Reason Mag Q1 2014 Seite 30

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understanding that vigirious shaking causes thicker turbidites For the 1700 earthquake scientists estimated its magnitude at about 9 0 The magnitude of other events were estimated by scaling the event s speci c turbidite thickness to that of the 1700 earth quake with the constraint that the total seis mic energy released by those earthquakes is not larger than the total energy available from the tectonic plate convergence The estimated magnitudes for the 19 full rupture events ranged from 8 7 to 9 4 The magnitude of the other 22 events all fell below 8 8 The use of assumptions makes it impossible to report the magnitudes with ironclad accuracy but the numbers represent a reasonable set of possibilities Probable maximum earthquake magnitude In part I of Megaquakes as seen in Reason issue 4 2013 we use the probable maximum earthquake magnitude concept instead of absolute maximum magnitude We employed the same methodology as in the previous arti cle to estimate the probable maximum earth quake magnitude except that we used both instrumentally recorded and turbidite based events for this subduction zone This is the rst application of paleoseismic data the turbi dites to this methodology Figure 4 illustrates the earthquake magnitude frequency distribu tions based on instrumentally recorded earth quake data red dots turbidite data green dots the theoretical tapered Gutenberg Richter TGR model black curves and the one used in the 2008 national seismic hazard maps by the United States Geological Survey USGS blue curve The TGR model nicely connects the instrumentally recorded and giant turbidite based events However discrepancies are observed at magnitudes less than 8 5 The median TGR model forecasts a higher rate than what the extent of the event The physical properties of the cores were then used to determine the thickness of each turbidite This study con rms that the region suf fered 19 megaquakes throughout the past 10 000 years each of which possibly rup tured the entire Cascadia margin Moreover the team uncovered 22 earthquakes that involved just the southern part of the fault The magnitude of prehistoric earthquakes Knowing how often earthquakes occur is invaluable In order to determine the proba ble maximum earthquake magnitude though we need to understand the magnitudes of the prehistoric earthquakes To achieve this we began by analyzing the turbite data As expected larger earthquakes cre ate larger rupture and slip areas along the fault plane The approximate rupture length is inferred from the extent of the area cov ered by the turbidite deposits The average slip size is estimated by the multiplication of the time interval between consecutive events and the tectonic plate convergence rate assuming that each great earthquake releases all the tectonic energy accumu lated since the previous event These cal culations are key factors in estimating the earthquakes magnitude Another method to estimate magni tude is by analyzing turbidite thicknesses LARGE EARTHQUAKES TRIGGER UNDERWATER LANDSLIDES DENSITY CURRENTS FLOW INTO DEEP WATER AND SETTLE TO FORM TURBIDITE DEPOSITS REPEATED LAYERS OF TURBIDITES UNDER THE SEAFLOOR RECORD PAST EARTHQUAKE EVENTS EVIDENCE OF PAST EARTHQUAKES Figure 2 Sketch of the formation of turbidites caused by earthquakes Credit Kathleen Cantner American Geosciences Institute AGI Professor Goldfinger s study confirms that the region suffered 19 megaquakes throughout the past 10 000 years each of which possibly ruptured the entire Cascadia margin Moreover the team uncovered 22 earthquakes that involved just the southern part of the fault 30 Reason ISSUE 1 2014 theSCIENCE