Tsunami Warning and Preparedness An Assessment of the U.S. Tsunami Program and the Nation's Preparedness Efforts (2011) / Chapter Skim
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Appendix G: Magnitudes from C. Richter to Mwp and the W phase
Appendix G: Magnitudes from C. Richter to Mwp and the W phase
Pages 249-256
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From page 249... ...
The definition of ML was very important because all the subsequent magnitude scales have been tied to this initial algorithm. The concept was soon extended worldwide using a combination of measurements on body and surface waves at teleseismic distances, leading to the definition of two standardized algorithms, a body-wave magnitude mb measured on short-period P-waves at a target frequency of 1 Hz, and a surface wave magnitude Ms measured at a period of 20 s.
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From page 250... ...
reveal a sudden increase in seismic moment release as late as one or two minutes into their source process; they constitute another class of events violating scaling laws. In lay terms, at the initiation of a seismic rupture, does Mother Nature really know how large the final product will be?
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From page 251... ...
SHORTCOMINGS OF THE MwP ALGORITHM As compelling as the Mwp concept may be, it suffers from having never been subjected to the necessary and independent exercise of being tested on synthetic seismograms (i.e., computational renditions of the wavetrains expected to be recorded on a given seismic instrument for a particular scenario of earthquake size, rupture parameters [including for example the case of "tsunami earthquakes"] , and receiver geometry)
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From page 252... ...
. Such limitations in the operational aspects of Mwp were actually recognized by the operators of the centers during the development of their algorithms, through a comparison of their results with published earthquake magnitudes obtained by the Harvard CMT project using geophysical inverse procedures.6 These authors documented that Mwp measurements become increasingly deficient when the magnitude of the earthquake increases, and they proposed to incorporate in the final algorithm (i.e., the one presently used at the TWCs)
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From page 253... ...
The W phase arrives following the P-wave and before conventional surface waves, and can be regarded as a very low frequency, fast propagating group of waves. Following a number of investigations, the systematic use of the W phase has now been implemented at the National Earthquake Information Center (NEIC)
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From page 254... ...
Figure Pete Davis, The response of the GSN standard seismometer has been shaped to be nominally flat to ground velocity between about 2.8 mHz and 5 Hz. For frequencies below 2.8 mHz, the seismometer's output has been shaped so that at zero frequency its output is zero.
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From page 255... ...
PTWC staff indicated that they are in the process of implementing a W phase algorithm, but a careful vetting of the algorithm before it can be reliably applied will be required. Recommendation: Before implementing the W phase algorithm in TWC operations, the NOAA Tsunami Program should validate the W algorithm to both a sufficient dataset of synthetic seismograms and to waveforms from past great earthquakes, paying particular attention to its performance in "tsunami earthquakes" and to the assessment of a lower magnitude bound for its domain of applicability.
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