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Precursory seismicity changes prior to major earthquakes along the Sumatra-Andaman subduction zone: a region-time-length algorithm approach
© Sukrungsri and Pailoplee. 2015
- Received: 23 February 2015
- Accepted: 10 June 2015
- Published: 23 June 2015
In this study, we investigated the precursory seismicity changes related to the major earthquakes posed along the Sumatra-Andaman subduction zone (SASZ) using the region-time-length (RTL) algorithm. Based on the suitable RTL characteristics of r 0 = 100 km and t 0 = 2 years, the anomalous RTL score representing the quiescence stage mostly started 0.1–5.2 years before the subsequent major earthquake, while no activation stage was illustrated. For the spatial investigation, the RTL anomalies also clearly illustrated the location of the subsequent major earthquakes. Thus, in order to determine the prospective areas of upcoming earthquakes, the series of RTL maps calculated during the recent 5-year (2010–2014) time span was used. The obtained results reveal four risk areas along the SASZ that might pose a major earthquake in the future, namely (i) Sittwe city, western Myanmar; (ii) offshore northern Nicobar Islands; (iii) Aceh city, northernmost of Sumatra Island; and (iv) offshore western Sumatra Island. Therefore, both a tsunami hazard in the Indian Ocean and a seismic hazard in the far-field cities should be recognized urgently.
- RTL algorithm
- Sumatra-Andaman subduction zone
Using statistical seismology, the possibility of applying the frequency-magnitude distribution (FMD; Gutenberg and Richter 1944) model as an earthquake precursor was demonstrated along the SASZ (e.g., Nuannin et al. 2005; Pailoplee and Choowong 2014). In particular, utilizing the 50 closest earthquakes in an individual site of interest, Nuannin et al. (2005) found that a low-FMD b-value was related to a high accumulated tectonic stress and so implied a prospective area of upcoming earthquakes. Thereafter, using Nuannin et al.’s (2005) assumption, the comparative low-FMD b-value areas along the northern segment of the SASZ were evaluated, revealing that (i) Sittwe city, western Myanmar, and (ii) offshore northern Nicobar Islands might be subject to strong-to-major earthquakes soon (Pailoplee et al. 2013). However, up to 2014, all the seismic risk areas mentioned above are still quiescent and so should be monitored carefully.
Since the existence of quiescent and activation stages of earthquakes was reported (Sobolev 1995), an alternative statistical method called the region-time-length (RTL) algorithm was developed to investigate such stages of seismicity (Sobolev and Tyupkin 1997, 1999). As a result of extended practice, a number of RTL investigations have revealed the successful correlation between the quiescent and/or activation stages and the subsequent moderate-to-major earthquakes in various seismogenic settings, such as the Mw-7.2 Kobe earthquake, Japan (Huang et al. 2001), Mw-6.8 Nemuro earthquake, Japan (Huang and Sobolev 2002), Mw-7.3 Izmit earthquake, Turkey (Huang et al. 2002), MS-7.3 Tottori earthquake, Japan (Huang and Nagao 2002), earthquakes with MS ≥ 5.0 in northern China (Jiang et al. 2004), earthquakes with MS ≥ 6.0 in the Yunnan area (Liu and Su 2006), Mw-7.3 Chi-Chi earthquake, Taiwan (Chen and Wu 2006), MS-8.0 Wenchuan earthquake, China (Huang 2008), and the latest hazardous event of the Mw-9.0 Tohoku earthquake, Japan (Huang and Ding 2012). In order to constrain the prospective earthquake sources proposed previously by Pailoplee et al. (2013), the RTL algorithm was applied in this study to the most up-to-date seismicity data recorded along the SASZ.
According to Eq. (7), the RTL score varies between −1 and 1, where a RTL score of <0 or >0 implies a quiescent or activation stage, respectively, when the background RTL score = 0.
Dataset and completeness
Using the assumption suggested by Gardner and Knopoff (1974), 102,017 events of earthquakes were defined as dependent foreshocks or aftershocks and so were eliminated. The completeness of the earthquake-detecting procedure was checked using the FMD power-law. Based on the assumption of the entire-magnitude range (Woessner and Wiemer 2005), a magnitude of completeness (Mc) of Mw-4.6 was found to cover most parts of the SASZ (Fig. 2c).
In addition, by recognizing the GENAS algorithm (Habermann 1983, 1987), the mainshock data was found to have a consistent seismicity with Mw ≥ 4.6 during 1980–2014. The straight line of the cumulative number of earthquakes (Fig. 2d) indicates no obvious man-made change in the bulk seismicity rate (Wyss 1991; Zuniga and Wiemer 1999). Therefore, all of the remaining 1668 mainshocks with a Mw ≥ 4.6 recorded during 1980–2014 were used in this RTL investigation.
List of earthquake events with Mw ≥ 7.0 posed along the SASZ during 1980–2014
Although only 4 years of seismicity data (1980–1984) were used, the graph illustrates a significant drop in the RTL score at 1984.69 followed by the Mw-7.3 earthquake in November 17th, 1984 (Fig. 3a). In the case of the devastating Mw-9.0 earthquake at the end of 2004, the quiescence stage was evident from 2002.56 and reached its minimum RTL value (−0.58) at 2002.87 (Fig. 3c).
