Magnitude Limits of Subduction Zone Earthquakes

Abstract

Maximum earthquake magnitude (m x ) is a critical parameter in seismic hazard and risk analysis. However, some recent large earthquakes have shown that most of the existing methods for estimating m x are inadequate. Moreover, m x itself is ill-defined because its meaning largely depends on the context, and it usually cannot be inferred using existing data without associating it with a time interval. In this study, we use probable maximum earthquake magnitude within a time period of interest, m p T, to replace m x . The term m p T contains not only the information of magnitude limit but also the occurrence rate of the extreme events. We estimate m p T for circumPacific subduction zones using tapered Gutenberg-Richter (TGR) distributions. The estimation of the two TGR parameters, β-value and corner magnitude (m c ), is performed using the maximum-likelihood method with the constraint from tectonic moment rate. To populate the TGR, the rates of smaller earthquakes are needed. We apply the Whole Earth model, a high-resolution global estimate of the rate of m ≥ 5 earthquakes, to estimate these rates. The uncertainties of m p T are calculated using Monte-Carlo simulation. Our results show that most of the circum-Pacific subduction zones can generate m ≥ 8:5 earthquakes over a 250-year interval, m ≥ 8:8 earthquakes over a 500-year interval, and m ≥ 9:0 earthquakes over a 10,000-year interval. For the Cascadia subduction zone, we include the 10,000-year paleoseismic record based on turbidite studies to supplement the limited instrumental earthquake data. Our results show that over a 500-year period, m ≥ 8:8 earthquakes are expected in this zone; over a 1000-year period, m ≥ 9:0 earthquakes are expected; and over a 10,000-year period, m ≥ 9:3 earthquakes are expected.

Determination of Mmax from Background Seismicity and Moment Conservation

“…In global studies, Bird and Kagan (2004) found the m c of continental strike-slip faults to be 8:01 0:47 −0:21 and for subduction zones 9:58 0:48 −0:46 . Other estimates for subduction zones include m c 9 − 9:7 (Kagan and Jackson, 2013) and m p T ≥ 9, in which T 10;000 yrs (Rong et al, 2014).…”

“…The largest recorded earthquake in instrumental records is the 1992 M w 7.1 Cape Mendocino event (Oppenheimer et al, 1993); there are very few other earthquakes here on record that were even medium-sized. In our analysis, we use a convergence rate of 40 5 mm=yr (Wilson, 1993;Wech and Creager, 2011) and a dip of 10° (Savage et al, 1991;Rong et al, 2014). The vertical extent of the seismogenic zone has been debated, from 15 km (Dragert et al, 1994) to 20-30 km (Williams et al, 2011;Hyndman, 2013;Rong et al, 2014); here we take a value of 25 10 km, giving an average seismogenic width of 125-150 km.…”

“…In our analysis, we use a convergence rate of 40 5 mm=yr (Wilson, 1993;Wech and Creager, 2011) and a dip of 10° (Savage et al, 1991;Rong et al, 2014). The vertical extent of the seismogenic zone has been debated, from 15 km (Dragert et al, 1994) to 20-30 km (Williams et al, 2011;Hyndman, 2013;Rong et al, 2014); here we take a value of 25 10 km, giving an average seismogenic width of 125-150 km. We select instrumental earthquakes on the down-dip side, and as with other subduction zones, we take the coupled fraction to be 45%.…”

“…There is, however, a physical limit, due to the fact that strain due to seismicity must match the strain resulting from geological motion across the fault on average over geological time (Brune, 1968). If the fault-slip rate is known, additional constraints on the seismicity model can thus be derived based on the seismic moment budget (e.g., Molnar, 1979;Ader et al, 2012;Kagan and Jackson, 2013;Rong et al, 2014;Avouac, 2015;Bird et al, 2015;Stevens and Avouac, 2016). There is now abundant evidence that seismic slip occurs in locations where faults were locked previously; this information can be gathered from geodetic observations (e.g., Avouac, 2015).…”

Probabilistic Seismic and Tsunami Hazard Analysis Conditioned on a Megathrust Rupture of the Cascadia Subduction Zone

“…Several models have been proposed to address the shortcomings of the GR recurrence law (Kagan, 1997(Kagan, , 2002aBird and Kagan, 2004). For SZs, such as the CSZ used as an example in this paper, a Tapered Gutenberg-Richter (TGR) distribution has been shown to be a robust model (Rong et al, 2014). The TGR is expressed as a function of the seismic moment M 0 , instead of magnitude m and an exponential taper is applied to the number of events with very large seismic moment.…”