| Title | Magnitudes at close and very close distances in eastern Canada: issues and recommendations |
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| Licence | Please note the adoption of the Open Government Licence - Canada
supersedes any previous licences. |
| Author | Bent, A L |
| Source | Geological Survey of Canada, Open File 8232, 2018, 32 pages, https://doi.org/10.4095/308131  Open Access |
| Image |  |
| Year | 2018 |
| Publisher | Natural Resources Canada |
| Document | open file |
| Lang. | English |
| Media | on-line; digital |
| File format | pdf |
| Province | New Brunswick; Newfoundland and Labrador; Nova Scotia; Prince Edward Island; Quebec; Ontario; Nunavut; Eastern offshore region; Northern offshore region |
| NTS | 1; 2; 3; 10; 11; 12; 13; 14; 15; 16; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37; 38; 39; 40; 41; 42; 43; 44; 45; 46; 47; 48; 49; 52; 53; 54; 55; 56; 57; 58; 59 |
| Lat/Long WENS | -95.0000 -50.0000 80.0000 40.0000 |
| Subjects | geophysics; earthquakes; earthquake magnitudes; seismicity; Methodology |
| Illustrations | tables; location maps; plots; bar graphs |
| Program | Canadian Hazard Information Service |
| Released | 2018 04 23 |
| Abstract | The Nuttli (MN) scale is the most frequently used magnitude scale in eastern Canada. It is based on the amplitude of the Lg phase and therefore is not appropriate for distances of less than 50 km where
the Lg phase is not developed. The original Richter, ML, scale developed for use in California and known to be inappropriate for eastern North America, is used in eastern Canada only when the Lg phase is highly attenuated or non-existent, generally
for earthquakes occurring in oceanic crust or small earthquakes that are not recorded at distances of greater than 50 km. This study focuses on the latter by establishing a magnitude relation between MN calculated at appropriate distances and MN or
ML calculated at close distances. The magnitude relation would enable magnitudes for very small earthquakes to be calculated from a larger number of stations and a magnitude recurrence relation to be established over a wider magnitude range. It would
also enable a direct comparison of earthquakes from a particular region recorded only locally with those recorded over a wider distance range. Data from earthquakes occurring within several regions of eastern Canada for which MN was reported as the
preferred magnitude but where amplitude and period data from local stations were also measured and archived, ML and MN from stations at less than 50 km from the epicenter were calculated and compared to the published or event magnitudes. In most
regions ML underestimates the magnitude by more than 1 magnitude unit whereas the MN (<50 km) values were only about 0.1-0.2 units smaller than the presumed magnitude of the earthquake indicated by MN calculated at appropriate distances. A series of
conversion relations were developed and the effects of initial magnitude type, complexity of conversion relation and global vs. regional corrections were compared. The effect of using hypocentral distance instead of epicentral distance for
earthquakes at less than 50 km was evaluated but except for the closest and deepest events, the effect is not significant. Additionally, because precise depths are rarely determined for small earthquakes in eastern Canada hypocentral distances for
most regions would be approximations only. For distances from approximately 10 km to 50 km it is recommended that MN be calculated and that 0.11 be added to the value. These magnitudes can then be combined with MN magnitudes from distances of greater
than 50 km to determine the event magnitude. At distances of less than 10 km the conversion relations are not reliable and there appears to be more regional variation. Unless there are no stations at distances of 10 km or greater, magnitudes should
not be calculated from such close distances. When there is no alternative, the same procedure as for 10-50 km distances should be followed but it should be noted that the magnitudes might not be equivalents to regional MN. For routine seismicity
analysis, stations at extremely close distances are rarely essential for magnitude determination. However, in special cases usually related to focused studies of a particular location data from these distances may be crucial. In conjunction with the
study of magnitudes determined from close distances, the practice in eastern Canada of using only the MN equation for greater distances of the two equations proposed by Nuttli (1973) for all distances in eastern Canada was re-evaluated and found to
be a better choice than the two-equation option. However, it was also demonstrated that attenuation relation is not ideal for eastern Canada as there is a discrepancy between magnitudes calculated at distances greater and less than 4°. |
| Summary | (Plain Language Summary, not published)
The MN magnitude scale used in eastern Canada is not defined for distances of less than 50 km. The ML scale is used instead, but its attenuation
relations are not appropriate for eastern Canada. This study compares magnitudes at close and regional distances to develop a reliable conversion relation such that magnitudes from close stations can be objectively compared to magnitudes from more
distant stations. The recommendation for routine use is that the MN scale be used for distances greater than 10 km but that a 0.11 magnitude unit correction be applied for distances of 10-50 km. These corrections allow magnitudes to be calculated
from a higher number of stations and may increase the magnitude range used for seismic hazard assessments. Magnitudes from stations at distances less than 10 km should not be used unless the earthquake was too small to be recorded at greater
distances. In this case, the correction for 10-50 km should be applied but it should be noted that the resulting magnitude may not be accurate. |
| GEOSCAN ID | 308131 |
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