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PREPARED
SIX MONTH REPORT, WORK UP TO 31 JULY 2003
W2.2 Possible Precursory Seismic quiescence and b-value Changes.
Max Wyss
World Agency of Planetary Monitoring and Earthquake Risk Reduction, Route de
Malagnou 36A; 1208 Geneva, Switzerland, tel.: +41(0)79 749 4894,
[email protected].
Summary
So far, we have received the earthquake catalog up to the end of 1995 for
analysis. Thus, the only work that could be done is the analysis of the spatial and
temporal homogeneity with which the earthquakes were reported during this period. The
earthquake catalog before 1991.5 is far inferior to after it and had to be excluded from
analysis. It was found that within the seismograph network in southern Iceland that
existed since 1991, the reporting of earthquakes was homogeneous throughout (1991.5 to
1996.0), and complete at the level of 0.5, on both magnitude scales used (Mw and ML).
The coordinates of the polygon within which this was the case are the following.
-19.80
-19.80
-21.43
-21.43
63.62
64.30
64.30
63.62
Outside of this area, but also in southern Iceland, a strong change of reporting took place
in 1994 that we interpret as a magnitude shift of about -0.4 units. This change was not
associated with a change of the average b-value, but it mimicked a decrease of the
minimum magnitude of complete reporting. It follows that seismicity patterns can be
studied, using either magnitude scale, within the coordinates given above, and for the
entire period 1991.5 to 1996.0. Outside of this polygon, the data before 1994.5 cannot be
used together with the data after this date, unless the magnitudes are corrected.
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Heterogeneity of Reporting in the Earthquake Catalog
All earthquake catalogs that cover decades and a whole country are not
completely homogeneous in their reporting of events (e.g. [Habermann, 1986;
Habermann, 1987]). As seismograph networks develop, changes in the extent of
coverage and also in the magnitude scale occur commonly. Therefore, one has to
quantitatively define the extent of the catalog as a function of time, space and magnitude
band that is homogeneous, if one wants to study seismicity patterns. We have developed
methods for this purpose [e.g. Wyss, 1991], which we will apply to the Iceland data.
The minimum magnitude of complete reporting, Mc, is one of the parameters
whose behavior we have to understand in detail to define the part of the data set one can
use [e.g. Wiemer and Wyss, 2002]. Mc is measured as the smallest magnitude for which
the frequency-magnitude distribution, FMD, (equation 1) is valid.
N= a –b M
(1)
In equation (1), N is the cumulative number of events, M the magnitude, and a, as well as
b, are constants. Because b is inversely proportional to the mean magnitude in a sample,
changes of b-value contain information on differences in the physics of the process that
generates earthquakes. Therefore, mapping of the b-value on a small scale can reveal
differences in tectonic processes, but sometimes the interpretation of the cause of the
differences is not unique. We also need to verify, whether or not the magnitude scale
may have changed in time, before we interpret any observed b-value differences as due to
natural causes.
The Local Magnitude Scale, ML
It is an advantage for analyzes of seismicity patterns that two magnitudes are
reported in Iceland. The reporting properties of the two magnitude scales were separately
analyzed.
The map of south-western Iceland (Figure 1) shows the epicenters in the catalog
for 1991.5 through 1996.0, together with the locations of the seismograph stations. The
ones that had operated since 1991 and 1992 are shown as black squares. Two stations
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were added in 1993 (grey squares) and two more at the end of 1993 and in the middle of
1994 (triangles).
Fig. 1: Map showing epicenters for 1991.5-1996.0 (dots) and seismograph
stations. Those that operated since 1991 and 1992 are marked by solid black squares,
those since 1993 and 1994 by triangles. The polygon marks the area within which
Mc=0.5 and the reporting is homogeneous.
A
B
Fig. 2: Cumulative numbers of earthquakes as a function of time (A) within the
area covered by a seismograph network since 1991 (polygon in Figure1), and (B) outside
of it.
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The seismicity rate within the seismograph network was marked by strong,
swarm-like increases with time, whereas that outside show an approximately constant
rate, on average (Figures 2 A and B, respectively). After some investigation, it became
clear that in mid 1994 a change took place in the reporting. This was the time of strong
rate increase within the network, and had therefore attracted attention for checking the
nature of that change. However, it was found that an artificial change occurred not
within, but outside the network where the seismicity rate had not changed.
The Mc within the network remained approximately constant throughout the
reporting period (Figure 3A). However, outside the network, especially to the north of it,
a decrease in Mc occurred in mid 1994 (Figure 3B). The simplest interpretation of this
observation is to assume that the addition of the station to the north in 1994 caused this
change. This is probably in part correct. However, decreases in all parts of the outside
area (outside of the polygon in Figure 1) cannot be explained in this simple way because
volumes far from the added station could not have been strongly affected by it.
