baizura_jmm

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Outline

Introduction Background Importance of the Study

Theory and Methodology Velocity model for Malay Peninsula Focal mechanism solution using polarities and amplitude ratios Data used for determination of focal mechanism

Results and Discussion with related studies Focal mechanisms of four Bukit Tinggi events Comparison of the P and T axes distributions Observed polarities of the first motion of Pwaves

Conclusion2


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Locations of Earthquakes from 2007-2010

A network of 4 broadband and 3

short period sensor currentlyoperational in Malay Peninsula

(MMD).

A total of 30 weak and felt

earthquakes have been recordedin the Malay Peninsula from 2007

2010 (MMD Database as of

April 2011).

Most of these earthquakes are

located near the Bukit Tinggi faultzone and Kuala Lumpur fault

zone, which classified as inactive

(MMGD, 2008).


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Year Month No. EQEpicenter

(Lat/Long/Location)

Depth

(km)

Magnitude

(mb) MMI

2007

Nov 3 3.3 3.4 N 101-102 E Bukit Tinggi < 7 3.2 3.5 III

Dec 63.3 3.5 N 101-102 E Bukit Tinggi

< 52.6 3.5

III

2008

Jan 43.3 3.4 N 101-102 E Bukit Tinggi < 3 2.4 3.4

III

Mar 3

3.3 3.4 N 101-102 E Bukit Tinggi

Shallow

2.5 3.3

III

May 1 3.4 N 101.8 EBukit Tinggi Shallow

2.6 III

2009

Mar 1 3.9 N 102.5 E Jerantut 50 3.2 III

Apr 1 4.2 N 100.7 E Manjung 22 2.8 III

Oct 63.3 3.4 N 101-102 E Bukit Tinggi

< 10 1 - 2.8 III

Nov 4 2.7 2.8 N 102 E Kuala Pilah< 15 3.0 3.5

2.6 3.2 mlI

Dec 1 3.2 N 102 E Bukit Tinggi 5 2.2 I

2010 No Felt EQ in Malay Peninsula

Local Felt EQs in Malay Peninsula (2007-2010)

4 MMD, as of April 2011


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Distributions of the Local Felt EQs since its

first occurrence on November 30th, 2007

5

0

1

2

3

4

0 90 180 270 360 450 540 630 720 810

Day (s) since its first occurrence

Mantumb


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Importance of the Study

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Although hypocenters and magnitudes of these local

earthquakes are determined by the MMD, their focal

mechanisms have not been determined. Due to an absenceof the surface movement observed, these focal mechanisms

are important in improving our understanding of seismic

pattern and fault behavior in the Bukit Tinggi fault zone.

Since far field seismic is less of a threat, it is suggested to (re)focus on

local earthquake or near field earthquake, esp. in the vicinity of an 80-km

long Bentong fault, close to Bukit Tinggi fault line, where earthquake M3.5

has been recorded

-Jeffrey Chiang, the Chairman of Working Group

in Designing Building Code in Malaysia


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We selected three crust structure models, of which two are from

CRUST2.0 (http://igpweb.ucsd.edu/~gabi/crust2.html) and the other

one is model iasp91 (Kennet and Engdahl, 1991).

We compared the observed travel time differences between P and S

to those computed for these models. We performed the similar

comparison for the P-arrival. We calculated the root mean square

(RMS) for both comparisons using the following equation :

where is the i-th observed data, is the i-th calculated data

and is the total number of data.

Velocity Model for Malay Peninsula


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CRUST 2.0 is a global crustal

model at 2x2 degrees scale.

Composed of 360 key 1d-profilesand one of these is assigned to

each 2x2 degree cell.

Each profile is a 7 layer 1D-model

(ice, water, soft sediments, hardsediments, upper crust, middle

crust and lower crust .

For Malay Peninsula, models J1

and C2 are available fromCRUST2.0 .

http://igppweb.ucsd.edu/~gabi/crust2.html

Source :Crustal Model CRUST2 page

Crustal Profile based on CRUST 2.0


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Depth(k

m)

Velocity (km/s)

Comparison of Velocity Structure between

CRUST2.0 (J1 and C2), and iasp91

Vs Vp


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Observing P and S Onset (IPM)

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Time series of vertical channel (a) and horizontal channel (b), of IPM

station for the BUkit Tinggi event occurred at 02:13 UTC, November30, 2007 with magnitude of 3.5mb. The upper, middle and lower

traces are velocity, squared velocity, and integration of squared

velocity, respectively

(a) (b)


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ObservedS-P(s)

Theoretical S-P (s)

S-P Travel-time Differences

Obs = 6.36

J1 = 6.79C2 = 6.84

Iasp91 = 6.5


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Comparison of P- phase Arrivals

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P-h

asearrival(M

MDPicking)

Theoretical P-arrival


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Comparisons of RMS

among JI, C2 and iasp91

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RMS VELOCITY MODEL

J1 C2 iasp91

S-P travel time all 2.65 2.68 2.57

excluding large

residuals (>3s)

1.44 1.44 1.25

P-phase arrival 1.49 1.01 0.85

The lowest RMS are consistently obtained by model iasp91, which suggests

that model iasp91 predicts the observed travel times better than the modelsfrom CRUST2.0.

