Our objectives were to use modem geodetic data, especially those derived from space techniques like Very Long Baseline Interferometry (VLBI), Satellite Laser Ranging (SLR), and the Global Positioning System (GPS) to infer crustal deformation in southern California and relate it to plate tectonics and earthquake hazard. To do this, we needed to collect some original data, write computer programs to determine positions of survey markers from geodetic observables, interpret time dependent positions in terms of velocity and earthquake caused episodic displacements, and construct a model to explain these velocities and displacements in terms of fault slip and plate movements

Jackson, David D.

English

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NASA-CR-196621
Final Technical Report
NASA Grant NAG 5-447
Tectonic Deformation in Southern California
November 1, 1993
David D. Jackson
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Objectives
Our objectives were to use modern geodetic data, especially those derived from
space techniques like Very Long Baseline Interferometry (VLBI), Satellite Laser
Ranging (SLR), and the Global Positioning System (GPS) to infer crustal
deformation in southern California and relate it to plate tectonics and earthquake
hazard. To do this, we needed to collect some original data, write computer
programs to determine positions of survey markers from geodetic observables,
interpret time dependent positions in terms of velocity and earthquake caused
episodic displacements, and construct a model to explain these velocities and
displacements in terms of fault slip and plate movements.
Accomplishments:
We made original GPS measurements, especially near NASA VLBI sites at
Pasadena, Pinon Flat, Vandenberg AFB, Mojave, Santa Paula, and Palos
Verdes, in southem California. We combined these data with existing VLBI data,
and with data from the USGS trilateration networks in southern California, to
measure the apparent vector velocity of survey markers relative to each other.
We developed computer programs for adjusting VLBI, SLR, GPS, trilateration,
and triangulation data to determine the instantaneous displacements due to
earthquakes, and the velocity of gradual changes between earthquakes of the
relative locations of survey monuments with respect to each other. These
programs use prior estimates of the coseismic displacements and the aseismic
velocity field in a Bayesian scheme to better estimate the velocities and
earthquake displacements. The programs also calculate the covariance matrix
that describes the uncertainties of the parameter estimates, and their
dependencies upon one another.
We developed a computer model to explain aseismic velocities resulting from
slip of seismic faults at depth. The model assumes that the crust is composed of
elastic blocks, whose boundaries are the faults that cause earthquakes. The
faults are assumed to slip freely, without friction, below some depth called the
locking depth. Above this depth, the faults are subject to friction, although they
may "creep," or slip at a steady rate, in spite of this friction. The computer model
allows us to estimate the translation velocity of the blocks, the rotation rate of
(NASA-CR-194621) TECTONIC
OEFORMATION IN SOUTHERN CALIFORNIA
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2each block about a vertical axis through its center, and the rate of aseismic
creep on the upper surface of each fault segment. The relative slip rate and the
rate of convergence at depth on the individual faults is also determined, because
these quantities are functions of the block parameters mentioned above..
We applied this model to interpret available geodetic data for several parts of
the world. These applications were essential to developing the model itself, and
gave us a breadth of experience essential for interpreting the deformation in
southern California.
Results
In the Hollister area of central California, both the San Andreas and Calaveras
fault display significant creep near the surface. In fact most of the block motion
can be explained by creep and deep aseismic slip, so that there is very little
elastic deformation of the blocks. Large earthquakes, such as those that occur
further to the southeast, are very improbable at Hollister, although they can be
expected on the San Andreas fault to the Northwest of Hollister..
The longitudinal valley in Taiwan shows both seismic and geodetic evidence of
convergence on dip-slip faults and right lateral strike slip faulting. The measured
velocities of surface geodetic monuments indicates that both the convergence
and strike slip deformation are confined to a very narrow zone in the longitudinal
valley, implying that there is considerable aseismic creep, similar to that
observed at Hollister. However, there is some elastic straining of the crust, and
