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TABLE 1Magnitudes and Stress Calculations for the Three LargestCanterbury EventsDateMw(regional)Mw(teleseismic) Ms Meτ a(Mpa)9/03/2010 7.10 6.97 7.30 7.99 15.852/22/2011 6.30 6.12 6.30 6.75 4.106/13/2011 6.00 6.00 6.00 6.70 6.26Choy 1992; Choy and Boatwright 1995; Singh and Ordaz1994; Baltay et al. 2011). Many others have proposed a momentdependence on τ a (e.g., Abercrombie 1995; Kanamori et al.1993). τ a has important implications for earthquake hazardstudies, as increasing τ a leads to stronger ground motions. Usingteleseismically determined Es and Mo (George Choy, personalcommunication), we solve for τ a of the three largest Canterburyevents (Table 1). We assume constant bulk regional propertiesfor density and shear-wave velocity to determine rigidity. Thesequantities are informed by the region’s 3D shear-wave velocitymodel (Eberhart-Phillips et al. 2010). Values of rigiditymight be underestimated for the February and June events, asit is possible that these events occurred on surfaces that are cutby intrusions from the 6 Ma volcanic activity that resulted inthe volcanic edifice in the vicinity of the earthquakes. In thisscenario, the faults would be locally strengthened by the intrusionsand the energy release would be dominated by a subregionof the fault, greatly increasing the localized τ a .Due to the frequency dependent nature of both scatteringand attenuation, it is difficult to measure Es over a broadfrequency range (Ide et al. 2003). In this study, we utilize estimatesof Mo that are derived from teleseismic data to maintainconsistency with the teleseismically determined Es. Greaterattenuation of high-frequency energy also dictates that teleseismicestimates of Es are minimum values of actual radiatedenergy in highly attenuating regions. Ideally, energy andmoment estimates from regional data would be used to estimateτ a . However, such techniques require refined knowledgeof local attenuation structure and site responses that werepreviously only coarsely resolved. Ongoing studies are refiningthese properties (Kaiser et al. 2011) and should allow forregional estimates in the near future.We estimate the τ a of the September event to be the highestof the three earthquakes, at ~16 MPa. τ a of the February andJune events are ~5 and ~6 MPa respectively. Intraplate earthquakestypically have larger τ a than interplate events. This isalso true of the South Island, New Zealand, where recent largeevents along the Puysegur subduction zone (Fry et al. 2010)have τ a ~ 0.2MPa. We also compare the three Canterburyevents to data from four sequences of Hokkaido, Japan, eventscompiled by Baltay et al. 2011 (Figure 1) and the 2007 M 6.8event that occurred on the Hikurangi subduction zone, NorthIsland, New Zealand. The Iwate Miyagi event is a particularlyappropriate analogue as the character of the recorded near-fieldwaveforms is similar to that of the 22 February Canterburyevent (Fry et al. 2011, page 846 of this issue).The stresses calculated from the Canterbury events areremarkably high compared to global averages. The Canterburyearthquakes are also on average higher than τ a values obtainedfrom reported stress drops (most between 10 and 15 MPa) ofintraplate events in eastern North America (Atkinson andBoore 2006). For shallow subduction events, Choy et al. (2001)▲ ▲ Figure 1. Apparent stress plotted as a function of Mw for data from Japan (Baltay et al. 2011) and New Zealand. Ch 04 is the 2004Chuetsu earthquake and aftershock sequence; CO 07 is the 2007 Chuetsu-Oki earthquake and aftershock sequence; Kam is the repeatingearthquake sequence off-shore Kamaishi, Iwate; IM 08 is the 2008 Iwate-Miyagi earthquake and aftershock sequence; GB 07 isthe 2007 M 6.8 Gisborne, New Zealand, earthquake; Puy are recent earthquakes on the Puysegur subduction zone, South Island, NewZealand; Can are the largest events of the 2010–2011 Canterbury sequence.834 Seismological Research Letters Volume 82, Number 6 November/December 2011