Verification And Application Of Surface Wave From Ambient Noise

Ambient seismic noise research is coupled with the signal research in seismology, both of which can date back to the very beginning of the modern seismology. About one decade ago, it was found that the time domain Empirical Green’s Function (EGF) between two seismic stations can be extracted from cross-correlation of ambient seismic noise. The researches based on ambient noise have exploded since then. The ambient seismic Noise Cross-Correlation Function (NCF) has been widely used to do tomogra-phy (known as ANT),velocity changes monitoring and earthquake location calibration, etc. Almost all of these reseaches are based on the assumption that the accuracte EGF can be extracted, and the travel time of the surface wave from the NCF is extracted in a single station pairs. However, the accuracy of the travel time of surface wave is worthy of being tested, before we do any further applications using NCF.We design an experiment to test the accuracy of short period (10-30 s) surface wave from NCF based on a ground truth event occurred in Sichuan, China. We deployed a seismometer at the epicenter of the earthquake in order to record continuous ambient seismic noise. The NCFs are calculated between this "reference" station with remote stations. The accuracy of the NCFs are tested by comparing both the waveforms and dispersions with that from earthquake records. We find that the phase velocity of short period surface wave from ambient noise is consistent with that from earthquake with the distance larger than 1000 km with difference of about 0.3%. However, in the shorter interstation distance (<500 km), some of the paths show obvious mismatches with dif-ference of (3%). It probably indicates that the ambient seismic noise is not diffuse enough for short interstation distance. More verification works are needed to testify the mismatch at short interstation distance.Based on another ground truth earthquake occurred in Italy, combined with global and regional surface wave phase velocity measurement we testify the accuracy of long period surface wave from ambient noise. The long period (50-250 s) phase velocities from ambient noise are consistent with that from earthquakes, with the mean difference around zero, and standard deviation of 1%. The verification wok demonstrate that the long period phase velocity from surface wave of ambient noise is feasible and accurate enough to do tomography.We review three different methods on calibrating the location of earthquake using the ambient noise. To use ambient noise cross-correlation function to locate the earth-quake, we need at least one seismic station close to the earthquake; it work well in the place where the stations are sparsely distributed. Using a seismic array in Austrilia we testify the location method. It is found that the accuracy is up to 2 km with good distri-bution of remote stations, and the mislocation almost has nothing to do with the distance between the reference station and target station. However, the depth of the earthquake is ignored during location calibration; therefore in the future we should take the source depth into account.The ambient noise has been widely used to study the earth structure, however most of them using only short period surface waves (<50 s). Tomography using surface wave form ambient noise (ANT) have many advantages over traditional tomography method based on earthquakes, for example, the initial phase for CCF is clear; the ray coverage relies on station distributions not earthquakes etc. Since both the short and long period surface wave from ambient noise have been testified, we use broadband surface wave from ambient noise to construct the 3D shear wave velocity structure of the US. We first calculate the cross-correlation among all the USAarray stations within US continent; af-ter that we extract very broadband (10-150 s) Rayleigh wave phase velocities. Then, we invert for the phase velocity maps within US using 2D finite frequency sensitivity kernels based on Born approximation. Based on the phase velocity maps, We invert for the 1D shear wave velocity profiles at each grid point using Markov chain Monte Carlo (MCMC) method. Finally a new 3D isotropic shear wave velocity model is constructed by assembling all of the 1D profiles. The new model covers the lithosphere and asthen-sphere structure, which extends deeper than most of previous ANT studies. Our new model is consistent with previous models and geological features. This new 3D model can be used as the initial model of future regional detail model studies. However, due to the drawbacks of the imaging based on surface waves, more independent observa-tions (e.g. body waves and Z/H ratio) should be combined together to study the earth’s structure with better constraints.