On October 20 2005 at 21:16 GMT, a magnitude mN 4.3 earthquake occurred in the southern part of Georgian Bay, approximately 12 km north of Thornbury, Ontario, Canada (latitude 44.67° N and longitude 80.46° W). This earthquake is the largest one in southern Ontario recorded by a local seismograph network and is of particular interest due to its location 90 km from a proposed long-term storage facility for high-level nuclear waste. The earthquake was felt along the southern shore of Georgian Bay with maximum intensity of IV MM. During the first 24 hours after the earthquake occurred, four portable ORION seismograph systems were installed to record possible aftershocks. The main shock on October 20 2005 was preceded by a foreshock 30 sec before it, and was followed by 5 aftershocks within 4-day period. All the epicenters of the foreshock and aftershocks were within 2.5 km from the epicenter of the main shock. The large amount of available data from the recently installed broad-band POLARIS seismograph stations, as well as the permanent CNSN stations and the temporary stations, gave us a unique opportunity to study the parameters of this event. The analysis of the foreshock-main shock-aftershock sequence indicated focal depths around 7 to 12 km. The focal mechanism calculated from the polarities of P-arrivals showed predominantly thrust mechanism of the main shock, with nodal planes oriented almost NW-SE. The focal mechanism is very similar to the predominant focal mechanism of the earthquakes in Western Quebec Seismic Zone but different from the predominant strike-slip focal mechanisms south of Lake Erie and the oblique slip mechanisms in western Lake Ontario. Aeromagnetic data reveal a prominent NW-SE structural fabric for the basement rocks beneath Georgian Bay, in good agreement with the orientation of the nodal planes. This structural fabric probably reflects mafic dykes (the Matachewan dyke swarm). The spectra of S-waves, recorded at 13 bedrock stations, were fitted with Brune’s model and used to calculate the seismic moment (3.6e+14 N.m), source radius (~ 400 m), stress drop (~ 20 bars), and moment magnitude (Mw 3.7). This seismic moment and calculated focal mechanism were used as initial approximation for seismic moment tensor inversion. The results of the inversion showed correspondence between the seismic moment and double-couple focal mechanism calculated from the moment tensor

The conventional methods for determining the magnitude of an earthquake such as Richter magnitude, Nuttli magnitude etc. are based mainly on peak-to-peak amplitudes of different phases on the seismic trace. These magnitude scales were developed in the past during the days when we had only paper seismic recordings. The conventional method for determining moment magnitude obtained from seismic moment is measured actually as the low frequency level of the displacement spectrum. Both of the above mentioned methods ignore a lot of the information, which is provided in a modern digital 3-component seismogram. In this study we explore the applicability of total signal energy as the main parameter to be used in magnitude estimation. Over 2100 three-component seismic traces from 258 local and regional earthquakes recorded on the Southern Ontario Seismic /POLARIS networks were used in this study. To relate the new energy magnitude scale to the old ones, including Mw, we have calculated most known magnitude types and the seismic moment of the earthquakes. To carry out this work, an automatic procedure was developed for measuring the peak-to peak amplitudes, periods, duration, and signal energy for each seismic trace. For calculation of the seismic moment, an iterative technique was developed to separate the effects of source functions from site response and geometrical spreading and attenuation effects. We have compared our energy magnitude measurements with the other well-known magnitude measurements by monitoring the solution errors. Our results show that the measurements of total seismic signal energy in both the P- and S-wave trains can improve the precision of the earthquake magnitude significantly and reduce much of the scatter found in conventional magnitude measurements.

