The Trembling Earth

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Landers @ 20

My favorite earthquake photo ever: dextral offset of a desert road surrounded by Joshua trees, taken by geologist Kerry Sieh during a reconnaissance trip five hours after the 1992 Landers earthquake.

Today is another noteworthy anniversary. [Quakiversary, as I'll start calling them if I'm not careful and don't restrain my incessant compulsion to merge phrases through obnoxious portmanteau-ing of things.] It marks twenty years since California’s largest earthquake in the last six decades, an earthquake that once again transformed the way we understand fault rupture.

Twenty years ago, on June 28, 1992, Angelenos and others throughout the desert southwest were rolled from their slumber by a massive 5am shock from the Mojave. The M7.3 earthquake ripped erratically through small California desert towns just north of Joshua Tree National Park (then a National Monument). The rupture jumped from one fault to another, linking together fractures in the ground that experts had previously considered discontinuous, unconnected, and thus capable of producing only relatively smaller earthquakes.

Fresh fault rupture passes directly beneath/through a house in Landers, CA after the 1992 earthquake. This scene illustrates the value of setting structures back from active faults as outlined in California’s Alquist-Priolo Zoning Act.

The towns of Yucca Valley and Landers were hit exceptionally hard, with violent shaking and in many cases primary ground rupture ripping through homes and buildings. Other desert towns–Twentynine Palms, Palm Springs, Barstow, Victorville–and the San Bernardino mountains experienced a lengthy period of strong shaking. The vast but more distant populations of Los Angeles and Las Vegas were rocked strongly and slowly, undoubtedly recognizing that something serious was happening somewhere… else. This was not their earthquake.

USGS Community Internet Intensity Map for the June 28, 1992 M7.3 Landers earthquake

Aftershocks rumbled and rattled as people began their days in premature wakefulness, and as details of the disruption in the desert towns occupied the headline spot in the 8am news, a sudden sharper jolt tore outwards through Los Angeles.

Have a look at the event occurring in CNN’s national feed. Skip ahead to 3:00 for the full experience.

The second earthquake, this one of magnitude 6.5, had been unleashed closer to L.A., rather far from the 5am mainshock and on a completely different fault system, centered near the resort town of Big Bear in the San Bernardino Mountains. Now aftershocks were emanating from the mountains above town and the giant tear out in the desert.

Basic map showing location and time of major earthquakes in the 1992 Landers sequence, and their aftershocks. Mapped June 28 fault ruptures are in yellow, other faults in black. The Los Angeles metropolitan area occupies the freeway-filled basin in the lower left.

Field teams from the USGS, Caltech, and other institutions around southern California mobilized immediately to investigate the damage and ground rupture from this earthquake, and thus is it one of the best recorded and documented earthquakes of recent decades. It was the first so have such extensive immediate field coverage.

To mark the current anniversary, the Desert Sun has a slideshow of pictures from the early 90s aftermath, along with some eyewitness reminiscence.

http://www.mydesert.com/article/20120628/NEWS0805/206280319/Landers-quake-still-vivid-minds-survivors

The Landers sequence is my favorite illustration of earthquake behavior because it covered all the basics: There were immediate foreshocks–all <M3–preceding the event by minutes, but a sequence of substantial quakes near the mainshock epicenter had occurred back in April. The aftershocks began immediately, and quickly delineated the extent of fault rupture. Then there’s the triggered earthquake and its own aftershocks. Much research has focused on the stress changes induced by the 7.3 mainshock and how they led to the 6.5 Big Bear quake that happened three hours later, outside of the strict aftershock zone.

We learned a lot about seismicity from this sequence of quakes, but its more lasting impact has been the recognition that nearby faults can link together and rupture in a single earthquake. The Landers earthquake broke open five separate, previously mapped faults, connecting them to each other through a network of unmapped structures that hadn’t been visible in the landscape prior. Since then many more earthquakes have occurred in this manor, including notably its Mojave successor the 1999 M7.1 Hector Mine earthquake, as well as the recent M7.2 El Mayor-Cucapah earthquake just south of the border.

Looking eastward down Linn Road on a 2010 visit to Landers, CA. The 1992 earthquake right-laterally offset the entire street grid several meters, so its effect is still quite visible today, 20 years later.

The well studied rupture from this earthquake is still evident out in the desert as subtle, rain-softened scarps stretching northward from Joshua Tree NP. The entire town of Landers was literally rent in two, with the eastern half sliding meters southward and the western half lurching to the north. The grid of streets still shows this spectacular offset. The whole area is worth a visit if you’re doing some fault-finding tourism, and especially if you love the Mojave.

Yours Truly perched next to the 1992 rupture scarp of the Emerson Fault near Galway Lake Road, site of the maximum slip. The photo is from 2007, 15 years after the quake, and the scarp has deteriorated since then, but will remain a conspicuous step in the landscape for millennia.

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12 responses to “Landers @ 20

  1. Ron Schott June 29, 2012 at 4:01 am

    I was about 150 miles away from the epicenter when the Landers quake struck and I’m sad to say I slept thru it. My field assistant felt it and once we heard radio reports of where it was and what had happened we took a day off to go find the fault trace. It took us most of a day to find the good ground offsets, but I got a couple of great slide photos of offset features (most of which are packed away right now, or I’d scan and share them). I remember particularly the right laterally offset chalk parking space lines in the dirt lot behind a small Baptist(?) church in Landers. The best offsets though were out in the desert along the Emerson Fault, if I recall correctly. We got there fairly late in the day and nearly got ourselves lost in the desert when the sun set. It was an unforgettable experience. Thanks for the reminder of its anniversary.

