# Presentation: 067604

- Global and Local Arrays

## Topics covered: 

- Types of seismic arrays
- How arrays are used
- Example global networks

## Seismic array: 

- Has common time base (one clock)
- Has common recording center

*Recorded by the SOSN/POLARIS Seismic Network*

*Waves remain coherent as they propagate across the array*

**Notes:**
An array is a local collection of instruments that have a coherent response to the recorded seismic energy. Historically timing used to be a problem and so arrays were connected to the same clock, but with accurate timing provided by GPS this is not a problem now. 

## An array can be used as an antenna to determine the direction from which the seismic waves arrive.  This process, called beamforming, tells where the earthquake is located.

- An array can be used as an antenna to determine the direction from which the seismic waves arrive.  This process, called _*beamforming*_, tells where the earthquake is located.

**Notes:**
In the same way that a radio antenna can be used to locate the source of radio waves and array can be used to study an earthquake rupture. By summing the energy from different azimuths constructive interference will give the incidence angle of the energy on the array. 

##  The NORSAR Array

-  The NORSAR Array

## Seismic traces recorded at 4 sub-arrays of the NORSAR array in Norway

- Each group of 5 traces is from one sub-array.
- Inconsistent amplitude behaviour of the phases due to differences in the site responses of the rocks beneath each sub-array

## Array examples

- WRA

- GBA

- YKA

## Mantle velocity structure and arrays

**Model 1: **
- Smooth increase in velocity with depth
- Upper mantle travel-time curve has no triplications

**Model 2**
- This model has a 400 km and 650 km transition zone
- The travel-time curve has 2 triplications
**                  **

**Notes:**
An historical use of arrays was the identification of the 410 and 660 km discontinuities. Because the individual peaks and troughs of the phases can be correlated across the array, the apparent velocity of phases can be accurately calculated. This analysis showed branches (triplications) in the mantle arrivals indicating there must be steps in velocity in the mantle.

## Two phases are seen arriving at the array. The distance to the earthquake was 21.69 degrees.
Slowness measurements shown on the right graph clearly shows the two phases are arriving with different slowness

- A record section from the Yellowknife array in Canada

**Two phases are seen arriving at the array. The distance to the earthquake was 21.69 degrees.**
**Slowness measurements shown on the right graph clearly shows the two phases are arriving with different slowness**

**This indicates the presence of 2 travel-time branches**
**Numerous slowness measurements from many arrays confirmed that upper mantle has 2 discontinuities or transition zones.**

##                    Upper Mantle Ray Paths

-                    Upper Mantle Ray Paths

## Seismic network: 

- Each station’s clock is independent
- Data recording may be at the station or at a common data center

- Waves may not be coherent as they propagate across the network

**Notes:**
The differences between networks and arrays are smaller now then in the past. In fact, the same configuration of instruments may be considered an array for one event and a network for another, depending on the location of the earthquake. The figure here and the one on slide 3 are from the same stations, for the earlier figure the earthquake was teleseismic and the event was coherent across the stations. Here the event is local and so contains more high frequency energy which is clearly not coherent, so in this case the stations are a network.

## In a network, the location of an earthquake is found by triangulation.  Information from all stations is used.  

- In a network, the location of an earthquake is found by _*triangulation*_.  Information from all stations is used.

**Notes:**
While arrays can be used to investigate the source with beamforming techniques, networks are used to locate earthquakes using triangulation. This figure shows the basic idea, with the distance from each of three stations calculated from the phase arrival times and these distances (represented as circles) used to find the epicenter from the crossing point on the circles. 

## The Earth acts like a filter!

**Notes:**
An important factor in considering the differences between arrays and networks is the attenuation of seismic energy. High frequencies are attenuated more rapidly then lower frequencies as the energy travels away from the focus. This is often referred to as the Earth filter, as the removal of high frequencies by attenuation has the same effect as filtering. It is because of this effect that distant events may be coherent across and array. Whereas local events contain more high frequency energy and are incoherent.

## IRIS GSN SYSTEM

- Global networks choose instruments capable of recording long period waves.  Local arrays may use short period sensors to record local earthquakes.

## The 2004 Sumatra-Andaman Islands quake radiated very long period seismic waves that were recorded by global networks.

**Notes:**
The long period surface waves and very long period normal modes generated by the Sumatra quake were not recorded in networks and arrays of high frequency instruments designed to look at local events.

