Saguenay Earthquake Research Articles

Vs30 Mapping of the Greater Montreal Region Using Multiple Data Sources

The metropolitan community of Montreal (MMC) is located in Eastern Canada and included in the western Quebec seismic zone characterized by shallow crustal earthquakes and moderate seismicity. Most of the urbanized areas are settled close to the Saint-Lawrence River and its tributaries and within the region, delimiting the extension of the clay deposits from the Champlain Sea. The influence of these recent and soft deposits on seismic waves has been observed after the 1988 M5.8 Saguenay earthquake and has proven to be crucial in seismic hazard analysis. The shear-wave velocity Vs averaged over the 30 m of soil, abbreviated Vs30, is one of the most used parameters to characterize the site condition and its influence on seismic waves. Since 2000, a site condition model has been developed for the municipalities of Montreal and Laval, combining seismic and borehole data for risk mitigation purposes. The paper presents an extended version of the Vs30 mapping for the entire region of the MMC, which accounts for half of the population of Quebec, including additional ambient noise recordings, recently updated borehole datasets, geological vector map and unpublished seismic refraction data to derive Vs profiles. The estimated Vs30 values for thousands of sites are then interpolated on a regular grid of 0.01 degrees using the inverse distance weighted interpolation approach. Regions with the lowest estimated Vs30 values where site amplification could be expected on seismic waves are in the Northeastern part and in the Southwest of the MMC. The map expresses in terms of site classes is compared with intensity values derived from citizen observations after recent felt. In general, the highest reported intensity values are found in regions with the lowest Vs30 values on the map. Areas where this rule does not apply, should be investigated further. This site condition model can be used in seismic hazard and risk analysis.

Large deformation dynamic analysis of progressive failure in layered clayey slopes under seismic loading using the particle finite element method

This paper presents the failure analysis of layered clayey slopes with emphasis on the combined effect of the clay’s weakening behavior and the seismic loading using the particle finite element method (PFEM). Diverse failure mechanisms have been disclosed via the PFEM modelling when the strain-weakening behavior of clay is concerned. In contrast to a single layered slope exhibiting either a shallow or a deep failure mode, a layered slope may undergo both failure modes with a time interval in between. Seismic loadings also enlarge the scale of slope failure in clays with weakening behavior. The failure of a real layered slope (i.e. the 1988 Saint-Adelphe landslide, Canada) triggered by the Saguenay earthquake is also studied in this paper. The simulation results reveal that the choice of the strain-softening value controls the slip surface of the landslide and the amplification effect is important in the triggering of the landslide.

Experimental and numerical investigation of the Saint-Adelphe landslide after the 1988 Saguenay earthquake

On 25 November 1988, after the Saguenay earthquake, a landslide occurred in the municipality of Saint-Adelphe. The soil profile indicated that the deposit was composed of a stiff clay crustal layer overlying sensitive plastic clay with a soft-to-stiff consistency. A geotechnical investigation was carried out in situ and in the laboratory and included the use of a new seismic simulator to develop a geotechnical model of the Saint-Adelphe clay. The model was incorporated in a finite-difference slope stability analysis before and during the earthquake. The results showed the development of plastic zones and the generation of pore water pressure, but the global safety factor remained above unity. A post-seismic analysis that utilized a strain-softening behavior model showed the initiation and propagation of the plastic zone, as well as the development of a failure surface close to the observed failure surface. Therefore, it is proposed that the Saint-Adelphe landslide could be explained by a progressive failure mechanism.

Further studies on the 1988MW5.9 Saguenay, Quebec, earthquake sequence

Many small earthquakes occur annually in Eastern Canada, but moderate to strong earthquakes are infrequent. The 25 November 1988 MW5.9 Saguenay mainshock remains the largest earthquake in the last 80 years in eastern North America. In this article, some aspects of that earthquake sequence were re-analyzed using several modern methods. The regional depth-phase modeling procedure was used to refine the focal depths for the foreshock, the aftershocks, and other MN≥ 2.5 regional earthquakes. The hypocenters of 10 earthquakes were relocated using hypoDD. The spatial distribution of eight relocated hypocenters defines the rupture plane of the mainshock. The moment tensor for the mainshock was retrieved using three-component long-period surface wave records at station HRV (Harvard seismograph station) with additional constraints from P-wave polarities. One nodal plane is conclusively identified to be close to the rupture plane, and its strike is similar to the trend of the south wall of the Saguenay Graben. Based on the consistency between the strike of the nodal plane and the trend of the Graben, as well as the deep focal depth distribution, we suggest that the Saguenay earthquake sequence is related to the reactivation of one of the faults of the Saguenay Graben.

