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Decay and Expansion of the Early Aftershock Activity Following the 2011, Mw9.0 Tohoku Earthquake O

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Decay and Expansion of the Early Aftershock Activity Following the 2011, Mw9.0 Tohoku Earthquake O GEOPHYSICAL RESEARCH LETTERS, VOL. 39, L18309, doi:10.1029/2012GL052797, 2012 Decay and expansion of the early aftershock activity following the 2011, Mw9.0 Tohoku earthquake O. Lengliné,1,2 B. Enescu,3,4 Z. Peng,5 and K. Shiomi3 Received 19 June 2012; revised 20 August 2012; accepted 20 August 2012; published 28 September 2012. [1] The 2011, Mw9.0 Tohoku earthquake was followed by waveform matching technique has been proven successful in an abundant amount of seismicity providing a unique detecting low frequency earthquakes (LFE) from tremor sig- opportunity to analyze the triggering mechanism of great nals [e.g., Shelly et al., 2007] and recovering missing earth- earthquakes. Although the Tohoku earthquake occurred quakes during aftershock [e.g., Lengliné and Marsan, 2009; close to a dense seismic network, many aftershocks that Peng and Zhao,2009],foreshock[Bouchon et al., 2011; occurred in the first few hours after the mainshock are Kato et al., 2012] and remotely triggered earthquake sequen- not recorded in the earthquake catalogs. Here we use a ces [Meng et al., 2012]. Such a technique is capable of iden- template waveform approach to recover as many as possible tifying earthquakes even when the signal to noise ratio at missing events in the first 12 hours following the Tohoku individual stations is relatively low. It also provides the pos- mainshock. Our analysis is able to detect about 1.4 times sibility of estimating the position and magnitude of the more events than those listed in the High Sensitivity detected events, based on the location and magnitude of the Seismograph (Hi-net) earthquake catalog. Combining our template signal. new dataset with earthquakes that occurred at latter times, [3] The 2011, Mw9.0 Tohoku megathrust earthquake pro- we are able to observe a continuous decay of the aftershock duced an unprecedented amount of seismic activity provid- rate and along strike expansion of aftershock area. We ing a unique opportunity to analyze the earthquake triggering relate the latter observation to the occurrence of post- caused by such a mega-earthquake. However, because the seismic slip over the deep interface. Citation: Lengliné, O., aforementioned incompleteness of the catalog, especially B. Enescu, Z. Peng, and K. Shiomi (2012), Decay and expansion immediately after the mainshock, the genuine patterns of of the early aftershock activity following the 2011, Mw9.0 Tohoku seismicity changes remain to be revealed. earthquake, Geophys. Res. Lett., 39, L18309, doi:10.1029/ [4] We use the waveform matching technique to identify 2012GL052797. aftershocks in the first 12 hours following the 2011 Tohoku earthquake. We show that our method is capable of recov- 1. Introduction ering many missed aftershocks even by the most complete earthquake catalog during this time period (the Hi-net cata- [2] Early aftershock activity carries useful information log). Magnitude and position of the newly detected events regarding the triggering mechanism of earthquakes [e.g., are also estimated. We find an apparent continuous decay of Helmstetter and Shaw, 2009]. Accurate detections and the aftershock rate during the entire time interval investi- characterization of this activity are thus needed to interpret gated in this study. Our results also suggest that the after- the evolution of aftershock sequences. Quantifying this activ- shocks, in the down-dip portion of the subduction interface, ity is also important when analyzing earthquake rate changes expanded southward, extending the initial size of the after- caused by sudden stress variations [e.g., Toda et al., 2011]. shock area. Unfortunately, numerous earthquakes are often missing in earthquake catalogs following large events due to a large 2. Data number of events occurring in a short time interval and to the coda-noise generated by the large events of the sequence [e.g., [5] We use continuous seismic recording from 227 stations Kilb et al., 2007]. To overcome this issue, earthquake detec- of the Japanese High Sensitivity Seismograph Network tion can be improved using the correlation of the continuous (Hi-net), operated by NIED (Figure 1). All stations are seismic signal with pre-determined template events. The recording at a sampling rate of 100 Hz and for most sites all the 3-component seismograms were available, resulting in 661 continuous records. Most of the continuous waveforms 1IPGS, CNRS, Université de Strasbourg, Strasbourg, France. are 12 hours and 10 minutes long and start on March 11, 2011 2Seismological Laboratory, California Institute of Technology, Pasadena, at 14:45:00 local time, i.e., about 1 minute before the main- California, USA. 3 shock origin time (14:46:18). Template events are selected as Earthquake and Volcano Research Unit, National Research Institute for all m > 3.5 events of the Hi-net catalogue that occurred in the Earth Science and Disaster Prevention (NIED), Tsukuba, Japan. 