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Measuring Earthquake Size Earlyideas LECTURE 4 MEASURING EARTHQUAKE SIZE EARLYIDEAS •Describe damage inflicted by earthquake →→ "INTENSITY Scales" Always written with roman numerals (IV,VII, XI, etc.) Dynamic connection: Intensity should express ground acceleration BUT... Shortcomings of Intensity Scales •Not directly related to earthquake source •Damage obviously distance-depen- dent •Needs population to report damage •Affected by site response Example of Intensity maps for 1886 Charleston, USA, earthquake. EARTHQUAKE MAGNITUDES →→ •Anessentially empirical concept, intr- •TTo thisot day,his day,measurements have oduced by Richter [1935], long before measurementsremained larhagelyve ad hoc,especially at anyphysical understanding of earth- remainedshort largelydistances. quakesources ad hoc, especially at short distances. [Bolt, 1987] PROGRESS in the 1940s •Apply worldwide •Try (!!) to justify theoretically →→ Leads to first worldwide quantified catalogue of earthquakes "Seismicity of the Earth" Gutenbergand Richter [1944; 1954] B. Gutenberg, 1958 "MODERN" MAGNITUDES Standardized at Prague meeting of the IUGG (1961) •Use Body (P)Wav estodefine short period magnitude, mb around a period of 1 second Q(∆, h) A m = log + Q(∆; h) h b 10 T •Use Surface (Rayleigh)wav e to define = "Long"-period magnitude, Ms,at T 20 seconds A M = log + 1. 66 log ∆+3. 3 s 10 T 10 ∆ Still largely empirical; Constants not justified [Okal, 1989] BODY-WAVE MAGNITUDE mb From first-arriving wave trains (" P "Wav es) *Should be measured at period close to 1 second SUMATRA−ANDAMAN, 26 DEC 2004 Station CTA(Charter Towers, Queensland, Australia); ∆=55° •Remove instrument response •Band-pass filter between 0.3 and 3 seconds •Select windowof80seconds duration around P wave •Apply Body-wav e Magnitude formula A m = log + Q(∆; h)(A in microns) b 10 T mb = 7. 2 SURFACE-WAV EMAGNITUDE Ms From later Surface-wave train (" Rayleigh "Wav es) *Should be measured at Period of 20 seconds SUMATRA−ANDAMAN, 26 DEC 2004 Station CTA(Charter Towers, Queensland, Australia); ∆=55° •Remove instrument response •Band-pass filter between 15 and 25 seconds •Select windowof11minutes duration around Rayleigh wav e •Apply Surface-wav e Magnitude formula A M = log + 1. 66 log ∆+3. 3 (A in microns) s 10 T 10 Ms = 8. 19 EARTHQUAKES TAKE TIME TOOCCUR •The larger the earthquake, the longer the source ("Scaling Law"). •Measuring large earthquakes at small periods simply misses their true size. •Inthe case of Sumatra, full size available only from normal modes. mb Ms TSUNAMI GPS EARTHQUAKE SOURCE GEOMETRY Fr omSingle Force to Double-Couple The physical representation of an earthquakesource is a system of forces known as a Double-Couple,the direction of the forces in each couple being the direction of slip on the fault and the direction of the normal to the fault plane. Normal to the Fault Single Force Direction of Slip Double Couple [Stein and Wysession, 2002] Mathematically,the system of forces is described by a Second-Order Symmetric Deviatoric TENSOR (3 angles and a scalar). SEISMIC MOMENT The double-couple representing a seismic source is quanti- fied through its moment, which represents the common torque of the opposing couples. It is a real physical quantity,called the seismic moment and its expression is: = µ ∆ M0 ∫Σ udS where µ is the rigidity of the medium, ∆u the slip between the fault walls at each point of the fault, and the integral is taken overthe surface of faulting. In particular,for a rectangular fault of length L and width W, = µ ⋅ ⋅∆ M0 LW u M0 is measured in dyn*cm (or N*m). Note that Kanamori [1977] has introduced a so-called "moment magnitude" Mw givenby 2 M = log M − 16. 