New field work, combined with analysis of high-resolution aerial photographs, digital elevation models, and satellite imagery, has identified an
active fault that is traceable for ∼90 km across the Seymchan Basin
and is part of the Ulakhan fault system, which is believed to form the
Okhotsk–North America plate boundary. Age dating of alluvial fan sediments
in a channel system that is disturbed by fault activity suggests the current
scarp is a result of a series of large earthquakes (≥Mw7.5)
that have occurred since 11.6±2.7 ka. A possible channel feature offset
by 62±4 m associated with these sediments yields a slip rate of 5.3±1.3 mm yr-1, in broad agreement with rates suggested from global
plate tectonics. Our results clearly identify the Ulakhan fault as the
Okhotsk–North America plate boundary and show that tectonic strain release
is strongly concentrated on the boundaries of Okhotsk. In light of our
results, the likelihood of recurrence of Mw7.5 earthquakes is
high, suggesting a previously underestimated seismic hazard across the
region.
Introduction
Since the earliest days of plate tectonics, the Eurasia (Eur)
and North America (NAm) plate boundary zone in northeastern Asia was recognised
as a likely location for smaller blocks and micro-plates, even if these could
not be precisely identified at the time . The existence of
an Okhotsk plate (Okh), encompassing a region including parts of northern
Japan, most of the Kamchatka Peninsula, and Sakhalin Island, as well as a
significant continental region north of the Okhotsk Sea, has been suggested
by multiple studies using the usual plate-tectonic inverse methods
. The northernmost portion of Okh forms a
broadly triangular region squeezed between the converging Eur and NAm plates, whose pole
of rotation lies more or less on their mutual boundary (Fig. 1), and a little
way north of the NAm–Eur–Okh triple junction at the apex of Okh
. The northern end of Okh is also a
region of diffuse seismicity and has undergone several relatively large (up
to Mw6.4) “intraplate” earthquakes
. However, the boundaries of this
northern portion of the plate have remained difficult to clearly demonstrate,
mostly for two reasons. One is the low deformation rate (≤5 mm yr-1 contraction–extrusion) leading to small numbers of
earthquakes with well-defined focal mechanisms, usable in plate-tectonic
studies. A second is the likelihood of elastic transient deformation
affecting much of the region due to its proximity to locked plate boundaries,
which makes rigid plate-tectonic interpretations of the limited amount of GPS
data available problematic .
Tectonics of the Okhotsk plate and surroundings. Earthquakes >Mw5.0 are shown by red dots. Earthquakes >Mw6.0 are
shown with focal mechanisms. Locations of Eur–NAm and Okh–NAm poles used to
calculate relative motion vectors (blue arrows: Okh–NAm; red arrows: Eur–NAm) shown by blue and red stars. The black box shows the location of Fig. 2 and
of the study area, and the town of Seymchan. The region is principally considered
a part of the Eurasia–North American plate boundary zone, and as such has
increasingly been recognised to consist of a number of smaller plates or
micro-plates, which can be proven to provide statistically better fits to
global plate-tectonic data. Nevertheless, there is a considerable amount of
intraplate seismicity in the region, especially northwestern Okh.
The Eur–Okh boundary is considered to run north from Sakhalin Island, where
it is well defined by the Mw7.0 Neftegorsk earthquake and
aftershock sequences , and to continue into generally
north trending, fault systems on land . The
NAm–Okh boundary is believed to correspond to the locally named Ulakhan fault
system, which is traceable over distances of ∼800 km and trends
approximately northwest . Whilst the large earthquake on
the Eur–NAm boundary (the largest non-subduction-related earthquake in the
region) gives strong evidence for an active plate boundary event within the
modern seismic record, it is noticeable that no such observations have been
made for the Ulakhan fault. Instead, the largest recorded earthquake in this
region is apparently “intraplate”, and a small number of ∼M5.0
earthquakes are all that have been associated with the trace of the Ulakhan
fault and hence the assumed Okh–NAm boundary itself. This seems somewhat
surprising, until one considers that the estimated displacement rates on this
boundary are of the order of ∼5 mm yr-1, over a fault system >1000 km in total length. thus argued that under various
assumptions about how the northern portion of the Okhotsk plate moves
relative to Eur and NAm, recurrence times for large earthquakes due to
interplate motions could easily be >1000 years for any particular segment
of the fault, but that events of Mw8.0 were possible.
Certain tectonic features along the suggested Okh–NAm plate boundary have
long been recognised as possible evidence of active tectonics. The most
remarkable of these is the Seymchan basin (also known as the Seymchan–Buyunda
basin), a ∼150km×∼60km northwest–southeast-trending depression, which forms a natural point of confluence for the Kolyma
and Buyunda rivers (Fig. 2). Geological and geomorphological evidence,
including Oligo-Miocene age, fluvio-lacustrine deposits and several apparent
depocentres with >1000 m of fill and ongoing sedimentation through the
Quaternary , suggests this basin has existed since some
time in the mid-Tertiary to early Tertiary (late Paleogene–early Neogene). As such, it
forms part of a series of sporadically distributed basins across the northern
Okhotsk region, including offshore regions, which initially formed in a generally
extensional–transtensional late Paleogene–early Neogene tectonic regime
.
Morphology and drainage of the Seymchan–Buyunda basin, showing
Buyunda fan structure, palaeodrainage networks on fan surface, subsidiary fans
along southern basin edge, Neogene sedimentary outcrops and the trace of the
fault segment across the Seymchan Basin, identified from aerial photographs,
satellite imagery and the TANDEM-X 0.4 arcsec DEM.
Today, the Kolyma and Buyunda rivers flow into the southern side of the
basin and cross the trace of the Ulakhan fault. Further downstream, on the
northern edge of the basin, the Buyunda river flows into the Kolyma. Whilst
the Kolyma is currently incising into the basin floor, the Buyunda has, at
least until very recently, been in a depositional regime and has built a
large alluvial fan on the basin margin.
There are a number of smaller alluvial fans crossing the southern margin of
the basin and the Ulakhan fault, between the Buyunda and Kolyma rivers over a
distance of about 50 km. In general, little was known about the intersection
between the Seymchan basin fill, characterised by the preserved alluvial fans
and associated alluvial basin deposits, and the Ulakhan fault. In the rest
of this paper, we describe the evidence for recent large earthquakes
affecting the Seymchan Basin segment of the Ulakhan fault, as well as
speculating on longer-term slip rates derived from new age determinations.
