The 1998 Earthquake Sequence South of Long Valley Caldera, California: Hints of Magmatic Involvement

Abstract

A significant episode of seismic and geodetic unrest took place at Long Valley Caldera, California, beginning in the summer of 1997. Activity through late May of 1998 was concentrated in and around the south moat and the south margin of the resurgent dome. The Sierran Nevada block (SNB) region to the south/southeast remained relatively quiet until a M 5.1 event occurred there on 9 June 1998 (UT). A second M 5.1 event followed on 15 July (UT); both events were followed by appreciable aftershock sequences. An additional, distinct burst of activity began on 1 August 1998. The number of events in the August sequence (over the first week or two) was similar to the aftershock sequence of the 15 July 1998 M 5.1 event, but the later sequence was not associated with any events larger than M 4.3. All of the summer 1998 SNB activity was considered tectonic rather than magmatic; in general the SNB is considered an unlikely location for future eruptions. However, the August sequence-an "aftershock sequence without a mainshock"-is suggestive of a strain event larger than the cumulative seismotectonic strain release. Moreover, a careful examination of waveforms from the August sequence reveals a small handful of events whose spectral signature is strikingly harmonic. We investigate the waveforms of these events using spectral, autocorrelation, and empirical Green's function techniques and conclude that they were most likely associated with a fluid-controlled source. Our observations suggest that there may have been some degree of magma or magma-derived fluid involvement in the 1998 SNB sequence.

Radiative transfer theory for estimation of the seismic moment

“…Assuming an omega‐square source model according to ω| M Ml (ω)| = M Ml 0 /(1 + f / f c ) 2 (Aki & Richards 1980, p. 823) one can use to calculate an approximate source displacement spectrum if the corner frequency f c is known. Further assuming a stress drop σ of 10 MPa, it can be obtained from (Hough et al 2000). This approximate source spectrum is plotted as thin dashed line in Fig.…”

Source Properties of Earthquakes near the Salton Sea Triggered by the 16 October 1999 M 7.1 Hector Mine, California, Earthquake

“…For example, Hough et al (2000) infer a stress drop on the order of 0.003 to 0.02 MPa for a M 2.7 Long Valley earthquake that is inferred to have had a fluidcontrolled source process.…”

“…Such evidence can be subtle. Hough et al (2000) recently presented spectral evidence of a fluid-controlled source process for (nontriggered) earthquakes in the Long Valley region, and the waveforms of these events were shown to be unremarkable at first glance. (Available data were insufficient to investigate possible nondouble-couple components to their focal mechanisms.…”

A survey of 71 earthquake bursts across southern California: Exploring the role of pore fluid pressure fluctuations and aseismic slip as drivers

“…The observation of nondouble‐couple source mechanisms for large earthquakes at the time of swarms [ Dreger et al , 2000] supports various magma‐related scenarios for episodicity and time migration of those swarms. More directly, upward and sideways migration rates ranging from 30 m/day to 0.5 km/hr, possibly due to intrusion and overpressure of rising and spreading magmatic fluid, have been noted [ Hill et al , 1990; Hough et al , 2000; Prejean et al , 2003]. …”