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Three sub-events composing the 2011 off the Pacific coast of Tohoku Earthquake (Mw 9.0) inferred from rupture imaging by back-projecting teleseismic P waves
Abstract
High-quality vertical component seismograms of teleseismic P waves
recorded at 151 stations of the European seismic network have been used
to image the rupture process of the 2011 Tohoku Earthquake rapidly by a
two-step back-projection method. The spatio-temporal distribution of
rupture fronts suggests that the earthquake ruptured northeastwards and
southwestwards over a total length of more than 340 km during at least
143 s. The fact that three fault segments ruptured at regions having
different lateral heterogeneities implies that the earthquake comprised
three sub-events. The first sub-event ruptured northeastwards during the
first 25 s, and then turned to the northwest direction. The second
sub-event ruptured at a relatively high speed of 2.78˜4.70 km/s.
The third sub-event ruptured with a direction variation from southwest
to southeast near the latitude of 37°. In addition, considering that
the first front of the second sub-event appeared at 74.6 s and was about
28 km away from the epicenter, we propose that the second sub-event
might have been triggered by the localized increase in tectonic stress
in the vicinity of the hypocenter that resulted from the rupture of the
first sub-event.
... Then the radiation begins to propagate along the strike direction and stop around 39 • in the north and continue to propagate to the south. These results are generally consistent with results from other BP methods (Ishii 2011;Koper et al. 2011a;Meng et al. 2011;Zhang et al. 2011). The radiation sources from the time domain BP in Wang & Mori (2011) are further to the trench compared to others. ...
... Comparison of the seismic radiation imaging from different back projection methods at 1 Hz. (a) Wavelet-based CS (this study), (b) conventional CS, (c) MUSIC from EU array, (d) MUSIC from US array (Meng et al. 2011), (e) time domain BP ) and (f) time domain BP (Zhang et al. 2011). The colour of each dot in (a)-(d) shows the source time. ...
We propose a new wavelet-based compressive sensing method to image coseismic radiation sources with teleseismic array data. Compared to the sliding-window-based technique, our method avoids choosing the window length by taking advantages of the adaptive resolution feature of the wavelet transform. Thus artefacts caused by inappropriate window length and waveformtruncation can be removed. Aswaveforms usually become less aligned at late arrivals due to the increasing distance of actual radiation sources with respect to the hypocentre, we introduce a time-correction scheme to realign waveforms with the reference point obtained from beamforming at every time step. Synthetic tests indicate that the new method can better resolve the seismic radiation sources with more reliable locations and source times. To demonstrate its feasibility, we apply the method to the Mw 9.0 Tohoku earthquake. We find similar spatial rupture and seismic radiation patterns, such as repeating rupture in the region close to the hypocentre and along-dip frequency-dependent radiation, as indicated by previous models from finite-fault inversion and back projection. In addition, our results show structure-controlled rupture characteristics, with the seismic radiation in the first 80 s mainly confined in the off-Miyagi high-wave-speed region. The rupture speed shows clear along-dip variations, with updip rupture slower than that of downdip, which corresponds well with the depth-varying characteristics along the megathrust.
... Obtaining high-quality seismic data with real-time accessibility has enabled vigorous progress in array data processing technologies. For example, by simply shifting and stacking the seismic waveforms observed at seismic arrays to conduct backprojection, the source extent and duration of an earthquake can be determined in great detail (Ishii et al., 2005;Krüger and Ohrnberger, 2005), and the frequency dependence and other characteristics of the earthquake's sources can be revealed with unprecedented accuracy (Vallée et al., 2008;Koper et al., 2011;Meng et al., 2011;Yao et al., 2011;Zhang et al., 2011;Roten et al., 2012;Satriano et al., 2012;Fan et al., 2019). The backprojection algorithm is a simple and robust approach for processing such data; it has been implemented by several institutions (such as Incorporated Research Institutions for Seismology) and is run in an automated fashion using dense regional or global seismic data. ...