Field work and remote sensing data
In 2011 and 2012 we visited and surveyed segments of a pronounced scarp that
crosses the Buyunda alluvial fan, where it enters the Seymchan Basin. We also
took samples of alluvial material for age determinations. We subsequently
obtained an archive of aerial photographs along the trace of the scarp, and
the TANDEM-X 0.4 arcsec (∼12 m) resolution DEM (digital elevation
model) for the region (Fig. 3a). We have georeferenced the aerial photographs
using satellite data or the DEM as ground control points and compiled an
aerial photo mosaic which can be manipulated using standard GIS software
(Fig. 3b). We now present the results of our analysis of these data.
Study area. (a) Detail of the Buyunda fan and the fault trace
across it, showing lakes, scarp and ridge, thermokarst, shutter ridges, and
subsidiary alluvial fans. (b) Detail of georeferenced aerial photo
mosaic used for interpretation. Red dots show sample site locations.
(c) Landsat false colour image (567 infrared bands) of same area.
Saturated zone at the toe of Okhotnik fan shows up as lighter colours. Scarp and
ridge is noticeably “drier” in this region. Older fan-top channels
(previous course of Buyunda) appear “fresher” or more clearly defined and
sharper the further east they are, suggesting fan surface may be
progressively younger in this direction.
Tectonic geomorphology of the Ulakhan fault across the Buyunda fan
The Buyunda alluvial fan builds two distinct lobes where it enters the
Seymchan Basin at its southeastern end (Figs. 2 and 3). The inner lobe
surface is steeper and preserves a dense network of braided-fan distributary
systems, some of which may have been active until recently. The outer lobe
has gentler gradients and is fed today by input from several smaller fans
along its southern edge (Fig. 2). The lobes together occupy ∼20 %
of the modern-day basin floor: the inner lobe measures ∼190000 km2, the outer lobe (including the inner lobe) ∼480000 km2. The fan is asymmetric and has a long axis oriented
towards the northwest (Fig. 2).
The Buyunda river, which enters the basin through a narrow north-trending
gorge, flows along the eastern side of the alluvial fan and bypasses its
earlier apex. The Buyunda river forms a braided channel, ∼1 km wide,
which trends almost linearly northeast, parallel to the fan edge, before
looping around an isolated remnant of the Jurassic basin floor and turning
northwest, along the northern basin margin. A second braided channel system,
which from its sharp definition in satellite imagery (Fig. 3c) seems very
recently abandoned, runs almost exactly parallel to the present-day system, on
its western side, at 2–5 km distance. This earlier course of the river
flowed to the west of the Jurassic remnant. Several similar braided channel
systems, representing earlier generations of the Buyunda river, occur across
the fan surface but become progressively more difficult to distinguish from
one another in a westerly direction (Fig. 2).
In 2012, we visited the western edge of the fan where it intercepts the
fault, which is marked by a lake (Lake Rovnoye) (Figs. 3, 4, 5),
approximately 1 km long and roughly triangular in shape. Immediately east
of it, there is a pronounced 3–5 m high scarp, trending in a roughly ESE
direction (105∘). Adjacent to the scarp to its north are a series of
10–50 m wide basins (Fig. 4). We were able to follow these basins and the
scarp for ∼1.2 km to the ESE, before a noticeable break at the point
where a second small lake is found, also visible on aerial imagery, where
there are three right-stepping en échelon scarps for a distance of ∼900 m (Figs. 3 and 4). These have variable orientations, with the two, more
southerly lying scarps trending close to E–W, whilst the isolated segment of
the main scarp further north trends ∼125∘. There is also
another small lake and basin at the tip of the most southerly of the three scarps
in this region. At this point, the scarps merge again and continue as a
single line. The scarps bound a wider (up to 500 m) uplifted portion of the
fan to the south, also visible from DEM data.
(a) Un-interpreted aerial photo picture of ridge and scarp,
showing sample localities (red stars) and names. Outline of Fig. 5 indicated.
(b) Interpreted image showing ridge area south of scarp (dark
shaded), fossil channel system (light blue outline) and offset channel bank
feature (light blue solid shading). Also shown are linear velocity vectors
based on Okh–NAm poles of and .
The basins along the scarp showed signs of intermittent flooding and
transport of some alluvial material, including occasional deposits of pebbles
up to 5 cm diameter. Field reconnaissance, aerial photographs and satellite
imagery suggest that present-day drainage runs west-northwest into Lake
Rovnoye, and parallel to the scarp (Figs. 3, 5).
(a) Magnification of aerial photo showing interpreted,
northwest-oriented palaeodrainage and sample localities ul1 and ul2,
interpreted to be offset as part of abandoned channel system.
(b) Aerial photo with interpretation, showing offset palaeo-channel
feature. (c) Landsat (567 band) detail, showing hydrology of scarp
and ridge with a dry ridge (dark coloured area) bounded north and south by
saturated zone (light coloured area). For explanation of hydrology, see
text.
In 2011, we visited a region immediately ESE of the 2012 campaign, starting
from the point where the main road on the Buyunda fan intersects the apparent
trace of the fault. Here, we encountered a series of ∼100–200 m long,
10–20 m wide depressions, almost symmetric in profile, with a total relief
of 5–8 m from the deepest point (Fig. 6). These basins follow a fairly
continuous 110–120∘ trend to the point where they intersect with the
2012 field area.
Field photographs and interpretations of the main fault scarp near
Lake Rovnoye (a) and a depression with a smaller scarp further
east (c). An aerial photograph (b) shows approximate locations
of the views and view orientations.
The aerial photographs and DEM show the wider fluvial and tectonic
geomorphology of the Buyunda fan in the vicinity of the scarp (Figs. 3, 4).
The pronounced scarp across the Buyunda fan, visited in 2012, can be traced
over a distance of ∼3.5 km, starting immediately to the east of Lake
Rovnoye. The scarp shows a consistent 1.5–5 m relief (southern side
higher), as can be seen by both fault-normal topographic profiles and relief
maps from the DEM (Fig. 7). It terminates into the series of narrow,
elongated depressions encountered in 2011, which form a linear trend oriented
115∘, and extend for a further 2.5 km eastwards. There are
approximately 10 of these depressions, with smaller vertical offsets than on
the scarp, further west. Still further eastwards, the series of smaller
depressions give way to three much larger (∼500 m ×100–200 m
wide and 10 m deep) basins, aligned in the same 115∘ orientation and
spread over a distance of ∼2.5 km. These may be dried-out thermokarst
lakes e.g.. These also coincide with the apparently
more recently active part of the fan surface, judging by the still fresh
appearance of the preserved braided channel systems in satellite imagery.
