SEISMIC Characterization of Giant or Anomalous Sub Duct Ion Earth Qua Les

8/9/2019 SEISMIC Characterization of Giant or Anomalous Sub Duct Ion Earth Qua Les

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SEISMIC CHARACTERIZA

GIANT and/or ANOMAL

SUBDUCTION EARTHQU

Emile A. OKAL

Department of Earth and Planetary Sc

Northwestern University

Evanston, IL 60208

[email protected]

July, 2008


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THE CHALLENGE

Design evalution methods which will correctly retrieve the

tsunami potential of an earthquake

(i.e., the long-period behavior of the source)

in as little time as possible.

Note that we want method[s] which will

WORK in EXCEPTIONAL CASES

(Giant events and Anomalous [slow] ones).

We test on the most recent large events:

Mantle magnitudeMm (Improved version)

Slowness parameter

Body-wav e integratorMwp

High Frequency body wave duration 1/3


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MmMm and TREMORS[Okal and Talandier, 1989]

DesignNEWMagnitude Scale,Mm ,

using mantle Rayleigh wav es,

with variablevariable period

Directly related to seismic momentM0

All constants justified theoretically Incorporate into Detection Algorithms to

AUTOMATE PROCESS

* Implemented,

Papeete, Tahiti (1991),

PTWC (1999)

Mm = log10 X() + CD + CS + C0

Mm = log10 M0 20


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Mm : Variable period Mantle Mag

Introduced by Okal and Talandier[1989]

Performance on very large datasets evaluated by Weinstei

In use

Recent Im

Boost periods up t

Regress and compare

Mm

= a1

* f + b1

Mm

= a1

* f + b1

(all

Mm = a2 * f + b2Mm = a2 * f + b2 (hig

Mm = a3 * f + b3Mm = a3 * f + b3 (low

Devise algorithm to e

* If earthquake big (b1

* Else, explore event sl

If earthquake is slo

If earthquake is nothen KEEP bb

Otherwise, AVERA

This admittedly empirical algorithm

Mm av = 8. 90 0. 035 * f

SUMATRA, 2004


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DefineEstimated Energy, EE

EE = (1 + q)16

5

[a/g(15;)]2

(Fest)2

max

min

2 u()

2e t

*() d

Scale to Moment through = log10EE

M0

Scaling laws predict = 4. 92.

Tsunami earthquakes characterized byDeficient (as much as 1.5 units).

Now being implemented at Papeete and PTWC

Nicaragua, 1992Java, 1994

Chimbote,

Peru, 1996

.5

ESTIMATED ENERGY and PARAMETER

Fr om BODY WAVES

[Boatwright and Choy, 1986; Newman and Okal, 1998]


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MwP

Idea: Try to recover the full moment information from the P

waves which arrive faster than the Rayleigh wav es.

Note that formula for P waves inv olves

TIME DERIVATIVE of MOMENT FUNCTION, XX

Idea is to compute TIME INTEGRAL ofP wave deformation torecover X, and hence static momentM0.

Problems: Instrument records velocity, so double integration

needed; noisy at long periods;NOT tested on large earthquakes.

MwpMwp : EXAMPLE

OKUSHIRI, Japan EART

Harvard CMT:M0M0

Station PFO ( = 77. 1)

Raw

( Velocity)

Ground Motion

Integrated

ground motion

M0 = 5. 3 1027M0 = 5. 3 1027 dyn-cm

[J. Hebde

[Tsuboi, 1996]


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A simple [trivial ?], robust measurement

[Ni et al., 2005]

Duration of source from High-Frequency (24 Hz)

