Retrieval of Interstation Local Body Waves From Teleseismic Coda Correlations

We retrieve the local P wave empirical Green's functions between the elements of five different regional arrays across the globe by cross‐correlating and bin stacking the teleseismic earthquake coda waves recorded at each array. The stack is made using the coda of P and S wave phases for events in the distance range from 40° to 50° from the center of the array. With a sequence of time windows along the coda the various body wave arrivals can be tracked, using record sections constructed by binning the stacked interstation correlograms in less than 1‐km distance increments. The correlation of the coda part of each principal seismic phase produces highly coherent interstation arrivals for different analysis windows. Such arrivals can be reproduced by just stacking 100 arrivals from a pool of more than a thousand events, showing the stability of the observed Green's functions. Modeling for the structure beneath the Warramunga array in the Northern Territory, Australia, demonstrates that these arrivals correspond to multiply reflected arrivals from layers at different depths. The recovery of high‐frequency interstation body waves from the teleseismic earthquake coda opens the prospect of conducting local high‐resolution seismic imaging with teleseismic energy

AusMoho: the variation of Moho depth in Australia

Since 2004 more than 7000 km of full-crustal reflection profiles have been collected across Australia to give a total of more than 11000 km, providing valuable new constraints on crustal structure. A further set of hitherto unexploited results comes fro

Stacking autocorrelograms to map Moho depth with high spatial resolution in southeastern Australia

Current estimates of Moho depth in southeastern Australia are based on sparse sampling. The results are augmented with 180 new Moho estimates constructed from spatial stacks of crustal P wave reflectivity derived from autocorrelograms at over 750 stations. The spatial stacks of reflectivity are constructed using a Gaussian with half width 0.5°. Picks of the base of crustal reflectivity are made with the aid of the previous Moho model, based on the sparser data, and knowledge of the variation in the character of the crust-mantle boundary across the region. Good ties can be made to previous results from deep reflection profiling. The new information fills in many holes in coverage and provides a Moho map with closer ties to geological provinces. The procedure exploits the continuous records at the stations and just the vertical components and so can be applied to older data for which receiver function techniques cannot be used

Crustal Imaging With Bayesian Inversion of Teleseismic P Wave Coda Autocorrelation

The autocorrelation of the seismic transmission response of a layered medium(autocorrelogram), in the presence of a free surface, corresponds to the reflection response. Despite manystudies on the imaging of local structures through retrieval and forward modeling of stackedautocorrelograms, there is limited work on the inversion of these data. In this study, we demonstrate thatthe probabilistic inversion of autocorrelograms is efficientand can be used as an alternative imaging toolwhen other approaches are not applicable. Here, we calculate autocorrelograms of teleseismicPwave codarecorded on more than 1,200 permanent and temporary seismic stations across Australia and utilize aBayesian framework to invert these data for crustal imaging. The results show patterns of structuresconsistent with those seen in previous crustal models constructed from receiver function, seismicreflection, and refraction methods. The new approach can therefore image large-scale crustal structurescomparable to those from other seismological methods

In situ lower crustal accretion by melt sill injection revealed by seismic layering

Oceanic crust is formed at mid-ocean spreading centres by a combination of magmatic, tectonic and hydrothermal processes. The crust formed by magmatic process consists of an upper crust generally composed of basaltic dikes and lava flows and a lower crust presumed to mainly contain homogeneous gabbro whereas that by tectonic process can be very heterogeneous and may even contain mantle rocks. Although the formation and evolution of the upper crust are well known from geophysical and drilling results, those for the lower crust remain a matter of debate. Using a full waveform inversion method applied to wide-angle seismic data, here we report the presence of layering in the lower oceanic crust formed at the slow spreading Mid-Atlantic Ridge, ~7-12 Ma in age, revealing that the lower crust is formed mainly by in situ cooling and crystallisation of melt sills at different depths by the injection of magma from the mantle. These layers are 400-600 m thick with alternate high and low velocities, with ± 100-200 m/s velocity variation, and cover over a million-year old crust, suggesting that the crustal accretion by melt sill intrusions beneath the ridge axis is a stable process. We also find that the upper crust is ~400 m thinner than that from conventional travel-time analysis. Taken together, these discoveries suggest that the magmatism plays more important roles in the crustal accretion process at slow spreading ridges than previously realised, and that in-situ lower crustal accretion is the main process for the formation of lower oceanic crust

