The magnitude 5.4 earthquake on July 2, 1922 is the first New Brunswick earthquake recorded by seismographs, although the stations were outside the province (Humphreys 1922a, 1922b). No instrumentally determined magnitude was reported for this latter event. The first recorded earthquakes in the province for which instrumentally calculated magnitudes were reported occurred on January 4, 1930 and September 30, 1937, for which a M_Ls of 4.6 and 5.0 are listed respectively by Smith (1966). Because Smith did not have a correct epicentre for this event, the magnitude for the 1937 earthquake was recalculated to be an M_N = 4.4 by Burke (2009).

However, it was not until the 1960s that a network of seismographs was available in eastern Canada to allow the recording of earthquakes with magnitudes as low as 2 to 3 in the province. For the period 1970 to 1982, a single vertical component station (UNB) was operated for the Geophysical Division of the Earth Physics Branch at the University of New Brunswick in Fredericton. Aſter the magnitude 5.7 Miramichi earthquake on January 9, 1982, seismometers were installed in the province at Edmundston (EBN), Caledonia Mountain (LNM), about 40 km, south of Moncton, McKendrick Lake (KLN), about 20 km southeast of the Miramichi earthquake epicentre and St. George (GGN). These seismometers formed part of the Eastern Canada Telemetered Network (ECTN) from 1982 until the early 1990s. Today, only seismometer installations at Caledonia Mountain and St. George remain within the province (Fig. 2). These latter stations are now broad band, three component stations, whose traces can be accessed on the Internet (Natural Resources of Canada 2010b).

A database of Canadian earthquakes from 1985 onwards is available on the Earthquakes Canada website (Natural Resources of Canada 2010d). Lists and maps for selected regions can be obtained from this database by specifying latitudes and longitudes for a search area. A map showing the epicentres for earthquakes with magnitudes of 3 or greater in New Brunswick for the period 1985 to the present is shown in Fig. 3. Data prior to 1985 is not included in this data base, which omits the 1982 sequence of earthquakes in the Miramichi area of the Central Highlands; the largest New Brunswick earthquake events recorded by seismographs. Epicentres for New Brunswick events for this period are plotted in Fig. 4. On both maps, the main events plot in the Central Highlands sub-zone (Burke 2004), where there has been a concentration of historical earthquakes (see below) and in the Passamaquoddy Bay and Moncton sub-zones (Fig. 1).

The 1982 Miramichi earthquake has been the only New Brunswick event to be subjected to a comprehensive study of geology and effects in the instrumentally recorded period. Basham and Adams (1984) report that the mainshock M 5.7 occurred January 9 and was followed by M 5.1 aſtershock three and a half hours later and by an M 5.4 aſtershock two and a half days later on January 11; field instruments recorded a further 237 aſter shocks in the 25 hour period between January 13–14th. Minor shocks (aſtershocks?) associated with this system are still occurring to this date. Focal mechanism studies of the main event and larger aſtershocks indicate a north trending conjugate V pattern of thrust faulting (e.g., Wetmiller et al. 1984; Basham and Kind 1986). On-site examinations by Basham and Adams (1984) of diorite outcrops close to the epicentre showed a few minor cracks, small-scale thrusting along a pre-existing joint (possibly glacitectonic), and a pop-up feature attributed to unloading during excavation, but no surface expression of a causative fault was found. In compressional tec-tonic settings, faults tend to develop as splays or blind thrusts and might not reach the surface (Wheeler and Johnston, 1992). Where fractures may have reached the surface the heavy vegetation and cover of glacial deposits would make small displacements extremely difficult to find. In these areas, sand blows and other seismic-generated disturbance of surface sediments may be the only evidence of a recent event. However, the Miramichi event occurred during conditions of frozen ground and was followed by 0.5 m of fresh snow before investigators could get to the site (Basham and Adams 1984, p. 117).

Fortunately, this 5.7 magnitude earthquake and large aſter-shocks were centred in a remote part of the Central Highlands. A full building damage survey was made aſter the event (Pernica and Maurenbrechen, 1982). Minor damage was reported in a few communities, at distances ranging between 60 to 115 kms from the epicentre. Cracks in foundation walls, concrete block walls and plaster-lathe wood partitions were noted, although in many cases these tended to be caused by the widening of pre-existing cracks. Damage to stairwell walls was observed in a senior citizens’ home in Bathurst (100 km from epicentre) and a hospital in Perth-Andover (85 km from epicentre). Gyproc panels were cracked in several retail stores, particularly in a one-storey store in Chatham (85 km from epicentre), where floor to ceiling cracks were developed or widened (see Fig. 2 for locations).

The historical seismicity of New Brunswick was investigated initially as part of a much larger study for the whole of eastern Canada by Smith (1962, 1966). Historical earthquakes of the region were included in a study of relationship to geological structure by Rast et al. (1979) and in a review of seismicity of the Maritime Provinces by Burke (1984). Leblanc and Burke (1985) studied the four largest known historical earthquakes in the Maine-New Brunswick region.

