Observation of Core Phase ScS from the Mw 9.0 Tohoku-Oki Earthquake with High-Rate GPS

Abstract

High-rate Global Positioning Systems (GPS) observations can record ground motions from moderate to strong earthquakes at distances of a few kilometers up to thousands of kilometers (Larson et al., 2003; Bock et al., 2004; Ohta et al., 2006;Wang et al., 2007; Davis and Smalley, 2009; Delouis et al., 2010; Shi et al., 2010; Yue and Lay, 2011). These observations detect strong signals such as S wave and surface waves (Kouba, 2003; Larson et al., 2003; Irwan et al., 2004; Ohta et al., 2006;Wang et al., 2007; Davis and Smalley, 2009; Shi et al., 2010), as well as relatively weaker seismic phases such as P waves (Avallone et al., 2011). Seismic phases arriving later than surface waves have been rarely reported. Although it has been confirmed that arbitrarily large seismic signals are recorded with high-rate GPS (Bock et al., 2004), we assess here the lower limit of signals from earthquakes that can be recovered from high-rate GPS records. Benefits of exploiting high-rate GPS in earthquake source studies include the ability to detect arbitrarily large dynamic ground motions that do not clip in the presence of strong ground motion and accurate recording of ground displacements without needing to doubly integrate acceleration records of strong-motion seismometers (Graizer, 2010). As a result, high-rate GPS records are now routinely used in studies of earthquake source parameters and rupture process independently of, or jointly with, seismological observations (Miyazaki et al., 2003; Ji et al., 2004; Bilich et al., 2008; Yokota et al., 2009; Avallone et al., 2011; Yue and Lay, 2011; Bock et al., 2011; Crowell et al., 2012). Though whole waveforms of high-rate GPS records have been employed in modeling earthquake source processes, it is not certain whether the wiggles following surface waves are true ground-motion signals or contamination from GPS signal propagation effects such as multipath or ionospheric distortion. Accordingly, very few phases following the arrival time of surface waves have been confirmed on high-rate GPS records, primarily because of the inherently lower signal-to-noise ratio (SNR). Identification and retrieval of the weaker secondary arrivals from high-rate GPS records would further confirm capability of GPS in recording ground motions, and thus lead to improved broadband seismograms via collated GPS and seismometers (Bock et al., 2011). Among the later arrivals, the seismic phase ScS is expected to be strong due to the complete reflection at the core–mantle boundary between the solid mantle and the liquid outer core where shear waves disappear. Moreover, ScS is far behind surface waves at close distances (less than 20°) where contamination of the surface-wave coda is weak. Here, we present a record section of seismograms retrieved from high-rate GPS from the 2011 Tohoku earthquake wherein we observe a weak but coherent seismic phase that we associate with ScS. This inference is supported by comparison of the high-rate GPS records with seismic records and synthetic seismograms.