Geophys. J. Int. (2002) 150, 724–736 The 1933 Ms = 7.3 Baffin Bay earthquake: strike-slip faulting along the northeastern Canadian passive margin Allison L. Bent National Earthquake Hazards Program, Geological Survey of Canada, 1 Observatory Cres., Ottawa, Ontario, Canada, K1A 0Y3. E-mail: [email protected] Accepted 2002 March 18. Received 2002 March 14; in original form 2001 May 18 SUMMARY The 1933 November 20 (Ms = 7.3) Baffin Bay earthquake is one of the largest instrumentally recorded passive margin earthquakes. Analysis of seismograms of this earthquake shows strong evidence for strike-slip faulting, which contrasts with the generally accepted belief that Baffin Bay is dominated by thrust faulting. The best-fitting solution consists of a large strike-slip subevent (strike 172◦, dip 82◦, rake 6◦) followed by two smaller oblique-thrust subevents (strike 190◦, dip 30◦, rake 62◦). All subevents occur at a depth of about 10 km. An instrumental moment magnitude of 7.4 was determined. Preliminary analysis of subsequent large (magnitude ≥6.0) earthquakes in Baffin Bay finds additional evidence for strike-slip faulting in the region. The results for Baffin Bay, together with those for other passive margin earthquakes, suggest strike- slip faulting may be more prevalent in these regions than was previously believed. Key words: earthquake source mechanism, passive margin, waveform analysis. dependent events. Current Canadian seismic hazard maps (Basham INTRODUCTION et al. 1997) show the hazard in Baffin Bay to be comparable to The earthquake (Ms = 7.3) that occurred beneath Baffin Bay on that of coastal British Columbia although, obviously, the risk to the 1933 November 20 (23:21:35.7 UT, 73.07◦N, 70.01◦W; Fig. 1) is population is considerably lower. the largest instrumentally recorded earthquake to have occurred Based on a small number of existing focal mechanisms, it has gen- along the passive margin of North America and, possibly the largest erally been believed that Baffin Bay is dominated by thrust faulting passive margin earthquake worldwide. Coincidentally, it is also the while earthquakes on Baffin Island are normal faulting events (Stein largest known earthquake north of the Arctic Circle. Despite its im- et al. 1979, 1989). The results of the present study based on detailed portance, this earthquake has not been as well studied as most other waveform modelling of the 1933 earthquake and analysis of first- large eastern Canadian earthquakes. motion data from subsequent large earthquakes, however, provide In spite of its size, the 1933 earthquake did not result in any strong evidence for strike-slip faulting in Baffin Bay. damage because of its offshore location combined with the sparse population of the adjacent onshore areas. There are no known felt reports from Baffin Island or elsewhere in northern Canada. On the REGIONAL SEISMOTECTONICS other hand, there are no reports to confirm that the earthquake was not felt. Belatedly, the London Times (1933 November 28) reported It had been believed that Baffin Bay was formed by seafloor spread- briefly that the earthquake had been felt in Greenland (Fig. 1) in the ing between 60 and 40 Ma (Jackson et al. 1979), but more recent region from Upernivik to southern Upernivik (a distance of roughly evidence suggests that the seafloor spreading began much earlier- 65 km), but not in either Thule (560 km north of Upernivik) or Disko around 69 Ma (Roest & Srivastava 1989). Wetmiller (1974) used the Bay (480 km south of Upernivik). Very few details were provided, absence of Lgwaves from earthquakes for which wave trains crossed but the absence of any mention of even superficial damage suggests the centre of the bay to infer that it is still underlain by oceanic crust. that the intensity in the Upernivik region (550 km from the epicentre) It has been difficult to precisely define the ocean–continent bound- was no more than IV on the modified Mercalli scale. ary owing to the thick sediments in Baffin Bay (Keen et al. 1972a). Prior to this earthquake Baffin Bay had been believed to be There is evidence for faulting in the basement rocks and older sed- aseismic, but subsequent improved seismic monitoring in northern iments in Baffin Bay (Keen et al. 1972a) and for slumping, which Canada has shown the Baffin Bay region to be very active (Basham could be seismically related, in the younger sediments (Keen et al. et al. 1982; also see Fig. 1 of the present paper). Subsequent to 1933, 1972b). there have been four earthquakes of magnitude 6.0 or greater in Although Baffin Bay is now known to be a very active seismic Baffin Bay and one on Baffin Island. Qamar (1974) suggests that the zone, considerably less is known about it relative to the seismic Baffin Bay events are aftershocks of the 1933 earthquake and not in- zones in southern Canada. Prior to the 