Similar depressions also occur in other places off-trend of the scarp.
Topographic profiles, showing vertical offsets across the scarp on the
Buyunda fan. Offsets are close to maximum near Lake Rovnoye (profiles 1 and
2) but diminish eastwards over a distance of ∼5 km. From Lake
Rovnoye westwards there is also no vertical offset. Dashed lines are
estimated average elevation of surface either side of fault for determining
vertical offset.
The linear features we have identified are aligned close to parallel to
various predicted Okh–NAm linear velocity vectors
. There is an especially close match to
the present-day GPS-based REVEL global Euler vector ,
with magnitude ∼6 mm yr-1. The best-fitting Euler vector based
on spreading rates and earthquake focal mechanisms, and hence considered a
longer-term estimate (up to 3.5 Myr) , is slightly more
oblique to the trend and of lower magnitude (∼4 mm yr-1)
(Fig. 4).
We can thus trace a linear feature across the inner fan surface made up from
west to east of a distinct fault scarp, a series of narrow elongated
depressions, and a series of possible dry, thermokarst lakes over a total
distance of ∼10 km. The linear trend terminates in the modern-day
Buyunda river channel. Given its linear nature, the sometimes pronounced
scarp, and the fact that the trend of the feature fits closely with predicted
Okh–NAm motion from global Euler vectors, we suggest that this is evidence of
a recently active, mostly strike-slip fault with a left-lateral offset,
likely to be the Okh–NAm plate boundary. We now present arguments for
potentially large earthquakes and ruptures along it.
Buyunda fan surface and hydrology
Although today the Buyunda river is in a braided channel to the east of the
Buyunda fan and is actively incising its earlier fan deposits, the fan
surface is composed of several generations of earlier braided channel systems
representing earlier courses of the Buyunda river. These have complicated,
discordant relationships to one another, but in general they become less
distinct and presumably older in a westerly direction (Fig. 3).
One of the most commonly used means to establish fault slip rates in
strike-slip regimes are offset markers, such as alluvial channels, terraces
or other stable land forms where they cross a fault
e.g.. It is
often hypothesised that some channels may be offset by a single earthquake
and simultaneously abandoned by the stream that flows into them. Under these
circumstances, a channel will become a passive marker for the current and all
subsequent earthquake offsets. However, there are a number of potential
problems with this idea. Firstly, the channel may have already been inactive
for other reasons, prior to an offsetting earthquake, and associated ages of
channel deposits would therefore not be synchronised with the start of offset
motion. This is probably only relevant for cases where single earthquake
offsets are being measured. In cases where multiple offsets have occurred,
the significance of a time delay between abandonment and the beginning of
offsets will become less as more earthquakes occur. Secondly, channels may
re-establish flow between offset segments in a phenomenon known as dog-legging
, and hence depositional ages within channels will be
younger than the timing of offsetting motion. Dog-legging may also occur
where a new channel exploits an existing fault scarp along part of its
length, causing a deflection in its course which is unrelated to seismic
events. The best tectonic offset markers are generally linear features such
as edges of incised terraces or straight segments of a
channel. However, in many cases, associating age determinations of
sedimentary features with their offsets is problematic.
The Buyunda fan inner lobe surface is crossed by many channels which are
intersected by the fault scarp. In general, these form abandoned, braided
systems of similar character to the present-day Buyunda river (Fig. 3).
Braiding leads to continuously curved features, which makes identifying
tectonic offsets more difficult. The eastern end of the fan in the area of
the larger thermokarst features nevertheless contains a number of channels
with straight segments that cross the fault, often almost perpendicular to
it, and none of which appear to be offset. We suggest that this region is the
youngest portion of the fan surface and was active after the last major
earthquake on this fault segment, which obliterates traces of the active
scarp. This system is bounded on its western edge by a channel which turns
fault-parallel for a distance of ∼350 m, which could be interpreted as
a fault offset but actually appears to mark the edge of this particular
generation of deposits (Fig. 3).
Further westwards towards the region of the pronounced fault scarp, the
braided channels are less distinct in aerial and satellite imagery (Fig. 4).
The fault scarp builds a ridge which cuts through drainage. To the south and
north of the ridge, aerial and multispectral LANDSAT imagery shows saturated
zones and fossil drainage, suggested by the light colours of the satellite
image (Fig. 5). This was also confirmed in the field, where we found
substantial areas of persistent surface water. Between them, the scarp
builds a “dry” zone (Fig. 4), with dryness implied by the dark colours of
the satellite image and also confirmed in the field. The hydrology of this
region is dominated by outflow from the adjacent Okhotnik river and fan
system, and the fault scarp which forms a step in the regional water table.
The area directly north of the scarp is a topographic low on the edge of the
inner lobe of the Buyunda fan. Water presumably percolates into the scarp
area, fed from the Okhotnik river. The step due to the scarp offsets the
hydraulic gradient within the fan deposits, leaving the scarp dry and the
areas to its north and south saturated. The saturated zones highlight a
fossil channel system which formed a pre-scarp drainage system. The channels with
higher permeability sediments are distinctly light coloured in satellite
imagery (Fig. 5). The fossil channel system drained to the northwest.
At the westernmost end of the scarp and ridge, directly adjacent to lake
Rovnoye, there are two small areas of raised topography, visible on the
aerial photographs and the DEM (Fig. 5). The southernmost of these forms the
main fault scarp at this point as can be seen from fault-normal topographic
profiles (Fig. 7). The fault thus passes between the two topographic highs,
before terminating and resuming a few metres south along the main scarp. The
topographic features both have straight, eastern edges, trending to the
north. They also appear as two darker spots on LANDSAT imagery (Fig. 5c),
again suggesting an apparent offset. These features are interpreted as
channel banks of an earlier north-draining system, the fossil remnants of
which can be seen ∼500 m further north (Fig. 4b). The channel banks
are cut by the trace of the fault and offset by 62±4 m in a
fault-parallel direction, which we interpret to be a measure of fault offset
in this location. We suggest abandonment of the channel may have occurred due
to the fault scarp and ridge that were formed during earthquakes, blocking
and shutting down the existing drainage. Subsequently, new drainage developed
parallel to the scarp and began to flow into lake Rovnoye at the scarp's
western end (Fig. 5a).
