ForumReply
doi:10.1130/ G36217Y.1 Lidar reveals uniform Alpine fault offsets and bimodal plate boundary rupture
behavior, New Zealand
Gregory P. De Pascale1,2, Mark C. Quigley2, and
2
Tim R. Davies
1
Fugro Geotechnical (NZ), Middleton, Christchurch, 8024, New Zealand
Department of Geological Sciences, University of Canterbury,
Christchurch 8140, New Zealand
2
In our recent Geology article (De Pascale et al., 2014) we use “bimodal rupture behavior” as a possible explanation for significant variations in
the rupture length (Lrup) and moment magnitude (Mw) during paleoearthquakes along New Zealand’s plate boundary Alpine fault (AF). We
compile paleoseismic data (De Pascale et al., 2014, our figure 1) to
highlight large variations in rupture extents and associated Mw in
successive AF ruptures: ca. A.D. 1717 (Lrup ≥ 380 km; Mw 7.9 ± 0.3),
A.D. 1600 ± 60 (Lrup 250 ± 50 km; Mw 7.6 ± 0.3), and A.D. 1440 ± 50
(Lrup 475 ± 125 km; Mw 7.9 ± 0.4) (Lrup and Mw estimates from Sutherland et al., 2007; De Pascale and Langridge, 2012). We point out that the
timing, extent, and Mw of AF paleo-earthquakes are commonly derived
from a mixture of on-fault indicators of surface faulting (e.g., dated fault
displacements in trenches) and off-fault independent proxies for strong
ground motions (i.e., shaking records; e.g., rockfalls, dunes, landslides,
growth disruptions in trees, and lacustrine turbidites). Comparison of
these records shows apparent large spatial and temporal variations in the
timing and recurrence intervals of earthquakes attributed to the AF (e.g.,
329 ± 68 yr [Berryman et al., 2012]; <90 to 110 yr based on dunes [Wells
and Goff, 2007]; 260 ± 70 yr based on lacustrine turbidites [Howarth et
al., 2012]). We argue that models with only successive “full” AF ruptures
(e.g., characteristic Mw earthquakes) do not satisfactorily explain these
paleoseismic records unless some of the earthquakes responsible for
shaking records originated from slip along other faults; expected
coseismic slip along the central AF for successive “full” ruptures at ≤110
yr intervals (i.e., the three most recent events) would greatly exceed
geologic and geodetically determined AF slip rates. We suggest an
alternative possibility: “bimodal behavior” where the AF exhibits both
partial (Lrup <300 km; Mw 6.5–7.8) and full (Lrup ≥ 300 km; Mw ≥ 7.9)
ruptures. Because the ca. 1600 event appears to have all the characteristics of a partial rupture (i.e., Lrup from 200 to 300 km, Mw ~7.6; Sutherland et al., 2007; Stirling et al., 2012; and Langridge et al.’s [2014]
Comment), and the 1717 event was a full rupture, it appears that the AF
may exhibit “bimodal” behavior (De Pascale et al., 2014).
In their first criticism of our paper, Langridge et al. state that we did
not identify a bimodal displacement pattern along the AF. In our study,
we measured offsets in the field and on lidar-derived topography for the
first time along the AF and suggested these were most easily explained as
7.1 ± 2.1 m slip increments along the central AF. We explicitly stated
that “resolution limited detectability… only offsets >2 m could be
detected in the topographic data; however…, smaller offsets (e.g., <2 m
to 6.5 m) should be visible on the ground, although none were found
during fieldwork. Based on the uncertainties in our data and challenging
field conditions, offsets smaller than 6.5 m could exist (and thus we may
be missing events).” However, our new AF offset compilation does show
coseismic slip increments to the north of our sites, ranging from <3 m
to >7.5 m (De Pascale et al., 2014, our figure 2) that imply slip and Lrup
variability (Sutherland et al., 2007) during successive ruptures—i.e.,
bimodal behavior.
We do not provide geological age control for any measured displacements in our study (Langridge et al.’s point 2) because the focus of
this study was to document and interpret dextral displacements. Instead,
we followed the established approach summarized by Grant Ludwig
(2013), who states “where dates of paleoearthquakes are not available,
recurrence time (Tr) may be estimated for a fault using the relationship
Tr = D/V where V is the slip rate and D (slip) is average displacement.”
