Real-time W phase inversion during the 2011 off the Pacific coast of Tohoku Earthquake
© 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. 2011
Received: 8 April 2011
Accepted: 24 May 2011
Published: 27 September 2011
The real time W phase source inversion algorithm was independently running at three organizations (USGS, PTWC and IPGS) at the time of the 2011 off the Pacific coast of Tohoku Earthquake. Valuable results for tsunami warning purposes were obtained 20 min after the event origin time. Within the next hour, as more data became available, the W phase solutions improved, and converged to a common result (Mw ≈ 9.0, dip ≈ 14°). A post-mortem W phase analysis using data selection based on pre-event noise confirmed the Mw = 9.0 result and yielded a best double couple given by (strike/dip/rake = 196°/12°/85°). We also ran the algorithm with increasingly longer periods (T ≈ 1500 sec) to test for the possibility of additional slow slip. The seismic moment remained stable, confirming the prior results.
Many moment tensor inversion codes are currently used at organizations providing real-time solutions of earthquakes. However, it is usually difficult to handle very large earthquakes (Mw ≥ 8.0) with conventional real-time source inversion techniques. Until recently, it was often necessary to wait for at least several hours to obtain a reliable point source solution. One of the difficulties encountered is the clipping of seismograms by signals from great earthquakes. The large spatial and temporal scales of great events call for the use of very long period data, but these records are often clipped for such large events even at teleseismic distances at the arrival of the first surface wave trains.
The W phase source inversion algorithm was specifically developed to handle very large earthquakes like the recent 2011 off the Pacific coast of Tohoku Earthquake (hereafter, abbreviated to 2011 Tohoku Earthquake) and the 2010 Maule, Chile, earthquake. This method exploits the long period content of the broadband seismic record (200 sec- 1000 sec) preceding the arrival of the surface waves. The interest in this new algorithm has rapidly grown because of its ability to quickly provide reliable source parameters of large earthquakes, which can subsequently be used as input for other applications such as ShakeMap generation (e.g. USGS, 2011a), tsunami propagation modeling, finite source inversions (e.g. USGS, 2011b), etc.
2. Real-Time Results
Real-Time W phase solutions obtained for the 2011 Tohoku Earthquake. The real-time instances of the W phase algorithm are running with a fixed depth, specified for each solution in the table.
USGS automatic trigger (internal)
PTWC automatic trigger 1
PTWC automatic trigger 2
PTWC manual trigger
IPGS automatic trigger 1
USGS automatic trigger (internal)
1 h 02 min
1 h 30 min
IPGS automatic trigger 2
3. Post-mortem Results
In this section we attempt to improve the real-time W-phase solutions by using a subset of higher quality channels of seismic data. We retrieved the LH channels belonging to the FDSN, GSN and STS1 global virtual networks of IRIS within epicentral distances of 90°. Most of these channels used belong to the BK, CI, CN, G, GE, IC, II, IU, MN, and US networks, and four sensors are commonly used: STS-1, STS-2, KS-5400 and CMG-3T. We use time domain deconvolution to retrieve ground displacement and then filter between 1 and 5 mHz.
3.1 First-order magnitude estimation
3.2 Centroid moment tensor solution
As discussed in Section 2, the initial real-time W phase solutions at all three institutions provided quite similar solutions, with Mw estimates ranging from 8.8 to 9.0, and fault dip values distributed between 10° and 20°, depending on the assumed initial depth. We explore here the possibility of narrowing this uncertainty by improving the overall signal-to-noise ratio through the careful selection of low noise stations, so as to best resolve the moment tensor elements. Since manual data selection is always questionable because it involves some subjective assessment of the waveform quality, a fully automated screening is used here. The screening scheme and criteria are detailed as follows.
4. Scalar Seismic Moment at Very Long Period
Summary of results for the application of the W phase source inversion algorithm with longer period frequency bands. Run “0” yields dip = 11.9°. This value is used to fix the dip in subsequent runs, to avoid the moment-dip trade-off effect.
Zero-trace (standard algorithm)
Double couple, Fixed dip
Double couple, Fixed dip
Double couple, Fixed dip
Double couple, Fixed dip
No significant variation of Mw with frequency can be seen in this experiment. The small observed variation of Mw is one order of magnitude smaller than the variation caused by dip or depth uncertainties.
5. Discussion and Conclusion
We reported here on the performance of the W phase source inversion algorithm for the 2011 Tohoku Earthquake. The algorithm was independently running at three organizations at the time of the 2011 Tohoku Earthquake (USGS, PTWC and IPGS). Results good enough for tsunami warning purposes were obtained at the USGS and the PTWC within 20 min of the event origin time. Over the next hour, the W phase solutions improved, and converged to a common result, as more data became available. Also, the centroid location, around 38.5° and 143.0°, indicated that the source was offshore, fairly close to the Japan Trench. Knowing the large magnitude, Mw ≈ 9, and the offshore location soon after the origin time can be a key element for suggesting the occurrence of an extraordinary event and the necessity for initiating large-scale rapid emergency activities.
Since it took 20 min to obtain the first solution using the global data, a faster method is desirable for regional tsunami warning purposes.
Earlier, we demonstrated (Kanamori and Rivera, 2008b; Rivera and Kanamori, 2009) that if regional data such as the Japanese F-net data are available in real-time, it is possible to shorten the 20 min solution-time to as little as 6 min. Thus, with the advent of high-quality seismic data and high-rate GPS data at regional distances, we believe that the W phase inversion method can yield key information for effective, rapid regional tsunami warning. For this event we tested the inversion using the F-net data after they became available on April 26, 2011. Using the data from 5° to 12°, we obtained a solution with Mw = 9.1 and a best double couple given by (strike/dip/rake = 201°/10°/92°). Because of the limited azimuthal coverage, the centroid location cannot be constrained well. Nevertheless, the solution is adequate for early warning purposes. In this case, the entire analysis would have been completed about 7 min after O.T.
For the 2011 Tohoku Earthquake, the first tsunami arrived at the coastal areas in about 15 min. It is difficult to determine the exact tsunami arrival times at Miyako, Ka-maishi, Ofunato, and Ayukawa (the most severely affected cities) because of clipping of tide gauge records. However, a GPS wave gauge GPS1 operated by Port and Airport Research Institute (PARI) is located just offshore of Kamaishi. On this record (PARI, 2011), after the disturbance due to the arrival of seismic waves and ground subsidence, the water level started rising gradually at 15:01 JST (approximately 15 min after O.T.), then rapidly at 15:07:30 JST (approximately 21 min after O.T.), and reached a maximum at 15:12 JST (approximately 26 min after O.T.).
To improve the real-time results, post-mortem W phase inversions have been conducted. The strategy was to improve the signal-to-noise ratio by including a pre-inversion screening scheme using additional criterion based on measured long period pre-event noise. The solution obtained is an almost pure double couple (non-double couple = 0.7%), with a scalar moment of 4.26e+29 dyn-cm (Mw = 9.02) and a shallow dipping fault plane of 11.9°. For the purpose of testing for a possible seismic moment variation at very long periods, we performed additional W phase inversions using increasingly long period band pass filters. We found no evidence of such variation, thus confirming the scalar moment value discussed above.
We thank Dr. A. Hutko and an anonymous reviewer for their helpful comments on the manuscript. This work uses Federation of Digital Seismic Networks (FDSN) seismic data. The Incorporated Research Institutions for Seismology (IRIS) Data Management System (DMS) and the F-Net (NIED) data centres were used to access the data.
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