Abstract
T waves propagate in the so-called SOFAR channel of minimum sound velocity acting as a waveguide for acoustic energy in the world's oceans. They can be excited by sources in the solid Earth such as earthquakes through conversion of seismic energy into acoustic waves at the solid-liquid interfaces. We present a historical perspective of the investigations of such conversions. In the context of geometrical optics, a sloping interface provides a mechanism for the penetration of the SOFAR channel after a series of reflections in the liquid wedge. This process, known as "downslope conversion", successfully explained many characteristics of earthquake-generated T waves, but has severe limitations, notably regarding "abyssal" T phases, generated under flat oceanic basins. We review theoretical developments based on mode theory which describe coupling between elastic and acoustic modes under scattering by structural heterogeneities located at the ocean bottom, and which are becoming increasingly successful at modeling the waveshapes of abyssal T phases. As a particular form of seismic wave emanating from an earthquake, T waves can provide insight into seismic sources in the oceanic environment. We review the application of T waves to the detection of small earthquakes in marine basins, discuss the retrieval of seismic source properties from T-phase waveforms, and show that several algorithms combining measurements of their amplitude and duration can yield information on source rupture, and more specifically detect the presence of source slowness. In particular, anomalously slow earthquakes such as the so-called "tsunami earthquakes" are poor T-wave generators, and more generally, T-phase amplitudes and tsunami generation are not found to correlate. In the context of the Comprehensive Nuclear-Test Ban Treaty, hydroacoustics has been recognized as a monitoring technology, and the deployment of state-of-the-art receivers at eleven sites will significantly improve long-range detection capabilities and open up new opportunities for the investigation of acoustic sources, including earthquakes, in the oceanic environment.
| Original language | English (US) |
|---|---|
| Title of host publication | Advances in Geophysics |
| Publisher | Academic Press Inc |
| Pages | 1-65 |
| Number of pages | 65 |
| ISBN (Print) | 0123742315, 9780123742315 |
| DOIs | |
| State | Published - 2008 |
Publication series
| Name | Advances in Geophysics |
|---|---|
| Volume | 49 |
| ISSN (Print) | 0065-2687 |
Funding
I am grateful to Ralph Stephen and Robert Odom, conveners of the workshop on “Seismo-acoustic applications in Marine Geology and Geophysics”, held at Woods Hole in March 2004, for inviting me to present a keynote lecture, and encouraging me to write up a review paper on its topic. My work on T phases has benefited from many years of regular cooperation with the Commissariat à l'Energie Atomique (France), and particularly from joint research with Jacques Talandier, Dominique Reymond and Olivier Hyvernaud. Several aspects of T-phase quantification were supported by the Department of Defense, most recently under Contract DTRA01-00-C-0065, and regarding tsunami hazard by the National Science Foundation, under Grant CMS-03-01054. I thank Library Staff at Northwestern University, the University of Chicago and Harvard University for help with the retrieval of historical references. Most maps were drafted using the GMT software ( Wessel and Smith, 1991 ).
ASJC Scopus subject areas
- Geophysics