Teleseismic P wavetrain

26 DEC 2004

t = 559 s

28 MAR 2005

t = 177 s

DEVELOP ALGORITH

HIGH-FREQUENCY P

TONGA, 3 May 2006 C

= 37

P PcP PP

ORIGINAL

FILTER

COMPU

1/3 (at 1/3 Maxim1/4 (at 1/4 Maxim

[Reymond and

1/31/3


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90

90

120

120

150

150

180

180

210

210

240

240

270

270

300

300

330

330

60 -

30 -

0 0

30 3

60 6

KURILES -- 13 JAN 2007

TAIWAN -- 26 DEC 2006

PERU -- 15 AUG 2007

NEW ZEALAND -- 30 SEP 2007MOLUCCAS -- 21 JAN 2007

SOLOMON Is. -- 01 APR 2007

BENGKULU I -- 12 SEP 2007

BENGKULU II -- 12 SEP 2007

BENGKULU III -- 13 SEP 2007

NO. CHILE -- 14 NOV 2007

SANTA CRUZ -- 05 SEP 2007

THESE ALGORITHMS WERE APPLIED IN

QUASI-REAL TIME

(i.e., following receipt of tsunami bulletins if during working hours)

TO FOURTEEN RECENT EARTHQUAKES

(JULY 2006 NOVEMBER 2007)

JAVA -- 17 JUL 2006HAWAII -- 15 OCT 2006

KURILES -- 15 NOV 2006

plus SUMATRA -- 26 DEC 2004


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102 104 106 108 110 112 114 116 118 120 122

-16 -16

-14 -14

-12 -12

-10 -10

-8 -8

-6 -6

-4 -426 MAY 2006YOGYAKARTA

17 JUL 2006 02 JUN 1994 19 AUG 1977

Bengkulu

Jakarta

Surabaya

Cilacap

P. Tritis

Str

aits

Sun

da J A V A

SUMATRA

L.S.

B.

SUMBA

FLORES

11 SEP 1921

Tsunami

Earthquake

Tsunami

Earthquake

27 SEP 1937

Typical "Tsunami Earthquake"; 700 killed by tsunami

Carbon copy of 1994 event, 600 km to the East

T.E. : Event whose tsunami is stronger than suggested by itsseismic magnitudes [Kanamori, 1972].

JAVA -- 17 JULY 2006

mb = 5. 9; Ms = 7. 7;

ImprovedMm = 7.90

On the high side, but clearly catching the

low-frequency size of the event Mwp = 7. 17 (time domain); 7.19 (frequency domain)

Underestimates Seismic Moment by factor of6.5

1/3 = 95 seconds; clearly VERY LONG SOURCE

M0 = 4. 6 1027 dyn*cm [CMT]

Slow Event, = 6. 13


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141 144 147 150 153 156 159

42 42

45 45

48 48

51 51

54 54

01-May-1915

15-Nov-2006

S e a o f

O k h o t s k

Pacific Ocean

152 153 154 155

46 46

47 47

48 48

49 49

0 50 100

km

Simushir

Matua

Shiashkotan

13-Jan-2007

01-May-1915

15-Nov-2006

SIMUSHIR (Central Kuril Is.) 15 NOVEMB

M0 = 3. 5 1028 dyn*cm

First large earthquake in the Centra

The event is not slow, but may be dela

Local effects surveyed in S

Run-up reaches 10 m inSimushir (Dushnaya Bay)and up to 1520 m onMatua.

The latter figures arehigher than expected, andcould result from localtopography (bays, cliffs) orlocalized lanslides.

Fortunately, these islands

are presently unpopulated

(even by bears...).

[courtesy J. Bourgeo

Matua I.

(12 km long)


11/46


M0 = 3. 5 1028 dyn*cm

ImprovedMm = 8.75 - OK

Mwp = 7. 91 (T.D.) and 7. 88 (F.D.)

Moment deficient by a factor of4

.... a Long Story! STAY TUNED

1/3 = 48 seconds

NOT A SLOW EARTHQUAKE

KURILES

Outer Rise N

M0 = 1. 6

ImprovedMm =

Mwp = 8. 05 (T.D

Moment values

= 4. 76 R

1/3 = 47 secon

REGULAR EAR

TREND toward


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THE CASE of the 2006 KURILES EVENT

By contrast, the 2006 Kuriles event has a complex character:

It does exhibit some Lateness, like the 2001 Peruvian

earthquake (It takes 30 seconds to reach a maximum ).

But it is not "slow", as | | falls rapidly after 50 seconds

indicating the end of the source.

It remains "Weak" (corresponding to a low strain

release), as the maximum value of is low (5.8).

But this weakness is not an artifact of a slow source.

THE CASE of for the 2006 KURILES EVENT

For this earthquake, the calculation of depends on the

delay imposed on the sampling window.


