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Article AbstractsF.-C. Lin, V. C. Tsai, B. Schmandt, Z. Duputel, and Z. Zhan, 2013. Extracting seismic core phases with array interferometry, Geophys. Res. Lett., v. 40, p. 1-5
Seismic body waves that sample Earth's core are indispensable for
studying the most remote regions of the planet. Traditional
core phase studies rely on well-defined earthquake signals,
which are spatially and temporally limited. We show that, by
stacking ambient-noise cross-correlations between USArray seismometers,
body wave phases reflected off the outer core (ScS), and twice
refracted through the inner core (PKIKP2) can be clearly extracted.
Temporal correlation between the amplitude of these core phases and
global seismicity suggests that the signals originate from distant
earthquakes and emerge due to array interferometry. Similar results
from a seismic array in New Zealand demonstrate that our approach is
applicable in other regions and with fewer station pairs. Extraction of
core phases by interferometry can significantly improve the spatial
sampling of the deep Earth because the technique can be applied
anywhere broadband seismic arrays exist. Z. Zhan, D. Helmberger, M. Simons, H. Kanamori, W. Wu, N. Cubas, Z. Duputel, J.-P. Avouac, R. Chu, V.C. Tsai, K.W. Hudnut, S. Ni, E. Hetland and F.H. Ortega Culaciati, 2012. Earthquakes with anonalously steep dip in the source region of the 2011 Tohoku-Oki earthquake and possible explanations, Earth Planet Sci. Lett., v. 353-354, p. 121-133.
The 2011 Mw9.1 Tohoku-Oki earthquake had unusually large
slip (over 50 m) concentrated in a relatively small region,
with local stress drop inferred to be 5-10 times larger than
that found for typical megathrust earthquakes. Here we
conduct a detailed analysis of foreshocks and aftershocks
(Mw 5.5-7.5) sampling this megathrust zone for possible
clues regarding such differences in seismic excitation. We
find that events occurring in the region that experienced
large slip during the Mw9.1 event had steeper dip angles (by
5-10deg) than the surrounding plate interface. This
discrepancy cannot be explained by a single smooth plate
interface. We provide three possible explanations. In Model
I, the oceanic plate undergoes two sharp breaks in slope,
which were not imaged well in previous seismic
surveys. These break-points may have acted as strong seismic
barriers in previous seismic ruptures, but may have failed
in and contributed to the complex rupture pattern of the
Tohoku-Oki earthquake. In Model II, the discrepancy of dip
angles is caused by a rough plate interface, which in turn
may be the underlying cause for the overall strong coupling
and concentrated energy-release. In Model III, the
earthquakes with steeper dip angles did not occur on the
plate interface, but on nearby steeper subfaults. Since the
differences in dip angle are only 5-10deg, this last
explanation would imply that the main fault has about the
same strength as the nearby subfaults, rather than much
weaker. A relatively uniform fault zone with both the main
fault and the subfaults inside is consistent with Model
III. Higher resolution source locations and improved models
of the velocity structure of the megathrust fault zone are
necessary to resolve these issues. Z. Duputel, H. Kanamori, V.C. Tsai, L. Rivera, L. Meng, J.-P. Ampuero and J. Stock, 2012. The 2012 Sumatra great earthquake sequence, Earth Planet Sci. Lett., v. 351-352, p. 247-257.