Rapid seismic intensity maps for damaging earthquakes enable the swift implementation of earthquake disaster mitigation action, issuance of accurate tsunami warnings, and prevention of associated secondary disasters. However, many countries lack dense local seismic observation networks, making it infeasible to obtain accurate seismic intensity maps of earthquakes within a few hours, particularly for earthquakes that have considerable source extents. In this study, we developed a new algorithm for rapidly obtaining seismic intensity maps of damaging earthquakes. With our model, source energy radiation is acquired using backprojection, and then the locations and relative amplitudes of the fault geometry and subevents are determined. Peak ground accelerations and peak ground velocities (PGVs) are subsequently calculated based on ground-motion prediction equations and the distribution of the estimated subevents. PGVs are then further site-corrected using the VS30 database (Wald and Allen, 2007; Heath et al., 2020). The algorithm was applied to the 2008 Mw 7.9 Wenchuan and 2010 Mw 6.9 Yushu earthquakes, and the resulting seismic intensity maps were highly similar to those generated by field surveys. The algorithm is simple and straightforward to use, and local real-time instrument observations are not required. Calculations can be performed automatically, and reliable seismic intensity maps can be issued within 30 min following damaging earthquakes. The model’s application may assist greatly with rescue and recovery efforts, and enable tsunami hazards to be evaluated immediately following earthquakes, particularly in regions lacking dense observation networks.
... To infer a comprehensive model for the rupture of the 2013 deep Okhotsk earthquake, we combined two different ruptureassessment methods: one is the multi-array BP method (Kiser and Ishii, 2012;Zhang et al., 2016), modified from the BP method (Ishii et al., 2005;Xu et al., 2009;Zhang and Ge, 2010;Zhang et al., 2011), and the other one is the multi-subevent inversion (Kikuchi and Kanamori, 1991). The back-projection analyses were carried out in two different frequency bands (Koper et al., 2011;Wang and Mori, 2011;Lay et al., 2012;Yao et al., 2013;Fan and Shearer, 2015) using data from USArray (TA) and European seismic network (EU) (Figure 1), while the multi-subevent inversion was performed using the data from the Global Seismic network (GSN) ( Figure 2). ...
The rupture mechanisms of deep-focus (>300 km) earthquakes in subducting slabs of oceanic lithosphere are not well understood and different from brittle failure associated with shallow (<70 km) earthquakes. Here, we argue that dehydration embrittlement, often invoked as a mechanism for intermediate-depth earthquakes, is a plausible alternative model for this deep earthquake. Our argument is based upon the orientation and size of the plane that ruptured during the deep, 2013 Mw 8.3 Sea of Okhotsk earthquake, its rupture velocity and radiation efficiency, as well as diverse evidence of water subducting as deep as the transition zone and below. The rupture process of this earthquake has been inferred from back-projecting dual-band seismograms recorded at hundreds of seismic stations in North America and Europe, as well as by fitting P-wave trains recorded at dozens of globally distributed stations. If our inferences are correct, the entirety of the subducting Pacific lithosphere cannot be completely dry at deep, transition-zone depths, and other deep-focus earthquakes may also be associated with deep dehydration reactions.
... In this method, the array signal processing technology is used to analyze the seismic waves recorded at the seismic network, so as to image the position of the source rupture front and obtain the spatial-temporal distribution image of the source rupture process. The back-projection method has been successfully applied to several major global earthquakes Kiser and Ishii, 2011;Meng et al., 2011;Wang and Mori, 2011;Zhang et al., 2011;Koper et al., 2012;Yao et al., 2012;Fan and Shearer, 2015;Meng et al., 2016;Wang and Mori, 2016;Liu et al., 2017). At present, it has become a widely used method for source process imaging. ...
A ruptured front obtained from high-frequency energy radiation is the key to understand the complex source. It is commonly observed that rupture fronts derived from different arrays often show some variations due to the obvious difference of the positioning accuracy of the far-field array between the azimuth and the epicentral distance. We developed a new multi-array back-projection method based on the classical back-projection method and applied the method to the 2015 M W7.8 Nepal earthquake. The back azimuth information with small error is separated from the classical back-projection results, and the azimuth intersection of multiple arrays is used to obtain more accurate spatial and temporal distribution information of the source rupture fronts.
... In this study, we estimated the source duration by performing backprojection in a large regional array (Wang et al., 2017). By stacking a large number of waveforms, backprojection method can overcome potentially strong site effects, cancel out coda waves, and consider the directivity effects in waveforms, thereby, obtaining source duration within a high resolution Krüger and Ohrnberger, 2005;Honda et al., 2011;Meng et al., 2011;Zhang et al., 2011;Koper et al., 2012;Satriano et al., 2014;Yagi and Okuwaki, 2015). The origin times, locations, depths, and moment magnitudes are from the U.S. Geological Survey (USGS). ...