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150 165 180 195

40

50

60

D = 6300 (

15-Nov-2006

2006 KURIL TSUNAMI DID SIGNIFICANT

in CRESCENT CITY, California

Harbor struck 8.5 hours afterseismic O.T.

Damage reached US$ 700,000.

Wave height reached 1.7 m (pk-to-pk) on local tide gauge

Damage resulting from (i) beamingof some tsunami energy towardsNorthern California; (ii) non-linearamplification by bay and harbor.

Tidal gauge record

Damage to docks in harborDi

Docks H, G, Fseverely damaged

[Uslu, 2007]


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152 15

0

SUBSEQUENT TSUNAMIS (ctd.)

5. Solomon Is., 01 April 2007 [ The Miracle ? ]

M0 = 1. 6 1028 dyn*cm

Preliminary Results:[H. Fritz, pers. comm., April 2007]

Local Tsunami, resulting in significant damage on several islands

More than 500 houses destroyed;

The community apparently had the r

(probably conditioned by the me

during a volcano-seismic swa


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SOLOMON ISLANDS -- 01 APRIL 2007

M0 = 1. 6 1028 dyn*cm

ImprovedMm = 8.57

On the high side (by a factor of2)

Mwp = 7. 88 (T.D.) and 7. 92 (F.D.)

Moment values slightly low (by a factor of2).

= 5. 35

1/3 = 46 seconds

REGULAR EARTHQUAKE; no SURPRISES!


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PERU -- 15 AUGUST 2007

M0 = 1. 11 1028 dyn*cm

ImprovedMm = 8.28 -- OK

Mwp = 7. 85 (T.D.) and 7. 89 (F.D.)

Moment values slightly low (by a factor of2).

= 5. 41

1/3 = 53 seconds

ANOTHER STRAIGHTFORWARD CASE!

(Unlike 1974 event trending towards SLOWNESS)

NOTES: Considerable damage due to the earthquake (520 deaths)

Moderate tsunami (only 3 deaths) Earthquake rupture penetrated Nazca Ridge

[Fritz et al., 2007]


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90

90

92

92

94

94

96

96

98

98

100

100

102

102

104

104

106

106

-6 -6

-4 -4

-2 -2

0 0

2 2

4 4

6 6

8 8

10 10

12 12

14 14

PDE 12-SEP to 28-SEP 2007CMT to 11-SEP-2007

I

II

III

AND THEN... THE SEPTEMBERThree large events in the Mentawai Gap!

[but tame, by Sumatra standards] I 12 SEP,II 12 SEP,

III 13 SEP,

These events are cle

expected repeat of the 1

Moment of I much to

II. is apparently an

deeper limit of the fau III is negligible in ter

Loss of life relatively

Tsunami moderate (ne r fi

[J. Borrero,pers. comm., 2007]


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BENGKULU -- 12 SEP 2007

(I) --- 11:10 GMT

M0 = 5. 05 1028 dyn*cm

ImprovedMm = 8.77 - OK

Mwp = 8. 12 (T.D.) and 8. 07 (F.D.)

Moment deficient by a factor of3

= 5. 78

1/3 = 50 seconds

TREND TOWARD SLOWNESS

(II) -

Aftershock at b

M0 = 1. 5 1028

ImprovedMm =

Mwp = 7. 92 (T.D

Moment defic

= 5. 54

1/3 = 55 secon


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SUMATRA 12-SEP-2007 (before RUNUP)

-70

-60

-50

-40

-30

-20

-10

0

10

20

-7

-6

-5

-4

-3

-2

-1

1

2

-30 -20 -10 0 10 20 30 40 50 60 70 80 90 100 110 120

-30 -20 -10 0 10 20 30 40 50 60 70 80 90 100 110 120

0.05 0.10 0.20 0.30 0.40 0.50 1.00 2.00 3.00 5.00

AMPLITUDE (m)

REAL-TIME SIMULATION

Based on the moment obtained using Improved MmMm, atsunami simulation was performed in real-time, and theattached map of maximum expectable amplitude on the highseas forwarded to Dr. Chris.J. Hartnady (Umvoto, Cape

To wn).

