Shake, Rattle & Roll: Living with Faults

Shake, Rattle & Roll:

LIVING WITH FAULTS

STORY & PHOTOS BY MARK M c LAUGHLIN

Extreme drought, record-breaking heat waves and an elevated risk for destructive wild res are making headlines this summer, but many residents of the Tahoe Sierra are marveling at the number of relatively strong earthquakes the region has experienced recently. Seismic activity is expected in mountain-building territory riddled with fracture zones and fault lines, but what is unusual this year is the frequency, strength and sensible movement of these temblors. It has been a hot topic of conversation: “Did you feel that earthquake?” Seismologists wonder if the shaking is a precursor to the overdue Big One.

Over the past several months, the Tahoe Sierra has been rattled by clusters of earthquakes and swarms of aftershocks. e unusual seismic activity has continued with many small tremors that few people notice or only register on a seismograph, an instrument that measures earthquake strength and characteristics. e shaking started in late April but then a sharp 4.7-magnitude quake struck near Truckee on May 10 and got everyone’s attention. at was quickly followed by 2.7 magnitude aftershock. Barely two weeks later, on May 28, a 4.2 temblor jolted the region along a fault line underneath Lake Tahoe about 5 miles from Dollar Point. at was followed by at least six more shakers.

On July 8, a powerful 6.0 quake struck the Eastern Sierra near the CaliforniaNevada state line between Lake Tahoe and Mono Lake. (A potentially damaging magnitude-6 earthquake or larger generally strikes every decade or so along the Sierra Front.) Fortunately, there were no

reports of injuries or deaths, but storekeepers reported running for cover as merchandise fell from walls and shelves. Several lucky motorists were spared when large rocks shaken loose rumbled down onto Highway 395 near the town of Walker, closing an important artery. e initial jolt was followed by a spate of a dozen aftershocks. e unnerving tremors were enough for authorities to evacuate Reno City Hall. e July 8 seismic shock was felt as far away as Las Vegas and San Francisco. Graham Kent, University of Nevada, Reno professor and director of Nevada Seismology Laboratory (NSL) con rmed that it was “the strongest earthquake in almost 25 years.” ere have been 33 signi cant earthquakes near this epicenter over the past century, but it seems a bit eerie that the current geological display is happening during the 115th annual anniversary of the 1906 San Francisco earthquake and re that destroyed the city. at violent 7.8-magnitude temblor shook most of California, western Nevada and southern Oregon, and spiked seismographs as far away as Germany and Japan.

Several lucky motorists were spared when large rocks shaken loose rumbled down onto Highway 395 near the town of Walker closing an important artery.

Earthquake fault escarpment at Genoa, Nev. | Mark McLaughlin

ing to an NSL 2003 Technical Report on “Pre-1900 Earthquakes in Western Nevada,” the urban corridor that includes Reno, Carson City and booming communities along the eastern Sierra Nevada Front sits on some of the most active faults in the tremulous Silver State. (Out of all 50 states, Nevada ranks fourth for seismic activity behind Alaska, California and Hawaii.) California and Nevada together

MAJOR QUAKE OVERDUE

account for about 80 percent of all quakes in the 48 contiguous states.

Scientists at NSL have been warning for years that a major earthquake along the Eastern Sierra is long overdue. e region averages a magnitude 7 event about every 30 years, and it has been more than 60 years since the last one. ere were seven 6.5-magnitude, or larger, temblors from 1915 to 1954, but none since. Scientists today use a moment magnitude scale to measure quakes; the once-familiar Richter scale has been largely abandoned.

Sierra Crest at Lone Pine. | Mark McLaughlin

SIERRA CONSTANTLY MOVING

In the Far West, terra rma is an illusion. e magni cent Sierra Nevada is roughly 400 miles long and 45 miles wide, sculpted by glacial ice into steep, deep canyons and elevated peaks exceeding 14,000 feet high. To the casual hiker, climber or skier, the range may seem stable, but it is really in a constant state of uplift that helps o set the weathering process that would otherwise slowly wear it down. Mountains rise along faults and earthquakes result from this building process. Lake Tahoe lies on the western rim of the Basin and Range Province, one of the most seismically active regions in the United States.

Scientists studying the bottom of Big Blue have found evidence of huge landslides likely triggered by ancient earthquakes of magnitude 7.0 or greater. Chunks of rock the size of city blocks ripped from surrounding mountains, smashed into the lake during powerful temblors thousands of years ago. Geologists have also warned that several faults that lie directly beneath and west of Lake

Tahoe are thought capable of generating quakes with a magnitude su cient to produce tsunami waves exceeding 30 feet high in the Tahoe Basin. In lakes this water sloshing, back-and-forth movement is referred to as a seiche, or standing wave.

Read more local history at TheTahoeWeekly.com

A MOUNTAIN RANGE GROWS

Few people ever see a mountain range grow, but at 5:12 a.m. on the morning of March 26, 1872, residents of Lone Pine did. e small hamlet is located on the Sierra east slope near the Owens Valley, about 350 miles south of Reno. Sleeping residents were awakened by a sound like rumbling thunder accompanied by the crash of falling dishes, mirrors and glassware. e heaving ground broke plaster from ceilings and tossed lled water pitchers into the air. Out of the 62 buildings lining Main Street, 52 eventually crashed to the ground in piles of rubble. Falling bricks and debris killed 27 people. From then until 6:30 a.m. there were not less than 50 distinct shocks that leveled the town. At Big Pine, 43 miles to the north, a surface ssure opened from 50 to 200 feet wide and 20 feet deep, running close along the base of the Sierra Nevada. e Lone Pine earthquake has been estimated at close to an 8.0 magnitude and its severe jolting reportedly stopped clocks in Truckee.

In Sacramento, where the shaking lasted for 90 seconds, buildings rocked alarmingly, as panicked residents poured into the streets in fear. Hard-rock miners working the night shift far below in the dark shafts and drifts beneath Virginia City, Nev., claimed that the “sensations in the bowels of the Earth were very disagreeable” and they would “in nitely prefer to being on the surface during earthquakes,” a typical Comstock understatement. e dramatic 1872 Lone Pine earthquake represents powerful seismic forces that literally uplifted the Sierra Range just 149 years ago — the blink of an eye in geologic time. Vertical displacement was measured at nearly 17 feet with a lateral movement near 40 feet. It left no doubt in the minds of residents that their beloved mountain range could still grow in leaps and bounds. 