- Email: [email protected]
Global seismicity of 2003: centroid–moment-tensor solutions for 1087 earthquakes
Physics of the Earth and Planetary Interiors 148 (2005) 327–351
Global seismicity of 2003: centroid–moment-tensor solutions for 1087 earthquakes G. Ekstr¨om∗ , A.M. Dziewo´nski, N.N. Maternovskaya, M. Nettles Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138, USA Received 3 September 2004; accepted 13 September 2004
Abstract Centroid–moment-tensor (CMT) solutions are presented for 1087 earthquakes that occurred during 2003. The solutions are obtained using the method of Dziewonski et al. [Dziewonski, A.M., Chou, T.-A., Woodward, J.H., 1981. Determination of earthquake source parameters from waveform data for studies of global and regional seismicity. J. Geophys. Res. 86, 2825–2852] and applying corrections for aspherical Earth structure as represented by the whole-mantle shear-velocity model SH8/U4L8 of Dziewonski and Woodward [Dziewonski, A.M., Woodward, R.L., 1992. Acoustic imaging at the planetary scale. In: Emert, H., Harjes, H.-P. (Eds.), Acoustical Imaging, vol. 19. Plenum Press, New York, pp. 785–797]. The model of inelastic attenuation of Durek and Ekstr¨om [Durek, J.J., Ekstr¨om, G., 1996. A radial model of inelasticity consistent with long-period surface wave attenuation. Bull. Seism. Soc. Am. 86, 144–158] is used to predict the decay of the waveforms. The focal mechanisms of the largest, or otherwise significant, earthquakes of 2003 are reviewed. © 2004 Elsevier B.V. All rights reserved. Keywords: Global seismicity; Centroid–moment-tensor solution; Earthquakes
1. Results This is a report on the global seismicity of 2003 as investigated using the centroid–moment-tensor (CMT) technique. This report is the latest contribution in a series of seismicity reports prepared by the Harvard Seis-
∗ Corresponding author. Tel.: +1 617 495 2351; fax: +1 617 495 8839. E-mail address: [email protected] (G. Ekstr¨om).
0031-9201/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.pepi.2004.09.006
mology group and published in Physics of the Earth and Planetary Interiors since 1983. This report contains the moment tensors and centroid parameters that are the primary results of our analysis. Principal axes and fault-plane parameters can be derived straightforwardly from the tabulated moment-tensor parameters using well-known geometrical relationships (Aki and Richards, 2002). Electronic access to the Harvard CMT catalog, including derived source parameters and graphical representations of the source mechanisms, is provided via the Harvard Seismology web site, http://www.seismology.harvard.edu.
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The CMT method is described in detail by Dziewonski et al. (1981), with later enhancements (incorporation of mantle-wave data) given by Dziewonski and Woodhouse (1983). The 1983 paper is the first example of the application of the CMT method to the systematic study of global seismicity; it contains solutions for 201 events that occurred during 1981. The CMT method was refined by Woodhouse and Dziewonski (1984) with the introduction of corrections for aspherical Earth structure. A brief description of this development is presented in the report for the first quarter of 1984 (Dziewonski et al., 1984). Model M84C of Woodhouse and Dziewonski (1984) was used to represent three-dimensional (3-D) Earth structure; the lower mantle was assumed to be spherically symmetric. Beginning with the third quarter of 1991 (Dziewonski et al., 1992), we use the wholemantle shear-velocity model of Dziewonski and Woodward (1992). Dziewonski et al. (1992) demonstrated that better accounting for 3-D Earth structure improves variance reduction and, consequently, the quality of the CMT determinations. Beginning with January 1994, we compute the decay of amplitude due to attenuation using model QL6 of Durek and Ekstr¨om (1996). References for the earlier CMT reports can be found in Dziewonski et al. (2003). We analyzed 1087 earthquakes for 2003. This is the largest number of earthquakes analyzed in a single year since the start of the CMT project. The addition of these solutions to the Harvard CMT catalog brings the total
number of earthquakes in the catalog for 1976–2003 to 20,855. The total moment release for this 28-year period is 1.05 × 1030 dyn cm. We use seismograms from the Incorporated Research Institutions for Seismology (IRIS)–USGS Global Seismographic Network (GSN) in the CMT analysis. A total of 119,867 seismograms from 122 different stations were included in the analysis for 2003. Fig. 1 shows the geographical distribution of contributing stations. The most frequently used station was ANMO (Albuquerque, New Mexico) which provided waveforms for the analysis of 912 earthquakes. Fig. 2 shows the number of earthquakes analyzed during each of the last 28 years; the number of events with magnitude MW ≥ 6.5 is shown with a darker shade. While these large earthquakes contribute only ∼5% of the total number of events analyzed, they contribute 90–95% of the total moment release (Fig. 3). Smaller events, however, carry valuable seismotectonic information, particularly in regions of low seismicity. Source parameters for all events analyzed are given in Table 1. The number in the first column is the sequence number for each event analyzed in 2003. An asterisk following the event number indicates that mantlewave data were included in the analysis. The sequence number is followed by the day, month, and origin time of the event. The origin time listed is that of the centroid solution, with the estimated standard deviation; δt0 indicates the time shift (in seconds) with respect to the time
Fig. 1. Map showing the locations of the 122 stations of the Global Seismographic Network that contributed seismograms to CMT analysis in 2003. Stations that contributed data to more than 200 earthquakes are shown by hexagons, other stations are shown by squares.