It was received in South Africa at 16:18 GMT (18:18 localtime), 4.5 hours before the tsunami reached Port Elizabeth.


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BENGKULU (III) -- 13 SEP 2007 03:35 GMT

Event triggered to the North of Fault zone of (I)

M0 = 4. 36 1026 dyn*cm

ImprovedMm = 7.66

Moment too large by a factor of10 !!

Mwp = 7. 69 (T.D.) and 7. 42 (F.D.)


= 4. 87

1/3 = 38 seconds

Mm VALUES BECOME VERY LARGE BEYOND 250 s

Yet, DURATION MEASURED BY1/3IS SHORT !!


21/46

BENGKULU (III) -- 13 SEP 2007 03:35 GMT

Event triggered to the North of Fault zone of (I)

M0 = 4. 36 1026 dyn*cm

ImprovedMm = 7.66


Mwp = 7. 69 (T.D.) and 7. 42 (F.D.)


= 4. 87

1/3 = 38 seconds

Mm VALUES BECOME VERY LARGE BEYOND 250 s

Yet, DURATION MEASURED BY1/3IS SHORT !!

PROBABLE EXPLANATION:

Surface waves used in Mm are contaminated by

multiple passages (principallyR4) from pre vious,

much larger, event (II), only 3.7 hours earlier.

Mwp may also be contaminated by coda of (II)


22/46

(66 recent events)

Improved Mm algorithm gives accurate values for mostev ents, including "Tsunami Earthquakes"

Sumatra 2004 remains somewhat underestimated

[ Expected, given duration of event comparable to

lowest usable frequency ]

SUMATRA -- 26 DEC 2004

JAVA -- 17 JUL 2006

HAWAII -- 15 OCT 2006




PERU -- 15 AUG 2007N.Z. -- 30 SEP 2007

MOLUCCAS -- 21 JAN 2007

SOLOMON Is. -- 01 APR 2007

BENGKULU I -- 12 SEP 2007

BENGKULU II -- 12 SEP 2007

BENGKULU III -- 13 SEP 2007

NO. CHILE -- 14 NOV 2007

SANTA CRUZ -- 02 SEP 2007

REPORT CARD : Mm (Improved)

A _

Only Bengkulu (III) event is grossly over-estimated, due to

contamination by previous event at lowermost frequencies.


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REPORT CARD : PARAMETER

A _ Correctly identifies SLOW "TSUNAMI EARTHQUAKES"

JAVA 2006 SUMATRA 2004

Identifies "SNAPPY" (Often Intraplate) EVENTS

KURILES 2007 TAIWAN 2006 HAWAII 2006

Has trouble distinguishing between Truly Slow and

DELAYED (Late) Events (KURILES 2006).

log10M0 (dyn*cm)

l

og10

EE

(erg)


24/46

Time-domain Computation Fourier-doma

log10 M0 (dyn*cm) log10 M0

Mmp

Mmp

Mwp

Mwp

Integrate P wave ground motion (in far field) to obtain sei

[In practice, integrate ground velocity twice].

Problems:

Algorithm fails to recognize truly great earthquakes

Also, mis-handles slow or late ones

REPORT CARD :

SUMATRA 2004 NIAS 2005 BENGKULU

JAVA 2006


25/46

Mwp Recent developments

Compilation ofMwp for a dataset of 64 recent events

shows a systematic correlation between slowness(expressed through ) and the residual of Mwp with respect

to published moment.

Residual

M

wp

Residual

M

wp

This indicates that the standard Mwp algorithm suffers

from the same inadaptation to exceptional events (slow

or gigantic) as other methodologies.

SUMATRA -- 26 DEC 2004

JAVA -- 17 JUL 2006KURILES -- 15 NOV 2006

= log10 [EE

/M0 ]


NIAS -- 28 MAR 2005

BENGKULU (I) -- 12 SEP 2007



26/46

Mwp

More Problems:

Theory valid only in far-field.

Yet, applied undiscriminately in both near- and far-

fields.

Length of window / Frequency band never satisfacto-

rily resolved.

Influence of depth phases / triplications not sorted out.

Empirical patches for big events (change h ??)

unsatisfactory.

In time domain algorithm, instrument responsenot flat at long periods.