The rapid and reliable estimation of the magnitudes of large earthquakes is critical for determining the potential shaking damage and tsunami hazards. The primary challenge to rapidly and accurately estimating the magnitude of large earthquakes is the need to wait for the full waveform in order to calculate the source parameters. We used data of the M≥7.0 shallow earthquakes in Japan from 2008 to 2016, recorded at 10°–60° (regional to teleseismic distances), to establish an operational method to quickly determine their magnitudes. Our results suggest that earthquake magnitudes can be estimated accurately 6–12 min after their origin times. The only time‐limiting factor on our method is the epicentral distances to the seismic stations. For the case of the 2011 great Tohoku earthquake, the magnitude was estimated as M 8.9–9.1 at 6–12 min after the origin time. Resolutions of the results were further investigated by bootstrap and jackknife tests and subarray analysis. Therefore, we propose building a system for determining the magnitude of large earthquakes in and around Japan using real‐time seismic data in China and worldwide. This will assist in disaster mitigation immediately after a damaging earthquake, especially for the purpose of tsunami evacuation and emergency rescue.
... We use the software TauP (Crotwell et al., 1999) and the velocity structure IASPEI 1991 (Kennett and Engdahl, 1991) to calculate the travel time between grid points and the seismic stations. In order to overcome the structure heterogeneities beneath the Alaska stations that are not taken into account in the IASPEI 1991 velocity model, we apply a station correction procedure (Wang et al., 2016;Fan and Shearer, 2015;Yao et al., 2013;Satriano et al., 2012;Zhang et al., 2011;Ishii et al., 2005) to further improve the travel time calculations. We align the first 60 and 10 s after P arrivals of the waveforms to the epicenter with filters 0.05 to 0.5, and 0.1 to 0.5 Hz, respectively, to constrain the initial source location to the epicenter determined by the USGS. ...
We apply a novel method to estimate the magnitude of the 23 January 2018 M7.9 Alaska earth-quake using seismic stations recorded at local to regional distances in Alaska, US. We determine the source duration from back-projection results derived from the Alaska stations in a relatively compact azimuth range. Then we calculate the maximum P-wave displacements recorded on a wide azimuth range at distances of 8 to 15 degrees. Combining the source duration and the maximum P-wave displacements, we obtain magnitudes of 7.86–8.03 for the 23 January 2018 earthquake in 3–5 min, very close to the Mw 7.9 determined by the USGS and GCMT. This example validates the new approach for determining magnitude of large earthquakes using local to regional stations, and its time efficiency that magnitudes of large earthquakes can be accurately estimated within in 3–5 min after origin time. Therefore, further application of this new method would help accurate estimation of size of earthquakes that occur off shore and might cause tsunami hazards.
... Meng et al. (2011) and Simons et al. (2011) applied the Multiple Signal Classification (MUSIC) algorithm to 0.5-1 Hz data from USArray and Europe, finding a similar two-stage evolution of the radiation (Fig. 4a), with initial slow expansion at 1 km/ s and then faster expansion to the north and southwest after 35 s. Other early back-projections are generally similar (e.g., Ishii, 2011;Wang and Mori, 2011a;Zhang et al., 2011;Kiser and Ishii, 2012;Satriano et al., 2014). In an extension of short-period back-projection methods, Yao et al. (2012) apply an iterative method with subevent signal stripping and correlation alignment. ...
The 2011 March 11 Tohoku-oki great (Mw 9.1) earthquake ruptured the plate boundary megathrust fault offshore of northern Honshu with estimates of shallow slip of 50 m and more near the trench. Non-uniform slip extended ~ 220 km across the width and ~ 400 km along strike of the subduction zone. Extensive data provided by regional networks of seismic and geodetic stations in Japan and global networks of broadband seismic stations, regional and global ocean bottom pressure sensors and sea level measurement stations, seafloor GPS/Acoustic displacement sites, repeated multi-channel reflection images, extensive coastal runup and inundation observations, and in situ sampling of the shallow fault zone materials and temperature perturbation, make the event the best-recorded and most extensively studied great earthquake to date. An effort is made here to identify the more robust attributes of the rupture as well as less well constrained, but likely features. Other issues involve the degree to which the rupture corresponded to geodetically-defined preceding slip-deficit regions, the influence of re-rupture of slip regions for large events in the past few centuries, and relationships of coseismic slip to precursory slow slip, foreshocks, aftershocks, afterslip, and relocking of the megathrust. Frictional properties associated with the slip heterogeneity and in situ measurements of frictional heating of the shallow fault zone support low stress during shallow sliding and near-total shear stress drop of ~ 10–30 MPa in large-slip regions in the shallow megathrust. The roles of fault morphology, sediments, fluids, and dynamical processes in the rupture behavior continue to be examined; consensus has not yet been achieved. The possibility of secondary sources of tsunami excitation such as inelastic deformation of the sedimentary wedge or submarine slumping remains undemonstrated; dislocation models in an elastic continuum appear to sufficiently account for most mainshock observations, although afterslip and viscoelastic processes remain contested.