The conventional methods for rapid determination of earthquake magnitude are based mainly on peak-to-peak amplitudes of specific phases on a seismic trace. Today, broadband digital records are readily accessible in real time, enabling the use of more information from a seismogram for rapid magnitude calculation. The aim of this work is to introduce a new magnitude scale for routine seismological analysis, denoted ME (P-wave, S-wave+coda, or both). This magnitude scale uses the signal energy and is illustrated here with a case study from southern Ontario/western Quebec (Canada). Traditional types of magnitude scales, based on the estimated maximum velocity (mb) and Richter local magnitude (ML), as well as the moment magnitude (MW), and some other magnitude types, based on the coda energy (MCoda) and ehvelope area (MEnv) are also computed for the study area for comparative purposes. Ihe proposed approach employed for this study can be easily applied to any other region of the world. The developed automatic procedure allowed the simultaneous computation of different magnitudes and different trace components and types of waves. The data used for this research are from 238 well-recorded earthquakes between 1991 and 2006 in southern Ontario/western Quebec/northern Ohio/northern NY State (1.0 < mN < 5.5). The results of our work show that, in general, magnitude values based on signal energy ME give less scattered estimates than magnitude values based on peak-to-peak measurements. We recommend using ME (S + coda) scale (vertical component) for quantifying the earthquakes in the study area in the future. The magnitude formula for this scale is given by ME = 0.5log ẼS + 0.92logD + 3.56 + S, where ẼS is the signal energy defined here (∑vs2 Δt, vS is measured in mm/s, Δt is the sample interval in seconds), D is the epicentral distance in km, and S is the station correction. The new ME magnitude can be used for a quick estimate of the MW magnitude for the study region using the relationship: MW = ME - 0.51 (for earthquakes with ME ≥ 2.6), obtained here.

A Mw 3.1 earthquake occured in Lake Ontario along the United States-Canada border, about 30 km south from Port Hope, Ontario, Canada, on 4 August 2004. Despite its small size, the shock was very well recorded by broadband seismographic stations deployed in recent years in Ontario, Canada, and in New York State. More than 40 broadband stations at local and regional ranges provided high-quality digital data. Waveform data analysis constrained the source at a depth of 4 (±2) km, which places the shock in the shallow Precambrian basement beneath Paleozoic platform deposits. The source mechanism from the regional waveform inversion for the double-couple moment tensor is predominantly strike-slip faulting. A NS striking (8°) nodal plane dipping to the east (dip = 59°) is the likely fault plane which represents right-lateral strike-slip motion. The subhorizontal P-axis orientation (trend = 234° and plunge = 12°) is consistent with the maximum horizontal compressional stress (SHmax) direction in eastern North America. Although the 4 August 2004 event is a small shock and has the seismic moment of M0 = 4.45 (±2.30) × 1013 Nm, it is the largest instrumentally recorded earthquake that has occurred in Lake Ontario. This and other significant earthquakes in the region suggest a broad-scale strike-slip faulting stress regime with a shallow seismogenic layer in the Erie-Ontario Lowlands region. The shallow focal depths of earthquakes in the region increase the risk of higher ground shaking compared to other seismic zones in northeastern North America with a deeper seismogenic layer.

The Southern Ontario Seismic Network (SOSN) consists of eleven three-component short-period seismic stations, located mainly in the Toronto-Hamilton-Niagara area of Ontario, Canada. The network has been in operation by the University of Western Ontario (UWO) for Ontario Power Generation (OPG) since 1991 with the purpose of obtaining information on the seismicity and seismic hazards of a region of southern Ontario in which a number of nuclear power stations are located. Over the past decade, an average of more than ten local earthquakes per year in the western Lake Ontario area was detected by the SOSN. Most of the events were in the 2–3 magnitude (MN) range. The largest events during this time took place in the surrounding regions—Pymatuning, northwestern Pennsylvania (285 km southwest from Toronto, just south of Lake Erie, 25 September 1998, MN 5.4), northern Ontario/Quebec border (325 km north of Toronto, 1 January 2000, MN 5.2), Ashtabula, Ohio (262 km southwest of Toronto, 26 January 2001, MN 4.4), and Au Sable Forks, New York (436 km east of Toronto, 20 April 2002, MN 5.1). The largest earthquake (MN 3.8) in the western Lake Ontario region during the past ten years occurred on 26 November 1999 in Lake Ontario, 16 km southeast of the town of Pickering, which lies just east of Toronto. The estimated location uncertainty (±2 km) is significantly better than that which was possible before 1991. The focal depths, though poorly constrained for most events, are shown to lie in the 3–15 km range, well within the Grenvillian rocks of the Precambrian Shield. The new seismicity map shows that a definite pattern is emerging in the SOSN data set in Lake Ontario, one which is significantly different from the past historical earthquake patterns obtained when the instrumental coverage was poor. Most events occur in scattered clusters in the western part of Lake Ontario and the northwestern corner of New York State. The area of seismicity does not extend significantly to the north of western Lake Ontario and appears to end to the west rather abruptly along a 30 km small fault line running from south of Hamilton in a north-northeasterly direction to Burlington, Ontario. Although the area of seismicity coincides with a region of linear magnetic anomaly trends (suggesting a strong structural fabric in the basement rocks), the correlation of seismicity of the new SOSN data set with magnetic lineaments is still unclear. The cause of the seismicity is speculated to be related to water flows along various fissures below the lake. It is known from induced seismicity studies of reservoirs that the presence of fluids can cause earthquakes by changing the pore pressure and reducing the friction along any faults which may be present. From seismic reflection studies, dipping structures and shear zones have been imaged to extend southeastward under Lake Ontario. This may explain why most of the earthquakes are occurring under the lake or southeast of the lake.

