W PHASE ANALYSIS FOR TSUNAMI WARNING
Tri Handayani∗
MEE08175

Supervisor: Kenji Satake∗∗
ABSTRACT

We applied W phase inversion for the large (M>7) Indonesian earthquakes that occurred in the periodof 2004-2009 by using global data obtained from IRIS DMC. The W phase inversion was made for the
PDE location, optimum centroid location and the GCMT location. These three locations gave similar
inversion results; both moment magnitude and focal mechanism were generally in very good
agreement with the GCMT solutions. Comparison of moment magnitudes Mw indicates that seismic
moments from the GCMT and W phase inversions are larger than that from body waves for three
events that caused damaging tsunamis (the 2004 Aceh, 2005 Nias and 2006 Java earthquakes).
Therefore it is necessary to perform analysis of long-period seismic waves to accurately evaluate thetsunami potential. Tsunami simulation was performed for the 2006 Java, 2007 Bengkulu (mainshock),2008 Sulawesi and 2009 Papua (mainshock) events for the three different centroid locations and sea
floor deformations based on the W phase inversion results. The simulation shows that these threemodels yields similar tsunami arrival times and heights on the coasts where the tsunami arrival times
are more than 30 minutes. Based on the W phase inversion and tsunami simulation results we suggestthat the PDE location yields good enough solution so that it can be used in real time situation for thewarning purpose. For coastal regions near the earthquake source with tsunami arrives less than 30
minutes, W phase analysis cannot be used for the first tsunami warning. Because W phase analysis
provides long-period information on the earthquake source and can be performed faster than the
GCMT inversion, it can be used to update the tsunami warning at BMKG (Agency for Meteorology
Climatology and Geophysics).
Keywords: W phase inversion, Indonesia, Large earthquakes, Tsunami warning.
1. INTRODUCTION
Agency for Meteorology Climatology and Geophysics (BMKG) Indonesia has been assigned the tasks
to produce the tsunami messages during the first five minutes of earthquakes. The main information oftsunami warning is in estimating the tsunami potential of an earthquake from the seismic parameterssuch as hypocenter and magnitude. Based on BMKG experiences, however the tsunami warning based
only on hypocenter and magnitude sometimes disagrees with observation, producing false alarms or
missing alarms. Tsunami potential of an earthquake can be underestimated for very large event (suchas the 2004 Aceh) and tsunami earthquake (such as the 2006 Java). The objective of this study is toperform rapid determination of seismic source parameters with sufficient accuracy for the issuance oftsunami warning. For this aim, we perform W phase inversion and tsunami simulation.
2. DATAWe used 1 sample-per-second data from vertical component of broadband records (LHZ channel) for
station within 90o taken from the database at the IRIS Data Management System(http://www.iris.washington.edu/dms/wilber.html). For event analysis, we selected 10 Indonesian
events with large magnitude (M>7) occurred in the period 2004-2009. We downloaded data from
Global CMT catalog (http://www.globalcmt.org) at the same period to compare with our inversion
results (Fig.1).
∗

Agency for Meteorology Climatology and Geophysics (BMKG), Indonesia.
Professor, Earthquake Research Institute (ERI), University of Tokyo, Japan.

