Precursory seismic quiescence before the M w = 8.3 Tokachi‐oki, Japan, earthquake on 26 September 2003 revealed by a re‐examined earthquake catalog

The 2003 Tokachi‐oki earthquake ( M w = 8.3) occurred on 26 September 2003 off the pacific coast of Hokkaido, Japan. In the present study, an earthquake catalog is used that lists 2,000 earthquakes with M ≥ 3.3. All of the earthquake waveforms were recorded by the Institute of Seismology and Volcanology, Hokkaido University. In the present study, these waveforms are manually re‐examined, and hypocenters and magnitudes are re‐calculated. A detailed analysis of the re‐determined earthquake catalog between 1994 and 2003 using a gridding technique (ZMAP) shows that the 2003 Tokachi‐oki earthquake is preceded by two neighboring seismic quiescence anomalies that start around the beginning of 1999, and last about 5 years, until the main shock occurs. These quiescence anomalies are located around the asperity ruptured by the main shock, and the Z ‐values are +3.9 and +4.0 for a time window of T w = 4 years, using a sample size of N = 100 earthquakes. The detected seismic quiescences can be interpreted as being caused by a decrease of 50% in the stressing rate based on Dieterich's theory. It is proposed that a quasi‐static pre‐slip occurs at the northeastern edge of the asperity ruptured by the main shock, and lasts for five years until the main shock occurs. By calculating the change in the Coulomb failure stress (ΔCFS), it is found that negative ΔCFS areas are consistent with the two quiescence anomalies, and a positive ΔCFS area corresponds to the hypocenter of the main shock, indicating that the quasi‐static pre‐slip model is a plausible one.