We analyzed over 3000 Fourier spectra from 370 earthquakes of energy magnitude (M-E) 1.1-6.0 recorded by the Southern Ontario Seismic Network (SOSN)/POLARIS networks during the period 1991-2010 in the area of southern Ontario and western Quebec. We employed a range of velocity stacking methods to significantly reduce the problem of variability due to wave scattering. This enabled us to determine underlying nonrandom spectral features, including source effects, site effects, and anelastic attenuation effects on spectral shape. The analysis technique is that we stack the velocity spectra of the whole observed data set into one or two bins and then compare that sum (the observed stack) with the theoretical expectation for corresponding stacks of simulated signals (the theoretical stack) for a given set of input parameters. A grid-search technique is used to find the input-parameter combination that optimizes the agreement between the observed and theoretical stacks. By stacking the spectra in different ways, different underlying spectral features are explored. We find the method works surprisingly well, allowing us to determine the apparent anelastic attenuation effects on the spectral shape, the average effect of site response, and some basic features of the source spectra. The key results of our paper: (1) there is no unique pair of values of the coefficients Q(0) and n of the frequency-dependent Quality factor relationship Q=Q(0)f(n), but there exist pairs of Q(0) and n along a curve in Q(0)-n space that are equivalent in terms of their effect on spectral shape; (2) the relationship between log corner frequency and energy magnitude (M-E) is linear, with a slope close to (-0.22) that is consistent with constant-width faulting for the studied small-to-moderate events; (3) the relation between moment magnitude M and energy magnitude M-E was found to be M = 8/9 M-E.

A three-station broadband network was installed around the Bruce Nuclear site at the beginning of August 2007, to monitor microearthquakes within a 50-km radius of the plant. The seismic network was equipped with borehole stations installed at cased boreholes at depths of 25, 27 and 40 m, and temporary surface stations at the same sites. The aim of the doubled identical equipment (Geotech Instruments' KS2000) was to compare the records of local, regional and teleseismic events, and seismic noise and to obtain results about the noise reduction, attenuation and the site response at each station. During the design and installation of the seismic network different geophysical surveys were carried out: refraction seismic profiles, vertical seismic profiling, and noise level measurements at different depths along the borehole. The obtained velocity models were used for modeling of the site response and finally comparison with the real data from the parallel borehole-surface recordings, and measured predominant frequencies using the Nakamura's HVSR method. The real noise reduction estimated from the parallel recordings was compared with the predicted 10 dB noise reduction. A practical conclusion was drawn out about the optimum borehole depths for instrument installation based on the noise reduction / attenuation balance and signal-to-noise ratio with the depth. The seismic threshold magnitude for the monitored area estimated at the design stage was compared with the threshold magnitude obtained from the real data.