Surface deformation of Long Valley caldera and Mono Basin, California, investigated with the SBAS-InSAR approach

Abstract

We investigate the surface deformation of the eastern California area that includes Long Valley caldera and Mono Basin. We apply the SAR Interferometry (InSAR) algorithm referred to as Small BAseline Subset (SBAS) approach that allows us to generate mean deformation velocity maps and displacement time series for the investigated area. The results presented in this work represent an advancement of previous InSAR studies of the area that are mostly focused on the deformation affecting the caldera. In particular, the proposed analysis is based on 21 SAR data acquired by the ERS-1/2 sensors during the 1992–2000 time interval, and demonstrates the capability of the SBAS procedure to identify and analyze displacement patterns at different spatial scales for the overall area spanning approximately 5000 km². Two previously unreported localized deformation effects have been detected at Paoha Island, located within the Mono Lake, and in the McGee Creek area within the Sierra Nevada mountains, a zone to the south of the Long Valley caldera. In addition a spatially extended uplift effect, which strongly affects the caldera, has been identified and analyzed in detail. The InSAR results clearly show that the displacement phenomena affecting the Long Valley caldera have a maximum in correspondence of the resurgent dome and are characterized by the sequence of three different effects: a 1992–1997 uplift background, a 1997–1998 unrest phenomenon and a 1998–2000 subsidence phase. Moreover, the analysis of the retrieved displacement time series allows us to map the extent of the zone with a temporal deformation behavior highly correlated with the detected three-phases deformation pattern: background uplift-unrest-subsidence. We show that the mapped area clearly extends outside the northern part of the caldera slopes; accordingly, we suggest that future inversion models take this new evidence into account. The final discussion is dedicated to a comparison between the retrieved InSAR measurements and a set of GPS and leveling data, confirming the validity of the results achieved through the SBAS-InSAR analysis.

Introduction

The Long Valley caldera and Mono Basin are located in eastern California, in an area surrounded to the west by the Sierra Nevada and to the east by the White Mountains. The Long Valley caldera, an east–west elongate oval depression with an extent of approximately 17 km by 32 km, was formed about 0.76 Ma B.P. (Hill & Prejean, 2005). Subsequently, between 0.76 and 0.6 Ma B.P., the caldera uplift and the rhyolitic lava flows eruption, referred to as early rhyolite, formed the resurgent dome (Bailey, 1989, Hill and Prejean, 2005).

The Mono Basin consists of several landforms belonging to different volcanic centers such as the Mono and Inyo chains, the Paoha and Negit Islands and some others (see Bursik & Sieh, 1989) with the centers of the basin being formed within the last 40,000 years.

Starting from the end of the 1970s and continuing to present, the Long Valley caldera and the Mono Basin have experienced seismicity and surface deformation above the background levels (Hill et al., 1991, Langbein et al., 1993, Langbein et al., 1995, Langbein, 2003, Newman et al., 2006). The most recent of these phenomena affected the Long Valley caldera, characterized by a significant unrest period occurring between 1997 and 1998. In this case, an inflation phenomenon slowly started in mid-1997 and then exponentially increased in late 1997, before rapidly returning to quiescence by mid-1998. The effects of this unrest episode were recorded via the exploitation of several geophysical instruments deployed in the area (including seismometers, EDM, dilatometers, tiltmeters and GPS) and via a leveling campaign that was carried out in 1997, following those performed in 1995 and 1992 (Battaglia et al., 2003a, Battaglia et al., 2003b, Battaglia et al., 2003c, Langbein, 2003).

We investigate in this study the deformation of Long Valley caldera and Mono Basin by applying the differential SAR Interferometry (InSAR) technique (Gabriel et al., 1989, Massonnet et al., 1993). This approach generates spatially dense ground deformation maps, obtained by computing the phase difference (interferogram) of two temporally separated SAR images, with the measured displacements representing the projection of the surface deformation on the SAR sensor line of sight (LOS).

In particular, the InSAR results presented here have been obtained by applying the algorithm referred to as Small BAseline Subset (SBAS) approach (Berardino et al., 2002), and represent an advancement over previous InSAR studies of the area. Most of the existing literature is focused on the deformation affecting the Long Valley caldera only (Fialko et al., 2001, Hooper et al., 2004, Newman et al., 2006, Thatcher and Massonnet, 1997). In contrast, we considered an expanded investigation zone extending for approximately 5000 km², including Long Valley caldera and Mono Basin, and we identified and analyzed both localized and large scale deformation effects, some of these being previously unreported. Moreover, the works of Fialko et al. (2001), Newman et al. (2006) and Thatcher and Massonnet (1997) utilize a small number of interferograms spanning specific deformation episodes and generated by using conventional InSAR processing. We use a significantly larger amount of SAR data and interferograms in order to follow the evolution of the detected deformation phenomena via the generation of displacement time series relevant to the whole period of observation. In this sense, our analysis is more similar to the work of Hooper et al. (2004) that, however, is just focused on the central area of the caldera and is carried out by using a Persistent Scatterers Interferometry (PSI) technique operating on full resolution (single-look) interferograms. In contrast, the applied SBAS approach allows us to analyze averaged (multilook) interferograms, thus reducing the amount of data to be processed and being very well adapted to geophysical phenomena over large spatial scales.

The paper is organized as follows: first, the geological setting and the recent dynamics of Long Valley caldera and Mono Basin are presented. Subsequently, the SBAS algorithm is described, highlighting the key characteristics and the main steps of the applied procedure. This analysis is followed by the presentation and discussion of the computed SBAS-InSAR results. A comparison with independent geodetic data is then presented. The last section is focused on the main conclusions of the work and on future developments. In the Appendix, analytical details about the SBAS technique are given.