The Significant Earthquakes list contains all the earthquakes processed by the PRSN (Puerto Rico Seismic Network) that meet the following criteria: felt in Puerto Rico and the Virgin Islands, not felt but with a magnitude greater or equal to 3.5 in the Puerto Rico and the Virgin Island region and earthquakes outside the area of responsibility of the PRSN with a magnitude greater or equal to 6.0 in the Caribbean, Gulf of Mexico and the Atlantic. The earthquakes marked in red color are reported as felt.
The magnitude is a measure of the amount of seismic energy released by an earthquake. It is determined taking a base-10 logarithmic scale of the biggest ground movement. The magnitude can be measured in different forms using the seismograms. Some methods are based on body waves (which travel through great depths inside the earth) and other superficial waves. However, all of the methods are designed to work in a specific range of magnitudes. Sometimes preliminary magnitudes can be estimated even though the data that has come in is not sufficient for a completely accurate measurement. For example, the Tsunami Warning Centers can calculate a preliminary magnitude and location for an event with minimum data to make an estimate. In these cases, time is an important factor as it needs to be released quickly to emit a tsunami message, so these preliminary magnitudes are enough in case the information is needed urgently. These preliminary measurements might be less accurate and are updated as more data comes in.
|< 4.0||0 - 400 km||Based on the duration of the earthquake. To measure it, the decay time of the amplitude from the seismogram is used. It is often used to measure the magnitude on saturated seismograms, due to the limited dynamic of analogue instrumentation, it is impossible to measure maximum amplitudes.|
|2.0 - 5.4||0 - 400 km||The magnitude originally defined by Richter & Gutenberg for local earthquakes in 1935. Based on the maximum amplitude registered by Wood-Anderson torsion seismograms. Even though these instruments are no longer used, ML magnitudes are calculated using modern instruments with the appropriate adjustments.|
|5.0 - 8.0||20 - 180 degrees y
<40 km deep
|Calculated for shallow & far away earthquakes. It is based on the Rayleigh superficial wave amplitude measured in a period of around 20 seconds.|
|> 3.5||All||Based on the momentum of the earthquake. This is equal to the amount of ground rigidity times the average amount of displacement in the fault near the rupture area.|
|4.0 - 7.0||16 - 100 degrees
(Only for deep earthquakes)
|Based on the P wave amplitude (Body Wave). This scale is most appropriate for deep earthquakes.|
|PRSN2||Xmag||< 4.5||Local magnitude calculated using the average of the amplitudes using the 1992 Eaton Equation in the HYPO 2000 program. Also appears as m.1 in the earthquake search tool.|
|Fmag||< 4.5||Local magnitude calculated using the average of CODAS (duration) with the 1992 Eaton Equation in the HYPO 2000 program. Also appears as m.2 in the earthquake search tool.|
|NTWC3||Mwp||> 5.5||TAll||Provides an estimate based on the magnitude of the first P wave displacements.|
Agency, Puerto Rico Seismic Network or Tsunami Warning Center that reported the earthquake or provided the data for the localization of the earthquake. Some agencies are the Puerto Rico Seismic Network, the United States Geological Survey or the National Tsunami Warning Center.
The date & time of origin of an earthquake established by the moment of the fault rupture that generated the earthquake. In our notation (YYYY/MM/DD) The year is the first four digits, followed by the month then the day; followed by the hour, minute and seconds (hh:mm:ss.ss) in which the earthquake originated. In our web page, two time formats are used: UTC (Universal Time Coordinated) & PR Local Time (UTC-4). The UTC time format is also known as GMT (Greenwich Mean Time) . The hour used in our main page is the local PR time. In our main catalogue the time used is the UTC time. Seismologists use UTC time to prevent confusions by time zones & AM/PM time. However when you click on an event to see more information you will see both UTC & PR local time.
An earthquake begins with a rupture on the inside of the Earth (hypocenter). It's projection to the surface is known as epicenter, and the distance between the epicenter and hypocenter is known as focal depth. When we provide localization information, we refer to the geographic coordinates where the epicenter is located as Latitude & Longitude. The latitude is the number of degrees North or South from the equator and ranges between 0-90 between both poles. The longitude ranges between 0 and 180 degrees East or West from the Greenwich meridian line. For earthquakes with the best possible location (located in zones with dense seismogram networks), the uncertainty of the hypocenter varies by 100 meters horizontally and 300 meters vertically. For far away or global earthquakes the uncertainty increases to dozens of kilometers.
