantle. While for Rayleigh waves and arrival times of body waves, corrections to the zero order coefficients of b are needed at layers in the mantle. The interpretation of the results of the waveform analyses indicates strong anisotropy in the upper mantle. The anisotropy also occurs, to a lesser extent, at layers beneath the upper mantle .

Location of Aftershocks of the 4 April 2010 Mw 7.2 El Mayor-Cucapah Earthquake of Baja California, Mexico

A magnitude M w 7.2 earthquake occurred on 4 April 2010 at approximately 50 km southwest of the city of Mexicali, Baja California, Mexico. The regional networks Red Sismica del Noroeste de Mexico (RESNOM) that the Centro de Investigacion Cientifica y de Educacion Superior de Ensenada (CICESE) operates and the Southern California Seismic Network (SCSN) located the main event between the sierras El Mayor and Cucapah at the southeastern end of the Pescadores fault. Twenty-four hours after the origin time of the main event, a temporal seismic network was installed around the faults that ruptured during this event. We used body-wave arrival times recorded by the first 14 stations installed to obtain precise hypocentral locations of this important sequence of earthquakes. Most of the aftershocks located on 6 and 7 April are distributed near the traces of the Pescadores and Cucapah faults, particularly at the southern end of these faults. The spatial distribution of the epicenters indicates that these faults and possibly others may extend southeast of Sierra Cucapah where the faults were buried by sediments and were activated during the seismic sequence. In this area, north of the Sierra El Mayor, where no faults were previously mapped, the epicenters align in the northwest–southeast direction. Most hypocenters have depths less than 15 km, suggesting that an important portion of the seismic slip was shallow. Based on the distribution of focal depths of the aftershocks, we infer that the seismogenic zone must be between 5 and 10 km.

The Long-Lasting Aftershock Series of the 3 May 1887 Mw 7.5 Sonora Earthquake in the Mexican Basin and Range Province

We study local and regional body-wave arrival times from several seis- mic networks to better define the active regional fault pattern in the epicentral region of the 3 May 1887 Mw 7.5 Sonora, Mexico (southern Basin and Range Province) earthquake. We determine hypocenter coordinates of earthquakes that originated between 2003 and 2007 from arrival times recorded by the local network RESNES (Red Sismica del Noreste de Sonora) and stations of the Network of Autonomously Recording Seismographs (NARS)-Baja array. For events between April and December 2007, we also incorporated arrival times from USArray stations located within 150 km of the United States-Mexico border. We first obtained preliminary earthquake loca- tions with the Hypoinverse program (Klein, 2002) and then relocated these initial hypocenter coordinates with the source-specific station term (SSST) method (Lin and Shearer, 2005). Most relocated epicenters cluster in the upper crust near the faults that ruptured during the 1887 earthquake and can be interpreted to be part of its long- lasting series of aftershocks. The region of aftershock activity extends, along the same fault zone, 40-50 km south of the documented southern tip of the 1887 rupture and includes faults in the epicentral region of the 17 May 1913 (Imax VIII, MI 5.0-0.4) and 18 December 1923 (Imax IX, MI 5.7-0.4) Granados-Huasabas, Sonora, earthquakes, which themselves are likely to be aftershocks of the 1887 event. The long aftershock duration can be explained by the unusually large magnitude of the mainshock and by the low slip rates and long mainshock recurrence times of the faults that ruptured in 1887.

Depth of the Moho discontinuity beneath the Japanese islands estimated by traveltime analysis

The crustal structure beneath the Japanese islands, including depth distributions of the Conrad and Moho discontinuities, was estimated using a tomographic inversion of regional body wave arrival times. Depth distributions of the bottom of the surface layer, the Conrad, and the Moho were modeled with two‐dimensional B spline functions, while velocity distributions in layers were expressed by three‐dimensional B spline functions. The depth of the discontinuities and the velocity in the layers were estimated simultaneously by the least squares method. The velocity structure was sequentially estimated from shallower parts to deeper parts to avoid correlation between them. This sequential analysis provided improved depth resolution. The deepest region of the Moho discontinuity was located in central Honshu, reaching about 40 km. The Moho discontinuity was generally deep in the central part of the islands, whereas it was relatively shallow in the Kanto, southwestern Chubu, and Chugoku districts in Honshu and in northern Kyushu. Some of the shallow Moho regions would be related to graben formation due to tensile tectonic stress since the Miocene. The results were compared with those of seismic refraction surveys and receiver function analyses, and it was found that the obtained model was consistent with many of these studies.

