2009 AGU Fall Meeting abstracts
B.
Enescu, T. Takeda, K. Obara, S. Sekine, W. Suzuki, Y. Asano, Y.
Yukutake, The 2008 Iwate-Miyagi Inland Earthquake (Mw6.9): The
Distribution and Focal Mechanism Solution of Aftershocks in relation to
the Crustal Structure, S44B-06
Abstract. Several
large earthquakes, including the 2008 Iwate-Miyagi Inland earthquake
(Mw6.9), occurred recently in the High Strain Rate Zone of Japan. In
the framework of a large national project of observations and research
in this region, we investigate the earthquake distribution, crustal
structure and aftershock focal mechanisms in the area of the 2008
Iwate-Miyagi sequence. The detailed aftershock distribution, spanning
roughly from south to north, shows a conjugate fault plane structure
(i.e., planes dipping towards NW and SE, respectively) that is
particularly clear in the northern part of the aftershock area.
However, such a conjugate fault structure is also apparent south of the
mainshock. To clarify the relation between seismicity and faults, we
further investigate the earthquake focal mechanisms. While most
aftershocks have a focal mechanism consistent with that of the
mainshock, some others are strike slip events but with the same
orientation (NW-SE) of the compression axis as that of the mainshock.
Moreover, there is a distinct group of thrust events located south from
the mainshock that have fault planes dipping to north or south. Our
results indicate that at least in the south there is one fault
structure dipping to NW (likely the mainshock fault) and several
clusters of earthquakes with different focal mechanisms that cannot be
easily associated with a fault. Taking advantage of the NIED dense
linear seismic array located south of the mainshock, we investigate the
detailed crustal structure on a profile positioned along the linear
array (and perpendicular to the fault). We use the P- and S-wave
arrival times recorded by the NIED array stations, as well as other
stations in the area, in a tomographic inversion using the Lotos
software (Koulakov, 2009). The velocity structure under the NIED
array shows two relatively low-velocity, shallow structures, separated
by a higher velocity body that continues in the deeper part. The
presence of such a higher velocity structure is also highlighted by the
analysis of seismograms recorded at array stations. The main fault
(dipping towards NW), which can be delineated from the distribution of
aftershocks, is located at the transition between lower and higher
velocities. To explain the distribution and the various focal mechanism
solutions of the aftershocks, we estimated the Coulomb stress changes
caused by the mainshock in the area. We used the slip distribution of
Suzuki et al. (2009) and computed the static stress changes on
optimum-oriented faults. The stress changes can explain well the most
important characteristics of the aftershock distribution. We are
presently computing the stress changes on the two conjugate fault
planes of aftershocks for which focal mechanisms are available. To
eliminate possible artifacts in the deltaCFF distribution close to the
fault, we remove from the analysis the aftershocks that are closer than
2km from the fault. Preliminary results indicate that most of the
aftershocks occurred in areas of positive Coulomb stress changes.
S. Toda, B.
Enescu, R. Stein, CoulombExpress: Automated near-realtime forecasts of
earthquake-induced stress transfer and expected seismicity rate
changes, S22C-02 (INVITED)
Abstract. A principal tenet of the
Coulomb hypothesis is that stress increases promote, and decreases
inhibit, fault failure. In support of such a simple hypothesis, a
growing number of studies have found that seismicity rates climb where
the stress increases and fall where the stress drops. However, they are
all evaluated retrospectively, which may permit unintentionial bias to
enter into data interpretation. Further, retrospective tests do not
contribute to earthquake disaster mitigation. In particular, the
probability rate for triggered seismicity is highest immediately after
a mainshock, as suggested by rate/state friction. Thus, to make the
stress-based earthquake forecasting rapidly available and to permit
objective prospective testing, we have developed CoulombExpress, an
automatic stress calculation system, which uses near real-time
information, such as earthquake magnitude, location, depth, and its
rapid moment tensor solution. Two versions of CoulombExpress,
“CoulombExpress Global” and “CoulombExpress Regional” are in progress.
1) CoulombExpress Global: The system quickly computes the Coulomb
stress change caused by an M≥6 around the globe. It automatically
accesses the USGS National Earthquake Information Center parameters. In
order of time, the system uses the NEIC W-phase, central, and body-wave
moment tensor solutions, when available, to make the two-nodal-plane
source fault models using the empirical scaling relations of Wells and
Coppersmith [1994]. The stress changes are resolved on receiver faults
parallel to the sources and also on both nodal planes of the nearby
1967-2005 Global CMT earthquakes, as stand-in's for active faults. The
color-coded displays allow the viewer to grasp where and by how much
the off-fault aftershocks might become active. Keeping long-term
records of our results in an official archive will allow evaluators to
examine rigorously the forecasting skills of our model.
