Source parameters of the 2011 Yellow Sea earthquake (ML 5.3): Different features from earthquakes on the Korean Peninsula
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB 2012
Received: 3 June 2011
Accepted: 12 December 2011
Published: 28 June 2012
A moderate earthquake of magnitude ML 5.3 occurred in the Yellow Sea on January 12, 2011. We estimated the source parameters and found that the quake was a shallow strike-slip fault event with a moment magnitude of 4.6. The stress drop of this event, 1.2–2.0 MPa, is lower than that of moderate earthquakes inland and at the eastern offshore of the Korean Peninsula, and also that of the typical value for shallow intraplate earthquakes. A stronger event (M ∼ 6) in the southern Yellow Sea in 1984 was previously reported to have a low stress drop. Therefore the low stress drop is probably characteristic of earthquakes in the Yellow Sea region. We found that aftershocks of the 2011 Yellow Sea event, with magnitudes greater than 2, occurred for about 5 days, while similar-sized aftershocks of some inland earthquakes of the Korean Peninsula continued for several hours only. The lower stress drop and greater active aftershocks in the Yellow Sea region might reflect a different tectonic setting from that on the Korean Peninsula.
Earthquake information discussed in this study. Parameters for the 1984 event and ISC magnitude are from ISC catalog, and others from the KMA catalog.
Origin time (UTC)
Latitude (° N)
Southern Yellow Sea
The Korean Peninsula and northeastern China are thought to be located on the Amur Plate (Heki, 1999; Bird, 2003; Petit and Fournier, 2005). The boundaries of the Amur Plate suggested by Bird (2003) and Petit and Fournier (2005) are shown in Fig. 1. As can be seen in the figure, these suggested boundaries are mostly similar but are vastly different around the Yellow Sea and Mongolia. The 2011 Yellow Sea event occurred along the boundary of the Amur Plate suggested by Bird (2003) but was slightly distant from that suggested by Petit and Fournier (2005). Although we cannot judge which proposed boundary is correct, tectonic conditions around the 2011 Yellow Sea event might be different from those on the Korean Peninsula. If so, the earthquake generation mechanism and the seismic features between the two regions, near the 2011 event and on the Korean Peninsula, may be different.
The 2007 Odaesan earthquake, which is one of the largest inland events to have occurred in South Korea in the last 30 years, has been extensively researched (Jo and Baag, 2007; Kim and Park, 2010; Kim et al., 2010; Park and Hahm, 2010). Furthermore, several studies have investigated the source parameters of earthquakes around the Korean Peninsula (Park and Mori, 2005; Park et al., 2007; Choi, 2009, 2010; Jun and Jeon, 2010). In this study, we have analyzed the source parameters of the 2011 Yellow Sea event and compared them to those of Korean earthquakes, including the 2007 Odaesan event, to determine the differences, if any, in the characteristics of earthquake generation in the Yellow Sea and the Korean Peninsula.
2. Source Parameters
Station information used for the analyses of the 2011 event. (KMA: Korea Meteorological Administration, F-net: Broadband Seismograph Network, National Research Institute for Earth Science and Disaster Prevention, Japan, KIGAM: Korea Institute of Geology, Mining and Materials, IC: New China Digital Seismograph Network.)
Epicentral distance (km)
Comparison of earthquake parameters. (CEA: China Earthquake Administration, USGS: U.S. Geological Survey.)
Origin time (UTC)
Next, we estimated the fault radius, rise time, and stress drop of the 2011 Yellow Sea event. We deconvolved the source time function using an empirical Green’s function method (Mori and Frankel, 1990; Park and Mori, 2005; Park and Hahm, 2010). To use this method, a small earthquake is needed for the empirical Green’s function. Since none of the aftershocks or foreshocks was reported, we identified them from continuous waveform data at the HUK station (Fig. 3). Three more stations, JDO, GOS and JJU, at similar epicentral distances (Fig. 3), were also used to search for the small events. However, they had lower signal-to-noise ratios (JDO, GOS), or an ambient noise with similar periods as earthquake signals (JJU). We identified a small earthquake as a foreshock or an aftershock if it was observed at other stations (JDO, GOS, or JJU) as well, or if it had a similar P-S time and shape of waveforms to the mainshock. We looked at waveforms of 5 and 10 days long before, and after, the mainshock, respectively, and found no foreshocks and 16 aftershocks.
3. Seismic Activities
To generalize the seismic activity on the Korean Peninsula, we looked at other earthquakes. Because 100-Hz continuous data have been available only since 2007, only two inland events could be analyzed. One is the 2008 Gongju event with a local magnitude of 3.4, and the other is the 2009 Andong event with a magnitude of 4.0. The earthquake parameters are listed in Table 1 and the epicenters are shown in Fig. 9(b). For these two events, we investigated fore- and after-shocks in the same way as the 2007 and 2011 events. For the 2008 Gongju event, we used the vertical seismogram at the TEJ station at an epicentral distance of 11.3 km and found 10 aftershocks. Unfortunately, about 60 hours of data were unavailable for this event and we could not detect any fore shocks, even if there might have been some. We used vertical waveform data from the ADO station of a distance of 22.9 km for the 2009 Andong event and found 12 aftershocks. There were no foreshocks for the 2009 event.
