A preliminary report on the
earthquake in Andaman islands region (Mwp: 7.7)
on 11th August, 2009
An earthquake of magnitude, Mwp:7.7 occurred on 11th August, 2009 at 0126
hours IST in north Andaman islands region. The revised preliminary hypo-central
parameters of this earthquake, as estimated by the Real Time Seismic Monitoring
Network (RTSMN) of IMD, are given below:
Date of occurrence : 11/08/2009
Time : 01:26 hours (IST)
Magnitude : 7.7 (Mwp)
Focal Depth : 17 Km.
Epicentre
Latitude & Longitude : 14.1° N & 92.9° E
° °
Region : Andaman Islands
The earthquake information has been disseminated to all concerned user
agencies as per standard operating procedures. The event, which comes under the
category of “Great earthquake” (M>7.0), was reported widely felt in whole of
Andaman islands region. The event clipped the records (S-wave) of the seismic
observatory at Diglipur, which is located about 110 km south of the epicenter. The
records (Surface wave portion) of the Port Blair observatory, which is located about
290 km. south of the epicenter, also got clipped. The event was also reported mildly
felt in parts of Chennai, Bhubaneshwar and Shillong. The HQ office of IMD remained
in constant touch with the in-charge at Port-Blair observatory, who informed that till
0325 IST of 11th August, 2009 no information on any unusual sea-level changes was
received by them. As reported by INCOIS, Hyderabad, the earthquake did not
generate any tsunamis.
Copies of screen shots showing the summary of event location and various
magnitude estimates are enclosed. The event is located using a total of 68 seismic
phases recorded by the Indian and global stations spread uniformly in all azimuths
across the globe (Annexure-1). The Root Mean Square (RMS) error in the
estimation of the location is 1.1 sec, which is well within the accepted error bars for
regional events of this nature. The details of various magnitude estimates including
the Mwp, which is normally used by real time monitoring systems for early warning of
tsunamis, are given in the screen shot at Annexure-2.
The preliminary faulting mechanism, as estimated through Centroid Moment
Tensor (CMT) and Moment Tensor (MT) solutions, indicate predominant normal
faulting for this earthquake (Annexures-3&4). It is known that events of normal
faulting mechanism do not produce significant tsunamis. The location of the event is
about 80 km east of the surface trace of the main Sunda trench, which separates the
Indian and Burmese plates (Annexure-5). The CMT and MT solutions are obtained
from waveform modeling making use of Body and Surface waves respectively. The
process essentially involves in simulating the synthetic seismograms by assuming a
known source, path and instrumental characteristics. These synthetic seismograms
are then matched with the observed ones at various stations in an attempt to arrive
at the characteristics of the source, which produces the best synthetic seismograms
matching with the observed ones.
A few sample seismic depth sections across the Andaman & Nicobar island
arc region based on the past earthquake data in the region are given in Annexure-6.
It may be seen from these depth sections that the events falling on the western side
of the active seismic zone (shown in red color) are mostly of shallow nature and
indicate an average focal depth of less than 25 km in the region (A-A’ profile), where
the subject earthquake has occurred. The events show larger focal depths as one
moves away westwards from the main arc. The centroid moment depth (29.1 km.)
indicated by the CMT solution for the present event matches very well with these
observations. However, it is necessary to carry out a more detailed analysis of the
data for arriving at more precise estimates of focal depth for this event.
***
Annexure-1
Annexure-2
Annexure-3
Annexure-4
Annexure-5
Annexure-6