4Earth Evolution Sciences Department, Faculty of Life and Environmental first 8 days following the mainshock, around the mainshock Sciences, University of Tsukuba, Tsukuba, Japan. rupture plane. This selection leaves us with 1401 template 5School of Earth and Atmospheric Sciences, Georgia Institute of events, which are distributed all over the fault plane area Technology, Atlanta, Georgia, USA. (Figure 1). For each template event waveform record, we Corresponding author: O. Lengliné, IPGS, CNRS, Université de extract windows around the P- and S-wave arrival times, at Strasbourg, 5 rue René Descartes, F-67084 Strasbourg CEDEX, all stations and on components where picking times, made by France. ([email protected]) visual inspection by Hi-net operators, are available. The ©2012. American Geophysical Union. All Rights Reserved. windows are 512 points long (5.12 s) and start 128 points 0094-8276/12/2012GL052797 L18309 1of6 L18309 LENGLINÉ ET AL.: TOHOKU EARLY AFTERSHOCKS L18309 record in the frequency range [1–6] Hz (see Text S1 in the auxiliary material).1 For each template event and recording station we obtain a continuous coherence signal that covers the full 12 hours and 10 minutes time interval. The coher- ence signals are shifted by the corresponding P-wave or S-wave travel times from the template event to the recording station, followed by the stacking of all the coherence signals corresponding to a template event. We finally obtain a mean coherence, R(t), by taking an average over the 24 highest coherence values obtained either from S- or P-wave win- dows at each time step. If an event has less than 24 records, it is not processed. 3.2. Selection of Possible Detections [7] Out of the 1401 template events, 1344 have at least 24 records and were assigned a coherence function R(t). For each of theses events, we assign a detection when R(t) exceeded a certain threshold, S. As employed in previous studies [e.g., Peng and Zhao, 2009], we define this threshold from the distribution of the coherence values. We set, S as mR +5sR, where mR and sR are the mean and standard deviation of R(t) (Figure 2). We finally filter out duplicate detections, i.e., detections obtained from different template events that correspond to the same detected event (see methods in Text S1). We get 881 new events for the first 12 hours after the mainshock. This set complements the 629 events already present in the Hi-net catalog that occurred in the first 12 hours (including the template events) and leads Figure 1. Map of the studied area off the coast of Honshu, to an overall number of 1510 earthquakes. For each detected Japan (boundaries for selecting detections are shown by event we assign the time for which the value of R(t) is the black dashed lines). Blue triangles are Hi-net stations for highest as the origin time and assume its location to be the which we have at least one record. Circles represent the tem- same as that of the corresponding template event. plate events used in this study and circles size is related to [8] As a test of our detection procedure, we consider the event magnitude. Both the coseismic (color-coded) and post- template event shown in Figure 2. We are able to detect a seismic slip (black contours) are updates of those obtained by close aftershock, included in the catalog, which occurred 16 Ozawa et al. [2011]. The coseismic model is estimated from minutes after the template event. The epicentral distance the onshore GPS data and seafloor GPS/acoustic observa- between these two events is less than 2 km and they have tions (Geospatial Information Authority of Japan, Onshore similar magnitude (5.3 and 5.1). Figure 2 shows that at and offshore crustal deformation and the coseismic slip dis- almost all stations, a coherence higher than the background tribution of the 2011 Tohoku-Oki earthquake, 2011, http:// value emerges at the time of the wave arrival of the second www.gsi.go.jp/cais/topic110520-index.html) and the after- event. At some stations far from the template event, we can slip distribution is based on the GPS data until October 30, still observe an increase of coherence whereas no picks were 2011 (Geospatial Information Authority of Japan, Postseismic made for the second event. This suggests a better perfor- deformation and the afterslip distribution estimated from mance of the correlation technique compared to manual GEONET data, 2011, http://www.gsi.go.jp/cais/topic110314- picking for identifying phase arrival of closely located index.html). The yellow and blue star (almost overlapping in events, particularly when the signal to noise ratio is small. the figure) represent the locations of the template and detected earthquake, respectively, shown as example in Figure 2. The 3.3. Magnitude dark line marks the strike of the subduction trench where earth- [9] We first derive a scaling relation between the magni- quakes are projected in Figure 4, the cross marks the origin of tude difference for pairs of template events and their wave- this projection; the distance along strike is labeled.
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