1 w 3 10 0 The retrievalofthe seismic moment M0 from seismological data is a relatively complexprocedure. While the equations relating the double-couple to the observable seismic wav eforms are indeed linear,they involvenot only the scalar moment M0,but rather the vari- ous elements of the double-couple, which makeupthe com- ponents of a Second-Order Symmetric Deviatoric Tensor. The inversion of these components is a difficult problem in theoretical seismology,which requires, at the minimum, a very large dataset. This is whyitmay be interesting to develop and apply methodologies which explore the low-frequencypart of the seismic spectrum, while at the same time keeping the con- cept of a single number,namely the philosophyofa magnitude scale. TSUNAMI WARNING: THE CHALLENGE •Upon detection of a teleseismic earthquake, assess in real-time its tsunami potential. • HINT: Tsunami being lowfrequencyisgenerated by longest periods in seismic source ("static moment M0"). • PROBLEM: Most popular measure of seismic source size, sur- face wav e magnitude Ms ,saturates for large earthquakes. FAR-FIELD TSUNAMI DANGER EXTREME PROBABLE LOW NIL [Geller, 1976] Ms SATURATES AROUND 8.2 MMmm and TREMORS [Okal and Talandier, 1989] •Design NEW Magnitude Scale, Mm , using mantle Rayleigh wav es, with variable period •Directly related to seismic moment M0 •All constants justified theoretically •Incorporate into Detection Algorithms to AUTOMATE PROCESS *Implemented, Papeete, Tahiti (1991), PTWC (1999) ORIGIN of MANTLE MAGNITUDE Mm •Now possible to makemeasurement in Fourier space (frequencydomain) •Spectral amplitude of Rayleigh wav e controlled by Source, Propagation: ω ∆ − a π e 2UQ 1 ω = ⋅ ⋅ −1/2 − 1/2 − 3/2 ⋅ X( ) a sR l K0 iqR l K1 pR l K2 M0 √ 2 √ sin∆ U →→ The Ki depend only on frequencyand source depth; →→ Trigonometric factors sR, qR, pR depend only on the geometry of focal mechanism and source-receiverlayout. Conversely,one can extract M0 from = − = ω + + + Mm log10 M0 20 log10 X( ) CD CS C0 where 1 aω ∆ • C = log sin ∆+log e ⋅ (distance correction) D 2 10 10 2UQ • CS is a Source correction which, at each frequency, can be built theoretically by av eraging the source terms overmanygeometries and source depths. π • C0 is a locking constant, predicted theoretically (depends only on and Earth radius a). TREMORS Single-Station Algorithm for Automated Detection and Evaluation of Far-Field Tsunami Risk Jacques Talandier, Emile A. Okal, Dominique Reymond, 1991 •Automatic detection of distant earthquake •Automatic Location of Epicenter •Automatic computation of the event’s Mantle Magnitude = ω + + − Mm log10 X( ) CD CS 0. 90 from spectral amplitude X(ω )ofsurface (Rayleigh) seismic wav esat the longest possible periods (250 to 300 seconds) AV O IDS MAGNITUDE SATURATION •Allows quasi-real time estimation of tsunami risk •Operational at Laboratoire de Ge´ophysique, Tahiti since 1991. •Also in use at Pacific Tsunami Warning Center,Ewa Beach; Chile. RECALL... TREMORS: EXAMPLE OF APPLICATION Kurile Is. Earthquake, 04 OCT 1994, Station: TKK (Chuuk, Micronesia) •Detection: Analyse signal level compared to previous minute. •Location : S − P givesdistance (36° or 4000 km). Geometry of P wave giv esazimuth. •Estimate seismic moment → Fourier-transform Rayleigh wave (highlighted) → At each period, compute = = spectral amplitude, correct for T 55 s; Mm 8. 