The equatorial Indian Ocean is a well known place of active
intraplate deformation defying the conventional view of
rigid plates separated by narrow boundaries where
deformation is confined. On 11 April 2012, this region was
hit in a couple of hours by two of the largest strike-slip
earthquakes ever recorded (moment magnitudes Mw=8.6 and
Mw=8.2). Broadband seismological observations of the Mw=8.6
mainshock indicate a large centroid depth (~30 km) and
remarkable rupture complexity. Detailed study of the
surface-wave directivity and moment rate functions clearly
indicates the partition of the rupture into at least two
distinct subevents. To account for these observations, we
developed a procedure to invert for multiple-point-source
parameters. The optimum source model at long period consists
of two point sources separated by about 209 km with
magnitudes Mw=8.5 and 8.3. To explain the remaining
discrepancies between predicted and observed surface waves,
we can refine this model by adding directivity along the
WNW–ESE axis. However, we do not exclude more complicated
models. To analyze the Mw=8.2 aftershock, we removed the
perturbation due to large surface-wave arrivals of the
Mw=8.6 mainshock by subtracting the corresponding synthetics
computed for the two-subevent model. Analysis of the
surface-wave amplitudes suggests that the Mw=8.2 aftershock
had a large centroid depth between 30 km and 40 km. This
major earthquake sequence brings a new perspective to the
seismotectonics of the equatorial Indian Ocean and reveals
active deep lithospheric deformation. L. Meng, J.-P. Ampuero, J. Stock, Z. Duputel, Y. Luo and V.C. Tsai, 2012. Earthquake in a Maze: Compressional Rupture Branching During the 2012 Mw 8.6 Sumatra earthquake, Science, v. 337, no 6095, p. 724-726.
Seismological observations of the 2012 moment magnitude 8.6
Sumatra earthquake reveal unprecedented complexity of
dynamic rupture. The surprisingly large magnitude results
from the combination of deep extent, high stress drop, and
rupture of multiple faults. Back-projection source imaging
indicates that the rupture occurred on distinct planes in an
orthogonal conjugate fault system, with relatively slow
rupture speed. The east-southeast–west-northwest ruptures
add a new dimension to the seismotectonics of the Wharton
Basin, which was previously thought to be controlled by
north-south strike-slip faulting. The rupture turned twice
into the compressive quadrant, against the preferred
branching direction predicted by dynamic Coulomb stress
calculations. Orthogonal faulting and compressional
branching indicate that rupture was controlled by a
pressure-insensitive strength of the deep oceanic
lithosphere.
Z. Duputel, L. Rivera, Y. Fukahata and H. Kanamori, 2012. Uncertainty estimations for seismic source inversions, Accepted for publication in Geophysical Journal International.
Source inversion is a widely used practice in seismology.
Magnitudes, moment tensors, slip distributions are now
routinely calculated and disseminated whenever an earthquake
occurs. The accuracy of such models depends on many aspects
like the event magnitude, the data coverage and the data
quality (instrument response, isolation, timing,
etc.). Here, like in any observational problem, the error
estimation should be part of the solution. It is however
very rare to find a source inversion algorithm which
includes realistic error analyses, and the solutions are
often given without any estimates of uncertainties. Our goal
here is to stress the importance of such estimation and to
explore different techniques aimed at achieving such
analyses. In this perspective, we use the W phase source
inversion algorithm recently developed to provide fast CMT
estimations for large earthquakes. We focus in particular on
the linear-inverse problem of estimating the moment tensor
components at a given source location. We assume that the
initial probability densities can be modeled by Gaussian
distributions. Formally, we can separate two sources of
error which generally contribute to the model parameter
uncertainties. The first source of uncertainty is the error
introduced by the more or less imperfect data. This is
carried by the covariance matrix for the data
(Cd). The second source of uncertainty,
often overlooked, is associated with modeling error or
mismodeling. This is represented by the covariance matrix on
the theory, CT. Among the different
sources of mismodeling, we focus here on the modeling error
associated with the mislocation of the centroid
position. Both Cd and CT
describe probability densities in the data space and it is
well known that it is in fact CD
= Cd + CT that should be
included into the error propagation process. In source
inversion problems, like in many other fields of geophysics,
the data covariance (CD) is often
considered as diagonal or even proportional to the identity
matrix. In the present work, we demonstrate the importance
of using a more realistic form for CD. If
we incorporate accurate covariance components during the
inversion process, it refines the posterior error estimates
but also improves the solution itself. We discuss these
issues using several synthetic tests and by applying the W
phase source inversion algorithm to several large
earthquakes such as the recent 2011 Tohoku-oki earthquake.
Z. Duputel, L. Rivera, H. Kanamori, G. Hayes, 2012. W phase fast source inversion for moderate to large earhquakes (1990 - 2010), Geophysical Journal International, v. 189, iss. 2, p. 1125-1147.