Seismic isolation retrofitting solution for an existing steel cable-stayed bridge.

This paper investigated the seismic retrofitting of an existing cable-stayed bridge through the use of a seismic isolation system. The bridge is situated in a high seismic zone. During the Saguenay earthquake 1988, one of the anchorage plates of the bridge supports failed. Herein, several configurations of seismic isolation system were considered to identify an appropriate solution for the seismic retrofitting of the bridge in both the longitudinal and transverse directions. A three-dimensional model of the bridge was created, and its seismic behavior studied through nonlinear dynamic time-history analysis. The comparative performance study among the five retrofitting configurations showed that the partial seismic isolation of the bridge led to an enhancement of the seismic response of the bridge in one direction only. However, the overall seismic response of the cable-stayed bridge substantially improved in the longitudinal and transverse directions in cases where the isolation systems were utilized between the supports and the deck-tower connection of the bridge.

A new classification of earthquake-induced landslide event sizes based on seismotectonic, topographic, climatic and geologic factors

This paper reviews the classical and some particular factors contributing to earthquake-triggered landslide activity. This analysis should help predict more accurately landslide event sizes, both in terms of potential numbers and affected area. It also highlights that some occurrences, especially those very far from the hypocentre/activated fault, cannot be predicted by state-of-the-art methods. Particular attention will be paid to the effects of deep focal earthquakes in Central Asia and to other extremely distant landslide activations in other regions of the world (e.g. Saguenay earthquake 1988, Canada). The classification of seismically induced landslides and the related ‘event sizes’ is based on five main factors: ‘Intensity’, ‘Fault factor’, ‘Topographic energy’, ‘Climatic background conditions’, ‘Lithological factor’. Most of these data were extracted from papers, but topographic inputs were checked by analyzing the affected region in Google Earth. The combination and relative weight of the factors was tested through comparison with well documented events and complemented by our studies of earthquake-triggered landslides in Central Asia. The highest relative weight (6) was attributed to the ‘Fault factor’; the other factors all received a smaller relative weight (2–4). The high weight of the ‘Fault factor’ (based on the location in/outside the mountain range, the fault type and length) is strongly constrained by the importance of the Wenchuan earthquake that, for example, triggered far more landslides in 2008 than the Nepal earthquake in 2015: the main difference is that the fault activated by the Wenchuan earthquake created an extensive surface rupture within the Longmenshan Range marked by a very high topographic energy while the one activated by the Nepal earthquake ruptured the surface in the frontal part of the Himalayas where the slopes are less steep and high. Finally, the calibrated factor combination was applied to almost 100 other earthquake events for which some landslide information was available. This comparison revealed the ability of the classification to provide a reasonable estimate of the number of triggered landslides and of the size of the affected area. According to this prediction, the most severe earthquake-triggered landslide event of the last one hundred years would actually be the Wenchuan earthquake in 2008 followed by the 1950 Assam earthquake in India – considering that the dominating role of the Wenchuan earthquake data (including the availability of a complete landslide inventory) for the weighting of the factors strongly influences and may even bias this result. The strongest landslide impacts on human life in recent history were caused by the Haiyuan-Gansu earthquake in 1920 – ranked as third most severe event according to our classification: its size is due to a combination of high shaking intensity, an important ‘Fault factor’ and the extreme susceptibility of the regional loess cover to slope failure, while the surface morphology of the affected area is much smoother than the one affected by the Wenchuan 2008 or the Nepal 2015 earthquakes. The main goal of the classification of earthquake-triggered landslide events is to help improve total seismic hazard assessment over short and longer terms. Considering the general performance of the classification-prediction, it can be seen that the prediction either fits or overestimates the known/observed number of triggered landslides for a series of earthquakes, while it often underestimates the size of the affected area. For several events (especially the older ones), the overestimation of the number of landslides can be partly explained by the incompleteness of the published catalogues. The underestimation of the extension of the area, however, is real – as some particularities cannot be taken into account by such a general approach: notably, we used the same seismic intensity attenuation for all events, while attenuation laws are dependent on regional tectonic and geological conditions. In this regard, it is likely that the far-distant triggering of landslides, e.g., by the 1988 Saguenay earthquake (and the related extreme extension of affected area) is due to a very low attenuation of seismic energy within the North American plate. Far-distant triggering of landslides in Central Asia can be explained by the susceptibility of slopes covered by thick soft soils to failure under the effect of low-frequency shaking induced by distant earthquakes, especially by the deep focal earthquakes in the Pamir – Hindukush seismic region. Such deep focal and high magnitude (> > 7) earthquakes are also found in Europe, first of all in the Vrancea region (Romania). For this area as well as for the South Tien Shan we computed possible landslide event sizes related to some future earthquake scenarios.