Earthquakes Canada maintains a comprehensive computer list of earthquakes as the National Earthquake Data Base (NEDB), formerly called the Canadian Earthquake Epicentre File (CEEF). Burke (2009) recently completed a study of historical earthquakes of New Brunswick for the years 1811–1960 and found evidence of 38 earthquakes, previously unlisted in NEDB. Most of the previously unlisted earthquakes were of relatively small magnitude and were only reported in one or two local newspapers, which may explain their absence from the NEDB. However, four moderate events in the Central Highlands were also identified by Burke (2009) that may have been missed previously because of their remote location far from larger communities. These moderate events were of magnitudes: 3.9 on March 16, 1863; 4.0 on December 18, 1903; 4.4 on March 20, 1911; and 3.9 on March 30, 1925.

The Modified Mercalli (MM) intensity scale is used to assign intensity levels from I to XII for an event (e.g., Reiter 1990; Bolt 1995). Table 1 lists the six largest earthquakes to have occurred during this period. The magnitudes and epicentres of the first five events are based upon the felt area and areas within the MM IV isoseismal. The epicentre and magnitude of the 1937 earthquake were determined by both instrumentally recorded data and felt data.

In the accounts of earthquakes in newspapers and other documentary sources, there is a wide range of reported effects for New Brunswick earthquakes, from simply ‘felt’ (MM II or III) to reports of ‘chimneys collapsing and walls cracking’ (MM VI–VII). The most common effects reported are the ‘rattling of dishes or windows’ and ‘people were awaken’ by the event (MM IV).

In Table 2, a summary of the Modified Mercalli intensity levels for those New Brunswick communities that reported having experienced the effects of these earthquakes is presented. As expected, most of the larger intensity values are listed where communities are close to the epicentre of an event. For example, intensity values of VI–VII have been assigned to St. Stephen for the 1817 and 1904 Passamaquoddy Bay earthquakes, because of chimney damage reported from there. Bricks falling from chimneys and cracking of plaster on walls in Moncton and nearby Hopewell, at the time of the 1855 earthquake, indicate these communities experienced intensities of VI and are in the epicentral region. Great rents in roadways and the river bank along the Petitcdiac River were observed for this latter event. Chimneys were knocked down or displaced in central New Brunswick communities at the time of the 1869 earthquake, which has been assigned a location in the Central Highlands (Burke 2009). Some springs appeared and others disappeared along the SW Miramichi River valley.

However, closeness to the epicentre is not required for damage to occur, particularly where local site conditions amplify the ground motion. This is the case for those places situated on unconsolidated deposits, as illustrated by the intensity values at Fredericton (discussed below), where early historical settlement had taken place on a river plain of thick alluvial sediments. Effects reported from there caused by quite distant earthquakes, such as those of 1855 and 1904, give intensity values that appear to be higher by one or two units than expected for the response at distances of 150 and 100 km respectively from the epicentres of these events.

Seven large regional earthquakes in the historical period from 1860 to 1960 have had noticeable effects in New Brunswick. Four of these earthquakes occurred in the Charlevoix region of Quebec, two in the state of New Hampshire in the USA and one occurred on the Grand Banks, in the Atlantic Ocean, south of Newfoundland (Fig. 5, Table 3). The distances from the epicentres of the Quebec earthquakes vary from about 160 km to Edmundston, in northwest New Brunswick, to about 500 km to Sackville, a community in the south east part of the province. Distances between the epicentral region of the New Hampshire earthquakes and New Brunswick vary between 360 km on the western boundary to 620 km on the eastern boundary of the province. The distances to the epicentre of the Grand Banks earthquake are 670 km from Sackville to about 1000 km from Edmundston respectively.

During these earthquakes, general shaking of buildings, rattling of dishes and awakening of people was reported in many communities throughout New Brunswick. Intensity levels reached IV–V in many places (Table 4). At some localities, effects associated with intensity levels as high as VI were reported, probably caused by local sites conditions. Such effects were observed in the Fredericton area of New Brunswick, during the 1929 Grand Banks earthquake, approximately 830 km distant (Burke and Slauenwhite 1987). During that event, buildings in the city core, which were sited over thick deposits of glacimarine/glacilacustrine silt and clay, were seen to vibrate more intensely than surrounding areas sited on bedrock with thin glacial cover. For the 1925 Charlevoix earthquake, at places close to the New Brunswick – Quebec border, damage to buildings was reported. Unstable chimneys fell at Campbellton, partitions of walls were separated and glass broken at Dalhousie, and bricks were dislodged from the flue of a chimney in Clearview (Fig. 2). The most significant effect was damage to a chimney in a house in Grand Falls that caused a fire to start and raze the structure. Flow from artesian wells at Newcastle was immediately enhanced aſter the 1925 Quebec earthquake.

Deterministic studies include the examination of bedrock faults or overlying deformed sediments caused by underling or nearby fault-generated seismic activity (e.g., Reiter, 1990; Tuttle and Seeber, 1991; Clague et al. 1992; Doughty et al. 2010). Such studies can contribute valuable information and enable extrapolation of magnitude, timing and number of seismic occurrences beyond the historical record (e.g., Tuttle et al. 1996; Rajendran 2000). However, caution must be exercised when interpreting suspected tectonic deformation in glaciated areas, where similar-looking structures can be produced by a variety of mechanisms associated with glaciations (i.e., Broster 1991). Examples of two recent studies in New Brunswick are presented below with additional comments on other factors of concern for site assessment.