2) CoulombExpress Regional: This is a forecasting system that computes
not only Coulomb stresses but also the space-time seismicity rate
evolution (Toda et al., JGR, 2005) for a particular region or country
where high-quality local data are available. It has been implemented
for M≥4 in Japan first (with California to follow). Moderate
earthquakes are implemented as point sources to generate local stress
perturbations. Since the local catalog is available, we analyze the
background rate of seismicity, aftershock durations, and regional G-R
parameters needed to translate the calculated stress changes into
expected seismicity rate changes. We have built matrices of assumed
receiver fault planes (strike, dip, and rake) based on focal mechanism
data and structural controls. Since stress increase should immediately
increase the seismicity rate, we select and use for further
calculations the maximum stress changes throughout the various receiver
planes and calculation depths. We will submit this regional model to
CSEP (Collabratory for the Study of Earthquake Predictability), Japan,
starting at the end of 2009.
Z. Peng, B.
Enescu, P. Zhao, S. Hainzl, Detecting early
aftershocks in California and Japan based on a matched filter
technique, S54A-06
Abstract. A large shallow earthquake is immediately followed by
numerous aftershocks with a significant portion missing in existing
earthquake catalogs, mainly due to masking of the mainshock coda and
overlapping arrivals. Recovering these missing early aftershocks is
important for understanding the physical mechanisms of earthquake
triggering, and tracking post-seismic deformation around the mainshock
rupture zone. An effective way of detecting and locating those missing
early aftershocks is the matched filter technique. It utilizes
waveforms or travel time information of existing events as a template,
or “matched filter”, to search for similar patterns in the continuous
recordings as a suggestive of an event. Here we apply this technique to
systematically detect early aftershocks of moderate-size events in
California and Japan. In the first study, we use waveforms of relocated
events along the Parkfield section of the San Andreas Fault (SAF) as
templates, and scan through continuous waveforms for 3 days around the
2004 Mw6.0 Parkfield earthquake to detect missing aftershocks. We
identify 11 times more aftershocks than reported in the standard
Northern California Seismic Network (NCSN) catalog. The newly detected
aftershocks show clear migration in both along-strike and down-dip
directions with logarithmic time since the mainshock, consistent with
the numerical simulations on expansions of aftershocks caused by
propagating afterslip. The cumulative number of early aftershocks
increases linearly with postseismic deformation in the first 2 days,
suggesting that early aftershocks could be driven by significant
afterslip along the SAF induced by the Parkfield mainshock. In the
second study, we apply the same technique to detect and locate early
aftershocks of the 2008 Mw6.9 Iwate-Miyagi Nairiku earthquake in
northeastern Honshu, Japan. The existing aftershock locations show
complex spatial distributions with many occurring away from the
mainshock fault plane. Well-resolved focal mechanism solutions of
aftershocks show considerable variations compared to the mainshock
focal mechanism. However, most of the off-fault events occur in areas
of positive Coulomb stress changes. A small coefficient of friction
(~0.2) provides in general a better consistency between stress changes
and aftershock locations. After detecting the missing events, our next
step is to examine the spatio-temporal migration patterns to quantify
whether early aftershocks were driven by coseismic stress changes,
afterslip, fluid migration, or a combination of these factors.
T. Takeda, H.
Sato, K. Obara, B. Enescu, NW-SE Trending
Fault-Segmentation Boundaries in the High-Strain-Rate Zone of Japan,
T53B-1578
Abstract. In the high-strain-rate zone of Japan, located in the
eastern margin of the Sea of Japan, large earthquakes of magnitudes up
to 7.5 have often took place. Recently, two M6.8 earthquakes occurred
in 2004 and 2007, showing reverse fault mechanisms, with a NW-SE
compression. The aftershock distributions of both mainshocks are
oriented in a NE-SW direction and seem to have a sharp cut-off boundary
at the northeastern edges by a common NW-SE line. This implies the
existence of a structural boundary trending in the NW-SE direction.
