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3. Seismicity of Southern California* by Charle S F

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3. Seismicity of Southern California* by Charle S F 3. SEISMICITY OF SOUTHERN CALIFORNIA* BY CHARLE S F. RICHTER t AND B ENO GUTENBERG l Evidence for regional seismicity is of four kinds : ( 1) geological ment of only a few inches along the line of the Manix fault; Instru­ field observation of fault phenomena, ( 2) historical documents, ( 3) mental locations of epicenters of aftershocks aligned nearly at right instrumental recording, and ( 4) fi eld investigation immediately after angles to this fault, suggesting that the observed displacement is li earthquakes. secondary result of a larger displacement on a fault with different Historical and instrumental data cover a very small part of ge­ strike in the basement rocks; (6) July 21, 1952. Arvin-Tehachapi ological time, and thus constitute only a snapshot of the record, so earthquake, Kern County; probably thrust faulting, with surface to speak. They may furnish positive evidence of seismicity, but expression obscured and complicated by large-scale slumping and failure of earthquakes to occur on a given fault during a period of sliding; White Wolf fault. less than two centuries is no proof of quiescence. On the other hand, The historical record begins with a strong earthquake felt by the identifying faults as active on the basis of field evidence alone im­ Portola expedition on July 28, 1769, when the explorers were in plies an assumption that there have been no significant permanent camp along the Santa Ana River near the present townsite of Olive. changes in seismicity in a few tens of thousands of years. This Subsequent information for all of California is extremely scanty assumption is reasonable, but it does not necessarily apply without until 1850, and in southern California the record is imperfect for exception. an even longer period of time. Especially in the desert areas, even Faults are commonly identified as active on the basis of such field a major earthquake j!Ould have escaped notice until about 1887, when evidence as small scarps, especially if these cut young alluvial fans; the seismological station at Lick Observatory was established. In this slickensides in modern deposits; offsets in minor drainage channels; same year the first catalogue of California earthquakes was pub­ and sag ponds. There are differences, however, in the apparent age lished. of these features. Many small features along the San Andreas fault The greater earthquakes known to have occurred in the region of are very fresh in appearance, and are known or believed to have California and Nevada are: (1) January 9, 1857, San Andreas fault originated during the earthquakes of 1857 and 1906. The rift topog­ (see above); (2) March 26, 1872, Owens Valley (see above); (3) raphy along the Garlock fault includes many sag ponds, low scarps, April 18, 1906, San Andreas fault: epicenter north of San Fran­ and other ge ol o~t ic ally young features, but relatively few of these cisco; instrumental magnitude 8t; ( 4) October 1, 1915, Pleasant seem to be truly fresh. This suggests that no major earthquake has Valley, south of Winnemucca, Nevada; instrumental magnitude 7t; broken the surface along the Garlock fault within the last few ( 5) .July 21, 1952, Kern County; instrumental magnitude between centuries. 71 and 7t (investigation in progress). Known occurrences of fault-trace phenomena associated with re­ Several less well known shocks, as those in 1812, 1836, 1838, 1852, corded earthquakes in southern California are: ( 1) January 9, 1857. and 1868, may have been comparable with these. San Andreas fault, from Carrizo Plain to San Bernardino, and pos­ sibly farther in both directions; (2) March 26, 1872. Owens Valley; California forms a small part of the circum-Pacific belt of seismic faulting, chiefly vertical with some strike-slip movement, along the activity (figs. 1, 2), which accounts for roughly 80 percent of the line of the east front of the Alabama Hills (not along the major seismicity of the earth. Of this, southern California contributes be­ Sierra Nevada scarp several miles west); (3) March 10, 1:J22. San tween 1 and 1 percent. Most of the circum-Pacific belt is composed Andreas fault in the region of Cholame; large cracks, .possibly not of arcuate structures that are associated with deep as well as shallow primary fault-trace effects; (4) May 18,1940. Imperial Valley; fault earthquakes, very active volcanism, folding, and thrust faulting. trace at least 40 miles long, extending from the vicinity of Imperial California is the largest ·of several sectors in which the structures and Brawley southeastward into Mexico; right-hand strike-slip move­ are not evidently arcuate, where only shallow earthquakes occur, ment, reaching a maximum displacement of 19 feet near the inter­ and where the tectonics are primarily those of block faulting, includ­ national boundary; scarps as much as 4 feet high; ( 5) April 10, ing long, narrow rift zones associated chiefly with strike-slip faults. 