Here - Stuff

Large Apparent Stresses from the CanterburyEarthquakes of 2010 and 2011B. Fry and M. C. GerstenbergerB. Fry and M. C. GerstenbergerGNS ScienceINTRODUCTIONAn earthquake of Mw 6.1–6.3 1 (Beavan et al. 2011, page 789of this issue) that struck Christchurch, New Zealand, on 22February (21 February, UTC) produced recorded groundmotion acceleration over 2 g. The event caused widespread damagewith dense recordings of non-linear site behavior. Globally,dense near-field recordings of shallow intraplate earthquakesare rare. It is possible that extreme ground motions are commonwith this type of earthquake and that their rarity is merelya function of inadequate seismic sampling in the near field ofsuch low-probability, high-potency events. To better define thenature of these events, we calculate apparent stress (τ a ) of thethree largest earthquakes in the Canterbury sequence and comparethem to global and regional data. We then place recordedPGA and spectral accelerations into the context of regionaland global ground motion prediction equations and discussthe implications of high-stress events for future seismic hazardestimates for the region. For the February event, we also brieflyexplore the implications of directivity on measured groundmotions in central Christchurch.The earthquakes that occurred in the Canterbury regionof the South Island, New Zealand, from September 2010 tothe present have disproportionately large energy magnitudes(Me) to their moment magnitudes (Mw). They have producedthe largest ground motions ever measured in New Zealand.The sequence began with the Mw 7.1 earthquake that occurredabout 40 km west of the city of Christchurch on 4 September2010. The maximum recorded ground acceleration recordedduring the event was over 1.25 g, which was experienced nearthe intersection of the triggering thrust on which the rupturebegan and the strike-slip Greendale fault that carried mostof the moment in the earthquake (Gledhill et al. 2010). Peakground accelerations (PGA) in the central business districtof Christchurch averaged between about 0.2 and 0.3 g. Thesemotions were sufficient to generate liquefaction in areas of thecity. The highest recorded acceleration in the greater metropolitanarea was 0.61 g in a suburb on the southern edge of the citythat has since proved to be prone to strong site amplification.On 22 February 2011, an Mw 6.3 thrust earthquake occurred1. Mw estimates for this earthquake have ranged from 6.1 (USGS) to6.2 (Beavan et al. 2011, page 789 of this issue). To be conservative inour comparison to observed ground motions, we have used Mw 6.3 inall calculations.on a structure below the southern suburbs of the city at about7 km epicentral distance from the center of Christchurch. Thisearthquake produced extreme motions in Christchurch (Fryet al. 2011, page 846 of this issue). Maximum PGA, consideringboth horizontal and vertical components, was over 2.2g with two other recordings in the city greater than 1 g andaverage PGA in the central business district between about0.6 and 0.8 g. This intense shaking damaged many buildingsin the central business district of the city (~5–8 km epicentraldistance) and triggered widespread liquefaction (Kaiser et al.2011). On 13 June 2011, the city was again subject to intenseshaking from a nearby, shallow Mw 6.0 earthquake (Beavan etal. 2011, page 789 of this issue). Measured accelerations fromthat event were also extreme, with measured PGA over 2 g in asoutheastern suburb of the city. Taken together, this sequencehas produced widespread destruction and more than 180 fatalitiesin Christchurch.HIGH APPARENT STRESS (τ a )The faults that failed in the September 2010 Mw 7.1, theFebruary 2011 Mw 6.3, and the June 2011 Mw 6.0 earthquakeswere likely very strong, with high amounts of friction.Typically, faults in slowly deforming areas with long earthquakerecurrence intervals exhibit this attribute, as increasingdeformation typically decreases fault strength by reducingheterogeneities on the fault surface (e.g., Ben-Zion andSammis 2003). Subsequently, the radiated energies (Es) forthe three events were high for their given moments. Radiatedenergy can be determined from high-frequency velocity records(Boatwright and Choy 1986) and can be used to directly calculateMe (Choy and Boatwright 1995). Compared to theseismic moment, which is derived from displacement records,energy magnitudes are more indicative of the shaking potentialof an earthquake. Es estimates from analysis of broadband Pwaves provide Me of Me = 7.99, Me = 6.75, and Me = 6.7 forthe three events (George Choy, personal communication).Apparent stress is defined as the product of rigidity (μ) and Esper unit moment (τ a = (μ × Es)/Mo) (Wyss and Brune 1968),or the amount of stress per unit moment. There is considerabledebate regarding the scaling of τ a with earthquake moment.Aki (1957) asserts that earthquakes are self-similar, implyingthat τ a is not dependent on seismic moment. This assertionis supported by numerous other studies (e.g., Boatwright anddoi: 10.1785/gssrl.82.6.833Seismological Research Letters Volume 82, Number 6 November/December 2011 833