1933 earthquake, the region 724 C 2002 RAS 1933 Ms = 7.3 Baffin Bay earthquake 725 50˚W 60˚W 90˚W 80˚W 70˚W 2000 m 1934, 1947 1933 1933 reported felt 1945 BAFFIN BAY Upernivik 70˚N 1957 1933 reported not felt er 1963 Clyde Riv geographic 70˚N Disko Bay reference Igloolik BAFFIN ISLAND DAVIS STRAIT Iqaluit 60˚N 50˚W 60˚W 60˚N 90˚W 70˚W 80˚W Figure 1. Seismicity in and near Baffin Bay. Circles (scaled to magnitude) indicate epicentres of earthquakes of magnitude less than 6.0. Larger earthquakes are represented by stars and date. Earthquakes of magnitude 5.0 and greater are plotted for the period 1900–1996, magnitudes 4.0–4.9 for 1960–1996, 3.0–3.9 for 1970–1996 and 2.0–2.9 for 1980–1996. See the text for completeness periods for various magnitudes. Epicentres are from the Canadian Earthquake Epicentre File (CEEF). The 2000 m bathymetry contour is indicated by the dashed line. Black triangles indicate communities in which the London Times reported that the 1933 earthquake had been felt; white triangles are communities in which the earthquake had been reported not felt; grey triangles are communities shown for geographic reference only. was believed to be aseismic (Lee 1937). Earthquakes of magnitude Baffin Island side of the 2000 m bathymetric contour (Basham et al. 6.0 and greater subsequent to 1933 (1934, 1945, 1947 and 1957) 1977; see Fig. 1 of the present paper). To date no one has been are noted in the International Seismological Summary (ISS) and able correlate the seismicity with particular geological structures or similar summaries, but it was only with the expansion of the Cana- geophysical anomalies. It has been suggested (for example, Stein dian seismograph network in the north during the 1950s and 1960s et al. 1979) that seismicity in the region is related to the stresses that these earthquakes could be put into any kind of regional con- associated with post-glacial rebound. text. Basham et al. (1982) estimate that the earthquake catalogue for Baffin Bay is complete above the magnitude 7.0 level since 1920, magnitude 5.5 since 1950, magnitude 4.0 since 1968 and is RELOCATION OF EPICENTRES incomplete for magnitudes less than 4.0 for all time periods. This contrasts sharply with the Charlevoix (Quebec) seismic zone in the Although the 1933 earthquake was virtually ignored in Canada at long-settled St Lawrence Valley where the completeness years for the time of its occurrence, it received more attention elsewhere. the same magnitude levels are estimated to be 1660, 1900 and 1937, The Canadian seismograph station bulletin reports only a few phase respectively, and where earthquakes of magnitude less than 0.0 can readings for stations in eastern Canada with a brief note that they now be routinely located by a dense local seismograph network. probably corresponded to an earthquake located by the US Coast Historical seismic activity is not uniformly distributed throughout and Geodetic Survey. Routine locations (Fig. 2) were reported by Baffin Bay but is concentrated in northwestern Baffin Bay on the the International Seismological Summary (ISS), Bureau Central C 2002 RAS, GJI, 150, 724–736 726 A. L. Bent 74W 70W 66W EPICENTERS DETERMINED FOR 1933 BAFFIN BAY EARTHQUAKE 75N km 0 50 100 # this study ISS 73N # JSA BCIS USCGS Rajko & Linden (1935) Lee (1937) Gutenberg & Richter BAFFIN IS. (1954) Qamar (1974) 71N 74W 70W 66W Figure 2. Epicentres of the 1933 Baffin Bay earthquake determined by various sources. ISS is the International Seismological Summary, JSA is the Jesuit Seismological Association, BCIS is the Bureau Central International de Seismologie´ and USCGS is the US Coast and Geodetic Survey. The uncertainty in the location obtained in this study is of the order of the symbol size. International de Seismologie´ (BCIS), the United States Coast and for each earthquake with no station corrections. Station corrections Geodetic Survey (USCGS), the Jesuit Seismological Association were then calculated based on the mean residual for each station (JSA) and by Gutenberg & Richter (1954). The earthquake was re- (or region) from the preliminary relocations, and then the inversion located as part of several research projects. Rajko & Linden (1935) was rerun using the station corrections. Individual station correc- located the earthquake in a study of Arctic seismicity. Lee (1937), tions were determined for those stations that reported P arrival times noting that the seismograph stations in North America, Europe and for at least four of the five events. Regional corrections were deter- Japan covered a broad azimuthal but narrow distance range, relo- mined for the remaining stations. These corrections, as well as all cated the earthquake in an effort to improve the available travel- traveltime residuals, are tabulated in Bent (1998a) and are of the time tables, particularly for S waves.