Correlated transient fluid pulsing and seismic tremor in the Costa Rica subduction zone

Abstract

Continuous measurements of fluid flow were made over a six month period across the Nicoya Peninsula, Costa Rica (Pacific), convergent margin utilizing osmotically-driven fluid flow meters designed to quantify both inflow and outflow rates on the order of ∼10^− 5 to 3 cm/d. Significant transience in flow was observed through the surface of the forearc. Three periods of correlated flow signals were seen on the subduction forearc among three instruments located in the out-of-sequence thrust (OOST) zone over along-margin strike distances of ∼30 km. Amplitudes of ground velocity recorded on collocated ocean bottom seismometers (OBS) increase during the three correlated flow events. The seismic signal has frequency characteristics that resemble volcanic and non-volcanic tremor. We hypothesize that repeated plate boundary slow slip events, potentially originating at the up dip limit of the seismogenic zone, generate the observed signals within the toe of the forearc. We propose a model in which the poro-elastic stress/strain field around a series of creep dislocations simultaneously forces flow through fracture networks in the forearc and oceanic basement rocks and induces diffuse flow through the shallow sediments. The former generates the seismic tremor-like noise recorded by the OBSs and the latter generates the flow transients recorded by the fluid flow meters. We suggest that high sensitivity fluid flow meters can be utilized to detect transient tectonic strain events in offshore environments where traditional geodetic techniques lack resolution or are not possible.

Introduction

The nature of temporal changes in slip processes on subduction faults has a broad range of socio-economic implications as these plate boundaries generate destructive earthquakes and tsunamis and account for the majority of world-wide seismic energy release [1]. Low dip angle subduction megathrusts are often thought to have a relatively broad simple down-dip zonation in fault coupling consisting of well- or partially-coupled plate interfaces capable of releasing significant seismic moment, commonly termed seismogenic zones, sandwiched between upper and lower aseismic zones [2]. These zones have unique mechanical, hydrological, and time dependent rupture responses that interact to generate the seismic cycle. However, we suspect that subduction megathrusts in systems like the Middle America Trench, can be highly laterally heterogeneous and should be thought of as having a spectrum of potential slip responses rather than a rigid partitioning of the plate boundary into seismic and aseismic regions. Indeed, a wide range of questions remain regarding the dynamics of convergent plate boundary processes. The occurrence of tsunami earthquakes and recent studies of continuous land-based geodetic data suggest that episodic displacement can occur within ‘aseismic’ regions, but the mechanics and necessary frictional properties are poorly defined (e.g., [3], [4], [5], [6]]. Evidence is also emerging for subtle pre-rupture signals that indicate fault breakdown prior to earthquake initiation [7], [8], [9], [10], although the predictability aspect of such signals remains mired in controversy. We would like to characterize the nature and controls on spatial changes in plate coupling (e.g., [11]), the impact of episodic strain on fluid migration patterns and fault stability (e.g., [12]), and the temporal behavior of plate boundary ruptures across a broad range of periods ranging from rapid seismic slip to ultra-slow slip and aseismic creep [5], [6], [13], [14], [15].

One fundamental problem for subduction zone studies is that the bulk of the region generating large interplate earthquakes lies offshore, where slow slip and creep events that radiate little or no energy at seismic or tsunami frequencies cannot be observed or studied adequately. To fully understand subduction zone dynamics a variety of geodetic techniques need to be established that are suited to offshore continental margin environments including such methods as acoustic/GPS [16], [17], [18] and borehole pore pressure and strain methods. These ODP borehole pore pressure measurements have lately revealed potential strain transients in permeable upper oceanic basement of the Juan de Fuca plate [19]. Presumably seismic and aseismic events would yield similar pore pressure transients in wells in creeping subduction faults. Other relatively inexpensive offshore geodetic techniques also need to be developed to supplement the spatial coverage of the borehole or the other observation methods. We show data from a pilot study off the Pacific coast of Costa Rica that address this problem utilizing new instrumentation that measures fluid flux at the seafloor.