Elsevier

Earth-Science Reviews

Volume 52, Issue 4, February 2001, Pages 261-297
Earth-Science Reviews

Influence of tidal effects on the periodicity of earthquake activity in diverse geological settings with particular emphasis on submarine hydrothermal systems

https://doi.org/10.1016/S0012-8252(00)00031-3Get rights and content

Abstract

Hydrothermal earthquakes may be defined as earthquakes caused by the hydrofracturing of the oceanic crust by submarine hydrothermal fluids. It is suggested that they correspond to the cracking of the ocean crust at the brittle–ductile transition that marks the downward limit of seawater circulation into the ocean crust. Semi-diurnal variations in submarine hydrothermal discharge in backarc settings and at the crests of mid-ocean ridges have been detected by hydrophone but no influence of tidal effects on the periodicity of microearthquakes around submarine hydrothermal vents has been observed so far. Although tidal stresses are too small to initiate earthquakes themselves, they may trigger earthquake activity when imposed on slowly increasing tectonic stress. These effects therefore tend to be observed in geologically active regions where stress levels in the earth's crust are high. The influence of tides on the periodicity of microearthquakes tends to be complex because the effects of ocean and earth tides are not uniform at the surface of the earth and because the microearthquakes occur in a wide variety of geological settings. Earth tides cause compression and dilatation of the oceanic crust which may be the most significant factor in initiating changes in fluid flow rate and microseismicity in submarine hydrothermal systems. Much of the information now available on the tidal periodicity of microearthquakes is based on relatively short-term monitoring which lacks statistical validity. The development and installation of deep-sea monitoring stations will permit rigorous long-term monitoring programmes to be carried out, which will lead to a better understanding of the nature and periodicity of microearthquakes around submarine hydrothermal vents.

Introduction

The influence of tides on earthquakes has been known for a long time. Omori (1908) referred to them as secondary causes of earthquakes. This is appropriate terminology because tidal accelerations of the earth are ±1 μm s−2, about 10−7 of the earth's gravity (9.81 m s−2) (Wenzel, 1997). Nonetheless, earth tides have the largest periodic stress variation in the earth's crust with cyclic variations of 0.001–0.01 MPa. Although these are small compared to tectonic stresses, the rate of tidal stress can reach 0.001 MPa h−1, which may exceed tectonic stress rates in some cases (McNutt and Beavan, 1981; Curchin and Pennington, 1987). These tidal stresses are not big enough to initiate earthquakes themselves but they may trigger earthquake activity when imposed on slowly increasing tectonic stress Klein, 1976, Tsuruoka et al., 1995, Emter, 1997. It should be borne in mind that a large part of the earth's crust is stressed almost to its failure strength such that an increase in shear strength of less than 1 MPa can induce failure (Scholz, 1992). Indeed, the earth's crust can be thought of as being in a critical state (Hirata and Imoto, 1991).

Within ocean basins, two tidal phenomena need to be considered:

(1) Ocean tides reflect the semi-diurnal vertical rise and fall of ocean waters as a result of the gravitational attraction of the moon and sun upon the earth and have fluctuations in the range 0–1 m (Schwiderski, 1980). Recent evidence has shown that approximately 1012 W (I TW) representing 25–30% of the total dissipation of tidal energy occurs in the deep ocean (Egbert and Ray, 2000).

(2) Earth tides are the response of the solid earth to the same forces as the ocean tides and have fluctuations in the range 0.07–0.15 m (Jackson, 1987). These motions are damped by internal friction in the earth such that the maximum strain lags behind the response expected in a totally elastic earth (Burton, 1986). Earth tides result in compression of the solid earth at low tide and expansion at high tide Melchior, 1966, Rinehart, 1980. The influence of earth tides on geyser activity has been documented by Rinehart, 1972a, Rinehart, 1972b. Earth tides may also have controlled the cyclicity of the submarine hydrothermal activity, which resulted in the formation of Proterozoic banded iron formations about 2500 Ma Williams, 1989, Williams, 1990. According to Lockner and Beeler (1999), regions of high fluid pressure, active geothermal or volcanic areas or zones of low effective normal stress are areas where correlation of earth tides with earthquakes may occur.

Microearthquakes have been defined pragmatically by marine geophysicists as having magnitudes less than 4 (i.e. a magnitude less than the threshold of regional and teleseismic networks) (Macdonald and Mudie, 1974). However, the Japanese Group of Earthquake Prediction Research had previously defined them, perhaps more rigorously, in 1962 as having magnitudes in the range 3–1 (Utsu, 1997). The American Geological Institute, on the other hand, considers them to have magnitudes of less than 2 but acknowledges that this limit is arbitrary and may vary according to the user (Jackson, 1987). In this study, we use the term microearthquake in a general sense without putting precise boundaries on their magnitudes.

