This paper was presented at the Intraslab Earthquakes in the Cascadia Subduction System: Science and Hazards workshop in Victoria, British Columbia, September 2000. It will be released in the spring of 2001 (along with the other papers from the conference) as a joint USGS/GSC Open File
Knowledge of in-slab
to improve seismic hazard estimates for
southwestern British Columbia
John Adams and Stephen Halchuk
Geological Survey of Canada, 7 Observatory Cres.,
Ottawa, Canada, K1A 0Y3
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In-slab earthquakes (earthquakes within the subducting Juan de Fuca
plate) make the major contribution to seismic hazard for the Strait of
Georgia region of British Columbia. These earthquakes dominate the
hazard, despite their depth, because they have a higher rate and cause
stronger shaking than the crustal earthquakes. Key knowledge of
in-slab earthquakes needed to improve seismic hazard estimates for
southwestern British Columbia includes the constraints on the spatial
distribution, rate and maximum size of the earthquakes, the ground
motions to be expected, the nature of the earthquake sources, and the
structure and properties of the lithosphere through which the waves
Seismic hazard for the Strait of Georgia region of British Columbia
(including Vancouver, Victoria, and a substantial fraction of B.C.'s
population) comes from three sources: crustal seismicity in the North
American plate, great earthquakes of the Cascadia subduction zone on
the interface between the North American and subducting Juan de Fuca
plate, and deep earthquakes within the subducting slab ("in-slab"
earthquakes). It is, however, dominated by the contribution from
in-slab earthquakes. In Canada's 4th Generation seismic
hazard model (see Adams et al., 1999a, 1999b, 2000) these earthquakes
dominate the hazard despite their greater depth, firstly because they
occur at a rate up to 5-fold higher per unit area than the shallower
crustal earthquakes, and secondly because their predicted shaking is
stronger than crustal events of the same size (see below). Thus when
attempts are made to improve the estimation of seismic hazard for
southwestern B.C. a great deal can be gained by better understanding
We raise the following series of questions to highlight the
knowledge of in-slab earthquakes we believe is needed to improve
seismic hazard estimates for southwestern British Columbia. Some of
the differences that result from the current level of uncertainty are
demonstrated on Fig. 1 , a
comparison of the GSC and USGS deaggregated hazard for Bellingham,
Washington. Clear differences are seen in the fraction of the hazard
coming from in-slab versus crustal earthquakes, and in the contribution
from earthquakes larger than magnitude 7.
What is the spatial distribution likely for
future earthquakes within the slab?
The GSC's 4th Generation hazard maps use three source
zones to model deep earthquake distribution, GEO and PUG for one
probabilistic model and GSP for the other, reflecting uncertainty in
the future location of damaging deep earthquakes (Fig. 2 ). What is needed are
geological or geophysical reasons to constrain the up-dip, down-dip,
northern and southern extent of the deep seismicity. Although the two
probabilistic models attempt to model the range of possible
distributions, the level of hazard is strongly controlled by the active
PUG zone. This is especially important for Vancouver, as the northern
boundary of PUG lies under the city and generates a steep gradient in
hazard across the city (Fig. 3 ). Fairly large changes in hazard for
communities in this gradient zone could result from slight adjustments
to the source zone boundary, perhaps as the result of new significant
earthquake activity outside the currently-defined PUG zone, or a
recognition that certain regions within the boundary are (and will
continue to be) aseismic. Improved geological/geophysical constraints
might identify these regions and so refine the hazard estimates.
What is the rate of activity?
The rate of large earthquakes is a function of the rate of activity
for small earthquakes (a-value or alpha for the magnitude-recurrence
curve) and the slope of the magnitude-recurrence curve (b-value or
beta). Alpha is quite well determined in aggregate, but it is unknown
whether it truly varies in space (as it appears to during the
historical record), and if so, why it should vary. The GSC uses a
source zone approach which assumes uniform rates within each source
zone (which may not be valid); the USGS uses spatially smoothing of
past activity, which assumes that the locations of future large
earthquakes will precisely mimic the smoothed distribution of the small
earthquakes (which may not be valid either).
The slope of the curve (beta) represents the relationship between
the number of small and big earthquakes. For PUG it is distinctively
flatter than for most crustal source zones such as the crustal
earthquake zone, CASR, which overlies it (Fig. 4 ; two curves are shown
for the crustal earthquakes (CASR), one representing the mathematical
fit to the observed rates and the other - dashed -accommodating the
observed higher rate of M>6.5 earthquakes). The value used in the
Canadian hazard model is much lower (beta=1.01, b=0.44) than that used
by the USGS (=1.5, b=0.65) for its deep earthquakes. No sound
explanations exist for the different empirical values of beta, though a
study of world-wide in-slab earthquakes might confirm the
reasonableness of the value chosen, and provide insight into the
reasons for such a low value.