Since revealing structural constraints on fault segmentation is
important for strong motion prediction and earthquake hazard
assessment, we investigate here the boundary that controls the fault
segmentation, using accurately located hypocenters and their focal
mechanisms. We investigate earthquakes which occurred in the target
area from 2001 to 2004 since later data is dominated by aftershocks
that can obscure subtle structural features. The hypocentral
distribution shows three major linear alignments with a strike of a
NW-SE direction. Two of them are located near an estimated epicenter of
a large earthquake in 1828, and the other is located in the northern
part of the aftershock area of the 2004 event. To assess the accurate
spatial distribution of hypocenters, we relocated these earthquake
alignments by using the double-difference method (Waldhauser and
Ellsworth, 2000) and classified the focal mechanism using the criterion
of Frohlich (1992). As a result, two alignments near the 1828
earthquake appear more concentrated and show subvertical planar
distributions, 2.5-3 km wide x 3-5 km deep. The average focal mechanism
has a P-axis E-W oriented. These observations indicate that the both
alignments have seismic activity along NW-SE trending strike-slip
faults. On the other hand, the alignment near the 2004 earthquake
separates into two main groups and does not show a clear NW-SE trending
strike-slip fault since it includes not only strike-slip type but also
reverse type of focal mechanisms. However the existence of strike-slip
mechanisms with N-S compression is supporting the existence of a NW-SE
trending strike-slip fault. These results indicate the existence of
NW-SE trending strike-slip faults, that is, structural boundaries
having the same orientation. Sato (1994) suggests that in this area a
rift structure, formed as a result of normal faulting when the Sea of
Japan opened in the Miocene, is presently reactivated as a reverse
fault by stress field inversion. Therefore the seismic activity would
be controlled by the rift structure and be bounded by a transfer fault
with a NW-SE strike. For example, there is a NW-SE trending lateral
ramp between the Shitada hill and the Niitsu anticline, which is
bounding the source fault of the 1828 earthquake. It is worth noting
that the alignment is located at this boundary and distributes along a
steep gravity anomaly gradient. There are many linear alignments other
than those previously mentioned. Linear alignment analysis using
various geophysical methods could result in a more accurate detection
of the boundaries that constrain fault segmentation in the
high-strain-rate zone of Japan.
M. Cocco, S. Hainzl,
J. Woessner, B. Enescu, F. Catalli, A. Lombardi, Sensitivity study of
forecasted aftershock seismicity based on Coulomb stress calculation
and rate- and state-dependent frictional response, S22C-05 (INVITED)
It is nowadays well established that both
Coulomb stress perturbations and the rate- and state-dependent
frictional response of fault populations are needed to model the
spatial and temporal evolution of seismicity. This represents the most
popular physics-based approach to forecast the rate of earthquake
production and its performances have to be verified with respect to
alternative statistical methods. Despite the numerous applications of
Coulomb stress interactions, a rigorous validation of the forecasting
capabilities is still missing. In this work, we use the Dieterich
(1994) physics-based approach to simulate the spatio-temporal evolution
of seismicity caused by stress changes applied to an infinite
population of nucleating patches modelled through a rate- and
state-dependent friction law. According to this model, seismicity rate
changes depend on the amplitude of stress perturbation, the physical
constitutive properties of faults (represented by the parameter Aσ),
the stressing rate and the background seismicity rate of the study
area. In order to apply this model in a predictive manner, we need to
understand the variability of input physical model parameters and their
correlations. We first discuss the impact of uncertainties in model
parameters and, in particular, in computed coseismic stress
perturbations on the seismicity rate changes forecasted through the
frictional model. We aim to understand how the variability of Coulomb
stress changes affects the correlation between predicted and observed
changes in the rate of earthquake production. We use the aftershock
activity following the 1992 M 7.3 Landers (California) earthquake as
one of our case studies. We analyze the variability of stress changes
resulting from the use of different published slip distributions. We
find that the standard deviation of the uncertainty is of the same size
as the absolute stress change and that their ratio, the coefficient of
variation (CV), is approximately constant in space. Second, we
demonstrate that all model parameters are strongly correlated for
physical and statistical reasons. We discuss this correlation
emphasizing that the estimations of the background seismicity rate,
stressing rate and Aσ parameter are strongly correlated to reproduce
the observed aftershock productivity. Our results demonstrate the
impact of these model parameters on the Omori-like aftershock decay
(the c-value and the productivity of the Omori law), implying a p-value
smaller or equal to 1. Finally, we discuss an optimal strategy to
constrain model parameters for near-real time forecasts. Our case
studies demonstrate that accounting for realistic uncertainties in
stress changes as well as for the correlation among model parameters
strongly improves the forecasting performances, although the original
deterministic approach is converted into a statistical method.