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This report presents the results of a study of the coupling of seismic energy to the SOFAR channel and the propagation of acoustic energy from source to receiver. The results of an investigation of T-phase amplitude level vs. earthquake magnitude are presented for events in the Pacific ocean. The poor correlation between these parameters suggests that the generation of detectable T-phases is dependent not only upon event magnitude and depth, source mechanism, and transmission loss from source to receiver, but also on details of the source bathymetry which govern the coupling of seismic to acoustic energy. Despite the complexities of the data, several features of T-phase data that are readily observable are: (1) T-phase energy for submarine earthquakes drops off rapidly at frequencies greater than 16 Hz, and (2) that the nature of the T-phase coda is strongly dependent on the geographic source area, thus T-phases from the same area tend to be very similar.
In September 1996, the United Nations General Assembly adopted the Comprehensive Nuclear-Test-Ban Treaty (CTBT), prohibiting nuclear explosions worldwide, in all environments. The treaty calls for a global verification system, including a network of 321 monitoring stations distributed around the globe, a data communications network, an international data centre (IDC), and on-site inspections, to verify compliance. A global hydroacoustic monitoring system is being planned and implemented for verification of the CTBT. Much of the research conducted over the past several decades on acoustic surveillance of the oceans, formerly driven by the need to detect and track submarines, is now being applied to the development of effective monitoring methods to verify compliance with the CTBT. The aim of this volume on Hydroacoustic Monitoring of the CTBT is to summarize the research being conducted in this field and to provide basic references for future research. Much of the new research emphasizes major advances in understanding the coupling of ocean acoustic waves with elastic waves in the solid Earth. Topics covered include source excitation, detection and classification of events generating hydroacoustic signals, discrimination between underwater explosions and naturally occurring events, as well as topics in coupling of acoustic to seismic wavefields.
Hydroacoustic tertiary (T- ) waves are seismically generated acoustic waves that propagate over great distances in the ocean sound channel with little loss in signal strength. Hydrophone recorded T-waves can provide a lower earthquake detection threshold and an improved epicenter location accuracy for oceanic earthquakes than land-based seismic networks. Thus detection and location of NE Pacific ocean earthquakes along the Blanco Transform Fault (BTFZ) and Gorda plate using the U.S. Navy's SOSUS (SOund SUrveillance System) hydrophone arrays afford greater insight into the current state of stress and crustal deformation mechanics than previously available. Acoustic earthquake information combined with bathymetry, submersible observations, earthquake source-parameter estimates, petrologic samples, and water-column chemistry renders a new tectonic view of the southern Juan de Fuca plate boundaries. Chapter 2 discusses development of seismo-acoustic analysis techniques using the well-documented April 1992 Cape Mendocino earthquake sequence. Findings include a hydrophone detection threshold estimate (M~2.4), and T-wave propagation path modeling to approximate earthquake acoustic source energy. Empirical analyses indicate that acoustic energy provides a reasonable magnitude and seismic moment estimate of oceanic earthquakes not detected by seismic networks. Chapters 3 documents a probable volcanogenic T-wave event swarm along a pull-apart basin within the western BTFZ during January 1994. Response efforts yielded evidence of anomalous water-column 3He concentrations, pillow-lava volcanism, and the first discovery of active hydrothermal vents along an oceanic fracture zone. Chapter 4 discusses the detection of a NE-SW trending microearthquake band along the mid-Gorda plate which was active from initiation of SOSUS recording in August 1991 through July 1992, then abruptly ceased. It is proposed that eventual termination of the Gorda plate seismicity band is due to strain reduction associated with the Cape Mendocino earthquake sequence. Chapter 5 combines bathymetric, hydro-acoustic, seismic, submersible, and gravity data to investigate the active tectonics of the transform parallel Blanco Ridge (BR), along the eastern BTFZ. The BR formation mechanism preferred here is uplift through strike-slip motion (with a normal component) followed by formation and intrusion of mantle-derived serpentinized-peridotite into the shallow ocean crust. The conclusion considers a potential link between the deformation patterns observed along the BTFZ and Gorda plate regions.