Together the magnitude recurrence parameters explain some of the
difference in hazard. Figure 4 shows the activity rates of PUG and the
overlying CASR crustal earthquake source. At magnitude 6, the
predicted rate of in-slab earthquakes is 3 to 10 times the rate of
crustal earthquakes, thus accounting for the larger hazard contribution
from the former. On Figure 4 the curve representing the USGS slope is
drawn through the magnitude 4 point on our PUG magnitude recurrence
curve. As to be expected, the steeper USGS slope predicts a rate of
M>6 earthquakes only one-third the GSC rate, and thus explains some
of the hazard difference.
How large can the in-slab earthquakes get?
The largest historical in-slab event occurred in 1949, of moment
magnitude about 6.9. Compared with recent earthquakes, almost nothing
is known about the rupture parameters of this earthquake, such its
depth extent, fault length or stress drop. Some geophysical
constraints such as temperature in the slab are believed to limit the
thickness of brittle rock thus restricting fault width; larger
earthquakes therefore require greater fault lengths or greater slip (or
both). The GSC model currently allows an upper bound magnitude of 7.3
for PUG (with an uncertainty range of 7.1 - 7.6) as shown on Fig. 5 ,
presuming that a future large earthquake could extend deeper into the
slab, or have larger displacement, or rupture a longer fault (perhaps
through cascading rupture segments as demonstrated during the Landers
earthquake). The USGS in 1997 adopted an upper bound magnitude of
7.0. Because of the high rate for these large earthquakes (due to the
small b value), their contribution to the total seismic hazard is not
trivial (for the GSC's results about 14-24%, dependent on model, of the
seismic hazard comes from earthquakes larger than the 1949 one). More
work in understanding the 1949 and 1965 earthquakes together with the
geological/geophysical conditions might allow tighter constraints on
the largest possible earthquakes.
How reliable are the current strong ground
Both the GSC and USGS use the Youngs et al. 1997 relations to
compute seismic hazard from the in-slab earthquakes. These relations
concluded that in-slab earthquakes produce ground motions 40% larger
than ground motions from adjacent subduction interface earthquakes
(Fig. 6 ), but this is not completely accepted. On the one hand the
Youngs et al. relations have been criticised as being based on rather
sketchy data and upon no long period data at all (Atkinson and Boore,
this volume), on the other the qualitative differences in damage
between interface and in-slab earthquakes (e.g. Kirby, this volume)
argue that there is almost certainly a quantitative difference in
excitation, perhaps even larger than 40%. Considerable work is needed
to determine if the 40% "premium" for in-slab earthquakes is realistic,
implausible, or too small, and whether the premium applies to all
periods or just to the shorter ones. The comparison of the in-slab and
crustal (using the Boore et al., 1997 relations) earthquake motions
(Fig. 6 ) indicates that at essentially all the distances significantly
contributing to the hazard the ground motions from a 50-km-deep in-slab
earthquake are expected to exceed those from a similar-sized 10-km-deep
What are the typical seismic sources we have
to contend with?
Our knowledge of the seismic source can affect our decision on which
strong ground motion relations to use. Most earthquakes will probably
have normal faulting mechanisms, but undetermined is the degree to
which rupture directivity effects are important, particularly if
ruptures tend to rupture upwards from their nucleation point (Fig. 7 ).
If as a first approximation the in-slab earthquakes are described as
Brune sources, what are their stress drops? If as a refinement they
are described as realistic, elasto-dynamic sources, what are the key
parameters (e.g. rupture velocity, source elongation, complete or
fractional stress drop, source complexity/episodic rupture, fault
roughness, etc) that affect the spectral shapes of the source as
radiated towards the overlying urban areas? Do in-slab source
acceleration spectra have intermediate (omega-1) slopes, and
if so, over what frequency band? Haddon (1996) showed that typical Mw
= 6 eastern earthquake sources have omega-1 slopes for about
1 decade of frequency above a lower corner, and that the high frequency
(f>1 Hz) levels exceed those associated with a Brune model for a
Mw=6, 100 bar stress drop event by a factor of three, and approach
those for a Brune model source a full magnitude larger (see the
velocity spectra on Fig. 8 ). The intermediate slopes are consequent on
high rupture velocities, rupture directivity effects involving
asymmetrical ruptures, episodic ruptures, and partial stress drop
events. Therefore, given records of small earthquakes, source scaling
parameters correctly incorporating these factors are needed to
synthesize the ground motions for potentially damaging earthquakes.
What are the crustal/mantle properties (e.g.
Q, velocity layering, dipping layers) that affect the radiated energy
between the source and the site where hazard is needed?
