Multiple Observations of the Prompt Elastogravity Signals Heralding Direct Seismic Waves

Abstract

The recent first observations of the prompt elastogravity signals (PEGS) induced by the 2011 Mw = 9.1 Tohoku megathrust earthquake generated interest in how these signals might best be observed, especially for lower‐magnitude events. Simulations of these signals preceding the direct P wave, for different depths and focal mechanisms, first reveal that shallow strike‐slip earthquakes offer a better detection potential than subduction megathrust earthquakes. Consistently, clear PEGS are observed at several broadband seismometers during the 2012 Mw = 8.6 Wharton Basin earthquake. Due to their short source durations, large deep earthquakes are then shown to have an even larger detection potential, confirmed by the successful seismological observations for the 2018 Mw = 8.2 Fiji and 1994 Mw = 8.2 Bolivia earthquakes. Detection is even improved when an earthquake is recorded by a number of good‐quality stations, allowing for stacking techniques. Thanks to the deployment of the USArray network across Alaska, the recent 2018 Mw = 7.9 off‐Alaska earthquake (strike slip) is thus observed with an excellent signal‐to‐noise ratio. Array stacking is also shown to reveal the PEGS induced by the large 2010 Mw = 8.8 Maule megathrust earthquake, for which individual observations are impeded by the long‐lasting radiation generated by a distant large earthquake. As a whole, we show new observations and successful modeling of the PEGS for five earthquakes in the 7.9–8.8 magnitude range. These findings demonstrate that, even without considering promising future instruments, the PEGS detection is not restricted to exceptional events, confirming their potential for magnitude and focal mechanism determination within the few minutes following a large earthquake.

Earthquakes Within Earthquakes: Patterns in Rupture Complexity

“…Previous studies have found that early P wave arrivals carry information about the overall earthquake size (Iio, 1995;Beroza & Ellsworth, 1996;Allen & Kanamori, 2003;Olson & Allen, 2005), but such correlations bear large uncertainties that necessitate additional information, such as attenuation parameters, in order to conduct meaningful early magnitude estimation (Wu et al, 2006;Colombelli et al, 2014;Noda & Ellsworth, 2016). The direct P wave arrival, however, represents only a small fraction of the available and potentially deterministic information (Vallée et al, 2017;Vallée & Juhel, 2019).…”

Comment on “Earthquake-induced prompt gravity signals identified in dense array data in Japan” by Kimura et al.

“…Recent progress in the research on PEGS has yielded new advances that go far beyond the K19 study. Readers interested in how PEGS can be optimally observed may refer to the more sophisticated stacking approaches described by Montagner et al (2016) and Vallée and Juhel (2019). Vallée and Juhel (2019) also show how multiple PEGS observations made for earthquakes of different focal mechanisms and depths are accurately modeled by the methods described by V17 and Juhel et al (2019).…”

“…Readers interested in how PEGS can be optimally observed may refer to the more sophisticated stacking approaches described by Montagner et al (2016) and Vallée and Juhel (2019). Vallée and Juhel (2019) also show how multiple PEGS observations made for earthquakes of different focal mechanisms and depths are accurately modeled by the methods described by V17 and Juhel et al (2019). Therefore, the remaining challenges today are no longer to show that PEGS are well understood, modeled, and observed for magnitudes larger than 8, but to lower this magnitude threshold and to reduce the detection delay, in order to make PEGS even more valuable for early warning systems.…”

“…The study of prompt elastogravity signals (PEGS) generated by earthquakes is now becoming a mature research area. After the pioneering works in modeling (Harms et al 2015;Harms 2016;Heaton 2017) and observation (Montagner et al 2016), PEGS have been directly observed, understood and modeled in the last 2 years (Vallée et al 2017;Juhel et al 2018Juhel et al , 2019Vallée and Juhel 2019). In particular, Vallée et al (2017) (hereafter referred to as V17) showed that the data from regional high-quality broadband sensors recording the 2011 Tohoku earthquake exhibit the distinctive features of PEGS.…”

Pre-P gravity signals from dynamic earthquake rupture: modelling and observations

“…For the quantitative use of prompt gravity signals in the future, these forward models await validation using authorized data that has reduced uncertainty. For the reduction of uncertainty in data, the use of stacked traces of observed waveforms will be useful [5,6,24]( §3). Range Seismograph Network of Japan (F-net) [3].…”