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Static Stress Changes and Fault Interaction Related to the 1985 Nahanni Earthquakes, Western Canada

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Static Stress Changes and Fault Interaction Related to the 1985 Nahanni Earthquakes, Western Canada Proceedings of the 1st IASME / WSEAS International Conference on Geology and Seismology (GES'07), Portoroz, Slovenia, May 15-17, 2007 75 Static Stress Changes and Fault Interaction Related to the 1985 Nahanni Earthquakes, Western Canada ALI O.ONCEL Earth Sciences Department King-Fahd University of Petroleum&Minerals P.O.Box 1946, Dhahran 31261 SAUDI ARABIA oncel@kfup http://faculty.kfupm.edu.sa/ES/oncel/ Abstract: - The 1985 Nahanni earthquakes (05/10/1985, Mw=6.6; 23/12/1985, Mw=6.8) were the largest observed events of the past 100 years not only in the Nahanni region of Canada but in the NE Cordillera. Their short interevent period (about two months) between the two mainshocks and their shallow depth (about 8 km) make them interesting since those characters for seismicity has rarely been reported. I compute the changes in static stresses along optimally oriented planes, caused by these events using the slip models of Hartzell et al. [1994] for the mainshock sources to investigate the possibility of dip-slip interaction and compare the relation between the patterns of aftershock seismicity and static stress. Thus, I then combine our coseismic stress changes with the regional stress to determine the induced stress changes for explaning short-term field- recorded aftershock seismicity nearly in one month and medium-term seismicity (1985-2002). The static stress change generated by the 5 October event onto the fault plane of December event are calculated and concluded that the occurrence of the 23 December Nahanni event seems triggered since the shallow major asperity along the fault plane is appeared to be induced by stress due to October earthquake but the deep intermediate asperity, which is very near to epicenter, seems partly discouraged, that may be a cause for delaying the onset time of December event for about two months. Subsequently, in 1988, a large event (M6.2) and its aftershocks, occurred in a region where the static stress level was increased by the 1985 Nahanni earthquakes. Key-Words: Coulomb Stress, Fault Modeling, Interaction, Shadow Zone, Earthquake 1 Introduction the previous 12 yrs, a possible sign that the area was Understanding short-term variability of preparing for future seismic activity. The time (79 seismicity parameters are very important to have days) and distance (5 km) between the two large some insights on short and medium-term seismic 1985 earthquakes (at depths of about 8 km) suggest hazard especially for active-faults of stable a physical relationship between the two events. In tectonics. Thus, earthquake-induced static stress this paper, static stress changes generated onto changes based on dislocation models of fault-slip dipping plane by the 1985 Nahanni events (Fig.1) provide a mechanism for understanding the spatial are examined to explain the aftershocks of 1985- variability of aftershocks [e.g., 1, 2]. Modeling of 1986 and subsequent mainshocks. short-term seismic hazard, based on the Coulomb- failure approach appears to be a tool for defining the 2 Problem Formulation potential locations of future mainshocks [e.g., 3; 4; 2.1. Data 5; 6; 7] as well as comparing to recent seismic Defining fault rupture planes for Canadian activity. earthquakes is not easy and is seldom obvious I test this hypothesis with data from a cluster of due to the general lack of surface