∗∗

1Figure 1. Focal mechanism of events analyzed.
3. METHOD
3.1. W Phase Inversion
W phase is a distinct long period phase, arriving right after the P phase and carries long period
information of the source at a much faster speed than surface waves. It has the group velocity ranging
from 4.5 to 9 km/s with period ranging from 100 s to 1000 s (Kanamori, 1993). This study is based on
the existing study done by Kanamori and Rivera (2008). We adopted their W phase inversion
technique and followed the procedure given by them. We deconvolved the vertical component of
global broadband seismograms to the acceleration time series then band-pass filtering them in the time
domain followed by twice integration of acceleration time series to get displacement. Fig. 2 shows anexample of the vertical displacement after deconvolution and band-pass filter (0.001-0.005Hz) for the2009 Papua (mainshock) Mw 7.6 at the several stations ranging from distance 13o until 70o. We used atime window of W phase with duration of 15∆ sec (∆ is epicentral distance in degrees) after P arrival.
In W phase inversion, we assume a spatial point source (the centroid location) which isvaried in time with a given time history. If the centroid location and the source time history are known,
the inversion is linear with respect to the moment tensor element. We use a triangular source function
to represent the moment rate function, defined by two parameters half duration th (the half width of thetriangular moment rate function) and the centroid delay td (the temporal position of the centre of thetriangle measured from the assumed origin time). We made the W phase inversion with three different
locations. First, we fixed centroid location at PDE location then we estimate the best time delay as the
optimized td by a simple 1-D grid search minimizing the root mean square (RMS) of the waveform
misfit. Secondly, we estimate the optimum Centroid location by spatial grid search using the above td.
Thirdly, we use the Global CMT
parameters (GCMT location,
GCMT td and th). We set th=td forthe inversion of PDE locationand optimum Centroid location.
We used band-pass filter 0.001 to
0.005 Hz for Mw>=7.5 and 0.002
to 0.006 Hz for Mw<7.5. We
concatenated them in order of
distance then we compute thesynthetics for a unit source foreach station using the Green’s
functions by convolving themwith a triangular source timefunction.
Figure 2. Vertical displacement of the 2009 Papua;
as the result of retrieval the W phase.

23.2. Tsunami Simulation
In this study we use TUNAMI-N2 code of tsunami numerical model. It is for investigating near fieldtsunami based on tsunami numerical simulation with the staggered leap-frog scheme, and wasdeveloped by Dr. Fumihiko Imamura et al at Tohoku University. TUNAMI-N2 program uses thebathymetry of the area as input data. In this study we use bathymetry data of GEBCO (General
Bathymetric Chart of the Oceans) which are digitized from nautical charts and gridded with an intervalof one minute; http://www.gebco.net/data_and_products/gridded_bathymetry_data/. For initialcondition of tsunami simulation, we need to provide fault parameters to compute sea floor deformation.
The fault parameters need to be computed are: the location of the fault coordinates (latitude, longitude
and depth), the fault length (L), the width (W), the strike angle ( φ ), the dip angle ( δ ), the rake angle( λ ), slip amount ( u ) (Okada, 1985). In this study, we compute fault parameters based on W phase
inversion results. We assumed that centroid locations (latitude, longitude, depth) of the W phase
inversion are located at the fault centre hence we needed to calculate the coordinates of the left cornerof fault as the input in TUNAMI-N2.
4. RESULTS AND DISCUSSION
4.1. W Phase Inversion
W phase inversion results can provide the source mechanism and seismic moment of the earthquake. In
this study, we analyzed 10 events for the W phase
inversion and compared with the result from Global
CMT solutions. Here, we show the results of W
phase inversion for the three events (Fig. 3):
the 2005 Nias, 2006 Java and 2007 Molucca
earthquakes. The overall focal mechanism andmoment magnitude Mw were very similar for thethree kinds of locations: PDE location, optimized
centroid location and GCMT location. These threelocations gave similar inversion results; both
moment magnitude and focal mechanism weregenerallyin very good agreement with the
GCMT solutions. Fig. 4 compares the concatenatedobserved (black traces) and synthetics (red traces) W
phases for the 2005 Nias earthquake. The match
between the observed and synthetic of W phases isvery good; error misfit RMS is 0.241.
Figure 3. Example of W phase inversion results(focal mechanism and Mw).

Figure 4. Example of concatenated W phase: observed (black traces) and synthetic (red traces).
Synthetic traces are almost overlapped on the observed traces.