An improved seismicity picture of the Southern Tyrrhenian area by the use of OBS and land-based networks: the TYDE experiment

The problem of large location uncertainties for seismicity occurring in the Southern Tyrrhenian Sea have been partially exceeded during the implementation of the long-term scientific mission of the TYrrhenian Deep sea Experiment (TYDE), which allowed the installation of 14 wide-band Ocean Bottom Seismometers (OBS) and Hydrophones (OBH) in the period December 2000 – May 2001 on the sea-bottom floor around the Aeolian and Ustica Islands. Local events recorded at landstations have been observed also on the seismograms of the Ocean Bottom Seismic Network (OBSN). Moreover, some hundreds of low magnitude events, undetected from the land networks, have been recorded. We combined the readings of body wave arrival times from OBS-OBH with those from landstations to localise seismic events. We focused our study on three clusters of events representative of the seismic activity of the area: (i) “deep” events, (ii) Ustica (iii) NE-Sicily. The analysis of the integrated data set of the seismicity off-shore and on-shore, obtained from the combined land-OBS seismic network (Ustica sequence and Deep events), has improved locations in terms of RMS residuals, azimuthal gap, epicentral and hypocentral errors. Moreover, further classes of events have been analysed: the first one includes some local events that could be located only by integrating single trigger readings from the few available land-stations with the OBSN-data; the second one comprises local events that have been detected only by the OBS-OBH stations. In particular, the last cluster underlines the importance of an OBSN in the Tyrrhenian deep basin to reveal its unknown intense micro-seismicity, permitting to better understand both the tectonic and geodynamic picture of the area.

Seismic location improvements from an OBS/H temporary network in Southern Tyrrhenian Sea

We present the first investigation performed on the seismicity of Southern Tyrrhenian Sea, off-shore Sicily with the contribution of data from broad-band ocean bottom seismometers and hydrophones (OBS/H). Offshore data were recorded during the TYrrhenian Deep sea Experiment (TYDE) from December 2000 to May 2001 in the Southern Tyrrhenian Sea. Hypocenter locations of a cluster of 53 seismic events occurred in March 2001 in north-eastern Sicily were estimated by the integration of land (permanent network) and offshore (temporary network) data and compared with locations estimated from land data only. The scatter of the cluster was evaluated by dispersion parameters. The off-shore data significantly reduced the scatter of the swarm hypocenters also restricting the depth range of the cluster. Moreover, space trends of the event distribution originally shown by the land data were only partially confirmed by the land-sea joint data. In order to assess the efficiency in terms of hypocenter mislocations in the subject area, of a land-sea integrated network with respect to a land-based network, we performed simulations by assuming a grid distribution of earthquakes and a recent local 3D velocity model, computing synthetic arrival times of body waves to the stations of both network configurations (integrated and land-based) perturbing the computed times and relocating earthquakes by inversion. The results of the synthetic tests demonstrated that the presence of sea bottom stations in the Tyrrhenian basin can reduce the mislocations of large magnitude and/or superficial earthquakes in the southernmost Calabria and Messina Strait and of low magnitude and/or deep earthquakes in north-eastern Sicily. The major accuracy of synthetic earthquake locations obtained including OBS/H data provides an additional support to the interpretation of the cluster occurred in March 2001 and to the opportunity of long-term installation of an off-shore network like TYDE in the study region.

Seismic tomographic imaging ofP- andS-waves velocity perturbations in the upper mantle beneath Iran

SUMMARY The inverse tomography method has been used to study the P- and S-waves velocity structure of the crust and upper mantle underneath Iran. The method, based on the principle of source‐receiver reciprocity, allows for tomographic studies of regions with sparse distribution of seismic stations if the region has sufficient seismicity. The arrival times of body waves from earthquakes in the study area as reported in the ISC catalogue (1964‐1996) at all available epicentral distances are used for calculation of residual arrival times. Prior to inversion we have relocated hypocentres based on a 1-D spherical earth’s model taking into account variable crustal thickness and surface topography. During the inversion seismic sources are further relocated simultaneously with the calculation of velocity perturbations. With a series of synthetic tests we demonstrate the power of the algorithm and the data to reconstruct introduced anomalies using the ray paths of the real data set and taking into account the measurement errors and outliers. The velocity anomalies show that the crust and upper mantle beneath the Iranian Plateau comprises a low velocity domain between the Arabian Plate and the Caspian Block. This is in agreement with global tomographic models, and also tectonic models, in which active Iranian plateau is trapped between the stable Turan plate in the north and the Arabian shield in the south. Our results show clear evidence of the mainly aseismic subduction of the oceanic crust of the Oman Sea underneath the Iranian Plateau. However, along the Zagros suture zone, the subduction pattern is more complex than at Makran where the collision of the two plates is highly seismic.