Regional Spectral Analysis of Three Moderate Earthquakes in Northeastern North America

Abstract We analyze Fourier spectra obtained from the horizontal components of broadband and accelerogram data from the 1997 Cap-Rouge, the 2002 Ausable Forks, and the 2005 Riviere-du-Loup earthquakes, recorded by Canadian and American stations sited on rock at hypocentral distances from 23 to 602 km. We check the recorded spectra closely for anomalies that might result from site resonance or source effects. We use Beresnev and Atkinson’s (1997) near-surface velocity structures and Boore and Joyner’s (1997) quarter-wave method to estimate site response at hard- and soft-rock sites. We revise the Street et al. (1975) model for geometrical spreading, adopting a crossover distance of r o =50 km instead of 100 km. We obtain an average attenuation of Q =410±25 f 0.50±0.03 for S + L g +surface waves with ray paths in the Appalachian and southeastern Grenville Provinces. We correct the recorded spectra for attenuation and site response to estimate source spectral shape and radiated energy for these three earthquakes and the 1988 M 5.8 Saguenay earthquake. The Brune stress drops range from 130 to 419 bars, and the apparent stresses range from 39 to 63 bars. The corrected source spectral shapes of these earthquakes are somewhat variable for frequencies from 0.2 to 2 Hz, falling slightly below the fitted Brune spectra.

Determination of Stress Parameters for Eight Well-Recorded Earthquakes in Eastern North America

We determined the stress parameter, Δσ, for the eight earthquakes stud- ied by Atkinson and Boore (2006), using an updated dataset and a revised point- source stochastic model that captures the effect of a finite fault. We consider four geometrical-spreading functions, ranging from 1=R at all distances to two- or three- part functions. The Δσ values are sensitive to the rate of geometrical spreading at close distances, with 1=R 1:3 spreading implying much higher Δσ than models with 1=R spreading. The important difference in ground motions of most engineering con- cern, however, arises not from whether the geometrical spreading is 1=R 1:3 or 1=R at close distances, but from whether a region of flat or increasing geometrical spreading at intermediate distances is present, as long as Δσ is constrained by data that are largely at distances of 100 km-800 km. The simple 1=R model fits the sparse data for the eight events as well as do more complex models determined from larger datasets (where the larger datasets were used in our previous ground-motion predic- tion equations); this suggests that uncertainty in attenuation rates is an important com- ponent of epistemic uncertainty in ground-motion modeling. For the attenuation model used by Atkinson and Boore (2006), the average value of Δσ from the point- source model ranges from 180 bars to 250 bars, depending on whether or not the stress parameter from the 1988 Saguenay earthquake is included in the average. We also find that Δσ for a given earthquake is sensitive to its moment magnitude M, with a change of 0.1 magnitude units producing a factor of 1.3 change in the derived Δσ.