Rapid characterization of the earthquake source and of its
effects is a growing field of interest. Until recently, it
still took several hours to determine the first order
attributes of a great earthquake (e.g., Mw>=7.5), even in a
well instrumented region. The main limiting factors were
data saturation, the interference of different phases and
the time duration and spatial extent of the source
rupture. To accelerate centroid moment tensor (CMT)
determinations, we have developed a source inversion
algorithm based on modeling of the W phase, a very long
period phase (100s - 1000s) arriving at the same time as
the P wave. The purpose of this work is to finely tune and
validate the algorithm for large-to-moderate-sized
earthquakes using three components of W phase ground motion
at teleseismic distances. To that end, the point source
parameters of all Mw>=6.5 earthquakes that occurred between
1990 and 2010 (815 events) are determined using Federation
of Digital Seismograph Networks (FDSN), Global Seismographic
Network broadband stations (GSN_BROADBAND) and STS1 global
virtual networks of the Incorporated Research Institutions
for Seismology Data Management Center (IRIS DMC). For each
event, a preliminary magnitude obtained from W phase
amplitudes is used to estimate the initial moment rate
function (MRF) half duration and to define the corner
frequencies of the passband filter that will be applied to
the waveforms. Starting from these initial parameters, the
seismic moment tensor is calculated using a preliminary
location as a first approximation of the centroid. A full
CMT inversion is then conducted for centroid timing and
location determination. Comparisons with Harvard and Global
CMT solutions highlight the robustness of W phase CMT
solutions at teleseismic distances. The differences in Mw
rarely exceed 0.2 and the source mechanisms are very similar
to one another. Difficulties arise when a target earthquake
is shortly (e.g., within 10 hours) preceded by another large
earthquake, which disturbs the waveforms of the target
event. To deal with such difficult situations, we remove the
perturbation caused by earlier disturbing events by
subtracting the corresponding synthetics from the data. The
CMT parameters for the disturbed event can then be retrieved
using the residual seismograms. We also explore the
feasibility of obtaining source parameters of smaller
earthquakes in the range 6.0<=Mw<6.5. Results suggest
that the W phase inversion can be implemented reliably for
the majority of earthquakes of Mw=6 or larger.
S. Barde-Cabusson, A. Finizola, A. Peltier, M. Chaput, N. Taquet, S. Dumont, Z. Duputel, A. Guy A., L. Mathieu, S. Saumet, F. Sorbadère, M. Vieille, 2012. Structural control of collapse events inferred by self-potential mapping on the Piton de la Fournaise volcano (La Réunion Island). J. Volcanol. Geotherm. Res., v. 209-210, p. 9-18.
Field surveys were performed on the terminal cone of Piton
de la Fournaise in 2006 and 2008 to precisely map the self
potential (SP) signal and determine the zonation of the
hydrothermal activity both on the flanks of the cone and in
the summit area, including inside the Bory and Dolomieu
craters. SP maps inside the craters have been performed 8
months before the 5-7 April 2007 caldera collapse. Zonations
appear both at the scale of the cone and of the summit and
allow new interpretation of the electrical signal
distribution on the terminal cone of Piton de la
Fournaise. Superimposed to the SP maxima linked to the
rift-zones, several areas of SP maxima associated with
collapse structures have been detected: (1) in the summit
area, the Bory and Dolomieu craters show the strongest SP
values with amplitudes exceeding 2 V with respect to the
base of the cone, and with a sharp lateral variation to the
East, corresponding to the inner boundary of the Dolomieu
caldera, collapsed on 5-7 April 2007, and (2) in the paleo
pit craters surrounding the summit which show amplitudes
similar to the Dolomieu-Bory craters. The analysis of the
variations of the signal with time evidences a modification
of the fluid flow pattern with a higher associated SP
signature to the east in 2008. We interpret the
amplification of fluid flow to the east in 2008 as a
consequence of the eastward motion of the eastern flank of
the volcano during the April 2007 eruption. The acquisition
of SP data during two periods separated by the April 2007
eruption turns out to be a good opportunity to correlate the
SP signal to the Piton de la Fournaise structure and to its
evolution in term of hydrothermal and eruptive activity.
