Rupture evolution and high-frequency radiation during mega/large earthquakes, resolved by hybrid backprojection technique

Ryo Okuwaki, & Yuji Yagi

Published September 12, 2016, SCEC Contribution #6392, 2016 SCEC Annual Meeting Poster #067 (PDF)

Tracking spatio-temporal evolution of high-frequency (HF) sources during rupture is a key to understand nature of earthquake rupture process since HF radiation reflects an abrupt change of rupture velocity and/or slip-rate. We have been developing hybrid backprojection (HBP) technique; a scheme for tracking HF sources at high resolution by stacking cross-correlation functions of observed waveforms and theoretically computed Green’s functions.

The HBP method overcomes a difficulty in ensuring a depth resolution in projected images and mitigates signal-delayed artifacts built in conventional backprojection (BP) methods due to larger amplitude of reflected phases than that of direct P-phase by using information of relative timing and amplitude between P-phase and depth phases (pP, sP phases) in a cross-correlation process. These features of the HBP method enable us to integrate HF sources with slip distribution obtained by using the waveform inversion and provide us more detailed view of rupture evolution, e.g., cascading rupture or rupture-stopping mechanism, which had been difficult to be considered with conventional BP methods. In addition, an explicit use of Green’s functions, which depend on depth and focal mechanism of assumed source knot, can draw how geometric complexity in fault system has an effect upon rupture propagation via tracking HF sources with the HBP method.

In the meeting, we will present the recent observations of the mega/large earthquakes (2008 Wenchuan China/2010 Maule Chile/2015 Gorkha Nepal/2015 Illapel Chile) applying the HBP method together with the waveform inversion, and discuss diverse patterns of rupture evolution highlighting the heterogeneous distribution of stress state or frictional property along the fault, and the role of geometric barriers in rupture propagation. Through applications of the HBP method, we have encountered an artifact showing that the strength of HF signals is systematically dependent on depth, and found that, in some cases, rupture direction is not well resolved due to source-receiver geometry. We will also present these problems we are now facing, and explore some tentative ways to solve them.

Citation
Okuwaki, R., & Yagi, Y. (2016, 09). Rupture evolution and high-frequency radiation during mega/large earthquakes, resolved by hybrid backprojection technique. Poster Presentation at 2016 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)