This study highlights the 2D rupture propagation of 2005 M7.6 Kashmir earthquake. Beamforming and multi signals classification (MUSIC) back projection techniques are applied on the recorded seismic waves of teleseismic seismometers to model the rupture propagation of 2005 M7.6 Kashmir earthquake. These techniques are robust enable to model propagated rupture extents and rupture phases as the event data is available with the teleseismic seismometers. The imaging of the 2005 M7.6 Kashmir earthquake source propagation with beamforming technique shows three phases of rupture propagation of 10, 10, and 25 s with the velocity of 0, 2.2, and 1.9 km/s respectively. While resolving the rupture propagation with MUSIC, it is observed that rupture propagated 26 and 14 s covering distance of 50 and 31 km respectively. Both techniques confirm the bidirectional rupture propagation with various velocities and time. Both the rupture speed and duration vary due to both applied technique resolutions. Our findings are quite consistent with the published slip models of the Kashmir earthquake. The modeling of 2005 Kashmir earthquake propagation provides key information about the mechanics of Bagh-Balakot Thrust, western Himalaya side.
This chapter introduces the principle of the back-projection method in the frame of the seismic wave propagation. This provides the solid basis for the technique. To further validate the method, synthesized waveforms are generated to apply the back-projection the procedure. The resultant imaging supports the argument that the relative back-projection method can mitigate the “swimming” artifacts in the traditional back-projection imaging results.
On February 27, 2017, at 06:34:00 (UTC) an Mw 8.8 megathrust earthquake struck nearby Concepcion, South Chile, where the Nazca plate is subducting beneath the overriding South American plate. The earthquake was located at (35.905°S, 72.733°W) at a focal depth of 35 km (provided by USGS).
Finite-source rupture models for the great 11 March 2011 off the Pacific coast of Tohoku (M
w 9.0) Earthquake obtained by inversions of seismic waves and geodetic observations are used to reconstruct deep-water tsunami recordings from DART buoys near Japan. One model is from least-squares inversion of teleseismic P waves, and another from iterative least-squares search-based joint inversion of teleseismic P waves, short-arc Rayleigh wave relative source time functions, and high-rate GPS observations from northern Honshu. These rupture model inversions impose similar kinematic constraints on the rupture growth, and both have concentrations of slip of up to 42 m up-dip from the hypocenter, with substantial slip extending to the trench. Tsunami surface elevations were computed using the model NEOWAVE, which includes a vertical momentum equation and a non-hydrostatic pressure term in the nonlinear shallow-water equations to account for the time-history of seafloor deformation and propagation of weakly dispersive tsunami waves. Kinematic seafloor deformations were computed using the Okada solutions for the rupture models. Good matches to the tsunami arrival times and waveforms are achieved for the DART recordings for models with slip extending all the way to the trench, whereas shifting fault slip toward the coast degrades the predictions.
The frequency-dependent rupture process of the 11 March 2011
Mw 9.0 off the Pacific coast of Tohoku Earthquake is examined
using backprojection (BP) imaging with teleseismic short-period
(˜1 s) P waves, and finite faulting models (FFMs) of the seismic
moment and slip distributions inverted from broadband (>3 s)
teleseismic P waves, Rayleigh waves and regional continuous GPS ground
motions. Robust features of the BPs are initial down-dip propagation of
the short-period energy source with a slow rupture speed (˜1
km/s), followed by faster (2-3 km/s) rupture that progresses
southwestward beneath the Honshu coastline. The FFMs indicate initial
slow down-dip expansion of the rupture followed by concentrated
long-period radiation up-dip of the hypocenter, then southwestward
expansion of the rupture. We explore whether these differences
correspond to real variations in energy release over the fault plane or
represent uncertainties in the respective approaches. Tests of the BP
results involve (1) comparisons with backprojection of synthetic P waves
generated for the FFMs, and (2) comparisons of backprojection locations
for aftershocks with corresponding NEIC and JMA locations. The data
indicate that the down-dip environment radiates higher relative levels
of short-period radiation than the up-dip regime for this great
earthquake, consistent with large-scale segmentation of the frictional
properties of the megathrust.
