Based on the full waveform simulation, we obtain the maximum magnitudes of eigenvalues of the dynamic strain tensor given as the representative maximum strain load at each position. Figure 3a shows the spatial distribution of maximum strain changes on the plate interface of Slab2. The value of strain changes generally decreases with distance from the epicenter. Anomalies of substantial strain changes are observed in the Guerrero and Jalisco regions to the northwest along the trench, where the plate interfaces are shallower than those in other regions. In the Jalisco region, the stress/strain changes have larger values within the tremor region than in the shallower or deeper regions (Fig. 3b). The maximum strain changes in the Jalisco tremor region are estimated at approximately 1 × 10− 6, which is one to two orders of magnitude larger than the changes due to the earth tide. Note that these values may underestimate the actual ones because the simulated maximum amplitudes of surface waves underestimate the actual maximum amplitudes.
We then estimate the stress changes resolved for the mechanism of the tectonic tremor. We assume that the tremor occurs on the plate interface with a mechanism having a strike of 319°, a dip of 29° and a rake of 90° for a fault plane consistent with the subducting slab geometry. Since the exact location of the triggered tremor is not available, we substitute the median epicenter of ambient tremors using the tremor catalog in Jalisco by Idehara et al. (2014), which is located approximately 50 km to the east of station CJIG. We estimate the change in the Coulomb failure stress at the hypocenter on the plate interface, Δτ + µ’Δσn, where the effective friction coefficient µ’ is assumed to be 0.2 and Δτ and Δσn are the shear stress change in the slip direction and the normal stress change on the fault plane (negative for compression), respectively (Fig. 4). The waveforms are bandpass filtered at 0.01 to 0.10 Hz. The Lamé parameters are set by the structure model in Table 1 for the synthetic case (Fig. 4a). We also use a linear kernel approach that computes continuous waveforms spanning the full spectrum at depth (Miyazawa and Brodsky 2008; Miyazawa 2019) to calculate the dynamic stress change resolved for the tremor mechanism at the depth of the plate interface. In this method, the stress changes associated with passing surface waves can be obtained from surface observations. For simplicity, we assume fundamental mode Rayleigh and Love waves propagating in an isotropic elastic medium. The Lamé parameters λ and µ are both 66 GPa at this depth, and the other parameters are the same as those described in Miyazawa (2019). Figure 5b shows the stress changes beneath station CJIG at a depth of 45 km. A systematic time difference in the arrival times of stress peaks between the simulation and estimation based on the observations occurs because of the difference in the location where we obtained the stress changes. As we cannot locate the triggered tremor source, we do not correct for traveltime from the tremor source to station CJIG to directly compare the stress changes and corresponding tremor amplitudes. However, the triggering stresses for the three tremors in Fig. 1 may correspond to the three peaks of 34.6 kPa, 38.2 kPa, and 9.1 kPa at times from 300 s to 370 s for the simulation (Fig. 4a) and of 14.7 kPa, 28.4 kPa, and 22.6 kPa from 310 s to 380 s for the estimation from the observations (Fig. 4b). The differences in these values appear primarily because the parameters and structures assumed in the simulation and estimation models are different and probably because station CJIG is located closer to the trench where the amplitudes of seismic waves become smaller than those in the tremor region.
Large stress changes could likely trigger tremors; however, the observation of three tremors may not be enough to clarify the relationship between the amplitudes of triggered tremors and triggering stress, as shown by Miyazawa and Brodsky (2008). Since the tremor hypocenters are not obtained, we simply compare the peak values of tremor amplitude in the vertical component and the triggering stress (Fig. 5), where the peak tremor amplitudes are measured from the envelope of high-pass filtered waveforms (Fig. 1a) with 1 Hz lowpass filtering and the stress changes are estimated by the waveforms recorded at CJIG (Fig. 4b). Two more small triggered tremors following three large ones are also used. Simulated values are not used here because the largest surface waves are not reconstructed. If we assume that the triggered tremors occurred at the same place, the relationship between the triggered tremor amplitude and the triggering stress derived by Miyazawa and Brodsky (2008) can be applicable to the peak values. The peak tremor amplitude, Tamp, is represented by
where a is an empirically derived constant, σ is the background stress (Dieterich 1994) and C is a constant. The parameters are obtained as aσ = 56.84 kPa and C = 0.01965, which may not be robust due to the small number of datasets. The model curve and those when aσ is 1, 10 and 100 kPa are shown in Fig. 5, while there is no significant difference for aσ > 100 kPa in this stress range. The relationship in our study may follow this model when aσ ranges from 10 kPa to 100 kPa (Fig. 5), which is consistent with the case of tremor triggering found in the Nankai region (Miyazawa and Brodsky 2008).