GEOPHYSICAL RESEARCH LETTERS, VOL. 17, NO. 1, PAGES 21-24, JANUARY 1990 OBSERVATIONS OF TELESEISMIC SHEAI•-W'AVE SPLITTING IN THE BASIN AND RANGE FROM PORTABLE AND PERMANENT STATIONS M. K. Savage Seismological Laboratory, MacKay School of Mines,University of Nevada-Reno • P. G. Silver Department of Terrestrial Magnetism, Carnegie Institution of Washington R.. P. Meyer Department of Geology and Geophysics, Universityof Wisconsin-Madison Abstract. Observations of shear-wave splitting were ob- tained from temporary and permanent stations in western Nevada and southern California. Measurements of fast po- larization azimuth q5 and delay time 6t were made on both SKS and direct S phases. For stations in the northern Basin and Rangethe results were consistent, yielding an average of (d, •) = (+75 ñ 8', 0.9ñ 0.3s). One station in thesouthern Basinand Rangegave(+40 ñ 11', 0.4 ñ 0.1 s) and one in the MojaveDesert gave(-54 ñ 3', 1.2 ñ 0.1 s). 6• = 0.9s is consistent with a 100kin-thick mantle layer characterized by 4% anisotropy. For the northern stations,the anisotropy ap- pears unrelated to the present-day extension or absolute plate motion. Rather, we suggest that it is 'fossil'anisotropy asso- ciatedwith pre-Miocene extension, whose directionis about +68'. For the California station •5 is nearly parallel to the strike of the San AndreasFault system,and is attributed to shear strain associated with relative plate motion. Introduction The recording of teleseismic shearwaves from temporary arrays opens up a new source of information about the crust and upper mantlethat, until recently, hasbeenavailable only from permanent stations. One immediateapplication is the measurement of shear-wave splitting and the characterization of crust and mantle anisotropy. As a first step in exploiting this information, wehavedeployed 12 three-component digital seismographs on a NNW-SSE transect through the northern Basin and Range of Nevada, from April 27 through May 31, 1988(Figure 1). As a control, we havealsoanalyzed records from the four-stationpermanent array of the University of Nevada, Reno (UNR), and two stations of the Lawrence Liv- ermore NationalLaboratory (LLNL). Shear wavesplittingmeasurements are based on the fact that in an anisotropicmedium, two shear-waves are present, with differentvelocities and orthogonal polarizations (see Crampin, !981). The basic measurements are the fast polar- ization direction, •, and the delaytime, 5t, between the fast and slowphases. For mantle anisotropy, q5 is related to the principal strain directions through the lattice preferred orien- tation (LPO)ofmantle minerals, while 5t depends on both the magnitude of anisotropy (and thus strain)andthe length of 1Now at California Department of Conservation, Division of Mines and Geology, Sacramento. Copyright 1990 by American Geophysical Union. Paper number 89GL03140. 0094-8276/90/89GL-0314 0503.00 the anisotropic path. Thesemeasurements provide excellent lateral resolution and havebeen relatedto various geologic and tectonic processes. [e.g. Ando et al., !983; Kind et al., 1985; Silver and Chan, 1988, hereafter SC]. WCN SNF 1.5s SAF I I) '. I ]l ' I •,/ Gap, GF Mojave ß 1.0s © 0.5s Fig. 1. Shear-wave splitting measurements. Arrows desig- nate average fast polarizationdirection •5from the best mea- surements for each station; size of circles is proportional to •. Numbered stations are portable;WCN, DNY, BMN, and MNA were operated by UNR; MNV andLAC by LLNL. MNA, MNV, and 10 werecolocated. Alsoshown, average of all sta- tions northof andincluding MNA, (AVG), absolute platemo- tiondirection (APM)[Minster andJordan, 1978], Miocene ex- tension direction (ME) and present extension direction (PE) [Zoback et al., 1981]. Surface traces of San Andreas (SAF), Gatlock (GF) faults, Sierra Nevada Frontal Fault Zone (SNF), and several faults in Nevada and Mojave Desert are shown. Boundary of magmatic gap [Eaton,!982] alsoshown. Dia- monds designate portable instruments without at least one we!l-constrained measurement. 21