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Download Download Research in Geophysics 2016; volume 5:5730 Time dependent seismicity Fuca and North America plates corresponds to the Cascadia subduction zone (CSZ), where Correspondence: Evangelos Christou, Planetary along the western coast the giant M9 earthquake occurred in 1700. Sciences Humboldt Universität zu Berlin, of Canada The present study was motivated by the Jablonskistraße 14, Berlin, Germany. occurrence of the 2012 Haida Gwaii islands Tel.: +49.15258299952. Evangelos V. Christou, George Karakaisis, (formerly the Queen Charlotte islands) earth- E-mail: [email protected] Emmanuel Scordilis quake (M=7.7). This was the first major thrust Key words: Time dependent seismicity; Canada; 1 Department of Geophysics, School of event recorded along the strike-slip QCF. In Haida Gwaii 2012 earthquake. this article we present the results of the appli- Geology, Aristotle University of cation of two time-dependent seismicity mod- Acknowledgements: Evangelos Christou grateful- Thessaloniki, Greece els in an attempt to retrospectively predict the ly acknowledges Dr. G. Papadopoulos (Chair) and 2012 mainshock. Moreover, we apply both mod- the Organizing Committee of the International els searching for future strong earthquakes Workshop Mega Earthquakes and Tsunamis in Subduction Zones for the financial support. Maps along the western coast of Canada. Abstract were made using Generic Mapping Tools GMT 4.5 (Wessel and Smith, 1998). We used data from the Models applied and data National Resources Canada (On-line Bulletin), Decelerating generation of intermediate The first of the two time-dependent seismic- http://earthquakescanada.nrcan.gc.ca/stndon/NE magnitude earthquakes (preshocks) in a nar- ity models is based on the triggering of a main- DB-BNDS/bull-eng.php, Nat. Res. Can., (last row region (seismogenic region) and acceler- shock by its preshocks and is called decelerat- accessed: July 23, 2014). We thank the anony- ating generation of relatively larger such mous reviewer for the helpful comments and sug- ing-accelerating seismic strain (D-AS) model. gestions. earthquakes in a broader region (critical There is reliable evidence that an increase in region) has been proposed as an appropriate the occurrence rate of intermediate-magni- Conference presentation: this work has been pre- model for intermediate-term earthquake pre- tude shocks has been observed in a broad area sented in the International Workshop Mega diction. We examined the seismic activity before strong earthquakes.2-8 Recently, howev- Earthquakes and Tsunamis in Subduction Zones- which preceded the Mw=7.7 (October 28, 2012) er, skepticism has been expressed on the reli- Forecasting Approaches and Implications for thrust event that occurred off the west coast of ability of these observations,9,10 which is dis- Hazard Assessment, held in Rhodes Island, Greece (2014). Haida Gwaii, Canada (formerly the Queen cussed later. Quantification of the accelerating Charlotte islands), by applying the decelerat- pattern of these earthquakes that occur in this Received for publication: 24 November 2014. ing-accelerating seismic strain model. We 11 broad region before a mainshock, showed Revision received: 6 December 2015. found that this mainshock was preceded by a that the cumulative Benioff strain, S(t), can be Accepted for publication: 22 December 2015. pronounced accelerating seismic sequence expressed by the following power law: with the time to the mainshock, as well as by This work is licensed under a Creative Commons an equally easily identifiable decelerating seis- Attribution NonCommercial 4.0 License (CC BY- mic sequence. Both precursory seismic (1) NC 4.0). sequences occurred in different space, time and magnitude windows. The behavior of pre- ©Copyright E.V. Christou et al., 2016 where tc is the origin time of the mainshock Licensee PAGEPress, Italy vious mainshocks that occurred close to the and A, B, m, are parameters calculated by the Research in Geophysics 2016; 5:5730 2012 earthquake was also examined by the available data (with m<1, B<0). The quantity doi:10.4081/rg.2016.5730 time and magnitude predictable regional S(t), which is considered as a measure of the model. preshock seismic deformation at time t, is An attempt was also made to identify such seismic strain patterns, which may also be defined as , where Ei is the (M=6.3-9.0) shallow (h<100 km) mainshocks related to the generation of strong mainshocks worldwide.15 Tests performed on synthetic cat- along the western coast of Canada. seismic energy of the ith preshocks and n(t) is alogues15,16 and retrospective predictions of the number of preshocks occurred up to time t. recent strong mainshocks have been used to On the other hand, it has also been observed evaluate the model whereas forward tests led Non commercialthat in the narrow (focal) region of anuse ensuing only to the successful intermediate-term prediction Introduction mainshock, a seismic excitation is followed by of