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USGS Earthquake Moment Tensor Catalog
,50 USGS Earthquake Moment Tensor Catalog Stuart A. Sipkin US Geological Survey, Denver, USA
1. I n t r o d u c t i o n Since 1981, the US Geological Survey's National Earthquake Information Center (NEIC) has been computing moment tensor solutions for all moderate-to-large sized earthquakes. From 1981 through the first half of 1982, a moment tensor inversion was attempted for all earthquakes with a magnitude, mb or Ms, of 6.5 or greater. From the second half of 1982 through 1994, an inversion was attempted for all earthquakes with an mb magnitude of 5.8 or greater (5.7 for intermediate- and deep-focus earthquakes). Beginning in 1995, an inversion was attempted for all earthquakes with a magnitude, mb or Ms, of 5.5 or greater. In the earlier parts of the catalog, completeness is compromised because of sparse station coverage for many parts of the Earth. Station coverage has steadily improved, and is mainly responsible for the ability to move the magnitude thresholds lower with time. The solutions are published on a monthly basis in the Preliminary Determination of Epicenters, the Bulletin of the Seismological Society of America, and Seismological Research Letters. Annual compilations are published in Physics of the Earth and Planetary Interiors. These currently include solutions for earthquakes that occurred from 1980 through 2000 (Sipkin, 1986b, 1987; Sipkin and Needham, 1989, 1991, 1992, 1993, 1994a,b; Sipkin and Zirbes, 1996, 1997; Sipkin et al., 1998, 1999, 2000a,b, 2002).
2. M e t h o d The algorithm is described in Sipkin (1982, 1986b). Briefly, the inversion procedure is based on multichannel signalenhancement theory (Robinson, 1967). In this algorithm the farfield Green's functions are the multichannel input; the observed seismograms are the desired output; and the moment-rate tensor is the convolution filter operating on the input. Because the convolution filter found is not only a signal-enhancement filter
but is also a noise-rejection filter, arrivals in the waveform that are not specifically accounted for in the Green's functions (such as those generated by near-receiver or near-source structure, other than pP and sP phases), and that are not coherent across the suite of seismograms to be inverted, are regarded as noise and do not affect the solution. In this formalism, the system of equations to be solved are in the form of a "block Toeplitz" matrix, so it can be quickly solved using recursive techniques. These techniques are equivalent to using recurrence relations in orthogonal polynomial theory. The details of these recursive techniques can be found in Robinson (1967), Claerbout (1976), or Oppenheim (1978). This inversion technique is applied to digitally recorded broadband P-waveform data from US National Network (USNSN) and Global Seismograph Network (GSN) stations that have good signal-to-noise ratios and that are located at epicentral distances between 30 ~ and 95 ~ The solution is constrained to be purely deviatoric, but not to be a pure double couple. The source depth is determined by varying the trial depth at 1-km intervals to find the focal depth that minimizes the misfit to the data. Because the P-wave forms contain both pP and sP phases, as well as direct P-wave arrivals, this procedure generally is highly sensitive to source depth. The source depth also depends somewhat on the velocity model used in the inversion. The model currently used is ak135 (Kennett et al., 1995). Although this model is quite adequate for computing the effects of mantle structure, its crustal structure may be too simple. This will not affect the mechanism found but may bias the depth estimate. Experiments involving the introduction of more realistic crustal layers yield differences on the order of 2-4 km (Sipkin, 1989), smaller than the intrinsic uncertainty of 5-10 km determined using both inversions of "realistic" synthetic seismograms (Sipkin, 1986a) and comparison with a set of well-located earthquakes (Engdahl et al., 1998). An advantage that using body-wave seismograms has over the use of surface waves is better resolution of source depth for shallow sources. The 823
824 inversion kernels for two of the moment tensor elements approach zero as the source depth approaches the Earth's surface. This occurs more rapidly at longer periods. Although the use of higher-frequency body waves does not eliminate this problem, it does ameliorate it. The deviatoric moment tensor can be decomposed into a double couple and a remainder term in an infinite number of ways. We prefer the decomposition suggested by Knopoff and Randall (1970) in which the moment tensor is decomposed into a "best" double couple and a compensated linear-vector dipole (CLVD). This decomposition is preferred to decomposing the moment tensor into a major and minor double couple because it is unique, whereas there are an infinite number of major and minor double-couple combinations. In addition, the "best" double couple and CLVD share the same principal axes; the major and minor double-couples will, in general, have equivalent force systems with differing directions. Using the preferred decomposition, the moment of the "best" double couple, M0, is 1/2(el - e3) where el is the largest positive eigenvalue, corresponding to the T axis, and e3 is the largest negative eigenvalue, corresponding to the P axis. The contribution of the CLVD component to the normalized total moment, ~QCLVO,is 2]eminl/]emax[, where emin is the eigenvalue with the smallest absolute value and emax is the eigenvalue with the largest absolute value. The percentage double couple is then 100 • (1 -/~CLVO).
