SUMMARY
New empirical traveltime curves for the major seismic phases have been derived from the catalogues of the International Seismological Centre by relocating events by using P readings, depth phases and the iasp91 traveltimes, and then re-associating phase picks. A smoothed set of traveltime tables is extracted by a robust procedure which gives estimates of the variance of the traveltimes for each phase branch. This set of smoothed empirical times is then used to construct a range of radial velocity profiles, which are assessed against a number of different measures of the level of fit between the empirical times and the predictions of the models. These measures are constructed from weighted sums of L2 misfits for individual phases. The weights are chosen to provide a measure of the probable reliability of the picks for the different phases.

A preferred model, ak135, is proposed which gives a significantly better fit to a broad range of phases than is provided by the iasp91 and sp6 models. The differences in velocity between ak135 and these models are generally quite small except at the boundary of the inner core, where reduced velocity gradients are needed to achieve satisfactory performance for PKP differential time data.

The potential resolution of velocity structure has been assessed with the aid of a non-linear search procedure in which 5000 models have been generated in bounds about ak135. Msfit calculations are performed for each of the phases in the empirical traveltime sets, and the models are then sorted using different overall measures of misfit. The best 100 models for each criterion are displayed in a model density plot which indicates the consistency of the different models. The interaction of information from different phases can be analysed by comparing the different misfit measures. Structure in the mantle is well resolved except at the base, and ak135 provides a good representation of core velocities.

/pdf/constraints-on-seismic-velocities-in-the-earth-from-3nhrct811a.pdf

Constraints on seismic velocities in the Earth from traveltimes

[1] A global set of present plate boundaries on the Earth is presented in digital form. Most come from sources in the literature. A few boundaries are newly interpreted from topography, volcanism, and/or seismicity, taking into account relative plate velocities from magnetic anomalies, moment tensor solutions, and/or geodesy. In addition to the 14 large plates whose motion was described by the NUVEL-1A poles (Africa, Antarctica, Arabia, Australia, Caribbean, Cocos, Eurasia, India, Juan de Fuca, Nazca, North America, Pacific, Philippine Sea, South America), model PB2002 includes 38 small plates (Okhotsk, Amur, Yangtze, Okinawa, Sunda, Burma, Molucca Sea, Banda Sea, Timor, Birds Head, Maoke, Caroline, Mariana, North Bismarck, Manus, South Bismarck, Solomon Sea, Woodlark, New Hebrides, Conway Reef, Balmoral Reef, Futuna, Niuafo'ou, Tonga, Kermadec, Rivera, Galapagos, Easter, Juan Fernandez, Panama, North Andes, Altiplano, Shetland, Scotia, Sandwich, Aegean Sea, Anatolia, Somalia), for a total of 52 plates. No attempt is made to divide the Alps-Persia-Tibet mountain belt, the Philippine Islands, the Peruvian Andes, the Sierras Pampeanas, or the California-Nevada zone of dextral transtension into plates; instead, they are designated as “orogens” in which this plate model is not expected to be accurate. The cumulative-number/area distribution for this model follows a power law for plates with areas between 0.002 and 1 steradian. Departure from this scaling at the small-plate end suggests that future work is very likely to define more very small plates within the orogens. The model is presented in four digital files: a set of plate boundary segments; a set of plate outlines; a set of outlines of the orogens; and a table of characteristics of each digitization step along plate boundaries, including estimated relative velocity vector and classification into one of 7 types (continental convergence zone, continental transform fault, continental rift, oceanic spreading ridge, oceanic transform fault, oceanic convergent boundary, subduction zone). Total length, mean velocity, and total rate of area production/destruction are computed for each class; the global rate of area production and destruction is 0.108 m2/s, which is higher than in previous models because of the incorporation of back-arc spreading.

An updated digital model of plate boundaries

Seismic tomography based on P-wave travel times and improved earthquake locations provides further evidence for mantle-wide convective flow. The use of body waves makes it possible to resolve long, narrow structures in the lower mantle some of which can be followed to sites of present-day plate convergence at the Earth's surface. The transition from subduction-related linear structures in the mid-mantle to long-wavelength heterogeneity near the core-mantle boundary remains enigmatic, but at least some slab segments seem to sink to the bottom of the mantle.

/pdf/evidence-for-deep-mantle-circulation-from-global-tomography-3unqcjsa05.pdf

Evidence for deep mantle circulation from global tomography

Topographic data are combined with data on structure magmatism, seismicity, and paleomagnetism to support a simple kinematical model for the late Cenozoic evolution of the central Andes. The model interrelates Andean uplift, a changing geometry of the subducted Nazca plate, and a changing outline of the leading edge of the South American plate. It is argued that the late Cenozoic uplift of the Andes is a result of thermal thinning of the lithosphere and crustal thickening produced by crustal shortening.