large earthquakes are can be expected here. An important difference between
the longitudinal valley of Taiwan and the Hollister area is that the block motion in
Taiwan includes a strong component of convergence. Taiwan is the only place in
the world, to our knowledge, where there is direct evidence of rapid aseismic
convergence (that is creep on a dip-slip fault) on land.
Around the Japanese islands, we used VLBI data and excellent triangulation
data collected over the last century to determine the pattern of straining caused
by plate tectonics in the area. Here the major plate boundary is offshore, so the
total plate motion must be inferred from plate tectonic models. The strain rate
within the islands is only about half of what would be observed if the Pacific
plate rate were moving at the modeled rate relative to the Eurasian plate, and if
the plates were locked together at depths shallower than 20 km. Because the
plate model has been confirmed by VLBI measurements on Pacific islands, we
believe the most likely explanation for the low strain rate on Japan is that the
plates are not fully locked above 20 km. Approximately 50 % of the expected
plate displacement may occur aseismically. This finding is consistent with the
seismic history of japan over the last 400 years; there have been less seismic
moment release than would be expected for a fully elastic plate model.
3Near Parkfield in central California, geodetic data are consistent with the
observation that surface creep is prevalent to the northwest, but does not occur
to any large extent to the southeast of Parkfield. We found weak evidence for an
"asperity", or zone where creep is locally inhibited, near the focal zone of the
1966 Parkfield earthquake. However, the data can be fit rather well by a simpler
model with no asperity; the asperity is not statistically significant. A consequence
is that the geodetic data do not require "characteristic" earthquakes, with a size
controlled by the dimensions of the asperity, as suggested by some other
investigators.
Further south, on a network crossing the San Andreas fault at Cholame and
extending westward through the southern Coast Ranges, the deformation is
consistent with a simple model in which the San Andreas fault is locked to a
depth of 20 km. The estimated slip rate is entirely consistent with the
geologically estimated slip rate of 34 mm/yr, at Wallace Creek, within our
geodetic network. In fact the simple elastic model, corrected for direct effects of
known historic earthquakes, is consistent with geologic data spanning several
thousand year, triangulation data spanning over 100 years, trilateration data
spanning about 20 years, and GPS data spanning just 4 years. We conclude
that any temporal variations of strain rate are no greater than 20% on any of
these time scales. Taken together, these measurements provide the first direct
evidence that so tightly constrains the amount of inelastic strain in the crust.
In the broader southern California region, we find the strain to be much more
widespread in the transverse ranges than further north or further south. The
strain pattern here is not consistent with a simple model of elastic strain resulting
from slip on locked faults, unless the locking depth exceeds the thickness of the
crust. As it is unlikely that the faults are locked to such great depth, we believe
the displacement is shared among many smaller faults, and that the slip rate on
the Mojave section of the San Andreas itself is considerable less than that on
the Cholame section. An implication is that the seismic hazard is spread more
uniformly throughout southern California, and closer to the populated regions of
Los Angeles and Orange counties.
4Publications:
The following papers resulted from work supported by this grant:
Matsu'ura, M., D. Jackson, and A. Cheng, Dislocation model for aseismic
deformation at Hollister, California, J. GeoDhvs. Res.. 91, 12,661-12,674_, 1986.
Cheng, A., D. Jackson, and M. Matsu'ura, Aseismic crustal deformation in the
Transverse Ranges of southern California, Tecton0physics, 144, 159-180, 1987.
Sung, L.Y., and D. Jackson, Geodetic evidence of the seismic potential at
Parkfield, California, Geophys. Res. Lett.. 15, 820-823, 1988.
Shen, Z.K., and D.D. Jackson, Global Positioning System reoccupation of early
triangulation sites: tectonic deformation of the southern Coast Ranges, J.
Geoohvs. Res.. 98, 9931-9946, 1993.
Feigl, K., D. Agnew, Y. Bock, D. Dong, A. Donnellan, B. Hager, T. Herring, D.
Jackson, T. Jordan, R. King, S. Larsen, KoLarson, M. Murray, Z. Shen, and F.
Webb, Measurement of the velocity field of central and southern California,
1984-1991, J_._ _ in press, 1993.


Tectonic deformation in southern California

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