V. Tsai, G.P. Hayes, Z. Duputel, 2011. Constraints on the Long-Period Moment-Dip Tradeoff for the Tohoku Earthquake, Geophysical Research Letters, v. 38, L00G17.
Since the work of Kanamori and Given (1981), it has been recognized that shallow,
pure dip-slip earthquakes excite long-period surface waves such that it is difficult
to independently constrain the moment (M0) and the dip (δ) of the source mechanism,
with only the product M0 sin(2δ) being well constrained. Because of this, it is
often assumed that the primary discrepancies between the moments of shallow, thrust
earthquakes are due to this moment-dip tradeoff. In this work, we quantify how severe
this moment-dip tradeoff is depending on the depth of the earthquake, the station
distribution, the closeness of the mechanism to pure dip-slip, and the quality of the
data. We find that both long-period Rayleigh and Love wave modes have moment-dip
resolving power even for shallow events, especially when stations are close to certain
azimuths with respect to mechanism strike and when source depth is well determined.
We apply these results to USGS W-phase inversions of thei recent M9.0 Tohoku, Japan
earthquake and estimate the likely uncertainties in dip and moment associated with
the moment-dip tradeoff. After discussing some of the important sources of moment
and dip error, we suggest two methods for potentially improving this uncertainty.
Z. Duputel, L. Rivera, H. Kanamori, G. P. Hayes, B. Hirsorn, S. Weinstein, 2011. Real-time W phase inversion during the 2011 off the Pacific coast of Tohoku Earthquake, Earth Planets and Space, v. 63, no. 7, p. 535-539.
The real time W phase source inversion algorithm was independently
running at three organizations (USGS, PTWC and IPGS) at the time of
the 2011 Off the Pacific Coast of Tohoku earthquake. Valuable results
for tsunami warning purposes were obtained 20 min after the event
origin time. Within the next hour, as more data became available, the
W phase solutions improved, and converged to a common result (Mw ~ 9.0, dip ~ 14°).
A post-mortem W phase analysis using data selection based on pre-event
noise confirmed the Mw = 9.0 result and yielded a best double couple given
by (strike/dip/rake = 196°/12°/85°). We also ran the algorithm with
increasingly longer periods (T ~ 1500 sec) to test for the possibility of
additional slow slip. The seismic moment remained stable, confirming the
prior results
F. Massin, V. Ferrazzini, P. Bachèlery, A. Nercessian, Z. Duputel, T. Staudacher, 2011. Structures and evolution of the plumbing system of Piton de la Fournaise volcano inferred from clustering of 2007 eruptive cycle seismicity. Journal of Volcanology and Geothermal Research, v. 202, iss. 1-2, p. 96-106.
Analysis of seismic activity associated with the eruptions
of 2007, which led to the collapse of the Dolomieu crater on
April 5th, reveals the link between the seismicity and the
magma transfers at Piton de la Fournaise. Three eruptive
phases occurred on February 18th, March 30th and April 2nd,
2007, at the summit, 2000 m, and 600 m high on the
South-East flank respectively, illustrating the three types
of eruptions defined for the current Piton de la Fournaise
activity. We use cross-correlation of seismic waveforms and
clustering to improve the earthquake locations and determine
the best-constrained focal mechanisms (with an average of 78
P phase polarities). The pre-eruptive seismicity of the
February and March eruptions is composed of time extended
clusters that also preceded other distal eruptions from 2000
to 2007. Our analysis shows that the seismic swarm prior to
the February eruption initiated the intrusion that led to
the April 2007 eruption. The seismicity preceding the
Dolomieu crater collapse consists of numerous, but
time-limited, clusters and specific seismic activity that
accompanied the Dolomieu crater collapse. From April 1st to
April 5th, earthquakes with CLVD mechanisms combined with
normal faulting sources occurred between 0.8 and 0 km asl,
until the complete rupture of the shallow magma storage
roof. This collapse induced the propagation of a
de-pressurization front and triggered a migration of
seismicity from 0 to − 8 km along a very narrow path. Z. Duputel, M. Cara, L. Rivera, G. Herquel, 2010. Improving the analysis and inversion of multimode Rayleigh-wave dispersion by using group-delay time information observed on arrays of high-frequency sensors, Geophysics, v. 75, iss. 2, R13.