The Southern Extension and Reactivations of the Clarendon-Linden Fault System

Gas seeps located near Pike, New York, on the inferred location of the central fault of the Clarendon-Linden Fault System (CLF), were initiated during, or slightly after, the Saguenay earthquake, 1988. Analyses of the gas show that the gas seeps have a Devonian shale source, and a nearby well suggests that the source may be as deep as 330 m. Thus, CLF fractures extending to depths possibly as great as 300+ m were probably opened as a local response to the Saguenay event, allowing the gas to be vented. Published data show that the CLF extends from Lake Ontario to slightly south of Pike. Previously, there were insufficient data available to enable investigators to determine if the fault system continued farther south. Our soil gas survey and analyses of well logs indicate the CLF continues south into Allegany County, which borders the state of Pennsylvania. Closely-spaced wells suggest that the central fault of the CLF is probably a series of step faults, even in units above the Silurian evaporite section, although a monocline cannot be ruled out. Growth-fault geometries of the CLF suggest that the CLF experienced motion during the Taconic and Acadian orogenies. Isopach maps, coupled with the proposed southward extrapolation of the CLF, suggest that in Late Devonian times the CLF motion history recorded the passage of the Appalachian foreland basin axis across the CLF.

The Saguenay Fjord, Quebec, Canada: integrating marine geotechnical and geophysical data for spatial seismic slope stability and hazard assessment

In 1996 a major flood occurred in the Saguenay region, Quebec, Canada, delivering several km 3 of sediment to the Saguenay Fjord. Such sediments covered large areas of the, until then, largely contaminated fjord bottom, thus providing a natural capping layer. Recent swath bathymetry data have also shown that sediment landslides are widely present in the upper section of the Saguenay Fjord, and therefore, should a new event occur, it would probably expose the old contaminated sediments. Landslides in the Upper Saguenay Fjord are most probably due to earthquakes given its proximity to the Charlevoix seismic region and to that of the 1988 Saguenay earthquake. In consequence, this study tries to characterize the permanent ground deformations induced by different earthquake scenarios from which shallow sediment landslides could be triggered. The study follows a Newmark analysis in which, firstly, the seismic slope performance is assessed, secondly, the seismic hazard analyzed, and finally an evaluation of the seismic landslide hazard is made. The study is based on slope gradients obtained from EM1000 multibeam bathymetry data as well as water content and undrained shear strength measurements made in box and gravity cores. Ground motions integrating local site conditions were simulated using synthetic time histories. The study assumes the region of the 1988 Saguenay earthquake as the most likely source area for earthquakes capable of inducing large ground motions in the Upper Saguenay region. Accordingly, we have analyzed several shaking intensities to deduce that generalized sediment displacements will begin to occur when moment magnitudes exceed 6. Major displacements, failure, and subsequent landslides could occur only from earthquake moment magnitudes exceeding 6.75.

Linking Historical Intensity Observations with Ground-motion Relations for Eastern North America

Modified Mercalli Intensity (MMI) data are the only available ground-motion data for most of the large historical earthquakes that have occurred in eastern North America (ENA). Are these historical observations consistent with expected intensity levels based on regional ground-motion relations? It is important to answer this question, as it bears on the reliability of the results of seismic hazard analyses. An empirical study of California strong-motion records and associated MMI values has shown that observed MMI is a function of response spectral amplitudes, magnitude, and distance (Atkinson and Sonley, 2000). Comparison of strong-motion and MMI data from the 1988 Saguenay, Quebec, earthquake confirms applicability of the California relations to ENA, at least for epicentral distances of 150 km or less. I therefore apply the relationships of Atkinson and Sonley (2000) to predict the MMI for historical ENA earthquakes as a function of distance, assuming that the response spectral amplitudes follow those of the ENA ground-motion relations of Atkinson and Boore (1995). The ground motions are adjusted to show the expected MMI for a range of soil conditions, including hard rock, firm ground, and soft soil. The predicted MMI values for firm-ground to soft-soil conditions are consistent with historical observations from most ENA earthquakes, with typical discrepancies representing overprediction or underprediction of observations by about 1 MMI unit. Specifically, observed intensities from the 1886 Charleston earthquake ( M 7.3) and the 1925 Charlevoix earthquake ( M 6.4) are weaker than predicted by about one intensity unit, while observed intensities from the 1811-1812 New Madrid earthquakes ( M 8) are larger than predicted by about one unit. Intensities from the 1968 Illinois earthquake ( M 5.4) and the 1988 Saguenay earthquake ( M 5.8) are larger than predicted by about 1.5 intensity units. On the whole, it is concluded that the historical MMI data are consistent with currently used ENA ground-motion relationships.