A reliable interpretation of crustal and mantle properties is
needed to assess and adjust the strong ground motion relations and to
perform forward modelling to determine the consequences of scenario
earthquakes. For example, if crustal conditions differ significantly
from Mexico, a source of much in-slab earthquake data, how do we adjust
strong ground motion parameters derived from a worldwide dataset?
What scenario earthquakes should be adopted
for Vancouver and Victoria? How can the use of empirical Green's
functions improve hazard estimates?
Deaggregations like Figure 1 indicate the magnitude and distance of
the earthquakes contributing to the seismic hazard and are the starting
point for design earthquake scenarios. Use of empirical Green's
functions can improve hazard estimates (by effectively accounting for
all path complexity), but still require much knowledge about the
seismic source so that the source scaling can be done appropriately.
Hence future improvements will depend critically on our ability to
understand what will happen during the larger earthquakes, and our best
insight to that will come from analysis of the past large Puget Sound
Different assumptions were adopted by the USGS in 1997 and the GSC
in 1994-1999 and resulted in different estimates of seismic hazard for
the US and Canadian territory overlying these in-slab earthquakes.
Reconciling these estimates and refining them towards the true hazard
will involve better answers to the questions raised above.
Adams, J., Weichert, D.H., and Halchuk, S. Trial seismic hazard maps
of Canada -1999: 2%/50 year values for selected Canadian cities.
Geological Survey of Canada Open File 3724, 107 pp., 1999a. Available
Adams, J., Weichert, D.H., and Halchuk, S. Lowering the
probability level - Fourth generation seismic hazard results for Canada
at the 2% in 50 year probability level. Proceedings 8th Canadian
Conference on Earthquake Engineering, Vancouver 13-16th June 1999, p.
Adams, J., Halchuk, S., and Weichert, D.H. Lower probability
hazard, better performance? Understanding the shape of the hazard
curves from Canada's Fourth Generation seismic hazard results. Paper
1555, 12th World Conference on Earthquake Engineering, Auckland, 30th
January - 4th February 2000, 8 pp., 2000.
Boore,D.M., Joyner,W.B., and Fumal,T.E. 1997. Equations for
estimating horizontal response spectra and peak acceleration from
western North America earthquakes: a summary of recent work. Seism.
Res. Lett., v. 68, p. 128-153
Haddon, R.A.W Earthquake source spectra for Eastern North America.
Bulletin of the Seismological Society of America, v.86, p 1300-1313,
Youngs, R.R., Chiou, S.-J., Silva, W.J., and Humphrey, J.R. Strong
ground motion relationships for subduction zone earthquakes.
Seismological Research Letters, v. 68, p. 58-73, 1997.
Fig. 1 . Seismic hazard
deaggregations of 0.2 s spectral acceleration values at 2%/50 years for
Bellingham show the GSC results are dominated by the contribution from
in-slab earthquakes, unlike the 1997 USGS results.
Fig. 2 . Selected in-slab
earthquakes (>35 km) in the Puget Lowlands / southwestern B.C., and
the alternative source zones used to model them for the GSC's
4th Generation seismic hazard maps.
Fig. 3 . Hazard map from the
GSC model ('H') using the PUG source. Contours, for
0.2 s spectral acceleration and 2%/50 year probability, are in %g.
Note how the steep gradient near Vancouver is dependant on the position
of the PUG boundary.
Fig. 4 . Magnitude-recurrence curves and observed activity rates
(dots with error bars) for in-slab (red) and crustal (black)
earthquakes for the Puget Sound. Both CASR curves have been reduced by
a factor of 6.2 to account for the larger area of CASR relative to the
PUG (see inset). The scaled USGS relation (blue) for deep earthquakes
is also shown.
Fig. 5 . Magnitude-recurrence curve for PUG (like Fig. 4), showing
the upper and lower uncertainty bounds and range of upper bound
magnitudes. For comparison, the curve used for the USGS calculations
is shown in blue.
Fig. 6 . A comparison of expected ground motions from adjacent
interface and in-slab earthquakes compared to those from similar-sized
crustal earthquakes (black).
Fig. 7 . Ground motions above a typical in-slab normal faulting
earthquake may depend critically on the rupture plane location, rupture
plane dip direction, and the asymmetry of the rupture relative to the
hypocentre, all contributing to directivity effects.
Fig. 8 . According to Haddon the velocity spectra of typical eastern
earthquakes sources are flat for about 1 decade of frequency above
their lower corner, and their high frequency (f>1 Hz) levels exceed
those associated with the corresponding Brune model event by a factor
of three, approaching the shaking of a Brune event one magnitude larger
(Redrawn from Haddon 1996).