19,106 research outputs found
Power Law Distributions of Offspring and Generation Numbers in Branching Models of Earthquake Triggering
We consider a general stochastic branching process, which is relevant to
earthquakes as well as to many other systems, and we study the distributions of
the total number of offsprings (direct and indirect aftershocks in seismicity)
and of the total number of generations before extinction. We apply our results
to a branching model of triggered seismicity, the ETAS (epidemic-type
aftershock sequence) model. The ETAS model assumes that each earthquake can
trigger other earthquakes (``aftershocks''). An aftershock sequence results in
this model from the cascade of aftershocks of each past earthquake. Due to the
large fluctuations of the number of aftershocks triggered directly by any
earthquake (``fertility''), there is a large variability of the total number of
aftershocks from one sequence to another, for the same mainshock magnitude. We
study the regime where the distribution of fertilities mu is characterized by a
power law ~1/\mu^(1+gamma). For earthquakes, we expect such a power-law
distribution of fertilities with gamma = b/alpha based on the Gutenberg-Richter
magnitude distribution ~10^(-bm) and on the increase ~10^(alpha m) of the
number of aftershocks with the mainshock magnitude m. We derive the asymptotic
distributions p_r(r) and p_g(g) of the total number r of offsprings and of the
total number g of generations until extinction following a mainshock. In the
regime \gamma<2 relevant for earhquakes, for which the distribution of
fertilities has an infinite variance, we find p_r(r)~1/r^(1+1/gamma) and
p_g(g)~1/g^(1+1/(gamma -1)). These predictions are checked by numerical
simulations.Comment: revtex, 12 pages, 2 ps figures. In press in Pure and Applied
Geophysics (2004
Earthquake source parameters and fault kinematics in the Eastern California Shear Zone
Based on waveform data from a profile of aftershocks following the
north-south trace of the June 28, 1992 Landers rupture across the Mojave
desert, we construct a new velocity model for the Mojave region which features
a thin, slow crust. Using this model, we obtain source parameters, including
depth and duration, for each of the aftershocks in the profile, and in
addition, any significant (M>3.7) Joshua Tree--Landers aftershock between
April, 1992 and October, 1994 for which coherent TERRAscope data were
available. In all, we determine source parameters and stress-drops for 45
significant (M_w > 4) earthquakes associated with the Joshua Tree and Landers
sequences, using a waveform grid-search algorithm. Stress drops for these
earthquakes appear to vary systematically with location, with respect to
previous seismic activity, proximity to previous rupture (i.e., with respect to
the Landers rupture), and with tectonic province. In general, for areas north
of the Pinto Mountain fault, stress-drops of aftershocks located off the faults
involved with the Landers rupture are higher than those located on the fault,
with the exception of aftershocks on the newly recognized Kickapoo (Landers)
fault. Stress drops are moderate south of the Pinto Mountain fault, where there
is a history of seismic swarms but no single through-going fault. In contrast
to aftershocks in the eastern Transverse ranges, and related to the 1992 Big
Bear, California, sequence, Landers events show no clear relationship between
stress-drop and depth. Instead, higher stress-drop aftershocks appear to
correlate with activity on nascent faults, or those which experienced
relatively small slip during mainshock rupture.Comment: 27 pages, 15 figures, to appear in Bull. Seism. Soc. A
Unified Scaling Law for Earthquakes
We show that the distribution of waiting times between earthquakes occurring
in California obeys a simple unified scaling law valid from tens of seconds to
tens of years, see Eq. (1) and Fig. 4. The short time clustering, commonly
referred to as aftershocks, is nothing but the short time limit of the general
hierarchical properties of earthquakes. There is no unique operational way of
distinguishing between main shocks and aftershocks. In the unified law, the
Gutenberg-Richter b-value, the exponent -1 of the Omori law for aftershocks,
and the fractal dimension d_f of earthquakes appear as critical indices.Comment: 4 pages, 4 figure
Bath's law Derived from the Gutenberg-Richter law and from Aftershock Properties
The empirical Bath's law states that the average difference in magnitude
between a mainshock and its largest aftershock is 1.2, regardless of the
mainshock magnitude. Following Vere-Jones [1969] and Console et al. [2003], we
show that the origin of Bath's law is to be found in the selection procedure
used to define mainshocks and aftershocks rather than in any difference in the
mechanisms controlling the magnitude of the mainshock and of the aftershocks.
