GNGTS – Atti del 19° Convegno Nazionale / 12.14
E. Casarotti, A. Piersanti, F. P. Lucente and E. Boschi
Istituto Nazionale di Geofisica e Vulcanologia, Roma
GLOBAL POSTSEISMIC STRESS DIFFUSION
AND FAULT INTERACTION AT LONG DISTANCES
The issue of remote triggering of earthquakes and of fault interactions at great
distances have received much attention in the scientific literature of the last years.
The idea was thrown out by Romanowicz (1993) who suggested that the pattern of
seismic energy release was characterized by alternating periods of prevalent strikeslip and dip-slip activity at the opposite margins of the pacific ring. Though
subsequent investigations have shown that the proposed correlation has a large
probability to be observed by chance (Johnsons and Sheridan, 1997), the debate on
the remote triggering of earthquakes is far from being ended since new evidences
have been put forward.
Pollitz et al. (1998) have addressed for the first time the problem of the
interaction between earthquakes at great distances in a work based on a theoretical
model. They studied the pattern of the deformation rate field associated with the
great earthquakes which occurred during the fifties and the sixties in the Alaska
region and its spatio-temporal correlation with the seismic activity in the north Pacific
area. Their conclusions suggested that exists a correlation between the maximum
deformation rate field and the seismic activity in California in the eighties and that the
viscosity value of the asthenosphere maximizing this correlation is 51017 Pa s.
Recently Marzocchi (2000), analyzing the catalog of the world strongest earthquakes
of this century has found that earthquakes with seismic moment greater than 51021
N m are responsible for an increase of the seismic activity at distances of 500010000 km with a delay time of 10-25 years. This time lag has been interpreted as
due to the delayed response of a viscoelastic asthenosphere while the spatial scale
involved represents the typical distance between opposite margins of the Pacific ring
which experienced the largest number of strong earthquakes in this century.
Despite the positive results of the two last cited works we feel that the scientific
community is rather skeptical about the possibility of fault interactions at great
distances (e.g. Kerr, 1998). The bottom line here is that, given the low magnitude of
the stress transferred to distant faults (Casarotti, 1999; Antonioli et al., 1997) the
results of statistical investigations alone or a single modellistic evidence are not
sufficient to validate the positive hypothesis. Our goal is to perform a detailed
analysis of the features of postseismic stress diffusion on a global scale and to study
systematically the effects of the transferred stress to test the plausibility of the
hypothesis of long distance fault interaction.
We explicitly compute the value of the variation of the Coulomb Failure
Function (CFF) associated with the major earthquakes of this century on more than
8000 seismogenic structures that have ruptured in the last 25 years. The source for
our simulation is the CMT catalog which contains more than 9000 lithospheric events
in the area of our interest. The Coulomb Failure Function technique has been widely
adopted in studies of earthquake stress triggering on local and regional scale (see
Stein, 1999 for a review). The computation of the variation of the Coulomb stress
GNGTS – Atti del 19° Convegno Nazionale / 12.14
(CFF) can be accomplished after discriminating between the two complementary
nodal plane resulting from CMT inversion.
The initial list of events used in this study includes all the earthquakes, reported
in the CMT catalogue, occurred down to 80 km depth in the 120/300 lon. and –
60/80 lat. range. A set of 9658 matching the aforementioned criteria was selected.
The CMT solutions provide strike, dip, and rake for the two nodal plane of the
best double couple for each event (Dziewonski et al., 1981; 1983). So we had to
discriminate the actual fault plane between the two possible CMT planes, from our
selection of 9658 earthquakes scattered on half the globe and occurring in different
tectonic environments. A possible solution to this issue is the most trivial to figure
out, at least for the thrust and the normal fault type mechanisms: it comes out from
the basics of the rock fault mechanics, in the form of the Coulomb failure criterion
(e.g. Ranalli, 1995). Crosscorrelating the strike, dip and rake angles of the two nodal
planes for the thrust type earthquakes occurring along the Aleutian trench zone, we
noticed that the planes with dip angles < 45° mainly show strike directions in
agreement with both the subduction direction and versus (that are strikes between
200 and 300), while their complementary nodal planes have strike directions
between 0 and 100, which are incompatible with a northward subducting plate
boundary. So we have found a key: for the thrust type earthquakes we select the
plane with dip angle < 45°, while for the normal type events we select the plane with
dip angle > 45°. The efficacy of this selection criterion and the reliability of the
selected strikes, was verified by an a-posteriori visual and statistical inspection over
each plate boundary segment characterized by both approximately uniform strike
and dominating fault type mechanism. The choice of the actual fault plane for the
strike slip type earthquakes was made isolating the groups of events occurring along
the same tectonic structures (i.e. the same transform fault) end selecting case by
case the appropriate mechanism between the left and right-lateral.
At the end of this selection process we have obtained 8131 fault plane (off
9658):
5282 thrust, 1805 normal and 979 strike slip mechanisms.
To compute the postseimisc stress field associated with the major earthquakes
of the century we have improved and optimized a model of global postseismic
deformation developed by Piersanti et al. (1995; 1997) (Casarotti, 1998).
To perform our simulation we have considered eight of the largest earthquakes
of the century: these eight earthquakes alone account for more than 70% of the total
seismic moment release of this century (Pacheco and sykes, 1992). In the following
we will refer to them as "source" earthquakes. We have computed the cumulative
postseismic stress field due to the source earthquakes at the epicentral coordinates
and origin time of each of the 8131 events selected from the CMT catalog. The next
step has been the projection of the computed stress field on the focal plane of the
each earthquake in order to retrieve the variation of the Coulomb Stress (CFF). The
optimum depth to compute the postseismic CFF has not received a unique solution
yet, the best candidate are the hypocentral depth and the middle of the fault plane
[e.g. Stein, 1999 and references therein] (none of these informations is retrievable
from the CMT solutions). Our choice has been to compute the CFF, given the depth
of the CMT solution, at the nearest depth with respect to four sample depths in the
lithosphere (10, 25, 55, 80 km). We remind that, given the extremely large epicentral
distances involved in our analysis this is not a critical parameter here.
Our results show that the the distribution of favoured and not favoured events is
far from being uniform. This striking evidence, supported by a strong statisitical
GNGTS – Atti del 19° Convegno Nazionale / 12.14
significance, indicates the existence of a physical connection among the different
patterns of release of seismic moment by the plate margins of the Pacific area. Our
results are better interpreted in theframework of the theory of earthquake generation
as a self-organized critical phenomenon but essentially the nature of this physical
connection isstill an open question.
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