The conventional local tomography can determine the 3D seismic velocity structure right beneath a seismic network, but it cannot determine the 3D structure outside a seismic network. In this study we show that such a limitation of tomography can be overcome if many earthquakes occur outside a seismic network. In the northeast Japan arc earthquakes occur actively from the Japan Trench to the Pacific coast. We detected and used sP-depth phase to relocate the suboceanic earthquakes accurately, and then used P- and S-wave arrival times from the relocated suboceanic events and the earthquakes under the northeast Japan land area to determine the 3D P- and S-wave velocity and Poisson's ratio structures of the entire northeast Japan arc from the Japan Trench to the Japan Sea coast. Our results exhibit strong lateral heterogeneities under the forearc region. The mainshock hypocenters of large interplate earthquakes (M 7.0-8.2) mainly cluster near the boundaries of high and low velocity and Poisson's ratio above the subducting Pacific slab. A few large earthquakes are located in areas with high velocity and low Poisson's ratio. These results suggest that lateral heterogeneities on the slab boundary can affect the rupture nucleation of large thrust earthquakes and the degree of interplate seismic coupling.
The seismic body wave tomography method has been improved and extended to adapt to a general velocity structure with a number of complexity shaped seismic velocity discontinuities and with three-dimensional variations in the velocities in the modeling space. This method is applied to 18,679 arrival times from 470 shallow and intermediate-depth earthquakes in order to study P and S wave tomographic images beneath northeastern Japan. In addition to first P and S wave arrivals, clear later arrivals of SP waves converted at the Moho and PS and SP waves converted at the upper boundary of the subducted Pacific plate are also used in the inversion. High-resolution P and S wave tomographic images down to a depth of 200 km have been determined. Large velocity variations amounting to 6 percent for P wave and 10 percent for S wave are revealed in the crust and upper mantle. In the crust, low-velocity (low-V) zones exist beneath active volcanoes. In the upper mantle, the low-V zones dip towards the west from the volcanic front. A high-velocity (high-V) zone corresponding to the subducted Pacific plate is clearly delineated. Most earthquakes in the lower plane of the double-planed deep seismic zone are found to occur in relatively high-V areas.
1] We image the rupture of the 28 March 2005 Sumatra Mw 8.6 earthquake by back-projecting teleseismic P waves recorded by the Global Seismic Network and the Japanese Hi-net to their source. The back-projected energy suggests that the rupture started slowly, had a total duration of about 120 s, and propagated at 2.9 to 3.3 km/s from the hypocenter in two different directions: first toward the north for $100 km and then, after a $40 s delay, toward the southeast for $200 km. Our images are consistent with a rupture area of $40,000 km 2 , the locations of the first day of aftershocks, and the Harvard CMT Mw of 8.6, which implies an average slip of $6 m. The earthquake is similar in its location, size, and geometry to a Mw $8.5 event in 1861. Our estimated average slip is consistent with a partially coupled subduction interface, GPS forearc velocities, and the $59 mm/yr convergence rate if the 2005 earthquake released elastic strain that accumulated over many hundreds of years rather than just since the last 1861 event.
The 11 March 2011 off the Pacific coast of Tohoku (M
w 9.0) Earthquake ruptured a 200 km wide megathrust fault, with average displacements of ~15–20 m. Early estimates of the co-seismic slip distribution using seismic, geodetic and tsunami observations vary significantly in the placement of slip, particularly in the vicinity of the trench. All methods have difficulty resolving the up-dip extent of rupture; onshore geodetic inversions have limited sensitivity to slip far offshore, seismic inversions have instabilities in seismic moment estimation as subfault segments get very shallow, and tsunami inversions average over the total region of ocean bottom uplift. Seismic wave estimates depend strongly on the velocity structure used in the model, which affects both seismic moment estimation and inferred mapping to slip. We explore these ideas using a least-squares inversion of teleseismic P-waves that yields surprisingly large fault displacements (up to ~60 m) at shallow depth under a protrusion of the upper plate into the trench. This model provides good prediction of GPS static displacements on Honshu. We emphasize the importance of poorly-constrained rigidity variations with depth for estimating fault displacement near the trench. The possibility of large slip at very shallow depth holds implications for up-dip strain accumulation and tsunamigenic earthquake potential of megathrusts elsewhere.
