Seismological Research Letters Volume 83, Number 3 May/June 2012 505 doi: 10.1785/gssrl.83.3.505 Regional Centroid-Moment-Tensor Analysis for Earthquakes in Canada and Adjacent Regions: An Update Honn Kao, Shao-Ju Shan, Allison Bent, Catherine Woodgold, Garry Rogers, John F. Cassidy, and John Ristau Honn Kao, 1 Shao-Ju Shan, 1 Allison Bent, 2 Catherine Woodgold, 2 Garry Rogers, 1 John F. Cassidy, 1 and John Ristau 3   INTRODUCTION Reliable determination of earthquake source parameters is a subject of fundamental importance for seismological research. It also provides critical observational constraints to the study of deformation and stress within the lithosphere. A compre- hensive catalog of earthquake source parameters is not only essential to the understanding of global and regional tectonics, but also ofers critical information on geological structures that may have engineering, economic, and hazard implications. In addition to the origin time, hypocenter, and mag- nitude, an earthquake’s source parameters include its focal mechanism, source time function, and the spatiotemporal dis- tribution of slip. However, unless the source dimension is con- siderably larger than the wavelengths of propagating seismic waves ( e.g., P or S), the source time function and slip distribu- tion of a seismic event are di icult to resolve, meaning that it is efectively a point-source. he focal mechanism, on the other hand, is directly related to fault movement in the source region and can be determined by several diferent methods even if the event is an efective point-source. he simplest way to deter- mine an earthquake’s focal mechanism is the “irst-motion” method that derives the orientation of the fault plane (strike and dip) and the relative movement between the hanging wall and the foot wall (rake) from the polarity and distribution of irst arrivals on the focal sphere (Aki and Richards 1980). A big drawback of this method is that it needs a large number of well-distributed stations with good data quality to suiciently cover the focal sphere. he process of picking irst arrivals and determining their polarity of motion can also be time-consum- ing, labor-intensive, and oten ambiguous, even for well-experi- enced data analysts. One breakthrough in seismology during the past several decades was the realization that a seismic source can be mathe- matically represented by a moment tensor consisting of six inde- pendent elements (Aki and Richards 1980). Furthermore, it is documented that observed seismic waveforms can be modeled by a linear combination of the six elements convolved with the Green’s functions derived from the property of the Earth along the propagating path and seismograph instrument response (Dziewonski et al. 1981). his theoretical advance enabled the routine practice of the centroid-moment-tensor (CMT) inver - sion for all large and moderate-size earthquakes (moment mag- nitude, M w ~ 5.5 or larger) using long-period data (<0.02 Hz) from global seismic networks (Dziewonski et al. 1981). With rapid improvements in digital seismic instruments, increasing knowledge of the earth velocity model at the regional scale, and the trend of establishing regional broadband seismic networks since the late 1980s, routine determination of moment-tensor solutions has became possible for earthquakes with M w as small as ~4 using regional waveforms with higher frequency contents (between 0.025 and 0.1 Hz) (Ekström et al. 1986; Ritsema and Lay 1995; Dreger et al. 1998; Ammon 2001). he systematic practice of regional CMT inversion for earthquakes in western Canada and adjacent regions was established at the Paciic Geoscience Centre (PGC) of the Geological Survey of Canada (GSC) in 2001 (Ristau 2004; Ristau et al. 2007), mainly as a result of the upgrade of the Canadian National Seismograph Network (CNSN) to con- tinuous digital recording with broadband sensors. Meanwhile, the regional CMT analysis was extended for some events as far back as 1995 when CNSN began upgrading. As of October 2010, the CNSN consists of 154 stations (Figure 1). Among them, 93 stations are equipped with broadband sensors suitable for regional CMT analysis. here have been several major changes to the original prac- tice of regional CMT inversion since 2001, including improved procedures for digital data processing, modiications to the inversion algorithm, inclusion of additional new or upgraded seismic stations, and the versatile means of distributing and publishing inversion results. One purpose of this paper is to document these changes. We also report new regional CMT solutions and provide updated information on the current operation of regional CMT analysis at GSC and on the avail- ability of inversion results to the research community as well 1. Geological Survey of Canada, Paci ic Geoscience Centre, Sidney, British Columbia, Canada 2. Geological Survey of Canada, Ottawa, Ontario, Canada 3. GNS Science, Lower Hutt, New Zealand