Buyunda fan age determinations and offset rate estimates
We collected sediment samples from four sites along the scarp for age
determinations using both optically and infrared stimulated luminescence of
quartz and feldspars (OSL and IRSL) using the SAR (single aliquot
regenerative dose) protocol according to , as well as
organic material for 14C from one further site (see
Appendices A and B for details of the method, sampling and laboratory
procedures). Samples came from the region directly adjacent to and north of
the scarp from five pits (Fig. 4), although from one pit (sample 5) we only
recovered organic material. Samples 1 and 2 are on both sides of the offset
channel feature we have identified, at the westernmost edge of the scarp.
Samples 3, 4 and 5 are from the broad zone of channel deposits that we can
identify from aerial photographs. Samples 1–4 were analysed with both OSL
and IRSL. Sample 5 was 14C only. Sample pits encountered
fine–coarse grained, sandy material, with occasional evidence of graded and
cross bedding. Small pebbles sometimes formed the base of cross beds. We
believe we sampled a mixture of channel or possibly overbank deposits of a
fossilised fan-top channel system, probably part of an old course of the
palaeo-Buyunda river when it flowed generally to the northwest, in a direct
line to its confluence with the Kolyma river. Sample collection and
processing procedures are described in the Appendix. As part of the sampling
procedure, we also recovered two samples from different faces of several
pits, which are referred to as (a) and (b) samples in this paper.
Samples were generally classified as being either well bleached where quartz
(OSL) and feldspar (IRSL) ages are consistent, or partially bleached where
quartz and feldspar ages differ significantly. In general, feldspar ages are
only considered indicative of true ages of channel deposits when they closely
match quartz ages (see Appendix for explanation of methodology and data
tables). If feldspar ages do not match quartz ages for an individual sample,
only the quartz ages are taken to be representative of the true age of the
deposit. From the reliable data there is a range of ages (8.85–14.3 kyr)
from samples 1–4, with a mean age of 11.6±2.7 kyr (Table A1, Fig. 8).
The ages suggest an early Holocene abandonment of this part of the fan and
by extension may date the first uplift of the scarp and ridge structure that
we suggest reorganised the drainage in this region. These are probably the
first genuinely physical age determinations of Quaternary sediments carried
out in this region , and certainly the first employing OSL.
Combining the mean age with the associated 62±4 m offset gives a slip
rate of 5.3±1.3 mm yr-1, which agrees with the modern-day plate-tectonic estimate of Okh–NAm motion at this point .
Usable luminescence and 14C age data from the five sample
sites along the Ulakhan fault scarp (for locations see Fig. 4). Raw data,
including from partially bleached IRSL samples not considered in age
determinations, can be found in the Tables A1, A2 and B1. Data are separated
into (a) and (b) samples, which refer to separate samples from different
sides of the same pit.
The poorly bleached feldspar samples have a wide range of ages
(12.3–45.6 kyr) (Table A2). Poor bleaching reflects the fact that feldspar
luminescence has not been reset during the latest transport and depositional
episode (see Appendix). It is quite likely that the Buyunda fan sediments
have been reworked from other deposits in the Buyunda river system, and it
may be that the ages reflect earlier episodes of transport and deposition in
other parts of the drainage basin from which the sediments have been
reworked, but there is no method available for quantifying this possibility.
14C dating of sample 5 gives a far younger age than the
neighbouring OSL sites. This is not particularly surprising, given the
possible ways of introducing organic material into the subsurface long after
deposition has occurred. We suggest that the consistency of the OSL results,
probably reflecting time since channel abandonment, make it likely that the
14C age is post-depositional.
Basin-wide fault and scarp features
The scarp we encountered in 2011 and 2012 is linked to a fault that can be
followed across most of the Seymchan–Buyunda basin. Using remote sensing
data, we can trace the fault westwards from the alluvial fan (Fig. 3). Aerial
photo coverage also overlaps with parts of this region. The fault extends
∼90 km in total to the northwestern edge of the basin where it may
also offset Neogene continental clastic deposits. Immediately west of Lake
Rovnoye, aerial photographs show several small lakes which may sit between
an overlapping en échelon portion of the fault (Fig. 3). A linear trace then
runs to the eastern edge of the incised Kolyma river, marked in places by
shutter ridges (Fig. 3). Several smaller rivers cross this part of the fault
and have built fans across it. Some of the fan edges suggest left-lateral
offsets, but this is not consistently obvious. Linking of offset distributary
channels (i.e. identifying consistent, left-lateral offsets) from one side of
the fault to the other is also difficult, although this is often the case in
strike-slip fault systems e.g..
Where the Kolyma river enters the Seymchan Basin, it emerges from a deeply
incised gorge in Jurassic and Triassic bedrock to the south and has incised a
delta-shaped region into the basin infill (Fig. 2). The fault is thus not
discernable in this region. West of the Kolyma, however, the fault segment
re-emerges, approximately in line with the end of the trace to the east of the
river and forming the northern boundary of Neogene outcrops on the south
side of the basin. The segment finally terminates in what is apparently a
second scarp but with the opposite uplift polarity to that on the Buyunda fan
(Fig. 9). This second scarp appears to be somewhat enhanced by erosion along
the fault trace, but the offset of the basin floor is clear from the fault-normal topographic profiles (Fig. 9). The northern side of the fault is
uplifted by up to 3–5 m, just as the southern side is uplifted by a similar
amount at the eastern termination of the fault segment on the Buyunda fan. As
we discuss in the next section, these two linked scarps and peak uplifts can
be related to elastic dislocation models of earthquakes on strike-slip fault
segments.
Topographic profiles, showing vertical offsets across presumed scarp
at the western end of the Seymchan Basin, assumed to mark the termination of
the 90 km Ulakhan fault segment. Offsets reach a similar maximum value to
the scarp on the Buyunda fan and diminish rapidly to the west, but more
gradually, over a distance of ∼5 km, to the east. Sense of offset is
reversed relative to the Buyunda scarp (Fig. 7).
