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determine the magnitudes of such earthquakes Delouis et al., 2009; Lomax and Michelini, 2009, which is relatively long enough if it is applied for a tsunami early warning system.
Another method to determine earthquake magnitudes can be obtained from a moment tensor, which is considered as the most accurate method of determining magnitudes for major
earthquakes. This includes the GLOBAL Centroid Moment Tensor CMT, commonly known as
�
W CMT
, also providing the origin time, epicentre and depth of the corresponding event. The GLOBAL CMT remains the most powerful, accurate method to date Dziewonski et al.,
1981; Ekström, 1994, where procedures for running the method are based on measurements of a long period of seismic waves S and the recorded surface waves Kawakatsu, 1995. The
research question is that how significantly different the common parameters: magnitude, origin time, epicentre and depth issued by the Ina-TEWS and the GLOBAL CMT are.
2. Materials and Methods
Data sets used in this study were all earthquake events that occured in Indonesia during years 2012-2015 available at
https:inatews.bmkg.go.id with permission from the Puslitbang
BMKG authority
for use
of this
study and
available at
http:www.globalcmt.orgCMTsearch.html . From the data available, we analysed and
estimated the magnitudes, origin times and depths of all the events using one of Open Source Physics
OSP Tools,
that is,
DataTool program
available for
free at
http:www.opensourcephysics.orgwebdocsTools.cfm?t=Datatool. This program is a data
analysis tool for plotting and fitting data from data sets organised into columns to make easy for visual graphing. We also made use of the Generic Mapping Tools GMT, which is freely
available at http:gmt.soest.hawaii.edu
for manipulating geographic and Cartesian data sets in plotting a map of epicentres.
3. Results and Discussions
Figure 1. Data comparison of magnitude measurements between BMKG red full-circles and the reference straight line provided by the GLOBAL CMT catalog, where earthquake events with the same
magnitudes measured by both institutions are marked as black squares lying on the line.
We compared common earthquake parameters used by BMKG and the GLOBAL CMT: magnitudes, origin times, depths and epicenters of all earthquakes occured in Indonesia
between 2012 and 2015. The results for all parameters are provided here in consecutively
Proceedings of MatricesFor IITTEP – ICoMaNSEd 2015
ISBN: 978-602-74204-0-3
Physics Page 221
separate graphs for each. Figure 1 describes distribution of eartquake magnitudes for all events issued by BMKG marked as red full-circles in majority and several black squares as
they are lying on the black straight line, which is defined as a reference for magnitude measurements given by the GLOBAL CMT catalog. With the majority of all cases with
various magnitudes ranging from 4.8 to 7.1 positioning outside the reference line, we conclude that the measurements of magnitudes performed by the two authorities do not agree
well one to another.
Figure 2 shows origin time data estimated by BMKG in red dots spreading, again, large areas outside a series of other data in black forming a straight line as references provided by the
GLOBAL CMT catalog.
Figure 2. Data comparison of origin time measurements between BMKG red full-circles and the data series provided by the GLOBAL CMT catalog, where earthquake events that occurred at the same times
observed by both institutions are marked as black squares in the series.
As with magnitude measurements, a large number of red dots are distributed away from the series-formed straight line, and thus the origin time measurements by both institutions are
inarguably different. The estimates of origin time in many cases are expected as accurate as possible for a tsunami early warning to reduce disaster risks to minimum. Considering this,
the data of origin time for all cases during a period of 4 years from 2012 to 2015 provided by BMKG are not reliable ones.
Figure 3 demonstrates another significant difference in depth measurements between BMKG and the GLOBAL CMT catalog. A large deviation of measured depths issued by BMKG from
reference depths given by the GLOBAL CMT catalog is observed particularly in cases where the depths are less than 100 km. This is not beneficial for a tsunami alert as the Ina-TEWS
releases a warning for events occurring at a depth of less than 100 km. Thus, the depth estimates performed by the Ina-TEWS are also questionable.
Proceedings of MatricesFor IITTEP – ICoMaNSEd 2015
ISBN: 978-602-74204-0-3
Physics Page 222
Figure 3. Data comparison of depth measurements between BMKG red full-circles and the reference line provided by the GLOBAL CMT catalog, where earthquake events with the same depths measured by
both institutions are marked as black squares in the line.
Figure 4. A map of places of interest in this study, showing distribution of epicentres from all cases predicted by BMKG using Ina-TEWS for automatic processing when an event occurs.
Figure 5. A map of places of interest in this study, showing distribution of epicentres from all cases provided by the GLOBAL CMT catalog.
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Among the common parameters, data of epicentres given by BMKG for all events during 2012-2015 are consistent with those provided by the GLOBAL CMT catalog. This is clearly
seen in two figures for comparison. Figure 4 shows distribution of epicentres, spanning from the west in northern Sumatra that includes Aceh and westcoast of Sumatra to the east in
eastern provinces that include northern parts of Sulawesi, Maluku and Papua, with each location is marked as red dots, which is very much the same pattern given by the GLOBAL
CMT catalog marked as green dots in Figure 5. One plausible reason for the coincidence of the above data given by both institutions is that earthquake epicentres are determined by
e
ither seismic stations or satellites using measurements at the Earth’s surface with minimum disturbed noises relative to measurements of other parameters, such as magnitude, origin time
and depth. Keep this in mind, with only one parameter agrees well with the GLOBAL CMT catalog we cannot rely on earthquake analyses using the common parameters usually used by
BMKG. For better prediction of potentially tsunamigenic events at various depths and magnitudes as well as better estimates of origin times and epicentres, we need to examine and
propose new tsunami parameters. As pointed out by Madlazim 2011; 2013, the need for rapid information of high accuracy is a must for appropriate tsunami assessment in order to
minimise false warnings that may lead to fatal decisions concerning with hazard mitigation.
The weakness of the existing methods in determining earthquake magnitudes accurrately and quickly indicates that two main factors required for a tsunami alert triggered by a major
earthquake below the sea level are accuracy and rapidness of information. These shortcomings must be then overcome to assess the magnitude of a destructive tsunamigenic
earthquake accurately and quickly as they are key solutions to the problem of a reliable tsunami early warning system Katsumata et al., 2013. If this is achieved, then disaster risk
reduction program run in the country particularly victims due to disastrous events, such as earthquakes and tsunamis, can be reduced to a minimum.
4. Conclusions