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Fig. 5 shows comparison of Mw estimated from our W phase inversion with GCMT solutions. They arevery similar with difference 0.1 or smaller, except for the 2004 Aceh earthquake. This indicates that
inversion of W phase yields results which are very similar to the GCMT solutions although only earlier
parts of seismograms (before surface wave arrival) are used while GCMT uses both body and surface
waves. The difference for the 2004 Aceh event between our result and the GCMT solution might be due
to either difference of power of very long-period component (> 200 sec) or inappropriate CMT locationof the GCMT inversion.
9.5

9.5

Damaging
Tsunami9.0

Mw (W phase)

Mw (W phase)

9.0

2005

8.5

8.0

8.0

7.5

7.0

7.0

8.0

2005

8.5

7.5

7.5

2004

2006

2006

7.0

Damaging
Tsunami

2004

8.5

9.0

7.0

9.5

7.5

8.0

8.5

9.0

9.5

Mw (GCMT)

Mw (USGS, body wave)

Figure 5. Comparison of
Mw (W phase) and Mw (GCMT).

Figure 6. Comparison of
Mw (W phase) and Mw (USGS, body wave).

Fig. 6 shows the comparison of Mw (W phase) versus Mw (USGS). Mw (USGS) is
determined by using body wave (period about 25 sec) and Mw (GCMT) is determined by using bodyand surface waves (period 50-150 sec), while Mw (W phase) is determined by using W phase which isvery long period wave (period 200-1000 sec). From Fig. 6 we can see that most of the events scatteraround the line y = x , except for the 2004 Aceh, 2005 Nias and 2006 Java earthquakes; their Mw (Wphase) are larger than Mw (USGS). Because these three events generated damaging tsunamis,
estimation of Mw based on long-period waves is essential to estimate the tsunami potential. The
GCMT analysis takes longer time as it includes both body and surface waves. The W phase inversion,on the other hand, does not include surface wave and can be performed with earlier (< 20 min) parts ofseismograms recorded globally. It is therefore useful to examine the tsunami potential.
4.2. Tsunami Simulation
We performed numerical simulation of four tsunami events(the 2006 Java earthquake, the 2007 Bengkulu mainshock, the2008 Sulawesi earthquake and the 2009 Papua mainshock)using W phase inversion results as the initial conditions offault parameters and sea floor deformation. Here we show one
example of tsunami simulation results for the 2007 Bengkulu
mainshock. Based on W phase inversion results we obtained
source parameters (focal mechanism and fault parameters areshown in Fig. 7). Then we used those parameters to performtsunami simulation for three models of fault locations; PDE
location, GCMT location and optimum centroid location.
Figure 7. Focal mechanism and fault
models for tsunami simulation.

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Figure 8. Distribution of maximum tsunami heights for simulation of PDE location, GCMT location andoptimum centroid location; asterisk refers to centroid, triangle refers to outpoint at coastal region.
In general, the tsunami simulation results show that the distribution of maximum tsunami wave
heights along the southern Sumatra coast
has a similar trend for three models of fault
locations (PDE Location, GCMT locationand optimum centroid location). The
maximum tsunami heights are distributed atarea in front of tsunami source (Fig. 8).
Large wave height is computed at Bengkulu
by about 1.4 m (Fig. 9) for models of
GCMT and optimum centroid, while in caseof PDE, the highest wave are calculated at
Bengkunat and Manna. In this case, PDE
location is closer to Manna, while thelocation of GCMT and optimum centroid
are closer to Bengkulu. From Fig. 9, we cansee that for case of GCMT, the tsunami
arrives at Bengkulu about 20 minutes earlier
than cases of PDE and optimum centroid.
On the other hand, tsunami arrives at Manna,
10 minutes earlier for the PDE case.
Figure 9. Comparison of tsunami waveform for PDE(green lines), optimum (blue lines) and GCMT (red lines).
We have shown that W phase source inversion yields consistent results with the GCMT
solutions for each centroid location: PDE location, optimized centroid location and GCMT location.
However GCMT is not suitable for warning purpose since it takes more time to determine it in realtime situation. The W phase inversion with PDE parameters can save time, because the hypocentral
location and origin time can be determined from P wave arrivals just after the occurrence ofearthquake. Based on tsunami simulation results we found that each model (PDE, optimum centroid
and GCMT) gives the similar tsunami heights and tsunami arrival times for the coastal area farther
from the source where the tsunami arrives at more than 30 minutes. Using the PDE location in W
phase inversion, within 20-30 minutes we can determine the seismic source with sufficient accuracy.
Following that time we can perform tsunami simulation to predict the tsunami arrival times andtsunami heights at the coastal regions. But for the region nearby the source where tsunami arrives inless than 30 minutes, the available time for warning is too late. The disadvantage of using W phase
inversion is the long time waiting for the data collection (because we need long period wave data
which is can reach within 15-20 minutes), but by using this method we can get the sufficient accuracyof seismic source and the inversion is easy to be applied.

