SNARL lecture explains the difference between intensity and magnitude
When it comes to documenting earthquakes, there’s magnitude, and then there’s intensity … and the two aren’t the same thing. During Tuesday evening’s Sierra Nevada Aquatic Research Lab (SNARL) lecture at the Green Church, Susan Hough, earthquake specialist with the US Geological Survey in Pasadena, detailed the difference between the two as part of her talk, “The 1872 Lone Pine Earthquake and Recent Activity in Long Valley.”
Hough, who was hounded by TV news media for information following the recent earthquake in Japan, has a resume which includes studies of the 1995 Ridgecrest quakes and their aftershocks, and the period of unrest that rumbled through the Eastern Sierra in 1998.
During the 1872 Lone Pine quake, one observer happened to be no less than John Muir, who reportedly ran out to experience what he called “a noble earthquake.” How big was it? Good question. In 1872, seismometers hadn’t been invented yet, and wouldn’t appear until the turn of the century. Pre-seismometer magnitudes can only be estimated, but researchers have other ways to figure out intensity. And, yes, there is a difference.
Magnitude, as per Charles Richter’s scale, characterizes the size of an earthquake by measuring (indirectly) the energy released. By contrast, intensity sizes up the local effects and potential for damage produced by an earthquake on the Earth’s surface as it affects humans, animals, structures, and natural objects, including everything from trees to bodies of water.
The Mercalli intensity scale, which dates back to the late 1800s, started with the widely used simple 10-degree Rossi-Forel scale, and today is based on a 12-degree Roman numeral system, in which I is virtually unnoticed, V corresponds to feeling the quake, buildings shaking and so on, VII denotes average amounts of damage, and X-XII indicate almost or complete destruction.
“We look at accounts,” Hough explained. “What was damaged, what did people feel?” Those are then lined up against maps of damages and comparisons of similar events that were able to be measured later, such as the 1992 Landers quake, and perhaps a closer match: the 1906 San Francisco earthquake.
“You can get an intensity value from anyone who was there to document what happened,” Hough said. “Intensity seems subjective, but it’s really about how buildings respond to various levels of shaking.” One Pacific Northwest quake, she said, was assessed by the amount of damage sustained to various chimneys in the area. “You have to interpret the accounts consistently, and not be swayed by any ‘drama’ that can be involved in some of them.”
In the case of the 1872 quake, there are about 150 accounts that can be used, as well as some photographic accounts of the aftermath.
Did you feel it?
Up until the proliferation of the Internet, the USGS fielded postcards to document intensity accounts. In 1999, for example, the USGS received 25,000 postcard accounts of the Hector Mine quake near Barstow in 1999. That temblor, which hit at about 2:30 a.m., “woke up everyone but the dead, and shook like gangbusters,” yet only ended up with an intensity rating of 2-3.
Hough pointed out one interesting feature about earthquakes that most of us probably never considered: big earthquakes don’t necessarily require big faults. The assumption, she said, is that major events only happen along major fault lines, but the 1872 Lone Pine quake occurred along a fault that wasn’t even close to the size and complexity of the San Andreas Fault, which spawned the 1906 San Francisco disaster.
In a head-to-head comparison, Hough showed that both were felt in many of the same places, even though the geographic area was much larger during the 1906 event.
The 1872 event was felt in many parts of San Diego, Los Angeles and other parts of southern and northern California, as far north as Sacramento and as far east as Elko, Nev. Whereas in 1906, people 200 miles or more away from the San Andreas Fault really didn’t notice the quake much, and intensity accounts drop off dramatically beyond that point.
In terms of magnitude, Hough argues that a M7.6 for the 1906 San Francisco is “implausibly low,” given the evidence suggests more than that. The 1872 Lone Pine event, 1872, however, Hough thinks was bigger, perhaps as much as an M7.9.
Can any of this research help predict a quake? No, but there might be ways to find some possible areas of likelihood. After an earthquake happens, it releases a lot of stress and things go quiet for a while. Renewed activity along a fault has prompted geologists to consider the hypothetical possibility of “stress shadows.” Increased stress can build up at the ends of a section where lots of stress was released, essentially moving some of the stress from one place to another due to sheer mechanics.
If a major intensity event occurred in Lone Pine, where else could one pop up? For example, more than 100 years after the Lone Pine quake, the entire corridor has been really quiet, but on Tuesday, just before Hough’s lecture, she said a M4.5 occurred near Keeler, Calif., a ghost town located 13 miles east of Lone Pine. “That got my attention,” she quipped.
Music to our ears?
The last major volcanic event locally happened about 760,000 years ago, though smaller events have been documented as early as 500 years ago, and most of those were north of Mammoth Lakes. Domes rising can lead to “earthquakes” and faults, which are really crust cracks due to the dome uplift.
In 1998, Hough placed several instruments at various points in the area, including the SNARL lab headquarters, Hot Creek, the Sheriff’s Substation below Mammoth, Tom’s Place and June Lake and recorded many quakes.
A few down to the south didn’t exhibit the normal wave patterns, and looked “messy.” Earthquakes are not unlike musical instruments and produce certain “tones.” Energy in those indicated that fluid was moving underground and causing cracks, giving off different “harmonics.”
Eruptions, unlike earthquakes, produce an escalating series of events that help geologists predict them. The episodes of unrest, however, can happen pretty suddenly. “When things get going, they get going pretty fast!” Hough said the long-term evolution, following historic eruptions in the north, could mean an eruption in the south, but more than likely in the distant future, not any time soon. “Don’t expect an eruption at Tom’s Place tomorrow,” she advised.
Check out tomorrow’s (May 10) SNARL lecture at 7 p.m. at the Green Church. Dr. Thomas Stephenson, Program Leader for the Sierra Nevada Bighorn Sheep Recovery will discuss “Sierra Nevada Bighorn Sheep: The Science of Recovery.”