rupture. Despite large earthquakes in Canada. The 1985 Nahanni this challenge, the aftershock surveys by the earthquakes (05/10/1985, Mw=6.6; 23/12/1985, Geological Survey of Canada provide significant Mw=6.8) were an unexpected sequence of large clues for choosing the rupture planes of the Nahanni earthquakes in the Northwest Territories of Canada, events [9]. In addition, similar rupture parameters a region with no record of historical large for the 1985 Nahanni events are found by Hartzell et earthquakes [8]. Two smaller events (14/08/1974- al. [10], Choy and Boatwright [11] and Wetmiller et MN=4.1 and 14/01/1977-MN=2.8, where MN is al., [9], providing encouragement that these are Nuttli magnitude, used to measure eastern Canada robustly modeled events whose effects I can earthquakes) had occurred in the Nahanni region in examine in the near-field. In this paper, I use the Proceedings of the 1st IASME / WSEAS International Conference on Geology and Seismology (GES'07), Portoroz, Slovenia, May 15-17, 2007 76 Slip(m) 0.2 0.20 0.00 0.12 1.40 1.57 2.47 2.97 2.55 2.75 1.02 0.56 1.37 0.14 0.00 0.00 3.40 3.20 3.00 0.07 0.00 0.20 1.45 0.53 0.29 0.70 0.42 1.52 0.22 0.00 0.37 0.47 0.00 0.00 2.80 100 km 2.60 0.43 0.37 0.09 0.46 1.24 0.42 0.64 0.85 1.55 0.95 0.00 0.90 0.63 0.00 0.11 2.40 3.30 to 4.50 0.09 0.02 0.88 3.14 2.89 1.84 1.00 0.50 1.63 2.24 0.00 0.18 0.00 0.00 0.00 2.20 2.00 0.00 0.37 2.16 2.31 2.08 0.14 0.00 1.49 1.06 0.80 0.26 1.06 0.31 0.00 0.00 1.80 4.50 to 6.00 km 1.60 0.00 0.44 0.00 2.27 3.29 0.00 0.00 0.78 0.55 0.00 0.00 0.90 0.70 0.00 0.12 1.40 68 1.20 0.05 0.00 0.00 2.04 3.83 0.00 0.00 0.61 0.30 0.00 0.00 0.00 0.22 0.00 0.62 1.00 6.00 t0 8.00 0.80 0.01 0.00 0.00 0.46 2.79 0.00 0.00 0.00 0.00 0.17 0.24 0.07 0.00 0.00 0.00 0.60 0.40 Distance Down Dip Distance 0.17 0.00 0.00 0.00 0.20 0.00 0.00 0.00 0.27 0.14 0.00 0.00 0.00 0.01 0.00 0.20 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.39 0.00 0.22 0.06 CANADA 17.4 -0.20 a) 0 km 40 N.W.T Slip(m) 2 0.34 0.92 2.20 2.45 1.17 0.97 0.06 1.42 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 3.40 66 3.20 MACKENZIE 3.00 0.03 0.52 0.39 0.81 1.65 1.37 0.99 1.34 1.93 0.98 0.70 0.03 0.09 0.00 0.00 0.00 0.00 0.00 2.80 MTNS 2.60 0.14 0.00 0.22 0.26 0.43 0.60 2.03 3.49 0.43 0.57 0.17 0.36 0.52 0.86 0.83 0.41 0.92 0.53 2.40 2.20 0.21 0.02 0.00 0.00 0.00 1.21 4.84 3.56 2.15 1.17 0.92 0.68 1.65 0.38 0.71 0.18 0.22 0.00 2.00 km 1.80 0.00 0.00 0.00 0.00 0.00 0.40 2.41 0.22 0.20 0.64 1.02 1.51 0.68 1.57 0.00 0.00 0.02 0.00 1.60 Mw=6.6 1.40 05/10/1985 0.00 0.00 0.00 0.00 0.00 0.08 1.80 0.76 0.64 0.36 0.02 0.00 0.00 0.18 0.25 0.00 0.00 0.00 1.20 1.00 64 Distance Down Dip 0.00 0.00 0.00 0.00 0.00 0.00 1.01 2.27 3.22 0.50 0.20 0.22 0.09 0.00 0.00 0.00 0.00 0.00 0.80 0.60 0.00 0.00 0.00 0.00 0.00 0.00 0.00 1.73 3.51 0.97 0.00 0.00 0.23 0.00 0.00 0.00 0.00 0.00 0.40 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.21 0.20 20 0.00 b) 0 km 48 Distance Along Strike fo 62 CORDILLERA Fig.2. Contoured slip subfaults and asperities, in Mw=6. 2 meters for a) the 5 October Nahanni earthquake, the 25/03/1988 area of the largest asperity is 46 km2 and its average Mw=6. 8 slip is 2.51 m and b) the 23 December Nahanni 60 23/12/1985 earthquake. The area of the largest asperity is 31 2 -136 -134 -132 -130 -128 -126 -124 -122 km and its average slip is 2.7 m. Fig.1. Seismicity map of the Nahanni fault zone for earthquakes with MN>3.3 between January 1985 and where apparent friction µ’=µ[1-β]. µis the June 2002. Harvard focal mechanisms solutions for two coefficient of friction, ∆σis the induced change in events are located in the map.
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