We have assembled a large suite of T-phase observations recorded on hydrophones located at Point Sur and Wake Island, in an attempt to understand how acoustic energy from underwater earthquakes is coupled to the sound channel and bow the sound propagates from source to receiver. The events cover a wide range of locations in the North Pacific. We find that T-phases observable at Point Sur are generated only in a limited number of geographical regions, the Aleutian Islands and Kodiak Island regions, south of Japan, and the southern portion of the Kuril Islands region. Too few records have been assembled to date for Wake Island to discern any pattern in the locations of observable T-phases. A signal to noise (SNR) measure was determined for each event. We found that there was no clear pattern in the SNR vs. magnitude when all the events were examined together. We narrowed the region of study to a small cluster of events in the Andreof Island region of the Aleutian Island chain in order to eliminate variables such as source mechanism, transmission path, and event depth. For this limited data set, we found that the SNR as observed at Pt. Sur was more strongly dependent upon event magnitude, however, there was still a considerable degree of scatter. This scatter is probably due to near source effects, such as the slope of the seafloor in the region of the events.
The destructive force of earthquakes has stimulated human inquiry since ancient times, yet the scientific study of earthquakes is a surprisingly recent endeavor. Instrumental recordings of earthquakes were not made until the second half of the 19th century, and the primary mechanism for generating seismic waves was not identified until the beginning of the 20th century. From this recent start, a range of laboratory, field, and theoretical investigations have developed into a vigorous new discipline: the science of earthquakes. As a basic science, it provides a comprehensive understanding of earthquake behavior and related phenomena in the Earth and other terrestrial planets. As an applied science, it provides a knowledge base of great practical value for a global society whose infrastructure is built on the Earth's active crust. This book describes the growth and origins of earthquake science and identifies research and data collection efforts that will strengthen the scientific and social contributions of this exciting new discipline.
This book introduces a framework of tsunami modelling from generation to propagation, aimed at application to the new observation started in Japan after the devastating tsunami of the 2011 Tohoku-Oki earthquake. About 150 seismic and tsunami sensors were deployed in a wide region off the Pacific coast of eastern Japan in order to catch tsunami generation inside the focal area, which makes a clear departure from conventional observations that detect tsunamis far from the source region. In order to exploit the full potential of this new observation system, it is not enough to model tsunami generation simply by static sea-bottom deformation caused by an earthquake. This book explains dynamic tsunami generation and sea-bottom deformation by kinematic earthquake faulting, in which seismic and acoustic waves are also included in addition to static sea-bottom deformation. It then systematically derives basic tsunami equations from the fundamental equations of motions. The author also illustrates the details of numerical schemes and their applications to tsunami records, making sound linkages among these topics to naturally understand how a tsunami is physically or mathematically described. This book will be a comprehensive guide for graduate students and young researchers to start their research activities smoothly.
Many coastal areas of the United States are at risk for tsunamis. After the catastrophic 2004 tsunami in the Indian Ocean, legislation was passed to expand U.S. tsunami warning capabilities. Since then, the nation has made progress in several related areas on both the federal and state levels. At the federal level, NOAA has improved the ability to detect and forecast tsunamis by expanding the sensor network. Other federal and state activities to increase tsunami safety include: improvements to tsunami hazard and evacuation maps for many coastal communities; vulnerability assessments of some coastal populations in several states; and new efforts to increase public awareness of the hazard and how to respond. Tsunami Warning and Preparedness explores the advances made in tsunami detection and preparedness, and identifies the challenges that still remain. The book describes areas of research and development that would improve tsunami education, preparation, and detection, especially with tsunamis that arrive less than an hour after the triggering event. It asserts that seamless coordination between the two Tsunami Warning Centers and clear communications to local officials and the public could create a timely and effective response to coastal communities facing a pending tsuanami. According to Tsunami Warning and Preparedness, minimizing future losses to the nation from tsunamis requires persistent progress across the broad spectrum of efforts including: risk assessment, public education, government coordination, detection and forecasting, and warning-center operations. The book also suggests designing effective interagency exercises, using professional emergency-management standards to prepare communities, and prioritizing funding based on tsunami risk.