Middle Valley September 2001: Event Description

  
 

 

 
 
  (click for full-size)
map of epicenters < 2 min. error
Fig. 1: Swarm epicenters with errors < 2 minutes and historical epicenters.
graph of time vs. distance plot of all epicenters
Fig. 2: Time vs. distance plot of all events < 5 minutes error.
graph of time vs. distance plot of epicenters < 5 min error lat/long
Fig. 3: Time vs. distance plot of events < 5 min. error and estimated migration rate.

Event Description :

The land seismic network operated by the Pacific Geoscience Center of the Geological Survey of Canada has recorded and produced moment tensor solutions for several of the larger events, indicating a mixture of normal and strike-slip motion. Strike-slip events are oriented parallel to Juan de Fuca orientation (15 deg) and are likely associated with the

 

Middle Valley September 2001: Event Migration

  
 

 

 
 

Event Migration:

epicenters through 9/12
Epicenters through 09/12/01.
epicenters through 9/20
Epicenters through 09/20/01

 

  
     
   
 
 

ridge system. The complex interplay between the Nootka fault and the JdF volcanic system results in a diffuse triple junction with correspondingly complex stress fields. Further detailed examination of the joint data sets will be required to unravel the total picture. A response effort is being discussed by the combined event response team funded by NSF and NOAA. A detailed description of the site can be found in Davis, E.E., and H. Villinger, 1992, Proceedings of the Ocean Drilling Program, Initial Reports, vol 139, pp 9-41.

Epicenter Maps:
Figure 1 shows the epicenters with latitude and longitude errors < 2 minutes. The epicenters seem to follow the structure of the eastern middle valley faults and "wrap around" the intersection with the Sovanco. Also comparing this with an earlier plot, it looks like these latest events are generally further south along Middle Valley than previous events.

Distance-time Plots:
Migration of earthquake epicenters during the Middle Valley earthquake sequence. The earthquakes are plotted in distance relative to an average location of the Dead Dog and Bent Hill Vents and the ODP Holes 856,857,&858. The earthquakes have clearly migrated southward along the eastern section of Middle Valley during the 10 days of the swarm, currently coming to within 5 km of the vents and drill holes.

Figure 2 shows events with latitude and longitude errors < 5 minutes and Figure 3 shows same events with an estimated migration rate.