The 1663 M~7 Charelvoix earthquake is historically the largest shock in the region, so this event is of particular importance in the interpretation of stress changes in the study area. In this section, we explore the stress interaction between the 1663 earthquake and current background seismicity in the CSZ (1978 to the present) under the hypothesis that small Coulomb stress changes (∆CFF ≥ 0.1 bar, Stein, 1999) can promote events. Based on the assumed threshold of 0.1 bar (Stein, 1991), we examine whether the background activity in the CSZ is promoted in the region of stress increase (∆CFF ≥ 0.1 bar) or inhabited in the region of stress decrease (∆CFF ≤ - 0.1 bar). The Coulomb stress calculation requires defining the slip direction and geometry of the faults that ruptured during the 1663
earthquake. From historical and geological lines of evidence, it is usually concluded that the epicentral area of the 1663 earthquake was in the vicinity of La Malbaie (Figure 4..2), where many profound geological effects were noted (Smith, 1962, Gouin, 2001). In our Coulomb stress modeling, we constrain the location of the centre of the fault center based on the Smith (1962) epicenter at -70.1°W, 47.6°N. We start with the assumption of thrust faulting for the 1663 earthquake on a fault plane (strike: N35°, dip: 60°, rake: 90°) that follows the
orientation of the major rift faults at the CSZ. This assumption is well constrained based on the focal mechanism of the recent largest earthquakes in CSZ that show thrust faulting on north-east south-west striking nodal planes (Bent et al, 2003), with the suggestion being that the 1663 earthquake may have the same focal mechanism (Ebel, 2011). However, we have tested alternative focal mechanisms in a later discussion.
To calculate ∆CFF, we estimate the rupture parameters (slip, rupture dimension) for the 1663 earthquake based on empirical relations that scale the source parameters based on the
magnitude of the earthquake (e.g Johnston, 1993; Wells and Coppersmith, 1994). Since the magnitude estimate of the 1663 earthquake is associated with a high level of uncertainty (at least several tenths of a magnitude unit), the rupture parameters are also highly uncertain. We start with an initial model with parameters chosen to be consistent with a M7.0 earthquake; we address the issue of the uncertainty in the assumed rupture model in more detail in later discussions. Using the Johnston (1993) scaling relation, it is assumed that the event
produced 2.9 m of reverse slip on a 30 km × 11 km rupture plane. It should be noted that the Wells and Coppersmith (1994) relation for thrust events estimate the rupture dimension of 48 km × 22 km for a M7.0 earthquake. Since the Johnston (1993) empirical model is developed based on intraplate earthquakes, it is preferred in our study.
Figure 4.3 depicts the resulting pattern of stress changes, along with the locations of recent seismicity (1978 to the present) from the unified CCSC catalog. It can be seen that regions of positive ∆CFF correlate relatively well with the concentration of recent microseismicity in the CSZ. To further test the correlation, we calculate the Coulomb stress change on a 0.005 degree × 0.005 degree horizontal grid. We associate each earthquake in the CCSC catalog to the closest grid node to estimate the Coulomb stress change at the event’s hypocentral location. We find that ~80% of earthquakes experience positive Coulomb stress change (∆CFF ≥ 0.1 bar) that would promote failure and only ~18% of the earthquakes show negative stress change (∆CFF ≤ - 0.1 bar) that would inhabit failure. Cross sections of the Coulomb stress changes through the center of the fault indicate that lobes of enhanced failure condition (positive ∆CFF) correspond to the distinct zones of activity that are observed in the CSZ. Additionally, the aseismic slab reported in previous studies coincides with the region of reduced Coulomb stress. The general correlation between the stress changes generated by the 1663 event with our assumed rupture parameters and recent microseismicity patterns may
suggest that the 1663 earthquake exerts significant influence over the spatial distribution of contemporary (from 1978 to the present) low-magnitude earthquakes. Indeed, it appears that current seismicity may well be attributable to stress response from the 1663 earthquake. With our assumed rupture model, the 1663 earthquake exerts only modest stress changes on the areas in which the larger recent events (M≥3.5 since 1978) have been located (except for a cluster of seismicity in the northeast).Moreover, with regard to the largest historic
earthquakes (also superimposed on Figure 4.3), three out of four events have occurred in regions with minimal stress changes, in the range from -0.17 to 0.17 bar. Considering the large uncertainty (at least tens of kilometers) associated with the location of these historic events, we cannot infer whether or not the stress changes imparted by the 1663 earthquake may have accelerated the occurrence of those events. On the other hand, localization of the recent larger earthquakes outside the region with strong enhanced failure conditions may suggest that the 1663 earthquake does not influence the occurrence of the larger events in the CSZ. This suggestion, however, depends significantly on the geometry of the seismic source assumed. It is likely that all the seismicity in the CSZ, including both microearthquakes and larger events, could be explained by assuming a larger source fault. We return to this point later, in the discussion.
Figure 4-3 Static stress field produced by the 1663 M7.0 event for the focal mechanism shown (strike 35, dip 60, rake 90). Top panel is map view at 10 km depth, in which the fault rupture area is shown as a white large rectangle (hosting 2.9m reverse slip). The white star represents the location of the 1663 earthquake. The green circles indicates the earthquake hypocenters, based on the unified CCSC catalog (since 1978,
depth≤30km).Thebeachballinthebottomrightindicatesthefaultfocalmechanism. The lower panel shows the Coulomb stress changes in a cross-section along the profile A-B, together with earthquake hypocenters within a 20-km-wide band. The dash line indicates the 10 km depth of the map view. Note the general coincidence of