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Fig. 2. Cumulative number of CMT solutions for individual years, 1976–2003. Solutions for events with MW ≥ 6.5 are shown using a darker shade.
Fig. 3. Cumulative seismic moment for individual years; the cumulative moment of earthquakes with MW ≥ 6.5 is shown using a darker shade. The unit on the vertical axis is dyn cm.
Table 1 Centroid–moment-tensor solutions for 1087 earthquakes of 2003
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reported by the National Earthquake Information Service (NEIS) in its Preliminary Determination of Epicenters (PDE). The hypocentral coordinates are also those of the centroid location: δφ0 and δλ0 indicate the perturbations obtained with respect to the PDE epicenter and δh0 is the perturbation with respect to the PDE depth. When no standard deviation is given, the parameter was held fixed in the inversion. For some shallow events, we constrain the source depth by modeling broad-band teleseismic P waves. In these cases, the letter B follows the focal depth in the table. The half duration (Half Drtn) of the earthquake is a fixed parameter in the inversion, estimated using the 1/3 formula th = 1.05 × 10−8 M0 , where th is the half duration in seconds and M0 is the scalar moment in dyn cm. The source time function is modeled as a boxcar. The scale factor (10ex ) is the number by which the scalar seismic moment and the moment-tensor elements must be multiplied to obtain a result in dyn cm. The entries in the table represent the exponent (ex) values. The scalar moment (M0 ) is defined as M0 = (σmax − σmin )/2, where σmax and σmin are the maximum and minimum eigenvalues of the moment tensor. The elements of the moment tensor are given in the standard spherical coordinate system (Gilbert and Dziewonski, 1975). In Cartesian coordinates, Mrr = Mzz , Mθθ = Mxx , Mφφ = Myy , Mrθ = Mxz , Mrφ = −Myz , and Mθφ = −Mxy (see Aki and Richards, 2002). The CMT solutions are constrained to have no isotropic component, so that Mrr + Mθθ + Mφφ = 0. In some cases, the elements Mrθ and Mrφ are also constrained to zero because of the instability of the solution. In these cases, the corresponding values and standard errors are omitted in the table.
2. Large and noteworthy earthquakes of 2003 Of the earthquakes analyzed for 2003, 43 had a magnitude MW (Kanamori, 1977) greater than or equal to 6.5 (M0 ≥ 7 × 1025 dyn cm). The geographical distribution of these events, including a graphical representation of the moment tensors, is shown in Fig. 4.
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Fig. 4. Centroid locations and moment tensors for the 43 largest (MW ≥ 6.5) shallow events of 2003. Plate boundaries are shown by gray lines. The full moment tensor is shown by shading; the nodal lines of the best-double-couple focal mechanism are also shown. The size of the focal mechanisms is a linear function of magnitude. The label above the focal mechanism gives the sequence number of the earthquake, as indicated in Table 1.