New empirical traveltime curves for the major seismic phases have been derived from the catalogues of the International Seismological Centre by relocating events by using P readings, depth phases and the iasp91 traveltimes, and then re-associating phase picks. A smoothed set of traveltime tables is extracted by a robust procedure which gives estimates of the variance of the traveltimes for each phase branch. This set of smoothed empirical times is then used to construct a range of radial velocity profiles, which are assessed against a number of different measures of the level of fit between the empirical times and the predictions of the models. These measures are constructed from weighted sums of L2 misfits for individual phases. The weights are chosen to provide a measure of the probable reliability of the picks for the different phases.
A preferred model, ak135, is proposed which gives a significantly better fit to a broad range of phases than is provided by the iasp91 and sp6 models. The differences in velocity between ak135 and these models are generally quite small except at the boundary of the inner core, where reduced velocity gradients are needed to achieve satisfactory performance for PKP differential time data.
The potential resolution of velocity structure has been assessed with the aid of a non-linear search procedure in which 5000 models have been generated in bounds about ak135. Msfit calculations are performed for each of the phases in the empirical traveltime sets, and the models are then sorted using different overall measures of misfit. The best 100 models for each criterion are displayed in a model density plot which indicates the consistency of the different models. The interaction of information from different phases can be analysed by comparing the different misfit measures. Structure in the mantle is well resolved except at the base, and ak135 provides a good representation of core velocities.
On 12 May 2008, the Mw 7.9 Wenchuan earthquake occurred along the Longmen Shan fault in the northwestern Sichuan province of China, resulting in huge damage as well as loss of human lives. Rapid estimation of the rupture process is essential for emergency response. In this paper, the relative back-projection method based on nonplane-wave array technology was applied to reconstruct the rupture process of this earthquake. First, the principle of the relative back-projection method was briefly introduced. Then the method was used to obtain the rupture process using the broadband data of 16 stations from the Australian National Seismic Network from the Incorporated Research Institutions for Seismology (IRIS) Data Management Center. The results showed that this earthquake ruptured mostly unilaterally along the Longmen Shan fault. Its rupture time was about 98 s, rupture length was about 290 km, and mean rupture velocity was about 3.0 km/s. Starting from the initial rupture point, epicenter (31.00 deg N, 103
Since their development in the 1960s, seismic arrays have given a new impulse to seismology. Recordings from many uniform seismometers in a well-defined, closely spaced configuration produce high-quality and homogeneous data sets, which can be used to study the Earth's structure in great detail. Apart from an improvement of the signal-to-noise ratio due to the simple summation of the individual array recordings, seismological arrays can be used in many different ways to study the fine-scale structure of the Earth's interior. They have helped to study such different structures as the interior of volcanos, continental crust and lithosphere, global variations of seismic velocities in the mantle, the core-mantle boundary and the structure of the inner core. For this purpose many different, specialized array techniques have been developed and applied to an increasing number of high-quality array data sets. Most array methods use the ability of seismic arrays to measure the vector velocity of an incident wav
The Mw 7.9 Wenchuan earthquake of 12 May 2008 was the most destructive Chinese earthquake since the 1976 Tangshan event. Tens of thousands of people were killed, hundreds of thousands were injured, and millions were left homeless. Here we infer the detailed rupture process of the Wenchuan earthquake by back-projecting teleseismic P energy from several arrays of seismometers. This technique has only recently become feasible and is potentially faster than traditional finite-fault inversion of teleseismic body waves; therefore, it may reduce the notification time to emergency response agencies. Using the IRIS DMC, we collected 255 vertical component broadband P waves at 30-95 deg from the epicenter. We found that at periods of 5 s and greater, nearly all of these P waves were coherent enough to be used in a global array. We applied a simple down-sampling heuristic to define a global subarray of 70 stations that reduced the asymmetry and sidelobes of the array response function (ARF). We also considered th