Just after the first light of dawn, his house was slammed by a sudden, gigantic shock. Then, as he ran out onto Tupper Street near downtown Santa Rosa, J.W. Brown noticed a “great noise” approaching from the west. He could see the tops of trees waving in that direction, and as he watched, the motion and roaring sound came racing towards him. He grabbed a small tree for support, but the shaking tore it from his grasp. As the ground swayed in waves “about 2 feet high and 15 feet long”, he watched the tall dome of the nearby courthouse sway west, then east, and back, and with the third swing, collapse.
Brown’s first-hand account of that April morning in 1906 offers a glimpse of what residents today might experience when the next big quake hits Sonoma County. His testimony appeared in the official 1908 Lawson report on the San Francisco Earthquake, which included interviews on the violent shaking’s disastrous effects in Santa Rosa.
Scientists at the US Geological Service, who constantly analyze and monitor California earthquakes, confirm the North Bay may now be due, and perhaps even overdue, for one as powerful as 7.2 in magnitude, or possibly greater.
So, what can we expect it to be like when it happens? Many of today’s county residents may not have experienced a large earthquake, authorities say, and so may not fully appreciate what’s in store.
To understand what’s coming, scientists have to peer beneath the earth’s surface, and back in time.
To the casual observer, the ground and landscape seem permanently fixed and firm. But to Suzanne Hecker, a research geologist with the USGS in Menlo Park, it looks quite different. Northern California is riding atop a giant slab of earth’s crust, which is grinding past another plate beneath the Pacific Ocean. The titanic slow motion collision between the two has fractured the state from Baja to Mendocino County, and is steadily driving the western slice of California northward at about the speed that fingernails grow.
The intense force of this ongoing continental grinding has broken the upper crust into large crack-like features we call faults. As most Californians know, the broken crust slides along these faults, driven by the motion of the great plates. Unfortunately, because the sliding is uneven, the faults can lag or lock. Then, when enough stress accumulates, they will suddenly lurch past each other, releasing tremendous energy, in earthquakes.
Sonoma County is bracketed by two such active faults, the San Andreas on the west, and the Rodgers Creek on the east. Hecker studies the Rodgers Creek fault. She was a key member of a team who recently confirmed for the first time that Rodgers runs beneath the center of Santa Rosa, using advanced laser ground mapping techniques.
While the county has other active faults, these two are the most worrisome for a particular reason. How intensely an earthquake shakes depends on how much of the fault has ruptured. Longer ruptures generate earthquakes with increasingly higher magnitude. Which means that longer faults can produce larger quakes.
Janet Watt, a research geophysicist with the USGS in Santa Cruz, also studies the Rodgers Creek fault, and last year made an important discovery: the Rodgers Creek is almost certainly connected under San Pablo Bay to the Hayward fault, which runs along the East Bay hills. The Hayward fault, which has been steadily accumulating stress since it last erupted with an estimated magnitude 6.8 quake, in 1868, is considered by some to be one of the most dangerous in the state.