Rupture process of the 1944 Tonankai earthquake (M_s 8.1) from the inversion of teleseismic and regional seismograms

Abstract

[1] We digitized teleseismic and regional records of the 1944 Tonankai earthquake. We used the multiple time window method to invert these records for the spatial and temporal distribution of slip and rake. We assume a 220 Â 140 km fault with a spacing of 20 Â 20 km and a maximum rupture velocity of 2.5 km/s. The inversion resolved an asperity along the accretionary wedge and under the Shima and Atsumi Peninsulas. The rupture propagated from southwest to northeast, and there was little slip near the hypocenter, consistent with the absence of uplift in the overlying submarine forearc basin. The peak slip is 2.3 m, and the total seismic moment is 2.4 Â 10 28 dyn cm (M w 8.2). We compared the predicted and observed vertical geodetic displacements and inferred that the limit of rupture is consistent with the change from subsidence to uplift west of the Atsumi Peninsula. This suggested that the earthquake did not rupture the plate interface in the Tokai gap. We modeled the tsunami using sea-bottom displacements generated from this slip model. The tide gauge amplitudes and frequency content are consistent with those predicted. Resolution tests from inverting synthetic seismograms computed using a simplified slip model indicated that the combination of teleseismic and regional data sets best recovers the asperity locations and peak slips given similar station geometry and degrees of freedom. An inversion of the synthetic regional data set indicates that slip artifacts can occur early in the rupture process due to many clipped records while the inversion of synthetic teleseismic data set had less resolution and recovered only 50% of the peak slip.

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“…It has a 1300 y historical record of recurring and typically tsunamigenic great earthquakes, including the 1944 Tonankai M 8.2 and 1946 Nankaido M 8.3 earthquakes (Ando, 1975;Hori et al, 2004). The rupture area and zone of tsunami generation for the 1944 event are now reasonably well understood (Ichinose et al, 2003;Baba et al, 2006). Land-based geodetic studies suggest that the plate boundary thrust here is strongly locked (Miyazaki and Heki, 2001).…”

The 2016 Mw 5.9 earthquake off the southeastern coast of Mie Prefecture as an indicator of preparatory processes of the next Nankai Trough megathrust earthquake

“…The Nankai Trough, where the Philippine Sea plate (PSP) subducts beneath the Amur plate (Bird 2003;DeMets et al 2010), is one such region where megathrust earthquakes and tsunamis have occurred at intervals of 100 to 150 years (e.g., Ando 1975;Ishibashi 2004). The 1944 Tonankai (M w 8.2; Ichinose et al 2003) and 1946 Nankai (M w 8.4; Murotani et al 2015) earthquakes were two such devastating events, which ruptured the eastern and western segments of the trough, respectively (Fig. 1), causing severe damage in western and central Japan.…”

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“…First, the rupture area of the most recent great earthquake, the 1944 Tonankai M 8.2 event, is well constrained by recent seismic and tsunami waveform inversions (e.g., Tanioka and Satake, 2001;Kikuchi et al, 2003). Slip inversion studies suggest that past coseismic rupture events in this region have clearly extended to shallow enough depth to be within the reach of current drilling technologies (Ichinose et al, 2003;, and an updip zone of large slip has been identified and targeted (Fig. F1B, F2A).…”

“…The Nankai Trough region has a 1300 y historical record of recurring great earthquakes that are typically tsunamigenic, including the 1944 Tonankai M 8.2 and 1946 Nankaido M 8.3 earthquakes (Ando, 1975;Hori et al, 2004). The rupture area and zone of tsunami generation for the 1944 event are now reasonably well understood and includes Stage 1 and Stage 2 hanging wall drill sites (Ichinose et al, 2003;. Land-based geodetic studies suggest that the plate boundary thrust is currently strongly locked (Miyazaki and Heki, 2001), and the relatively low level of microseismicity near the updip limits of the 1940s earthquakes implies significant interseismic strain accumulation on the megathrust (Obana et al, 2001).…”

“…F1B, F2A). Notably, coseismic slip during events like the 1944 Tonankai earthquake may have occurred on the megasplay fault in addition to the plate boundary décollement (Ichinose et al, 2003;Baba et al, 2006). The megasplay fault is therefore a primary drilling target, equal in importance to the basal décollement.…”