Elastic displacement modelling and scarp polarity
Elastic displacement theory has long been applied to analysis of co-seismic
slip in earthquakes. Although there are many degrees of sophistication in
these models today e.g., the simplest case of a
vertical strike-slip fault which reaches the surface and was the basis of
Chinnery's () first application of the method to
earthquake slip problems is adequate for our purposes. The method calculates
the displacement components around a Volterra dislocation in an elastic half-space, by solving the equations of elasticity for boundary conditions of
stress-free bounding surfaces, using a Green's function method
. As such, a Volterra
discontinuity is a surface with constant offset or displacement across it.
This is a reasonable first approximation to a fault that has undergone an
earthquake displacement. The parameters in the model are fault half-length,
L; fault depth from the surface, D (in the case where the fault
intercepts the surface); and fault slip, U. The results are given as the
three displacement components, parallel to x, y and z axes, in the
volume surrounding the fault, with x and z chosen to contain the fault
plane in the case of a vertical strike-slip fault. For our analysis, we only
use the vertical displacements since these pertain most directly to scarp
formation.
Theoretical predictions of slip on a left-lateral strike-slip fault have two
interesting properties. The first is that the vertical motions on both sides of
the fault reach their maximums at the tips of the propagating zone of fault
slip. Hence, the largest vertical offset corresponding to a scarp would be
expected at the two ends of a single rupture (Fig. 10). The Seymchan Basin
fault shows two such pronounced scarps linked by a continuous fault.
Elastic dislocation model for a 90 km fault segment (half-length,
L=45 km), 10 km deep, reaching the surface, with 15 m average slip.
Contours show vertical displacements (co-seismic) for such a slip event. Fault
is vertical. Also shown is the topography around the two scarps at the
terminations of the 90 km fault segment.
The second property of interest is the polarity of uplift at the two tips.
The predictions of an elastic dislocation model fit the polarity found in the
Seymchan Basin, with the “south” side up in the east and the “north”
side up in the west. The simplest way to achieve this geomorphology on a
fault is a rupture, or series of ruptures, along the entire length of the
fault we have traced across the basin. In other words this 90 km fault
segment fails, probably repeatedly, as a single entity.
Uplift amounts and scarp offsets can also be correlated with total slip,
fault depth and fault length for single events from the model. Assuming
there has been little modification of the average surface offset across the
scarp by surface processes, we need to model ∼3–5 m of uplift. For
example, a single 15 m slip event on a 90 km fault segment, 10 km deep,
would produce ∼1.15 m maximum uplift giving a maximum total vertical
offset 2.3 m (Fig. 10). We therefore think it is likely that the fault scarp
has been produced by several (probably 3–5) separate earthquakes over time,
which would fit well with our finding of a ∼11.5 kyr age for a 62±4 m offset across the fault, and the earlier results of
suggesting recurrence times of large earthquakes on segments of the Ulakhan
fault to be ∼1–3 kyr.
DiscussionSeismological and kinematic data
There have been no large earthquakes (>Mw7.0) on the plate
bounding faults of the northwestern portion of Okh within the instrumental
seismic record. The most northerly large earthquake on the boundary of Okh
(excluding those along the segment of the Pacific subduction zone) was the
Neftegorsk event at the northern end of Sakhalin Island, on the Eur–Okh
boundary. Otherwise, excepting the Mw6.7 Illin-Tas earthquake
in 2013, which occurred on the Eur–NAm boundary, north of the triple junction
with Okh , the largest event in the region was the
intraplate Mw6.4 Artyk event in 1971, which lay within the
Okhotsk plate. This lack of data led and
to suggest that there were a number of ways to accommodate
Eur–NAm convergence by internal and plate boundary strain within northwestern
Okh (Fig. 11). These were an effectively rigid, plate-like “extrusion” of
the northwestern corner of Okh from between converging Eur and NAm, with the
majority of tectonic displacement occurring on the plate bounding faults; a
northwestern corner of Okh consisting of a series of blocks, mostly elongated
north–south, moving independently of one another by relatively even amounts
over many earthquake cycles, and thus no single clear plate-bounding fault.
Although some internal deformation is occurring within Okh as shown by the
1971 intraplate earthquake, it is 1 order of magnitude smaller than that
expected for full release of plate-tectonic strain according to
. GPS data from northwestern Okh are sparse and have been
interpreted in a variety of ways . Our
field observations on the Ulakhan fault give a first insight into the
palaeo-seismology of the region and add important data in this context.
Graphic showing two hypothesised modes of tectonic strain release in
the northwestern corner of Okhotsk. (a) Region composed of
independent blocks acting as independent micro-plates, with average relative
slip across them fairly even. This means there is no distinct Okhotsk–North
America or Okhotsk–Eurasia plate boundary. (b) Region composed of
blocks that, although they can slip relative to one another, are mostly
“stuck together” by compressive force of converging Eurasia–North America.
In this case, the behaviour of northwest Okhotsk is plate-like, even if there
is some intraplate deformation.
Palaeo-seismology, earthquake recurrence and seismic hazard
The fault scarp we encountered requires a recent earthquake or series of
earthquakes of large magnitude and also forms part of a single 90 km fault
segment that we can trace across the Seymchan–Buyunda basin. The opposite
uplift polarities at the likely tips of this segment are a morpho-tectonic
signature probably uniquely explained by rupture or repeated ruptures on a
single fault segment, as suggested by elastic dislocation modelling. The
magnitude of uplift implied by the Ulakhan fault scarp over ∼11.5 kyr
also matches well with that predicted for the combination of a 90 km fault
segment and the slip magnitude available due to plate-tectonic strain
accumulation in this time period .
Our interpretation of the field observations has several implications. It
confirms a potentially significant seismic hazard in the region, with a
likelihood of ≥Mw7.5 earthquakes occurring within the
Seymchan Basin (the 90 km length 10 km deep 15 m slip event modelled in
this paper is equivalent to an Mw7.7 earthquake), and hence
affecting both populated areas and large infrastructure, in particular the
Kolyma hydro-electric dam located at Ust Srednekan (Fig. 2). It firmly
constrains the location of the plate boundary to follow the trace of the
Ulakhan fault, and suggests the slip and strain partitioning due to plate-tectonic motions is concentrated (>90%=5–6 mm yr-1) on the
plate boundaries. This in turn implies that the internal strain of the
northwestern Okhotsk plate is confined to the release of small amounts (probably
<0.5 mm yr-1) of accumulated slip. This may mean that the largest
possible intraplate earthquakes are no bigger than the Mw6.4
Artyk earthquake of 1971, although ultimately this will also depend on their
frequency.
