Archaeoseismology and Palaeoseismology in the Alpine ... - Tierra

Fault system within Central segment is presented by
sequences of shear, extension, and thrust structures
composing zone of left‐lateral strike‐slip, in which vertical
offsets could be found fragmentary (Solonenko et al.,
1966). There are contradictory interpretations of vertical
offset character. Amplitude of vertical offset does not
exceed 3.3 m, of horizontal slip – 1 m (Kurushin et al.,
2007).
Eastern segment is located at the pediment of Udokan
Range and is composed by extension structures (grabens).
Observed steep dipping of the fault to the North is
evidence of normal faulting (Kurushin et al., 2007).
Amplitude of the offset varies along the fault from 0 to
1.5 m. There are no traces of horizontal slip at this fault
segment (Kurushin, 1963; Solonenko et al., 1966).
Total length of definitely proved surface faulting is ca. 20
km. But in some publications pointed out that Eastern
segment might be extended for another 5 km (e.g.
Solonanko et al., 1985).
Macroseismic information is summarized in Fig. 6, based
on questionnaires collected in the Institute of the Physics
of the Earth, RAS in 1958 and data from (Solonenko et al.,
1958). Though the data spatial coverage is unfavourable
for compilation of a reliable isoseismal map (unpopulated
epicentral area and eastern part) we can draw a very
large felt area (over 700 km from surface faulting zone).
DISCUSSION
Fig. 6: Isoseismal map of the Muya earthquake.
Epicentral intensity (Io) of the Muya earthquake can be
assessed based on the total length of the surface faulting.
Its well‐documented length is 20 km plus 5 km of doubtful
piece. According to the ESI2007 scale (Michetti et al.,
2007) epicentral intensity X corresponds to 20‐25 km total
rupture length. Maximum vertical offset is 3.3 m. For
normal faulting this value is also in agreement with
intensity X 1 . Therefore, both assessments give consistent
result, which from one hand proves existence of
correlation between SRL and offset along it, from the
other hand is evidence of accurate evaluation of Io being
1 To avoid confusion Roman letters are used for ESI2007 scale
and Arabic numbers are for macroseismic intensities
1 st INQUA‐IGCP‐567 International Workshop on Earthquake Archaeology and Palaeoseismology
158
X degrees. Nearest to the surface faulting zone localities
are at 50 km distance from it. This large distance can
explain why maximum observed macroseismic intensity is
much lower than Io and does not exceed 8 degrees.
Absence of localities in the epicentral area explains also
the anomalously large size of 7 degree isoseismal
compared to lower value isoseismals: certainly within the
isoseismal 7 has to be higher intensities. If for epicentral
intensity assessment of the Muya earthquake we
considered only macroseismic effects in localities, as it is
suggested by EMS98 (European Macroseismic Scale,
1998), Io would be underestimated at least by two
degrees.
Epicenter instrumental location differs more than 100 km
according to different seismological agencies and
publications. Closest locations to the surface faulting zone
are solutions by Vvedenskaya and Balakina (1960) and by
Balakina et al. (1960) (Fig. 5a). But it is very possible that,
deriving it they used also position of surface faulting,
therefore, it is not completely instrumental. Hypocentre
depth given by (Kondorskaya and Shebalin, 1977) is 15
km, by Doser (1991) – 10 km; the deepest solution (22
km) gives Balakina et al. (1972). Macroseismic data is
consistent with 20 km depth. First of all, this depth
together with M=7.6 is in agreement with 700 km of felt
radius. Second, isometric form of isoseismals (Fig. 6) is
characteristic to relatively deep source. Relatively short
surface faulting zone (20‐25 km) also agrees with deeper
source. For comparison, we could mention two
earthquakes with M=7.4: the Neftegorsk, 1995, which
accompanied by 46 km of SRL and Altai, 2003, ‐ by 70 km
of SRL. According to Wells and Coppersmith (1994) 20‐25
km of SRL corresponds to M=6.6, which is 1 unit less, than
the magnitude of Muya earthquake. It is clear, that only
small part of the rupture in the source of Muya
earthquake was exposed on surface. Possibly it is
characteristic to the regional seismicity. This hypothesis
can explain why the reported PSS lengths are < 45 km (see
Fig. 2).
First motion mechanism (Vvedenskaya, Balakina, 1960)
corresponds to normal faulting with left‐lateral strike‐slip
component along a WNW plane, and steeply dipping to
the South (Fig. 5a). In general, this solution is consistent
with the geometry of surface faults and kinematics of
faulting. But body wave‐form inversion at teleseismic
distances gives different solution (Doser, 1991): three
sub‐events are recognized, each almost pure strike‐slip
with negligible normal fault component. Data quality does
not permit spatial resolution of sub‐events. Solution with
sub‐events is in much better accordance with observed
segmentation of surface fault zone, but is in contradiction
with the fact, that vertical offset everywhere larger than
horizontal slip. A possible reason might be the following.
Under extension stresses, strike‐slip movement along en
echelon sub‐parallel faults, which are not co‐axial,
requires normal faulting at surface to accommodate shear
deformation. According to this hypothesis, normal fault
component is not representative for the source in general
and its amplitude can be anomalously large, compared to
the surface fault length (as it was noted in Fig. 2).