Evidence of Changes of Seismic Properties in the Entire Crust Beneath Japan After the M_w 9.0, 2011 Tohoku‐oki Earthquake

Abstract

Studies of mechanical responses of the Earth crust to large earthquakes can provide us with unique insights into the processes of stress buildup and release. As a complement to geodetic methods that derive crustal strain dynamics from surface observations (e.g., GPS, InSAR), noise‐based seismic velocity monitoring directly probes the mechanical state of the crust, at depth and continuously in time. We investigate the responses of the crust to the Mw 9.0, 2011 Tohoku‐oki earthquake. In addition to the Hi‐net short‐period sensors, we use Hi‐net tiltmeters as long‐period seismometers (8–50 s) to sample the crust below 5 km in depth. The spatial distribution of the strong velocity decreases at short periods appears to be limited to the region of strong ground shaking induced by the 2011 Tohoku‐oki earthquake, while the long‐period velocity changes correlate well with the modeled static strain induced by viscoelastic relaxation and afterslip at depth. Amplitudes of coseismic velocity changes decrease with increasing depth. The temporal evolution of velocity changes in different period bands shows that the maximum drops in the velocity at long periods are delayed in time with respect to the occurrence of the Tohoku‐oki earthquake. The inversion of seismic velocity changes at depth illustrates how S wave velocities evolve down to 40 km at a regional scale after a major earthquake.

Seasonal and Coseismic Velocity Variation in the Region of L'Aquila From Single Station Measurements and Implications for Crustal Rheology

“…This latter value agrees with other estimates based on tidal-dv/v responses (Takano et al, 2014;Yamamura et al, 2003). Our estimations are also in agreement with dilatant induced velocity reduction during slow slip in the lower crust (Rivet et al, 2011;Wang et al, 2019).…”

“…The energy velocity is determined by the equipartition state. We take the theoretical equipartition ratio as 10.4 and 3 for a Poisson solid (Margerin et al, 2000;Weaver, 1982), the same as Wang et al (2019), for further calculation. We solved for the body wave depth sensitivity normalized to 30 km depth with each layer 1 km thick layer.…”

Monitoring of velocity changes based on seismic ambient noise: A brief review and perspective

“…Studies of the mechanical responses of the earth’s crust to large earthquakes can provide us with unique insights into the processes of stress buildup and release at depth (Bürgmann and Dresen, 2008). At this stage, we mainly observe a rapid coseismic velocity reduction followed by a slow postseismic exponential recovery process (Field et al, 1998; Wegler and Sens‐Schönfelder, 2007; Brenguier et al, 2008a, Xu ZJ and Song XD, 2009; Sawazaki et al, 2009; Chen JH et al, 2010; Cheng X et al, 2010; Zhao PP et al, 2012; Froment et al, 2013; Acarel et al, 2014; Brenguier et al, 2014; Liu ZK et al, 2014; Hong TK et al, 2017; Liu ZK et al, 2018; Wang QY et al, 2019; Poli et al, 2020). The predominant mechanism underlying the observed earthquake‐related velocity drop is referred to as shallow dynamic shaking (Sleep, 2015) and deep stress changes by the earthquake.…”

“…Here we present a recent study by Wang QY et al (2019) related to the 2011 M w 9.0 Tohoku‐Oki earthquake off the coast of Japan. The authors identified depth (frequency)‐dependent seismic velocity changes.…”

“…This order of magnitude is much higher than the classical results from coda wave‐based methods. Consequently, the existence of repetitive events from both artificial sources (Yang W et al, 2018; Wang QY et al, 2019) and repeating earthquakes (Sawazaki et al, 2015) extends the possibility of the combined use (Hirose et al, 2017) of ambient noise and other sources to detect changes in seismic wave velocity. Multiple approaches to monitoring can also effectively complement each other to improve the temporal and spatial resolution and extend the scope and sustainability of a region that can be monitored.…”