The wider question of recurrence times of large earthquakes on individual
fault segments can also be partly addressed by our new results.
considered two possible scenarios for strain release along
the plate boundaries of Okh, whilst assuming that rigid, plate-like extrusion
occurred. In the first, strain was only seismically released along the
Okh–NAm boundary (∼1150 km total length). This was considered a
possibility due to the absence of any seismicity, or indeed any clearly
defined structure for the plate boundary, along Eur–Okh, which would creep
aseismically instead. In this case, the average recurrence times for large
earthquakes on any segment of the Okh–NAm boundary were estimated to lie
between ∼0.7 and 1.2 kyr. The second scenario had seismic strain release
occurring along both Okh–NAm and the adjacent portion of Okh–Eur (total fault
length ∼2500 km). In this case, recurrence times were estimated at
∼3.0–4.9 kyr. The lower estimate was based on average earthquake sizes
Mw7.6–7.8. The 3 kyr recurrence interval, with 62±4 m
total slip and a suggested age for the offset channel of ∼11.5 kyr for
the Seymchan segment of the Ulakhan fault, would give four earthquakes of
average slip ∼15 m. Assuming a 15 km rupture depth and a 90 km
length gives Mw7.7 per event.
Earthquake size and fault dimensions
One of the key questions in earthquake seismology is the nature of any
relationship between rupture length and average fault slip.
analysed a large number of earthquakes from around the globe, and according
to their empirical formulae, a single earthquake on a 90 km long fault
segment should yield an average ∼1.9–2.8 m slip. Our estimate of
∼15 m slip in a single event on the Ulakhan fault seems large in this
context. However, the natural example of the 1855 Mw8.1
Wairarapa earthquake in New Zealand, which has a relatively well-constrained
rupture length of ∼145 km and average slip ∼12–16 m, shows that
much higher displacement–length ratios for strike-slip faults are possible
. The Wairarapa fault is
interesting as an analogue for the Ulakhan fault in several other ways.
Studies have shown it has a Holocene slip rate of ∼6–12 mm yr-1, broadly comparable to that suspected for the Ulakhan,
and is mostly undergoing strike-slip displacement
. There have also been a
sequence of large earthquakes (∼11) on this segment of the Wairarapa
through the Holocene, demonstrated by differential uplift of a series of
river and beach terraces adjacent to the fault
. In general, the Wairarapa fault provides a
well-constrained example of the behaviour we hypothesise for the Ulakhan.
A comparison with the Wairarapa fault scarp, with a known source age,
relatively well-constrained magnitude and similar kinematics, is also
interesting for our study. The Wairarapa fault has multiple offset terrace
levels, giving a high-resolution earthquake “stratigraphy” in the
landscape and confirming the repeated Holocene ruptures on the segment.
These terraces are due to a longer-wavelength landscape uplift pattern
around the fault of up to 5 m per earthquake over a wide area (maximum
terrace uplift today ∼40 m). This broader uplift pattern is the main
driver of terrace formation and abandonment. The scarp from the 1855
earthquake shows a 1–2 m vertical offset and remains visible in many places
today . Elastic modelling of the
Wairarapa earthquake has suggested a listric fault
geometry, partly to account for the broader uplift field. The Ulakhan fault
may also be creating uplifted fluvial terraces in a similar way. However, it
is difficult to separate tectonic from glacio-eustatic and other base-level-related signals. More generally, the Holocene behaviour of the
Seymchan–Buyunda segment of the Ulakhan fault seems to be quite well modelled
by a simple vertical strike-slip fault. However, further analysis is
probably required in this context.
Large-scale tectonics
An interesting aspect of the northwestern corner of Okh is its tectonic
situation, as a narrow sliver of a small plate caught in compression between
much larger converging ones (Eur and NAm) (Fig. 11). Despite the convergent
motion of Eur and NAm, the resultant motion along the boundaries of Okh is
generally believed to be strike-slip with northwestern Okh moving towards the
south, perpendicular to convergence . Due to the
proximity of the Eur–NAm Euler pole to the region, the rates of convergence,
and hence overall rates of slip on boundaries, are low (∼5 mm yr-1). There are few, if any, other places on earth directly
comparable to this. In terms of deformation rates, northern China is broadly
similar . However, northern China is a zone of intraplate
deformation. It is also the place with the longest historical record of
seismicity in existence, which has allowed unique insights into the nature of
slowly deforming regions, faulting and seismicity . It
appears that northern China is composed of a system of linked faults across
the plate interior, and these move to some degree in coordinated fashion
. It is only possible to establish this due to the
2000-year record of earthquakes there. By contrast, our work suggests that
northwestern Okhotsk is more plate-like with slip concentrated on discrete
plate bounding faults, even if there may be relatively large intraplate
earthquakes occurring too. It is thus interesting that plate-like behaviour
can apparently persist into the realm of very low deformation rates (<5 mm yr-1). At the same time, the Ulakhan fault system may exhibit
similar long-term behaviour to much faster slipping strike-slip faults. The
North Anatolian Fault (NAF) slips at 23 mm yr-1 and has a length of
∼1000 km , a length broadly similar to the
Ulakhan system. The NAF is also segmented, and sequential earthquake
migration over longer time periods on different segments has been observed
within historical records , in a similar fashion
to that now postulated at much slower rates along the Ulakhan fault.
Conclusions
We have documented a fault scarp on the Buyunda alluvial fan, at
the eastern end of a ∼90 km segment of the Ulakhan fault in the
Seymchan Basin in northeastern Russia. We suggest the scarp is indicative of a
series of large earthquakes affecting the segment. At the western end of this
segment, which we can trace almost continuously in the landscape from remote
sensing data, we find a probable scarp of opposite uplift polarity to that in
the east. We suggest these two scarps mark the ends of a fault segment that
ruptures repeatedly as a single entity. The good fit of the uplift pattern we
have found in the field to that generated by simple elastic dislocation
models of left-lateral strike-slip faults of the appropriate magnitude and
slip also tends to confirm the idea of a single fault segment.