## Global Networks

**Notes:**
The GSN is a global network of instruments run by two institutions (largely for historical reasons, they used to work independently). The data collected on this network are archived at the IRIS Data Management System in Seattle and are freely available to anyone over the internet, with no restriction on use of the data.

## GSN Goals:

- develop high quality seismic data acquisition equipment
- deploy systems globally at 2000 km spacing
- make data openly available to all who request a copy

**Notes:**
The GSN aims to have a coarse instrument spacing around the whole globe. The system was designed to answer scientific questions about the Earth’s deep structure, rather than for hazard mitigation or seismic observatory operations. 

## GSN data are used to study the internal seismic structure of the Earth:

- Courtesy of Adam Dziewonski, Harvard University

**Notes:**
These are examples of the global tomography that has been done using GSN stations. The global coverage of high quality instruments has allowed small variations in the earth’s internal structure to be resolved, addressing questions such as heat transfer within the Earth, the fate of subduction plates and the structure of mantle convection.

## Slide 19

**Notes:**
One of the non-academic uses of global networks is the monitoring of nuclear explosions. The monitoring of nuclear testing is based on a network of seismic stations, hydroacoustic stations and radionuclide monitoring. However, seismic observations are key to the monitoring and the CTBT provides support for research into improved earthquake location and focal mechanism solutions.

## Slide 20

## Slide 21

**Notes:**
The FDSN is the coordinating organization that bring together national networks into a virtual global network. The FDSN promotes digital broadband seismology globally, promoting data exchange, instrument standardization, site selection, interoperability between networks, ocean bottom seismometers. Unlike the GSN it supports both a backbone of 300 globally distributed stations and regional, national and local networks. The FDSN maintains a network of data management centers, supported primarily through the IRIS DMS and promote real time data access and open access.

FDSN members in the region are: Indonesia, Thailand, Malaysia, Australia, Japan and the PTWC

## Regional networks

- The future Earth coverage will depend increasingly on regional and national networks.
- These	are supported for surveillance and alert,
- often have more stable budget and recognition
- are less interested in technical developments
- require real-time data availability and processing
- are less strict about VBB standards
- participate less in FDSN activities
- rarely have science under their mandate
- Extreme challenge to organize an efficient data exchange
- National priorities and requirements, restrictions to data access

## German GEOFON Network

**Notes:**
The German GEOFON Network is an example of a nationally supported, science driven network

## French GEOSCOPE Network

**Notes:**
Geoscope is another nationally supported global network.

## Japan HiNET

**Notes:**
HiNET is an amazingly dense network of stations and is used by the Japanese as a network for science and tsunami warnings. For teleseismic events it can be used as an array, as was done for the Sumatra earthquake.

## Chinese Digital Seismic Stations

**Notes:**
The Chinese have recently dramatically increased their network and now have over 670 broadband stations, compared to 11 in 1986.

## Plans for the Indian Ocean

- Courtesy L. Kong

**Notes:**
There are plans to hugely improve the station density in the region of the Indian Ocean; however, the ocean is always going to present a large obstacle to getting good data coverage for tsunami warnings. 

## A note on noise at seismic stations.

- The quietest stations in the world are on continents.

- Stations near the ocean are noisier.

**Notes:**
Noise affects the performance of networks and arrays. The two figures are recordings of the power at different wavelengths for two stations. On the left the station is in a continental interior (Siberia) and on the right the station is on land, but nearer to the ocean (San Diego, California). The station near the coast is much nosier in the ~1-10 sec range. 

## This peak is caused by ocean waves breaking on coastlines.

**Notes:**
The dashed black lines are the envelope of noise recorded at observatories, the small peak shown in the box is generated by waves breaking against the coastlines.

## This peak is caused by ocean waves also.

**Notes:**
The bigger, longer wavelength peak is also generated by the oceans, but is related to deep ocean waves putting pressure on the seabed.

## An important source of long period noise is changes in air pressure.

**Notes:**
Vertical components (light blue on this figure) tend to be quieter at the longer wavelengths, as at these period the noise is mostly due to air pressure. Pressure changes tend to cause slight tilting of the ground putting noise into the horizontals put not the vertical component.

## If possible, locate seismic stations away from the coast where noise conditions are better (noise is lower).

**Notes:**
For scientific study low noise is important so we try to locate stations away from the coast; however for this region that can be a problem as there is a lot of coast!