This is something that really needs a picture, but I will try to explain without one. A simple seismograph works by taking a pen attached to a fairly heavy weight and hanging it from a string. This above a rotating drum so that the pen draws a line as the rotates. The drum and the support from which the weighted pen hang are both firmily attached to the ground. When the ground shakes, the weight on the pen keeps it from moving with the ground. However the drum, being fermly attached to the ground moves with it. This causes the line to wiggle on the drum. The more violent the quake the grater the amplitude of the wiggle. This is a simple seismogragh. Modern seismographs use electronics to increase the sensitivity of the instrument and to allow the transmission of the readings to remote locations so that the seismologist does not need to go to each one to find out what has happened where.

I have no idea but go on to this website cos it will tell you www.ugs.com

"Seismometers are instruments that measure and record motions of the ground, including those of seismic waves generated by earthquakes, nuclear explosions, and other seismic sources. Records of seismic waves allow seismologists to map the interior of the Earth, and locate and measure the size of these different sources. The word derives from the Greek σεισμÏŒς, seismós, a shaking or quake, from the verb σείω, seíō, to shake; and μέτρον, métron, measure. Seismograph is another Greek term from seismós and γράφω, gráphō, to draw. It is often used to mean seismometer, though it is more applicable to the older instruments in which the measuring and recording of ground motion were combined than to modern systems, in which these functions are separated. Both types provide a continuous record of ground motion; this distinguishes them from seismoscopes, which merely indicate that motion has occurred, perhaps with some simple measure of how large it was." "Inertial seismometers have: A mass, usually called the inertial mass, that can move relative to the instrument frame, but is attached to it by a system (such as a spring) that will hold it fixed relative to the frame if there is no motion, and also damp out any motions once the motion of the frame stops. A means of recording the motion of the mass relative to the frame, or the force needed to keep it from moving. Any motion of the ground moves the frame. The mass tends not to move because of its inertia, and by measuring the motion between the frame and the mass the motion of the ground can be determined, even though the mass does move. Early seismometers used optical levers or mechanical linkages to amplify the small motions involved, recording on soot-covered paper or photographic paper. Modern instruments use electronics. In some systems, the mass is held nearly motionless relative to the frame by an electronic negative feedback loop. The motion of the mass relative to the frame is measured, and the feedback loop applies a magnetic or electrostatic force to keep the mass nearly motionless. The voltage needed to produce this force is the output of the seismometer, which is recorded digitally. In other systems the mass is allowed to move, and its motion produces a voltage in a coil attached to the mass and moving through the magnetic field of a magnet attached to the frame. This design is often used in the geophones used in seismic surveys for oil and gas. Professional seismic observatories usually have instruments measuring three axes: north-south, east-west, and up-down. If only one axis can be measured, this is usually the vertical because it is less noisy and gives better records of some seismic waves. The foundation of a seismic station is critical. A professional station is sometimes mounted on bedrock. The best mountings may be in deep boreholes, which avoid thermal effects, ground noise and tilting from weather and tides. Amateur, or less exotic instruments are often mounted in insulated enclosures on small buried piers of unreinforced concrete. Reinforcing rods and aggregates would distort the pier as the temperature changes. A site should always be surveyed for ground noise with a temporary installation before pouring the pier and laying conduit." [this article describes also some modern intruments:] "4 Modern instruments 4.1 Teleseismometers 4.2 Strong-motion seismometers 4.3 Other forms 4.4 Modern recording 4.5 Interconnected seismometers" Source and further information: http://en.wikipedia.org/wiki/Seismometer