The Tr we derive from this method and our displacements match
independently derived Tr from other methods only if slip at our study
sites during the A.D. 1600 event was either minimal or did not occur.
Ongoing studies will test and refine the timing of central AF displacements as proposed in De Pascale et al. (2014).
Langridge et al. state that we “do not accommodate the lack of
moderate to large earthquakes in the historical record.” Our suggestion
of bimodal behavior does not require that partial ruptures must occur at
intervals between every full AF rupture, and the short historical record
in New Zealand (since ca. 1840) leaves a >120 yr time gap between the
most recent major AF event (i.e., 1717) and the onset of historical
earthquake observations. Langridge et al. attribute all off-fault shaking
records in the region to AF events, despite the presence of many other
nearby active seismic sources (e.g., Cox et al., 2012) that are capable of
generating strong ground motions and resultant geomorphic responses
near the AF (a non-isolated fault). Note that the 1826 event (De Pascale
et al., 2014, our figure 1; Wells and Goff, 2007), which was likely a
Puysegur subduction event near (but not along) the AF, generated offfault shaking records that extend to the central AF and, perhaps more
importantly, generated a tsunami along the west coast of the South
Island (including up the Whataroa River toward our offset sites) which
was extremely unlikely to be generated during a dominantly strike-slip
AF event. Near any plate boundary, all seismic sources must be
considered when attempting to interpret the behavior of any one fault
from off-fault records alone.
REFERENCES CITED
Berryman, K.R., Cochran, U.A., Clark, K.J., Biasi, G.P., Langridge, R.M., and
Villamor, P., 2012, Major earthquakes occur regularly on an isolated plate
boundary fault: Science, v. 336, p. 1690–1693, doi:10.1126/science
.1218959.
Cox, S.C., Stirling, M.W., Herman, F., Gerstenberger, M., and Ristau, J., 2012,
Potentially active faults in the rapidly eroding landscape adjacent to the
Alpine Fault, central Southern Alps, New Zealand: Tectonics, v. 31,
doi:10.1029/2011TC003038.
De Pascale, G.P., and Langridge, R.M., 2012, New on-fault evidence for a great
earthquake in A.D. 1717, central Alpine fault, New Zealand: Geology, v. 40,
p. 791–794, doi:10.1130/G33363.1.
De Pascale, G.P., Quigley, M.C., and Davies, T.R.H., 2014, Lidar reveals
uniform Alpine fault offsets and bimodal plate boundary rupture behavior,
New Zealand: Geology, v. 42, p. 411–414, doi:10.1130/G35100.1.
Grant Ludwig, L., 2013, Historical Seismicity—Paleoseismology, in Schubert,
G., ed., Treatise on Geophysics, Volume 4, Earthquake Seismology: Elsevier, p. 567–589, doi:10.1016/B978-044452748-6.00080-8.
Howarth, J.D., Fitzsimons, S.J., Norris, R.J., and Jacobsen, G.E., 2012, Lake
sediments record cycles of sediment flux driven by large earthquakes on the
Alpine fault, New Zealand: Geology, v. 40, p. 1091–1094,
doi:10.1130/G33486.1.
Langridge, R.M., Howarth, J., Cochran, U., Stirling, M., Villamor, P.,
Sutherland, R., Berryman, K., Townend, J., and Norris, R., 2014, Lidar
reveals uniform Alpine fault offsets and bimodal plate boundary rupture
behavior, New Zealand: Comment: Geology, v.42, p. e351, doi:10.1130
/G35935C.1.
Stirling, M.W., et al., 2012, National Seismic Hazard Model for New Zealand:
2010 Update: Bulletin of the Seismological Society of America, v. 102, p.
1514–1542, doi:10.1785/0120110170.
Sutherland, R., et al., 2007, Do great earthquakes occur on the Alpine Fault in
central South Island, New Zealand? in Okaya, D., et al., eds., A continental
plate boundary: Tectonics at South Island, New Zealand: American Geophysical Union Geophysical Monograph 175, p. 235–251.
Wells, A., and Goff, J., 2007, Coastal dunes in Westland, New Zealand, provide
a record of paleoseismic activity on the Alpine fault: Geology, v. 35, p.
731–734, doi:10.1130/G23554A.1.
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