Imaging the ruptures of the 2009 Samoan and Sumatran earthquakes using broadband network back-projections: Results and limitations

Applications of teleseismic P-wave back-projection to image gross characteristics of large earthquake finite-source ruptures have been enabled by ready availability of large digital data sets. Imaging with short-period data from dense arrays or broadband data from global networks can place constraints on rupture attributes that otherwise have to be treated parametrically in conventional modeling and inversion procedures. Back-projection imaging may constrain choice of fault plane and rupture direction, velocity, duration and length for large (M>~8.0) earthquakes, and can robustly locate early aftershocks embedded in mainshock surface waves. Back-projection methods seek locations of coherent energy release from the source region, ideally associated with down-going P wave energy. For shallow events, depth phase arrivals can produce artifacts in back-projection images that appear as secondary or even prominent features with incorrect apparent source locations and times, and such effects need to be recognized. We apply broadband P-wave back-projection imaging to the 29 September 2009 Samoa (Mw8.2) and 30 September 2009 Sumatra (Mw7.6) earthquakes using data from globally distributed broadband stations and compare results to back-projections of synthetic seismograms from finite-source models for these events to evaluate the artifacts from depth phases. Back-projection images for the great normal-faulting Samoa event feature two prominent bright spots, which could be interpreted to correspond to two distinct slip patches, one near the epicenter in the outer trench slope and the other approximately 80 km to the west near the plate boundary megathrust where many aftershocks occurred. This interpretation is at odds with finite-fault modeling results, which indicate a predominantly bilateral rupture in the NW-SE direction on a steeply dipping trench slope fault, with rupture extending about 60 km in each direction. Back-projections of data and synthetic seismograms from the finite-fault modeling with common source-receiver geometries are nearly identical, with both having the two prominent bright regions as well as many secondary features. The prominent feature to the west is an artifact resulting from constructive interference of azimuthally varying depth phases; this coincidentally projects in the same region as many aftershocks. Similar analysis for the Sumatra event yields back-projections of data and synthetics for a finite-fault model that closely match and indicate a very compact source with virtually all of the coherent radiation located within 20km of the epicenter. Weak features in the back-projections with arrival times consistent with pP and sP are visible offset from the epicenter. Back-projection methods can be useful for constraining aspects of large earthquake rupture processes, particularly if the variation of waveforms across the imaging network is small, but it is always important to assess what features of the back-projections are artifacts from the path geometry or depth phases to avoid misinterpreting the images, particularly when using globally distributed stations rather than large-aperture arrays.