Sipkin
approximately 30. Since 1995 the average number of stations per event has remained at this level.
4. Moment Tensor Inversion Code An important caveat is that the moment tensor source code was not originally intended for general distribution, and so is not very well documented. It is, however, included here (on the attached Handbook CD) for historical documentation purposes. The inversion software is modular. That is, an independent program handles each step of the process. The programs are bound by a UNIX shell script. This could also be accomplished on different operating systems with a VAX/VMS command file, a DOS batch file, etc. Many of the programs are for preprocessing (filtering, winnowing, and aligning) the data. All of these modules are contained in momten.tar.Z. The code also refers to several library modules--source code for each is contained in compressed tar files. The routines for recursively solving the system of equations can be found in Robinson (1967). momcon.tar.Z contains routines for converting from one type of source description (double-couple, principal axes, moment tensor, slip vector, etc.) to another (written by R.P. Buland, some routines modified by S.A. Sipkin). buplot.tar.Z contains the plotting package used (BUPLOT, written by R.P. Buland), but any plotting package of choice will do.
3. Moment Tensor Catalog The moment tensor solutions from 1981 through 2000, along with the hypocentral parameters determined by the NEIC, are listed in the files MT.LIS and FMECH.LIS included on the attached Handbook CD. These files, as well as updates, can also be downloaded via anonymous FTP at ghtftp.cr.usgs.gov in directory ./pub/momten. MT.LIS contains the elements of the moment tensors along with detailed event information; FMECH.LIS contains the decompositions into the principal axes and best double-couples; the file FMECH.DOC describes the various fields. As indicated in the Section 1, the catalog is not uniform over time. Beginning in 1981, the magnitude threshold was 6.5; in the second half of 1982, the threshold was reduced to 5.8 (5.7 for intermediate- and deep-focus earthquakes); and starting in 1995, the threshold was further reduced to 5.5. Since 1981 global coverage with broadband, digitally recording seismic stations has vastly improved. The improved station coverage is mainly responsible for the ability to move the magnitude thresholds lower with time. This is reflected in the annual average number of stations used per event. From 1981 through 1991, the average number of stations per event gradually rose from approximately 8 to approximately 13. By 1995, the average number of stations had rapidly increased to
Acknowledgments The staffs of the USGS National Earthquake Information Center, the USGS Albuquerque Seismological Laboratory, the IDA program at the Scripps Institution of Oceanography, and the Incorporated Research Institutions for Seismology make this work possible by operating and maintaining the US National Seismograph Network and the Global Seismograph Network and its data centers. Madeleine Zirbes wrote much of the preprocessing code and made several valuable suggestions for improving the procedure. Russell Needham, Madeleine Zirbes, and Charles Bufe did most of the routine data processing.
References Aki, K. and P.G. Richards (1980). "Quantitative Seismology," vol. 1, Freeman, San Francisco. Claerbout, J.F. (1976). "Fundamentals of Geophysical Data Processing," McGraw-Hill, New York. Engdahl, E.R., R. van der Hilst, and R. Buland (1998). Global teleseismic earthquake relocation with improved travel times and procedures for depth determination, Bull. Seismol. Soc. Am. 88, 722-743.
