Frequency-time Analysis Technique Research Articles

Western Java Ambient Noise Tomography: A Preliminary Result

Western Java, Indonesia, has at least three important active faults: the Cimandiri, Lembang, and Baribis Faults, which pose a great danger for the cities near them. It is crucial to have a better understanding of shallow crustal structure to delineate active faults and deep basins in order to support seismic hazard and disaster mitigation efforts in Indonesia. In this study, we perform ambient seismic noise tomography which can give better resolution of the shallow structure beneath western Java. We have deployed a seismometer network in the western to central Java region through a research collaboration program between the Bandung Institute of Technology (ITB) and the Australian National University (ANU). We deployed 70 seismometer stations from June to January 2017 to acquire ambient seismic noise data. As the first stage of the data processing, we will focus on conducting single data preparation and cross-correlation to retrieve an estimate of Green’s functions between station pairs. We also use the frequency-time analysis technique to obtain dispersion curves to measure the interstation group velocity. The group velocity is use as an input in tomography inversion. Our preliminary results show low velocity anomalies associated with sedimentary basins and a high velocity anomaly associated with the southern mountains.

VS models in the historical centre of Naples (Southern Italy) from noise cross-correlation

Ambient noise measurements have been performed in the historical centre of Naples (Southern Italy) along 40 paths (inter-receiver distances) with a length ranging from ~300 m to ~1550 m, for recording time variable from few hours to several months.Synchronous noise recordings at two receivers, 1-bit normalized, have been cross-correlated in order to estimate the empirical Green's function (surface wave train) of the medium between them. The fundamental-mode Raleigh waves have been extracted at periods ranging from 0.15 s to 1 s by using the frequency-time analysis technique. Average group velocity dispersion curves with error bar have been evaluated along 28 paths and inverted with a non-linear approach to get VS profiles extending to depths of ~50–300 m.Comparison of VS with some deep drillings and two down-hole measurements lets to argue important findings. First of all there is a good agreement with shallow (about 35 m) point down-hole measurements. The historical centre of Naples results to be characterized by a cover of man-made ground and pyroclastics of recent activity with thickness of 15–30 m and VS lower than 400 m/s. The products of the Neapolitan Yellow Tuff eruption have a thickness of about 80 m and are characterized by VS of 500–700 m/s in the shallower, altered and fractured part and of 700–900 m/s in the deeper, compact part. Below them, separated by a thin layer of the Campanian Ignimbrite that cannot be resolved by the data, ancient tuffs or marine sands are present which are locally responsible of velocity decrease. VS of 1100–1300 m/s, ascribable to intercalations of volcanic and sedimentary (marls, conglomerates and marly sands) rocks, are detected at ~200 m depth in the ancient part of the historical centre, rising to ~100 m depth, in the north-east of it.This study also gives an important contribution to microzoning studies. The top of the lithoid Neapolitan Yellow Tuff cannot be considered as the top of the seismic bedrock, being characterized by 500 m/s < VS < 800 m/s. The seismic bedrock with VS ≥ 800 m/s is deeper, sometimes at depths >100 m.

3D shallow structures in the Baogutu area, Karamay, determined by eikonal tomography of short-period ambient noise surface waves

Eikonal tomography based on ambient noise data is one of the most effective methods to reveal shallow earth structures. By tracking surface wave phase fronts, constructing travel time surfaces, and computing the gradients of travel time surfaces to generate phase velocity maps, eikonal tomography avoids the ray tracing and matrix construction and inversion in the traditional surface wave tomography methods.In this study, we collect continuous ambient noise data recorded by a dense seismic array in Karamay, Xinjiang to construct a 3D model of shallow structures using eikonal tomography. The seismic array consists of 35 stations with shortest interstation distance close to 1km. 890 empirical surface wave Green's functions (EGFs) between each station pair are retrieved by cross-correlating one or two months of continuous ambient noise data. From these EGFs, surface wave travel times in the frequency range of 1.8 to 4.0 Hz are measured by a frequency–time analysis technique (FTAN). Then, eikonal tomography is adopted to construct Rayleigh wave phase velocity maps and estimate the phase velocity uncertainties. Finally, we invert the obtained phase velocity dispersion curves for 1D shear velocity profiles and then assemble these 1D profiles to construct a 3D shear velocity model. Major velocity features of our 3D model are correlated well with the known geological features. A shallow east–west velocity discontinuity is observed, which clearly reflects the lithological change between Baogutu formation (C1b) and Xibeikulasi formation (C1x) of lower Carboniferous system. Low shear velocities are observed beneath the location of porphyry copper deposit (V), possibly related to stockwork fracture and hydrothermal brecciation developed during the intrusion of deep magma in forming the deposit.

Ambient noise Rayleigh wave tomography of New Zealand

SUMMARY We present the first New Zealand-wide study of surface wave dispersion, using ambient noise observed at 42 broad-band stations in the national seismic network (GeoNet) and the Global Seismic Network (GSN). Year-long vertical-component time-series recorded between 2005 April 1 and 2006 March 31 have been correlated with one another to yield estimated fundamental mode Rayleigh wave Green's functions. We filter these Green's functions to compute Rayleigh wave group dispersion curves at periods of 5–50 s, using a phase-matched filter, frequency–time analysis technique. The uncertainties of the measurements are estimated based on the temporal variation of the dispersion curves revealed by 12 overlapping 3-month stacks. After selecting the highest quality dispersion curve measurements, we compute group velocity maps from 7 to 25 s period. These maps, and 1-D shear wave velocity models at four selected locations, exhibit clear correlations with major geological structures, including the Taranaki and Canterbury Basins, the Hikurangi accretionary prism, and previously reported basement terrane boundaries.

