Stephanie Prejean, Stanford University Department of Geophysics

The Interaction Of Magmatic And Tectonic Processes in the Long Valley Caldera, Eastern California

  • DINNER MEETING - Tuesday, March 12, 2002
  • Location: Stanford University

  • 5:30 PM-Wine Tasting: Mitchell Bldg., first floor
  • 6:15 PM-Dinner: . . . Mitchell Bldg., first floor
  • 7:30 PM-Meeting: . . . Bloch Lecture Hall (TCSEQ Room 201)

    See map to Mitchell (Dinner)
    See map to Bloch (Lecture)
    See more info on the elegant new Teaching Center, Science and Engineering Quad TCSEQ Room 201 (Bloch Lecture Hall).

    Anyone wishing to attend the lecture only is welcome at no cost.

    This will be the 344th meeting since 1954.

    About the Speaker

    Stephanie is originally from southern Louisiana (hence the Cajun last name). She obtained her BS in Geology from Memphis State University in 1996, and subsequently came to Stanford to complete her Ph.D. under the supervision of Mark Zoback and Bill Ellsworth. Stephanie completed her Ph.D. on the seismology and tectonophysics of the Long Valley caldera area in February, 2002 and will present a general summary of that work in this PGS talk. In March of 2002, she will begin a two-year postdoc with David Hill, John Langbein, and Malcolm Johnston of the the USGS to continue her work on Long Valley.

    cross-section of a caldera

    Abstract of Stephanie G. Prejean's, Ph.D. Dissertation, Stanford University

    The Interaction of Tectonic and Magmatic Processes in the Long Valley caldera, California

    The Long Valley caldera of eastern California is a hazardous province where tectonic and magmatic processes interact to drive on-going seismicity and deformation. The caldera is located on the boundary between the Basin and Range province and the Sierra Nevada batholith along the actively extending Sierra Nevada range-bounding normal faults. It is not clear if Basin and Range tectonic extension drives magmatic intrusion in this area or if magmatic activity is independent of regional tectonic processes. Magmatic intrusion into the caldera and extensional faulting are temporally coupled, yet it is not clear how these processes mechanically interact and potentially trigger each other.

    In this dissertation, I investigate the interaction between tectonic and magmatic processes in the Long Valley caldera over a range of scales, with the goal of developing a comprehensive model for the observed activity in the caldera. To gain a first order understanding of the mechanics by which the Long Valley area deforms, local fault geometries need to be established. To this end I relocated seismicity in the greater Long Valley area. The resulting high-resolution locations reveal a systematic fabric of faults within the caldera and in the Sierra Nevada basement rock to its south. From the focal mechanisms associated with individual faults, I developed a kinematic model for seismic deformation in the Long Valley caldera. Seismicity within the caldera occurs primarily on a set of east/west-trending right-lateral faults in the caldera's south moat. Since the south moat is located in a left step of the Sierra Nevada range-bounding normal faults, the south moat shear zone in essence forms a "transform" zone between loci of Basin and Range extension. In the Sierra Nevada block, directly south of the caldera, tectonic extension is accommodated by an east-dipping oblique-normal fault and two left-lateral strike-slip faults in its hanging wall. The location of these faults in the footwall of the Sierra Nevada range-bounding normal fault at this latitude, suggests that Basin and Range extension is potentially cutting into the Sierra Nevada batholith in this area.

    To understand better the mechanical interaction of tectonic and magmatic processes at the regional scale, I performed a series of focal mechanism stress inversions in the caldera area. The inversions show that around the caldera the minimum compressive stress is perturbed from the more regional WNW-ESE direction to a NE-SW orientation. Dislocation modeling of the mapped stress field reveals that the stress perturbation cannot be explained solely by the intrusion of magma beneath the resurgent dome, but may reflect the large-scale left-stepping offset in the Sierran range-bounding normal faults. Thus, the direction of fault slip seems to be controlled by regional tectonic processes rather than local magmatic processes. This implies that Basin and Range extension governs activity in the caldera and possibly provides conduits for ascending magma.

    To understand the relationship between tectonic and magmatic processes at the scale of the earthquake source, I investigate the influence of magmatic activity on earthquakes by examining the source processes of earthquakes and by studying the spatial and temporal development of seismicity during a crisis episode. The great majority of earthquakes in the caldera region appear to be typical "tectonic" earthquakes with source parameters similar to those observed in non-volcanic regions. However, a small number of earthquakes show magmatic signatures. A close examination of a seismicity swarm on November 22, 1997 in the western south moat of the caldera reveals that the swarm was triggered directly by magmatically derived fluids. Thus, although earthquakes slip in accordance with the regional tectonic stress field, magmatic activity can trigger seismicity by decreasing the effective normal stress across faults.

    Reservations: The preferred way to make reservations is simply to email Janice Sellers at by Friday, March 8, tell her you will attend, commit to pay, and bring your payment to the meeting. Janice always emails a confirmation; if you don't get one, assume email crashed yet again and email her a second time. A check made to"PGS"is preferred, payable at the meeting.

    If you want to pay in advance:

    Stanford faculty and students: Please make dinner reservations by Friday, March 8. Contact Dr. Juhn Liou via his mailbox (and leave check), Geological and Environmental Sciences Office, Geocorner - Bldg. 320 (Rm. 118). Make checks out to"PGS."

    All others, including faculty and students from other Bay Area universities and colleges and USGS: Please make dinner reservations by Friday, March 8. Contact Janice Sellers, at Seismological Society of America, 201 Plaza Professional Building, El Cerrito, CA 94530, phone (510) 559-1780. Send check made out to"PGS"to Janice.

    Dinner is $26.00. Includes wine (5:30 to 6:15 PM.), dinner (6:15-7:30 PM.), tax, and tip.

    For students from all universities and colleges, the dinner, including the social half-hour, is $5.00 and is partially subsidized thanks to the School of Earth Sciences, Stanford University (Note, no-show reservations owe the full price).

    Dues for Academic Year 2001-2002 ($10.00) should be sent to Janice Sellers, Seismological Society of America, 201 Plaza Professional Building, El Cerrito, CA 94530. Janice's phone: (510) 559-1780.

    Officers: Gary Ernst, President; Mike Diggles, Vice President; Vicki Langenheim, Secretary; Janice Sellers, Treasurer; Adina Paytan, Field-Trip Czarina

    Campus map

    Date created: 12/05/2001
    Last modified: 03/01/2002
    Created by: Mike Diggles, Vice President, PGS.

    c/o U.S. Geological Survey, MS-951, 345 Middlefield Road, Menlo Park, CA 94025. (650) 329-5404. email Mike Diggles at

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