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Backprojection of the high-frequency radiation released during the Pisagua (Chile) earthquake (01/04/2014, Mw 8.1) and the Iquique aftershock (03/04/2014, Mw 7.6)
No full textNorthern Chile has recently been struck by the Mw 8.1 Pisagua earthquake, which occurred
on 01/04/2014 and partially filled the Iquique seismic gap. The Pisagua earthquake has been
preceded by intense foreshock activity which started in July 2013 and culminated in a cluster
of events in March 2014. We have inferred the rupture dynamics of the mainshock and of its
largest (Iquique) aftershock (03/04/2014, Mw 7.6) by backprojecting the high-frequency
seismic radiation released during the events and recorded by 310 stations of USArray.
The time-evolution of the high-frequency (1-4 Hz) energy radiated during the
mainshock shows that the rupture lasted about 80 s, with most of the energy released
between 25 s and 50 s from the onset. The cumulative energy emitted during the whole
rupture process mainly originated downdip the epicenter just off the coast line, approximately
in the latitude range 19.5°-20°S. This region falls at the down-dip side of the co-seismic slip
area, similarly to the case of the Maule earthquake (South Chile, 27/02/2010, Mw 8.8) and of
other large earthquakes. Differently from the Maule case, most aftershocks not located in the
area of large seismic radiation. The time-evolution of the coherent seismic radiation displays
an initial low-energy phase (lasting about 20s) during which the source starts to migrate from
the nucleation point at the epicenter towards the south-east, activating deeper parts of the
subduction interface. After reaching points close to the coast line (after ~30s from the onset),
the source moves back towards the epicenter mainly activating in sequence two patches of
the interface located around 20°S,70.5°W (F1) and 19.7°S,70.2°W (F2), shortly followed by
the activation of points close to the area of the largest co-seismic slip (F3). In the last ~15s of
the fracturing (F4), the re-activation of the area releasing energy during F2 is observed. Thus,
despite the simple bullseye co-seismic slip pattern, the history of energy radiation is quite
scattered, suggesting peculiar and sharply site-dependent frictional properties along this
segment of the Chilean subduction interface.
The Mw7.6 aftershock displays a similar time-evolution of the radiating source, with an
initial low-energy stage, during which the rupture front migrates from the epicenter towards
deeper zones. Most energy is released about 25s from the onset, when the rupture front
reaches points around 20.5°S,70.0°W
Applicability and bias of VP/VS estimates by P and S differential arrival times of spatially clustered earthquakes - why the method of Lin and Shearer (2007) will yield biased results in nearly all realistic configurations
No full textEstimating small-scale VP/VS variations at depth can be a powerful tool to infer lithology and hydration of a rock, with possible implications for frictional behavior. In principle, from the differential arrival times of P and S phases from a set of spatially clustered earthquakes, an estimate of the local VP/VS can be extracted, because the VP/VS is the scaling factor between the P and S differential times for each pair of earthquakes. We critically review the technique proposed by Lin and Shearer (2007), in which the mean value over all stations is subtracted from the differential arrival times of each pair of events in order to make the method independent of a priori information on origin times. The demeaned differential P and S arrival times are plotted on a plane, and the VP/VS ratio is estimated by fitting the points on this plane.