two strong earthquakes in the Aegean.17,18 a drop of seismicity, i.e., a seismic quiescence During the formulation of the D-AS model it Strong and large earthquakes along the period.12,13 Global data were used14 to show that was observed that each of the investigated western coast of Canada are not uncommon. intermediate magnitude preshocks in the focal During the instrumental period (since 1898, region form a decelerating pattern and that the mainshocks was preceded by a decelerating when the first seismograph of Milne type was time variation of the cumulative Benioff strain preshock sequence, generated in a relatively small region (seismogenic region) where the installed at Victoria) several M≥7.0 events up to the mainshock also follows a power-law occurred there. The majority of the strong (relation 1) but with a power value larger than mainshock is also located, and by an accelerat- earthquakes in this area are associated with one (m>1). That is, this pattern of decelerat- ing preshock sequence generated in a broader the motion between the three major lithos- ing strain in the focal region is formed of a region (critical region) and that both precurso- pheric plates, namely Pacific, North America seismic excitation followed by a decrease of ry seismic sequences have predictive proper- and Juan de Fuca (Figure 1). The boundary seismicity of intermediate magnitude ties, related to the ensuing mainshock. between Pacific and North America plates is a preshocks. Decelerating preshocks occur in different right lateral transform fault (Queen Charlotte The formulation of the D-AS model for inter- time, magnitude and space windows than the Fault, QCF) that extends from Vancouver mediate-term earthquake prediction was accelerating preshocks. The latter start earlier Island up to Alaska and the Fairweather Fault based on the examination of the patterns than the former (tsa>tsd), and their magnitudes (FF), whereas the boundary between Juan de described above, which preceded strong are larger the magnitudes of the decelerating [Research in Geophysics 2016; 5:5730] [page 1] Article preshocks. The strain acceleration, qa, as well parameter and Pd is the probability that a than the relatively short span of the earth- as the strain deceleration, qd, (also called qual- decelerating strain release fulfils relations (2) quake catalogues used, it is preferable to con- ity indexes), vary with the time to the main- and (4). Both quality indexes qd and qa are very sider seismogenic sources, i.e., circular shock.17 Both start with low values, attain their useful in searching for decelerating and accel- regions that include, in addition to the main largest values several years before the main- erating seismicity patterns since they attain fault where the largest mainshock occurs, shock occurrence and cease gradually about their largest values at the seismogenic and other smaller faults where smaller mainshocks three years before the mainshock, i.e., the critical region, respectively. Global observa- occur. On the basis of this idea, the time and seismic activity declines in the critical region tions15 resulted in the following cut-off values magnitude predictable regional (TIMAPR) and increases in the seismogenic region. of the parameters that describe the decelerat- model has been proposed,21 which makes use The radii r (km) of the seismogenic region ing and the accelerating precursory seismic of numerous interevent times of strong earth- and R (km) of the critical region, which are sequences: quakes (mainshocks) generated in a region, in assumed circular, are given by the relations:15 order to investigate the time-dependent seis- (8) micity of this region. This model has been developed by considering a large sample of (9) global data,21 which was used to derive the fol- (2) lowing two equations that relate the interevent The second of the models applied in the time, Tt (in years), until the next mainshock present work makes use of interevent times of and its expected magnitude, Mf, to the magni- (3) strong earthquakes in an area, which depend tude, Mp, of the previous mainshock in the on the rate of tectonic loading. However, since region examined, the long-term seismicity 1/2 such earthquakes, that occur on a single fault, level, Sd (the seismic strain rate in Joule per with M being the mainshock magnitude and sd usually have recurrence times much larger year) and the minimum mainshock magni- 1/2 4 2 and sa (in J /y × 10 km ) is the Benioff strain in the seismogenic and critical region, respec- tively. The following two relations hold also for the decelerating and accelerating preshock sequences:15 (4) (5) where tsd and tsa are the start times (in years) of the decelerating and accelerating preshock sequence, respectively. The curvature parameter, C, has been pro- posed19 as a measure of accelerating strain release and equals to the ratio of the RMS error of the power-law fit (Eq. 1) to the corre- sponding linear fir error.
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