Three major types of earthquakes can be recognized in the oceans: tectonic, volcanic and hydrothermal, although differences between these three are not fully understood (cf. Sykes, 1970). The relative importance of these three processes varies along mid-ocean ridges and between slow and fast spreading ridges Smith and Cann, 1992, Solomon and Toomey, 1992, Fornari and Embley, 1995, Auzende et al., 1995, Shaw and Lin, 1996, German and Parson, 1998, Macdonald, 1998.

Tectonic earthquakes are generated by changes in the tectonic stress field associated with fault activity, plate subduction, plate collision, or deformation in shallow sections of the earth's crust. At mid-ocean ridges, tectonic earthquakes of magnitude 5–7.5 are shallow focus (depth <60 km) (CPEMRC, 1985) and occur mainly along fracture zones as reported earlier by Francis, 1968a, Francis, 1968b. Earthquakes are also observed at the crests of mid-ocean ridges but it is usually difficult to decide whether they are generated by changes of stress due to ocean spreading (tectonic earthquakes) or by magma intrusion into the shallow oceanic crust (volcanic earthquakes).

Volcanic earthquakes are earthquakes associated with volcanic activity. Minakami (1960) has classified volcanic earthquakes into four types. Type A earthquakes occur as volcanic swarms originating from the base of the volcano at depths of 1–10 km and show no characteristic differences from tectonic earthquakes. Type B earthquakes also occur as volcanic swarms but at shallower depths, 0–1 km. Volcanic tremors occur when the magma chamber is rising. This is accompanied by a rise of the lava-lake level and tilting of the summit region of the volcano. Volcanic eruption is the final stage of volcanic activity. Earthquakes occurring during this phase are called explosive earthquakes or eruptive earthquakes.

Wolfe et al. (1995) have shown that the state of stress on the Mid-Atlantic Ridge is dominated by two processes, continuing tectonic extension and episodic magma emplacement. These authors recorded certain microearthquakes which they identified as volcanic earthquakes on the basis of their close association with the axial volcanic ridge and the clustering of some of these microearthquakes with anomalous characteristics beneath an axial volcano at 29°11′N. They suggested that these earthquakes were associated with the emplacement of magma. Subsequently, Delaney et al. (1998) have demonstrated that seismic swarms are related to the emplacement of dikes at the crest of mid-ocean ridges and elsewhere (cf. Cowen and Baker, 1998, Curewitz and Karson, 1998, Sohn et al., 1999a).

McNutt and Beavan (1981) were able to show that volcanic earthquakes at Pavlof Volcano in Alaska correlated with the solid earth tide. In particular, they showed that the volcanism is sensitive to changes in the ambient stress rate of about 2.5×10−7 MPa s−1 during times when the magma is near the earth's surface. In fact, most investigations of the tidal triggering of volcanic eruptions give positive results but nearly all are based on poor statistics (Emter, 1997).

Hydrothermal earthquakes may be defined as earthquakes caused by the hydrofracturing of the oceanic crust by submarine hydrothermal fluids. These microearthquakes are therefore related to the cracking of the ocean crust with the microearthquakes marking the depth of the brittle–ductile transition which denotes the downward limit of seawater circulation into the ocean crust (Glasby, 1998). Based on such considerations, the brittle–ductile transition may be considered to be a ‘propagating thermal front’ (Lister, 1977) (cf. Lister, 1983, Delaney et al., 1997a, Delaney et al., 1997b). The depth of the brittle–ductile transition is dependent on the ocean-spreading rate. It varies from less than 10 km on the Mid-Atlantic Ridge to less than 3 km on the East Pacific Rise Phipps Morgan and Chen, 1993, Glasby, 1998, Bach and Humphris, 1999. According to de Alteriis et al. (1998), this transition marks the 700–800°C isotherm at the crests of mid-ocean ridges. The flow of circulating seawater in the uppermost layers of ocean crust at the axes of mid-ocean ridges is confirmed by the high porosities and permeabilities of this layer and the abrupt transition to non-porous crust below Francis, 1976, McClain et al., 1985, Purdy, 1987, Vera et al., 1990, Lowell, 1992, Nesbitt and Tilley, 1992, Agar, 1994, Alt, 1995, Fornari and Embley, 1995, Schulz and Elderfield, 1997. Among the first to recognize the importance of hydraulic fracturing in generating microearthquakes of measurable magnitude were Knapp and Knight (1977).

In addition to the above, Montalto (1994) has identified seismic events from the La Fosse volcanic-hydrothermal system at Vulcano, Italy, resulting from the discharge of pressurized hot gases and vapours at the crater fumaroles. These events resemble volcano-tectonic earthquakes, although without any recognizable S-phases, and have been tentatively attributed to the hydraulic microfracturing of rocks that have been extensively altered.

