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Seismologists have never had a better understanding of earthquakes. But tragedy after tragedy shows that quakes still surprise and shock people with their mercurial behavior. Precise predictions of when and where quakes will occur, and how deadly they may be, are not yet possible. If, however, researchers could chronicle how quakes grow, they might be able to better forecast how powerful they will become.
The mightiest quakes are far from instantaneous. They can last minutes, which makes them less like a single subterranean blast and more like a series of explosions moving outward. A new study, published on Wednesday in Science Advances, explains that the outward journey of these explosions differs depending on the power of the quake.
That means that the final magnitude of a quake could be determined in as little as 10 to 15 seconds after it begins, and long before it ends.
Whether earthquakes of different sizes are distinguishable early in their rupture process is a subject of debate. Studies have shown that the frequency content of radiated seismic energy in the first seconds of earthquakes scales with magnitude, implying determinism. Other studies have shown that recordings of ground displacement from small to moderate-sized earthquakes are indistinguishable, implying a universal early rupture process. Regardless of how earthquakes start, events of different sizes must be distinguishable at some point. If that difference occurs before the rupture duration of the smaller event, this implies some level of determinism. We show through analysis of a database of source time functions and near-source displacement records that, after an initiation phase, ruptures of M7 to M9 earthquakes organize into a slip pulse, the kinematic properties of which scale with magnitude. Hence, early in the rupture process—after about 10 s—large and very large earthquakes can be distinguished.
We find that individual events of a given magnitude demonstrate substantial variability (Fig. 2), a reflection of natural differences between earthquake ruptures. However, their median behavior over a range of magnitude bins shows systematic differences that scale with magnitude. On average, earthquakes with a larger final magnitude grow faster early on in the source process.
These findings are relevant for hazard warning systems, in particular for earthquake early warning (EEW), where every second gained matters. The modern EEW paradigm centers on forecasting the shaking intensity at a site of interest some distance away from an earthquake. Recent studies have shown that uncertainty of an event’s final magnitude leads to a tradeoff between the possible warning time and the certainty of the ground motion forecast (7). At present, a fast warning can only be made with large uncertainty in the associated shaking. While waiting for more information about the source significantly reduces the uncertainty, this comes at the cost of shrinking the warning time and enlarging the blind zone where no warning is possible. Modern EEW systems rely on a combination of point source and finite fault algorithms, neither of which is designed to exploit weak determinism. Point source algorithms focus on the first 1 to 3 s of a P wave as measured by inertial seismic sensors (18–20); thus, it is unlikely that they will overcome the familiar problem of magnitude saturation for the largest events. Of course, for the smaller events in our dataset, by 10 to 15 s, the earthquakes are in the later portion of their source processes. As a result, from an early warning perspective, the results here would still leave room for future improvements in speeding up warning times. Finite fault algorithms (21–23) can more effectively deal with large magnitude events but rely on the entire rupture (or a significant portion of it) being complete before they produce reliable magnitude estimates.
“It’s a good speculative idea, we just need to fill it in before we can have a lot of confidence in it,” said John Vidale, a professor of seismology at the University of Southern California.
BUT they are talking of determining the scale of the quake in the first few seconds of it starting.
For these events, we find that the near-source records finish growing to their final value (or very close to it) in a time much shorter than the event duration. This behavior is a strong function of distance to the source. While the records will be affected by complexities in the STFs, such as multiple moment rate peaks, at close distances, and at long periods, HR-GNSS records will be mostly unaffected by elastic wave propagation and are dominated by the ramp-like growth to PGD and to the static offset. Thus, they more accurately reflect the behavior of slip at adjacent portions of the fault (10). At greater distances, this relationship breaks down and HR-GNSS records reflect mostly S waves and surface waves.
|BTW thanks for the post of anything other than politics.