Are such small magnitude earthquakes (current seismicity) accepted to correlate them with the neotectonics of the region? If yes, then please elaborate how one should approach, but if no, then please recommend other applications and scope in this regard.
Yes, they are related. The analysis of seismicity and its relation to tectonic depends on the number of seismic stations you have in the area and specifically about the failures who wants to study. If the seismic stations are distributed around the fault, you can make focal mechanisms to determine the mechanism of it. If you have broadband stations, could then, for earthquakes with magnitudes between 4 and 5 make the investment slack time to do other more specialized analyzes.
Not being a seismologist I can't really comment on this (surely very reasonable and expected) correlation. I am bit surprised that earthquake of magnitude up to 5 may be considered "microearthquakes". After all, their energy release is equivalent to the simultaneous explosion of more than 2 Hiroshima-class atomic bombs. I always thought the cut-off is more in the magnitude 2 range. Important papers to the subject of microearthquakes have been published by Gerassimos A. Papadopoulos, [email protected]
Friedemann Freund
I fully agree with Waldo, low energy earthquakes may be correlated to a single fault or fault system. They may be used to study the relationships among different faults and to analyze their mechanical interactions. They can be also used to have information on the possible role of fluids in the triggering mechanism of earthqaukes. Low energy events may be also triggered by anthropic activities (mining and geothermal activities). Finally, these earthquakes could give information for the identification of buried faults (if any).
First at all, microearthquakes are not low magnitude earthquakes. In the book "Elementary Seismology" by Richter, in 1958, you can read in page 16: "Unfortunately, the noun microseism is used in a different way. Microseisms, quite illogically, are not small earthquakes; they are more or less continuous disturbances in the ground recorded by seismographs. Most of them appear to be conected with weather ...".
Unfortunatelly, at the present time, both concepts are considered as synonyms.
Of course, low magnitude earthquakes, particularly in areas with low seismicity and low seismic hazard, can be an inestimable tool to know about the tectonics of the region. Beyond that, in order to display unknown tectonic features (faults) and its characteristics, from the distribution of well located epicenters and focal mechanism data.
Lot of things can be learned even from small earthquakes, From getting first ideas on scaling laws (earthquake energy and size of fault), to effects of wave propagation. All this information can be exploited for modeling larger earthquakes, which may have occured in the past. Indeed, being strong events rare and therefore lacking of instrumental data, seismologists often have to extrapolate somehow from smaller earthquakes (with all the caveats implied). As Jose said, valuable seismotectonic information can be obained, both from fault plane solutions, moment tensor inversion but also from the distribution of the seismicity, which may delineate tectonic structure. Many papers have been published on high-precision location with astonishingly high resolution of the fault geometries.
It´s true: microearthquakes, low magnitude earthquakes and microseisms are different seismic realities! But I think that our colleague Rakesh is interested in other subjects.... The analysis of earthquakes with magnitude lowers than 5 gives a lot of information about the present tectonic activity of the area considered. An accurate location of the events indicates which faults are active and their focal mechanisms inform about how these faults are moving. The analysis of the stress tensors offers very valuable information about the new distribution of the stresses after an earthquake and the magnitudes and the temporal occurrence of the seismic events cast light about the level of energy liberated through seismic waves and the way this process is occurring. The study of the coda waves of the records allows you to estimate the seismic attenuation in the area, a decisive factor in seismic hazard estimation…… You can see that this field of research is really exciting!
The seismic energy generated by the microearthquakes attenuates
rapidly at short distances. By virtue of this fact they are only observed
at shorter epicentral distances. Therefore, a dense seismic network
surrounding the faults of interest is a must to study the microearthquakes.
The large number of stations enables accurate locations of the microearthquakes
which in turn provide an image of the faults that generate the small size
events. Also, the seismological data acquired by a dense seismic network
can be used to obtain a tomographic image of the crust that accomodates
the faults by estimating a 3D velocity structure model.
Moreover, the focal mechanism of the microearhquakes can be retrieved from
the first motion polarities and the apmlitudes of the P- and S-waves as well
as from the waveform modeling. Once we obtain several focal mechanisms in
a region these focal mechanims can be used to obtain the orientation of
the principal stress that generate the microearthquakes.
Recently developed methods using waveform correlation to obtain precise relative locations have significantly increased the value of small earthquakes. Groups of locations using this method can clearly define fault lines rather than just indicating activity on faults defined from other information. For example swarms of earthquake re-locations in the Salton Sea area have shown precision such that they demonstrate activity proceeding along one fault line and then subsequently shifting onto another intersecting fault. Once the locations become this precise all of the other parameters that can be measured from the seismic signal are more meaningful for interpretation of fluid movement, rock types, or local deformation. In my work, geothermal exploration, these events can be used to understand whether the earthquakes are related to fluid injection, fault movement, or magmatism. Once we understand movement patterns on the faults we can define areas of local extension to apply as well targets. From an academic perspective seismic activity in these areas can reveal the configuration of the remarkable shear-extension fault-fracture meshes that comprise some geothermal systems.