Near-surface shear-velocity structure can be inferred from
multimode dispersion data. Several methods have been
developed to isolate the different modes from seismic
signals observed on linear arrays of sensors. Most
techniques analyze the wavefield through a
frequency-wavenumber (f-k) transform, paying little
attention to group-delay-time information. Moreover,
classical analyses are generally restricted to
fundamental-mode dispersion, limiting the resolution power
at depth. We have overcome the limitations of classical f-k
analysis by using a wavefield representation in the
group-velocity/phase-velocity (U-c) domain. We have then set
up a nonlinear inversion procedure, easily tractable on a
common field computer, to constrain the 1D vertical profile
of shear velocities. Applications to synthetic data and to a
set of actual records show that U-c diagrams greatly help to
separate dispersion information between different modes,
even when they are not detectable on usual f-k
diagrams. Tests on synthetic and actual data confirm that
the inversion procedure quickly converges to the expected
model. A. Mordret, A.D. Jolly, Z. Duputel, N. Fournier, 2009. Monitoring of phreatic eruptions using Interferometry on Retrieved Cross-Correlation Function from Ambient Seismic Noise : Results from Mt Ruapehu, New Zealand, Journal of Volcanology and Geothermal Research, v. 191, iss. 1-2, p. 46-59.
Since the last major eruption in 1995-96 Mt. Ruapehu has
erupted twice, on 4 October 2006 and on 25 September
2007. These events occurred without any clear precursors and
were mostly phreatic explosions. The technique of
"Interferometry on Retrieved Cross-Correlation Function from
Ambient Seismic Noise" (IRCCASN) is used to monitor subtle
temporal changes of Mt. Ruapehu's elastic properties. The
computation of Cross-Correlation Functions of seismic noise
recorded at several stations around the volcano allowed us
to observe variations during the 2006 eruption period. The
comparison between a Reference Cross-Correlation Function
and a Current Cross-Correlation Function allows us to infer
relative seismic velocity variations. A 0.8% decrease of
relative seismic velocity in the edifce, starting two days
before the 2006 eruption, was observed. This drop is due to
a reversible and ephemeral effect, which can be attributed
to a pressurization of a magma pocket beneath the east flank
of Ruapehu due to new magma entering a small reservoir. This
pressure increase produced an infation of the east flank of
Ruapehu and opened fractures in this area leading to a
localised drop of seismic velocity. We conducted the same
analysis for the 2007 eruption but no signifcant seismic
velocity variation was observed. This difference is possibly
due to the varying time scales of pressurization for the two
events and also the IRCCASN time resolution. An analysis of
the seismicity before these two eruptions allows us to
propose a conceptual model which explains the velocity drop
for the 2006 eruption and the lack of velocity variations
for the 2007 eruption. Since there was no surface
deformation recorded by the GeoNet GPS network, we modelled
the maximum radius and pressure change for a simple Mogi
point source at 5 km depth that produced no deformation. We
infered the maximum fresh magma volume of ~0.0017 km3
entering the reservoir as a detectability threshold for GPS
ground deformation measurments. S. Barde Cabusson, G. Levieux, J.F. Lénat, A. Finizola, A. Revil, M. Chaput, S. Dumont, Z. Duputel, A. Guy, L. Mathieu, S. Saumet, F. Sorbadère, M. Vieille, 2009. Transient self-potential anomalies associated with recent lava flows at Piton de la Fournaise volcano (Réunion Island, Indian Ocean). Journal of Volcanology and Geothermal Research, v. 187, p. 164-173.