Although hydrothermal earthquakes have not previously been specifically defined in the literature, several authors have presented evidence for their existence (cf. Glasby, 1998). For example, Riedesel et al. (1982) located very small microearthquakes with a maximum depth of 2–3 km within 1 km of submarine hydrothermal vents at the crest of the East Pacific Rise at 21°N. These were thought to represent the limit to the extent of hydrothermal circulation and brittle fracture above the axial magma chamber. These authors were the first to suggest that microearthquake activity around submarine hydrothermal vents could be caused by the fracturing of rocks as a result of hydrothermal circulation. Kong et al. (1992) proposed that variability in the maximum depth of microearthquake activity is related to variability in the depth of the brittle–ductile transition zone at 26°N on the Mid-Atlantic Ridge. Wolfe et al. (1995) also raised the possibility that microearthquakes located at depths of 3–7 km near the Broken Spur hydrothermal field on the Mid-Atlantic Ridge could be linked to hydrothermal activity. Mochizuki et al. (2000) similarly identified microearthquakes at depths of 3–7 km along the northern Reykjanes Ridge, which they also linked to hydrothermal activity. Sohn et al. (1995) studied the microseismicity at the Megaplume Site on the Juan de Fuca Ridge (cf. Hildebrand et al., 1997). They distinguished between two potential sources to explain the microearthquake waveform generated by the submarine hydrothermal systems: fracturing of the source rocks and vibrating fluid sources. Malahoff (1997) reported one family of earthquakes at Loihi Seamount, HI, generated by hydrothermal fluid boiling or other circulation effects.

Based on detailed seismological studies around a submarine hydrothermal vent field on the Endeavour segment of the Juan de Fuca Ridge, McClain et al. (1993) demonstrated that the distribution of earthquakes was bimodal with swarms of extremely small microearthquakes associated with cracking of the oceanic crust most frequent and local tectonic earthquakes much less frequent. A similar distribution had previously been obtained for the Mid-Atlantic Ridge by Francis (1968a). The relative frequency of these two types of earthquakes was thought to depend on the extent of faulting within the area with the faults providing the conduits for fluid flow. Recently, an earthquake swarm of apparent tectonic origin has been observed to induce temperature changes in hydrothermal vent fluids on the Endeavour segment of the Juan de Fuca Ridge 4–11 days after the first earthquake took place (Johnson et al. 2000).

The precise nature of hydrothermal earthquakes remains somewhat problematic. Early workers were unable to distinguish between volcanic, hydrothermal or magmatic processes for the origin of microearthquake swarms at the axes of mid-ocean ridges, although they did consider these earthquakes to conform to a Mogi type 3 pattern of activity Sykes, 1970, Macdonald and Mudie, 1974. Such earthquakes occur where there is a high stress concentration around numerous cracks and faults such that local fractures begin to occur under low stress conditions (Mogi, 1963). None of the papers mentioned in the previous paragraphs unambiguously characterizes hydrothermal earthquakes. In fact, these earthquakes are defined in terms of their origin rather than their wave form characteristics and there is probably a large overlap in the characteristics of microearthquake swarms of different origins. For example, Sohn et al. (1995) have recorded low-frequency spectral peaks around submarine hydrothermal vents which are similar to those from geysers and long-period events in volcanoes. Kong et al. (1992) have concluded that the variation in the maximum depth of microearthquake activity along the axes of mid-ocean ridge crests is mapping the depth of the brittle–ductile transition zone and argued that much longer deployments of ocean bottom seismometers are required to substantiate this.

In a key paper, Sohn et al. (1998) identified a cracking event at the crest of the East Pacific Rise at 9° 50′N. Four days after recording an intense swarm of microearthquake activity 1.0–1.3 km beneath the seafloor, the temperature of the associated hydrothermal vent fluids increased from 365°C to 373°C over a period of 1 week. The authors proposed that the microearthquake swarm was triggered by the fracturing and hydrothermal penetration of a 200–300-m-tall column of previously untapped rock. More detailed accounts of this work have been published by Fornari et al. (1998) and Sohn et al. (1999b).

Most recently, Singh et al. (1999) have demonstrated that the axial magma chamber on the southern East Pacific Rise at 14°S is about 50 m thick and is overlain by a 50–60-m roof. Above this, lies a 100–150-m-thick low-velocity layer, which is interpreted to be a hydrothermal fracture front that represents the base of hydrothermal circulation in the crust. The porosity of this layer is estimated to be about 6–7%. These observations support the idea that hydrothermal circulation could extend down to the roof of the magma chamber.

