Academic literature on the topic 'Teleseismic events'

Abstract An automatic detection algorithm has been developed which is capable of time P-phase arrivals of both local and teleseismic earthquakes, but rejects noise bursts and transient events. For each signal trace, the envelope function is calculated and passed through a nonlinear amplifier. The resulting signal is then subjected to a statistical analysis to yield arrival time, first motion, and a measure of reliability to be placed on the P-arrival pick. An incorporated dynamic threshold lets the algorithm become very sensitive; thus, even weak signals are timed precisely. During an extended performance evaluation on a data set comprising 789 P phases of local events and 1857 P phases of teleseismic events picked by an analyst, the automatic picker selected 66 per cent of the local phases and 90 per cent of the teleseismic phases. The accuracy of the automatic picks was “ideal” (i.e., could not be improved by the analyst) for 60 per cent of the local events and 63 per cent of the teleseismic events.

In this paper we consider the use of nonlinear dynamic (NLD) forecasting as a signal processing tool for seismic applications. The specific problem considered here arises in monitoring nuclear tests and nuclear treaty compliance, where the presence of ubiquitous background noise obscures the seismic signals associated with the tests. The problem is that the signal from a distant teleseismic event can be attenuated so that it is lost in the background noise, and since the noise overlaps the frequency band occupied by the teleseisms, frequency-based techniques provide only marginal improvements in detection capabilities. For the work in this paper, we studied a test set of actual seismic sensor data prepared by the Air Force Technical Applications Center (AFTAC). The data set was composed of background seismic noise which contained or had added to it a number of hidden teleseismic signals. This data was analyzed to determine if techniques of NLD forecasting could be used to detect the hidden signals. For this test case, it was possible to predict the behavior of the seismic background sufficiently well that when the predicted background behavior was removed, the hidden signals became evident. However, some of the weaker signals were very close to the residual noise level, so the ability to detect these events is compromised.

The Tripoli Seismic Array (TRISAR) is a small-aperture array designed to monitor and locate the seismicity in the area of Greece. In this study, its detection capabilities are discussed for regional and teleseismic events. A reference event list is compiled, consisting of events ofmb>5.0for regional and teleseismic distances (A>6°), according to the ISC On-line Bulletin. TRISAR automatically detected approximately 25% of these events over the entire investigated distance range. Although TRISAR slowness vector residuals are rather large, as expected for an array of such small aperture, the benefits resulting from the use of such a system for reporting regional and teleseismic activity is obvious.

Abstract. For seismic hazard assessment and earthquake hypocentre localization, detailed shear-wave velocity profiles are an important input parameter. Here, we present a method to construct a shear-wave velocity profiles for a deep unconsolidated sedimentary layer by using strong teleseismic phases and the ambient noise field. Gas extraction in the Groningen field, in the northern part of the Netherlands, is causing low-magnitude, induced seismic events. This region forms an excellent case study due to the presence of a permanent borehole network and detailed subsurface knowledge. Instead of conventional horizontal-to-vertical spectral ratios (H∕V ratios) from amplitude spectra, we calculate power spectral densities and use those as input for H∕V calculations. The strong teleseisms provide resonance recordings at low frequencies, where the seismic noise field is too weak to be recorded well with the employed geophones and accelerometers. The H∕V ratios of the ambient noise field are compared with several forward modelling approaches to quality check the teleseism-based shear-wave velocity profiles. Using the well-constrained depth of the sedimentary basin, we invert the H∕V ratios for velocity profiles. A close relationship is observed between the H∕V spectral ratios from the ambient noise field, shear-wave resonance frequencies and Rayleigh-wave ellipticity. By processing only five teleseismic events, we are able to derive shear-wave velocities for the deeper sedimentary sequence with a 7 % bias in comparison with the existing detailed velocity model for the Cenozoic sediments overlying the Groningen gas field. Furthermore, a relation between resonance frequency and unconsolidated sediment thickness is derived, to be used in other areas in the Netherlands, where detailed depth maps are not available.

