Bringing Seismicity into Focus: New Insights into
Fault Behavior from Accurate Earthquake Locations
Bokelmann(1), G.H.R., Beroza(1), G., Cole(2), A., Ellsworth(2), W.,
Schaff(1), D., Waldhauser(2), F., Zanzerkia(1), E.
(1) Stanford University;
(2) USGS, Menlo Park
Newly developed high-resolution images of seismicity on faults within
the San Andreas system reveal structure that was previously obscured by
earthquake location errors. By combining ordinary travel time picks
with
high-precision phase correlation measurements, improved earthquake
locations
reveal a wealth of detail previously hidden from view. In particular,
hypocenters on the Hayward, Calaveras and San Andreas faults show that
seismicity is highly organized in both space and time. Common features
from all of these faults include: 1) a narrow (25-100 m) width of the
fault core; 2) approximately horizontal streaks of hypocenters along
the fault plane; 3) multiple recurrence of earthquakes of the same size
at precisely the same spot on the fault (multiplets); and 4) regions
devoid
of earthquakes (holes). The narrow core of the fault is commonly
embedded
within a more diffuse volume of seismicity, which is itself often
comprised
of discrete narrow zones, i.e. faults. The streaks occur at multiple
depths of the fault core at many locations, and have typical lengths
of up to 5 km. Members of individual multiplets occur within a common
hypocentroid, and must repeatedly rupture the same area of the fault.
This suggests frictional or geometric controls on their location and
size. The holes are perhaps the most intriguing feature, as in the long
term, the fault must slip on these zones, either in earthquakes or
by steady or episodic creep. The nucleation zones of the M 6.1 1966
Parkfield, M 6.2 1984 Morgan Hill and M 6.9 1989 Loma Prieta
earthquakes all locate within holes near the base of the seismogenic
zone, suggesting that these holes represent zones of locking.
In contrast, the source zone of the 1992 slow earthquake at San Juan
Bautista on the San Andreas fault (Linde, et al., 1996) appears to
be coincident with shallow holes in the seismicity.
If such holes represent likely nucleation regions of future earthquakes
they suggest opportunities for focussed natural laboratory experiments
in the context of the Plate Boundary Observatory.
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