Illuminating Physical Facets of an Ecological Gem: Monterey Bay, California

By John P. Ryan, Monterey Bay Aquarium Research Institute

Bountifully occupying the heart of a National Marine Sanctuary, Monterey Bay, California, is an ecological gem. The biological richness of this ecosystem is supported largely by transport of deep, nutrient-rich waters into the shallow sunlit zone, a process by which photosynthetic primary production is stimulated to fuel a wonderful web of life (Figure 1).

Studying a dynamic, rapidly changing environment like Monterey Bay is quite challenging. An important tool in the studies of complex coastal ecosystems is synoptic imaging by remote sensing. For example, visible- and infrared-remote sensing has shown that Monterey Bay is a hotbed for red tides, a type of phytoplankton bloom that can cause harm. Process studies integrating remote sensing with in-situ observations have explored how such blooms can develop and lead to harmful effects. It is essential in these interdisciplinary studies to understand physical forcing by the atmosphere and ocean.

By describing ocean-surface roughness, Synthetic Aperture Radar (SAR) provides a unique description relevant to understanding physical processes and their ecosystem consequences. After exploring the use of SAR in interdisciplinary studies of Monterey Bay, the author was motivated to see what the greater archive of SAR observations of this region could reveal. The Alaska Satellite Facility’s (ASF) Web interface greatly facilitated searching and subsetting within the archive and batch processing with ASF’s MapReady software efficiently produced a collection of more than 2,400 images for further examination.

The most-frequently observed feature in the SAR archive of Monterey Bay is a distinct dark swath. Although variable in location, linearity, dimensions, and co-occurrence with other physical signals, manifestations of this signature shared the common characteristic of being oriented NW/SE across the northern bay, and most were limited to the northern bay (Figure 2). The dark swaths, representing areas of smooth sea-surface slicks, were observed in every month, except January. While 51% of the postseismic-surface deformation is interpreted as localized afterslip on or near the fault surfaces that slipped initially. The afterslip for each earthquake has its own unique signature. For the Nima-Gaize earthquake, both the initial rupture and the afterslip occurred on a pair of synthetic fault planes (Figure 4). For the March 2008 Yutian earthquake, there is significant afterslip concentrated towards the ends of the initial rupture, at shallow depth. The afterslip for the August 2008 Zhongba County earthquake occurred on both the coseismic rupture surface and another surface offset by a few kilometers. For the Nima-Gaize and Yutian earthquakes, sufficient data exists to create a time series of postseismic-surface deformation and can assess the time dependence of the afterslip. In both cases, rapid motion during the first couple of months gives way to a slower rate of deformation during the ensuing few months and by about 9 months after the coseismic rupture; most of the deformation has run its course. For the Nima-Gaize event, the total afterslip represents about 10% of the initial coseismic moment.

Also of interest is the exploration of rheological properties of the mid-to-lower crust, particularly in the light of the ongoing debate about the mechanical nature of the crust in Tibet. Another advantage of studying dip-slip earthquakes is that for such events, the deformation fields for localized afterslip and distributed viscoelastic relaxation look very different (unlike the case of strike-slip earthquakes, where the two mechanisms yield very similar surface-displacement patterns). Thus far, no evidence is seen in the postseismic deformation field of the events for distributed viscous flow at depth. For central Tibet, a lower bound of 5 x 1017 pascals can be placed on the viscosity of the lower crust and continued observation should allow for improvement on this constraint.

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