Earthquake depth distribution in western Bohemian Massif used as a constraint on the
local upper crustal rheological model
Petr Špaček
Institute of Physics of the Earth, Brno, Czech Republic and Geophysical Institute, Academy of
Science, Prague, Czech Republic
It is generally assumed that earthquakes can occur in such lithospheric domains where rocks
are deformed in a brittle fashion. Thus, considering the far-field-stress-controlled deformation
of intraplate regions we do not expect frictional events to occur beneath the brittle-plastic
transition of rocks (BPT) which deform at intraplate strain rates. Theoretically, using this
premise we could test the relevance of rheological parameters in seismically active regions,
provided that we would know all the other variables necessary for the solution of strength
equations. In practice, we always miss some information and we are forced to make estimates.
Nevertheless, there are regions for which such estimates can be made with smaller error than
for the most part.
Relatively good state of the art in geophysics and geology of the West Bohemian earthquake
swarm area allows for such attempt. In this region several thousands of earthquakes with local
magnitudes ML= -1.0 to 3.1 have been registered in last decade. The database of locations
from KRASNET seismological network (IPE Brno) represents a satisfactory collection for
statistical analysis of earthquake depth distribution. In the past the detailed measurements of
gravity field were carried out in this area. Both the DEKORP and 9HR reflection seismic
profiles and the CELEBRATION refraction profile run through here, giving constraints on
upper crustal velocity structure and composition. The KTB borehole is located only 50 km
away providing us with geothermal data from the depth of 9 km.
The focal depth distribution analysis in 13 sub-areas of the seismic region shows that
although the peak frequencies of the hypocenters are rather scattered, in 8 of them the
maximum focal depths lie between 12 and 13.5 km and in 3 sub-areas the maximum depths
are even larger. The maximum depths of earthquakes seem to correlate roughly with gravity
field (the higher the gravity, the higher the maximum focal depths). This supports the
expected dependence of crustal seismicity on its composition.
The rheological modelling is focused on the areas with the most pronounced negative
gravity anomalies, which belong to the highest negative anomalies within the whole
Bohemian Massif. Here, the typical focal depths of 9-10 km and maximum depths of 12.5 km
are characteristic. The low gravity, low elastic waves velocity and geology indicate quartzofeldspathic composition (modelled density ρ ≈ 2.61-2.72 g/cm3, Vp ≈ 5.95-6.15 at 9-12 km,
Švancara et al. 2000 and Hrubcová in prep.).
The thermal gradient 27-30°C/km is deduced from regional heat flow (80 mW/m2) and KTB
measurements. This corresponds to 243-270°C at depth of 9 km (Šafanda and Čermák 2000,
Clauser et al. 1997).
The strain rate estimate is always a problematic issue but the measurements of European
VLBI network indicate that the Central and North European horizontal strain rates outside the
Alps do not exceed 1.5-6×10-17 s-1 (Ward 1994, Tomasi et al. 1999). Given the intraplate
position of the region this seems to be reasonable (comp. Zoback and Townend 2001,
Anderson 1986).
Focal mechanisms indicate prevailing horizontal shearing and maximum principal stress
direction dipping at 0-30° to the SE (e.g. Vavryčuk 2002, Brudy et al. 1997). In the
rheological model we take maximum principal stress as being horizontal. A strike-slip on pre-
existing faults is assumed with frictional coefficient µ = 0.4 - 0.8 and average density of
overburden rocks ρ = 2.65 g/cm3. Pore-fluid pressure is taken as an unknown variable.
Considering the quartzo-feldspathic composition in the focal area and presuming that the
power-law creep controls the rheology of the plastic domain, we use appropriate rheological
parameters for wet granite and quartzite which are available in literature (Kirby 1983, Hansen
and Carter 1983). For each of these the highest and the lowest permissible temperature
gradients and strain rates are used (see above). None of these models is apt to locate the BPT
at or below the earthquake foci with the condition of near-hydrostatic pore-fluid pressure. The
rheology of wet quartz diorite (Carter and Tsenn 1987) permits the brittle behaviour at depth
of 10-10.5 km when assuming the highest strain rate and the lowest temperature. But the
rocks with such rheology usually contain significant amount of dark minerals and their
presence would be in contradiction with the geophysical data. There are two interpretations at
hand:
1) When we assume that the rheological parameters are valid and applicable, the earthquakes
must be induced by brittle deformation under low differential stresses. This can only be
explained with the existence of near-lithostatic pore-fluid pressures (λ ≈ 0.95).
2) The experimental data might not apply to natural conditions. In polyphase rocks the
brittle behaviour of rocks can not be excluded down to the depths where the plasticity of the
strongest mineral phase sets on (comp. Scholz 1988).
References:
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Brudy M. et al. (1997): Estimation of the complete stress tensor to 8 km depth in the KTB
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