Post-seismic deformations after the Landers, 1992 earthquake
GPS,
trilateration, strainmeter, and SAR interferometry (InSAR) data revealed
patterns of various scales in the surface deformation field associated
with post-seismic processes after the 1992 Landers earthquake. (1) A large
scale pattern consistent with after-slip on deep sections of the fault
was observed in all data sets including data from remote stations of the
PGGA. After-slip models based on GPS observations indicate up to 0.8-1
m of right-lateral slip on the Homestead Valley and Emerson faults (Savage
and Svarc, 1997). However, such models imply vertical movements of up to
4 cm in the 10-20 km range from the fault, which are inconsistent with
the range change observed in the InSAR data spanning 1-4 years after the
earthquake. Visco-elastic relaxation, or possibly poro-elastic rebound
caused by fluid flow in the shallow crust may be advocated to account for
the observed range change. (2) InSAR data revealed several centimeters
of post-seismic rebound in step-overs of the 1992 break with a characteristic
decay time of 0.7 years. Such a rebound can be explained by shallow crustal
fluid flow associated with the dissipation of pore pressure gradients caused
by co-seismic stress changes (see below). A model of the post-seismic rebound
in the Homestead Valley pull-apart using Poisson's ratio values of 0.27
and 0.35 for the drained and undrained upper crust, respectively accounts
for the observed surface uplift in the 3 years following the earthquake.
The large value of the undrained Poisson's ratio we derive here is probably
due to intense fracturing, brecciation, and fluid saturation in the gouge
rocks within the fault zone. In fact, this value corresponds to a seismic
velocity ratio Vp/Vs of 2.1, consistent with the observed low Vs values
of fault-zone guided waves at shallow depth (Li et al., 1997). (3) InSAR
and creepmeter data also revealed post-seismic surface creep along the
Eureka Peak and Burnt Mountain faults with a characteristic decay time
of 0.8 years. Co-seismic, dilatant hardening (locking process) followed
by post-seismic, pore pressure controlled fault creep provide a plausible
mechanism to account for the decay time of the observed slip rate along
this section of the fault. This work was presented at the 1997, AGU Fall
meeting by G. Peltzer, F. Rogez, P. Rosen, and K. Hudnut. You can also see
a HTML version of the JGR paper Poro-elastic
rebound along the Landers 1992 earthquake surface rupture by the same authors.
Savage J.C. and J.L. Svarc, Postseismic deformation associated
with the 1992 Mw=7.3 Landers earthquake, southern California, J. Geophys.
Res., 102, 7565-7577, 1997.
Li Y.G., W.L. Ellsworth, C.H. Thurber, P.E. Malin, and
K. Aki, Fault-zone guided waves from explosions in the San Andreas Fault
at Parkfield and Cienega Valley, California, Bull. Seism. Soc. Am., 87,
210-221, 1997
Postseismic Rebound in Fault Step-Overs Caused by Pore
Fluid Flow
Near-field strain induced by large crustal earthquakes results in changes
in pore fluid pressure which dissipate with time and produce surface deformation.
Synthetic aperture radar (SAR) interferometry revealed several centimeters
of post-seismic uplift in pull-apart structures and subsidence in a compressive
jog along the Landers 1992 earthquake surface rupture, with a relaxation
time of 270 ± 45 days. Such a post-seismic rebound may be explained
by the transition of the Poisson's ratio of the deformed volumes of rock
from undrained to drained conditions as pore fluid flow allows pore pressure
to return to hydrostatic equilibrium (Peltzer
et al., 1996).
Figure 1. Left panel: Three-pass interferogram of the Landers
area generated with ERS-1 SAR images acquired in the 3 years after the
1992 earthquake. White line depicts the 1992 surface rupture. Straight
lines in zoomed areas are profiles shown in right panel. Fault labels are
CR: Camp Rock fault, E: Emerson fault, K: Kickapoo fault, JV: Johnson Valley
fault, and HV: Homestead Valley fault. Right panel: Line of sight
surface displacement along profiles 1,2 and 3 shown in left panel for time
intervals shown in red, green, and blue. Dots are displacement of individual
image pixels within ~400 m from profile line and solid curves indicate
averaged values in ~160 m-long bins along profiles strike.
To map post-seismic displacement, we combined SAR images spanning three
different time intervals in the three years following the earthquake (Fig.
1). The interferogram shown in Fig. 1 covers 41 days after the event, starting
on 7 August 1992. The most striking features are the localized strain along
three sections of the 1992 surface rupture where the rupture changed direction
or jumped to another fault branch and formed two pull-apart structures
and a compressive jog (boxes in Fig. 1). The observed displacement in the
fault step-overs is consistent with surface uplift in the pull-apart structures
and subsidence in the conpressive jog, i.e., opposed to the direction of
the co-seismic movements (Fig. 2).
Figure 2. Sketch showing expected vertical surface movements in compressive
and extensive jogs of a right-lateral, strike-slip fault activated by an
earthquake.
We interpreted the observed post-seismic rebound as a progressive change
of the mechanical characteristics of the deformed volume of rock in the
vicinity of the 1992 surface break as pore fluid pressure gradients caused
by the earthquake dissipate. For a more complete discussion of this interpretation
please see the text
version of the paper by Peltzer et al., 1996.
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GPS,
trilateration, strainmeter, and SAR interferometry (InSAR) data revealed
patterns of various scales in the surface deformation field associated
with post-seismic processes after the 1992 Landers earthquake. (1) A large
scale pattern consistent with after-slip on deep sections of the fault
was observed in all data sets including data from remote stations of the
PGGA. After-slip models based on GPS observations indicate up to 0.8-1
m of right-lateral slip on the Homestead Valley and Emerson faults (Savage
and Svarc, 1997). However, such models imply vertical movements of up to
4 cm in the 10-20 km range from the fault, which are inconsistent with
the range change observed in the InSAR data spanning 1-4 years after the
earthquake. Visco-elastic relaxation, or possibly poro-elastic rebound
caused by fluid flow in the shallow crust may be advocated to account for
the observed range change. (2) InSAR data revealed several centimeters
of post-seismic rebound in step-overs of the 1992 break with a characteristic
decay time of 0.7 years. Such a rebound can be explained by shallow crustal
fluid flow associated with the dissipation of pore pressure gradients caused
by co-seismic stress changes (see below). A model of the post-seismic rebound
in the Homestead Valley pull-apart using Poisson's ratio values of 0.27
and 0.35 for the drained and undrained upper crust, respectively accounts
for the observed surface uplift in the 3 years following the earthquake.
The large value of the undrained Poisson's ratio we derive here is probably
due to intense fracturing, brecciation, and fluid saturation in the gouge
rocks within the fault zone. In fact, this value corresponds to a seismic
velocity ratio Vp/Vs of 2.1, consistent with the observed low Vs values
of fault-zone guided waves at shallow depth (Li et al., 1997). (3) InSAR
and creepmeter data also revealed post-seismic surface creep along the
Eureka Peak and Burnt Mountain faults with a characteristic decay time
of 0.8 years. Co-seismic, dilatant hardening (locking process) followed
by post-seismic, pore pressure controlled fault creep provide a plausible
mechanism to account for the decay time of the observed slip rate along
this section of the fault. This work was presented at the 1997, AGU Fall
meeting by G. Peltzer, F. Rogez, P. Rosen, and K. Hudnut. You can also see
a HTML version of the JGR paper Poro-elastic
rebound along the Landers 1992 earthquake surface rupture by the same authors. 