Fig. 5. Epicentral region of the September 25, MW = 8.3, Hokkaido earthquake. All shallow (h < 50 km) CMT solutions for 2003 are shown. Two events (497 and 518) preceded the main sequence and occurred approximately 100 km northeast of the future mainshock epicenter. Hexagons show centroid locations. The number above each focal mechanism refers to the sequence number in Table 1. The approximate plate boundary is shown by the gray line. The mainshock epicenter is shown by the black diamond.
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Fig. 6. Map showing the epicentral region of the July 15, MW = 7.5, Carlsberg Ridge earthquake. The ridge–transform plate boundary is shown by the gray line, and the centroid locations of earlier earthquakes (1976–2002) are shown with shaded hexagons. The centroids and focal mechanisms for earthquakes that occurred in 2003 are also shown. Note the large distance between the mainshock epicenter (black diamond) and mainshock centroid (black hexagon). The two aftershocks that lie off the plate boundary (July 21 and 27) show normal faulting.
Fig. 7. Map showing the epicentral region of the August 4, MW = 7.6, South Orkney Islands (Scotia Sea) earthquake. The six focal mechanisms without dates show the earlier (1976–2002) CMT solutions for the area. The open hexagons show the centroid locations of the earthquakes in the 2003 sequence, connected to the focal mechanisms with black lines.
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The largest earthquake of 2003 was the September 25, MW = 8.3, Hokkaido earthquake. This is the third largest earthquake in the CMT catalog, surpassed in moment only by the 1977 Sumba (MW = 8.3) and 2001 Peru (MW = 8.4) earthquakes. The Hokkaido earthquake was followed by a very rich aftershock sequence, shown in Fig. 5. The aftershock mechanisms are generally very consistent with that of the mainshock. The July 15, MW = 7.5, Carlsberg Ridge (Indian Ocean) earthquake was one of the more unusual earthquakes of 2003 (Antolik et al., 2003). The epicenter of the event lies on or near the ridge–transform plate boundary (Fig. 6). The centroid is, however, located nearly 180 km northeast of the epicenter, suggesting that the earthquake ruptured away from the plate boundary. The right–lateral strike–slip focal mechanism indicates motion that is opposite that on the transform fault near the epicenter. It seems likely that this event involved reactivation of a fracture-zone fault. The August 4, MW = 7.6, South Orkney Islands (Scotia Sea) earthquake occurred in the tectonically complex region along the southern boundary of the Scotia plate (Fig. 7). The historical seismicity, as represented in the ISC and USGS catalogs, is moderate, with the largest previous earthquake (in 1973) having an mb of 6.4. The moment tensor of the 2003 mainshock reflects a focal mechanism that is a combination of normal and strike–slip faulting, and the aftershocks show a mixture of strike–slip and normal motion.
Acknowledgements The analysis described here was performed using data from stations of the Global Seismographic Network, operated by the Albuquerque Seismological Laboratory of the U.S. Geological Survey and the IDA group at the University of California, San Diego, in cooperation with the Incorporated Research Institu-
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tions for Seismology (IRIS) and in particular its Data Management Center in Seattle. This research was supported by grant EAR-0207608 from the National Science Foundation.
