Modelling eruption cycles and decay of mud volcanoes
Anna Zoporowski*, Stephen A. MillerGeodynamics/Geophysics, Steinmann-Institute, University of Bonn, Nussallee 8,53115 Bonn, Germany
Marine and Petroleum Geology 26 (2009) 1879-1887
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Modelling eruption cycles and decay of mud volcanoes
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The basic mechanism of mud volcano formation is the release of high-pressure mudtrapped at ... mud volcanoes is the focus of the conceptual and mathematical modelproposed ...
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Abstract
Recent debates about the eruptive behavior of mud volcanoes and their activation mechanisms have been driven particularly by the LUSI eruption in Indonesia that resulted in huge commercial and cultural damages.
Numerical modeling of mud volcanoes, of which few exist, can provide insight into eruptive behavior and contribute to hazard evaluation. In this paper, we present a simple model to describe fluid escape from an underground reservoir through a conduit, extruded as a mud volcano at the surface.
The governing equations result in oscillatory behavior, and we study the influence of changes in rheological properties of surrounding rock and fluid characteristics of the mud on extrusion dynamics.
We focus on understanding long-term eruption behavior, flow cycles, and decay factors. Model results can be used to estimate the discharge rates and extruded volume from assumptions on the mud reservoir and conduit, or conversely, the reservoir or conduit properties from discharge rates.
Mud volcanoes result from the extrusion of gas- and watersaturated mud both in sub-aerial and in sub-marine environments.
This semi-liquid is forced through openings in the upper crust, sometimes producing massive quantities of mud on the surface, as evidenced by the recent LUSI mud volcano in Java, Indonesia.
Globally, the distribution of mud volcanoes shows about 1800 individual sites (Dimitrov, 2002), and extraterrestrial occurrences are also documented (Fortes and Grindrod, 2006; Skinner and Tanaka, 2007; Skinner and Mazzini, 2009).
The basic mechanism of mud volcano formation is the release of high-pressure mud trapped at depth.
Triggering mechanisms of the volcanoes are still debated, but various hypotheses include earthquake-triggering, fault failure, and drilling (Manga et al., 2009, and references therein).
The morphology of mud volcanoes varies, and includes conical vents and bubbling mud pools (Fig. 1(a)).
Some mud sources are shallow <1 km, while others are fed by reservoirs at depths of up to 6 km.
Vents range from the centimeter scale to several hundreds of meters (Aslan et al., 2001; Mazzini et al., 2009a,b).
Extruded material can include mud, gas, boulders of clay or other solid material, indicating that dike-like conduits form in response to over-pressured fluids flowing along permeable fractures, eroding the wall rock and evolving to an open vent (Bonini, 2007).
This study investigates some mechanical considerations of mud volcanism. While some mud fields are continuously active (with ongoing substantial seepage for more than 60 years of observation), other areas exhibit an alternation between periods of eruption and relative quiescence.
The time intervals between significant material escape during dormant stages vary from minutes to several days, but in many cases show a cyclic behavior.
For example, the Dashgil mud volcano in Azerbaijan is characterized by continuous pulsating venting of mud, water and gas (Hovland et al., 1997; Mazzini et al., 2009a,b).
Onshore in Trinidad, it has been shown that the vertical conduits allow the escape of gas-charged methane-rich cold seeps.
The mud volcanoes have cyclic phases of eruptions, where the initial sedimentary mobilization could have occurred from pore water in deep sandy reservoirs (Deville et al., 2006).
For the mud field along the Pede-Apennine margin, fault failure cycles (tectonic loading and unloading) are hypothesized to promote a long-term fluid release cycling, during which over-pressured fluids are periodically discharged from a reservoir through the creation or the reactivation of fractured systems (Bonini, 2007).
A highly studied case is the eruption of mud and gas called LUSI, that started 29th of May 2006 in North east Java (Davies et al., 2008; Mazzini et al., 2007).
The discharge rate rose from 5000 to 120,000 m3/d during the first eleven weeks, flooding large area of the Sidoarjo village.
The mud flow then was observed to pulsate, and the extruded volume again increased dramatically following earthquake swarms.
Although LUSI was perturbed several times, the mud flow shows a clear tendency to pulsate every few hours and to erupt in changing cycles – an important characteristic, that has lasted for more than two years and continues still.
The observed oscillatory behavior of mud volcanoes is the focus of the conceptual and mathematical model proposed here. We focus on the periodic characteristics and develop mathematical model equations where the solution for the material discharge rate oscillates naturally.
Results are presented that describe the solution and its dependence on initial conditions and physical parameters such as fluid properties and characteristics of the mud volcano.
In the case where an eruption or seepage process decays with time, results show the evolution towards an oscillatory state about equilibrium.
Integration of the calculated discharge rate provides information about the volume extruded at the surface, allowing comparisons and observational constraints on
Discussion and conclusions
We have developed a simple model to describe periodic mud volcano extrusion processes. One conduit geometry and two mud reservoir estimates were investigated. Our results demonstrate the dependence of the discharge rate and extruded mud volume solutions on the initial conditions and parameter choices.
We interpret the oscillation period of the discharge rate as time between eruptions or maximum seepage, and the oscillation amplitude as the eruption or seepage intensity. In addition,we presented solutions for the estimated mud volume extruded at the surface.
Using a deflating mud source reservoir, the initial reservoir volume was identified as the main influence on the decay time of the discharge process. The initial influx rate also affects the decay of the process, but the differences are less significant. However, the amplitude is strongly driven by the influx rate, which we presume to be initialized by a triggering event that we do not attempt to identify. Other oscillation amplitude driving forces are the initial discharge rate, interesting for cases of disturbance of ongoing fluid flows, and the mud viscosity. Other parameters such as the conduit cross-sectional area, height and mud compressibility mainly influence the oscillation period and therefore the time between eruption peaks.
Investigations of a non-deflating reservoir (e.g. persistent supply and influx rates) show similar dependence of the initial conditions and parameters, but revealed an important modification possibility for seepage settings lasting for decades, as a continous influx of material produces a pulsating discharge rate around a nonzero equilibrium.
Neglecting the initial triggering and conduit opening process, we examined mud ascent through an open mud volcano conduit. For application to real mud volcanoes, a combination of this approach with fractured rock models is more reasonable, that is, a fractured system at depth ending up in an open conduit in shallow regions close to the surface. This will be addressed in future studies.
Direct comparison with observations is difficult because of the lack of parameter constraints such as density, viscosity and discharge rates. The discharge rates of the LUSI mud volcano are reasonably well-constrained, but other parameters are not. For certain parameter combinations the oscillator equation is overdamped, i.e. the discharge rate function loses its periodic characteristic.
Therefore our models do not only cover cyclic mud volcano behavior, but can also be used for single eruption investigations (not included here). However, cyclic behavior is very common, and our model can be adjusted to different mud volcano settings. This simple model captures the basic dynamics of an eruption or seepage process, particularly the controls on the periodicity and decay time. Recognizing that initial values such as the influx rate function I and reservoir volume estimates, and that parameters influence the system behavior, this model can mimic a wide range of mud volcano eruption situations. Possible extensions of this model would be to include fully compressible flow, the use of a non-constant viscosity, conduit erosion, and introducing various (possibly arbitrary) influx rate rules. We suggest that this model forms the basis for comparisons with data on mud volcano discharge rates and mud extrusion to the surface.
Acknowledgements We thank Adriano Mazzini and two anonymous reviewers who helped to improve earlier versions of the manuscript.
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