Regarding the time span between the occurrences of an anomalous RTL score and subsequent major earthquakes, most case studies illustrated a short time period of 0.1–5.2 years (Fig. 3a–h). This indicates that monitoring of the RTL measurements with r 0 = 100 km and t 0 = 2 years may be useful for the intermediate-term (months, years) earthquake forecasting along the SASZ. Although the RTL anomalies appear almost 15 years before the Mw-8.6 earthquake of 2012, the calculated RTL score dropped obviously to −0.96 in mid-1997 (Fig. 3i).
Correlation coefficients of the RTL values between different characteristic parameters r 0 and t 0
r 0 = 100 km, t 0 = 2 years
r 0 = 75 km
r 0 = 125 km
t 0 = 1.75 years
t 0 = 2.25 years
Correlation of A and B
where m is the number of RTL score data available in the time span t 1 to t 2.
The RTL maps still illustrated clearly the anomalies at the offshore western Sumatra Island (Fig. 5b, c), where the anomalous RTL score seen 0.19 years during 1997.46–1997.65 was followed by the Mw-7.4 earthquake on November 2nd, 2002 (Fig. 5b) and the Mw-9.0 earthquake of December 26th, 2004, followed 0.12 years of the anomalous RTL score during 2002.87–2002.99 (Fig. 5c). During 2005, two RTL anomalies were evident along the SASZ, where the average RTL score decreased down to −0.3 to −0.8 during 2005.02–2005.44 followed by the Mw-8.6 and Mw-7.3 earthquakes on March 28th and July 24th of 2005 at the southern Nicobar Islands and the offshore western Sumatra Island, respectively (Fig. 5d, e).
Moreover, there were two additional precursory RTL scores that could be observed at the offshore western Sumatra Island during 2006.86–2008.05 (1.19 years) and 2007.70–2008.05 (0.35 years) (Fig. 5f, h), which were followed by earthquakes of Mw-7.4 and Mw-7.8 on February 20th, 2008 and April 6th, 2010, respectively. Meanwhile, for the offshore northern Nicobar Islands, the Mw-7.8 earthquake of August 10th, 2009, was within 1 year of the prominent RTL anomalies seen during 2008.62–2009.58 (Fig. 5g). For the latest major earthquake of the SASZ, only one obvious RTL anomaly was detected 0.23-year long during 1997.35–1997.58, and this preceded the Mw-8.6 earthquake of April 11th, 2012 (Fig. 5i). To test the statistical significance of the obtained RTL anomalies, the earthquake data were synthesized randomly in the study area according to stochastic process (Huang et al. 2002). Thereafter, the RTL anomalies at nine epicenters of the major earthquakes demonstrated here were evaluated. From 10,000 iterative tests with random data, it was revealed that almost all the anomalies obtained here did not result from a stochastic process (Fig. 4b). According to both the temporal and spatial relationship between the origin time and location of the precursory RTL score and the subsequent major earthquakes described above, we preferred and applied the condition of r 0 = 100 km and t 0 = 2 years for the present-day investigation of the RTL algorithm as shown in the next section (the “Present-day investigation” section).
Based on the FMD investigation using the seismicity data until 2010, Pailoplee et al. (2013) proposed two prospective areas of upcoming earthquakes, namely at Sittwe city and the offshore northern Nicobar Islands. This study, using the anomalous RTL values, concurs with the previous work of Pailoplee et al. (2013) and supports the high possibility of a major earthquake soon at Sittwe city and the northern Nicobar Islands.
In addition, it should be mentioned that other intense RTL anomalies were also detected around (i) Aceh city, northernmost of Sumatra Island, and (ii) offshore western Sumatra Island during 2010.43–2011.27 and 2012.35–2014.57, respectively (Figs. 5c and 6a). No significant earthquakes have been reported in the vicinity of these two regions since 2012, and so, these regions are additionally proposed as being high seismic risk areas that are likely to experience an earthquake in the near future.
We investigated retrospectively the seismicity changes prior to nine major earthquakes generated along the SASZ by applying the RTL algorithm with the completeness earthquake catalogue. Based on this iterative test, suitable r 0 (100 km) and t 0 (2 years) values were found and led the RTL algorithm to illustrate the meaningful quiescence anomalies prior to the occurrence of all nine major earthquakes, although no activation stage was prominent. For the duration between the detectable RTL anomalies and the subsequent major earthquakes, quiescence was most frequently detected in the range of 0.1–5.2 years before the earthquake, which provides the best opportunity for the application of this RTL algorithm to intermediate-term earthquake forecasting at the SASZ region. As a result, we also evaluated the spatial variations of the RTL anomalies with the most up-to-date seismicity data (2010–2014) and concluded that there are four regions that have a possibility to generate a major earthquake in the near future (Fig. 6). These were (i) Sittwe city, western Myanmar, (ii) offshore northern Nicobar Islands, (iii) Aceh city, northernmost of Sumatra Island, and (iv) offshore western Sumatra Island. Therefore, effective mitigation plans for both seismic and tsunami hazards should be developed and implemented.
This research was supported by the Ratchadapiseksomphot Endowment Fund 2015 of Chulalongkorn University (WCU-58-021-CC). Thanks are also extended to T. Pailoplee for the preparation of the draft manuscript. I thank the Publication Counseling Unit (PCU), Faculty of Science, Chulalongkorn University, for a critical review and improved English. I acknowledge thoughtful comments and suggestions by the editors and anonymous reviewers that enhanced the quality of this manuscript significantly.
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