825
USGS Earthquake Moment Tensor Catalog
Kennett, B.L.N., E.R. Engdahl, and R. Buland (1995). Constraints on seismic velocities in the Earth from traveltimes. Geophys. J. Int. 122, 108-124. Knopoff, L. and M.J. Randall (1970). The compensated linear-vector dipole: a possible mechanism for deep earthquakes, J. Geophys. Res. 75, 4957-4963. Oppenheim, A.V. (1978). "Applications of Digital Signal Processing" Prentice-Hall, Englewood Cliffs, NJ. Robinson, E.A. (1967). "Multichannel Time Series Analysis with Digital Computer Programs." Holden-Day, San Francisco. Sipkin, S.A. (1982). Estimation of earthquake source parameters by the inversion of waveform data: synthetic waveforms. Phys. Earth Planet. Inter. 30, 242-259. Sipkin, S.A. (1986a). Interpretation of non-double-couple earthquake source mechanisms derived from moment tensor inversion. J. Geophys. Res. 91, 531-547. Sipkin, S.A. (1986b). Estimation of earthquake source parameters by the inversion of waveform data: global seismicity, 1981-1983. Bull. Seismol. Soc. Am. 76, 1515-1541. Sipkin, S.A. (1987). Moment tensor solutions estimated using optimal filter theory for 51 selected earthquakes, 1980-1984. Phys. Earth Planet. Inter. 47, 67-79. Sipkin, S.A. (1989). Moment-tensor solutions for the 24 November 1987 Superstition Hills California earthquakes. Bull. Seismol. Soc. Am. 79, 493-499. Sipkin, S.A. and R.E. Needham (1989). Moment-tensor solutions estimated using optimal filter theory: global seismicity, 1984-1987. Phys. Earth Planet. Inter. 57, 233-259. Sipkin, S.A. and R.E. Needham (1991). Moment-tensor solutions estimated using optimal filter theory: global seismicity, 1988-1989. Phys. Earth Planet. Inter. 67, 221-230. Sipkin, S.A. and R.E. Needham (1992). Moment-tensor solutions estimated using optimal filter theory: global seismicity, 1990. Phys. Earth Planet. Inter. 70, 16-21. Sipkin, S.A. and R.E. Needham (1993). Moment-tensor solutions estimated using optimal filter theory: global seismicity, 1991. Phys. Earth Planet. Inter. 75, 199-204.
Sipkin, S.A. and R.E. Needham (1994a). Moment-tensor solutions estimated using optimal filter theory: global seismicity, 1992. Phys. Earth Planet. Inter. 82, 1-7. Sipkin, S.A. and R.E. Needham (1994b). Moment-tensor solutions estimated using optimal filter theory: global seismicity, 1993. Phys. Earth Planet. Inter. 86, 245-252. Sipkin, S.A. and M.D. Zirbes (1996). Moment-tensor solutions estimated using optimal filter theory: global seismicity, 1994. Phys. Earth Planet. Inter. 93, 139-146. Sipkin, S.A. and M.D. Zirbes (1997). Moment-tensor solutions estimated using optimal filter theory: global seismicity, 1995. Phys. Earth Planet. Inter. 101, 291-301. Sipkin, S.A., M.D. Zirbes, and C.G. Bufe (1998). Moment-tensor solutions estimated using optimal filter theory: global seismicity, 1996. Phys. Earth Planet. Inter. 109, 65-77. Sipkin, S.A., C.G. Bufe, and M.D. Zirbes (1999). Moment-tensor solutions estimated using optimal filter theory: global seismicity, 1997. Phys. Earth Planet. Inter. 114, 109-117. Sipkin, S.A., C.G. Bufe, and M.D. Zirbes (2000a). Moment-tensor solutions estimated using optimal filter theory: global seismicity, 1998. Phys. Earth Planet. Inter. 118, 169-179. Sipkin, S.A., C.G. Bufe, and M.D. Zirbes (2000b). Moment tensor solutions estimated using optimal filter theory: global seismicity, 1999. Phys. Earth Planet. Inter. 122, 147-159. Sipkin, S.A., C.G. Bufe, and M.D. Zirbes (2002). Moment tensor solutions estimated using optimal filter theory: global seismicity, 2000. Phys. Earth Planet. Inter., 130, 129-142.