Ambient noise Rayleigh wave tomography across Europe

We extend ambient noise surface wave tomography both in bandwidth (10–50 s period) and in geographical extent (across much of Europe) compared with previous applications. 12 months of ambient noise data from 2004 are analysed. The data are recorded at about 125 broad-band Seismic stations from the Global Seismic Network and the Orfeus Virtual European Broad-band seismic Network. Cross-correlations are computed in daily segments, stacked over 1 yr, and Rayleigh wave group dispersion curves from 8 to 50 s period are measured using a phase-matched filter, frequency time analysis technique. We estimate measurement uncertainties using the seasonal variation of the dispersion curves revealed in 3 month stacks. On average, uncertainties in group delays increase with period from ∼3 to ∼7 s from periods of 10 to 50 s, respectively. Group speed maps at periods from 10 to 50 s are estimated. The resulting path coverage is denser and displays a more uniform azimuthal distribution than from earthquake-emitted surface waves. The fit of the group speed maps to the ambient noise data is significantly improved below 30 s compared to the fit achieved with earthquake data. Average resolution is estimated to be about 100 km at 10 s period, but degrades with increasing period and toward the periphery of the study region. The resulting ambient noise group speed maps demonstrate significant agreement with known geological and tectonic features. In particular, the signatures of sedimentary basins and crustal thickness are revealed clearly in the maps. These results are evidence that surface wave tomography based on cross-correlations of long time-series of ambient noise data can be achieved over a broad period band on nearly a continental scale and yield higher resolution and more reliable group speed maps than based on traditional earthquake-based measurements.

3D S-Wave Velocity Structure of the Crust and Upper Mantle

In this paper, 238 Rayleigh wave path data are selected and processed by the matched-filtering frequency-time analysis technique and the grid dispersion inversion method to obtain the 3D S-wave velocity structure of China mainland and its adjacent sea regions. The results show that the velocity structure relates to geotectonic division, Bouguer gravity anomaly is basically controlled by the relief of Moho discontinuity, the buried depth of LVL in upper mantle concerns the surface heat flow deeply. In this paper, authors indicate the main characteristics of the velocity structure in tectonic active and stable regions.

Three-dimensional structure of the crust and upper mantle beneath the south China area from long period surface waves

In the present study the long period surface wave records of 238 wave-paths from 79 earthquakes within China and its adjacent regions received by 30 seismic network stations are measured by using the improved match-filtering frequency-time analysis technique and the grid dispersion inversion method to obtain the rayleigh pure-path dispersion values for 147 slant grids of 4° × 4° in this area, then a three-dimensional shear wave velocity model of the crust and upper mantle beneath south China area to a depth of 170 km is inversed.

Distribution characteristics of Rayleigh wave group velocity in China mainland and its adjacent sea areas

Making use of 75 earthquake data of China mainland and adjacent areas recorded by long period seismometers of 27 stations of China and 3 stations of WWSSN and processed by match-filtering frequency-time analysis technique and grid dispersion inversion, the authors obtain pure-path dispersion curves of Rayleigh surface wave in 147 grids in this paper. The distribution characteristics of group velocity are as follows:the China mainland and its adjacent sea areas are divided into two parts of east and west by South-North belt and are separated in blocks of south and north with boundaries of 44°–44°N,28°N (in the west part) and 28°–32°N (in the east part), the first and third boundaries may extend eastward into sea regions, in the west side of island arc and continental shelves, appears belt-form distribution of group velocity striping NE direction. These distribution characteristics correspond to zonation of tectonic structure. In addition, the results also indicate that the differences of group velocity dispersion curves exist between tectonic elements of next order. It is revealable for the differences of group velocity among different tectonic elements until periodT = 113s (corresponding depth is about 170km).

Three‐dimensional crust and upper mantle structure of the Eurasian Continent

Group velocities of fundamental mode Rayleigh waves of 109 wave paths within Eurasia have been measured by an improved frequency‐time analysis technique. Stochastic inversion theory is applied to the mixed‐path measurements to extract pure path group velocities of 10°×10° grid elements that form the subdivisions of Eurasia. Then a three‐dimensional crust and upper mantle structure of the Eurasian continent to a depth of 300 km is constructed. The average crustal thickness of Eurasia is found to be about 40 km. Abnormally thick crust from 50 to 65 km is obtained in areas of Tien Shan, the Altai mountains, Afghanistan, Pamir, Tibet, and Burma. Among them, Tibet has the least dense crust and highest elevation.This implies that at least part of the elevation of Tibet is due to isostatic compensation. The upper mantle seismic velocity of the tectonically active regions of Asia is considerably lower than that of the European and Siberian cratons. If high velocity implies high rigidity, this explains that large‐scale tectonic deformations of Eurasia occur in regions with weak upper mantle that deforms more plastically. Regions with stronger upper mantle remain relatively undeformed. This is especially evident for the Tertiary tectonic deformation as a consequence of the collision of India with Eurasia.