Quake pattern watched closely: How Reno temblors will affect faults at Lake Tahoe unclear

By Adam Jensen
Sun News Service
April 30, 2008

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Earthquake preparedness tips

• Make sure each member of your family knows what to do no matter where they are when earthquakes occur. • Know where your gas, electric and water-main shutoffs are, and how to turn them off if there is a leak or electrical short. Make sure older members of the family can shut off utilities. • Locate your nearest fire and police stations, and the emergency medical facility. • Talk to your neighbors about how could they help you, or you them, after an earthquake. • Secure household items that could cause injury if they fell during an earthquake. •Keep a disaster-preparedness kit including a fire extinguisher, adequate supplies of medications, a first-aid kit and handbook, flashlights with extra bulbs, portable radio, extra batteries, heavy-duty plastic bags, several days’ worth of canned or packaged food, a can opener, two weeks’ worth of water (allow at least 1 gallon per person per day), tablets or chlorine bleach for water purification, and a camp stove or barbecue grill. — Source: U.S. Geological Survey

As earthquakes continue to shake Reno, two reports released this month give new insight into the likely characteristics of a temblor at Lake Tahoe.

A 4.2-magnitude quake shook western Reno at 4:33 a.m. Monday, the latest in a series of quakes that has rocked the Mogul area since February.

Monday’s quake is considered an aftershock of a 4.7-magnitude quake that hit the area Friday, after which seismologists warned there was a slightly higher probability of a larger quake.

“Following an earthquake of this size, there is a small increase in the probability of a larger-magnitude earthquake in the west Reno area,” according to a statement from the Nevada Seismological Laboratory in Reno.

Earthquake activity is unpredictable and also could drop off, but scientists are watching the region closely.

“We’re still in kind of a 72-hour window for something that will be bigger,” said Diane dePolo, a network seismologist with the lab.

Whether the seismic activity west of Reno could stimulate movement along Lake Tahoe’s three major fault lines is unknown.

“We don’t have a magic crystal ball to see if something out here in Reno is going to affect something in Tahoe,” dePolo said. “Anyone in this area, even up in Tahoe, should be prepared for an earthquake.”

The chance of a magnitude-6.7 or greater quake hitting the Tahoe Basin is relatively small, according to an interagency study known as the Uniform California Earthquake Rupture Forecast that was released this month. Maps from the study show the basin ringed primarily in yellow, indicating there is a 1 percent chance of a 6.7-magnitude or larger quake hitting the area in the next 30 years.

A small portion of the East Shore shows a 10 percent chance of an earthquake of magnitude 6.7 or greater in the next 30 years, and a section of the West Shore shows a 0.1 percent chance of an earthquake of the same size in the same time period, according to the map.

The study was incorporated into the 2008 U.S. Geological Survey National Seismic Hazard Maps, which also were released this month. Seismic hazard maps, last completed in 2002, are used by insurance companies, engineers and a variety of government agencies in earthquake planning.

Included in the 2008 maps is a slightly greater chance of short-wavelength ground shaking, and a somewhat decreased chance of long-wavelength ground shaking from earthquakes originating near the surface at the South Shore, according to Mark Petersen, the U.S. Geological Survey’s lead on the hazard map project.

The shorter-wavelength quakes are more likely to damage shorter buildings, while the longer-wavelength earthquakes are likely to affect buildings more than 10 stories high, such as South Shore casinos, Petersen said.

Like dePolo, Petersen urged preparedness for residents of Nevada and California, two of the most seismically active areas in the United States.

“I think it’s clear that earthquakes do appear near Lake Tahoe,” Petersen said. “They have occurred in the past, and they will occur in the future.”