55. CONCLUSIONSThe W phase inversion results show that both moment magnitude and focal mechanism estimatedfrom W phase are generally in very good agreement with the Global CMT solutions. Comparison of
moment magnitudes from body waves, GCMT and W phase show that Mw from GCMT and W phase
inversion are larger than Mw from body waves for the 2004 Aceh, 2005 Nias and 2006 Java
earthquakes. These events caused damaging tsunamis. This indicates that it is necessary to perform
analysis of long-period seismic waves to accurately evaluate the tsunami potential. In general, tsunami
simulation results show that the distribution of maximum tsunami wave heights along the coastal
points has the similar trend for the PDE location, GCMT location and optimum centroid location. The
computed tsunami arrival times and heights are very similar for the three models except for the nearby
points (the area which very close to the source with the tsunami arrival time in less than 30 minutes).
Based on the W phase inversion and tsunami simulation results, we can conclude that the W phase
inversion using PDE location yields good enough solution so that we can use it in real time situation
for the warning purpose.
RECOMMENDATION
The goal of this W phase analysis is to perform a rapid determination of seismic source parameterswith sufficient accuracy which can be used in performing tsunami simulation to get prediction oftsunami heights and tsunami arrival times at the coastal region. Fig. 10 shows the flow chart of timeline for process warning information. The left side shows the current situation at BMKG while theright side shows W phase time frame which is recommended to be applied in BMKG. BMKG has
been assigned the tasks to produce the tsunami messages during the first 5 minutes of earthquake
occurrence. This tsunami message is the information about tsunami potential based on earthquake
location and magnitude which should be determined within 3 minutes after earthquake. In the next 30
minutes, BMKG target is to give the updating. Tsunami potential of earthquake can beunderestimated for very large event (such as the 2004 Aceh) and tsunami earthquake (such as the 2006
Java). Therefore we need to update the magnitude
which can be determined from long period wave (Wphase analysis) with more sufficient accuracy than
only from short period wave. Although for this aim
we need to wait at least 20 minutes in data collection
only, but after that we can do W phase inversion to
get source parameters (focal mechanism and moment
magnitude) which can be used in tsunami simulation
to predict tsunami arrival times and tsunami heights
at the coastal region. Then within 30 minutes we cancontribute in updating warning information for theregion where the tsunami arrival time is more than
30 minutes.
Figure 10. The time line for process warning
information at BMKG.
AKNOWLEDGEMENT
I would like to express my heartfelt gratitude to Prof. Luis Rivera and Prof. Hiroo Kanamori, for thesource code of W phase inversion program.
REFERENCES
Kanamori H., and Rivera L., 2008, Geophysics.J.Int., 175, 222-238.
Kanamori H., 1993, Geophysical Research Letters, Vol.20, No.16, Pages 1691-1694.

Imamura Fumihiko, 2006, DCRC, Tohoku University, Japan.
Okada Y., 1985, Bull. Seism. Soc. Am., 75, 1135-1154
PDE earthquake catalog; USGS, http://neic.usgs.gov/neis/epic/

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