F. Courboulex et al., Rupture complexity of a moderate intraplate earthquake in the Alps: the 1996 M5 Epagny-Annecy earthquake, GEOPHYS J I, 139(1), 1999, pp. 152-160
The magnitude 5 Epagny-Annecy earthquake of 1996 July 15 is the largest sei
smic event to have occurred in the Alps since the introduction of modern di
gital instrumentation. This strike-slip event was located on the Vuache Fau
lt, near the town of Annecy, in the northern French Alps.
The aim of our work was to retrieve the main parameters of the rupture proc
ess of this earthquake from seismograms recorded at local and regional dist
ances (20-300 km). To eliminate path and site effects from the seismograms,
we compared the main shock recordings at each station with those of the la
rgest aftershocks nearby. We used a combination of techniques, including pu
lse-width measurements and cross-correlation of velocity traces, comparison
of P-wave displacement pulses, and empirical Green's function deconvolutio
n, to retrieve the apparent duration of the rupture process as seen at each
station. Our results demonstrate that, in the absence of on-scale data, P-
wave pulse-width measurements on clipped signals can be misleading if the r
upture process is complex. In the case of the Annecy earthquake, comparison
s of on-scale P-wave displacement seismograms and the empirical Green's fun
ction deconvolutions show that the rupture process consisted of at least tw
o subevents separated by 0.2-0.3 s, and with a total duration of about 0.5
s. The systematic azimuthal dependence of both the shape and duration of th
e apparent source-time function is consistent with a nearly unilateral prop
agation of the main rupture phase in a southeast direction along the fault
plane and parallel to the direction of slip. An isochron analysis reveals t
hat the first subevent occurred slightly to the northwest of the nucleation
point but that the second subevent was located further to the southeast, t
hus confirming the overall rupture directivity towards the southeast. An in
terpretation of our results in light of the previously documented aftershoc
k distribution and of observations of ground cracks in the epicentral area
suggests that the main shock occurred on the Vuache Fault, and that rupture
in a northwest direction was inhibited by a right-lateral stepover in the
fault. Accordingly, the vast majority of the subsequent aftershocks, which
include several magnitude 3-4 events, occurred on a fault segment that is s
lightly offset from the inferred surface trace of the Vuache Fault and that
was activated by the main shock.