Self-potential signals are sensitive to various phenomena
including ground water flow (streaming potential), thermal
gradients (thermoelectric potential), and potentially rapid
fluid disruption associated with vaporization of water. We
describe transient self-potential anomalies observed over
recent (< 9 years) lava flows at Piton de la Fournaise
volcano (Reunion Island, Indian Ocean). Repeated
self-potential measurements are used to determine the decay
of the self-potential signals with time since the
emplacement of a set of lava flow. We performed a 9 km-long
self-potential profile in February 2004 in the Grand
Brulé area. This profile was repeated in July2003 and
August 2006. The second repetition of this profile crossed
eight lava flows emplaced between 1998 and 2005 during seven
eruptions of Piton de la Fournaise volcano. The
self-potential data show clear positive anomalies (up to 330
mV) and spatially correlated with the presence of recent
lava flows. The amplitude of the self-potential anomalies
decreases exponentially with the age of the lava flows with
a relaxation time of not, vert, similar 44 months. We
explain these anomalies by the shallow convection of
meteoric water and the associated streaming potential
distribution but we cannot exclude possible contributions
from the thermoelectric effect and the rapid fluid
disruption mechanism. This field case evidences for the
first time transient self-potential signals associated with
recent volcanic deposits. It can be also a shallow analogue
to understand the variation of self-potential signals in
active geothermal areas and transient self-potential signals
associated with dike intrusion at larger depths. The
empirical equation we proposed can also be used to diagnose
the cooling of recent lava flow on shield volcanoes. Z. Duputel, V. Ferrazzini, F. Brenguier, N. Shapiro, M. Campillo, A. Nercessian, 2009. Real time monitoring of relative velocity changes using ambient seismic noise at the Piton de la Fournaise volcano (La Réunion) from January 2006 to June 2007, Journal of Volcanology and Geothermal Research, v. 184, iss. 1-2, p. 164-173.
We present the results of a real time method based on
coda-wave interferometry from seismic noise
cross-correlation functions for relative seismic velocity
variations monitoring on a volcanic edifice. The ambient
seismic noise at the Piton de la Fournaise volcano on La
Réunion island is analyzed from January 2006 to June
2007. During this period, five eruptions occurred showing a
great diversity in eruption duration, intensity and eruptive
fissure location. Two different methods are used to compute
the velocity variations in order to compare their stability
in quasi real-time routine. We compare the obtained velocity
variations with the surface deformation observed by GPS and
extensometers networks. This allows us to identify and
quantify three major processes at the origin of seismic wave
velocity variations in the edifice. Firstly, the observed
pre-eruptive summit inflation is accompanied by a decrease
in seismic velocity. Secondly, the edifice deflation
following the opening of an eruptive fissure is
characterized by an increase of the velocity. Finally, the
summit caldera collapse generates a strong velocity
drop. Coda-wave interferometry from seismic noise
cross-correlation functions in quasi-real time may allow us
to forecast eruption and constrain the processes taking
place in the volcanic plumbing system. F. Brenguier, N.M. Shapiro, M.Campillo, V. Ferrazzini, Z. Duputel, O. Coutant, A. Nercessian, 2008. Towards forecasting volcanic eruptions using seismic noise. Nature Geoscience, v. 1, iss. 2, p. 126-130.
Volcanic eruptions are preceded by increased magma pressures, leading
to the inflation of volcanic edifices1. Ground deformation resulting
from volcano inflation can be revealed by various techniques such as
spaceborne radar interferometry2, or by strain- and
tiltmeters3. Monitoring this process in real time can provide us with
useful information to forecast volcanic eruptions. In some cases,
however, volcano inflation can be localized at depth with no
measurable effects at the surface, and despite considerable effort4, 5
monitoring changes in volcanic interiors has proven to be
difficult. Here we use the properties of ambient seismic noise
recorded over an 18-month interval to show that changes in the
interior of the Piton de la Fournaise volcano can be monitored
continuously by measuring very small relative seismic-velocity
perturbations, of the order of 0.05%. Decreases in seismic velocity a
few weeks before eruptions suggest pre-eruptive inflation of the
volcanic edifice, probably due to increased magma pressure. The
ability to record the inflation of volcanic edifices in this fashion
should improve our ability to forecast eruptions and their intensity
and potential environmental impact. |
Contact Address:
California Inst. of Tech. Telephone:
+1 626-395-3801 Email:
zacharie@gps.caltech.edu
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