78 excitation and distance; = = obtain Mm T 80 s; Mm 8. 71 → = = = Conclusion: Average Mm 8. 60 T 120 s; Mm 8. 60 = × 28 T = 150 s; M = 8. 55 (M0 4 10 dyn-cm). m × 3 = = T 200 s; Mm 8. 49 T = 250 s; M = 8. 40 Harvard solution (obtained later): × 7 m = × 28 = M0 3 10 dyn-cm (Mm 8. 48) TREMORS -- Operational Aspects •Performance against subsequently published values of M0 CONFIRMED: AVOIDS SATURATION TREMORS -- Operational Aspects •Response Time of TREMORS algorithm ATREMORS station at an epicentral distance of 15° can issue a useful warning for a shorelocated 400 km from the event. MM m CAN WORK at SHORTDISTANCES Tested by Okal and Talandier [1992] down to ∆=1. 5 °(165 km). MM m WORKS for GIGANTIC EVENTS Chile, 1960 Works evenonsev erely clipped records obtained on instru- ments with poor dynamic. [Okal and Talandier, 1991] MM m CAN WORK for HISTORICAL EVENTS 17 AUGUST 1906 -- Aleutian Islands Wiechert mechanical seismometer,Strasbourg 28 Mm = 8. 58; M0 = 3. 8 × 10 dyn*cm Important for reassessment of old events, based on very sparse datasets. THE INFAMOUS "TSUNAMI EARTHQUAKES" •Aparticular class of earthquakes defying seismic source scaling laws. Their tsunamis are much larger than expected from their seismic magni- tudes (even Mm). •Example: Nicaragua, 02 September 1992. THE EARTHQUAKE WAS NOTFELTATSOME BEACH COMMUNITIES, WHICH WERE DESTROYED BY THE WAVE 40 MINUTES LATER 170 killed, all by the tsunami, none by the earthquake El Popoyo, Nicaragua El Transito, Nicaragua "TSUNAMI EARTHQUAKES" • The Cause: Earthquakehas exceedingly slow rupture process releasing very little energy into high frequencies felt by humans and contributing to damage [Tanioka, 1997; Polet and Kanamori, 2000]. • The Challenge: Can we recognize them from their seismic wav esin[quasi-]real time? • The Solution: The Θ parameter [Newman and Okal, 1998] compares the "size" of the earth- quakeintwo different frequencybands. → Use generalized−P wavetrain (P, pP, sP). 1994 Jav a "Tsunami Earthquake" Station: TAU (Hobart, Tasmania) → Compute Energy Flux at station [Boatwright and Choy,1986] → IGNORE Focal mechanism and exact depth to effect source and distance corrections (keep the "quick and dirty "magnitude" philosophy). → Add representative contribution of S waves. → Define Estimated Energy,EE ω 2 max 16 [a/g(15; ∆)] * E E = (1 + q) ρα ∫ ω 2 u(ω ) 2 e ω t (ω ) ⋅ dω 5 (F est )2 ω min E → Θ= E Scale to Moment through log10 M0 → Scaling laws predict Θ=−4. 92. • Tsunami earthquakes characterized by Deficient Θ (as muchas1.5 units). .5 • Java,1994 Chimbote,• Nicaragua, 1992 Peru, 1996 • Now being implemented at Papeete and PTWC EXAMPLE of COMPUTATION of PARAMETER Θ JAPAN RED Events areSLOW(Θ≤−5. 8) Note that this event had a trend towards being FAST (Outer ridge event NOTatSUBDUCTION INTERFACE) PROBLEM with Θ forVERYLARGE EVENTS •The duration of the source (and hence of the P-wave train may be so long that the P wave interferes with subsequent phases (PP,even S) Example: Sumatra-Andaman Event, 26 DEC 2004 Station MSEY (Mahe´,Seychelles; ∆=41°).
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