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Not All Earthquakes Are the Same: Geodetic Data Quick to Define Magnitude

Researchers: Diego Melgar, University of Oregon and Gavin Hayes, U.S. Geological Survey

Written by Linda Rowan
31 May 2019


Summary

Analysis of source time functions and geodetic data show that within the first 20 seconds of a magnitude 7+ earthquake, the magnitude can be determined base on the characteristics of the slip pulse. This is a change from previous work regarding determinism or self-similar models of earthquakes. Beyond understanding earthquakes better, the results can help to more quickly determine the size of the event and help with preparedness, response and mitigation.


Observations

Earthquakes create a rupture, typically along a plane of weakness or high stress zone, that can be studied with seismic, geodetic and even satellite measurements of the rupture characteristics. A large magnitude event has a rupture process that can last for tens of seconds to minutes. Smaller earthquakes have shorter rupture processes. Most previous work shows that the rupture pulse looks the same regardless of earthquake magnitude (i.e. are self-similar), so there is no way to determine the magnitude early in the rupture process. Some work suggests the rupture process looks different, but only for medium size earthquakes (magnitude 3 to 8).

Deciphering whether earthquakes are self-similar could be critical for hazard warning and response. An early estimate would be helpful for event response for a large magnitude event (7+) with a long rupture duration and the most damaging effects.

Here the authors use source time functions (STFs) that measure the moment release over time and high rate Global Navigation Satellite Systems (HR-GNSS) that measure earthquake motion over time to investigate whether the rupture pulse looks different for large events.


Results

Twelve events of magnitude 7 to 9 were compared in terms of their moment acceleration at different time intervals from the STFs and the evolution of their peak ground deformation (PGD) from the HR-GNSS data. The results suggest that all earthquakes start with a chaotic growing rupture and then settle down into a linear growth that scales with the moment. Large magnitude events (7+) have a longer rupture process and take a longer time to transition from chaotic to linear. The time that it takes for the rupture to stabilize is dependent on the magnitude of the event and thus one can determine the magnitude of the event from the early growth of the rupture. Finding a way to quickly determine the magnitude of a large event could be a game changer for hazard warning and response because it may provide more time to warn and more time to know how and where to respond to more damaging earthquakes.

Related Links


Reference

D. Melgar, G. P. Hayes, Characterizing large earthquakes before rupture is complete. Sci. Adv. 5, eaav2032 (2019). doi: 10.1126/sciadv.aav2032.

Keywords

earthquake, rupture, self similar, magnitude, moment


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