We have created a digital age grid of the ocean floor with a grid node interval of 6 arc min using a self-consistent set of global isochrons and associated plate reconstruction poles. The age at each grid node was determined by linear interpolation between adjacent isochrons in the direction of spreading. Ages for ocean floor between the oldest identified magnetic anomalies and continental crust were interpolated by estimating the ages of passive continental margin segments from geological data and published plate models. We have constructed an age grid with error estimates for each grid cell as a function of (1) the error of ocean floor ages identified from magnetic anomalies along ship tracks and the age of the corresponding grid cells in our age grid, (2) the distance of a given grid cell to the nearest magnetic anomaly identification, and (3) the gradient of the age grid: i.e., larger errors are associated with high age gradients at fracture zones or other age discontinuities. Future applications of this digital grid include studies of the thermal and elastic structure of the lithosphere, the heat loss of the Earth, ridge-push forces through time, asymmetry of spreading, and providing constraints for seismic tomography and mantle convection models.

Digital isochrons of the world's ocean floor

We present a new tomographic model for P- and S-velocity anomalies beneath Europe (30°N-55°N, 5°W-40°E), extending in depth up to 700 km and constrained by inversion of data from the International Seismological Center (ISC) catalogue. The algorithm uses the traveltimes from events located in the study area recorded by all available worldwide stations, as well as times from teleseismic events recorded by European stations. The events from the ISC catalogue have been relocated and combined into composite events. All the traveltimes were corrected for crustal structure using the reference model EuCRUST-07. The resulting velocity anomalies show similar large-scale patterns as observed in previous studies, but have a higher resolution, which allows detection of some features in more detail. For example, it is now possible to assess the depth extension of the small slow velocity body beneath the Eifel region and Eger graben. The P and the S model show a good consistency in the uppermost 200 km below most of the European area and in some parts even in the deeper layers (e.g. beneath the Apennines and the Hellenic arc). The new model provides clear images of some principal features, which were previously detected in a limited number of studies, while the comparison between P- and S-velocity anomalies provides novel constraints to address on their nature (e.g. the gap in the Adriatic plate subducted below the central-southern Apennines). In this paper, we pay special attention to testing the reliability of the results. The random noise effect is evaluated using a test with independent inversion of two data subsets (with odd/even events). The spatial resolution is estimated using different checkerboard tests. Furthermore, we present a synthetic model with realistic patterns, which reproduces after performing forward and inverse modelling the same shape and amplitudes of the anomalies as in the case of the real data inversion. In this case, the parameters of the model can be used to assess the amplitudes of P and S anomalies that is critical for evaluation of other petrophysical parameters (temperature, density, composition, etc.) in the upper mantle. © 2009 The Authors Journal compilation © 2009 RAS.

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P‐ and S‐velocity anomalies in the upper mantle beneath Europe from tomographic inversion of ISC data

LOTOS code for local earthquake tomographic inversion: benchmarks for testing tomographic algorithms

‘hyper-calcic’) phases found in the xenoliths and the contaminated zones in Merapi feldspars.The xenoliths also exhibit micro-vesicular textures that can be linked to the CO2 liberation process seen in the experiments.This study, therefore, provides well-constrained petrological insights into the problem of crustal interaction at Merapi and points toward the substantial impact of such interaction on the volatile budget of the volcano.

/pdf/magma-carbonate-interaction-processes-and-associated-co2-4v7buf4fqs.pdf

Magma-carbonate interaction processes and associated CO2 release at Merapi volcano, Indonesia: insights from experimental petrology

We present the LOTOS-07 code for performing local earthquake tomographic inversion, which is freely available (see the [Data and Resources][1] section for the Web site). The initial data for the code are the arrival times from local seismicity and coordinates of the stations. It does not require any information about the sources. The calculations start from absolute location of sources and estimates of an optimal 1D-velocity model. Then the sources are relocated simultaneously with the 3D-velocity distribution during iterative coupled tomographic inversions. The code allows results to be compared based on node or cell parameterizations. The synthetic dataset used for testing the code is based on source–receiver configurations from a real experiment in Costa Rica. The travel times for this dataset are computed by 3D tracing through a rather complicated synthetic model and are perturbed with realistic noise. We also present a series of synthetic datasets with unknown sources and velocity models (see the [Data and Resources][1] section for the Web site) that can be used as blind benchmarks for testing different tomographic algorithms. We encourage other users of tomography algorithms to join the program on creating benchmarks that can be used to check existing codes.

 [1]: #sec-15

/pdf/lotos-code-for-local-earthquake-tomographic-inversion-1gcwcxgag0.pdf

LOTOS Code for Local Earthquake Tomographic Inversion: Benchmarks for Testing Tomographic Algorithms

Here we present the results of local source tomographic inversion beneath central Java. The data set was collected by a temporary seismic network. More than 100 stations were operated for almost half a year. About 13,000 P and S arrival times from 292 events were used to obtain three-dimensional (3-D) Vp, Vs, and Vp/Vs models of the crust and the mantle wedge beneath central Java. Source location and determination of the 3-D velocity models were performed simultaneously based on a new iterative tomographic algorithm, LOTOS-06. Final event locations clearly image the shape of the subduction zone beneath central Java. The dipping angle of the slab increases gradually from almost horizontal to about 70°. A double seismic zone is observed in the slab between 80 and 150 km depth. The most striking feature of the resulting P and S models is a pronounced low-velocity anomaly in the crust, just north of the volcanic arc (Merapi-Lawu anomaly (MLA)). An algorithm for estimation of the amplitude value, which is presented in the paper, shows that the difference between the fore arc and MLA velocities at a depth of 10 km reaches 30% and 36% in P and S models, respectively. The value of the Vp/Vs ratio inside the MLA is more than 1.9. This shows a probable high content of fluids and partial melts within the crust. In the upper mantle we observe an inclined low-velocity anomaly which links the cluster of seismicity at 100 km depth with MLA. This anomaly might reflect ascending paths of fluids released from the slab. The reliability of all these patterns was tested thoroughly.

http://www.ivan-art.com/science/PAPERS/2007_java_jgr.pdf

P and S velocity structure of the crust and the upper mantle beneath central Java from local tomography inversion

Merapi volcano in Central Java is one of the most active volcanoes in the world. Located at the eruption zone between the Eurasian and the Indo-Australian Plate, Merapi shows andesitic character, which is currently demonstrated by active dome formation, accompanying dome-avalanches and pyroclastic flows up to distances of 2 to 8 km. This type of volcanism exhibits a high degree of explosivity. Even small volcanic events are a permanent danger to life within the densely populated area at the flanks of the volcano. A large eruption like >6000 years ago would immediately endanger more than one million of inhabitants in the region around Yogyakarta, about 35 km from the summit. Due to this reason Merapi has been classified as a high-risk volcano and included into the list of 16 Decade Volcanoes by IAVCEI.

M. A. Purbawinata

Papers

P and S velocity structure of the crust and the upper mantle beneath central Java from local tomography inversion

The Merapi-Project — Interdisciplinary Monitoring of a High-Risk Volcano as a Basis for an Early Warning System