Macroseisinie Eviilence f o r the Fault P i a n e A . G . GALANOPOULOS Under the assumption that the earthquakes are the result of fault- ing under the action of a couple the amplitudes of the longitudinal waves and those of the transverse waves are zero in ali directions situated in the fault piane (Honda-Emura, 1957). Taking this info consider- ation, it is intuitively evident that the minimum radius of the felt area should occur in the direction of the fault piane. The unsymmetrical energy distribution from the hypocentre proved strong enough not to be masked by the influence of the inhomogeneity of the medium, es- peciallv of the upper layers, upon the isoseismal pattern (Keilis Borok, 1956). Such being the case, the minimum radius of the macroseismic area should be used auxiliarly in the case it is not possible with the liei]) of the initial motion of transverse waves or in other way to determine which of the two nodal planes for longitudinal waves in the focus was the actual fault piane. The macroseismic evidence presumably fails in case of dip-slip motion and reasonably is a striking one in the transcurrent type earth- quakes. The suggestion is illustrated by the data of some well studied shocks. The earthquake of No veni ber 2, 1954 near Sumbawa Tsland (8°. OS, 119°. OE) is a very typical example: strike of the fault piane N 1° E; azimuth of the fault movement N 182° E (Eitsema, 1957): diameter of the felt area in the E - W direction about 800 km and perpen- dicular hereto 400 km only (Ritsema, 1955). The Ivern County earthquake of July 21, 1952 started from the White Wolf fault. With the assumption that the strike of the earth- quake fault is in azimuth of about 50° from north towards east, the seismic solution led to the following results (Gutenberg, 1955): at the deptli of the source (about 10 miles) the fault piane has a dip of about 60° to 66° towards southeast; the slip along the fault was roughly up towards north in the upper block, down towards soutli in the lower; the vertical component of the slip was about 1.4 times that of the horizontal: the horizontal component produced a relative movement northeastward 190 A . G. G A L A N O P O U L O S in the upper block (southeast of the fault), southwestward in the lower (northwest of the fault). A s the motion was much closer to dip-slip motion than to strike-slip the macroseismic evidence is not conspicuous. However, the minimum radius of the felt area occurs clearly in the direction of the fault piane. The evidence is more clear under the as- Fig. 1. - Isoseismal map, Kern County earthquake of July 21, 1952, after F. Neumann and. W . Cloud (1955), and tlie direction of faulting in tlie main shock (35° 00' N, 119° 01' W ) after B. Gutenberg (1955). M A C R O S E I S M I C E V I D E N C E F O R T H E F A U L T P L A N E 191 sumption of the new solution obtained by G. Sutton and E. Berg (1958): fault piane striking ZsT 33° E, dipping 63° SE, motion up and north on the SE side, and a ratio of dip-slip to strfke-slip motion, r = 0.8. I n the Fairview Peak earthquake of December 16, 1951 the seismic solution obtained implies a fault striking N 11° W and dipping 62° Fig. 2. - Isoseismal map, Nevada earthquakes of Deeember 16, 1954, after W. C L O U D (19.57), and the seismic solution for the direction of faulting in the Fairview Peak earthquake (39° 17' N, 118° 07' W), after C. Romney (1957). to the east; motion along this piane was such that the east side of the fault moved toward 155° from north (measured clockwise) relative to the west side, and down at an angle of about 24° measured from the surface; the horizontal displacement should be about twice as large as the vertical component (Romney, 1957). The seismic solution was com- 192 A . G. G A L A N O P O U L O S pletely confìrmed by the results of triangulation and leveling by the U. S. Coast and Geodetic Survey. Although the maeroseismic effects of the Fairview Peak earthquake were disturbed by those of the Dixie Valley shock (39°. 8 X , 118°. 1 W ) originated more than 4 minutes later, from an epicentèr about 55 kilometers to the north, and the isoseismal map shows intensity distribution for the two shocks without regard to which sliock may have been the responsible agent, the minimum radius of the felt area occurs clearly in tlie direction of the fault piane. Fig. 3. - Intensity distribution in the area most strongly affected by the earthquake of Aprii 30, 1954, after A. Galanopoulos (1955) and the seismic solution obtained by J. Hodgson and J. Cock (1958). Another example with a large dip component ( — .301) is the So- phades earthquake of Aprii 30, 1954 (39°.3 X , 22°.2 E ) : The area of strong shaking centered in a point near the village Sophades shows clearly the relationship of the origin of the earthquake to the marginai fault of the southeastern side of the faulted basin of Karditsa (Galanopoulos, 1955). The solution obtained by J. Hodgson and J. Cock (1958) implies M A C R O S E I S M I C E V I D E N C E F O R T H E F A U L T P L A N E 193 a fault piane striking either X 86° E or N 46° W . The macroseismic evidence favours the b-plane striking N 46° W and dipping 78° towards southwest. Diameter of the felt area in the direction of the fault piane 380 km (Konitsa-Laurion) and in that of the auxiliary piane 140 km (Corfou-Lemnos). On grounds of geological logie the fault piane for the Samos earth- quake of July 16, 1955 (37°. 