[SIO GP Seminars] Friday Geophysics Seminar

[Peter Shearer]

Reminder:  Seminar TODAY


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Friday, June 15th, 3:00 PM
   (refreshments served at 2:45 PM)
   IGPP Munk Conference Room

Jian Zhang, Lamont

"Studies of Inner Core Motion and Seismotectonics Using Correlated  
Earthquakes"

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ABSTRACT
Correlated earthquakes generating similar waveforms can contribute high
precision in detecting inner core motion and relocating earthquakes. We
have found more than 100 high-quality earthquake doublets in sevel
subduction zones. Observations using these doublets provide evidence  
for a
temporal change of inner core properties beneath Central America,  
Central
Asia, and Canada respectively. On the other hand, observations on three
ray paths, of which the part in the inner core is nearly parallel to the
equatorial plane, show no temporal change of inner core travel times.  
Such
pattern of observations showing both presence and absence of inner core
travel-time change may be explained by the geometry and relative
directions of ray path, lateral velocity gradient, and particle  
motion due
to inner core super-rotation. For the Bucaramanga earthquake nest,
teleseismic high-precision relocation is achieved by applying a
double-difference (DD) algorithm to phase picks from EHB bulletin of
Engdahl et al. (1998) and waveform cross-correlation (WCC) measurements.
The DD relocation of all Bucaramanga nest events reveals that about 700
events (~ 70% of all events in the past 40 years) focus on a small
fault-like volume that has dimensions of 20km x 10km x 10km. Further
improvement of the pick-based DD relocation using WCC measurements on a
set of 33 similar events images a complex near-vertical weak zone with a
width of ~ 5 km. Focal mechanisms of the nest earthquakes are highly
variable. However, the P axes generally align with the orientation of  
the
relative plate movement between Nazca and Caribbean. We propose a
slab-slab interaction model, in which the nest may represent a contact
zone where two subducted slabs, belonging to the Nazca plate and the
Caribbean plate respectively, collide and slide past each other.
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