The depth (in kilometers) where the rupture of the earthquake starts. This depth can be relative to the sea level or the average elevation of the seismic stations. In the PRSN, the depth of an event is related to the average sea level. Depth can also be based on a reference, for example 25km. This occurs when the quality of the data obtained is not enough to calculate and the program establishes a predetermined measurement.
The name of a region is determined automatically by our locating system. The name of the region for earthquakes outside or our area of responsibility is assigned using the geographic-seismic regional scheme Flinn-Engdahl (proposed in 1965, defined in 1974 and revised in 1995). The limits of Flinn-Engdahl regions are defined in degree intervals and differ from established political and geographical limits. For example, if there is an earthquake in the Puerto Rico/Virgin Islands region, specifically in the Puerto Rico Trench, the system automatically assigns it a Puerto Rico region, even though the earthquake isn't inside of the island, but is a predetermined zone in the Flinn-Engdahl scheme.
The Puerto Rico & Virgin islands region is a zone inside of the latitudes 17.00°N to 20.00°N and the longitudes 63.50°W to 69.00°W. This includes Puerto Rico, The American/British Virgin Islands, and the East of the Dominican Republic. For earthquakes located inside of the Puerto Rico region, the PRSN has established seismicity zones. These zones were revised in 2004, so for events prior to that date, the zone could vary.Regions from 1986/01/07 to 2004/07/04:
The Puerto Rico Seismic Network provides distances from the epicenter of the earthquake to local places. These places can be towns, cities or other geographical locations. These distances could not be exact, due to localization errors (1km or more) or the imposibility of describing a place with just coordinates. We always try to provide a point of reference for the general public.
Since October 2009, the PRSN implemented the tsunami warning messaging in our bulletins and other online products. These messages follow agreements made by the PRSN and the National Tsunami Warning Center. These messages are a sentence long and contain the most important information related to the tsunami generation potential of an earthquake in the Puerto Rico/Virgin Islands region. Also included is the emission time (in UTC-4) for these messages as earthquake information could remain the same, but the tsunami messages might change. The following are the messages we provide in our tsunami messaging:
The intensity of an earthquake measures the earthquakes effects in different places. The intensity is determined in a location in specific, using the effects produced by the earthquake shake in objects, buildings, people and the terrain itself. The intensity scale that is generally used is the Modified Mercalli Intensity Scale. This scale goes from I (not felt) to XII (total destruction). The original scale (I to X) was created by Rossi-Forel in 1884, revised by Giuseppe Mercalli in 1906 and published by Harry O. Wood and Frank Neumann in 1931. The scale was improved by Charles Ritcher, who is known as the father of the Ritcher Scale. The intensity described by our products is obtained from reports obtained by the population, either through our Did You Feel It? web form, phone calls, shake maps (strong motion maps) or directly from the instrumentation.
The horizontal and vertical uncertainty of the localization is based on ERH and ERZ values. The uncertainty of a localization ranges from 100 to 300 meters horizontally for earthquakes with excellent localizations. The following parameters are used in the PRSN and provide information about the precision of the earthquake localization.
|rms||Root Mean Square is an error measurement that is obtained by comparing the average difference between the theoretical arrival time and the observed one in seconds, using our seismogram readings. The smaller the value, the better the localization. This depends on the precision of the velocity model, the weight assigned to the arrival times, and the procedure used to compute the earthquakes localization.|
|ERH||Horizontal error in meters|
|ERZ||Vertical error in meters|
|Gap||Maximum Azimuthal Gap between adyacent stations in degrees. Generally, the smaller the number, the better is the coverage from stations and the more accurate is the localization of the earthquake. Earthquakes with a gap that exceeds 180 degrees, usually have high ERH and ERZ values.|
|M||Type of Magnitude, explained above.|
|C||Quality of the earthquake localization. It is determined based on the RMS, ERH, ERZ, number of stations utilized and the geographic distribution. "A" is for a excellent localization and "D" for a poor one.|
Unique identification number for each event, generated using the date and time the earthquake occurred in UTC time. The format is YYYYMMDDhhmmss.
The number of phases used in the localization
Type of localization. A is automatic and R is revised by an analyst at the PRSN.