Editor's Note The moment tensor catalog and software files are given on the attached Handbook CD, under directory \ 5 0 Sipkin.
1. I n t r o d u c t i o n Since 1981, the US Geological Survey's National Earthquake Information Center (NEIC) has been computing moment tensor solutions for all moderate-to-large sized earthquakes. From 1981 through the first half of 1982, a moment tensor inversion was attempted for all earthquakes with a magnitude, mb or Ms, of 6.5 or greater. From the second half of 1982 through 1994, an inversion was attempted for all earthquakes with an mb magnitude of 5.8 or greater (5.7 for intermediate- and deep-focus earthquakes). Beginning in 1995, an inversion was attempted for all earthquakes with a magnitude, mb or Ms, of 5.5 or greater. In the earlier parts of the catalog, completeness is compromised because of sparse station coverage for many parts of the Earth. Station coverage has steadily improved, and is mainly responsible for the ability to move the magnitude thresholds lower with time. The solutions are published on a monthly basis in the Preliminary Determination of Epicenters, the Bulletin of the Seismological Society of America, and Seismological Research Letters. Annual compilations are published in Physics of the Earth and Planetary Interiors. These currently include solutions for earthquakes that occurred from 1980 through 2000 (Sipkin, 1986b, 1987; Sipkin and Needham, 1989, 1991, 1992, 1993, 1994a,b; Sipkin and Zirbes, 1996, 1997; Sipkin et al., 1998, 1999, 2000a,b, 2002).
2. M e t h o d The algorithm is described in Sipkin (1982, 1986b). Briefly, the inversion procedure is based on multichannel signalenhancement theory (Robinson, 1967). In this algorithm the farfield Green's functions are the multichannel input; the observed seismograms are the desired output; and the moment-rate tensor is the convolution filter operating on the input. Because the convolution filter found is not only a signal-enhancement filter
but is also a noise-rejection filter, arrivals in the waveform that are not specifically accounted for in the Green's functions (such as those generated by near-receiver or near-source structure, other than pP and sP phases), and that are not coherent across the suite of seismograms to be inverted, are regarded as noise and do not affect the solution. In this formalism, the system of equations to be solved are in the form of a "block Toeplitz" matrix, so it can be quickly solved using recursive techniques. These techniques are equivalent to using recurrence relations in orthogonal polynomial theory. The details of these recursive techniques can be found in Robinson (1967), Claerbout (1976), or Oppenheim (1978). This inversion technique is applied to digitally recorded broadband P-waveform data from US National Network (USNSN) and Global Seismograph Network (GSN) stations that have good signal-to-noise ratios and that are located at epicentral distances between 30 ~ and 95 ~ The solution is constrained to be purely deviatoric, but not to be a pure double couple. The source depth is determined by varying the trial depth at 1-km intervals to find the focal depth that minimizes the misfit to the data. Because the P-wave forms contain both pP and sP phases, as well as direct P-wave arrivals, this procedure generally is highly sensitive to source depth. The source depth also depends somewhat on the velocity model used in the inversion. The model currently used is ak135 (Kennett et al., 1995). Although this model is quite adequate for computing the effects of mantle structure, its crustal structure may be too simple. This will not affect the mechanism found but may bias the depth estimate. Experiments involving the introduction of more realistic crustal layers yield differences on the order of 2-4 km (Sipkin, 1989), smaller than the intrinsic uncertainty of 5-10 km determined using both inversions of "realistic" synthetic seismograms (Sipkin, 1986a) and comparison with a set of well-located earthquakes (Engdahl et al., 1998). An advantage that using body-wave seismograms has over the use of surface waves is better resolution of source depth for shallow sources. The 823
824 inversion kernels for two of the moment tensor elements approach zero as the source depth approaches the Earth's surface. This occurs more rapidly at longer periods. Although the use of higher-frequency body waves does not eliminate this problem, it does ameliorate it. The deviatoric moment tensor can be decomposed into a double couple and a remainder term in an infinite number of ways. We prefer the decomposition suggested by Knopoff and Randall (1970) in which the moment tensor is decomposed into a "best" double couple and a compensated linear-vector dipole (CLVD). This decomposition is preferred to decomposing the moment tensor into a major and minor double couple because it is unique, whereas there are an infinite number of major and minor double-couple combinations. In addition, the "best" double couple and CLVD share the same principal axes; the major and minor double-couples will, in general, have equivalent force systems with differing directions. Using the preferred decomposition, the moment of the "best" double couple, M0, is 1/2(el - e3) where el is the largest positive eigenvalue, corresponding to the T axis, and e3 is the largest negative eigenvalue, corresponding to the P axis. The contribution of the CLVD component to the normalized total moment, ~QCLVO,is 2]eminl/]emax[, where emin is the eigenvalue with the smallest absolute value and emax is the eigenvalue with the largest absolute value. The percentage double couple is then 100 • (1 -/~CLVO).