Dear Rakesh,
Since the colleagues have already answered the yes-or-no question above (it is a definitely yes) , I am about to suggest some papers to read, if you are interested. In the Pannonian Basin (and in most of the surrounding other basins, like Vienna Basin) the earthquakes are fortunately typically of M
For geological studies, microearthquakes can be used in various ways. Their location and even better their relocated location can highlight the active tectonic strucutres at depth (faults, brittle-ductile transition zones). From profiles, focal mechanisms and possibly multiplet geometry, the geometry of these structures can be inferred. Extrapolating the structure along dip back to the surface, corresponding mapped fault traces at the surface can be identified. In the presence of swarms, if a migration of earthquakes is noticed it coud indicate the presence of fluid circulations at depth. The direction of migration indicates the direction of the pore pressure pulse. First order estimates of permeability and hydraulic diffusivity can be obtained after assumptions on the fluid source location and the origin time of fluid infiltration. Studies based on analysis of focal mechanisms allow estimating the regiona stress field.
My new P-H pulses system can detect as low as M1.2-1.4 we have swarms of these earthquakes near Perth WA Australia o9n very active fault liens you can request files
Phil
First of all, it is important to know how much event you have ? having just few events is in my opinion useless. If you have a large number of events with a dense network having a good azimuthal coverage, in this case you can have use it for tectonic studies. In any case, you need to have precise location of these events to be allowed to linked them to any probable seismic sources.
on the other hand, if you have waveform dataset you can invert them and obtain information of source parameter.
THe coda can be inverted to locate structures that scatter seismic waves. See Chen and Long, Bull Seism Soc Am, 90,2, 512.
We add these to our database to learn from the events and see the time delay from the precursor to the EQ event I use Mathematica9 to build up my database which comes from my detectors and we are progressively adding other detection sites around Australia to RDF (Radio Direction Find Using standard Ham radio procedures)
I am in process of finalizing abstract with details
Phil
The combination of using the focal mechanism solutions and locations of large amounts of relocated earthquakes can be used to build the 3D subsurface active fault plane in case we are lack of other data to constrain the geometry. I think this modeling can be applied under the software Gocad, which is an industry 3D imaging application. The paper from Plesch et al. (2007) is for your reference.
We already have 3D modeling over areas of South America.
I will forward if interested. (but please wait a while as I need to get permissions for this.
We train our systems on these smaller M 1.2 etc so that the heuristic software can easily identify these closer swarms
Kindly check for Nakamura technique as that uses horizontal to vertical spectral ratio in order to determine the amplification ratio for the soil. Also it uses microtremor data similar to the one you are referring to. People have used the same at multiple locations. Kindly refer to the publication below for an elaborate discussion.
Nakamura Y. "A method for dynamic characteristics estimation of subsurface using ambient noise on the ground surface," Quarterly Report of Railway Technical Research Institute, 30, Issue 1, 25-33, 1989.
Hope this will be useful for the work.
Thank you
The answer is absolutely yes because the seismicity is a strong image obtained at the surface of the Earth of some more and/or less deep geological processes. But several conditions must be met (see for exemple the Abdelhakim Ayadi's answer). Sometimes M
I recoomend to search the key words "ambient noise tomography". This technique is also used for earthquakes M
Instrumental seismicity is crucial to understand the neotectonism of a certain area. However alone these may not be sufficient. Instrumental seismicity is relatively young (around 100 years). However "neotectonism" means geologically recent times which may be several orders older than the instrumental seismicity recordings. Thus a comprehensive geologic analysis of the neotectonism of a certain area must include historical earthquakes which can be found in historical earthquake catalogs or publication on trenches astride active faults.
So the answer relies greatly on the recurrence interval of the major events in your study area.
Meanwhile may things can be done with the data you have.
1) Check if the geographic distribution of the events you have can be correlated with a specific fault or set of faults. (This can be revealed through earthquake clusters sometimes). Off course after the assessing the accuracy of location of these events.
2)It would be nice to project the earthquakes along a specific line. Then plot these projected earthquakes versus depth. This may reveal the presence of fault planes (especially for elongated clusters).
3)You can decluster the data you have to remove the effects of foreshocks and aftershocks. Then you can compute frequency - magnitude distributions to find b - values and assess the seismic hazard of your area. But here it would be better to include historical seismicity data.
P.S. Here is a reference example from Lebanon:
Meirova, T. and Hofstetter, R. (2012). Observations of seismic activity in Southern Lebanon. J Seismol DOI 10.1007/s10950-012-9343-2.
Also by using these small magnitude earthquakes as an indicator of fluid flow and permeability. You can use these for geothermal exploration and resource assessment.