We use the ETAS model of seismicity, which provides a more realistic model of
aftershocks, based on (i) a universal Gutenberg-Richter (GR) law for all
earthquakes, and on (ii) the increase of the number of aftershocks with the
mainshock magnitude. Using numerical simulations of the ETAS model, we show
that this model is in good agreement with Bath's law in a certain range of the
model parameters.Comment: major revisions, in press in Geophys. Res. Let
Sub-critical and Super-critical Regimes in Epidemic Models of Earthquake Aftershocks
We present an analytical solution and numerical tests of the epidemic-type
aftershock (ETAS) model for aftershocks, which describes foreshocks,
aftershocks and mainshocks on the same footing. The occurrence rate of
aftershocks triggered by a single mainshock decreases with the time from the
mainshock according to the modified Omori law K/(t+c)^p with p=1+theta. A
mainshock at time t=0 triggers aftershocks according to the local Omori law,
that in turn trigger their own aftershocks and so on. The effective branching
parameter n, defined as the mean aftershock number triggered per event,
controls the transition between a sub-critical regime n<1 to a super-critical
regime n>1. In the sub-critical regime, we recover and document the crossover
from an Omori exponent 1-theta for t<t* to 1+theta for t<t* found previously in
[Sornette and Sornette, 1999a] for a special case of the ETAS model. In the
super-critical regime n>1 and theta>0, we find a novel transition from an Omori
decay law with exponent 1-theta fot t<t* to an explosive exponential increase
of the seismicity rate fot t>t*. The case theta<0 yields an infinite n-value.
In this case, we find another characteristic time tau controlling the crossover
from an Omori law with exponent 1-theta for t<tau, similar to the local law, to
an exponential increase at large times. These results can rationalize many of
the stylized facts reported for aftershock and foreshock sequences, such as (i)
the suggestion that a small p-value may be a precursor of a large earthquake,
(ii) the relative seismic quiescence sometimes observed before large
aftershocks, (iii) the positive correlation between b and p-values, (iv) the
observation that great earthquakes are sometimes preceded by a decrease of
b-value and (v) the acceleration of the seismicity preceding great earthquakes.Comment: Latex document of 41 pages + 6 eps figures + 1 tabl
Aftershock production rate of driven viscoelastic interfaces
We study analytically and by numerical simulations the statistics of the
aftershocks generated after large avalanches in models of interface depinning
that include viscoelastic relaxation effects. We find in all the analyzed cases
that the decay law of aftershocks with time can be understood by considering
the typical roughness of the interface and its evolution due to relaxation. In
models where there is a single viscoelastic relaxation time there is an
exponential decay of the number of aftershocks with time. In models in which
viscoelastic relaxation is wave-vector dependent we typically find a power law
dependence of the decay rate, compatible with the Omori law. The factors that
determine the value of the decay exponent are analyzed
Importance of direct and indirect triggered seismicity
Using the simple ETAS branching model of seismicity, which assumes that each
earthquake can trigger other earthquakes, we quantify the role played by the
cascade of triggered seismicity in controlling the rate of aftershock decay as
well as the overall level of seismicity in the presence of a constant external
seismicity source. We show that, in this model, the fraction of earthquakes in
the population that are aftershocks is equal to the fraction of aftershocks
that are indirectly triggered and is given by the average number of triggered
events per earthquake. Previous observations that a significant fraction of
earthquakes are triggered earthquakes therefore imply that most aftershocks are
indirectly triggered by the mainshock.Comment: Latex document of 17 pages + 2 postscript figure
Violation of the scaling relation and non-Markovian nature of earthquake aftershocks
The statistical properties of earthquake aftershocks are studied. The scaling
relation for the exponents of the Omori law and the power-law calm time
distribution (i.e., the interoccurrence time distribution), which is valid if a
sequence of aftershocks is a singular Markovian process, is carefully examined.
Data analysis shows significant violation of the scaling relation, implying the
non-Markovian nature of aftershocks.Comment: 11 pages, 2 figures, 1 table. Dedicated to Francois Bardou
(1968-2006
- …