PGAs as a function of time for all four events are compared in Fig. 9(a). Foreshocks were observed only for the 2007 Odaesan event. Aftershocks of earthquakes on the Korean Peninsula followed within 3 days, while those of the 2011 Yellow Sea event followed by as long as 5 days. The last aftershock of the 2011 event that we observed occurred about 118 hours after the mainshock. We investigated seismograms of 5 more days for the 2011 event, but could not find any other aftershocks. Because the epicentral distances of the stations are quite different from each other, it is impossible to compare the absolute number of aftershocks. Instead we compare the number of aftershocks that are large enough to be detected at a distance of 215 km. The broken horizontal lines on each graph indicate the relative values of the PGA corresponding to ML 2.0, to the PGA of the mainshock assuming ML reported by the KMA. Considering events larger than ML 2.0, inland earthquakes of the Korean Peninsula have very few aftershocks within several hours. However, aftershocks of the 2011 Yellow Sea event continued for at least 5 days. This may indicate that the seismic activity is relatively higher in the region around the 2011 Yellow Sea event than on the Korean Peninsula.
4. Discussion and Conclusion
We estimated the source parameters of a moderate 2011 Yellow Sea earthquake in order to see if there are any differences in the characteristics of earthquake generation in the Yellow Sea and on the Korean Peninsula. The 2011 Yellow Sea event is a shallow strike-slip fault event and the moment magnitude was 4.6. The fault radius, rise time, and stress drop were determined to be 1.5–1.8 km, about 0.2 s and 1.2–2.0 MPa, respectively.
The stress drop of this event, 1.2–2.0 MPa, is lower than that of many earthquakes on the Korean Peninsula, including the Odaesan earthquake in 2007 at more than 8 MPa (Jo and Baag, 2007; Kim et al., 2010; Park and Hahm, 2010), a moderate earthquake in 2004 (2004 M 5.3 event in Fig. 2) at about 7–14 MPa (Park and Mori, 2005), and the 1996 quake (1996 M 4.8 event in Fig. 2) at about 14 MPa (Choi, 2009). Chung and Brantley (1989) determined the stress drop of the 1984 M 6.0 (Ms 6.3) Southern Yellow Sea event (Fig. 2) to be 4.2 MPa and summarized the values of some large earthquakes (M s ∼ 7) in northern China as 10–20 MPa. They suggested that the southern Yellow Sea region is characterized by short recurrence intervals while the northern China region has very long recurrence intervals. Additionally, they argued that short recurrence intervals and low stress drops reflect a lower material strength in the region of the southern Yellow Sea. The 2011 Yellow Sea event also has a low stress drop, which is lower than the typical value for shallow intraplate earthquakes, similar to the 1984 Southern Yellow Sea event. Meanwhile, earthquakes on the Korean Peninsula and just off the east coast seem to have higher stress drops, indicating that they are typical intraplate events. Therefore, the Yellow Sea region may be in a different geologic, or tectonic, setting from that of the Korean Peninsula or northern China. Choi (2010) estimated stress drops of two moderate Yellow Sea events (2003 M 4.9 and 2003 M 5.0 events in Fig. 2) to be about 10–30 MPa. So the southern part of the Yellow Sea may be characterized by low stress drop.
From examination of continuous waveform data following the 2011 Yellow Sea earthquake, we found 16 aftershocks with magnitudes greater than 2 that occurred over 5 days. However, following three earthquakes on the Korean Peninsula, aftershocks with a magnitude greater than 2 followed within several hours. The low level of aftershock activity and high stress drop on the Korean Peninsula may indicate that earthquakes are not so easily triggered on the Korean Peninsula than in the Yellow Sea. The Korean Peninsula seems to be different from the Yellow Sea, but is, however, similar to northern China.
We used waveform data observed at seismic stations of the Korea Meteorological Administration, Korea Institute of Geology, Mining and Materials (KIGAM) and Broadband Seismograph Network (F-net) operated by the National Research Institute for Earth Science and Disaster Prevention (NIED). Some of the figures were made using the Generic Mapping Tools (GMT). Moment tensors were computed using the MTpackageV2.1 package developed by Douglas Dreger of the Berkeley Seismological Laboratory, and relating Green’s functions were computed using the FKRPROG software developed by Chandan Saikia of URS. This study was supported by the project of ‘Study on Development and Application of Earthquake Monitoring Techniques’ by the National Institute of Meteorological Research, Korea Meteorological Administration. We thank the EPS reviewers and editor for their helpful comments to improve the manuscript.
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