References Aki, K., Richards, P.G., 2002. Quantitative Seismology, second ed. University Science Books, Sausalito, California. Antolik, M., Abercrombie, R.E., Pan, J., Ekstr¨om, G., 2003. Rupture characteristics and aftershocks of the July 15, 2003 Carlsberg ‘H’ (Indian Ocean) MW 7.6 earthquake. EOS Trans. AGU 82, 1147. Durek, J.J., Ekstr¨om, G., 1996. A radial model of anelasticity consistent with long-period surface wave attenuation. Bull. Seism. Soc. Am. 86, 144–158. Dziewonski, A.M., Woodhouse, J.H., 1983. An experiment in the systematic study of global seismicity: centroid–moment tensor solutions for 201 moderate and large earthquakes of 1981. J. Geophys. Res. 88, 3247–3271. Dziewonski, A.M., Woodward, R.L., 1992. Acoustic imaging at the planetary scale. In: Emert, H., Harjes, H.-P. (Eds.), Acoustical Imaging, vol. 19. Plenum Press, pp. 785–797. Dziewonski, A.M., Chou, T.-A., Woodhouse, J.H., 1981. Determination of earthquake source parameters from waveform data for studies of global and regional seismicity. J. Geophys. Res. 86, 2825–2852. Dziewonski, A.M., Franzen, J.E., Woodhouse, J.H., 1984. Centroid– moment tensor solutions for July–September 1983. Phys. Earth Planet. Int. 34, 1–8. Dziewonski, A.M., Ekstr¨om, G., Salganik, M.P., 1992. Centroid– moment tensor solutions for July–September 1991. Phys. Earth Planet. Int. 72, 1–11. Dziewonski, A.M., Ekstr¨om, G., Maternovskaya, N.N., 2003. Centroid–moment tensor solutions for October–December, 2000. Phys. Earth Planet. Int. 136, 141–159. Gilbert, F., Dziewonski, A.M., 1975. An application of normal mode theory to the retrieval of structural parameters and source mechanisms from seismic spectra. Philos. Trans. R. Soc., Lond. Ser. A 278, 187–269. Kanamori, H., 1977. The energy release in great earthquakes. J. Geophys. Res. 82, 2981–2987. Woodhouse, J.H., Dziewonski, A.M., 1984. Mapping the upper mantle: three dimensional modeling of Earth structure by inversion of seismic waveforms. J. Geophys. Res. 89, 5953–5986.
Global seismicity of 2003: centroid–moment-tensor solutions for 1087 earthquakes G. Ekstr¨om∗ , A.M. Dziewo´nski, N.N. Maternovskaya, M. Nettles Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138, USA Received 3 September 2004; accepted 13 September 2004
Abstract Centroid–moment-tensor (CMT) solutions are presented for 1087 earthquakes that occurred during 2003. The solutions are obtained using the method of Dziewonski et al. [Dziewonski, A.M., Chou, T.-A., Woodward, J.H., 1981. Determination of earthquake source parameters from waveform data for studies of global and regional seismicity. J. Geophys. Res. 86, 2825–2852] and applying corrections for aspherical Earth structure as represented by the whole-mantle shear-velocity model SH8/U4L8 of Dziewonski and Woodward [Dziewonski, A.M., Woodward, R.L., 1992. Acoustic imaging at the planetary scale. In: Emert, H., Harjes, H.-P. (Eds.), Acoustical Imaging, vol. 19. Plenum Press, New York, pp. 785–797]. The model of inelastic attenuation of Durek and Ekstr¨om [Durek, J.J., Ekstr¨om, G., 1996. A radial model of inelasticity consistent with long-period surface wave attenuation. Bull. Seism. Soc. Am. 86, 144–158] is used to predict the decay of the waveforms. The focal mechanisms of the largest, or otherwise significant, earthquakes of 2003 are reviewed. © 2004 Elsevier B.V. All rights reserved. Keywords: Global seismicity; Centroid–moment-tensor solution; Earthquakes
1. Results This is a report on the global seismicity of 2003 as investigated using the centroid–moment-tensor (CMT) technique. This report is the latest contribution in a series of seismicity reports prepared by the Harvard Seis-
∗ Corresponding author. Tel.: +1 617 495 2351; fax: +1 617 495 8839. E-mail address: [email protected] (G. Ekstr¨om).
0031-9201/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.pepi.2004.09.006
mology group and published in Physics of the Earth and Planetary Interiors since 1983. This report contains the moment tensors and centroid parameters that are the primary results of our analysis. Principal axes and fault-plane parameters can be derived straightforwardly from the tabulated moment-tensor parameters using well-known geometrical relationships (Aki and Richards, 2002). Electronic access to the Harvard CMT catalog, including derived source parameters and graphical representations of the source mechanisms, is provided via the Harvard Seismology web site, http://www.seismology.harvard.edu.