We tested the method by both theoretical analysis and numerical simulations of P and S travel times in several velocity models. We found that the method returns exact values of VP/VS only in the case of a medium with homogeneous VP/VS , whereas, when a VP/VS gradient is present, the estimates are biased as an effect of systematic differences between P and S takeoff angles. We demonstrated that this bias arises from the demeaning of the arrival times over the stations. In layered models with VP/VS decreasing with depth, we found that VP/VS is overestimated or underestimated, respectively, for takeoff angles larger or smaller than 90°. In mosst realistic local earthquake monitoring settings, the take-off angles are not equally distributed but there will be a dominance of downward going rays, resulting in an overall bias. We calculated analytically the dependence of this bias on the takeoff angles. Additional simulations showed that the difference between the calculated and the expected VP/VS is reduced for simple horizontally layered velocity structures (<0.06), whereas it is 0.27 in a more realistic velocity model mimicking a subduction zone
Applicability and bias of the Vp/Vs estimates by differential arrival times of cluster of earthquakes
No full textEstimating small-scale V-P/V-S variations at depth can be a powerful tool to infer lithology and hydration of a rock, with possible implications for frictional behavior. In principle, from the differential arrival times of P and S phases from a set of spatially clustered earthquakes, an estimate of the local V-P/V-S can be extracted, because the V-P/V-S is the scaling factor between the P and S differential times for each pair of earthquakes. We critically review the technique proposed by Lin and Shearer (2007), in which the mean value over all stations is subtracted from the differential arrival times of each pair of events in order to make the method independent of a priori information on origin times. The demeaned differential P and S arrival times are plotted on a plane, and the V-P/V-S ratio is estimated by fitting the points on this plane.
We tested the method by both theoretical analysis and numerical simulations of P and S travel times in several velocity models. We found that the method returns exact values of V-P/V-S only in the case of a medium with homogeneous V-P/V-S, whereas, when a V-P/V-S gradient is present, the estimates are biased as an effect of systematic differences between P and S takeoff angles. We demonstrated that this bias arises from the demeaning of the arrival times over the stations. In layered models with V-P/V-S decreasing with depth, we found that V-P/V-S is overestimated or underestimated, respectively, for takeoff angles larger or smaller than 90 degrees. Moreover, we calculated analytically the dependence of this bias on the takeoff angles.
Our simulations also showed that the difference between the calculated and the expected V-P/V-S is reduced for simple horizontally layered velocity structures (< 0.06), whereas it is 0.27 in a more realistic velocity model mimicking a subduction zone
Inference of small-scale Vp/Vs ratio along the rupture area of the Tocopilla earthquake, Northern Chile (Mw 7.7, 14/11/2007)
No full textWe have inferred the Vp/Vs ratio along the segment of the Peru-Chile subduction margin corresponding to the rupture area of the Tocopilla earthquake (TE, Mw 7.7, 14/11/2007). This event nucleated in Northern Chile and broke the southern ~100 km of the ~500 km Northern Chile Southern Peru seismic gap, which had not seen an earthquake of this magnitude since the M~9 event of 1877. TE activated two main co-seismic slip patches: one around the epicenter and another north-east of the Mejillones Peninsula. We have applied the Lin and Shearer approach [1] to the aftershock sequence of TE. In this approach, the relative time shift between the S phases of a pair of nearby events at one station are plotted as function of the time shift between the P phases of the same pair. The process is repeated for a set of events. If the events are close enough to assume a uniform local Vp/Vs and the P-reciprocal wavefront can be approximated as planar, the points lay on a line, whose slope is an estimation of the local Vp/Vs. The technique is extended to a set of stations demeaning the time shifts from each pair of events. The time shifts are inferred maximizing the cross-correlation function between the event pairs. The technique has been applied to clusters of events sharing similar waveforms and spatially clustered hypocentres. We have adopted a robust linear L2 regression and have assigned a statistical error to the best fit. Most clusters are identified within a sub-vertical branch of the subduction interface hosting a major aftershock (Michilla earthquake, 16/12/2007, Ml 6.8) and its aftershocks. This branch falls inside the subducted Nazca Plate at depths of 40-50 km, north-east of the Mejillones Peninsula, and shows Vp/Vs mostly in the range 1.8-1.9. Clusters close to the Mejillones Peninsula and to the epicenter displays Vp/Vs around 1.7 and 1.8, respectively. References [1] - Lin, G., & Shearer, P. (2007). Estimating local Vp/Vs ratios within similar earthquake clusters. Bulletin of the Seismological Society of America, 97 (2), 379-388
Local Vp/Vs ratio in the vicinity of the Tocopilla (Chile) earthquake (Mw 7.7, 14/11/2007) inferred by differential P- and S- travel times
No full textWe have inferred the Vp/Vs ratio along the segment of the Peru-Chile subduction
margin corresponding to the rupture area of the Tocopilla earthquake (Mw 7.7, 14/11/2007).
