Determination Of Source Depth Research Articles

Using surface wave amplitude spectra to estimate the source depth of Sichuan Jiuzhaigou <italic>M</italic>7.0 earthquake

The accurate source depth of earthquake is important for understanding plate-tectonic processes and structure of seismic faults, identifying natural earthquake and non-natural earthquake. The amplitude formula of seismic surface wave shows that the amplitude of surface wave is closely related to the focal mechanism, the azimuth from epicenter to station and the source depth. When the focal mechanism and the observation position are determined, the excitation energy of surface wave is sensitive to source depth. The surface wave amplitude spectrum can be applied to determine the focal depth. The amplitude energy of shallow earthquake shows the “notch” phenomenon in band of frequency between 0.01 and 0.08 Hz in amplitude spectra, i.e. the attenuation of amplitude energy. The position of the frequency-dependent “notch” is related to the source depth. In this study, the relationship between the position of the “notch” and the source depth is analyzed by theoretical seismogram. We obtain the amplitude spectrums of the far-field at different source depths and then locate the source depth by means of comparing with observed seismic waveforms. By the amplitude spectrum of theoretical surface wave, small changes in the source depth can cause the variation of the amplitude spectrum so that the source depth of shallow earthquake can be accurately determined. The focal mechanism is coupled with the source depth. In order to analyze the influence of the disturbance of source mechanism to source depth determination, the focal mechanism of Jiuzhaigou earthquake was inversed, with disturbance of 10 degrees to strike, dip and rake. The results show only rather small changes in locating depth. In order to eliminate the influence of low frequency noise, shear wave and surface wave of short-period on amplitude spectra, this study adopts the AK135 velocity model to remove the dispersion. In this study, the seismic source depth of the Jiuzhaigou M 7.0 earthquake of 8 August 2017 in Sichuan province, China, was determined at 8.2 km by the energy attenuation characteristics of Rayleigh amplitude spectrum.

The downward continuation of the tilt angle

ABSTRACTThe tilt angle filter can be used both to enhance images of aeromagnetic data and as the basis of semi‐automatic interpretation methods. For vertically magnetised and vertically dipping contacts, rescaling by factor is equivalent to computing from sources with all depths decreased by a factor . This downward continuation is much less sensitive to noise than Fourier transform‐based downward continuation techniques, and because it reduces interference, it allows more accurate source depth determination from methods such as the tilt‐depth method. The method can be applied irrespective of the source and magnetisation type if the tilt angle of the analytic signal amplitude is used.

Source Depth Determination from Aeromagnetic Data of Ilesha, Southwest Nigeria, Using the Peters’ Half Slope Method

The slope method is one of the earliest magnetic depth estimate techniques which can determine the depth to the top of magnetic basement. Reduction-to-pole was applied to the aeromagnetic data of Ilesha southwest, Nigeria with sheet number 243. The data which was originally partitioned into different profiles was later processed to generate reduction-to-pole graph. The contour map of the study area was partitioned into four quadrants, with the highest magnetic anomaly occurring at the third quadrant (southwest region), while the northwest region has the lowest magnetic anomaly. The depth to top for very thin body, intermediate body and very thick body was determined using the Peter’s half slope method. These values range between 2.40 km and 10.60 km for the very thin body, 1.8 km to 7.93 km for the intermediate body and 1.41 km to 6.35 km for the very thick body. These results revealed the extent of the depth of the source of the magnetic anomaly that was produced by the aeromagnetic data of the study area.

The depth of the October 1981 off Chile outer-rise earthquake (Ms = 7.2) estimated by a comparison of several waveform inversion methods

Abstract The determination of source depth of the October 1981 (Ms = 7.2) outer-rise earthquake off the Chilean coast has important implications for the stress state of the subducting plate. We analyze the Chile earthquake using three different waveform inversion methods in order to check the consistency of these methods and to estimate the source depth. The methods we used are: These methods give consistent results which suggest that the centroid depth and depth extent of the Chile earthquake lies between 20 to 45 km, measured from the sea surface. This conclusion is contradictory to those obtained by the previous workers who prefer a shallow (<25 km) source depth. We found that the GDSN long-period data alone cannot constrain the source depth well. The shape of deconvolved source-time function is found to be strongly dependent on the assumption of point and distributed sources and the crustal structure used in the deconvolution. The source depth obtained in this study shows that the 1981 Chile outer-rise earthquake possibly occurred because of the compression associated with the bending of the subducting plate.

Source depth determination using multi-modal Rayleigh spectral ratios and linear discriminant analysis: A study of the reservoir-induced seismic sequence at Lake Kariba, Africa (September 1963 to August 1974)

abstract The source depths of 12 reservoir-induced seismic events at Lake Kariba were determined using the short- to long-period spectral ratios of multi-modal Rayleigh data recorded at two African stations. Source depths of less than 10 km were confirmed through the analysis of short-period pP-P time differentials. Two separate source-to-receiver crustal path models were employed in the analysis. An African shield model, with low-velocity surficial layers and a low-shear velocity zone between 7 and 18 km, was utilized to reproduce the observed multi-modal Rayleigh waves acquired from the Kariba to Nairobi path. The model is consistent with published evidence that the East African Rift System extends south to Lake Kariba. A conventional South African shield model was utilized for the Kariba to Pretoria path. Synthetic multi-modal Rayleigh waveforms were computed for a range of source depths for the two crustal models. The spectral ratios obtained from these waveforms provided the synthetic data base to which the observed Rayleigh wave spectral ratios were compared using linear discriminant analysis. Due to the limitations in the available data, the source depths could not be determined at a statistical level of confidence. However, the demonstration of the feasibility of the procedure should stimulate its application to many other problems associated with source parameter inversion.

Determination of source depth from the spectra of small explosions observed at long ranges

The acoustic pressure versus time signature of an underwater explosive source is a sensitive function of the charge weight and the depth of the explosion. If those parameters are known, then the amplitude of the primary shock pulse and the amplitudes and spacings of the following bubble pulses may be accurately predicted. The power spectrum of the signal is characterized by a scalloped form, which also is determined by the charge weight and depth. Even though the signal observed at long range is distorted due to propagation effects, the scalloped spectral structure is preserved. By using power cepstrum techniques, it is possible to determine the characteristic period of the spectrum. From the relation between the spectral period and the bubble pulse period, the exact detonation depth may be determined from a received shot signal if the charge weight is known. An example of such a measurement is presented in this paper. A series of signals from 1.8 lb Mk 61 SUS charges detonated at nominal depths of 60 ft was examined. The bubble pulse period of each source was determined from recordings made onboard the ship from which the shots were deployed. Shots were received at ranges of 250–310 NM at one location and at ranges of 625–700 NM at another location. The depths, as estimated at the remote receiver, were distributed about the onboard measurements with standard deviations of 0.49 ft at the shorter ranges and 0.52 ft at the longer ranges. Subject Classification: [43]30.75.