9 N, 27°. .1 E ) is the a-plane of the seismic solution (Hodgson-Cock, 1958): maximum radius of the felt area in the direction of the a-plane (N 40° E ) 200-270 km (Syros, Milos) and in that of the b-plane (N 50° W ) 360-440 km (Argalasti, Domokos); motion was strike-slip (dip component + .105). As another argument may be cited the Cephallenia earthquake of August 12, 1953 (Di Filippo-Marcelli, 1954, Hodgson-Cock, 1958): maximum radius of the felt area in the direction of the fault piane (N 62°. 5 E ) about 520 km ( I I at Catania) and in that of the auxiliary piane (N 31° W ) about 600 km ( I I I at Foggia). Under the above mentioned assumption of the fault mechanism the direction of maximum radiation of transverse waves is approximatelv at right angles to the piane of the fault. In most cases destructiveness produced by sliear waves is greater than that produced by other types of waves (Benioff-Gutenberg, 1955). Since the strike of the fault piane of the great majority of the shocks is directed more or less perpendicular to the seismic and structural zones (Ritsema, Honda-Emura, 1957), it is self-evident why the earthquakes are mostly felt over greater distances parallel to the structural lines than perpendicular thereto (Sieberg, 1932-1933). This must be considered one more confirmation of the reliability of the macroseismic evidence for specifying the fault piane and the auxiliary piane normal to the motion vector. ABSTRACT Since the amplitudes of the longitudinal waves and tliose of the trans- verse waves are zero in ali directions situated in the fault piane, the mini- mum radius of the felt area should occur in the direction of the fault piane. The unsymmetrical energy distribution from the hypocentre proved strong enough not to be masked by the influence of the inhomogeneity of the medium, especially of the upper layers upon the isoseismal pattern. Such being the case, the minimum radius of the macroseismic area should be used auxiliarly in the case it is not possible with the help of the initial motion of transverse waves or in other ivay to determine which of the two nodal planes for longi- 194 A. G. G A L A N O P O U L O S tudinal ivaves in the focus ivas the actual facult piane. The macroseismic evidence presumably fails in case of dipslip motion and reasonably is a strildng one in the transcurrent type earthquakes. Z USAMMENFASS UNG Im Falle einer Erdbebendislokation unter der Beanspruchung eines Scherungskràftepaars ist die Dislokationsebene im Hypozentrum eine Knotenebene fiir beide Vorlauferwellen. Die maximale Ausstrahlung von Sclierungswellen ist ungefahr senkrecht zu der Bewegungsebene. Die Lon- gitudinalwellen haben Minimalamplituden in der Riclitung der Bewe- gungsebene und senkrecht dazu. Fiir die Erdbebenwirkungen und besonders fiir die in grossen Epizentralentfernungen beobachteten makroseismischen Erscheinungen sind die Transversalwellen von alien Erdbebenwellen in den meisten Fàllen weitaus mehr schuldig. So muss die minimale Aus- breitung der makroseismischen Energie mit der Streichrichtung der Be- wegungsebene zusammenfàllen bzw. der minimale Durchmesser des makro- seismischen Schuttergebietes auf die Orientierung des Scherungskrdfte- paars im Hypozentrum hinioeisen. Wenn die Beivegungsrichtung senkrecht zum Streichen der Scherungsflache ist, sollen die makroseismischen Data mutmasslich versagen. Die makroseismischen Data sóllten besonders zu- treffen in dem zweiten Grenzfalle, wo die Bewegungsrichtung parallel zum Streichen der Scherungsflàche ist. Das Streichen der Bewegungsebene ver- làuft fiir die meisten Erdbeben mehr oder minder senkrecht zu den seismischen Zonen bzw. zu den Stórungszonen. Insoiveit die Erdbebenverwerfungen senkrecht zu der Faltenrichtung streichen, scheint die maximale Ausbrei- tung der Erdbebenenergie in der Richtung der Gebirgsketten physikalisch gut verstàndlich zu sein. RIASSUNTO Poiché l'ampiezza delle onde longitudinali e di quelle trasversali è zero in tutte le direzioni situate nel piano di faglia, il raggio minimo della zona deformata dovrebbe verificarsi in direzione dei piani di faglia. La asimmetrica propagazione dell'energia dall'ipocentro si è dimostrata abba- stanza forte ma non tanto da essere mascherata dall'inimogeneità del mezzo, e da riflettersi sull'andamento delle isosiste negli strati superiori. In questo caso, il raggio minimo della zona macrosismica dovrebbe essere preso in M A C R O S E I S M I C E V I D E N C E FOR T H E F A U L T P L A N E 1 9 5 considerazione soltanto quando non sia possibile usufruire dell'ausilio del movimento iniziale delle onde trasversali, o con qualche altro metodo, per determinare quale dei due piani nodali (risultanti dalla registrazione delle onde longitudinali) è stato il piano di faglia effettivo. L'uso di osservazioni macrosismiche cade presumibilmente in difetto nel caso di movimento di slittamento profondo, mentre diviene elemento positivo in un terremoto di scorrimento. 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R I T S E M A , R . - S O E T A D I , R . , The Earthquake of November 2, 19-54 near Sum- bawa Island, in: « Verk. », 47, 1-35, Djakarta 1955. —- On the Use of the Transverse ÌVaves in Earthquake Mechanism Studies and the Direction of Fault Displacement in SE Asian Earthquakes, in: « Yerh. », 52, 1-31, Djakarta 1957. - Earthquake-Generating Stress Systems in Southeast Asia, in: « Bull. Seisni. Soc. Am. », 47, 267-280, (1957). ROMNET, C., Seismic Waves from the Dixie Volley-Fairview Peak Earthquakes, in: «Bull. Seism. Soc. Am. », 47, 301-319 (1957). SIEBERG, A., Untersuchungen iiber Erdbeben und Bruchschollenbau ini óst- lichen Mittelmeergebiet, in: « Denkschr. Med.-Naturw. Ges. », 18, Jena 1932.. - Erdbebenforschung und ihre Yerwertung fiir Teclinik, Berqbau und Geo- logie, Jena 1933. SUTTON, G. - BERG, E., Direction of Faulting from First-Motion Studies, in: «Bull. Seism. Soe. Am. », 48. 117-127, (1958).