    • Austin Elliott June 29, 2012 at 5:15 am

      That sounds like a phenomenal experience: instant fault-finding adventure. We geomorphologists always want more of those. My PhD advisor’s Landers experience is pretty legendary. Maybe I should get him to tell it–I’ve always suggested he write a little essay about this–but the gist is that he was camping with friends in the San Bernardino Mountains. Lots of heavy shaking, out in nature with only the rocks and trees to jostle. The most notable thing he reports (other than the terror of not knowing where the quake hit and whether L.A. had just been wrecked) is the difference in sound between the more distant Landers quake/aftershocks and the much closer Big Bear quake/aftershocks, which occurred directly underfoot. The close ones made loud audible roars, while the distant ones were silent and bouncy. Frequency-dependent attenuation at work!

  2. sc100 June 29, 2012 at 4:03 am

    I was in Anaheim during this earthquake. I was 12 and my family was on a Southern California vacation. We were staying on the second floor of a motel and sleeping when the earthquake woke us up. The motion was VERY rolly. It definitely felt like you were out in the ocean riding big ocean swells. Nothing scary, you were just going along for the ride. After it was over, we went outside and everybody was out in the parking lot. Then we turned on the TV and watched the breaking news coverage. Interestingly, we didn’t feel the Big Bear quake. I don’t know where we were at the time, maybe driving around.

    • Austin Elliott June 29, 2012 at 5:23 am

      That’s quite a way to be introduced to SoCal! I suspect most people in quake-prone areas have some sense of the difference in urgency (maybe it’s just intuitive) between “sharp, jolting” quakes and “rolling” ones. SoCal has had enough geographic scattering of quakes that I think people recognize the rolling motion means you’re feeling a big quake that’s really hitting somewhere else, as opposed to the sharp, high frequency jolt of a nearby temblor.

      I am always fascinated by that period you mention, after an earthquake, when news and new information are slowly trickling in and everyone is realizing the location and scope of what happened. It makes a remarkably different perspective from our more common historical view of earthquakes as well constrained events whose effects we talk about factually. In the immediate aftermath there’s so much confusion and ongoing surprise as people reel from what happened.

      It’s also interesting how situationally dependent perception of earthquakes is. If you’re driving, or even running outside, it can be so easy to miss them–even sizable ones, obviously.

  3. terry (@shortstack81) June 29, 2012 at 6:06 am

    My grandmother took my cousin to California that summer and they spent it in San Diego, where one of my aunts was stationed at the time. It was his first plane ride and she promised him that there wouldn’t be an earthquake (he was terrified of them, and we really don’t have them in Pennsylvania.) Needless to say…the Earth had other ideas!

    I, of course, was jealous beyond words.

    (The Big Bear quake was captured on live international television via CNN, but I’ve yet to find any footage of it.)

    • Austin Elliott June 29, 2012 at 12:14 pm

      It’d be great to see (or hear) any media coverage of the quakes from that morning. The only footage I know of is from the Big Bear Solar Observatory, where four astronomers/technicians were checking on damage from the earlier quake and recorded their escape as the nearby 6.5 rocked them. I can’t find it anywhere on the net, but I have it in an old VHS documentary about earthquakes.

      Here’s the report of the observatory experience, for guidance if anyone wants to continue trying to track the video down:
      http://solarnews.nso.edu/1992/12_92.html#2

      • sc100 June 29, 2012 at 3:55 pm

        Here are some great YouTube videos of media coverage that morning. You’ll see some of the famous SoCal seismologists (Lucy Jones, Tom Heaton, etc.) but looking much younger!

        http://www.youtube.com/watch?v=LWms7nZZKh8
        http://www.youtube.com/watch?v=iQWbJX-HGuU
        http://www.youtube.com/watch?v=o4ucxltysUU
        http://www.youtube.com/watch?v=P54bZFB5kIE
        http://www.youtube.com/watch?v=VgYutThPmxk

      • terry (@shortstack81) July 2, 2012 at 9:16 am

        oh, I forgot part of the story. My cousin’s mother was watching CNN at the time of the Big Bear quake, having been informed that there was an earthquake that morning, and completely melted down.

        I don’t think either of them will ever go to California ever again.

  4. Uncle Tim July 1, 2012 at 6:22 am

    Fascinating stuff. Keep up the good work.

  5. Pingback: Stuff we linked to on Twitter last week | Highly Allochthonous

  6. Anthony July 17, 2012 at 10:52 am

    Remember this earthquake I was taking a summer bible class in Glendora when the Big Bear earthquake hit and it rocked the building I remember seeing the outside post sitting on Citrus Ave swaying the door was open and the door slam shut right after the earthquake.

  7. Christina August 27, 2012 at 3:13 pm

    I was living in Lucerne Valley at the time of the Big Bear quake. I remember being terrified because we couldn’t get out of the house. Things were shifting so badly the back door wouldn’t open and the front door was blocked by a china hutch that had fallen over. We spent the entire day sitting outside at our picnic bench listening to the radio broadcasts and watching the telephone poles sway with each aftershock. I was 16. Twenty years later I can recall with great detail everything from the terror of being woken up by an earthquake to the fascination of watching the telephone poles sway and the asphalt road ripple.

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