Several kinds of natural source signals are recorded by seismic exploration stations on the continental ice sheet in Eastern Dronning Maud Land, East Antarctica, during 2002 austral summer. They include not only tectonic earthquakes, but also ice-related phenomena possibly involving recent global climate change. The recorded signals are classified into (1) teleseismic events, (2) local ice quakes, and (3) unidentified events (X-phases). The teleseismic waves show the high signal-to-noise ratio in spite of the small magnitude of the event; this indicates that it is highly feasible to study not only the local shallow structure but also the deep structure of the earth by using teleseismic events. Frequency spectra of the all waveforms represent discordances along the observation seismic profile. The abrupt change of topography in the valley along the seismic profile might cause both the anomalous frequency content and travel times. Finally, an origin of the X-phases is speculated as the intraplate earthquakes or possibly large ice-quakes (glacial earthquakes) around Antarctica, involving global warming appeared in polar region.

Abstract Recent studies have shown that the Antarctic cryosphere is sensitive to external disturbances such as tidal stresses or dynamic stresses from remote large earthquakes. In this study, we systematically examine evidence of remotely triggered microseismicity around Mount (Mt.) Erebus, an active high elevation stratovolcano located on Ross Island, Antarctica. We detect microearthquakes recorded by multiple stations from the Mt. Erebus Volcano Observatory Seismic Network one day before and after 43 large teleseismic earthquakes, and find that seven large earthquakes (including the 2010 Mw 8.8 Maule, Chile, and 2012 Mw 8.6 Indian Ocean events) triggered local seismicity on the volcano, with most triggered events occurring during the passage of the shorter-period Rayleigh waves. In addition, their waveforms and locations for the triggered events are different when comparing with seismic events arising from the persistent small-scale eruptions, but similar to other detected events before and after the mainshocks. Based on the waveform characteristics and their locations, we infer that these triggered events are likely shallow icequakes triggered by dilatational stress perturbations from teleseismic surface waves. We show that teleseismic earthquakes with higher peak dynamic stress changes are more capable of triggering icequakes at Mt. Erebus. We also find that the icequakes in this study are more likely to be triggered during the austral summer months. Our study motivates the continued monitoring of Mount Erebus with dense seismic instrumentation to better understand interactions between dynamic seismic triggering, crospheric processes, and volcanic activity.

Abstract In 1991, during an experiment to compare low-frequency seismic noise on a basaltic and a sediment covered seafloor (NOBS), we recorded teleseisms on the Juan de Fuca ridge, the Gorda ridge, and the adjacent Cascadia Basin with the SNAG ocean-bottom seismometers (OBS). These data provide an indication of the type of data that may be obtained from future experiments to record teleseisms and may be helpful in designing these experiments and analyzing the results. We found that although seafloor noise is dominated by microseisms in the band 0.1 to 0.3 Hz, there is a well-developed minimum in noise from about 0.03 to 0.1 Hz (the noise notch). In this noise notch, teleseisms can be most easily detected. In the Cascadia area, the overall noise levels are such that only teleseismic events with magnitude greater than 6.5 were usefully recorded. A magnitude 6.6 event in the New Britain area (Δ = 89°) produced usable P- and surface-wave data only in this noise notch. In the band 0.03 to 0.1 Hz, the character of compressional waves is very sensitive to water depth and the type of sensor. We show that pressure sensors are especially sensitive to reverberation in the ocean and that motion sensors (seismometers) are less sensitive to ocean reverberations and will record teleseismic phases with less distortion than pressure sensors. The Cascadia data indicate enhanced P amplitudes at sites on the ridge axes that could be due to focusing caused by a low-velocity lens. These data suggest that amplitude information may be as, or even more, useful than P delay times for determining upper mantle structure.

