In this thesis I apply paleomagnetic techniques to paleoseismological problems. I investigate the use ofsecular-variation magnetostratigraphy to date prehistoric earthquakes; I identify liquefaction remanent magnetization(LRM), and I quantify coseismic deformation within a fault zone by measuring the rotation of paleomagnetic vectors.
In Chapter 2 I construct a secular-variation reference curve for southern California. For this curve I measurethree new well-constrained paleomagnetic directions: two from the Pallett Creek paleoseismological site at A.D. 1397-1480 and A.D. 1465-1495, and one from Panum Crater at A.D. 1325-1365. To these three directions I add the best ninedata points from the Sternberg secular-variation curve, five data points from Champion, and one point from the A.D. 1480eruption of Mt. St. Helens. I derive the error due to the non-dipole field that is added to these data by thegeographical correction to southern California. Combining these yields a secular variation curve for southernCalifornia covering the period A.D. 670 to 1910, with the best coverage in the range A.D. 1064 to 1505.
In Chapter 3 I apply this curve to a problem in southern California. Two paleoseismological sites in theSalton trough of southern California have sediments deposited by prehistoric Lake Cahuilla. At the Salt Creeksite I sampled sediments from three different lakes, and at the Indio site I sampled sediments from four differentlakes. Based upon the coinciding paleomagnetic directions I correlate the oldest lake sampled at Salt Creek with theoldest lake sampled at Indio. Furthermore, the penultimate lake at Indio does not appear to be present at Salt Creek.Using the secular variation curve I can assign the lakes at Salt Creek to broad age ranges of A.D. 800 to 1100, A.D.1100 to 1300, and A.D. 1300 to 1500. This example demonstrates the large uncertainties in the secular variationcurve and the need to construct curves from a limited geographical area.
Chapter 4 demonstrates that seismically induced liquefaction can cause resetting of detrital remanentmagnetization and acquisition of a liquefaction remanent magnetization (LRM). I sampled three different liquefactionfeatures, a sandbody formed in the Elsinore fault zone, diapirs from sediments of Mono Lake, and a sandblow in thesesame sediments. In every case the liquefaction features showed stable magnetization despite substantial physicaldisruption. In addition, in the case of the sandblow and the sandbody, the intensity of the natural remanentmagnetization increased by up to an order of magnitude.
In Chapter 5 I apply paleomagnetics to measuring the tectonic rotations in a 52 meter long transect across theSan Andreas fault zone at the Pallett Creek paleoseismological site. This site has presented asignificant problem because the brittle long-term average slip-rate across the fault is significantly less than theslip-rate from other nearby sites. I find sections adjacent to the fault with tectonic rotations of up to 30°. Ifinterpreted as block rotations, the non-brittle offset was 14.0+2.8, -2.1 meters in the last three earthquakes and8.5+1.0, -0.9 meters in the last two. Combined with the brittle offset in these events, the last three events allhad about 6 meters of total fault offset, even though the intervals between them were markedly different.
In Appendix 1 I present a detailed description of my standard sampling and demagnetization procedure.
In Appendix 2 I present a detailed discussion of the study at Panum Crater that yielded the well-constrainedpaleomagnetic direction for use in developing secular variation curve in Chapter 2. In addition, from samplingtwo distinctly different clast types in a block-and-ash flow deposit from Panum Crater, I find that this flow had acomplex emplacement and cooling history. Angular, glassy "lithic" blocks were emplaced at temperatures above 600° C.Some of these had cooled nearly completely, whereas others had cooled only to 450° C, when settling in the flow rotatedthe blocks slightly. The partially cooled blocks then finished cooling without further settling. Highlyvesicular, breadcrusted pumiceous clasts had not yet cooled to 600° C at the time of these rotations, because they show astable, well clustered, unidirectional magnetic vector.
