A fault runs through it: Montclair

The Hayward fault runs through the heart of Montclair, in the Oakland hills behind Piedmont.

Only in seismologists’ equations, and perhaps deep down in the crust, are faults smooth, flat planes. In the world, on the surface, faults are as ragged and variable as any other geological feature. The Hayward fault is more of a zone, from a few to a hundred meters in width, with several fractures running through it. Where two strands overlap, a block of ground between them may slump in tension or rise in compression, depending on how the strands are oriented. Where the ground slumps, a sag basin may form. Lake Temescal is an example of a sag basin, repurposed as a water feature. So, apparently, is Lake Aliso, the pond on the grounds of Mills College. Montclair Park’s duck pond, as it happens, is in a basin created during railroad construction, so contrary to popular opinion (including my own, until I checked the literature) it’s not a sag basin.

The first “great San Francisco earthquake” occurred 21 October 1868 on the Hayward fault. The epicenter appears to have been in southern San Leandro, and surface rupture extended from there all the way down to Fremont. In Montclair, the other direction along the fault, there was plenty of shaking of course. A trenching study, conducted along the third-base line of the little ballfield in Montclair Park, found no sign of movement along the fault there since the 1868 earthquake. But slow, silent motion does affect the fault in Montclair. The old fire station on Moraga Avenue has been rendered useless by aseismic creep, and some of the houses along the fault appear to show foundation disruption. But generally creep is invisible unless there is some structure that it affects, like the south curb of Medau Place, below. South of here, the fault crosses Route 13, reaching the other side at the head of Dimond Canyon.