10:45 AM - 11:00 AM
[SSS10-05] Re-determination of aftershock distribution using HYPODD: the 1969 and 1994 Hokkaido-toho-oki earthquakes and the 1975 tsunami earthquake
Keywords:HYPODD, Hokkaido-toho-oki earthquake, aftershock distribution
1. Introduction
Three earthquakes of different types occurred off the east coast of Hokkaido (off Shikotan Island). The first is the 1969 Hokkaido-toho-oki earthquake (Mw8.0; Ioki and Tanioka, 2016), a typical plate boundary megathrust earthquake; the second is the June 10, 1975 earthquake, a tsunami earthquake (Mw7.8; Ioki and Tanioka, 2016); the third is the 1994 Hokkaido-toho-oki earthquake (Mw8.3; Global CMT), an intra-slab earthquake. Among these three earthquakes, the fault plane of the 1994 earthquake is still controversial, with some claiming that the strike is parallel to the trench axis (e.g., Kikuchi and Kanamori, 1995; Katsumata et al., 1995) and others that it is almost perpendicular to the trench axis (e.g., Tanioka et al., 1995). In this study, aftershock distributions of three earthquakes were re-determined by HYPODD, and source faults of three earthquakes were examined.
2. Data
Hypocenter parameters and arrival times of P-wave are published in the ISC Bulletin, and they were used in this study. I selected aftershocks with a body wave magnitude (mb) equal to or larger than 5.3 and a depth of less than 100 km for 7 days after the main shock. I selected 80 seismic stations that satisfy the following conditions; the epicentral distance is smaller than 5000 km and there are P-wave arrival times data for all three earthquakes.
3. Method
For hypocenter determination, I used a double-difference earthquake location method, HYPODD (Waldhauser and Ellsworth, 2000). However, HYPODD calculates travel times assuming a horizontal layered velocity structure, but in order to use teleseismic P-waves, it is necessary to calculate travel times in a spherically stratified velocity structure of the Earth. In this study, I assumed a one-dimensional velocity structure iasp91 and calculated travel times using the iaspei-tau package (Kennett and Engdahl, 1991; Snoke, 2009). I used 25 aftershocks in 1969 including the main shock, 12 in 1975, and 29 in 1994, totaling 66. I generated 6382 sets of data by taking the double-difference when the hypocentral distance was less than 50 km. For the inversion, the LSQR method was applied with a damping of 30.
4. Results
The rms of the residual of P-wave was initially 2.36 s for the ISC hypocenters and decreased to 1.39 s (-41%) after two iterations of HYPODD. In the case of 1969 event, both the main shock and the aftershock are located near the upper boundary of the subducting Pacific plate, but their epicenters are more than 50 km away from the trench axis. In the case of 1975 event, both the main shock and the aftershock are located near the plate boundary as in the 1969 event, but the epicenter is more than 50 km away from the trench axis. No aftershock occurred in the large slip area near the trench axis, as claimed by Ioki and Tanioka (2016). The case of 1994 event is clearly different from 1969 and 1975 events, and hypocenter concentration near the plate boundary is not seen. The aftershocks are distributed on the plane that strikes almost perpendicularly to the trench axis and dips 30-40 degrees to the southwest. This aftershock distribution is consistent with the fault plane proposed by Tanioka et al. (1995).
Three earthquakes of different types occurred off the east coast of Hokkaido (off Shikotan Island). The first is the 1969 Hokkaido-toho-oki earthquake (Mw8.0; Ioki and Tanioka, 2016), a typical plate boundary megathrust earthquake; the second is the June 10, 1975 earthquake, a tsunami earthquake (Mw7.8; Ioki and Tanioka, 2016); the third is the 1994 Hokkaido-toho-oki earthquake (Mw8.3; Global CMT), an intra-slab earthquake. Among these three earthquakes, the fault plane of the 1994 earthquake is still controversial, with some claiming that the strike is parallel to the trench axis (e.g., Kikuchi and Kanamori, 1995; Katsumata et al., 1995) and others that it is almost perpendicular to the trench axis (e.g., Tanioka et al., 1995). In this study, aftershock distributions of three earthquakes were re-determined by HYPODD, and source faults of three earthquakes were examined.
2. Data
Hypocenter parameters and arrival times of P-wave are published in the ISC Bulletin, and they were used in this study. I selected aftershocks with a body wave magnitude (mb) equal to or larger than 5.3 and a depth of less than 100 km for 7 days after the main shock. I selected 80 seismic stations that satisfy the following conditions; the epicentral distance is smaller than 5000 km and there are P-wave arrival times data for all three earthquakes.
3. Method
For hypocenter determination, I used a double-difference earthquake location method, HYPODD (Waldhauser and Ellsworth, 2000). However, HYPODD calculates travel times assuming a horizontal layered velocity structure, but in order to use teleseismic P-waves, it is necessary to calculate travel times in a spherically stratified velocity structure of the Earth. In this study, I assumed a one-dimensional velocity structure iasp91 and calculated travel times using the iaspei-tau package (Kennett and Engdahl, 1991; Snoke, 2009). I used 25 aftershocks in 1969 including the main shock, 12 in 1975, and 29 in 1994, totaling 66. I generated 6382 sets of data by taking the double-difference when the hypocentral distance was less than 50 km. For the inversion, the LSQR method was applied with a damping of 30.
4. Results
The rms of the residual of P-wave was initially 2.36 s for the ISC hypocenters and decreased to 1.39 s (-41%) after two iterations of HYPODD. In the case of 1969 event, both the main shock and the aftershock are located near the upper boundary of the subducting Pacific plate, but their epicenters are more than 50 km away from the trench axis. In the case of 1975 event, both the main shock and the aftershock are located near the plate boundary as in the 1969 event, but the epicenter is more than 50 km away from the trench axis. No aftershock occurred in the large slip area near the trench axis, as claimed by Ioki and Tanioka (2016). The case of 1994 event is clearly different from 1969 and 1975 events, and hypocenter concentration near the plate boundary is not seen. The aftershocks are distributed on the plane that strikes almost perpendicularly to the trench axis and dips 30-40 degrees to the southwest. This aftershock distribution is consistent with the fault plane proposed by Tanioka et al. (1995).