← Back to Gallery

A Mechanism That Can Lead to a Slip Patch With Extremely Large Stress Drop

During the 2011 Tohoku earthquake (M_w9.0), pulse-like strong ground motions with a time width of approximately 1 - 2 s were observed. The most prominent pulse was included in the second wave packet observed in Miyagi prefecture. Nozu (2012) estimated that the pulse was generated at a small region located at 142.574E, 38.062N, at a depth of 28.3 km. A similar location was also suggested by Kurahashi and Irikura (2013). The short time constant associated with the pulse indicates that the pulse was generated, not from an SMGA that spans several tens of kilometers but from an SPGA (Nozu et al., 2012) that spans only several kilometers. Nozu et al. (2012) estimated that a seismic moment of approximately 2.1×10¹⁹ Nm was released at a region occupying only 10.5 km². Therefore, it is a slip patch with an extremely large stress drop. If we simply apply the Eshelby’s (1957) equation, a stress drop of 1504 MPa is estimated, although it is not reasonable to simply apply the equation because the slip outside the SPGA is nonzero. On the other hand, if we relate the initial slip velocity with the instantaneous stress drop as τ=ρβV₀/2 following Brune’s idea, a stress drop of 1026 MPa is estimated. The question is how we can interpret this large stress drop. Because the subduction interface is approximately horizontal and the depth is approximately 30 km, by assuming a density of 3×10³ kg/m³, the effective normal stress acting on the interface can be estimated as 600 MPa. Therefore, as long as we assume a static friction coefficient of 0.6, the shear strength only up to 360 MPa is available. Therefore, even if we assume that the stress is completely released (Hasegawa et al., 2011), the stress drop can be no more than 360 MPa. Therefore, I infer that cementation occurred locally at the plate interface before the 2011 event which resulted in apparent static friction coefficient exceeding 1.0. The SPGA may correspond to a region where the cementation occurred.

Presenting Author: Atsushi Nozu

Student Presenter: No

Day: 4/21/2021

Time: 3:45 PM - 4:45 PM Pacific

Additional Authors