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<br />. <br /> <br />The general nature of this earthquake belt is shown on the epicenter map, Figure 2. It <br />ha~ been estill'att.:'d t 11:.1. 1 earthquakl.'s in C:.lIifornia alld wt'stern Nevada represt'-nt about <br />90 pen.ent of the seismic activity of the eontiguous United States. <br />Geodetic surveys across the Imperial Valley segment of the Salton Trough <br />indicatl' that sheUf strain is continuing to build up in this rL'~i()l1. C:llculations of strain <br />rt'!t'':ls~ versuS strain accumulation sugg~st that till' seismic "habit" of tht" llorth~rl1 t'nd <br />of the Gulf Trough is one of relatively frequent moderate-sized earthquakes. as opposed <br />to the infrequt:1l1 greut eanl1quJkes that OCClIr along Ilort'lcrn ~t'gllll.'llts of till' San Andreas <br />system (such as the famed San Francisc.o earthquake of 1906\. <br />Rt'cl'nt seismic ~Ictivity has occurred Ilt'ar the llorthL"..lst margJll of the S~dtol1 <br />Trough but tllert' is 110 record of fault lllOVt'll1t'llt in the \'lIma area in historic time. <br />TilL' st'ismiL and geologic I.:'vidL'I1I..'L' together indic<Jte that earthquake d~lll1ag(' in till...' YUllla <br />;lrL';1 i'i kss likely frolll movt'rnents on local faults th~lJl on thosL' located l'arthl...'r west <br />,lJld south in the Salton Trough and tilL' north~rn Gulf of California r~gion. Eart!lquakl...'s <br />(,(,\ltned in tl1es~ ,HL'JS havt' CJlIst'd sub:-.tantial d,:lIllJge in tilt' YUllla arL'a in historic tillle; <br />at kast tlHI...'e sll\.:11 eVL'llts have occurred. <br />Information rcl.1kd to ~~lrt!Jqll.:Jkt's felt in tilL' YUTlW arL'<l hus bCl...'n rCL'on.kd <br />in one form or anoth!.;'r since :Jt IC~lst April 1770, ;l'lll morc than 100 events h,lVC b~I...'tl <br />reported through 1975. Between 177(i and 1931, the information is lar~ely in tht' form <br />of llt"wS accollnts. but sume is frolll thl' diarit"s of military.: pcrsonnL'l st:Jtiollt'd ,It Fort <br />)'lIma. In this p~ri()d of 155 Year~ mort' th3n 70 CVI...'nts felt at Yum3 wt'r~ r~cordcd. <br />At least one of these events (1852) had substantial energy release. Only a few events <br />were recoreled prior to 1852. It is probable' that strong shol.ks occurred for which there <br />is no record. <br />Beginning in 1932, ,I network of seVl'll seismic stations W:JS established in southl'rn <br />California by the California Institute of Technology at P"sadcn,.. The number of stations <br />incrt'Jst'J OVl'[ till' yt'Jrs ~lnd by Oeccmbl'r 1972, 39 stJtiolls were in operation for the <br />purpose of monitoring seismic activity. In 1972. the Seismologie~1 Laboratory of the <br />Institute reported 40 years of record anel listed a total of 15.340 seismic events in the <br />ul1ited States JI1(1 MeXICO Jlong tlle trend of tile San AnareJs Felldt lone. Evenis of <br /> <br />. <br /> <br />. <br /> <br />17 <br />