Age dating of a fluvial system that seems to have become abandoned due to
formation of the scarp on the western edge of the Buyunda alluvial fan
suggests the sequence of earthquakes causing abandonment began ∼11.5 ka. This age may also be associated with a 62±4 m left-lateral
offset of an ∼11.5 ka fluvial feature.
In general, our field data suggest that the Okhotsk–North America plate
boundary in this region slips at 5.3±1.3 mm yr-1, thereby
releasing almost all the available tectonic strain due to Eur–NAm plate
convergence . This slip rate is also
in agreement with local predictions from the Okh–NAm Euler vector.
The earthquake recurrence analysis in previous work and
comparison with the new field data suggests infrequent earthquakes of
relatively large magnitude (>Mw7.5, every ∼3 kyr) are
most likely responsible for the Holocene tectonic geomorphology of the
basin. Given that strain accumulation must be continuing to the present day,
the seismic hazard in the local area needs careful assessment. Perhaps the
most critical question now is when exactly did the last large earthquake
occur on this segment of the fault? Given sufficient resources, it may be
possible to determine this by trenching across the fault scarp.
Code availability
The FORTRAN code written for the purpose of georeferencing
the aerial photographs can be requested from the main author (David Hindle). It
may also be the subject of a future publication.
Data availability
A file of the x, y coordinates (longitude, latitude) of
the mapped segments of the Ulakhan fault, based on the TANDEM-X DEM and
aerial photo interpretation is available from the main author (David Hinde). A
GeoTIFF file of the composite scene of aerial photographs in geographic
coordinates may be requested for academic use only. These data are not to be
redistributed. Anyone wishing to have them must request them for themselves
directly from the main author (David Hindle).
OSL dating and results
Certain minerals like quartz and feldspar can store energy released by
radioactive decay. In the case of sedimentary material, this radioactivity is
derived to a major extent from isotopes of uranium (e.g. 238U) and
thorium (e.g. 232Th) and their daughter nuclides, as well as
potassium (40K), both in the sediments of interest and their
surroundings. This stored energy can be released by heating the minerals or
by exposure to light. The energy is released as light (luminescence) which
can be measured using a photomultiplier or a CCD camera. The higher the level of radioactivity and the longer the
duration of exposure, the more energy will have accumulated and the more
light will be released. Hence, for sediments, it is assumed that provided
their minerals are sufficiently exposed to sunlight during transport, prior
to deposition, in order to reset the radioactive stored energy, the total
amount of radioactive energy measured by luminescence (radioactive dose in
grays, Gy) can be divided by the dose rate (rate of energy supplied to the
minerals by radioactive decay within the sediments, measured in Grays per
year, Gy yr-1) to give the luminescence age of the rock
. In sediments, this age should usually reflect the time
since final deposition and burial.
Sample collection
Sediment was collected from pits dug in abandoned channels of the fan-top
drainage system of the Buyunda fan. The aim of the collection procedure was
to collect sediments that were buried and cut off from light, and hence began
accumulating radioactive energy derived only from the surrounding sediments
and internal crystalline sources. As any fresh exposure to light will release
this energy, sampling must be carried out in a light-proof way. Hence, we
used a metal sampler with an internal removable light-proof plastic sleeve,
to take samples from the pit wall, penetrating up to 20 cm. After removing
the sampler from the pit wall, the ends of the plastic sleeve were sealed
with light-proof tape immediately upon removal from the sampler, and then
stored in a light-proof bag. Sample pits were generally ∼1 m deep (the
depth at which permafrost was first encountered), and samples were taken at a
depth of 80 cm within the pit (Fig. A1). An example of the localities for
sample gathering is shown in Fig. A2. The sample pits showed thin soils
(generally 20–30 cm with roots evident to up to 50 cm. From 40 cm and
deeper, some sedimentary structure was visible, with faint cross-bedding
discernible, sometimes marked by basal pebble conglomerate layers. This
suggests channel-like flow, or possibly overbank sedimentation with flow.).
Sample pits and sampler usage on the Ulakhan fault scarp. Sampler is
inserted at the deepest part of the pit, usually by hammering, then extracted
by hand.
Sample sites of UL1 and UL2, both areas of slightly raised
topography, apparently offset by the Ulakhan fault, and an example of excavated
material from hole. Note the pebbles, some of which were from near the surface, but
most of which came from sample pit.
Sample preparation and measurement
Age determinations were carried out at the Klaus-Tschira-Archäeometrie-Zentrum, in Mannheim. Sediments were sieved to separate grain sizes of
100–200 µm (coarse) and less than 100 µm (fine).
Organic material was destroyed using Perdrogen (30 %). Acetic acid
(30 %) was used to remove carbonates. The coarse-grain fraction was split
into mineral fractions using heavy liquid separation (tungstate density 2.75
and 2.62 g cm-3) to extract quartz minerals. These were etched with
48 % hydrofluoric acid for 45 min to remove the outer 20 µm
of the grain that are influenced by alpha radiation and the material below
100 µm was removed by sieving. The fine-grain material was
further refined to a 4–11 µm fraction by settling in acetone. The
first step removes grains larger than 11 µm and a second step
excludes grain sizes smaller than 4 µm. Both
fractions, 100–200 and 4–11 µm, were then deposited onto steel
discs for measurement.
We used a standard Risø TL-DA-20 reader equipped with a
90Sr/90Y source for beta irradiation (strength
0.06 Gy s-1 for coarse grain, 0.08 Gy s-1 for fine grain) and
an alpha source 241Am for fine grain (strength
0.116 Gy s-1). Coarse- and fine-grain quartz was stimulated with
blue LEDs (470±20 nm) and the luminescence signal was detected using
7.5 µm Hoya U340 filters (280–370 nm), whereas
fine-grain feldspar was measured by stimulation with infrared LEDs (870±40 nm) and
detected in the blue range using filter BG3 and BG39 (3 mm each, ∼350–420 nm).
For all coarse-grain quartz samples, preheat tests have been made at 180,
200, 220, 240 and 260 ∘C at 2 aliquots each to determine a stable
preheat temperature. Each measurement cycle of the quartz samples included an
infrared measurement to bleach feldspar contamination. In addition for some
of the coarse-grain samples dose recovery tests have been performed on six
aliquots each. The measurement followed the suggestions of
and .