A
B
Fig. 3: Average Mc as a function of time inside (A) and north (B) of the polygon
in Figure 1. The number of events averaged was 100 and 150, respectively.
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The chief clue for what happened to the reporting in mid 1994 comes from the
magnitude signatures [Habermann, 1983] of the inside and outside areas. Within the
seismograph network, the annual rate was much larger in all magnitude bands in the
second period, but the distribution in small and large magnitude bands remained the same
(Figure 4A).
A
B
Fig. 4: Magnitude signatures comparing the seismicity before 1994.0 (circles) to
that after 1995.0 (crosses), (A) within the polygon in Figure 1, and (B) outside that
polygon. Plotted is the annual rate of events as a function of magnitude band.
Outside the seismograph network, the annual rate remained about the same but a
clear shift in magnitudes occurred (Figure 4B). In all magnitude bands above M=1.1,
fewer events were reported, below M=1.1 more events were reported, also in all bands.
The total number of more earthquakes in the small categories equaled approximately the
numbers lacking in the larger ones, such the over-all rate remained the same (Figure 2B).
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This pattern is the classical effect of a magnitude shift, brought about unintentionally
[e.g. Wyss, 1991; Wyss and Toya, 2000].
The difference of reporting within and outside the long-term network becomes
also clear from maps of minimum magnitude of complete reporting (Figure 5A). Within
the network (black rectangles in Figure 5), the reporting is of high quality at Mc=0.5.
Fig. 5: Map of the magnitude of complete reporting for the period 1991.5 to
1996.0. For the central part of the figure Mc remained constant for that period, but for
the peripheral parts the Mc shown is an average between pre- and post 1994 values
(Figure 3). (A) Local magnitudes, (B) moment magnitudes. Black squares mark station
operating since 1991, green square in the west marks the station introduced in 1992.
(This Figure should be viewed in color).
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Outside the network, Mc rapidly increases to 1<Mc<1.5, except in the western
area (-22 to -21.5 degrees longitude) were Mc is intermediate, which was probably due to
the relatively early installation of the station marked as a green square in that area
(Figures 1 and 5).
The Moment Magnitude Scale, Mw
The results analyzing the moment magnitude catalog closely paralleled those
reported above for the ML-catalog. As an example, Figure 5B shows the same pattern of
Mc as a function of space for both catalogs.
Conclusions
The following conclusions about the quality of the earthquake catalog for southwestern Iceland covering the years 1991.5 to 1996.0 are reached.
(1) The catalog is complete at a low level, Mc=0.5, within the seismograph
network that was in place from 1991 on (area defined by the polygon given in the
summary).
(2) Outside the long-term network, the catalog is complete only at the Mc>1 level.
(3) Inside the network, the magnitude scale shows no signs of artificial changes.
(4) Outside the long-term network, the magnitude scale was shifted by
approximately -0.4 units in mid 1994.
(5) The catalog inside the long-term network is well suited for seismicity pattern
analysis, especially for magnitudes above 0.4. Outside of the network, the catalog cannot
be used for studies of seismicity patterns, unless the magnitudes are corrected.
Acknowledgments
We thank S. Wiemer for the use of his software ZMAP [Wiemer, 2001].
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References
Habermann, R.E., Teleseismic detection in the Aleutian Island arc, J. Geophys. Res., 88,
5056-5064, 1983.
Habermann, R.E., A test of two techniques for recognizing systematic errors in
magnitude estimates using data from Parkfield, California, Bull. Seism. Soc. Am.,
76, 1660-1667, 1986.
Habermann, R.E., Man-made changes of Seismicity rates, Bull. Seism. Soc. Am., 77, 141159, 1987.
Wiemer, S., A software package to analyze seismicity: ZMAP, Seismological Research
Letters, 373-382, 2001.
Wiemer, S., and M. Wyss, Spatial and temporal variability of the b-value in seismogenic
volumes: An overview, Advances in Geophysics, 45, 259-302, 2002.
Wyss, M., Reporting history of the central Aleutians Seismograph network and the
quiescence preceding the 1986 Andreanof Island earthquake, Bulletin of the
Seismological Society of America, 81, 1231-1254, 1991.
Wyss, M., and Y. Toya, Is the Background Seismicity Produced at a Stationary
Poissonian Rate?, Bulletin of the Seismological Society of America, 90, 11741187, 2000.
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