Evaluation of models for earthquake source spectra in eastern North America

AbstractThere have been several relations proposed in the last few years to describe the amplitudes of ground motion in eastern North America (ENA). These relations differ significantly in their assumptions concerning the amplitude and shape of the spectrum of energy radiated from the earthquake source. In this article, we compare ground motions predicted for these source models against the sparse ENA ground-motion database. The source models evaluated include the two-corner models of Boatwright and Choy (1992), Atkinson (1993a), Haddon (1996), and Joyner (1997a,b), and the one-corner model of Brune [as independently implemented by Frankel et al. (1996) and by Toro et al. (1997)]. The database includes data from ENA mainshocks of M > 4 and historical ENA earthquakes of M > 5.5, for a total of 110 records from 11 events of 4 ≦ M ≦ 7.3, all recorded on rock. We also include 24 available rock records from 4 large earthquakes in other intraplate regions; conclusions are checked to determine whether they are sensitive to the addition of these non-ENA data.The Atkinson source model, as implemented in the ground-motion relations of Atkinson and Boore (1995), is the only model that provides unbiased ground-motion predictions over the entire period band of interest, from 0.1 to 10 sec. The source models of Frankel et al. (1996), Toro et al. (1997), and Joyner (1997a,b) all provide unbiased ground-motion estimates in the period range from 0.1 to 0.5 sec but overestimate motions at periods of 1 to 10 sec. The Haddon (1996) source model overpredicts motions at all periods, by factors of 2 to 10. These conclusions do not change significantly if data from non-ENA intraplate regions are excluded, although the tendency of all models toward overprediction of long-period amplitudes becomes more pronounced.The tendency of most proposed ENA source models to overestimate long-period motions is further confirmed by an evaluation of the relationship between Ms, a measure of the spectrum at 20-sec period, and moment magnitude. A worldwide catalog of shallow continental earthquakes (Triep and Sykes, 1996) is compared to the Ms-M relations implied by each of the source models. The Atkinson source model is consistent with these data, while other proposed ENA models overpredict the average Ms for a given M.The implications of MMI data from historical earthquakes are also addressed, by exploiting the correlation between felt area and high-frequency source spectral level. High-frequency spectral amplitudes, as specified by the Atkinson and Boore (1995), Frankel et al. (1996), Toro et al. (1997), and Joyner (1997a,b) source models, equal or exceed the levels inferred from the felt areas of most of the large ENA events, with the noteable exception of the Saguenay earthquake. By contrast, high-frequency spectral amplitudes specified by the Haddon (1996) source model agree with the felt area of the Saguenay earthquake but overpredict the felt areas of nearly all other large events. In general, models that fit the Saugenay data—be it intensity data, strong-ground-motion data, regional seismographic data, or telescismic data—will not fit the data from the remaining earthquakes.A source model derived from the California database, suitably modified for regional differences in crustal properties, is also evaluated. This model is not significantly different from the Atkinson model for ENA. There is an important practical application of this similarity, which we develop as an engineering tool: Empirical ground-motion relations for California may be modified to predict ENA ground motions from future large earthquakes.

Anatomy of a small intraplate earthquake: a dissection of its rupture characteristics using regional data

Summary The magnitude mbLg 5.0 Mont-Laurier earthquake of 1990 October 19, in Quebec, Canada, was one of the largest to have occurred in eastern North America during the past decade. High-frequency ground motions recorded on regional network instruments exceeded values anticipated for an event of its size by a factor of 3. A commonly favoured explanation for the discrepancy is that the source was a rare ‘high-stress’ event. In this paper, detailed fault-slip models are derived to fit waveform and spectral characteristics of the regional data. The results establish that the effective rupture stress was normal (about 100 bars), that the fault rupture developed asymmetrically, and that the average slip time for points inside the rupture area (approx. 0.1 s) was significantly less than that associated with the standard Brune (1970) source spectral model. The rupture area developed in at least four distinct episodes, each extending the previously ruptured area. Taken together with similar results for the mbLg 6.5 Saguenay earthquake of 1988 November, the results indicate that a widely used assumption in hazard analyses, that earthquake spectra are adequately represented by the standard Brune spectral model, is unreliable for the interpretation and prediction of strong ground motion.