Theory based on geometrical optics. Large events

require frequencies (5 mHz) implying wav elengthscomparable to Earths mantle thickness.

Operational details of algorithm unresolved. Fixes

derived empirically for small, regular events unappli-

cable to large or anomalous ones.


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64 earthquakes

2004 Sumatra event recognized as very long "Tsunami Earthquakes" also identified

(1/3 = 167 s; 1/4 = 291 s)

(Java, 2006; Nicaragua, 1992)

By contrast, the 2006 Kuriles earthquake is notfound to exhibit slowness.

This confirms its character as weak and late, but

not slow.

SUMATRA 2004

JAVA 2006NICARAGUA 1992

KURILES 2006

REPORT CARD : 1/31/3B


28/46

REPORT CARD : 1/31/3 (ctd.)

HOWEVER,

The method fails to convincingly identify

all tsunami earthquakes:

It misses

JAVA 1994CHIMBOTE, Peru 1996

ACCORDINGLY, it only earns a Bbut pending more research

with INCOMPLETE


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1/31/3 : EXTRA CREDIT ? Use 1/3 vs.EE

Idea: 1/3 expected to grow likeM1/30

Estimated Energy expected to grow likeM0

Hence 1/3 / (EE

)1/3

should be constant

DefineDuration Test

DT= log10 1/3 1

3log10E

E + 6. 43

Note: Constant 6.43 predictable theoretically from scaling laws

DT > 0.35correctly predicts ALL Slow Earthquakes

... but also includes one regular event (Costa-Rica, 1991)

log10M0 (dyn*cm)

D

T

NICARAGUA 1992

JAVA 1994

CHIMBOTE, Peru 1996

SUMATRA 2004

JAVA 2006

TIBET 2001

[ COSTA-RICA 1991 ]


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WMmWMm :

A "QUICK-and-DIRTY" MAGNITUDE

to QUANTIFYING the WW P

Emile A. OKAL

Department of Earth & Planetary SciNorthwestern University

Evanston, IL 60208

[email protected]

Western Pacific Meeting, American Geophysic

Cairns, Wednesday, 30 July 2008


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WWPhase: The Origin

[Kanamori, 1993]


32/46

What IS the WW Phas

A combination of multiply-reflected bod

pling the upper mantle at very low fre

5 mHz) and arriving between P andRayleig

It can also be regarded as a superpositio

overtones, i.e., of spheroidal modes of thequencies, with high group velocities (5. 5


33/46

0Sl 1Sl 2Sl 3Sl 4Sl 5Sl

WW PHASE as COMBINATION of SPHEROID


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EARLY INVESTIGATIONS (19939

Attempt to retrieve long-period behavior ofM

Wphase under the magnitude concept


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RECENT DEVELOPMENTS

In the wake of the 2004 Sumatra event, Lockwood and

Kanamori [2006] showed that the W phase was promi-

nently recorded world-wide and that its spectral amplitude

could be quantified.

Rivera and Kanamori [2007, 2008] later showed that Wphase signals could be inverted to obtained the ultra-long

period focal mechanism of the event.


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WMm: A NEW LOOK

In this framework, we re-open the question of the rapidrapid

quantification of the W phase, in the spirit of a magnitude

measurement:

"quick-and-dirty";

paying no attention to details such as focal mechanism

and exact depth;

but reconstructing reliably the behavior of the seismic

moment at ultra-long periods.

Based on our experience with the mantle magnitude Mm,

we seek to make a mesaurement, from the spectral ampli-

tude of the W phase, of the seismic moment M0 through a

Wmagnitude:

WMm = log10M0 20.


37/46

DESIGNING WMm

We recall the formula for the standard mantle magnitude Mm

Mm = log10X() + CD + CS + C0

for Rayleigh waves, where X is the spectral amplitude at angluar fre-quency , CD a distance correction, Cs a source correction dependingonly on , and C0 a locking constant, justifiable theoretically.

This formula is generally applicable to all waves expressing the super-position of normal modes of the Earth along a single continuousbranch, and as such was succesfully applied to Love waves, 1stRayleigh overtones, and even tsunamis [Okal and Titov, 2007].