This event nucleated in Northern Chile and broke the southern ~100 km of the ~500 km
Northern Chile Southern Peru seismic gap, which at the time had not seen an earthquake of
this magnitude since the M~9 event of 1877. Tocopilla event activated two main co-seismic
slip patches: one around the epicenter and another north-east of the Mejillones Peninsula.
We have applied the Lin and Shearer (2007) approach to the aftershock sequence of
the Tocopilla event. In this approach, the relative time shift between the S phases of a pair of
nearby events at one station are plotted as function of the time shifts between the P phases of
the same pair. The process is repeated for a cluster of events. If the events are close enough
to assume a locally uniform Vp/Vs ratio and the P-reciprocal wavefront can be approximated
as planar, the points lay on a line, whose slope is an estimation of the local Vp/Vs. The
technique is extended to a set of stations demeaning the time shifts from each pair of events.
The time shifts are inferred maximizing the cross-correlation function between the event pairs.
The technique has been applied to clusters of events sharing similar waveforms and
spatially clustered hypocentres. We have adopted a robust linear regression and have
assigned a statistical error to the best fit.
Clusters have been identified along the whole profile of the subducting slab, although
most clusters falls within a sub-vertical branch of the subduction interface hosting a major
aftershock (Michilla earthquake, 16/12/2007, Ml 6.8) and its aftershocks. This branch falls
inside the subducted Nazca Plate at depths of 40-50 km, north-east of the Mejillones
Peninsula, and shows Vp/Vs mostly in the range 1.7-1.8. Clusters of the plate interface
shallower than about 30 km show Vp/Vs around 1.9, while at intermediate depths (30-40 km)
Vp/Vs is ~1.8. We speculate on the existence of hydrated crust producing the highest Vp/Vs
(~1.9 or larger) observed at shallow depths
Repeating earthquakes on the Chile subduction zone following the Maule 2010 M 8.8 earthquake
No full textWe investigate repeating earthquakes (REs) on the Chile subduction zone, in the first 9 months following the Maule 2010 M 8.8 earthquake. Using the aftershock catalogue of approximately 30,000 events (Rietbrock et al., 2012) and the data from the International Maule Aftershock Dataset (IMAD), we identify 1550 clusters of small magnitude (Mw ~1.5-3) events showing similar waveforms (cross-correlation coefficients>0.9). Clusters are found from the surface to depths of ~60km, indicating the generation of RE on pre-existing crustal faults and slab interface. A particularly dense band of clustered seismicity runs NE-SW along the length of Chile at 37-47km depth on the slab/continent interface, apparently defining the limit of plate coupling (Rietbrock et al, 2012; Lange et al., 2012). Relocation of deep clusters, via the double difference method (hypoDD), reveal that they lie within a region of increased fluid content (interpreted from high Vp/Vs ratio (Hicks et al., 2012)), and define streaks of seismicity orientated down-dip. Moment tensor analysis of selected aftershocks shows that larger events M 4-5 are located at the interface or deeper in the slab (5-8km beneath the slab interface) and show thrust motion along the direction of the plate interface. REs, in contrast, show predominantly strike-slip motion and are located close to the interface. Temporal analysis also shows non-constant recurrence times of events within clusters, which we interpret as an indication that the seismicity in the deep clusters are driven by pulses of after-slip from the Maule 2010 event together with episodic fluid migration. We introduce a model of Mixed Mode Fault Slip (MMFS) to explain our observations, where aseismic sliding of trench sediments in the subduction channel build up stress on fragments of ocean crust, causing them to repeatedly fracture and generate repeating earthquakes
High-frequency seismic radiation from Maule earthquake (Mw 8.8, 27/02/2010) inferred from high-resolution backprojection analysis
No full textThe Maule earthquake (2010 February 27, Mw 8.8, Chile) broke the subduction megathrust along a previously locked segment. Based on an international aftershock deployment, catalogues of precisely located aftershocks have become available. Using 23 well-located aftershocks, we calibrate the classic teleseismic backprojection procedure to map the high-frequency seismic radiation emitted during the earthquake. The calibration corrects traveltimes in a standard earth model both with a static term specific to each station, and a ‘dynamic’ term specific to each combination of grid point and station. The second term has been interpolated over the whole slipping area by kriging, and is about an order of magnitude smaller than the static term. This procedure ensures that the teleseismic images of rupture development are properly located with respect to aftershocks recorded with local networks and does not depend on accurate hypocentre location of the main shock.