Sipkin
approximately 30. Since 1995 the average number of stations per event has remained at this level.
4. Moment Tensor Inversion Code An important caveat is that the moment tensor source code was not originally intended for general distribution, and so is not very well documented. It is, however, included here (on the attached Handbook CD) for historical documentation purposes. The inversion software is modular. That is, an independent program handles each step of the process. The programs are bound by a UNIX shell script. This could also be accomplished on different operating systems with a VAX/VMS command file, a DOS batch file, etc. Many of the programs are for preprocessing (filtering, winnowing, and aligning) the data. All of these modules are contained in momten.tar.Z. The code also refers to several library modules--source code for each is contained in compressed tar files. The routines for recursively solving the system of equations can be found in Robinson (1967). momcon.tar.Z contains routines for converting from one type of source description (double-couple, principal axes, moment tensor, slip vector, etc.) to another (written by R.P. Buland, some routines modified by S.A. Sipkin). buplot.tar.Z contains the plotting package used (BUPLOT, written by R.P. Buland), but any plotting package of choice will do.
3. Moment Tensor Catalog The moment tensor solutions from 1981 through 2000, along with the hypocentral parameters determined by the NEIC, are listed in the files MT.LIS and FMECH.LIS included on the attached Handbook CD. These files, as well as updates, can also be downloaded via anonymous FTP at ghtftp.cr.usgs.gov in directory ./pub/momten. MT.LIS contains the elements of the moment tensors along with detailed event information; FMECH.LIS contains the decompositions into the principal axes and best double-couples; the file FMECH.DOC describes the various fields. As indicated in the Section 1, the catalog is not uniform over time. Beginning in 1981, the magnitude threshold was 6.5; in the second half of 1982, the threshold was reduced to 5.8 (5.7 for intermediate- and deep-focus earthquakes); and starting in 1995, the threshold was further reduced to 5.5. Since 1981 global coverage with broadband, digitally recording seismic stations has vastly improved. The improved station coverage is mainly responsible for the ability to move the magnitude thresholds lower with time. This is reflected in the annual average number of stations used per event. From 1981 through 1991, the average number of stations per event gradually rose from approximately 8 to approximately 13. By 1995, the average number of stations had rapidly increased to
Acknowledgments The staffs of the USGS National Earthquake Information Center, the USGS Albuquerque Seismological Laboratory, the IDA program at the Scripps Institution of Oceanography, and the Incorporated Research Institutions for Seismology make this work possible by operating and maintaining the US National Seismograph Network and the Global Seismograph Network and its data centers. Madeleine Zirbes wrote much of the preprocessing code and made several valuable suggestions for improving the procedure. Russell Needham, Madeleine Zirbes, and Charles Bufe did most of the routine data processing.
References Aki, K. and P.G. Richards (1980). "Quantitative Seismology," vol. 1, Freeman, San Francisco. Claerbout, J.F. (1976). "Fundamentals of Geophysical Data Processing," McGraw-Hill, New York. Engdahl, E.R., R. van der Hilst, and R. Buland (1998). Global teleseismic earthquake relocation with improved travel times and procedures for depth determination, Bull. Seismol. Soc. Am. 88, 722-743.