This is consistent with the result obtained by Din (2011) in which crustal

thickness beneath station IPM is similar to the thickness of iasp91 (35km).

We used a velocity model from iasp91 for the calculation of take off angles.


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FOCMEC Software Package (Snoke, 1984)

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We used the FOCMEC software package available at http://www.iris.edu/software/downloads/processing/

This software determines focal mechanisms using polarities (P,SV, SH) and amplitude ratios (SV/P, SH/P, SV/SH).

Focmecprogram calculates all acceptable solutions. We can useFocpltfor further analysis and display.

Horizontal components are being rotated into radial andtransverse components to separate SV and SH.

SH to P amplitude ratio provides what could be considered almostindependent data. Polarities and ratios involving SH are likely to be

more reliable than those involving SV.


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Data Used to Determine Focal Mechanism

We requested three-componentbroadband waveform data recorded at

stations of IPM and KOM for four largest

Bukit Tinggi events (mb 3 - 3.5) using

BREQ_FAST (Batch REQuest, FAST),

which provides batch access to the IRIS

DMC archive.

This was done by emailing a formatted

file to [email protected]

YEAR MONTH DAY TIME(UTC)

EPICENTER DEPTH(KM)

MAGNITUDELAT LONG

2007 Nov 30 02:13 3.36N 101.80E 2.3 3.5 mb

2007 Nov 30 12:42 3.31N 101.84E 6.7 3.2 mb

2007 Dec 12 10:01 3.47N 101.79E 10 3.2 mb

2008 Jan 10 15:38 3.39N 101.73E 3.0 3.0 mb


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Rotation of the 02:13 UTC,November 30, 2007

Bukit Tinggi waveform data

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Coordinat

e

rota+on

East-West

North-South

Radial

Transverse

Ver+cal

Ver+cal


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Observing polarities of the 02:13 UTC

November 30, 2007 Bukit Tinggi event

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E-W

N-S

Radial

Transverse

Ver+cal


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Input Parameters for FOCMEC (1)

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Station Azimuth Take offAngle Key Log10 (S/P)IPM 325.23 46.17 DIPM 325.23 46.17 FIPM 325.23 46.17 LIPM 325.23 46.17 H 0.85281SHKOM

KOM 127.25127.25 46.1746.17 DFKOM 127.25 46.17 LKOM 127.25 46.17 H 0.77248SH

Input parameters for the November 30th, 2007, 02:13 UTC Bukit Tinggi

event.D = Dilatation, F = Away from event , L=Left and H = Log10(SH/

P)


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Focal Mechanism of the 02:13 UTC


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58 acceptable solutions

dip

strike

rake

Fault Plane 1 Fault Plane 2

F1

F2


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dip

strike

rake

Fault Plane 1 Fault Plane 2F1

F2


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December 12, 2007 Bukit Tinggi event

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rake

strike

dip


F2


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January 10, 2008 Bukit Tinggi event

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rake

strike

dip


F2


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Comparison of the P and T axes distribution

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02:13 UTC

Nov 30, 2007

3.5 mb

12:42UTC

Nov 30, 2007

3.2mb

10:01UTC

Dec 12, 2007

3.2 mb

15:38 UTC

Jan 10, 2008

3.0 mb

P, T and B are the maximum compressional, minimum compressional and

null axes, respectively).


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P-wave First Motion Polarities Observations

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The similar waveforms

imply that the

mechanisms of the

events are similar.

The observed polarity

at a given station

should be the same

for each event in

cluster (Hardebeckand Shearer, 2002).

Waveform data of the 02:13 UTC Nov, 30,2007(left) and the 15:38 UTC Jan 10, 2008 (right)

Waveform data of the 12:55 UTC Dec, 9,2007

(left) and the 00:50 UTC Mar 15, 2008 (right)


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Discussion on Focal Mechanism of

Bukit Tinggi with Related Studies

1. The orientation of P and T axes are differentfrom Simons et. al (2007): compression is in theNE-SW direction

2. One possible explanation : weak-zone-normalextension mechanisms. (Hurukawa and Imoto,

1992).

3.

Westward motion of the Sunda Plate cause theweak zone in the vicinity of the Bukit Tinggi

fault to rupture or open due to an extensional

movement in the East-West direction (Lat and

Ibrahim, 2009).

4. (Fatt et al., 2011) shows that epicenters of BukitTinggi events are scattered along the Bukit

Tinggi fault (striking NW-SE directions and

dipping toward the NE) and Kuala Lumpur fault

(striking NW-SE directions and dipping toward

the SW).

T

P


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Conclusions

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We obtained relatively well-constrained solutions of the eventswe analysed although the constraint for dip angles is weaker.

The focal mechanisms of the largest 3.5mb event is mostlystrike slip with some dip slip, while that of three events are strike

slip fault type.

The maximum compressional (P) axes of the largest 3.5mbevent are in the NNW-SSE direction while the three strike slip

events are in the NW-SE direction.

The minimum compressional (T) axes of the strike slip eventsare in the NE-SW direction.

Since there is no surface trace of ruptures this result isimportant to improve our understanding of these seismic activities.


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Thank You

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