1947. Southeast of Manix, Mojave Desert; left-hand strike-slip move- In evaluating both historical and instrumental.data, due attention should be paid to magnitude of the earthquakes. Frequently a com­ • Contribution No. 627, Division of the Geological Sciences, California Institute of T echnology. paratively moderate earthquake, such as the Long Beach earthquake t Professor of Seismology, California Institute of T echnology. l Professor of Geophysics, California Institute of Technology. of 1933, chances to originate in a thickly populated area and hence ( 19 ) 20 GEOLOGY OF SOUTHERN CALIFORNIA [Bull. 170 17 E 18 ,. 130 120 110 100 17 ow 16 0 ·~ LARGE SHALLOW EARTHQUAKES *MAGNITUDE 7-7.7, 1918-19~2 • MAGN ITUDE 75/4 OR OVER, 190 4 -19~2 M OD I~IE O )o! QLL WE IO E PK(; JE':TI O' l FIGURE 1. Large shallow earthquakes of the world, 1904-52. Chapt. IV] SEISMICITY-RICHTER AND GUTENBERG 21 Epicenter location, origin time, and trULgnitude of SOUle of the shocks terms of an arbitrary scale ranging from I to XII. High intensity shown in figure 2. may result because an earthquake of moderate magnitude originates nearby or because an earthquake of large magnitude originates at a Date Time Latitude Longitude Magnitude distance. Some results of experience with the magnitude scale can be tabu­ 1934. June 8 ... ........ 04:47 :45 36 N 120).1! w 6.0 I 952. No,·. 22 .... .... 07:46:38 35.8 N 121.2 w 6± lated as follows: 1922. 1\-!arch 10 . .... II :21:20 35% N 120)4 w 6).1! Magnitude Effects I 946. March I 5 ........ 13 :49:36 35.7 N 118 . I w 6)4 1952. J uly 21. ........ I 1:52:14 35.0 N 119 . 0 w 7.7 1 Only ooserved instrumentally. 1947. April 10 ......... 15:58:06 35. 0 N 116 .6 w 6.4 2 Can he hare!.\- felt (intensity II) n ear epicentPr. J916.0ct.23 . ......... 02:44 34.9 N 118. 9 w 5).1! ± 1927. N o,·. 4 ..... .... 13:50:43 34 ).1! N 121Y, w 7.3 41 F elt to distances of some 20 miles from the epicenter; may 1946. July 18 . ... ..... 14 :27:58 34.5 N 116.0 w 5% cause ~ light damage ( int e n s it~ · \'II ) in a ~ mall area. 1941. J uly I. ... ...... 07 :50:55 34.4 N 119.6 w 5.9 l\Io<l erately <l estructive (example : Long B each, Hl33 ; Santa 1925. J u ne 29 .......... 14:42 :16 6+ 34.3 N 11 9.8 w 6)4 Bar bara, Hl2:i) . 1930. Jan. 16 . ... ____ 00 :24:34 34.2 N 11 6.9 w 5)4 1935. Oct. 24 . __ .... __ 14:48:08 34. 1 N 11 6.8 w 5r.f 7+ :\Injor parthqual;e. ! 940. l\ !ay 18 _____ ___ __ 05:03:59 34. I N 11 6.3 w 5. 4 G reat rn rthqual;e ( 1872, 1006 ). 1923, J uly 23 ........ 07 :30 :26 34 N 117)4 w 6)4 H+ 194 7. J uly 24. _________ 22:10:47 34.0 N 11 6.5 w 5 .5 1949. May 2 __ ______ ___ 11:25:47 34.0 N 11 5.7 w 5 .9 Because of the late development of instrumental seismology, great 1930. August 31. ....... 00:40:36 33.9 N ll8 .6W 5)4 earthquakes of magnitude 8 and over can be catalogued completely 1948. Dec. 4 _... __ . ___ 23:43:17 33.9 N 116.4 w 6.5± 1941. Nov. 14. ........ 08:41 :36 33.8 N 118 .2 w 5 . 4 only for the period beginning in 1904 (fig. 1) . Such earthquakes are 1933. Oct. 2 .. __ . ..... 09:10:18 33.8 N 118.1 w 5 . 4 1918. April21 ......... 22 :32:25 33% N ll7 w 6.8 related to the major active structures. Mino:" earthquakes, on the I 933. March 11 ... ___ . 01:54:08 33.6 N 118.0 w 6)4 other hand, are more commonly related to minor faults and struc­ 1937. March 25 ___ ____ _ 16:49:03 33.5 N 11 6.5 w 6.0 1942. Oct. 22 . ......... 01:50 :38 33.3 N 115.7 w 5% tures, and the epice nters of the small, instrumentally recorded shocks 1950. July 28 ... .... 17 :50:48 33. 1 N 11 5.6 w 5. 4 are peppered over the region. 1950. July 29 ....•..... 14 :36:32 33. 1 N 11 5.6 w 5. 4 1942, Oct. 21 . __ . __ .. 16:22: 14 33.0 N 11 6.0 w 6Y, Since 1932, epice nters for sufficiently well r ecorded shocks have 195 1, D ec. 26 .
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