Multiscale full waveform inversion

We develop and apply a full waveform inversion method that incorporates seismic data on a wide range of spatio-temporal scales, thereby constraining the details of both crustal and upper-mantle structure. This is intended to further our understanding of crust-mantle interactions that shape the nature of plate tectonics, and to be a step towards improved tomographic models of strongly scale-dependent earth properties, such as attenuation and anisotropy. The inversion for detailed regional earth structure consistently embedded within a large-scale model requires locally refined numerical meshes that allow us to (1) model regional wave propagation at high frequencies, and (2) capture the inferred fine-scale heterogeneities. The smallest local grid spacing sets the upper bound of the largest possible time step used to iteratively advance the seismic wave field. This limitation leads to extreme computational costs in the presence of fine-scale structure, and it inhibits the construction of full waveform tomographic models that describe earth structure on multiple scales. To reduce computational requirements to a feasible level, we design a multigrid approach based on the decomposition of a multiscale earth model with widely varying grid spacings into a family of single-scale models where the grid spacing is approximately uniform. Each of the single-scale models contains a tractable number of grid points, which ensures computational efficiency. The multi-to-single-scale decomposition is the foundation of iterative, gradient-based optimization schemes that simultaneously and consistently invert data on all scales for one multi-scale model. We demonstrate the applicability of our method in a full waveform inversion for Eurasia, with a special focus on Anatolia where coverage is particularly dense. Continental-scale structure is constrained by complete seismic waveforms in the 30-200s period range. In addition to the well-known structural elements of the Eurasian mantle, our model reveals a variety of subtle features, such as the Armorican Massif, the Rhine Graben and the Massif Central. Anatolia is covered by waveforms with 8-200s period, meaning that the details of both crustal and mantle structure are resolved consistently. The final model contains numerous previously undiscovered structures, including the extension-related updoming of lower-crustal material beneath the Menderes Massif in western Anatolia. Furthermore, the final model for the Anatolian region confirms estimates of crustal depth from receiver function analysis, and it accurately explains cross-correlations of ambient seismic noise at 10s period that have not been used in the tomographic inversion. This provides strong independent evidence that detailed 3-D structure is well resolve

Imaging of upper crustal structure beneath East Java-Bali, Indonesia with ambient noise tomography

The complex geological structures in East Java and Bali provide important opportunities for natural resource exploitation, but also harbor perils associated with natural disasters. Such a condition makes the East Java region an important area for exploration of the subsurface seismic wave velocity structure, especially in its upper crust. We employed the ambient noise tomography method to image the upper crustal structure under this study area. We used seismic data recorded at 24 seismographs of BMKG spread over East Java and Bali. In addition, we installed 28 portable seismographs in East Java from April 2013 to January 2014 for 2–8 weeks, and we installed an additional 28 seismographs simultaneously throughout East Java from August 2015 to April 2016. We constructed inter-station Rayleigh wave Green’s functions through cross-correlations of the vertical component of seismic noise recordings at 1500 pairs of stations. We used the Neighborhood Algorithm to construct depth profiles of shear wave velocity (Vs). The main result obtained from this study is the thickness of sediment cover. East Java’s southern mountain zone is dominated by higher Vs, the Kendeng basin in the center is dominated by very low Vs, and the Rembang zone (to the North of Kendeng zone) is associated with medium Vs. The existence of structures with oil and gas potential in the Kendeng and Rembang zones can be identified by low Vs.The data used in this study were taken from BMKG (Free) and portable seismometers installed independently by the help of funding from the Australian Department of Foreign Affairs and Trade Grant 71982

Upper crustal structure of central Java, Indonesia, from transdimensional seismic ambient noise tomography

Delineating the crustal structure of central Java is crucial for understanding its complex tectonic setting. However, seismic imaging of the strong heterogeneity typical of such a tectonically active region can be challenging, particularly in the upper crust where velocity contrasts are strongest and steep bodywave ray paths provide poor resolution. To overcome these difficulties, we apply the technique of ambient noise tomography (ANT) to data collected during the Merapi Amphibious Experiment (MERAMEX), which covered central Java with a temporary deployment of over 120 seismometers during 2004 May-October. More than 5000 Rayleigh wave Green's functions were extracted by cross-correlating the noise simultaneously recorded at available station pairs. We applied a fully non-linear 2-D Bayesian probabilistic inversion technique to the retrieved traveltimes. Features in the derived tomographic images correlate well with previous studies, and some shallow structures that were not evident in previous studies are clearly imaged with ANT. The Kendeng Basin and several active volcanoes appear with very low group velocities, and anomalies with relatively high velocities can be interpreted in terms of crustal sutures and/or surface geological features

Seismic microzonation of Bandung basin from microtremor horizontal-to-vertical spectral ratios (HVSR)

Bandung is located on a thick sedimentary basin, which is mainly composed of volcanic rocks and the depositional remnants of an ancient lake. The high population density and vital infrastructure, surrounding by potential sources of earthquakes make Bandung vulnerable to earthquake impact. To study the seismic response of Bandung, a microzonation study is needed so as to facilitate disaster risk assessment and mitigation. The parameters which are mapped for the purpose of microzonation are the distribution of the dominant frequency (F0), the amplification factor (Am) and seismic susceptibility (Kg). We use the HVSR analysis method with microtremor/ambient seismic noise data. The microtremor data were taken from 58 measurement points from the Bandung Seismic Experiment network run from March to October 2014 in the Bandung basin and surrounding areas. The results show that the dominant peak frequency in the studied area ranges from 0.195 Hz to 7.016 Hz, amplification (A) spans from 1.6 to 11.3, and the value of the seismic susceptibility index (Kg) ranges from 0.6 to 245.6. The spatial distribution of seismic susceptibility index indicates that almost all areas of the Bandung basin have high susceptibility to earthquake hazard. The highest susceptibility is found in the eastern part of the Bandung basin that includes Bojongsoang, Rancaekek, Ciparay, Rancasari, and Majalaya, while the lower susceptibility zones are scattered in the hills and mountains around the basin of Bandung