HI everybody
I think that it is a very good question but we have to be more precise. Firstly, microearthquakes can be related either with earthquake swarms or aftershock sequences of a main shock. Secondly, microearthquakes can be clustered in a few zone or distributed along a possible fault zone. If true be told, any case you can use microearthquakes to obtain relevant geological information: 1- Time domain. The time dsitribution of microearthquakes with M
As indicated in the replies above, microearthquakes are standard tools for determining the locations and types of active faults or their subsurface parts. In particular, microearthquake clusters or swarms can be used for this purpose, making it possible to identify fault location with a relative location uncertainly of sometimes as little as metres to a few tens of metres. Microearthquakes are also a measure of the energy being dissipiated in a larger fault zone which hosts the microearthquake swarms. These topics relate to seismotectonics and earthquake mechanics.
A more applied use of microearthquakes is to map out potential or actual geothermal reservoirs. Enhanced Geothermal Systems (EGS) use the technique of 'hydroshearing' where fluid pressure in rock body within the crust is increased (through pumping) so as to induce slip on existing fractures, thereby generating microseismicity. By mapping the hypocenter or focus distribution of numerous microearthquakes, we obtain a three-dimensional view of the potential geothermal reservoir - commonly at 3-6 km depth. The potential reservoir is marked by a 'cloud' of hypocenters and is a rock volume where the permeability has increased - because of increased fracture apertures and interconnection as a consequence of the microfault slip and propagation.
Both these uses (and others) of microearthquakes - location of active faults or parts of faults and for mapping enhanced geothermal systems - are described in numerous papers and special issues, as well as in monographs and in Wikipedia.
Many other answers provide a spectrum of possibilities, but you have left out key information: duration of catalog, density and type of seismic recordings available to be analyzed, and relationship to larger earthquakes (e.g., could the seismicity be dominated by aftershocks?). At the worst possible end, you only have access to a catalog of earthquakes from a sparse network (or, heaven help you, the global network). You can play statistical games but realistically you'd be stuck fighting the completeness of the catalog. At the high end, if you have access to broadband recordings from a dense network you can determine depths accurately, get relative locations with very high precision [accuracy depends on velocity model], determine focal mechanisms, for somewhat larger events you can model waveforms to determine the fault plane, etc.
Also unclear is what you goal is-- "correlate [earthquakes] with the neotectonics of the region" meaning...? You want to try to map surface faults to depth? you want to see if current seismicity is reflective of the geologically estimated strain?
If you are working in the Himalaya (a guess from your affiliation), there is a rich literature on the seismicity as recorded from global networks, regional networks (e.g., Nepalese network) and temporary networks (e.g., HiCLIMB). As I expect you know, a lot of the primary data in the region is not widely available. Some of this work has focused on identifying the edge of locked zones of megathrusts to help estimate plausible magnitudes of megathrust earthquakes, some has been used to try and map the surface thrusts down into the subsurface, and some controversy has been associated with the presence of earthquakes in the lower crust vs upper mantle. These earthquakes have also been used to image the crust (P and S wavespeeds and even attenuation), but that is more distant from your neotectonic interests.
I think using earthquakes of M< 5 in a tomographic method for neotectonic structures at local or regional scales will be sufficient. The most critical thing is to obtain as many events with good azimuthal coverage so as to be able to get high resolution in your images.
Hello Everybody!
first of all I would like to thank you all for your prompt reply and providing the requested information. This is really a very healthy and high quality discussion which teach me a lot as well as it will certainly upskill my capabilities. And I apologize for mentioning earthquake of M
It depends on the available equipment or network. If you can monitor an area with several accelerometers you can get good informations on the active faults or even dicover new ones. You shoud read Courboulex (Peille earthquake in France). You can also compare with geomorphology data. For instance karsts are very good natural recorder of tiny movements.
Dear Rakesh,
In the geological records we can usually only detect events larger tan M 5.5.
Then we are talking about paleoseismic events of the past.
Modern earthquakes of M
http://www.ga.gov.au/earthquakes/recentQuakes.do?when=1&where=3&which=false&x=14&y=7
we have a swarm now near Perth WA Australia
If you have access to the raw records, then double-difference relocations can improve the imaging of the active faults. Many times they can then be related to individual faults, especially if focal mechanism fit with the trends. One thing to remember is the the larger faults- like the megathrust- are usually locked and quiet in microseismicity. The microseismicity shows the smaller faults that adjusting. I was just working with people on a swarm in southern Italy. 5000 events with the largest a M5.
Recurent small earthqake is indication of movement and adjustment of active faults. If they are swarm then they can be identified as volcanogenic or tectonic. Both the cases it is interesting for tectonic interpretation and prediction.
From my experience (Andean foreland); tectonic regime counts. In low-moderate strain rate regions, seismicity does not illuminate morphogenic (Neotectonic) structures, particularly under current shortening.
Even large EQs are hard to relate to major faults or features with neotectonic activity. We may probably need a much longer time window of instrumental seismicity ir order to find some geometric patterns that fit with the surficial structures.
Yes, I accept to correlate them with the Neotectonics of the region, I mean in my research area in Iraq, that I found a relationship between the small earthquake and Neotectonics process in that area !!