Contrasts in lithospheric structure within the Australian craton—insights from surface wave tomography

Contrasts in the seismic structure of the lithosphere within and between elements of the Australian Craton are imaged using surface wave tomography. New data from the WACRATON and TIGGER experiments are integrated with re-processed data from previous temporary deployments of broad-band seismometers and permanent seismic stations. The much improved path coverage in critical regions allows an interpretation of structures in the west of Australia, and a detailed comparison between different cratonic regions. Improvements to the waveform inversion procedure and a new multi-scale tomographic method increase the reliability of the tomographic images. In the shallowest part of the model (75 km) a region of lowered velocity is imaged beneath central Australia, and confirmed by the delayed arrival times of body waves for short paths. Within the cratonic lithosphere there is clearly structure at scale lengths of a few hundred kilometres; resolution tests indicate that path coverage within the continent is sufficient to reveal features of this size in the upper part of our model. In Western Australia, differences are seen beneath and within the Archaean cratons: at depths greater than 150 km faster velocities are imaged beneath the Yilgarn Craton than beneath the Pilbara Craton. In the complex North Australian Craton a fast wavespeed anomaly continuing to at least 250 km is observed below parts of the craton, suggesting the possibility of Archaean lithosphere underlying areas of dominantly Proterozoic surface geology.

Reference frequency of teleseismic body waves

Anelasticity of the Earth causes physical dispersion of seismic waves. The appropriate choice of reference frequency is important to determine arrival times of body waves, especially later arrival times, from broadband seismograms by fitting observed to synthetic waveforms. Our question is how to choose the reference frequency of physical dispersion, f0, consistent with the extensive data sets of hand‐picked body wave arrival times compiled through the long history of seismology. Setting the reference frequency to be the conventional 1 Hz, we measured S‐P differential travel times by waveform fitting. We found a systematic discrepancy between the measured and hand‐picked S‐P times that can be removed by changing f0 from 1 to 2 Hz. This change of f0 requires a correction in Q of the PREM model: (1) if Q is strictly frequency‐independent, we need a 15% increase in Q value or (2) if Q is fixed to the value given by PREM at a period of 200 s, we need to introduce a small frequency dependence in Q, where the dependence is expressed by the power law with an exponent of 0.04.

Imaging the mantle beneath Iceland using integrated seismological techniques

Using a combination of body wave and surface wave data sets to reveal the mantle plume and plume head, this study presents a tomographic image of the mantle structure beneath Iceland to 400 km depth. Data comes primarily from the PASSCAL‐HOTSPOT deployment of 30 broadband instruments over a period of 2 years, and is supplemented by data from the SIL and ICEMELT networks. Three sets of relative teleseismic body wave arrival times are generated through cross correlation: S and SKS arrivals at 0.03–0.1 Hz, and P and PKIKP arrivals at 0.03–0.1 and 0.8–2.0 Hz. Prior to inversion the crustal portion of the travel time anomalies is removed using the crustal model ICECRTb. This step has a significant effect on the mantle velocity variations imaged down to a depth of ∼250 km. Inversion of relative arrival times only provides information on lateral velocity variations. Surface waves are therefore used to provide absolute velocity information for the uppermost mantle beneath Iceland. The average wave number for the Love wave fundamental mode at 0.020 and 0.024 Hz is measured and used to invert for the average S velocity. Combination of the body wave and surface wave information reveals a predominantly horizontal low‐velocity anomaly extending from the Moho down to ∼250 km depth, interpreted as a plume head. Below the plume head a near‐cylindrical low‐velocity anomaly with a radius of ∼100 km and peak VP and VS anomalies of −2% and −4%, respectively, extends down to the maximum depth of resolution at 400 km. Within the plume head, in the uppermost mantle above the core of the plume, there is a relatively high velocity with a maximum VP and VS anomaly of +2%. This high‐velocity anomaly may be the result of the extreme degree of melt extraction necessary to generate the thick (46 km) crust in central Iceland. Comparison of the plume volumetric flux implied by our images, the crustal generation rate, and the degree of melting suggested by rare earth element inversions, suggests that (1) mantle material must be flowing horizontally away from the plume core faster than the overlying lithosphere and (2) the bulk of the plume material does not participate in melting beneath Iceland.

Optimal measurement of relative and absolute delay times by simulated annealing

Summary Conventional approaches to determine relative arrival times of body waves recorded by a local or a regional array rely on cross-correlations between pairs of traces. This problem is better posed as a non-linear inverse problem, which involves the minimization of a cost function that measures the difference between a reference unknown waveform that can be time delayed, and the observed data. The unknown parameters that are solved for, are the amplitude values at each time sample of the optimal waveform and its time delay defined for each trace. The problem, which has a large number of unknown parameters, is solved with simulated annealing. The algorithm is very efficient and can be used for a routine analysis of seismic data. The application of this method to several earthquakes recorded during different PASSCAL experiments demonstrates that it provides accurate and robust differential traveltime measurements even with very noisy data.