Travel times of principal P and S phases over small distances in southern California*

Abstract

Summary Study of arrival times of the principal phases in fifty of the larger and better recorded earthquakes in southern California resulted in the following travel times t (seconds) and apparent velocities V (km/sec.): Pt = 0.1793DV = 5.577St = −0.5 + 0.3066DV = 3.26Pyt = 1.2+0.1654ΔV = 6.047Syt = 2.1 + 0.274ΔV = 3.65Pnt = x + 0.124ΔV = 8.06Snt = y + 0.225ΔV = 4.45 Δ = epicentral distance, D2 = Δ2 + h2, h = focal depth. x and y depend on the region; the following are characteristic values Coastal areas,Mountain areas,NorthernSierralow valleyssoutheastern Calif.Owens ValleyNevadax67910±sec.y8½9½12½14±sec. The average true velocities of Py and Sy are about one-third of one per cent, those of Pn and Sn about one-half of one per cent, smaller than the corresponding apparent velocities. In the uppermost 50 km. the velocity increases with depth. The order of magnitude of this increase is roughly 1 per cent per 10 kilometers, but larger in the uppermost one or two kilometers. It can be found from a study of amplitudes (Gutenberg, 1943c); its effect on the travel times exceeds the limits of error by too small an amount to be ascertained beyond doubt from the data of the present paper. The curvature of the earth can be disregarded within the range of distances used (in general not exceeding 800 km.). The effect of “mountain roots” on the travel times of Pn and Sn is investigated. Reproduction of travel-time curves and recalculation of the thickness of the various layers must wait until a study of other (especially reflected) recorded phases now under way is finished. Preliminary values are 18 km. for the thickness of the granitic layer with small local variations, and about 35 km. for the total of the crustal layers in the coastal areas of southern California with an increase inland approaching twice that amount under the Sierra Nevada.