Quartz was usually extracted from the fine-sand fraction between 100 and
200 µm. We also used polymineral fine-grain samples of grain
sizes 4–11 µm to measure the feldspar signal. Quartz could also
be measured in this fraction. Measuring both quartz and feldspar from the
same grain size fraction enables us to interpret different aspects of
sedimentation. Quartz bleaches (i.e. releases luminescence energy due to
exposure to light) much faster than feldspar; hence, if ages of both minerals
overlap within errors the sediment was well bleached and the sedimentation
process was rather slow meaning grains were completely reset before
deposition and burial. If the ages differ significantly and the quartz age is
the younger of the two, it is likely that the sediment was accumulated abruptly and
minerals were not properly bleached (radioactive energy reset). In this case
only the quartz ages are significant as they reset their radioactive dose
quickly. Samples in this condition are referred to as “partially bleached”
and only the quartz ages (younger) from the sample are used.
Sample radial plots for OSL and IRSL measurements.
OSL and IRSL analysis: samples suitable for age determination either
from quartz (OSL) or well bleached feldspar (IRSL). Wa.Co. stands for water content (i.e. humidity).
MALSampleLong/latThUKC DRDROSL/FG/DeaWa.Co.Ageno.(∘ N, ∘ E)(ppm)(ppm)(%)(mGy a-1)(mGy a-1)IRSLCG(Gy)value(%)(kyr)10206ul1a153.03548.92.511.860.212.53I290 ∘CFG36.26±1.480.0816.510.0±0.762.4416±0.20±0.07±0.05±0.03±0.06OCG27.27±2.02±2.010.0±0.7610207ul1b8.512.251.880.212.90I290 ∘CFG38.68±2.360.051311.6±1.2±0.20±0.07±0.05±0.03±0.35OFG40.87±1.320.03±2.012.9±1.210208ul2a153.03498.352.272.040.213.15IFG42.3±2.860.0611.611.7±1.262.4421±0.18±0.06±0.05±0.03±0.35OFG48±2.090.03±2.014.3±1.35OCG34.44±0.4711.8±0.310209ul2b8.22.222.030.213.37I290 ∘CFG42±2.10.057.411.5±8.5±0.17±0.06±0.05±0.03±0.30OCG48.5±20.04±2.013.6±0.9510210ul3a153.05069.682.682.10.213.48OFG40.54±1.340.0310.511.0±1.05±0.19±0.06±0.05±0.03±0.40±2.010211ul3b62.44148.922.52.040.212.85OFG30.34±0.520.0221.810.0±0.65±0.21±0.07±0.05±0.03±0.25OCG24.71±0.640.02±2.09.05±0.310212ul4a153.045210.572.462.120.213.30OFG47.8±2.90.0421.814.0±1.262.4409±0.25±0.07±0.05±0.03±0.26OCG41.13±3.18±2.014.3±1.1510213ul4b9.792.622.460.214.24OFG39.24±0.50.031.08.85±0.55±0.21±0.07±0.06±0.03±0.35±2.0
FG/CG: fine grain/coarse grain; Th, U, K: thorium, uranium,
potassium content (required for calculating dose rate); a value: ratio of De
determined by β irradiation and by α irradiation; C DR: cosmic
dose rate, dependant on geographic coordinates; DR: dose rate from
radionuclide contents; De: equivalent dose, dose determined through OSL/IRSL
measurement; I290 ∘C: IR preheat 290 ∘C IRSL measurement
(see Table A3).
IRSL data unsuitable for age determinations
due to partial bleaching.
FG/CG: fine grain/coarse
grain; Th, U, K: thorium, uranium, potassium content (required for
calculating dose rate); a value: ratio of De determined by β
irradiation and by α irradiation; C DR: cosmic dose rate, dependant on
geographic coordinates; DR: dose rate from radionuclide contents; De:
equivalent dose, dose determined through OSL/IRSL measurement;
I290 ∘C: IR preheat 290 ∘C IRSL measurement (see Table A3);
I 50 ∘C, 125 ∘C, 290 ∘C: post–post-IR IRSL
measurement at elevated temperatures (see Table A3).
For feldspar a post-IR IR procedure was used, where an initial low-temperature IR bleach is carried out and then a series of measurements of the
remaining IRSL signal are made at elevated temperatures
and IR 290 measurements were made according to a procedure outlined by
. The measurement steps for IRSL and also for quartz OSL
are given in Table A3.
Examples of radial plots from OSL and IRSL measurements are shown in Fig. A3.
Results
The results are shown in Tables A1 and A2.
Measurement steps used for IR 290, post-IR
IR and OSL dating.
a As determined from preheat test.
b 20 ∘C lower than preheat temperature.
Radiocarbon dating
The organic material for radiocarbon age dating was pretreated to remove
carbonates and humic acids. It was then graphitised and measured with a
MICADAS (Mini Carbon Dating System, IONPLUS) AMS system. The results were
calibrated using Oxcal 4.2 and the IntCal13 dataset. Details of the
measurement procedure can be found in . Results of the
radiocarbon dating can be found in Table B1.
14C results.
MAMSLong/lat (∘ N, ∘ E)SampleMaterialAge 14Ccal BP (1σ)cal BP (2σ)Cal BP (mean)23960153.0401/62.4414ul5organic1125±261058–9831173–9601026±40Author contributions
DH undertook the field campaign in 2012, organised sample
collection and processing, and collated and interpreted the major part of the
data used in this project. He was the recipient of DFG grant HI-1409/3-1 and
project DEM_GEOL2124. BS accompanied the 2012 field work and provided
support in obtaining data. SL processed all OSL samples and provided
interpretations of the results. KM provided some data from a field visit in
2011.
Competing interests
The authors declare that they have no conflict of
interest.
Acknowledgements
The work of David Hindle was supported by DFG grant no. HI-1409/3-1. Access
to TANDEM-X data was granted by DLR (Deutsches Zentrum für Luft- und
Raumfahrt/German Aerospace Centre) under project DEM_GEOL2124. David Hindle
also acknowledges the support of NEISRI, the Seismological Branch of the
Russian Geophysical Survey in Magadan, as well as their station in Seymchan,
and their help with logistics and permits. Lothar Laake (mechanical workshop, Geoscience Centre Göttingen) manufactured the OSL samplers. Most of the figures in this paper were
processed and prepared using GMT v5.4 .
This open-access publication was funded by the University of
Göttingen.
Edited by: Bernhard Grasemann
Reviewed by: Valery Imaev and one anonymous referee
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