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The CMT method is described in detail by Dziewonski et al. (1981), with later enhancements (incorporation of mantle-wave data) given by Dziewonski and Woodhouse (1983). The 1983 paper is the first example of the application of the CMT method to the systematic study of global seismicity; it contains solutions for 201 events that occurred during 1981. The CMT method was refined by Woodhouse and Dziewonski (1984) with the introduction of corrections for aspherical Earth structure. A brief description of this development is presented in the report for the first quarter of 1984 (Dziewonski et al., 1984). Model M84C of Woodhouse and Dziewonski (1984) was used to represent three-dimensional (3-D) Earth structure; the lower mantle was assumed to be spherically symmetric. Beginning with the third quarter of 1991 (Dziewonski et al., 1992), we use the wholemantle shear-velocity model of Dziewonski and Woodward (1992). Dziewonski et al. (1992) demonstrated that better accounting for 3-D Earth structure improves variance reduction and, consequently, the quality of the CMT determinations. Beginning with January 1994, we compute the decay of amplitude due to attenuation using model QL6 of Durek and Ekstr¨om (1996). References for the earlier CMT reports can be found in Dziewonski et al. (2003). We analyzed 1087 earthquakes for 2003. This is the largest number of earthquakes analyzed in a single year since the start of the CMT project. The addition of these solutions to the Harvard CMT catalog brings the total
number of earthquakes in the catalog for 1976–2003 to 20,855. The total moment release for this 28-year period is 1.05 × 1030 dyn cm. We use seismograms from the Incorporated Research Institutions for Seismology (IRIS)–USGS Global Seismographic Network (GSN) in the CMT analysis. A total of 119,867 seismograms from 122 different stations were included in the analysis for 2003. Fig. 1 shows the geographical distribution of contributing stations. The most frequently used station was ANMO (Albuquerque, New Mexico) which provided waveforms for the analysis of 912 earthquakes. Fig. 2 shows the number of earthquakes analyzed during each of the last 28 years; the number of events with magnitude MW ≥ 6.5 is shown with a darker shade. While these large earthquakes contribute only ∼5% of the total number of events analyzed, they contribute 90–95% of the total moment release (Fig. 3). Smaller events, however, carry valuable seismotectonic information, particularly in regions of low seismicity. Source parameters for all events analyzed are given in Table 1. The number in the first column is the sequence number for each event analyzed in 2003. An asterisk following the event number indicates that mantlewave data were included in the analysis. The sequence number is followed by the day, month, and origin time of the event. The origin time listed is that of the centroid solution, with the estimated standard deviation; δt0 indicates the time shift (in seconds) with respect to the time
Fig. 1. Map showing the locations of the 122 stations of the Global Seismographic Network that contributed seismograms to CMT analysis in 2003. Stations that contributed data to more than 200 earthquakes are shown by hexagons, other stations are shown by squares.
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Fig. 2. Cumulative number of CMT solutions for individual years, 1976–2003. Solutions for events with MW ≥ 6.5 are shown using a darker shade.
Fig. 3. Cumulative seismic moment for individual years; the cumulative moment of earthquakes with MW ≥ 6.5 is shown using a darker shade. The unit on the vertical axis is dyn cm.
Table 1 Centroid–moment-tensor solutions for 1087 earthquakes of 2003
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reported by the National Earthquake Information Service (NEIS) in its Preliminary Determination of Epicenters (PDE). The hypocentral coordinates are also those of the centroid location: δφ0 and δλ0 indicate the perturbations obtained with respect to the PDE epicenter and δh0 is the perturbation with respect to the PDE depth. When no standard deviation is given, the parameter was held fixed in the inversion. For some shallow events, we constrain the source depth by modeling broad-band teleseismic P waves. In these cases, the letter B follows the focal depth in the table. The half duration (Half Drtn) of the earthquake is a fixed parameter in the inversion, estimated using the 1/3 formula th = 1.05 × 10−8 M0 , where th is the half duration in seconds and M0 is the scalar moment in dyn cm. The source time function is modeled as a boxcar. The scale factor (10ex ) is the number by which the scalar seismic moment and the moment-tensor elements must be multiplied to obtain a result in dyn cm. The entries in the table represent the exponent (ex) values. The scalar moment (M0 ) is defined as M0 = (σmax − σmin )/2, where σmax and σmin are the maximum and minimum eigenvalues of the moment tensor. The elements of the moment tensor are given in the standard spherical coordinate system (Gilbert and Dziewonski, 1975). In Cartesian coordinates, Mrr = Mzz , Mθθ = Mxx , Mφφ = Myy , Mrθ = Mxz , Mrφ = −Myz , and Mθφ = −Mxy (see Aki and Richards, 2002). The CMT solutions are constrained to have no isotropic component, so that Mrr + Mθθ + Mφφ = 0. In some cases, the elements Mrθ and Mrφ are also constrained to zero because of the instability of the solution. In these cases, the corresponding values and standard errors are omitted in the table.
