June 28, 2012

Landers @ 20

Posted by Austin Elliott

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.