A team of seismologists from UC Santa Cruz and California Institute of Technology has developed a new measurement of seismic energy that could help us understand why some earthquakes are more destructive than others.
Unlike magnitude scales, which indicate an earthquake’s relative size by measuring the amount of ground movement (or seismic waves with specific wave periods), the new measure – Radiated Energy Enhancement Factor (REEF) – reveals the complexity of different ruptures including variations in the amount and duration of slip along the fault.
Most people have heard of the Richter scale, but today’s most widely used measure of magnitude is “moment magnitude” (Mw), which measures ground movement created by “long-period” seismic waves, and is uniformly applicable to all sizes of earthquakes.
The problem is that the Mw scale only tells one part of the story, and earthquakes of a similar Mw magnitude can produce wildly varying amounts of radiated seismic energy. In the simplest terms, a large rupture that’s jerky and irregular radiates much more seismic energy than a similar magnitude rupture that’s smooth, especially at high-frequencies.
The amount of seismic energy has a significant bearing on how destructive an earthquake is, and, crucially, high-frequency, “short-period” waves are what often cause the most devastation to buildings.
REEF was developed in an effort to understand the different characteristics of the largest earthquakes, including the 2004 Sumatra earthquake (magnitude 9.2) and 2011 Tohoku earthquake in Japan (magnitude 9.1). The measure represents the ratio between an earthquake’s actual measured radiated energy to the minimum possible energy for an event of similar magnitude and rupture duration.
A map summarising the new REEF measure of seismic energy for earthquakes around the Pacific Ring of Fire shows distinct regional patterns
The researchers calculated REEF measurements for 119 recent major earthquakes of magnitudes 7.0 to 9.2 and found clear regional patterns, with some boundaries between tectonic plates having higher REEF ruptures on average than other zones.
“This indicates, for the first time, that energy release is influenced by regional properties of each fault zone,” Thorne Lay, co-author of the research and a professor of Earth and planetary sciences at UCSC, said.
The exact cause of some zones radiating higher energy is still under investigation, but could be connected to regional differences in the roughness of the faults, in fluid distribution on the faults, or in the sediments trapped in the fault zone, he said.
REEF could help seismologists to better understand earthquake mechanics and indeed how to manage earthquake hazards around the world, but Thorne doesn’t expect it to replace magnitude ratings altogether.
“REEF is a special normalisation of the radiated energy that immediately indicates how rough and complicated the event is, which is important. But Mw is critical for getting the long period motion correct that, for example, is related to the possible excitation of tsunami and to the total slip in the earthquake,” he told Alphr.
“Engineers still make use of ML (Richter) and other seismic magnitudes at specific periods so that they can utilise historical databases for which those are the only measures to compare with past building damage. So REEF will be useful, but it will supplement historical magnitudes.”
The research is published in Science Advances.
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