Some of you may have felt the magnitude 4.1 earthquake that rocked the Bay Area on Friday evening. Of course, some of you may not have felt it. The solid bedrock of Bernal Hill diminishes a lot of ground motion, so Bernalites may not feel some of the jolts and tremors that put other parts of the City on edge.
Regardless of whether or not you felt it, please take two minutes to fill out the USGS’ Did You Feel It questionnaire about this earthquake. Reports on who didn’t feel anything in an area where plenty of people did feel it are actually incredibly useful to scientists. Reports of earthquake intensity, from extreme to zero, can help highlight underlying the geology of the area. Retroactive shakemaps from Loma Prieta or 1906 show the strongest intensity in areas of artificial fill, and the weakest on solid bedrock. (ie: Bad for the Marina, good for Bernalwood.) So please fill this out! Filing a report for the smaller quakes is good practice for when a bigger one inevitably happens.
Friday’s earthquake was centered near San Jose. Relocations of the waveforms put the quake on the Calaveras Fault, at a depth of 7 km (which is a moderate depth for this kind of fault). This particular segment of the Calaveras is near where the Hayward Fault branches off from it; Some recent studies even suggest that the two faults are connected at depth. The 2007 Alum Rock earthquake — a M5.6 — wasn’t too far to the northwest of today’s event. The southern Calaveras Fault was also responsible for the M6.2 1984 Morgan Hill earthquake, which was widely felt over the entire Bay Area and beyond.
Lest you think that the only thing this earthquake has to do with Bernal Heights was to unsettle a couple of people who aren’t fortunate enough to have our solid foundation, an interesting tip-off came on Twitter shortly after the quake: apparently the land that contains the epicenter was once owned by a certain Jose de Jesus Bernal — the original Mexican landowner who gave our hill its name.
Fortunately, all of his land wasn’t so faulted as this chunk near San Jose. Bernal Hill has some small fault offsets in its chert, but those are not the kind that cause large earthquakes (or even small ones anymore). In other words, we’re solid!