825
USGS Earthquake Moment Tensor Catalog
Kennett, B.L.N., E.R. Engdahl, and R. Buland (1995). Constraints on seismic velocities in the Earth from traveltimes. Geophys. J. Int. 122, 108-124. Knopoff, L. and M.J. Randall (1970). The compensated linear-vector dipole: a possible mechanism for deep earthquakes, J. Geophys. Res. 75, 4957-4963. Oppenheim, A.V. (1978). "Applications of Digital Signal Processing" Prentice-Hall, Englewood Cliffs, NJ. Robinson, E.A. (1967). "Multichannel Time Series Analysis with Digital Computer Programs." Holden-Day, San Francisco. Sipkin, S.A. (1982). Estimation of earthquake source parameters by the inversion of waveform data: synthetic waveforms. Phys. Earth Planet. Inter. 30, 242-259. Sipkin, S.A. (1986a). Interpretation of non-double-couple earthquake source mechanisms derived from moment tensor inversion. J. Geophys. Res. 91, 531-547. Sipkin, S.A. (1986b). Estimation of earthquake source parameters by the inversion of waveform data: global seismicity, 1981-1983. Bull. Seismol. Soc. Am. 76, 1515-1541. Sipkin, S.A. (1987). Moment tensor solutions estimated using optimal filter theory for 51 selected earthquakes, 1980-1984. Phys. Earth Planet. Inter. 47, 67-79. Sipkin, S.A. (1989). Moment-tensor solutions for the 24 November 1987 Superstition Hills California earthquakes. Bull. Seismol. Soc. Am. 79, 493-499. Sipkin, S.A. and R.E. Needham (1989). Moment-tensor solutions estimated using optimal filter theory: global seismicity, 1984-1987. Phys. Earth Planet. Inter. 57, 233-259. Sipkin, S.A. and R.E. Needham (1991). Moment-tensor solutions estimated using optimal filter theory: global seismicity, 1988-1989. Phys. Earth Planet. Inter. 67, 221-230. Sipkin, S.A. and R.E. Needham (1992). Moment-tensor solutions estimated using optimal filter theory: global seismicity, 1990. Phys. Earth Planet. Inter. 70, 16-21. Sipkin, S.A. and R.E. Needham (1993). Moment-tensor solutions estimated using optimal filter theory: global seismicity, 1991. Phys. Earth Planet. Inter. 75, 199-204.
Sipkin, S.A. and R.E. Needham (1994a). Moment-tensor solutions estimated using optimal filter theory: global seismicity, 1992. Phys. Earth Planet. Inter. 82, 1-7. Sipkin, S.A. and R.E. Needham (1994b). Moment-tensor solutions estimated using optimal filter theory: global seismicity, 1993. Phys. Earth Planet. Inter. 86, 245-252. Sipkin, S.A. and M.D. Zirbes (1996). Moment-tensor solutions estimated using optimal filter theory: global seismicity, 1994. Phys. Earth Planet. Inter. 93, 139-146. Sipkin, S.A. and M.D. Zirbes (1997). Moment-tensor solutions estimated using optimal filter theory: global seismicity, 1995. Phys. Earth Planet. Inter. 101, 291-301. Sipkin, S.A., M.D. Zirbes, and C.G. Bufe (1998). Moment-tensor solutions estimated using optimal filter theory: global seismicity, 1996. Phys. Earth Planet. Inter. 109, 65-77. Sipkin, S.A., C.G. Bufe, and M.D. Zirbes (1999). Moment-tensor solutions estimated using optimal filter theory: global seismicity, 1997. Phys. Earth Planet. Inter. 114, 109-117. Sipkin, S.A., C.G. Bufe, and M.D. Zirbes (2000a). Moment-tensor solutions estimated using optimal filter theory: global seismicity, 1998. Phys. Earth Planet. Inter. 118, 169-179. Sipkin, S.A., C.G. Bufe, and M.D. Zirbes (2000b). Moment tensor solutions estimated using optimal filter theory: global seismicity, 1999. Phys. Earth Planet. Inter. 122, 147-159. Sipkin, S.A., C.G. Bufe, and M.D. Zirbes (2002). Moment tensor solutions estimated using optimal filter theory: global seismicity, 2000. Phys. Earth Planet. Inter., 130, 129-142.
Editor's Note The moment tensor catalog and software files are given on the attached Handbook CD, under directory \ 5 0 Sipkin.