2. Large and noteworthy earthquakes of 2003 Of the earthquakes analyzed for 2003, 43 had a magnitude MW (Kanamori, 1977) greater than or equal to 6.5 (M0 ≥ 7 × 1025 dyn cm). The geographical distribution of these events, including a graphical representation of the moment tensors, is shown in Fig. 4.
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Fig. 4. Centroid locations and moment tensors for the 43 largest (MW ≥ 6.5) shallow events of 2003. Plate boundaries are shown by gray lines. The full moment tensor is shown by shading; the nodal lines of the best-double-couple focal mechanism are also shown. The size of the focal mechanisms is a linear function of magnitude. The label above the focal mechanism gives the sequence number of the earthquake, as indicated in Table 1.
Fig. 5. Epicentral region of the September 25, MW = 8.3, Hokkaido earthquake. All shallow (h < 50 km) CMT solutions for 2003 are shown. Two events (497 and 518) preceded the main sequence and occurred approximately 100 km northeast of the future mainshock epicenter. Hexagons show centroid locations. The number above each focal mechanism refers to the sequence number in Table 1. The approximate plate boundary is shown by the gray line. The mainshock epicenter is shown by the black diamond.
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Fig. 6. Map showing the epicentral region of the July 15, MW = 7.5, Carlsberg Ridge earthquake. The ridge–transform plate boundary is shown by the gray line, and the centroid locations of earlier earthquakes (1976–2002) are shown with shaded hexagons. The centroids and focal mechanisms for earthquakes that occurred in 2003 are also shown. Note the large distance between the mainshock epicenter (black diamond) and mainshock centroid (black hexagon). The two aftershocks that lie off the plate boundary (July 21 and 27) show normal faulting.
Fig. 7. Map showing the epicentral region of the August 4, MW = 7.6, South Orkney Islands (Scotia Sea) earthquake. The six focal mechanisms without dates show the earlier (1976–2002) CMT solutions for the area. The open hexagons show the centroid locations of the earthquakes in the 2003 sequence, connected to the focal mechanisms with black lines.
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The largest earthquake of 2003 was the September 25, MW = 8.3, Hokkaido earthquake. This is the third largest earthquake in the CMT catalog, surpassed in moment only by the 1977 Sumba (MW = 8.3) and 2001 Peru (MW = 8.4) earthquakes. The Hokkaido earthquake was followed by a very rich aftershock sequence, shown in Fig. 5. The aftershock mechanisms are generally very consistent with that of the mainshock. The July 15, MW = 7.5, Carlsberg Ridge (Indian Ocean) earthquake was one of the more unusual earthquakes of 2003 (Antolik et al., 2003). The epicenter of the event lies on or near the ridge–transform plate boundary (Fig. 6). The centroid is, however, located nearly 180 km northeast of the epicenter, suggesting that the earthquake ruptured away from the plate boundary. The right–lateral strike–slip focal mechanism indicates motion that is opposite that on the transform fault near the epicenter. It seems likely that this event involved reactivation of a fracture-zone fault. The August 4, MW = 7.6, South Orkney Islands (Scotia Sea) earthquake occurred in the tectonically complex region along the southern boundary of the Scotia plate (Fig. 7). The historical seismicity, as represented in the ISC and USGS catalogs, is moderate, with the largest previous earthquake (in 1973) having an mb of 6.4. The moment tensor of the 2003 mainshock reflects a focal mechanism that is a combination of normal and strike–slip faulting, and the aftershocks show a mixture of strike–slip and normal motion.
Acknowledgements The analysis described here was performed using data from stations of the Global Seismographic Network, operated by the Albuquerque Seismological Laboratory of the U.S. Geological Survey and the IDA group at the University of California, San Diego, in cooperation with the Incorporated Research Institu-
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tions for Seismology (IRIS) and in particular its Data Management Center in Seattle. This research was supported by grant EAR-0207608 from the National Science Foundation.
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