/pdf/uplift-of-the-central-andean-plateau-and-bending-of-the-5kmhingx2v.pdf

Uplift of the Central Andean Plateau and bending of the Bolivian orocline

Recent global travel time tomography studies by Zhou [1996] and van der Hilst et al. [1997] have been performed with cell parameterizations of the order of those frequently used in regional tomography studies (i.e., with cell sizes of 1°–2°). These new global models constitute a considerable improvement over previous results that were obtained with rather coarse parameterizations (5° cells). The inferred structures are, however, of larger scale than is usually obtained in regional models, and it is not clear where and if individual cells are actually resolved. This study aims at resolving lateral heterogeneity on scales as small as 0.6° in the upper mantle and 1.2°–3° in the lower mantle. This allows for the adequate mapping of expected small-scale structures induced by, for example, lithosphere subduction, deep mantle upwellings, and mid-ocean ridges. There are three major contributions that allow for this advancement. First, we employ an irregular grid of nonoverlapping cells adapted to the heterogeneous sampling of the Earth's mantle by seismic waves [Spakman and Bijwaard, 1998]. Second, we exploit the global data set of Engdahl et al. [1998], which is a reprocessed version of the global data set of the International Seismological Centre. Their reprocessing included hypocenter redetermination and phase reidentification. Finally, we combine all data used (P, pP, and pwP phases) into nearly 5 million ray bundles with a limited spatial extent such that averaging over large mantle volumes is prevented while the signal-to-noise ratio is improved. In the approximate solution of the huge inverse problem we obtain a variance reduction of 57.1%. Synthetic sensitivity tests indicate horizontal resolution on the scale of the smallest cells (0.6° or 1.2°) in the shallow parts of subduction zones decreasing to approximately 2°–3° resolution in well-sampled regions in the lower mantle. Vertical resolution can be worse (up to several hundreds of kilometers) in subduction zones with rays predominantly pointing along dip. Important features of the solution are as follows: 100–200 km thick high-velocity slabs beneath all major subduction zones, sometimes flattening in the transition zone and sometimes directly penetrating into the lower mantle; large high-velocity anomalies in the lower mantle that have been attributed to subduction of the Tethys ocean and the Farallon plate; and low-velocity anomalies continuing across the 660 km discontinuity to hotspots at the surface under Iceland, east Africa, the Canary Islands, Yellowstone, and the Society Islands. Our findings corroborate that the 660 km boundary may resist but not prevent (present day) large-scale mass transfer from upper to lower mantle or vice versa. This observation confirms the results of previous, global mantle studies that employed coarser parameterizations.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/98JB02467

Closing the gap between regional and global travel time tomography

We report a “slow thrust slip event” that occurred beneath the Bungo Channel region, southwestern Japan. On Oct. and Dec., 1996, two Hyuganada earthquakes (both Mw = 6.7), followed by afterslips, occurred. In addition, a crustal movement characterized by an extremely slow rise was observed around the Bungo Channel, about 200 km north from the epicenters, and continued for about 300 days long. Assuming a slow slip on the plate boundary, we estimate its duration and surface displacements from GPS time series data by curve-fitting, and then, determine the fault slip distribution. We found that a slow slip without any earthquakes continued for nearly one year and released the seismic moment comparable to that of the Hyuganada earthquakes. Occurrence of the slow thrust slip event suggests that this kind of event may be a characteristic mode of stress release at a transition region of interplate coupling.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/1999GL010999

A slow thrust slip event following the two 1996 Hyuganada Earthquakes beneath the Bungo Channel, southwest Japan

Three-dimensional seismic structure beneath southwest Japan: The subducting Philippine Sea Plate

A numerical simulation of earthquake cycles along the Nankai Trough in southwest Japan: lateral variation in frictional property due to the slab geometry controls the nucleation position

A large-scale three-dimensional seismic velocity structure down to 650 km under the Japan Islands and the Sea of Japan has been investigated to clarify the subduction of lithospheric plates by applying the inversion method of Aki and others. About 3500 data of P-wave travel time residuals observed at 330 Japanese and foreign stations from 20 intermediate and deep-focus earthquakes around Japan were used to determine the velocity anomalies in a number of blocks and source corrections simultaneously. This analysis has well revealed lateral heterogeneities in the upper mantle above 450km. A high velocity zone corresponding to the descending Pacific plate has been clearly identified, particularly in the north Honshu arc, indicating good correlations with the horizontal and vertical distributions of a number of earthquakes around there. It was found that intermediate and deep-focus earthquakes take place mainly in the upper portions of the descending plate with a thickness of about 100km. The highest velocity anomalies are +5-6%(50-250km), +3%(350km), +2%(450km) within the plate, and about -2% down to 350km over a region on the continental side of the descending plate.

A large-scale three-dimensional seismic structure under the japan islands and the sea of japan

We simultaneously estimate 2.5 years of afterslip and viscoelastic relaxation, as well as coseismic slip, for the 2011 Tohoku-oki earthquake. Displacements at inland GPS and seafloor GPS/Acoustic stations are inverted using viscoelastic Green's functions for a model with an upper elastic layer and lower viscoelastic substrate. The result shows that afterslip is isolated from the rupture area and possibly asperities of historical earthquakes and has almost decayed by 10 September 2013, 2.5 years after the main shock. The inversion result also suggests that observed landward postseismic displacements at the seafloor GPS/Acoustic stations are caused by the viscoelastic relaxation, whereas trenchward displacements at inland stations are mainly an elastic response to afterslip.

/pdf/afterslip-and-viscoelastic-relaxation-following-the-2011-43d53pujth.pdf

Kazuro Hirahara

Papers

A slow thrust slip event following the two 1996 Hyuganada Earthquakes beneath the Bungo Channel, southwest Japan

Three-dimensional seismic structure beneath southwest Japan: The subducting Philippine Sea Plate

A numerical simulation of earthquake cycles along the Nankai Trough in southwest Japan: lateral variation in frictional property due to the slab geometry controls the nucleation position

A large-scale three-dimensional seismic structure under the japan islands and the sea of japan

Afterslip and viscoelastic relaxation following the 2011 Tohoku-oki earthquake (Mw9.0) inferred from inland GPS and seafloor GPS/Acoustic data