We track a bilateral rupture propagation lasting ∼160 s, with its dominant branch rupturing northeastwards at about 3 km s−1. The area of maximum energy emission is offset from the maximum coseismic slip but matches the zone where most plate interface aftershocks occur. Along dip, energy is preferentially released from two disconnected interface belts, and a distinct jump from the shallower belt to the deeper one is visible after about 20 s from the onset. However, both belts keep on being active until the end of the rupture. These belts approximately match the position of the interface aftershocks, which are split into two clusters of events at different depths, thus suggesting the existence of a repeated transition from stick-slip to creeping frictional regime
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
High-frequency seismic emission during Maule earthquake (Mw 8.8, 27/02/2010) inferred from high-resolution backprojection analysis of P waves
No full textSince its first application on Sumatra-Andaman earthquake, back-projection analysis has been widely exploited
to infer the time-evolution of the rupture fronts of mega-earthquakes. In this technique, selected seismic phases
recorded at teleseismic distances by a network of sensors are shifted according to a possible source position and
a velocity model, and a multichannel version of the cross-correlation function is estimated. In this way, the timedependent
map of the seismic energy emission in the source area can be inferred.
We have back-projected the mainshock of Maule earthquake (Mw 8.8), which nucleated on 27/02/2010 in central
Chile and is one of the largest earthquakes recorded in modern times. We have analyzed P phases filtered in the
frequency range (0.4-3) Hz recorded by three seismic arrays located in US, Africa and Antarctica. Relative time
shifts between sensors (inferred by maximizing the cross-correlation function) have been estimated with respect
to a 1D global velocity model (ak135) and have been refined introducing two corrections, a static correction and
a dynamic correction. The former is the time shift induced by local effects in the sensor area, whereas the latter
is the correction associated with the source-sensor path and is mostly affected by medium properties in the source
area. We have inferred these two corrections by analyzing the waveforms of 23 aftershocks and foreshocks with
high magnitude (>5.3). In detail, static correction was chosen as the mean time shift averaged over all the events
recorded by one station, while dynamic correction was the remaining part of the travel time after removing the 1D
model travel time and the static correction. Moreover, dynamic corrections (and hence the complete travel times)
have been interpolated over all the source area by Kriging, a spatial interpolation method.
Results show that high-frequency seismic energy emission mostly occurs along the coastline with a general northward
migration during the event. Specifically, in the first minute of the rupture process, the energy emission occurs
southerly from or close to the epicenter. Afterwards, seismic emission moves northwards, with a gap with respect
to the first emission zone, and a further northward migration occurs till the end of emission. Both the spatial gap of
seismic emission and the northward migration are in line with the results of other studies in the same area, whereas
we find a shallower emission area and different emission features in the zone close to the epicenter. Results for
different frequency bands and the analysis of secondary maxima of energy emission are being investigated. In
particular, we are shifting towards higher frequencies looking at the frequency bands (1-4) Hz and (2-8) Hz. The
former band displays an emission pattern similar to that of (0.4-3) Hz, but with a sharper gap of about 50 Km; the
latter band shows coherent arrivals only during the first 80 s, with a clear energy emission south of the epicenter at
the onset of the event and preserving the northward migration afterwards
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