The shot-profile migration approach of wave-equation migration generates subsurface images using the interferometric principle of crosscorrelating two passive wavefields. These wavefields are typically a source wavefield containing energy from an excited source and a receiver wavefield comprised of scattered-source wavefield energy by the discontinuous earth structure. Shot-profile migration can be recast as a novel way of imaging the earth’s lithosphere using teleseismic wavefield data, where the source wavefield is the directly arriving wavefront and the receiver wavefield is the following wavefield coda. We demonstrate that the shot-profile technique can be tailored to suit teleseismic acquisition geometry and wavefields. Assuming an acoustic framework and 2.5D experimental geometry, we develop procedures that enable kinematic and structural imaging (migration) using both transmission and free-surface reflected passive wavefields. Experiments with synthetic data demonstrate the method’s applicability and illustrate the negative imaging consequence of using inaccurate migration velocity profiles. We apply shot-profile migration to a suite of teleseismic events acquired during the IRIS-PASSCAL CASC-1993 experiment in central Oregon. The imaging results are interpreted to show the Juan de Fuca plate subducting beneath the North American plate. We attribute the observed dissimilarities between these results and other Juan de Fuca subduction- zone images to the combination of different imaging goals and the use of more accurate migration velocity profiles.

The aim of this thesis is to develop procedures for the modelling and inversion of teleseismic P and S waveforms which are as flexible as possible. This flexibility is necessary in order to obtain accurate source depth and mechanism estimates for small to moderate size events, such as those that are relevant in the context of monitoring the Comprehensive Nuclear-Test-Ban Treaty (CTBT). ¶ The main challenge for extending source depth and mechanism inversion methods to smaller events is to ensure that sufficiently accurate synthetic seismograms are available for comparison with observed records. An accurate phase-adaptive reflectivity method has therefore been developed, against which the performance of less computationally intensive approximations can be judged. The standard reflectivity method has been modified to allow for different crustal and upper mantle structures at the source and receiver, and the full effects of reverberations and conversions in these structures can be allowed for. Core reflections and refractions can also be included; these phases can become important at certain distance ranges. A slowness bundle approach has been developed, where a restricted slowness integration about the geometric slowness for the direct wave is undertaken at each frequency, allowing accurate results to be obtained whilst avoiding the expense of a full reflectivity technique. ¶ Inversion using the neighbourhood algorithm (NA) is performed for source depth, mechanism and time function, by modelling direct P and S and their surface reflections (pP, sP and pS, sS) at teleseismic distances. Both SV and SH data are exploited in the inversion, in addition to P data, in order to obtain improved constraints on the source mechanism, including any isotropic component. Good results are obtained using a simple generalised ray scheme, however, the use of a flexible derivative-free inversion method means that more accurate synthetics are able to be used in the inversion where appropriate. The NA makes use of only the rank of the data misfits, so that it is possible to employ any suitable misfit criterion. In the few cases where control on the source mechanism is limited, good depth resolution is still usually obtained. ¶The structures near the source and receiver play an important role in shaping the detail of the teleseismic waveforms. Although reasonable results can be achieved with simple synthetics and a standard velocity model, significant improvement can be made by modifying the representation of structure near the source and receiver. In the case of sub-oceanic events it is important to allow for the effects of water reverberations. The crustal structure near the receiver can also have quite a large influence on the waveforms through reverberations and conversions. This is exploited in receiver function inversion, which is again accomplished using the NA approach.

Thesis (Ph. D.)--Missouri University of Science and Technology, 2009.
Vita. The entire thesis text is included in file. Title from title screen of thesis/dissertation PDF file (viewed April 29, 2009) Includes bibliographical references (p. 119-131).

The aim of this thesis is to develop procedures for the modelling and inversion of teleseismic P and S waveforms which are as flexible as possible. This flexibility is necessary in order to obtain accurate source depth and mechanism estimates for small to moderate size events, such as those that are relevant in the context of monitoring the Comprehensive Nuclear-Test-Ban Treaty (CTBT). ¶ The main challenge for extending source depth and mechanism inversion methods to smaller events is to ensure that sufficiently accurate synthetic seismograms are available for comparison with observed records. An accurate phase-adaptive reflectivity method has therefore been developed, against which the performance of less computationally intensive approximations can be judged. The standard reflectivity method has been modified to allow for different crustal and upper mantle structures at the source and receiver, and the full effects of reverberations and conversions in these structures can be allowed for. Core reflections and refractions can also be included; these phases can become important at certain distance ranges. A slowness bundle approach has been developed, where a restricted slowness integration about the geometric slowness for the direct wave is undertaken at each frequency, allowing accurate results to be obtained whilst avoiding the expense of a full reflectivity technique. ¶ ...

