An earthquake of magnitude 5.7 (M5.7) struck in the Sierra Nevada just to the south of Lake Almanor in Lassen Volcanic Park on the evening of 23 May. The ‘quake, which is reported to have been felt across Northern California, was followed some hours later by a second tremor, measured at M4.9 near Canyondam, CA, but at the time of writing, no reports of damage or injuries had been received.
The Lake Almanor, California, Earthquake of 23 May
The tremor occurred within the heart of the northern Sierra Nevada, at a depth of 11km, in an area that is generally considered relatively tectonically stable compared to the eastern and western boundaries of the range. The USGS issued warnings of possible further tremors, with the likelihood of a large aftershock (at least M5.0) over the next seven days put at 36% and the chances of a tremor larger than the M5.7 itself put at 5-10%.
Although no detailed information is yet available from the United States Geological Survey on the mechanics causing the quake, fault maps of the area produced by California’s Department of Conservation clearly show the existence of faulting. The May 23 earthquake most probably occurred on or close to the northern end of the Indian Valley fault and is unusual because these faults appear not to have experienced significant recent fault movement (within the last 200 years).
Tectonic Setting of California and the Sierra Nevada
Seismic activity in California is largely associated with the San Andreas fault zone in the west, but in fact the whole region is complex and, in geological terms, relatively recent. The state is thought to have formed over the past 200 million years or so by the accretion of different slabs of crust, of varying origin, which collided obliquely and coalesced to form land.