| NSF Org: |
OCE Division Of Ocean Sciences |
| Recipient: |
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| Initial Amendment Date: | August 17, 2015 |
| Latest Amendment Date: | August 17, 2015 |
| Award Number: | 1538276 |
| Award Instrument: | Standard Grant |
| Program Manager: |
Candace Major
OCE Division Of Ocean Sciences GEO Directorate for Geosciences |
| Start Date: | August 15, 2015 |
| End Date: | July 31, 2018 (Estimated) |
| Total Intended Award Amount: | $227,583.00 |
| Total Awarded Amount to Date: | $227,583.00 |
| Funds Obligated to Date: |
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| History of Investigator: |
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| Recipient Sponsored Research Office: |
1608 4TH ST STE 201 BERKELEY CA US 94710-1749 (510)643-3891 |
| Sponsor Congressional District: |
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| Primary Place of Performance: |
2150 Shattuck Avenue, Suite #300 Berkeley CA US 94704-5940 |
| Primary Place of
Performance Congressional District: |
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| Unique Entity Identifier (UEI): |
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| Parent UEI: |
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| NSF Program(s): | Marine Geology and Geophysics |
| Primary Program Source: |
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| Program Reference Code(s): | |
| Program Element Code(s): |
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| Award Agency Code: | 4900 |
| Fund Agency Code: | 4900 |
| Assistance Listing Number(s): | 47.050 |
ABSTRACT
Seismometers are sensitive to tiny ground motions generated by distant earthquakes and other phenomena. As has long been known, in the absence of earthquakes, these instruments also record a background 'hum', a low level seismic signal originating in the earth that has energy cycling every 6-300 seconds. Broad waves called "infragravity waves" moving through the ocean are the source of most of the 'hum', imparting cyclic pressure on the seafloor as they travel through the seawater. This general relation between ocean 'forcing' on the seafloor and Earth's seismic 'hum' is understood, but how much is generated along the shallow margins of the ocean versus within the deep ocean basins, and why? This project investigates the relationship between the source region(s) of the 'hum' and the passage of major storms. It explores whether the shape of the coastal slopes influences how much hum energy is produced. In order to accomplish the work, seismometers both on the seafloor and on the continent will be used to combine information from the signals detected across the array of instruments, and point toward the location where the seafloor forcing occurred.
Through a collaboration between seismologists and oceanographers, the spatial variability of infragravity wave (IG) energy levels, the influence of ocean storms on that pattern, and the generation of microseisms and Earth's long period "hum" will all be documented. Broadband seismic and pressure spectra from the Cascadia amphibious array will be analyzed. Numerical models will be employed to explore the range of possible wave behaviors across the continental shelf. The coupling between wind-driven ocean waves, IG waves and the seafloor will be studied in a near-coastal region of the Pacific Northwest. The results of this study will contribute to the characterization of IG noise levels on the ocean floor, link this variability to the near shore IG wave climate, and provide guidance for future off-shore deployments of seismic and pressure arrays. Improved understanding of the sources of Earth?s hum may help design approaches to using long period seismic noise data for the study of deep earth structure. The west coast of North America is uniquely suited for this purpose as it is has been documented as a strong source region for the "hum". A graduate student will play a significant role in this cross-disciplinary research, obtaining experience in array processing techniques such as stacking, beamforming, and back projection.
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PROJECT OUTCOMES REPORT
Disclaimer
This Project Outcomes Report for the General Public is displayed verbatim as submitted by the Principal Investigator (PI) for this award. Any opinions, findings, and conclusions or recommendations expressed in this Report are those of the PI and do not necessarily reflect the views of the National Science Foundation; NSF has not approved or endorsed its content.
The earth's "hum" is the background low frequency seismic noise in the period range 50-300s that occurs continuously, in the absence of earthquakes. It is generated is mainly in the oceans and is thought to be due to interactions with the seafloor of long period ocean waves, called infragravity waves (IG), themselves generated by non-linear interactions between ordinary, short periods (~10-20s) ocean waves. The precise generation mechanism is not yet fully understood. Locating hum sources precisely in time and space in relation with storms and infragravity waves can help clarify this mechanism. A burst of strong hum events can be generated by large ocean storms. We have identified a 7 day period in december 2015 with no earthquakes of magnitude larger than 5.5 during which 3 different storms propagate from west to east across the Pacific ocean. Two of them (C1 and C2) hit the western coast of north America broadside, while the third one (A3) propagates in a more northerly direction along the Aleutian Islands towards Alaska.
Using seismic array analysis and back-projecting the seismic energy from seismic stations located around the Pacific ocean, averaged over 3 hour intervals, on a 5o x 5o grid, we show that the first 2 storms efficiently generate hum sources that follow in time and space the propagation of IG waves along the coast towards the north, as well as towards the open ocean. In contrast, the 3rd storm produces insignificant levels of hum. The direction of approach of the storms with respect to the west coast of north America is therefore an essential indicator of the efficiency of hum generation. We also find that shorter period hum is generated along the coast, while longer period hum extends far into the ocean basin, in agreement with what is expected from interactions of IG waves with the ocean floor (since longer period IG waves reach deeper below the ocean surface).
Combining information about the direction of propagation of storms in the north Pacific, with higher resolution IG wave models may make it possible to determine the distribution of hum sources, to first order, at high resolution, which will be useful in improving the resolution in studies of earth's mantle structure that rely on ambient noise tomography .
Last Modified: 10/01/2018
Modified by: Barbara A Romanowicz
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