博士
國立中央大學
地球物理研究所
97
The Yellowknife array (YKA), a small-aperture array, recorded many depth phase seismograms from earthquakes which occurred in Taiwan region. Such kinds of phases are unavailable to observe on Taiwan’s local seismic records. The depth phase(pP、pwP、sP、sS and so on) reflected from the surface then traveled to the recording station can provide unequivocal confirmation of focal depth. The pP-P interval depends strongly on focal depth and is independent of clock errors. It can give a good constrain for earthquakes locating that occurred out of seismic network and eliminate a trade-off relationship between the source depth and epicenter determination. It is a simple and direct way to estimate focal depth using the pP-P interval on teleseismic records. However, the pP phase propagates a little bit longer path than P and dissipates more energy, so that the amplitude of pP phase is normally smaller than direct P. In this study, we proposed to use slant stack procedures to reduce the effects of random noise in the data and to enhance the depth phase signal. The surface reflections arrive behind the direct arrival and may be interfered by scattered P wave energy from both the source and receiver sides. The shape of the interference packet changes with depth and this information supplies a means of estimating the source depth. Comparing the observations with suitable synthetic seismograms will help us to identify the accurate source depth. For this reason, we adopt a forward modeling way to calculate synthetic waveforms in different focal depths to help us to recognize depth phase arrival. Finally, the source depth search can be quantitatively represented by a cross-correlation analysis to find the accuracy focal depth range. First, we used two events which occurred in the seismic network to check our method is authentic. The result of this analysis showed our method indeed helped us easily control the range of depth and obtain a robust result. Moreover, using forward grid search to look for the best hypocenter location by S-P time difference of TSMIP records provided information about shallow 1-D S wave velocity stracture under the epicenter region at the same time. Second, we applied our method and procedure to analyze other earthquakes which ML>4.5 and occurred offshore northeastern and southwestern Taiwan. All events are outside of Taiwan seismic network. Some earthquakes were far away from the seismic network and some did not have a good station distribution. The analyzed results of these events showed we can easily control the range of depth and estimate optimal results. The events in offshore southwestern Taiwan were selected from Pingtung Earthquake sequence. In this study, we chose 6 events for analyzing and the best depth solutions were accurately obtained. Analyzed results show that most depths of the offshore Pingtung earthquake sequence are slightly shallower than that reported by CWBSN except for the 2nd event that was the biggest event of the Pingtung earthquake sequence. Combining analyses from near source and teleseismic observations, we concluded that the rupture properties of the 2nd event began at a shallow depth, continued to grow to a depth of 56km where the largest rupture occurred and released the most energy. Considering the arrival times of surface-reflected phases to locate an event depth, it can provide an excellent constrain in source depth. Moreover, accompanying with the forward waveform simulation in different source depth to help us to identify the accurate depth phase, we took the superiors of both methods to have a result with good stability and high depth resolution. Although this method is mainly applied for the events with a ML from 5.0 to 7.5, which features a complicate rupturing process and sometimes hardly to obtained an idea result from the comparing with theoretical simulation and observed waveforms. However, in overall speaking, it is a reliable method for the events outside of the array in source depth calculation. A simple but robust procedure to identify the depth of a seismic event has been developed and successfully demonstrated.

ABSTRACT Seismic tomography methods that use waves originating outside the volume being studied are subject to bias caused by unknown structure outside this volume. The bias is of the same mathematical order and similar magnitude as the local-structure effects being studied; failure to account for it can significantly corrupt derived structural models. This bias can be eliminated by adding to the inverse problem three unknown parameters specifying the direction and time for each incident wave, a procedure analogous to solving for event locations in local-earthquake and whole-mantle tomography. The forward problem is particularly simple: The first-order change in the arrival time at an observation point resulting from a perturbation to the incident-wave direction and time equals the change in the time of the perturbed incident wave at the point where the unperturbed ray entered the study volume. This consequence of Fermat’s principle apparently has not previously been recognized. Published teleseismic tomography models probably contain significant artifacts and need to be recomputed using the more complete theory.