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

The 2003 Tokachi-oki earthquake (Mw = 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 Tw = 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

Long-Term Seismic Quiescences and Great Earthquakes in and Around the Japan Subduction Zone Between 1975 and 2012

An earthquake catalog created by the International Seismological Center (ISC) was analyzed, including 3898 earthquakes located in and around Japan between January 1964 and June 2012 shallower than 60 km with the body wave magnitude of 5.0 or larger. Clustered events such as earthquake swarms and aftershocks were removed from the ISC catalog by using a stochastic declustering method based on Epidemic-Type Aftershock Sequence (ETAS) model. A detailed analysis of the earthquake catalog using a simple scanning technique (ZMAP) shows that the long-term seismic quiescences lasting more than 9 years were recognized ten times along the subduction zone in and around Japan. The three seismic quiescences among them were followed by three great earthquakes: the 1994 Hokkaido-toho-oki earthquake (M (w) 8.3), the 2003 Tokachi-oki earthquake (M (w) 8.3), and the 2011 Tohoku earthquake (M (w) 9.0). The remaining seven seismic quiescences were followed by no earthquake with the seismic moment M (0) 3.0 x 10(21) Nm (M (w) 8.25), which are candidates of the false alarm. The 2006 Kurile Islands earthquake (M (w) 8.3) was not preceded by the significant seismic quiescence, which is a case of the surprise occurrence. As a result, when limited to earthquakes with the seismic moment of M (0) 3.0 x 10(21) Nm, four earthquakes occurred between 1976 and 2012 in and around Japan, and three of them were preceded by the long-term seismic quiescence lasting more than 9 years

Automatic seismic tomography : Do you believe human or computer?

How much laboratory automation is possible in seismology? We conducted an experiment to compare seismic tomography images using arrival times picked by a computer and a human. If the computer gives the same tomographic image as that the human gives, seismologists have no need to insist upon a manual picking of arrival times. We found that unexpectedly a computer is able to provide rather good results. 1. In troduction In the near future seismologists should entrust a computer with all the process in a seismic tomography, which is a powerful method to image th

Slow slip event in the focal area of the 1975 Kurile tsunami earthquake inferred from unusual long-term seismic quiescence

Abstract In subduction zones, slow slip events (SSEs) have been observed in the portion deeper than the downdip edge of seismogenic zone. However, since it is far offshore from geodetic networks on land, shallow SSEs near the trench axis are hardly observed. Despite of less quantitative than seafloor geodetic observations, a method for inferring the shallow SSEs based on seismic quiescence was presented in this study. Unusual decrease in occurrence rate of M ≥ 5.0 earthquakes was found in the southwestern Kurile Islands. The occurrence rate was ∼1.3 events/year between 1977 and around 2004 and no earthquake was observed during 16 years after 2004. The spatial pattern of the seismic quiescence can be explained qualitatively by the Coulomb failure stress change due to shallow SSE and its fault plane is on the upper boundary of the subducting Pacific plate in the focal area of the 1975 tsunami earthquake