Simulated hard-rock motions in Saint Louis, Missouri, from large New Madrid earthquakes (Mw ≧ 6.5)

Abstract We simulated broadband hard-rock ground-motion time histories for the Saint Louis, Missouri, metropolitan area due to three large earthquakes (6.5 ≦ M w ≦ 7.5), each occurring on the largest fault segment of the New Madrid fault zone at a distance of about 200 km. A deterministic procedure was implemented to simulate ground motion for frequencies less than 1 Hz for which the propagation effects were calculated using full-wave theory. For signals above 1 Hz, a semi-empirical simulation approach was used. The fault dimensions were defined in conformity with a constant stress-drop model developed for eastern North America; the fault width was constrained to a maximum depth of 16 km following the occurrence of the natural seismicity. For each event, we simulated broadband time histories for ten randomly generated slip models at eight sites distributed in various locations in the city of Saint Louis. This study shows that the level of peak ground motions at hard-rock sites in Saint Louis is small; the largest average peak horizontal ground-motion acceleration simulated is only 26.45 cm/sec 2 (1.96 cm/sec for the velocity) even for the largest event. To validate the simulation results, we examined regional accelerograms and their peak values and response spectra recorded during the 25 November 1988 Saguenay earthquake ( M w = 5.8) at distances further than 230 km from its epicenter to be consistent with the distance of the city of Saint Louis from the New Madrid earthquake zone. The Saguenay earthquake observations seem consistent with simulation values obtained for the M w = 6.5 New Madrid earthquake. A preliminary equivalent linear analysis by applying these simulated motions at the base of a soil profile suggests that the level of ground motions is too weak to produce damaging effects in Saint Louis even for the largest event on the longest segment.

Earthquake source spectra in eastern North America

Abstract A new model for average earthquake S-wave source spectra in eastern North America (ENA) is derived. The model is consistent with fundamental elasto-dynamic principles, as well as the familiar spectral scaling laws appropriate to sources having geometrical and dynamical similarity. Observed characteristics of S-wave spectra in ENA are fully explained as simple consequences of directivity effects entailed by the classical crack rupture model (Kostrov, 1964; Madariaga, 1976), with “normal” effective stresses of the order of 100 bars and fractional stress drop (Brune, 1970): abnormally high stress drops are not required. Recognition of the fundamental role of directivity explains most features of observed data that are currently interpreted as “anomalous”. Among other things, neglect of such effects is shown to be responsible for the conclusion by Boatwright and Choy (1992) and Boore and Atkinson (1992) that high-frequency spectral levels for the magnitude 6 Saguenay earthquake of 1988 are anomalously large. Other important implications of the new model are that some previously inferred systematic variations in Brune stress drop, with magnitude, are simply artifacts of the invalid use of the standard Brune (1970, 1971) model assumption and that current estimates of Q in ENA, which depend on this assumption, are likely to be significantly too large for high frequencies. Associated uncertainties allow the possibility that effective stress does not vary significantly with magnitude, for earthquakes having magnitudes in the range of − 4 to + 7. The new model suggests that some currently favored source spectra may under-estimate spectral amplitudes of future larger earthquakes in ENA, by as much as a factor of 6, for some frequencies. The proposed new spectra are practically identical with those of the Brune standard model for an effective stress of 100 bars, for frequencies up to about twice the Brune corner frequency, but then increase to values 3 times larger over a range of about 4 in frequency. As such, the high-frequency spectral levels are equivalent to standard Brune model values for earthquakes of one full magnitude unit larger, with consequent important implications for seismic hazard estimates in the region.