The challenge regarding the W phase comes from its nature as asuperposition ofseveral overtone branches featuring widely differ-ent excitation functions as well as attenuation coefficients.

In particular, the latter preclude the easy definition of auniversal distance correction CD.

200

250

333

500

1000

QQ


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DESIGNING WMm (II)

Rather, we proceed by building a large database of synthetic W phases

for many focal mechanisms (6480 geometries varying three angles) and

epicentral distances (from 20 to 130 degrees).

Each synthetic is then Fourier-transformed, and the resulting 1321920spectral amplitudes X at 17 frequencies between 1 and 5 mHz kept

into a database.

For each of the 204 combinations of distance and angular frequency

, the 6480 spectral amplitudes are geometrically averaged [over focal

geometry] to obtain an ad hoc but theoretically derived correction:

C(, ) = 1

6480 focalgeometries

(, , )

log10X(,; , , )

Correction C(, )


39/46

The correction C(, ) may seem largely ad hoc,

but it is a modern relative of Gutenberg andRichters body-wave correction Q(; h), which is

still in use for the calculation ofmb.

At least, our correction is justified theoretically....

h [Richter, 1958]


40/46

WMm: EXAMPLE OF COMPUTATION

BENGKULU, Sumatra, 12 SEP 2007

M0 (CMT) = 5. 05 1028 dyn*cm

Station: HNR (Honiara, Solomon Is.), = 58

P S

W

Rayl.

Instrument Deconvolved (1 f 20 mHz)

U= 9 km/s + 15 minutes

Deconvolve

Instrument

Window

Record

F.F.T.

Then, at each

Frequency:

Read C(

;)

Apply : WMm = log10X() + C + 7. 0

Av erage Residual: r = 0. 09r = 0. 09 log. units


41/46

EXAMPLE: BENGKULU 2007 (ctd.) Results on 26 S

Residuals r= WMm

Our results suggest the elimination of the longest period (

and of stations at distances < 40.

The average moment from WMm then becomes M0 = 6. 35

compared to apublished CMTofM0 = 5. 05 1028 dyn*cm

The resulting residuals have an average r = 0. 10 0. 27r = 0. 10 0. 27 lo

Published CMT


42/46


43/46

ANALYZING and REMOVING the OU

04361 Sumatra: Undersampled, even at 800 s.

91112 Costa Rica: Too few stations, all in same

azimuth (early days of IRIS).

07256Bengkulu III: Follows previous, larger event;

Contaminated by multipleR from latter.

06360 Taiwan: Double event, followed 8 minutes later

by second shock, leading to contamination.

07353 Aleutian: ???

After remova

Av erage residual: r

Best Regression: WMm


44/46

APPLICATION to CHALLENGING CASES

Tsunami EarthquakesTsunami Earthquakes

94153 Java

96052 Chimbote, Peru

06198 Java

ALL MOMENTS CORRECTLY

ASSESSED USING WMmWMm

92246 Nicaragua

Overestimated; Too few Stations (early days of IRIS)

Difficult ("Late") Earthquakes

01174 Peru

06319 Kuriles

ALL MOMENTS CORRECTLY

ASSESSED USING WMmWMm


45/46

WMmWMm NOT AFFECTED by SLOWNESS

Residual r = WMm (log10M0 20)

plotted as a function of the

slowness parameter = log10(EE/M0).

Correlation Coefficient: 52%

without Sumatra 2004 : 47%

= log10(EE

/M0)

r

=

WMm

(log10

M0

20)

without outliers: 48%


46/46

CONCLUSIONS

The mantle magnitude formalism can be successfully

applied to the W phase, using a single distance/frequency

correction, which remains fully justifiable theoretically.

Distances less than 40 and periods longer than 800 s are

better avoided.

An adequate number of stations (> 10), well distributed in

azimuth, is necessary.

On the average, WMm slightly overestimates published

CMT moments, raising the question of the possible exis-

tence of ultra-low-frequency components to the source of

very large earthquakes.

Most importantly, WMm faithfully assesses tsunami

earthquakes and other challenging events.

REPORT CARD : WMmWMm B +

Documents

SEISMIC Characterization of Giant or Anomalous Sub Duct Ion Earth Qua Les