A new BIEM for rupture dynamics in half-space and its application to the 2008 Iwate-Miyagi Nairiku earthquake

Abstract

S U M M A R YWe developed a code for dynamic rupture propagation on arbitrarily shaped faults embedded in half-space using boundary integral equation method (BIEM). It is based on an existing code for the full-space case, in which virtual free surface elements were introduced. Computations in half-space are definitely necessary for the modelling of shallow dipping fault ruptures. First, we describe the implementation and conduct some validation tests that demonstrate the accuracy of the code. The validation tests are carried out by comparing the BIEM results both with the analytical static solution and with the seismic wavefield obtained by a discrete wavenumber method. Then, we apply the code to the study of the 2008 Iwate-Miyagi, Japan, earthquake (M w 6.9) rupture dynamics. The average stress drop was estimated to be rather large (16 MPa) in the asperity area, while the average strength excess was 5 MPa. The average fracture energy inside the asperity was found to be 36 MJ m −2 . Finally, the free surface effect examination showed that the stress drop and the fracture energy were overestimated at shallow depth in the full-space computation.

Stress field in the 2008 Iwate-Miyagi earthquake (M7.2) area

“…The Δ τ / τ for the 2008 IMEQ is ∼0.8 for a ∼24° rotation (Figure 11). As the average stress drop (Δ τ ) is generally smaller than 20 MPa [ Asano and Iwata , 2011; Hok and Fukuyama , 2011], the background deviatoric stress level is ∼25 MPa. Measurements in deep boreholes reveal the deviatoric stress on the order of 100 MPa at seismogenic depths [e.g., Townend and Zoback , 2000].…”

Complexity of the rupture process during the 2009 L’Aquila, Italy, earthquake

“…The local velocity is computed from the gradient of rupture times, following the procedure proposed by Pulido & Dalguer (2009) and Hok & Fukuyama (2011). To compute the local rupture velocity vectors at each point on the fault plane, we first evaluate the rupture slowness vectors, as the gradient of rupture times ( t ) along strike and dip directions (identified by unit vectors and ): …”

Equivalent formulae of stress Green’s functions for a constant slip rate on a triangular fault

“…We believe our expressions can be used to simulate the rupture propagation in some actual earthquakes (Hok and Fukuyama 2011;Hu et al 2017) due to the convenience in handling the complex geometry structures with a higher speed. We also intend to generalize the formulae into halfspace medium considering the effect of free surface (Zhang and Chen 2006) as the strong influence of a free surface usually occurs in some disastrous earthquakes.…”