NA-Pa Plate Boundary Wilson [1960] USGS Prof. Paper 1515

Faults: Northern California USGS Prof. Paper 1515

Faults: Southern California USGS Prof. Paper 1515

SCEC Clickable Fault Map

Evolution of North American - Pacific Plate Boundary USGS Prof. Paper 1515

Evolution of North American-Pacific Plate Boundary Modern SAF (4 Ma) & opening of Gulf CA

Atwater Animation [Atwater and Stock, 1998]

“Captured” North American Plate USGS Prof. Paper 1515 Inactive early transform with component of subduction Captured part of NA plate now moving northwesterly with Pacific plate

Evolution of San Andreas USGS Prof. Paper 1515; Powell & Weldon [1992] Ma20-10 Ma 10-2 Ma5-0 Ma2-0 Ma 60 km 220 km

Displacement of Pelona Schist USGS Prof. Paper 1515; Powell & Weldon [1992] Pelona Schist Orocopia Schist

USGS Prof. Paper 1515; Powell & Weldon [1992] Displacement of Pelona Schist 50 km right lateral slip by San Gabriel f. 210 km right lateral slip by San Andreas f.

Fault Slip Rate USGS Prof. Paper 1515

Historical Faulting USGS Prof. Paper 1515

Historical Faulting

Seismicity ( ) USGS Prof. Paper 1515

Probability for occurrence of major earthquake from

Past and Future Earthquakes on the San Andreas Fault [Weldon et al., 2005] Wrightwood and the earthquake cycle... [Weldon et al., 2004]

Exposed Trench, Pallett Creek A.D USGS Prof. Paper 1515 Fault zone intermittently covered by debris flows and peat layers Samples provide approx. event dates & offsets

Weldon et al. [2004] Wrightwood Trench fault/fissure stratigraphic contact

Weldon et al. [2004] Wrightwood Event Ages Mean recurrence interval: 105 yrs ( yrs) Mean slip: 3.2 m (0.7-7m)

Weldon et al. [2004] Tests of Slip/Time Predictable Models Slip-predictable: length of interseismic period predicts amount of slip in next event Time-predictable: offset of the last event predicts the time length until the next event

Weldon et al. [2004] Strain-Predictable Model?

Weldon et al. [2004] Strain-Predictable Model?

Weldon et al. [2005] Beyond Wrightwood “The current 148-year hiatus is probably not exceptional. However, no lull in the past 1600 years appears to have lasted more than ~ 200 years, and when the current hiatus ends, a substantial portion of the fault is likely to rupture, either as a single long rupture or a series of overlapping ruptures in a short time interval.”

Weldon et al. [2004, 2005] Rupture Scenarios 1. North and south ruptures w/overlap 2. Random rupture length 3. North 2/3’s rupture; south 1/3 rupture 4. Long ruptures w/small additional ones

Conclusions 1. Strain released in an earthquake is not simply that accumulated since the last event. 2. It appears likely that slip occurs at a wide range of strain levels and does not always release the same amount of strain. 3. Overlapping tails of essentially separate large ruptures may explain two small events in a short interval of time at a point, but this cannot explain the A.D. flurry of events with large displacements, or the past 1000 years of less-frequent, smaller-than-average events. 4. Given the rupture lengths of 1812 and 1857 events (~150, 300 km) & lack of substantial rupture in last 148 years, many doubt the possibility of frequent, small ruptures on the southern SAF.

References

S. California Topography USGS Prof. Paper 1515

Weldon et al. [2004]