Determination of Threshold Ground Acceleration for Soil Liquefaction Using the Strain and Stress Approaches

The evaluation of soil liquefaction using the stress approach utilizes the relative density and initial confining stress available at the layer subjected to liquefaction. The shear strain approach makes use of the shear modulus which is a more fundamental property of materials. The threshold ground acceleration values for forty layers at ten selected sites were evaluated using the stress and strain approaches. For the stress approach, the method proposed by Shibata and Teparaksa (1988) based on the concept of critical cone tip resistance was selected. For the strain approach, the method proposed by Dobry and others (1981) was employed. The threshold ground acceleration values computed using the stress approach were found to be two to four times higher than those computed using the strain approach. Several scenarios were tested in order to incorporate the effect of variations in the threshold shear strain and the ratio of (G/Gmax)t. The results of the stress and the strain methods are based on site response analyses performed using the program SHAKE91 and the input ground motions of the 1988 Saguenay earthquake.

Modeling of source rupture characteristics for the Saguenay earthquake of November 1988 : R. A. W. Haddon, Bulletin — Seismological Society of America, 85(2), 1995, pp 525–551

Modeling of source rupture characteristics for the Saguenay earthquake of November 1988 : R. A. W. Haddon, Bulletin — Seismological Society of America, 85(2), 1995, pp 525–551

Modeling of source rupture characteristics for the Saguenay earthquake of November 1988

AbstractThe magnitude mb = 6.0 Saguenay earthquake of 25 November 1988 in Quebec, Canada, was one of the largest to have occurred in eastern North America during the past half-century. Recorded high-frequency ground motions exceeded anticipated values for an event of its size by a factor of 10 on both the regional network and strong-motion instruments. Two proposed explanations for the discrepancy are (1) that the source was a rare “high stress drop” event and (2) that it was an asymmetrical “fractional stress drop” rupture (involving only normal effective stresses). In this article, detailed fault-slip models are derived to fit characteristics of strong-motion displacement, velocity, and acceleration data. The results establish that the effective rupture stress was normal (less than 100 bars), that the fault rupture was highly asymmetrical with respect to the point of rupture initiation, and that the average slip time for points within the rupture area (approx. 0.2 sec) was considerably less than that associated with the standard Brune (1970) source spectral model. The rupture area developed in a number of episodes, each widening or lengthening the previously ruptured area, which may explain the short average slip time. The results indicate that the widely used assumption in hazard analyses that earthquake spectra are adequately represented by the standard Brune (1970) complete stress drop model may be seriously unreliable for prediction of strong ground motion in eastern North America.

Rupture histories of eastern North American earthquakes

Abstract Digital, worldwide records of P and SH waves are used to invert for rupture histories of the recent, larger, eastern North American earthquakes. The data include a broad bandwidth to facilitate recovery of both source details and total moment. The following events are studied- 9 January 1982 New Brunswick; 5 October and 23 December 1985 Nahanni; 25 November 1988 Saguenay; and 25 December 1989 Ungava earthquakes. We employ a finite-fault, waveform inversion scheme that discretizes the slip history as a function of position on the fault. This formulation allows estimation of rise time and slip velocity, in addition to the spatial distribution of slip. Static stress drops averaged over the entire rupture surface range from a few tens of bars to just over 100 bars and are similar to values estimated for western United States earthquakes. Estimates of stress drop for spatially limited asperities are in the range of a few hundred bars, with the value for the Saguenay earthquake asperity being the largest (approaching 1 kbar). The variation in stress drop is considerable, but no evidence is seen for a scaling relation in which stress drop increases with moment. Maximum slip velocities on the fault also have a wide range (from 50 cm/sec to perhaps greater then 200 cm/sec), with the Saguenay and 5 October Nahanni earthquakes lying at the upper end of this range. Of the events studied, the Saguenay earthquake is unique in terms of its greater depth, spatially concentrated source, and large asperity stress drop.

Statistical analyses of damages to buildings in the 1988 Saguenay earthquake : P. Paultre, G. Lefebvre, J.-P. Devic & G. Cote, Canadian Journal of Civil Engineering, 20(6), 1993, pp 988–998

Statistical analyses of damages to buildings in the 1988 Saguenay earthquake : P. Paultre, G. Lefebvre, J.-P. Devic & G. Cote, Canadian Journal of Civil Engineering, 20(6), 1993, pp 988–998