According to Hildebrand et al. (1997), low seismicity rates are typical of fast-spreading ridges and high seismicity rates are typical of slow-spreading ridges (cf. de Alteriis et al., 1998). In their review of the patterns of hydrothermal circulation at slow-, intermediate- and fast-spreading ridges, Wilcock and Delaney (1996) have proposed that, at fast-spreading ridges such as the crest of the East Pacific Rise, emplacement of dikes is the primary mechanism for generating deep-axial permeability. By contrast, patterns of hydrothermal venting at slow-spreading ridges such as the crest of the Mid-Atlantic Ridge correspond to those at fast-spreading ridges only during periods of high magmatic activity. During periods of waning magmatic activity, near-surface spreading is tectonically controlled. In this case, active faults and fissures provide a pathway for the hydrothermal fluids to the downward-migrating cracking front. Patterns of hydrothermal microearthquake distribution at the crest of mid-ocean ridges are therefore dependent on the seafloor spreading rate.

To support the contention that hydrothermal earthquakes are of low magnitude, no earthquakes with magnitude >5 were recorded at the crest of the East Pacific Rise between 17°20′S and 18°40′S where submarine hydrothermal activity and hydrothermal plumes are virtually continuous (Baker and Urabe, 1996) during the period 1964–1977 (CPEMRC, 1985). Mazarovich and Sokolov (1998) have concluded that submarine hydrothermal fields on the Mid-Atlantic Ridge gravitate to the relatively stable (aseismic) parts of the rift zones which are the most favourable for the stable circulation of hydrothermal fluids. The hydrothermal fluids are therefore believed to migrate along the strike of the rift zones.

Submarine hydrothermal activity is known to be highly episodic with variability occurring on the scale of 106–10−6 years. According to Lowell et al. (1995), earthquake swarms at oceanic ridge crests tend to have duration in the range 10−2–10−6 years (90 h–30 s). However, swarms can sometimes last more than a month. Tidal effects can induce variability in the flux of hydrothermal fluids at submarine hydrothermal vents and therefore in the pattern of microseismicity at these vents.

In this paper, we would like to demonstrate the influence of ocean and earth tides on the periodicity of microearthquake swarm activity in a number of different geological settings. We then discuss ways in which tidal effects might influence microearthquake activity and submarine hydrothermal circulation and suggest new methods for the long-term monitoring of these phenomena in the deep sea. The timing of this review is particularly opportune because much fragmentary information is now available on the tidal periodicity of earthquakes based on relatively short-term monitoring which lacks statistical validity. The development and installation of deep-sea monitoring stations over the next few years will permit rigorous long-term monitoring programmes to be carried out in dynamic areas such as around submarine hydrothermal vents, which will result in a much more complete assessment of the factors leading to the generation of hydrothermal earthquakes in the deep sea.

Section snippets

Periodicity of microearthquakes and submarine hydrothermal venting

Tides can influence the periodicity of earthquakes and moonquakes (as observed by seismometer) and of submarine hydrothermal activity (as observed by hydrophone). Periodicity in the distribution of microearthquakes, moonquakes and submarine hydrothermal activity is known to occur in a number of different geological environments such as at the volcanic front off southern Honshu, Japan, in backarc settings such as in the Mariana and Okinawa Troughs, at the crest of mid-ocean ridges, in intraplate

The influence of tides on the periodicity of microearthquakes and submarine hydrothermal venting

In the previous section, we have demonstrated the strong effects of tides on hydrothermal circulation at a mid-oceanic ridge and in backarc basins and the strong influence of tides on volcanic activity and earthquakes and tectonic earthquakes on land. Although the effects of tides on earthquake activity at mid-ocean ridges have not been observed, it is reasonable to think that this is possible. In this section, we review some of the factors that may control the observed periodicity in

Monitoring

As noted by Sato et al. (1995), only seismometers and a hydrophone were used to monitor hydrothermal venting in the southern Mariana Trough. Future studies will require measurements of temperature, mass flux, vent fluid velocity and other parameters to understand the physical mechanisms of hydrothermal activity. Virtually, all the studies reported in this paper have involved relatively short-term observations, typically of the order of a week, although longer deployments are becoming more

Summary and conclusions

The influence of tides on the periodicity of microearthquakes has been observed in a number of geological settings. This phenomenon was first recorded during the earthquake swarm on the east coast of the Izu Peninsula in central Honshu, Japan, in 1930 which preceded the Kita–Izu Earthquake. Several other examples were subsequently recorded on the Izu Peninsula and on the volcanic front which extends southeastwards through Sagami Bay, the best studied of which occurred during the eruption of

Acknowledgments

Much of the work reported in this paper has been carried out by Japanese scientists. We thank our colleagues for their sterling efforts over many years. We also wish to thank Dr. B. Atwater (ERI), Professor G. Igarashi (University of Tokyo), Professor A. Malahoff (University of Hawaii) and Dr. T. Urabe (GSJ) for helpful discussions. The senior author (GPG) was a Visiting Professor and subsequently a European Union (EU) Visiting Professor at the Earthquake Research Institute, University of

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